L-type calcium channel regulation of dopamine activity in the ventral tegmental area to nucleus accumbens pathway: implications for substance use, mood disorders and co-morbidities

Abstract

L-type calcium channels (LTCCs), including the Cav1.2 and Cav1.3 LTCC subtypes, are important regulators of calcium entry into neurons, which mediates neurotransmitter release and synaptic plasticity. Cav1.2 and Cav1.3 are encoded by the CACNA1C and CACNA1D genes, respectively. These genes are implicated in substance use disorders and depression in humans, as demonstrated by genetic-wide association studies (GWAS). Pre-clinical models have also revealed a critical role of LTCCs on drug and mood related behavior, including the co-morbidity of substance use and mood disorders. Moreover, LTCCs have been shown to regulate the neuronal firing of dopamine (DA) neurons as well as drug and stress-induced plasticity within the ventral tegmental area (VTA) to nucleus accumbens (NAc) pathway. Thus, LTCCs are interesting targets for the treatment of neuropsychiatric diseases. In this review, we provide a brief introduction to voltage-gated calcium channels, specifically focusing on the LTCCs. We place particular emphasis on the ability of LTCCs to regulate DA neuronal activity and downstream signaling in the VTA to NAc pathway, and how such processes mediate substance use and mood disorder-related behavioral responses. We also discuss the bi-directional control of VTA LTCCs on drug and mood-related behaviors in pre-clinical models, with implications for co-morbid psychiatric diagnosis. We conclude with a section on the clinical implications of LTCC blockers, many which are already FDA approved as cardiac medications. Thus, pre-clinical and clinical work should examine the potential of LTCC blockers to be repurposed for neuropsychiatric illness.

Introduction

Voltage-gated calcium channels (VGCCs) are important protein-complexes found on excitable cells, including neurons, which mediates calcium entry (Yamakage and Namiki, 2002; Dolphin, 2006). VGCCs function as an assembly of five different channel subunits (α1, α2, β, γ, δ), with the α1 subunit forming the central pore of the channel. Different classes of VGCCs have been identified based on their unique pharmacological and electrophysical properties, including the L-type VGCCs (Cav1), the T-type VGCCs (Cav3) and the P/Q-type, R-type, and N-type VGCCs (Cav2) (Ertel et al., 2000; Catterall et al., 2005; Zamponi et al., 2015). L-type calcium channels (LTCCs) are activated at higher voltages, display slower activation kinetics, and are sensitive to the class of drugs known as dihydropyridines that block LTCCs (Tang et al., 1993; Lipscombe et al., 2004; Helton et al., 2005). LTCCs are widely expressed in the brain, specifically the Cav1.2 and Cav1.3 subtype, which play a critical role in regulating intracellular second messenger systems related to synaptic plasticity, gene expression, and regulation of neurotransmitter release (Deisseroth et al., 1998; Mermelstein et al., 2000; Nanou and Catterall, 2018; Heck et al., 2021). Specifically, Cav1.2 and Cav1.3 are found on mesolimbic dopamine (DA) neurons. Activation of LTCCs with the channel activator Bay K8644 induced burst firing of DA neurons, whereas blockade of LTCCs with nifedipine attenuated the burst firing induced by Bay K8644 (Liu et al., 2014). Furthermore, these LTCCs are critical regulators of drug-induced synaptic plasticity (Ford et al., 2009; Schierberl et al., 2011). DA neurons projecting from the ventral tegmental area (VTA) to terminal regions, including the nucleus accumbens (NAc) and prefrontal cortex, play a critical role in drug reinforcement and in mood-related behaviors (Cooper, 2002; Ikemoto and Wise, 2002; Chaudhury et al., 2013; Tye et al., 2013).

Impaired DA transmission has been implicated in both substance use and mood-related behaviors in rodents and humans (Salamone and Correa, 2012; Treadway et al., 2012; Nunes et al., 2022). Specifically, LTCCs have been implicated in neuropsychiatric disorders including substance use disorders, bipolar disorder, and depression (Pinggera and Striessnig, 2016; Kabir et al., 2017; Terrillion et al., 2017). Moreover, evidence from genome-wide association studies (GWAS) has demonstrated CACNA1C and CACNA1D risk gene variants associated with neuropsychiatric diseases (Sklar et al., 2008; Cosgrove et al., 2017; Kabir et al., 2017; Liu et al., 2022). Furthermore, mutations of Cav1.2 and Cav1.3 have been associated with Timothy syndrome and autism spectrum disorder, both which present with cognitive and psychiatric symptoms, (Gershon et al., 2014; Pinggera et al., 2015, 2017; Pinggera and Striessnig, 2016). Human imaging studies have correlated risk variants for LTCC subtypes to functional changes to neuronal processing and network connectivity in patients with bipolar disorder, schizophrenia, and substance use disorder (Gurung and Prata, 2015; Kabir et al., 2016; Romme et al., 2017). In this review, we discuss the significance of LTCCs regulation on behavioral, neurochemical and plasticity-related effects of substance and mood related behaviors in rodents. We place emphasis on the role of LTCC regulation of dopaminergic activity in the VTA and NAc pathway, in the context of behavioral responses to drug exposure and behavioral responses related to stress, anxiety, and motivation. Lastly, we conclude with a discussion of LTCCs as therapeutic targets for the treatment of both substance use disorder and mood-disorder related symptoms

L-type calcium channel regulation of DA activity and synaptic plasticity in the VTA to NAc pathway

The firing patterns of VTA DA neurons, in the presence of substances or stress, can mediate and modulate drug and mood-related behavior. DA neurons are spontaneously active and display two types of firing patterns: single-spike firing (i.e., tonic DA release) and burst firing (i.e., phasic DA release) (Floresco et al., 2003; Zhang et al., 2009; Schultz et al., 2015). The firing pattern of DA neurons controls the temporal dynamics of DA release, which can signal across multiple timescales (Grace et al., 2007; Schultz, 2007; Salamone and Correa, 2012; Saunders et al., 2013). Low frequency, single spike firing patterns contribute to the baseline level of extracellular DA, which can fluctuate over minutes to hours. DA levels can also fluctuate over a much faster timescale, reflected by burst firing of DA neurons at higher frequencies, which results in DA release and rapid and large increase in DA concentration. VTA LTCCs have been shown to have a bi-directional control over DA neuron activity. Activation of LTCCs with pharmacological agents such as Bay K8644 and FPL 64176 induces burst firing of DA neurons, while LTCC blockers, like nifedipine, attenuate the increase in burst firing induced by LTCC activators (Liu et al., 2007, 2014). Moreover, blockade of LTCCs attenuates cholinergic receptor stimulation-induced burst firing in DA neurons (Zhang et al., 2005). In mice, reduced levels of Cav1.2, has been shown to attenuate mesolimbic DA system function and a reduced sensitivity to pharmacological inhibition of dopamine transporters (Terrillion et al., 2017). Utilizing a genetic and pharmacological approach, Rajadhyaksha and colleagues have shown that the Cav1.3 is a critical regulator of single-spike firing, while Cav1.2 and Cav1.3 contribute to burst firing of VTA DA neurons (Liu et al., 2014). Moreover, activation of LTCCs is also implicated in modulating synaptic strength of DA neurons via actions on second messenger systems, including protein kinase C (PKC), Ca2+-calmodulin-dependent kinase II (CaMKII), and protein kinase A (PKA) (Bonci et al., 1998; Berridge et al., 2003). The role of PKC has been demonstrated to be an important mechanism for burst firing of DA neurons (Liu et al., 2007).

Infusion of the LTCC blocker isradipine into the VTA or administered systemically, attenuated cocaine and alcohol conditioned place preference (CPP), which was mediated by attenuation of glutamatergic-dependent synaptic plasticity due to cocaine and alcohol exposure (Degoulet et al., 2016). In our laboratory, using a rat model of cocaine self-administration followed by a period of forced abstinence, we have shown that cocaine exposure followed by abstinence alters the response of DA release to the LTCC blocker isradipine. During cocaine abstinence, isradipine increases tonic and phasic DA release in cocaine withdrawn rats compared to controls (Addy et al., 2018). Thus, cocaine administration, followed by abstinence, results in synaptic plasticity of the DA system, which may underlie craving and negative affective states during abstinence. Yet, further questions remain about the precise role of VTA LTCCs on VTA DA neurons in response to substance use and in mood disorders. For example, classical techniques such as electrophysiology coupled with modern approaches such as optogenetics can be used to determine the role of VTA LTCC conductance on DA neurons during cocaine abstinence. Taken together, this supports the notion that LTCCs in the mesolimbic DA system not only regulate DA neuron activity (Liu et al., 2014) but also mediates aspects of drug-induced plasticity underlying drug-seeking behavior. This points to the utility of ongoing investigations of the ability of VTA LTCC manipulations to alter behaviors, and attenuate the plasticity related to substance use and mood disorders.

L-type calcium channels mediate drug-related behavioral responses in rodents

The entry of calcium into neurons via LTCCs and its subsequent effects on calcium mediated second messenger systems such as calmodulin, has been demonstrated to play a role in the acute behavioral responses to psychostimulants, opiates, alcohol and nicotine (Pierce and Kalivas, 1997; Pierce et al., 1998; Licata and Pierce, 2003). Pre-clinical models have been used to examine the role of VTA LTCCs on several drug-related behaviors, including behavioral sensitization to psychostimulants, CPP, drug self-administration and drug-seeking behaviors. LTCC blockers including nimodipine, nifedipine, and diltiazem have been shown to attenuate the development and expression of behavioral sensitization to cocaine (Pierce and Kalivas, 1997; Giordano et al., 2010). For example, repeated stimulation of LTCCs in the VTA with the activator BayK 8644 produces a cross-sensitization to cocaine, as demonstrated by an increase in stereotypy and locomotor counts in rats that received both BayK 8644 and cocaine (Licata et al., 2000). In contrast, VTA infusion of the LTCC blocker isradipine prevents the acquisition of cocaine CPP, while promoting the extinction of both cocaine and alcohol CPP (Degoulet et al., 2016). Furthermore, LTCC blockers have also been shown to mediate the behavioral effects of other substance, such as morphine and nicotine, as reflected by LTCC inhibitor attenuation of CPP and drug self-administration (Kuzmin et al., 1992; Biala and Langwinski, 1996; Biała, 2003; Biala and Budzynska, 2006). In rodent studies using intravenous (i.v.) cocaine self-administration, the LTCC blockers isradipine and nimodipine have been shown to attenuate the reinforcing effects of both cocaine and morphine (Kuzmin et al., 1992). In contrast, work from our lab revealed that VTA specific administration of the LTCC inhibitor, isradipine, does not affect I.V. cocaine self-administration (Addy et al., 2018). Based on our data, it is certainly plausible that LTCCs in other brain areas are mediating the reinforcing effect of cocaine, while VTA LTCCs do not. Nevertheless, our lab demonstrated a significant effect of both global and VTA specific LTCC blockade to reduce cue-induced cocaine-seeking during periods of abstinence (Addy et al., 2018). In this model, rats are trained to self-administer cocaine intravenously, which was paired with a light + tone cue for ten days. Following day 10 of cocaine self-administration, rats enter a period of forced abstinence with no exposure to cocaine or its related cues. Interestingly, in our study, the mechanism by which isradipine attenuated cue-induced cocaine seeking was by increasing DA release in the NAc (Addy et al., 2018). These somewhat surprising findings add to the increasingly complex role of DA in mediating drug-related behaviors (Swinford-Jackson and Pierce, 2018).

The use of haploinsufficient mice, where one copy of the Cacna1c gene is silenced (Dao et al., 2010; Lee et al., 2012) has enabled the specific pharmacological manipulation of Cav1.3 receptors by LTCC blockers, which do not distinguish between Cav1.2 and Cav1.3. Experiments in these haploinsufficient mice demonstrate an attenuated locomotor sensitization response to DA transport blockers compared to control mice (Dao et al., 2010; Terrillion et al., 2017). Furthermore, genetically modified mice, which express dihydropyridine insensitive Cav1.2 receptors have been developed, thus enabling the ability to isolate the effects of Cav1.3 LTCC activation and blockade on drug-related behaviors. Activation of VTA Cav1.3 LTCCs in Cav1.2 insensitive mice led to an enhancement of cocaine CPP and the psychomotor effects of cocaine, while blockade of Cav1.3 LTCCs attenuated these cocaine behaviors (Martínez-Rivera et al., 2017). Taken together, this demonstrates the ability of Cav1.2 and Cav1.3 LTCCs to bi-directionally regulate the drug-related behavioral responses. Furthermore, a potential mechanism by which LTCCs can regulate these behavioral responses may be attributed to its effects on DA transmission in the VTA to NAc pathway. Taken together, this demonstrates a bi-directional control of LTCCs, whereby activation of LTCCs promotes drug-related behaviors, while blockade attenuates them.

L-type calcium channels mediate mood-related behavioral responses in rodents

In preclinical models, the VTA to NAc pathway has been shown to mediate behavioral responses related to anhedonia, stress, and anxiety as reflected in the sucrose preference test (SPT), elevated plus maze (EPM) and forced swim test (FST) (Chaudhury et al., 2013; Tye et al., 2013). Repeated VTA administration of the LTCC activator Bay K8644 to selectively activate Cav1.3 LTCCs in Cav1.2 DHP−/− mice, produces pro-depressive behavioral responses in the SPT and the FST, while also decreasing social interaction in the social approach paradigm. In Cav1.3 deficient mice, anti-depressant and anxiolytic-like phenotypes emerged as measured by the TST, FST, and EPM (Busquet et al., 2010). Thus, these data reveal a bi-directional control of VTA Cav1.3 LTCCs on mood-related behavioral responses in rodents. The Cav1.2 receptor has also been implicated in mediating behavioral responses in pre-clinical models related to mood. Reduced expression of Cacna1c in the NAc increases the susceptibility to subthreshold social defeat stress compared to control mice, while increasing anxiety like behavior on the EPM (Terrillion et al., 2017). The role of the Cav1.2 receptor subtypes in other brain regions beyond the VTA to NAc pathway have also been examined regarding mood-related behaviors in rodents. Selective knockdown of Cacna1c in serotonin neurons produces a pro-depressive behavioral phenotype, as demonstrated by a reduction of active coping behavior in the FST, which is attenuated by 5-HT1A receptor blockade (Ehlinger and Commons, 2019). This suggests an interesting relationship between LTCC and serotonin receptors, with their shared ability to modulate responses to stress and mood-like behavioral responses. In contrast, Cacna1c knockdown in the prefrontal cortex produced an anti-depressant like behavioral phenotype as measured by the SPT, TST, and FST in mice (Kabir et al., 2016, 2017). Taken together, this implicates a significant role of Cav1.2 and Cav1.3 LTCC subtypes in mediating mood-related behavioral responses on pre-clinical models. Future studies should begin to examine the role of these LTCC subtypes and how they mediate motivational processes such as effort-choice behavior, which are also significantly impaired in mood-disorders (Nunes et al., 2022).

Co-morbidities

The comorbidity of substance use and mood disorders are well documented (Saha et al., 2022). A diagnosis of one psychiatric disorder increases the risk of development of a co-morbidity, and is correlated with poorer treatment outcomes and a reduced quality of life (Volkow, 2004). For example, patients with depression-like conditions often exhibit debilitating fatigue in the absence of physical exertion (Corfield et al., 2016; Saligan et al., 2019). Thus, individuals with depression may seek out illicit substances as a coping strategy of self-medication to alleviate negative symptoms despite the increased risk of developing a substance use dependency (Khantzian, 1985; Bolton et al., 2009; Robinson et al., 2011; Turner et al., 2018). As discussed above, pre-clinical and clinical evidence supports a role of LTCC blockers in attenuating drug and mood-related behavioral responses. Yet, less work has investigated the role of LTCC Cav1.2 and Cav1.3 on co-morbid substance use and mood disorders. Despite the limited scientific study in this area, ongoing work from Rajadhyaksha and colleagues have begun to elucidate the role of LTCC channels mediating both drug and mood-disorder related behaviors (Kabir et al., 2017; Martínez-Rivera et al., 2017). Their findings reveal and demonstrate that VTA Cav1.3 LTCCs mediate cocaine, depressive, and social interaction-related behaviors, via modulation of AMPA receptor transmission in the NAc, which may serve as a link between substance use and mood disorders (Martínez-Rivera et al., 2017). Future pre-clinical experiments should examine the role LTCC blockers in the context of drug exposure and its effects on behavioral measures of depressive and anxiogenic-like behaviors. Current experiments in our laboratory are beginning to examine the relationship between I.V. delivered cocaine self-administration and its effects on mood-related behaviors, including on measures of motivated behavior in female and male rats. We hypothesize that during periods of cocaine abstinence, rats will exhibit a pro-depressive and anxiogenic behavioral phenotype that are mediated by VTA LTCCs.

Clinical implications

LTCC blockers are clinically available, with well-established dosing regimens, for the treatment of cardiac related conditions such as hypertension, arrhythmias, and cardiac ischemia (Ueng et al., 2011; Short et al., 2022). An intriguing question is whether individuals treated with LTCC blockers for cardiac conditions have lower indices of substance use and mood-related symptoms. We are unaware of any clinical data which has directly examined this relationship. Examining electronic health records is an interesting place to begin exploring this relationship. Recently, using this approach it was revealed that patients taking brain penetrant calcium channel blockers had a lower incidence of neuropsychiatric disorders (Colbourne and Harrison, 2022). Clinical studies have attempted to examine the role and efficacy of LTCC blockers, including nifedipine and isradipine, in alleviating substance mediated reward and euphoria, as well as symptoms associated with substance use disorders and other neuropsychiatric conditions (Muntaner et al., 1991; Suddath et al., 1991; Pazzaglia et al., 1998; Sofuoglu et al., 2003; Johnson et al., 2004, 2005; Ostacher et al., 2014). Yet, clinical studies investigating LTCC blocker efficacy have yielded mixed results (Muntaner et al., 1991; Malcolm et al., 1999, 2005). Some studies report no effect with isradipine and nimodipine, while others have reported a reduction in the subjective response to cocaine with nifedipine (Muntaner et al., 1991; Sofuoglu et al., 2003). Interestingly, one study demonstrated an enhancement of the subjective effects of cocaine with isradipine (Roache et al., 2005). It is certainly plausible that the mechanism by which isradipine increased the subjective effects of cocaine is its action on the cardiovascular system. Cocaine increases heart rate, blood pressure, and myocardial contractility, which is attenuated by LTCC blockers (Johnson et al., 2005). Nevertheless, the outcome where LTCC blockers were more effective was in the enhancement of cognitive function in individuals with cocaine use disorder, which may help to attenuate subjective effects of cocaine craving during periods of sobriety (Johnson et al., 2005). These findings of limited effects on cocaine reinforcement are also consistent with our pre-clinical findings showing isradipine does not affect cocaine-self administration, but is effective in reducing cue-seeking during cocaine abstinence (Addy et al., 2018). Thus, future clinical investigations should examine the effectiveness of LTCC blockers on cue-reactivity (Papini et al., 2020).

Initial clinical studies have begun to indicate a role of LTCC blockers as potential treatments for mood-disorders. In a small sample study, patients with bipolar depression treated who were given isradipine showed a reduction in negative mood, while two patients showed a full remission of symptoms (Ostacher et al., 2014). Despite isradipine showing promising results in the treatment of bipolar depression, the ability of currently available LTCC blockers to act as reliable thymoleptics, for mood disorders, is still unclear (Cipriani et al., 2016). Thus, further clinical inquiry into the role of LTCC blockers on mood disorders are warranted. Coupled with innovative approaches in drug development with LTCC compounds which can discriminate between Cav1.2 and Cav1.3 LTCC subtypes, may prove valuable in robust attenuation of symptoms related to substance use, mood-disorders, and its related co-morbidities.

Conclusion

Patients with substance use disorders are more likely to exhibit simultaneous physical, emotional distress, along with co-morbid mood disorders. Similarly, individuals with depression, anxiety and other mood disorders are at a greater risk for the use of illicit substances, possibly to alleviate negative affective states or symptoms. Further, treatment approaches and options are more challenging for those with co-morbid substance use and mood disorders. Thus, development of medications which can treat co-morbid diseases are needed. The ability of LTCCs to regulate DA in the VTA to NAc pathway makes LTCCs an appealing pharmacological treatment target. Pre-clinical and clinical studies show positive trends that LTCC blockers, which are currently prescribed for cardiovascular disease, are useful for treatment of substance use, mood disorders, and the co-morbidity between the two. Yet, problems persist with the clinical use of LTCC blockers as a treatment option for brain-based diseases. As discussed in this review, LTCCs express two subtypes in the brain, Cav1.2 and Cav1.3. Currently available LTCC blockers do not distinguish between these two subtypes, which may account for their effects on both cardiac and neuronal tissue and that may partially explain the mixed clinical findings. However, it is certainly plausible that development of selective agents for the LTCC subtypes maybe able to distinguish between Cav1.2 and Cav1.3 LTCCs in brain and heart. As a result of this LTCC subtype specificity, these compounds may prove to be effective at the treatment of psychiatric co-morbidities linked to LTCC function. Lastly, as VTA LTCCs mechanisms related to drug and mood induced plasticity are further elucidated, other proteins and signal transduction mechanisms linked to LTCC function may also provide useful targets for drug development.