Neurobehavioral Changes Arising from Early Life Dopamine Signaling Perturbations

Abstract

Dopamine (DA) signaling is critical to the modulation of multiple brain functions including locomotion, reinforcement, attention and cognition. The literature provides strong evidence that altered DA availability and actions can impact normal neurodevelopment, with both early and enduring consequences on anatomy, physiology and behavior. An appreciation for the developmental contributions of DA signaling to brain development is needed to guide efforts to preclude and remedy neurobehavioral disorders, such as attention-deficit/hyperactivity disorder, addiction, bipolar disorder, schizophrenia and autism spectrum disorder, each of which exhibits links to DA via genetic, cellular and/or pharmacological findings. In this review, we highlight research pursued in preclinical models that use genetic and pharmacological approaches to manipulate DA signaling at sensitive developmental stages, leading to changes at molecular, circuit and/or behavioral levels. We discuss how these alterations can be aligned with traits displayed by neuropsychiatric diseases. Lastly, we review human studies that evaluate contributions of developmental perturbations of DA systems to increased risk for neuropsychiatric disorders.

1. Introduction

Brain dopamine (DA) signaling exerts powerful neuromodulatory effects on multiple brain functions including movement initiation and coordination, reinforcement, and higher order cognition (Robbins, 2003). Not surprisingly, DA signaling dysfunction has been implicated in many neurological and psychiatric disorders including Parkinson’s disease (Lees et al., 2009), schizophrenia (Howes and Kapur, 2009), bipolar disorder (BPD) (Ashok et al., 2017) and addiction (Hyman et al., 2006). In humans, the robust expansion and synapse formation of DA pathways during early postnatal development, which continues through adolescence (Larsen et al., 2020; Rothmond et al., 2012; Webster et al., 2011), aligns with proposals that dopaminergic perturbations contribute to neurodevelopmental disorders, notably, attention-deficit/hyperactivity disorder (ADHD) (Gizer et al., 2009; Mazei-Robison et al., 2005; Yang et al., 2007) and autism spectrum disorder (ASD) (Hamilton et al., 2013; Pavăl, 2017). As reviewed below, evidence indicates that dysregulation of developmental DA signalling impacts brain function at the molecular, circuit and behavioral levels, with long-lasting effects on physiology and behavior that can persist throughout adulthood. In this review, we highlight findings obtained from preclinical animal models, where genetic and pharmacological manipulations of DA signaling that afford mechanistic conclusions are possible. Through these studies, DA perturbations, exerted constitutively or at critical stages of brain development, are shown to trigger multiple layers of developmental plasticities and to produce behavioral and cognitive alterations that can be aligned with the features of neuropsychiatric disorders.

2. Overview of DA system during neurodevelopment

In rodents, midbrain DA neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are specified and begin to differentiate between embryonic day (E) 12 and E15, followed by processes of axonal extension and synaptic maturation. Neurons from the SN and the VTA project via the medial forebrain bundle and arrive at the dorsal and ventral striatum (i.e. nucleus accumbens (NAc), respectively, around E14, with projections reaching the medial frontal cortex (mFC) around E18 (Kalsbeek et al., 1988; Voorn e al., 1988). The expansion and maturation of dopaminergic innervation in these regions continues postnatally, with a mature pattern reached around postnatal day (PND) 60 (Kalsbeek et al.,1988; Olson & Seiger, 1972). Interestingly, even before DA afferents have expanded through the striatum and cortex, these regions express DA receptors (Araki et al., 2007; Sillivan & Konradi, 2011). Although both D1-type DA receptors (D1Rs) and D2-type DA receptors (D2Rs) are expressed in higher levels in the striatum when compared to the mFC, consistent with the density of dopaminergic projections in this region, their developmental trajectory exhibit distinct patterns (Sillivan & Konradi, 2011). From E15 to E21, levels of D1-type DA receptors (D1Rs) exhibit a steeper increase in mFC as compared to striatum, whereas D2R exhibit a steeper increase in the striatum, suggesting that the functional identity of these neurons start being shaped embryonically (Sillivan & Konradi, 2011).

As with many neural pathways, the maturation of forebrain dopaminergic projections follows a course of expansion and then contraction, with several molecular determinants of DA signaling peaking during late adolescence (~PND28–50), then reaching mature levels at ~PND60–90, including DA biosynthetic enzymes, DA receptors, and the DA transporter (DAT) (Suri et al., 2015). Due to the intense postnatal development of forebrain circuits during this time, the adolescence is considered a sensitive period for internal and external factors to stimulate or disrupt normal brain development, and that, in humans, these factors may impact risk for neuropsychiatric disorders (Suri et al., 2015). Indeed, as we will discuss further, alterations in the development of DA neurons can induce normal or pathological development of neurons and circuits within dopaminoceptive regions (Lieberman et al., 2018; Ye et al., 2017).

The first two postnatal weeks in rodents is an active maturation period for the striatum (Novak, et al., 2013). During this period, a gene network comprised of transcription factors driving cell fate decisions gives rise to the expression of genes that confer a functional identity to medium spiny neurons (MSNs), the most abundant class of dopaminoceptive neurons (Novak, et al., 2013), including expression of D1R and D2Rs, as well as the DA-and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), a master postsynaptic regulator of DA signaling. Expression of the latter genes precedes expression of myelination genes, suggesting an important role for DA signalling in the maturation of striatal efferent projections (Novak, et al., 2013).

In vitro studies with rodent slices and neuronal cultures indicate that activation of DA receptors can regulate neuronal migration and neurite outgrowth in a cell- and region-dependent manner. Thus, in mouse embryonic slice cultures, activation of D1Rs promotes cortical GABAergic neuron migration whereas D2R agonism decreases migration (Crandall et al., 2004). In cortical neuronal cultures, D1R stimulation decreases neurite outgrowth whereas D2R stimulation increases neurite outgrowth (Li et al., 2013; Reinoso et al., 1996; Sillivan et al., 2011; Song et al., 2002). In striatal neuronal cultures, activation of D1R receptor promotes neurite outgrowth that is associated with increased abundance and complexity of neuritic arborization (Schmidt et al., 1996). In primary midbrain cultures, chronic activation of D2-type auto-receptors (D2ARs) has been reported to decrease the number of DA neuron axon terminals, an effect accompanied by a protein kinase A-dependent decrease in DA release (Fasano et al., 2008). Optogenetic experiments have provided in vivo evidence of frequency-dependence of developmental DA effects where phasic, but not tonic, dopaminergic activity increased the formation of mesofrontal axonal boutons in adolescent mice, while adult stimulation lead to no changes, and these effects were accompanied by increased circuit activity and suppressible by D2R agonism (Mastwal et al., 2014).

DA plays a role in dendritic spine formation and maturation of projection targets. Interestingly, both hyperdopaminergia and DA depletion can result in decreased density of dendritic spines of MSNs, suggesting that the formation and/or stabilization of these signaling compartments is finely tuned to the magnitude of DA signaling (Money and Stanwood, 2013). Indeed, co-culture of DA neurons with MSNs has been shown to promote the formation of immature, spine-like protrusions in the MSNs, an effect that can be reversed by D1R and D2R antagonists, whereas moderate pharmacological activation of these receptors in MSN monocultures can increase spine density (Fasano et al., 2013). Moreover, spine density of layer V cortical pyramidal neurons is reduced in adult D1R and D2R knockout mice (Wang et al., 2009). Pharmacological studies have provided further evidence of a role of DA in spine formation. Thus, a single administration at PND21 of either of the D2R agonists quinpirole or bromocriptine has been found to acutely decrease spine density of hippocampal CA1 neurons, whereas 5 day administration of the D2R antagonist eticlopride, increases spiny density (Jia et al., 2013). No difference was seen when quinpirole was administered to adult mice (at 8 or 12 weeks of age), suggesting that D2R regulates spine density in an age-dependent manner (Jia et al., 2013). Consistent with these studies, deletion of dysbindin protein, which results in increased surface expression of D2Rs, also promotes a decreased density of mature spines in the hippocampus of adolescent mice accompanied by an impairment in spatial working memory in adulthood (Jia et al., 2013). Presynaptic modulation of DA signaling capacity and dynamics has also been shown to influence postsynaptic spine maturation. For example, DAT ^−/− mice exhibit dendritic spine loss in striatal MSNs at 6 to 7 months of age (Berlanga et al., 2011). There are certainly discrepancies apparent in the literature concerning the direction of effects of DA signaling on spine formation and stability, likely due to region and circuit specificities, in vitro versus in vivo approaches, and the nature and intensity of DA stimulation (or antagonism) utilized across studies, and there are reasons to question how these manipulations relate to physiological conditions in vivo. A further, critical point for divergence, is likely the developmental time-point at which interventions are pursued. It is well appreciated that changes induced by constitutive gene knockouts may be different from those induced by time-limited pharmacological manipulations. Additionally, as discussed below, pharmacological manipulations of DA signaling pathways at different stages of development can elicit different outcomes at neuronal, circuit and behavioral levels. Nonetheless, the cumulative impression left by this work is that DA exerts powerful actions on the number and formation of spines, a critical signaling compartment where perturbations are likely to drive enduring changes in cell signaling and behavior.

In addition to structural changes, DA signalling during development modulates the maturation of electrophysiological properties of postsynaptic neurons. An increase in striatal DA neurotransmission precedes the maturation of MSN excitability that occurs during the fourth postnatal week (Lieberman et al., 2018). At PND28, MSNs begin to show defined up (depolarized) and down (hyperpolarized) states, characteristic of the mature electrophysiological transitions of these cells and similar to properties seen in the adult mouse (Tepper et al., 1998). Mouse D1R-expressing MSNs that are developmentally deficient in striatal DA fail to undergo maturation and maintain hyperexcitability, with evidence that this phenotype arises from altered phosphatidylinositol 4,5-biphosphate signaling (Lieberman et al., 2018). These deficits can be corrected by DA replacement when provided from birth, but not during adulthood, defining a temporal window of DA action during postnatal development that is required for the physiological maturation of MSNs (Lieberman et al., 2018). These findings are accompanied by a transient increase observed to occur in mouse VTA DA neuron activity until ~PND45, with higher non-bursting activity and longer bursts, activity that reduces and stabilizes once the animal reach adulthood (McCutcheon et al., 2012). In summary, adolescence comprises a period of intense developmental change and plasticity of DA neurons and their targets. As we discuss below, this sensitive period provides a framework for enduring effects of early life drugs of abuse that directly manipulate DA signaling.

3. Developmental effects of DA neuron pharmacological manipulations

3.1. Long-term behavioral changes associated with juvenile and adolescent exposure to dopaminergic drugs

Adolescence is a particularly sensitive period for the effects of psychostimulant drugs, representing a window when use can most readily lead to substance use disorders (SUDs), including addiction (Anthony & Petronis, 1995; Wong et al., 2013). Psychostimulants such as amphetamine (AMPH) and methylphenidate (MPH) are also used to treat ADHD (e.g. AMPH mixture-based treatment Adderall™ and MPH-based treatment Ritalin™), and ADHD is a disorder typically diagnosed during childhood and adolescence, with symptoms expected to be evident by the age of 12 (American Psychiatric Association, 2013). These psychostimulants, as well as cocaine (COC), target the DAT and through different mechanisms lead to elevations of extracellular DA. MPH and COC are DAT blockers, inhibiting DA reuptake, whereas AMPH acts as a competitive substrate and can also promote DAT-mediated DA efflux (Gnegy et al., 2004; Gowrishankar et al., 2014). Therefore, several studies have evaluated the abilities of psychostimulants to impact features of neurodevelopment and future risk for SUDs.

Along with AMPH, MPH is commonly prescribed for the treatment of ADHD in children and teenagers. However, the potential misuse and abuse of MPH by adolescents and adults remain of clinical concern. Exposure to a clinically-relevant dose of MPH during adolescence in mice has been shown to increase COC self-administration in adulthood, but not psychomotor responses to a COC challenge (Brandon et al., 2001) (Table 1). In spontaneous hypertensive rats (SHR), an admittedly controversial model for ADHD (Mergy et al., 2014b), greater breakpoints for COC self-administration, a measure thought to model reward motivation, have also been reported in adults if animals received MPH treatment during adolescence (Baskin et al., 2015). In addition, AMPH, MPH and COC exposure from PND22 to PND31 induced cross-locomotor sensitization to a subacute methamphetamine dose in adulthood, indicative of a long lasting plasticity of DA signaling pathways (Shanks et al., 2015).

Table 1.

Pharmacological and physiological perturbations on DA system during postnatal development

Perturbation	Age	Species	Cellular-level changes	Brain region	Electrophysiological changes	Behavioral consequences	Reference (s)
Amphetamine	PND22–31	Mouse	↑ in the expanse of DA innervation in adult mice ↓ density and total number of varicosities in adult mice	mPFC	N/A	↑ Conditioned locomotor activity (contextual salience attribution)	Reynolds et al., 2015
Low dose of Amphetamine	PND22–31	Mouse	No effect on DA innervation (in contrast with higher doses)	mPFC	N/A	Improved cognitive performance on go/no-go task	Cuesta et al., 2019
Amphetamine	PND22–31	Mouse	↓ dopamine varicosities in adult mice	oPFC	N/A	N/A	Hoops et al., 2018
Quinpirole and bromocriptine	PND21	Mouse	↓ spine density	HPC	N/A	N/A	Jia et al., 2013
6-OHDA	PND3–5	Rat	↑ 5-HT innervation	striatum	N/A	Spontaneous hyperactivity	Luthman et al., 1987; Stachowiak et al., 1984; Towle et al., 1989
6-OHDA	PND5	Rat	↓ 5-HT innervation	PFC	N/A	N/A	Cunningham et al., 2005
Methylphenidate	PND20–35	Rat	N/A	N/A	N/A	Aversion to cocaine-paired side	Andersen et al., 2002; Brenhouse et al., 2009
Methylphenidate	PND28–48	Rat	N/A	N/A	N/A	↑ cocaine self-administration	Brandon et al., 2001; Baskin et al., 2015
Methylphenidate	PND30–44	Mouse and rat	↓ synaptic active zone length ↑ spine density	PFC	N/A	Preference for a small and certain reward over a large and uncertain on a 2-choice operant behavior test	Kim et al., 2009; Cavaliere et al., 2012
Cocaine	PND28–39	Mouse and rat	N/A	N/A	N/A	↑ locomotor sensitization and drug seeking	Badanich & Kirstein, 2012; Valzachi et al., 2013
Cocaine	PND28–46	Mouse and rat	Enlarged dendritic spine head size; ↓ synapses; ↓ dendritic spine density	PFC	N/A	↑ perseverative errors; ↑ delay discounting; inability to develop contextual fear responses; ↑ anxiety-like behavior; ↑ habit-based behavior	Sillivan et al., 2011; Pope et al., 2016; DePoy et al., 2014; Zhu et al., 2018
Cocaine	PND28–42	Rat	Altered neuronal morphology; ↑ Astrocyte activity; ↓ neurogenesis	HPC	N/A	↑ anxiety-like behavior	Zhu et al., 2016; Garcia-Foster et al., 2017
Apomorphine	PND38–51	Rat	N/A	N/A	N/A	Impaired latent inhibition	Shao et al., 2010
L-Dopa	PND1–5	Mouse	Increased DA and DOPAC tissue content	Striatum	N/A	Hyperactivity in juvenile females, ↓ total liquid consumption and sucrose preference in males	De Matos et al., 2018
Phasic optical stimulation of VTA during adolescence	PND28–42	Mouse	↑ Formation of mesofrontal axonal varicosities adolescence	Frontal cortex	↑ Total LFP response, increased response duration, high frequency oscillations (4–55Hz)	Suppressed novelty induced and psychomotor activity following prior phasic stimulation	Mastwal et al., 2014

Abbreviations: DA, dopamine; PND, postnatal day; N/A, not assessed; mPFC, medial prefrontal cortex; oPFC, orbito prefrontal cortex; HPC, hippocampus; PFC, prefrontal cortex; VTA, ventral tegmental area; LFP, local field potential.

In contrast to these model system studies, studies in humans have provided evidence that early MPH treatment may actually have a protective effect on risk for later life SUDs (Wilens et al., 2003), and other studies in animal models concur with these findings. For example, a study by Andersen and colleagues (2002) reported that, depending on the dose, administration of MPH to juvenile rats leads to no preference or even aversion to COC-paired environments on a conditioned place preference paradigm, a test typically interpreted to reflect “drug-liking”, in adulthood. A decrease in COC-induced locomotor activation was also found (Andersen et al., 2002). When the MPH treatment was performed during adulthood, however, these mice did not show aversion to COC and exhibited normal locomotor responses, suggesting that the effects of MPH critically depend on when during development the treatments began (Andersen et al., 2002). A similar result was found by Brenhouse and colleagues, where the investigators demonstrated that administration of MPH to juvenile males resulted in aversion to COC-paired environments during adulthood, though, such treatments enhanced COC place preference when administered to female adolescent mice (Brenhouse, et al., 2009). In addition, some groups have shown no effect of MPH when given prior to, or during adolescence, on COC sensitivity during adulthood. For example, in mice, treatment with MPH from PND15–28 did not lead to an increase in locomotor sensitization to COC (Guerriero et al., 2006). Similarly, adolescent exposure to MPH in SHR rats, did not affect locomotor activation or conditioned place preference to COC in response to any of the 4 doses tested (Zhang-james et al., 2020). Interestingly, there are also reports that early MPH exposure in rats results in increased COC intake in a self-administration paradigm without affecting conditioned place preference, and remarkably this enhancement was only seen in male mice (Crawford et al., 2011). Therefore, for animal models, the literature is controversial and the effects of MPH on future drug abuse, unclear, and they appear to depend on the measures obtained. On the other hand, human studies thus far do not consistently point to increased potential for future SUDs after early treatment of ADHD subjects with MPH, and these studies are discussed later in this review. It is also important to note that the effects of psychostimulant drugs in wild-type animals differ from those in many animal models for ADHD (Gainetdinov, 2010). Therefore, although the effects of psychostimulant misuse in humans remains a concern, a direct causal evidence for MPH to increase risk for SUDs in a typically developing animal needs to be considered critically with respect to clinical relevance, particularly regarding treatments of subjects with ADHD.

Effects of MPH treatment during adolescence on later ethanol intake have also been investigated. One report shows that although a decrease in the locomotor depressant effect of a low dose of ethanol in adulthood was observed in animals treated with MPH in adolescence, suggesting a drug-induced difference in sensitivity to the locomotor effects of ethanol, a similar consumption of, and preference for ethanol compared with water was seen (Crowley et al., 2014). Additionally, another study revealed that repeated treatment with AMPH, but not with MPH, during early-mid adolescence enhanced ethanol intake during late adolescence in male, but not female rats (Ruiz et al., 2018). Conversely, in SHR rats, an increase in anxiety-related behaviors in both sexes and in ethanol consumption in female adult animals that have been treated with MPH during adolescence have been reported (Vendruscolo et al., 2008).

The inconsistency between findings regarding the effects of MPH and AMPH pre-and during adolescence on future substance use may be related to several methodological differences observed in the cited studies. Although most studies used 2 mg/kg MPH, a dose proposed to achieve similar blood levels of those used clinically, the doses on the cited studies ranged from 1–10 mg/kg. In addition, the start of treatment varied from PND11–35, with treatment duration ranging from 4 to 30 days. Considering the series of developmental events occurring from birth to adulthood (PND60), these administration regimens are spanning age ranges comprising multiple, dynamic neurodevelopmental stages. Although only four of these studies administered MPH orally, and using similar doses, one of them reported increased self-administration (Baskin et al., 2015), another one showed aversion to COC (Brenhouse et al., 2009), and the two others presented no difference in ethanol consumption (Crowley et al., 2014) or preference for a COC-paired environment (Zhang-james et al., 2020). For these studies, strain and species differences (i.e. C57Bl/6J mice, Sprague-Dawley rats and SHR rats), the time of the treatment - whether administered on dark vs light periods, may also have contributed to the contrasting results.

In addition to AMPH and MPH that are used therapeutically, psychostimulants such as COC, and a recent class of DAT targeted agents termed “bath salts” (Prosser and Nelson, 2012), are recreationally used and are frequently abused. COC addiction is one of the most prevalent drug abuse disorders and remains a global concern. Indeed, the estimated rates for COC manufacture and quantity seized have recently reached an all-time high (UNODC, 2019). Whereas strong evidence is lacking that therapeutic psychostimulant use increases risk for SUDs, multiple studies indicate that adolescent COC use confers increased susceptibility for later drug addiction (Anthony & Petronis, 1995; Wong et al., 2013). Intrinsic neurochemical differences between the developing versus adult brain may contribute to the higher vulnerability of adolescents to drugs of abuse. Adolescent rats show a larger increase as compared to adult animals in DA extracellular levels in the dorsolateral (DL) striatum, a region involved in habit formation ( Yin et al., 2004; Lipton et al., 2019), in response to different drugs of abuse (Corongiu et al., 2019). Moreover, cortical neurons that project to the NAc, an important component of the reward circuitry, express higher levels of D1Rs during adolescence in rodent models, a finding that correlates with increased COC conditioned place preference (Brenhouse et al., 2008). In addition, dopaminergic neurons in the VTA fire faster in adolescent rats than in adults (McCutcheon et al., 2012) and decreased inhibition along with higher variability in adolescent phasic neuronal activity in the orbitofrontal cortex was evident in a reward-motivated behavioral task (Sturman and Moghaddam, 2011).

Exposure to COC during adolescence in rats and mice not only leads to enhanced locomotor sensitization and COC-seeking behavior in adulthood (Badanich & Kirstein, 2012; Valzachi et al., 2013), but also to additional behavioral alterations relevant to neuropsychiatric disorders (Pope et al., 2016; Sillivan et al., 2011; Valzachi et al., 2013; Zhu et al., 2016). Additionally, rodent exposure to COC during adolescence results in the appearance in adulthood of despair and anxiety-like behaviors (Zhu et al., 2016), inability to develop contextual fear responses (Sillivan et al., 2011), more perseverative errors in a spatial-discrimination-reversal test (Pope et al., 2016) and faster delay discounting for a reward (Pope et al., 2016). Likewise, AMPH administration at a high dose during early adolescence in mice leads to impaired behavioral inhibition in adulthood (Reynolds et al., 2019). Noteworthy, this effect is not reproduced by a therapeutic mimetic dose of AMPH, which, conversely, improves adult cognitive performance in the go/no-go test (Reynolds et al., 2019; Cuesta et al., 2019).

The effects of increased DA signalling produced perinatally by systemic administration of the DA precursor L-dihydroxyphenylalanine (L-DOPA) has also been investigated (de Matos et al., 2018). Juvenile mice that have been treated with L-DOPA from PND1–5 exhibit behavioral differences that are sex-dependent (de Matos et al., 2018). Thus, hyperactivity in the open field test, with increased distance travelled and rearing, is seen in juvenile females but not in males, whereas a decrease in total liquid consumption and sucrose preference is observed only in males (de Matos et al., 2018). No differences in anxiety- or depression-like behaviors were reported for males or females (de Matos et al., 2018). These studies are interpreted to indicate that a transient perinatal DA increase leads to an increase in exploratory behavior in females and altered hedonic behavior in males during juvenile period (de Matos et al., 2018). Conclusions are limited due to the cellular complexity of these effects since L-DOPA can enter into, and be converted to DA in non-dopaminergic neurons such as those releasing serotonin or norepinephrine (reviewed in Chagraoui et al., 2020). Additionally, whether these behavioral changes persist into adulthood remains unknown.

3.2. Long-term cellular and molecular alterations associated with juvenile and adolescent pharmacological manipulation by dopaminergic drugs

The behavioral outcomes cited above are likely related to several morphological and functional alterations induced as consequence of psychostimulant exposure during postnatal development. Thus, repeated, high-dose AMPH exposure during adolescence, but not in adulthood, promotes an increase in the volume of dopaminergic innervation of the PFC in mice (Reynolds et al., 2015) though a reduction in TH-positive varicosities was observed (Reynolds et al., 2015). This AMPH effect appears to be dependent on the netrin-1 guidance cue receptor DCC (deleted in colorectal cancer) signalling, since mice lacking DCC within DA neurons do not exhibit such changes in DA synaptic connectivity in the mPFC (Reynolds et al., 2015). The alterations did not occur in the NAc, and no difference in soma size or number of DA neurons were found (Reynolds et al., 2015). The impact of AMPH on netrin-1 and DCC expression is of high relevance because this signalling provides guidance for dopaminergic axons when they innervate target regions, evidenced by targeting errors and ectopic growth of mesolimbic axons from the NAc to the PFC as a consequence of genetically-reduced DCC signalling (Hoops and Flores, 2017; Reynolds et al., 2018).

Noteworthy, the effects of AMPH administration on netrin-1/DCC regulation and mesocortical DA development during adolescence in mice appear to be dose-dependent. Whereas a high dose of AMPH during early adolescence was reported to reduce netrin-1 expression in the NAc and PFC and to decrease mesocortical dopaminergic connectivity and function, these effects were not observed after a lower dose that mimics therapeutic levels in humans (Reynolds et al., 2019; Cuesta et al., 2019; Reynolds et al., 2015). As previously discussed for MPH, although the question whether AMPH treatment may have detrimental effects on neurodevelopment is important, the intrinsic changes induced by disorders such as ADHD per se, and how these features interact with AMPH and other psychostimulant drugs in a therapeutically prescribed dosage need to be taken into account. In this sense, construct-valid animal models featuring altered DA signalling present an advantageous opportunity to investigate molecular mechanisms for the therapeutic and/or negative effects of these drugs with higher clinical relevance (Mergy et al., 2014b).

Morphological changes in response to MPH treatment during development have also been reported. Chronic MPH treatment starting at 4–5 weeks of age increases dendritic spine density in D1-MSNs of NAc core and shell, while for D2-MSNs this increase is seen only at the shell (Kim et al., 2009). MPH during the juvenile period was also reported to increase dendritic length and complexity in the dorsal anterior cingulate cortex of Octon degus rodents (Zehle et al., 2007). Important to note, these changes were observed shortly after the end of drug treatment and, therefore, could be transient. Conversely, MPH from PND30–44 resulted in increased synapse number with a reduction in the synaptic active zone length and an increase in spine density in the prefrontal cortex 6 weeks after the end of treatment (Cavaliere et al., 2012). These findings suggest that, in wild-type animals, MPH treatment pre- and during adolescence induces synaptic remodelling, likely in a long-lasting manner.

Adolescent COC exposure of rodents has also been associated with neuronal structural changes later in adulthood. Thus, decreases in spine density and synapse related proteins such as PSD-95 and synapsin I have been observed in the mPFC (Zhu et al., 2018). Moreover, persistent alterations of dendrite structure in the orbitofrontal cortex have been described, including shortened and less complex dendrites (DePoy et al., 2014). Interestingly, increased astrocyte activity in the hippocampus (Zhu et al., 2016) and deficient neurogenesis in rats during adulthood have been reported (García-Fuster et al., 2017), suggesting broader, enduring effects of adolescent COC exposure. Overall, these changes in synaptic remodeling may contribute to increased risk in adulthood of psychiatric disorders (Jordan and Andersen, 2017) following juvenile COC exposure.

Although it is natural to consider juvenile elevations in DA signalling given the actions of psychostimulants, a reduction in dopaminergic function and its impact on neurodevelopment has also been studied and has provided insights into developmental contributions made by DA. DA depletion studies have typically made use of DA neuron toxins (i.e. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA)), inhibitors of vesicular DA transport such as reserpine, or genetic models such as the Aphakia Pitx3 ^−/− mouse, where DA neurons do not differentiate (Hilario et al., 2016; Nunes et al., 2003; Ferro et al., 2005; Leão et al., 2015). In contrast to genetic models, toxin-induced lesions can be performed postnatally, allowing for normal midbrain DA neuron development to proceed prior to postnatal DA neuron ablation. However, both pharmacological and genetic models result in comparable loss of DA axons in the striatum (Niederkofler, et al., 2015). Adult mice that were neonatally administered 6-OHDA have been reported to exhibit altered levels of mRNAs involved in cytoskeleton formation and axon guidance, changes accompanied by altered cortical width (Krasnova et al., 2007). In addition to a reduction in cortical thickness, Bouchatta and colleagues have reported a decrease in spine density and diameter of anterior cingulate cortex pyramidal neurons in adult mice following neonatal DA depletion (Bouchatta et al., 2018). These findings suggest that developmental DA depletion can induce morphological plasticities in regions that typically receive dopaminergic projections.

Interestingly, DA depletion studies also demonstrate a major influence of DA on development of 5-HT projections, highlighting a functional and structural interplay between these systems throughout development (Niederkofler, et al., 2015). Thus, brains of adult mice with a knockout of Pitx3, a transcription factor required for the development of DA neurons in the substantia nigra (Nunes et al., 2003), or mice where DA neurons have been neonatally lesioned by MPTP or 6-OHDA administration show a pronounced increase in the density of dorsal striatal 5-HT axons, 5-HT tissue content, and extracellular 5-HT levels, as well as 5-HT2A receptor mRNA, suggesting that dopaminergic innervation and neurotransmission suppresses 5-HT innervation of DA target areas during early postnatal development (Luthman, et al., 1987; Smits, et al., 2008; Stachowiak, et al., 1984; Li et al., 2013). Noteworthy, these alterations seem to be region-dependent as unlike the striatum, in the PFC, Cunningham et al demonstrated that early postnatal DA neuron ablation results in a reduction of 5-HT innervation, suggesting that DA signaling in this region normally stimulates rather than suppresses 5-HT axonal arborizations (Cunningham, et al., 2005). To our knowledge, this surmise has not been tested with developmental exposure to specific DA receptor agonists or psychostimulants, or with genetic models of altered DA signaling. It also remains possible that DA signaling sustains a normal level of serotonergic input but does not enhance 5-HT inputs beyond densities dictated by other factors, such as neurotrophins.

4. Genetic manipulations of the DA system impacting postnatal neurodevelopment

Mutation of the DAT gene (Slc6a3) that yield alterations in DA signaling during development has been used to generate animal models of neuropsychiatric diseases, including ADHD, ASD and BPD. DAT KO mice and rats exhibit increased extracellular DA levels and a decrease in the amplitude of evoked DA release and DA total tissue content, indicative of elevated tonic DA signalling but reduced phasic DA signalling (Giros et al.,1996; Jones et al., 1998; Leo et al., 2018). These alterations are reflected in the most visibly pronounced behavioral phenotype exhibited by DAT KO mice - spontaneous hyperlocomotion in a novel environment. In addition, cognitive impairments including impaired extinction of habitual memory (Hironaka et al., 2004), spatial learning and memory deficits (Gainetdinov et al., 1999), impaired behavioral inhibition (Gainetdinov et al., 1999) and increased bias towards a positive tastant are observed (Costa et al., 2007) (Table 2). Notably, although the initial characterization for the DAT KO mice reported indifferent locomotor activity in response to COC and AMPH (Giros et al., 1996), further studies with DAT KO rats showed that the spontaneous hyperactivity of these animals can be counteracted by AMPH and MPH in both DAT KO mice and rats (Gainetdinov et al., 1999; Leo et al., 2018), suggesting targets other than DAT is involved in the calming effects of these psychostimulants. In addition to hyperlocomotion, DAT KO rats exhibit reduced preference for sucrose and a rigid pattern behavior accompanied by compulsive stereotypies in the intolerance-to-delay task (Cinque et al., 2018). On the structural side, multiscale analysis of MSNs of 6 to 7 month-old DAT KO mice using both light and electron microscopy techniques has revealed a highly localized loss of spines on the proximal portion dendrites as opposed to an overall morphological alteration, since no changes were found in dendritic length, number, or intersections, nor a difference in synapse to neuron ratio (Berlanga et al., 2011).

Table 2.

Impact on postnatal neurodevelopment of genetic manipulations of DA system

Perturbation	Species	Cellular-level changes	Brain region	Electrophysiological changes	Behavioral consequence	Reference (s)
Pitx3 −/−	Mouse	Lack of ontogenetic reduction in excitability (maturation) in dSPNs	Dorsal striatum	Elevated RMP and input resistance, more action potentials in response to injected current at P28	N/A	Lieberman et al., 2018
Pitx3 −/−	Mouse	Shorter dendritic trees ↓ spine density ↑ excitability	Striatum (dSPNs and iSPNs)	More action potentials evoked by depolarizing current and lower threshold	Impaired motor coordination (rotarod and beam transversal test)	Suarez et al., 2018
Pitx3 −/−	Mouse	↑ 5-HT innervation ↑ 5-HT2AR and SERT	Striatum	N/A	N/A	Li et al., 2013; Smits et al., 2008
Dysbindin mutant mice – increased surface D2R expression	Mouse	Spine deficiency	HPC	↓ mEPSC frequency	Impairment of spatial working memory	Jia et al., 2013
DAT +/−	Mouse	Decreased expression of Homer1a	PFC	N/A	Inattentive and impulsive phenotypes that can be rescued by low doses of amphetamine. Short-term memory deficits in adult females.	Mereu et al., 2017
DAT KD	Mouse	Long-lasting changes in cortico-striatal neurotransmission	Dorsal striatum	Altered amplitude and frequency of spontaneous glutamate receptor-mediated synaptic currents, shorter half-amplitude durations and faster decay times. No effect of AMPH or COC on spontaneous glutamate currents	↑ motivation, improved reversal learning	Wu et al., 2007; Milienne-Petiot et al., 2017
DAT −/−	Mouse	Highly localized loss of dendritic spines	Striatum MSNs	↓ amplitude of evoked DA release	Hyperactivity; impaired extinction of habitual memory; spatial learning and memory deficits; impaired behavioral inhibition	Berlanga et al., 2011; Hironaka et al., 2004; Jones et al., 1998; Gainetdinov et al., 1999;
DAT −/−	Rat	N/A	N/A	N/A	Hyperactivity working memory impairment, sensorimotor deficit; ↓ sensitivity to reward stimuli and compulsive stereotypy	Leo et al., 2018; Cinque et al., 2018
DAT Val559	Mouse	Anomalous DA efflux	Midbrain	Spontaneous and altered evoked D2R-mediated IPSCs	↑ motivation for reward, ↑ impulsivity; ↓ locomotor response to AMPH and MPH; Lack of cocaine-induced hyperlocomotion	Mazei-Robinson et al., 2008; Davis et al., 2018; Mergy et al., 2014; Stewart et al., 2019
DAT Val559	Mouse	↑ DAT surface expression	Striatum	Blunted AMPH- and 4-AP–Evoked DA Release, tonic presynaptic D2R stimulation	See above	Gowrishankar et al., 2018
DAT Met356	Mouse	Anomalous DA efflux and ↓ DA reuptake	Striatum	N/A	hyperlocomotion, altered social behavior, repetitive behavior	DiCarlo et al., 2019

Abbreviations: DA, dopamine; dSPNs, direct pathway spiny projection neurons; iSPNs, indirect pathway spiny projection neurons; RMP, resting membrane potential; N/A, not assessed; 5-HT, serotonin; 5-HT2AR, serotonin 2A receptor; SERT, serotonin transporter; HPC, hippocampus; mEPSC, miniature excitatory postsynaptic currents; PFC, prefrontal cortex; KD, knockdown; AMPH, amphetamine; COC, cocaine; MSNs, medium spiny neurons; D2R, dopamine D2 receptor; IPSCs, inhibitory postsynaptic currents; MPH, methylphenidate; DAT, dopamine transporter.

Similar to studies with DAT knockout mice, the genetically-induced DAT hypofunction evident in DAT+/− mice has been reported to produce hyperactivity that persists from adolescence to adulthood (Mereu et al., 2017; Wu, et al., 2007). In addition, adolescent male and female DAT ^+/− mice display recent memory impairments, though interestingly, this phenotype is lost once male mice reach adulthood, whereas females continue to exhibit this phenotype (Mereu et al., 2017). Noteworthy, this finding is consistent with a recent study showing that females with a childhood diagnosis of ADHD exhibit significantly more ADHD symptoms throughout adulthood when compared to males (Millenet et al., 2018). DAT heterozygosity also produces inattentive and impulsive phenotypes in adulthood, recapitulating ADHD-related phenotypes that can be rescued by low dose sub-chronic treatment with AMPH (Mereu et al., 2017). These behavioral alterations are accompanied by lower levels of the neuronal plasticity-associated protein Homer1a in PFC regions related to executive functions (Mereu et al., 2017).

In order to induce chronic hyperdopaminergia without a complete loss of DAT as a psychostimulant target and avoiding a growth retardation phenotype exhibited by the DAT-KO, a DAT knockdown (DAT-KD) mice, expressing 10% of wild-type levels, has been developed (Zhuang et al., 2001). Electrophysiological recordings from medium spiny neurons in the dorsal striatum of young adult DAT KD mice indicate alterations in both amplitude and frequency of spontaneous glutamate receptor-mediated synaptic currents in the MSNs, with shorter half-amplitude durations and faster decay times (Wu et al., 2007). These neurons also respond differently to stimulant drugs and D2R agonists and antagonists (Wu et al., 2007). Whereas in WT animals AMPH, COC and D2 agonist quinpirole reduced the frequency of spontaneous glutamate currents, in DAT-KD mice, either no changes or only small increases in frequency are observed (Wu et al., 2007). These findings indicate that developmentally increased extracellular DA levels leads to long-lasting changes in cortico-striatal neurotransmission that may be mediated by changes in D2R function (Wu et al., 2007).

DAT KO animals have provided compelling information as to the essential nature of DAT expression for proper control of DA signaling and DA-linked behaviors. However, lifelong deletion of the transporter leads to profound neuroadaptive changes that may or may not accompany more subtle changes in transporter structure, trafficking or gene expression. With a loss of DAT throughout life, these models also present challenges to the timing of key mechanistic insults. Use of conditional DAT KO/KD models can help address these issues. Additionally, use of constitutive DAT KO animals to model DA-associated behavioral disorders such as ADHD and BPD have inherently limited construct validity, given that humans harboring homozygous loss of function DAT mutations exhibit Infantile Parkinsonism/Dystonia, as opposed to hyperactivity or other ADHD-related symptoms (Kurian et al., 2009). In addition, the hyperlocomotion seen in DAT KO mice can be reversed by antidepressants (Gainetdinov et al., 1999), whereas these drugs do not suppress ADHD core symptoms.

In order to provide models that overcome limitations associated with a constitutive loss of DAT, we and others have generated mouse lines that express functional DAT coding variants identified in individuals with neuropsychiatric disorders, including ADHD, BPD and ASD (Mergy et al., 2014; Mergy et al., 2014b; Davis et al., 2018; Gowrishankar et al., 2018; DiCarlo et al., 2019). For example, we identified the rare DAT coding variant Val559 from two adolescent male siblings with ADHD (Mazei-Robison et al., 2005), a variant previously identified in a female with BPD (Grünhage et al., 2000) and later found in two unrelated boys with ASD (Bowton et al., 2014). We developed a Val559 knock-in (KI) mouse line where the variant is expressed at the native DAT gene locus as in the humans bearing the mutation. In vitro studies demonstrated that the variant displays anomalous DA efflux despite normal DA uptake (Mazei-Robison et al., 2008; Mazei-Robison, et al., 2005) whereas KI mice exhibit elevated basal DA levels and a reduced locomotor response to AMPH and MPH, with a complete locomotor insensitivity to COC (Mergy et al., 2014; Stewart et al., 2019). The latter effect has been traced to a serotonergic plasticity arising in the context of basal hyperdopaminergia generated by the DAT Val559 mutation, since COC-induced hyperlocomotion is evident in DAT Val559 treated with a COC analog that exhibits reduced SERT action, with genetic elimination of COC binding at SERT, and by a combination of COC and a 5-HT2C receptor antagonist (Stewart et al., 2019). Additionally, male DAT Val559 mice exhibit an elevated tissue content of 5-HT in the striatum and cortex, suggestive of increased neurotransmitter stores and/or axonal density (Mergy et al., 2014). Interestingly, although the DAT Val559 mice display locomotor insensitivity to COC, they still exhibit COC conditioned place preference, indicating that the rewarding effects of the drug are still present. Indeed, it has been shown that the DAT Val559 variant induces enhanced motivation for reward in a progressive ratio test using sucrose and increased impulsivity behaviors when they are required to delay responding in order to get a reward on the 5-choice serial reaction time task (5-CSRTT) (Davis, et al., 2018). Collectively, these behavioral findings indicate a stronger penetrance of the DAT Val559 variant in DA circuits such as the nigral-striatal pathway associated with locomotion and habitual responding, as compared to reward sensing mediated by dopaminergic VTA to NAc projections. As presynaptic D2-type autoreceptors regulate DAT in the dorsal but not ventral striatum, producing elevated surface levels of efflux prone transporters in the former but not the latter area (Gowrishankar et al., 2018), we propose that this difference in receptor coupling may underlie the differential actions of the DAT Val559 variant on both basal impulsivity and COC-induced locomotion as compared to acquisition of CPP. We wish to note that aforementioned studies were conducted in males only, owing to the male to female bias of ADHD and autism. Studies are underway to explore drug responses and potential circuit level differences of DAT/D2R coupling in females.

Noteworthy, the behavioral phenotypes of the DAT Val559 mice described above were observed during adolescence, highlighting the importance of adequate DAT function through this developmental stage and the likelihood that humans expressing this and other variants (Mergy et al., 2014b; Sakrikar et al., 2012) will display disorders associated with a juvenile onset. In this regard, a de novo coding mutation in the DAT gene has been identified in a subject with ASD encoding a Thr356Met substitution (Hamilton et al., 2013). De novo, functional coding mutations are extremely rare and when associated with a disease are thought to more likely be causal for the disorder than other heritable variants (Alonso-Gonzalez et al., 2018). Like the DAT Val559 variant, also identified in subjects with ASD, the Met356 variant drives anomalous DA efflux, though unlike DAT Val559, demonstrates significantly reduced DA uptake capacity. DAT Met356 KI was also found to induce hyperlocomotion in flies (Hamilton et al., 2013) and mice (DiCarlo et al., 2019), and in mice to display altered social and repetitive behaviors (DiCarlo et al., 2019). Along with the DAT Val559 model, the DAT Met356 mice and other construct-valid DAT KI models derived from humans with neurological disorders provide important tools to investigate DA-associated mechanisms that underlie traits which can be shared across multiple neuropsychiatric disorders depending on other genetic or environmental variables. Future studies with these models should provide insights into when altered biochemical and behavioral traits arise, determine whether DAT-dependent mechanisms can contribute to ADHD and ASD sex bias, link developmental changes in extracellular DA to altered cellular and circuit plasticity, and provide opportunities for novel therapeutic development in a construct-valid context.

As noted earlier, a reduction in dopaminergic function, as well as a tonic DA elevation, and its impact on neurodevelopment has made use of genetically-engineered models of DA depletion. Pitx3^−/− mice exhibit selective bilateral DA depletion in the striatum due to a developmental loss of DA neurons in the substantia nigra pars compacta around P1 (Nunes et al., 2003; Lieberman et al., 2018; Suarez, et al., 2018). This lack of DA innervation to the striatum produces PD-like motor symptoms, that can be reversed by acute L-DOPA treatment (Hwang et al., 2005; Suarez et al., 2018). Spiny projection neurons (SPNs) from Pitx3 ^−/− mice fail to undergo maturation of excitability, displaying at P28 significantly elevated resting membrane potential and input resistance and decreased rheobase compared to WT mice, effects that persisted into adulthood. In Pitx3 ^−/− mice, it has been demonstrated that both direct SPNs (dSPNs) and indirect SPNs (iSPNs) have shorter dendritic trees and lower spine density of mushroom and thin type spines at 3 to 4 months of age (Suarez et al., 2018). L-DOPA treatments reverse these spine changes in iSPNs but persists in dSNPNs (Suarez et al., 2018). Both SPNs in this model exhibited hyperexcitability, presenting more action potentials evoked by depolarizing currents (Suarez et al., 2018). These findings, along with the studies of DAT KI mice, indicate that genetic manipulations leading to excess or diminished dopaminergic signaling induce significant developmental effects that can be revealed through molecular, physiological and behavioral studies in model systems and thus could contribute to adolescent and adult human neurobehavioral disorders.

5. Developmental perturbations to the DA system and risk for neuropsychiatric disorders

As previously discussed, the developmental dynamics associated with the maturation of DA neuron excitability, of dopaminergic axonal arborization and synapse formation, of DA receptors and DAT expression, and of the synthesis and release of DA, still an area of active research, likely play important roles in determining the higher vulnerability of adolescents to drugs of abuse. Although there is strong evidence that COC use during adolescence greatly increases the risk for SUDs (Center for Behavioral Health Statistics and Quality, 2015; Jordan & Andersen, 2017; Pianca et al., 2016), adolescent use of stimulant medication such as AMPH mixtures and MPH in the course of treatment for ADHD does not appear to demonstrate such an effect (Humphreys et al., 2013; Molina et al., 2013). Indeed, a recent meta-analysis and a multimodal treatment study concluded that stimulant treatment for ADHD initiated in childhood neither protects nor increases the risk for SUD (Humphreys et al., 2013; Molina et al., 2013). However, another study reported that time of initiation of MPH treatment may determine outcomes with increased risk being associated with later initiation of stimulant treatment (8–12 years old) (Mannuzza et al., 2008). Use of animal models with construct validity for ADHD may be of particular utility in exploring such variables and the mechanisms that determine their impact.

In support for changes in developmental DA actions as impacting risk for neurobehavioral disorders, several human genetic studies have demonstrated an association between polymorphisms in dopaminergic genes and such conditions. Thus, polymorphisms in multiple DA receptors as well as DAT have been associated with risk for ADHD (Kustanovich et al., 2004; Mazei-Robison et al., 2008, 2005; Ribasés et al., 2012; Wu et al., 2012; Yang et al., 2007). Correspondingly, alterations in the density of DA receptors in several brain regions of ADHD patients have been reported (reviewed on Prince, 2008) and gene x environment interactions for DAT and DA receptors with maternal use of alcohol and nicotine have been found (Brookes et al., 2006; Cortese, 2012; Neuman et al., 2007). Although not usually diagnosed before young adulthood, schizophrenia, a disorder whose positive symptoms (e.g. hallucinations) are treated with D2-type DA receptor antagonists (Howes and Kapur, 2009), is increasingly being understood as a neurodevelopmental disorder, with both a strong genetic component and early environmental contributions such as maternal infection (Karam et al., 2010). Both the combination of common mutations with modest effects and rare mutations of high penetrance contribute to genetic risk for schizophrenia (Karam et al., 2010; Sebat et al., 2009). Common variation in the D2 DA receptor gene (DRD2) has been associated with risk for schizophrenia in genome wide association studies (Ripke et al., 2014) but also see (Edwards et al., 2016) and a reported schizophrenia risk allele in DRD2 has been linked to cortical thinning and consummatory anhedonia (Alfimova et al., 2019) as well as lateral prefrontal to amygdala connectivity in relation to emotion processing (Quarto et al., 2017). Treatment of mice with the D2 DA receptor directed antipsychotic drug haloperidol has been found to induce transcriptional changes enriched for genes located at schizophrenia risk loci in genome wide association studies (Kim et al., 2018). With respect to rare, but highly penetrant gene variation and schizophrenia risk, subjects with 22q11 deletion syndrome (22q11DS) exhibit 25–30 times increased risk of developing schizophrenia relative to the general population, and the contribution of 22q11DS to cases of severe early-onset schizophrenia is even higher (Karayiorgou & Gogos, 2004; Usiskin et al., 1999). This chromosomal abnormality disrupts, among others, the gene encoding catechol-O-methyltransferase (COMT), an enzyme involved in the metabolism of catecholamines, including DA. Variations in COMT activity may have particularly significant effects in the prefrontal cortex (Karayiorgou and Gogos, 2004) since DA projections to this region exhibits a lower density of DAT, resulting in extraneuronal contributions by COMT to DA inactivation. Together, these findings reinforce the plausibility that early life changes in DA signaling contribute to risk of schizophrenia in some individuals.

Previously, we noted that functional de novo and heritable mutations in the DAT gene (SLC6A3) have been identified in subjects with ASD (Bowton et al., 2014; Hamilton et al., 2013), a neurodevelopmental disorder characterized by impaired social communication and restricted repetitive patterns of behavior. These reports align with evidence of dopaminergic dysfunction underlying ASD-like behaviors (Dichter et al., 2012; Ernst et al., 1997; Pavăl, 2017). Gathering the data on the literature, a framework for a DA hypothesis of ASD has been proposed, suggesting that deficits in the mesocorticolimbic circuit could lead to social deficits, while the stereotyped behaviors could arise from a dysfunction in the nigrostriatal pathway (Pavăl, 2017). This view is supported by the observation that dysfunctions in these circuits can lead to ASD-like behavior in non-autistic subjects (Pavăl, 2017; Williams and Swedo, 2015). In addition, dopaminergic modulators, such as risperidone and aripiprazole, are clinically used for the management of ASD-associated behaviors (LeClerc and Easley, 2015). While mainly prescribed for reducing irritability, studies have reported a reduction in stereotypic behavior and an improvement in social behavior studies, although the results between studies are heterogenic and inconsistent (Pavăl, 2017). Thus far, the data supporting a coherent dopaminergic hypothesis for ASD is still very limited and further studies are necessary. Of note, however, a significant fraction of ASD subjects meet clinical criteria for ADHD (Avni et al., 2018; Hanson et al., 2013; Murray, 2010), a disorder with deeper roots in DA biology and treatment. Additionally, copy number variation (Kushima et al., 2018) and genome-wide association studies (The Brainstorm Consortium, 2018) point to overlap in risk loci between ASD and schizophrenia. Thus, perturbations in DA signaling likely contribute to multiple neuropsychiatric disorders with distinct trajectories emerging during periods of brain development that coincide with the birth, differentiation and maturation of DA neurons.

6. Closing remarks

The literature provides strong evidence that DA plays an important role in typical brain development, and that altered DA signalling can impair neurodevelopmental events such as migration and differentiation, neurite outgrowth, spine development and synaptogenesis. These cellular and circuit alterations may be long lasting and can impact cognitive and behavioral functions, underlying aspects of risk for neurodevelopmental/neuropsychiatric disorders. Although the mechanisms underlying these diseases are still not fully understood, it is increasingly clear that DA perturbations during development, genetic or pharmacological, are able to induce phenotypes in animal models that can be aligned with multiple neuropsychiatric disorders, including ADHD, ASD, schizophrenia, and SUDs.

Reproducibility issues between studies, as well as the inevitable gap between animal models and humans, are critical challenges in these efforts. Due to the clear ethical and technical limitations of studying neurobiology in humans, rodents and monkeys are frequently used as animal models. Despite the high degree of conservation for the ventral midbrain between mice and humans, single-cell RNA sequencing studies have described some differences in cell proliferation and development of dopaminergic neurons between these two species (La Manno et al., 2016). Monkeys are a more proximal animal model for human conditions than rodents, but mice and rats are used more frequently mainly due to practical advantages, including their suitability for genetic engineering and more invasive studies. Although differences in cortical organization between primates and rodents are known (Smith et al., 2014), functional homology and conserved DA dynamics within the striatum have been demonstrated in many studies (Balleine & O’Doherty, 2010; Calipari et al., 2012; Giros & Caron, 1993). However, differences in the innervation patterns of dopaminergic projections to cortical regions, especially the primary motor cortex, and of corticostriatal projections between rodents and primates have also been demonstrated (Smith et al., 2014) and species-specificities in DA metabolism and regulation of gene expression are also known (Burbulla et al., 2017). Although these differences have been discussed with higher relevance to Parkinson’s disease, it is possible that these or other species-specific differences could be relevant to the interpretation of models used to study neurodevelopmental disorders as well.

To some extent, the gap between human and animal studies is likely to be aided by the development of models with greater construct validity, such as KI mouse models produced from functional gene variants identified win subjects with neuropsychiatric disorders. Advancing our knowledge of the characteristics and circuit specific-specific penetrance of DA alterations, the ontogeny of DA-driven physiological and behavioral perturbations, and the gene and protein networks underlying CNS DA signaling and plasticity are likely to provide insights into the multiple brain disorders linked to perturbed DA signaling and suggest new opportunities for therapeutic intervention.

Highlights.

• Strong evidence indicates that DA plays a critical role in typical brain development in addition to its well-known adult actions.

• Altered DA signaling during juvenile and adolescent periods in rodents can lead to enduring changes in brain function.

• Long-lasting alterations arising from early-life DA signaling perturbations are evident at molecular, cellular and behavioral levels.

• Rodent models that express functional human gene variants can be helpful in linking altered DA signaling to juvenile onset neurobehavioral disorders.