Developmental regulation of excitatory-inhibitory synaptic balance in the prefrontal cortex during adolescence

Abstract

The prefrontal cortex (PFC) is a cortical structure involved in a variety of complex functions in the cognitive and affective domains. The intrinsic function of the PFC is defined by the interaction of local glutamatergic and GABAergic neurons and their modulation by long-range inputs. The ensuing interactions generate a ratio of excitation and inhibition (E-I) in each output neuron, a balance which is refined during the adolescent to adult transition. In this short review, we aim to describe how an increase in GABAergic transmission during adolescence modifies the E-I ratio in adults. We further discuss how this new setpoint may change the dynamics of PFC networks observed during the transition to adulthood.

Introduction

The prefrontal cortex (PFC) undergoes a period of protracted development that spans adolescence until early adulthood [1, 2]. It is thought such a delayed maturation is responsible for the acquisition of adult cognitive abilities later in life particularly in the domains of decision-making, attention, learning, memory, and affect regulation. Due to this developmental feature, the time window during which the PFC is susceptible to environmental interference is longer than for other cortical regions. When compounded with the increase in experimentation and risk-taking also observed during adolescence [3], PFC disruption can have negative consequences in the acquisition of adult behavior. Here we discuss how PFC maturation during adolescence can be understood as a function of the excitation-inhibition (E-I) balance.

The E-I ratio and PFC development

The biological underpinnings behind prefrontal maturation are only beginning to be understood. At the cellular level, PFC activity is defined by the interaction of local GABAergic interneurons and pyramidal cells as well as glutamatergic projections from other limbic structures, and most notably, frontal-projecting dopamine cells in the ventral tegmental area (VTA). Thus, the resulting PFC output function is broadly the sum of inhibitory (GABAergic) and excitatory (glutamatergic) transmission impinging onto local pyramidal neurons, regulated by a layer of neuromodulators including but not limited to catecholamines, acetylcholine, and serotonin. The computation of these synaptic signals by pyramidal neurons can be condensed into a ratio of excitatory over inhibitory activity [4], hereby referred to as E-I ratio. Synaptic integration by pyramidal neurons in the PFC is especially important in deep layers, which make up the bulk of prefrontal output with projections to limbic structures such as the amygdala and the VTA, and subcortical circuits including the basal ganglia and the thalamus [5].

The importance of GABAergic and glutamatergic transmission in PFC function was established early on by application of receptor agonists and antagonist directly into the PFC. These early studies demonstrate that either increasing or decreasing inhibitory signaling in the PFC of adult animals have drastic consequences for PFC-dependent behaviors [6]. Similarly, inhibition of glutamatergic activity has been shown to alter prefrontal physiology and associated behaviors [7, 8]. Based on these studies, it can be inferred that any deviation of the established E/I ratio can have profound effects in the adult PFC. Indeed, proof of concept studies have shown that acutely increasing the excitability of pyramidal cells in the prelimbic cortex is enough to disrupt PFC-dependent behaviors in adult animals [9]. However, fewer studies have defined how, and most importantly, when, the balance between excitation and inhibition is achieved. The answer to that question will allow us to define how susceptible is the E-I balance to outside experience and how much of it is already predisposed.

Studies in animal models

Over the years, our laboratory and others have documented a developmental recruitment of inhibitory transmission in the PFC up to late adolescence (postnatal day - P- 50 in rats) without a concurrent increase in basal excitatory transmission [10–13]. In other words, the E-I ratio of layer V pyramidal neurons is actively readjusting during adolescence due to the gain of local GABAergic transmission (Figure 1). This fits with the model of protracted development of the GABAergic component in the PFC during adolescence [14]. By using models of non-contingent drug administration, the data show that any insult experienced up to late adolescence (P50) is enough to disrupt the normal increase of GABAergic function. The same results were not true for adult animals indicating that any insult sustained during adolescence preferentially disrupts the normal development of PFC GABAergic function and local E-I balance [10, 12, 13, 15] (Figure 1).

Figure 1:

Adolescence is a critical period for synaptic activity in the PFC. A. The frequency of excitatory postsynaptic currents (EPSC) onto pyramidal output neurons remains relatively constant during the transition from adolescence to adulthood (blue line). Conversely, basal inhibitory postsynaptic currents (IPSC) increase sharply after postnatal day 40–45 (red line). Thus, the excitatory-inhibitory (E-I) ratio of synaptic activity became balanced after postnatal day 50 through adulthood. Dotted lines indicate the trajectory of IPSC and E-I ratio in the PFC upon disruption of NMDAR function [13] and parvalbumin (PV) expression [12] during adolescence. B. Characteristic developmental facilitation of EPSC frequency onto fast-spiking interneurons (FSI) in the PFC during adolescence. Dotted line illustrates trajectory of EPSC upon disruption of NMDAR function [13] and PV expression [12] in the PFC during adolescence.

The signal responsible for the increase in GABAergic transmission remains incompletely understood. Several lines of evidence point to glutamatergic afferents playing an important role. The PFC receives short and long-range glutamatergic projections from other cortical and subcortical structures, most notably, the hippocampus, the basolateral amygdala, and the thalamus [16, 17]. Many of these terminals contact the dendrites of pyramidal neurons with approximately 5% contacting GABAergic interneurons [18]. Although small, evidence suggests these glutamatergic inputs (e.g., vGlut1-positive) increase during adolescence, particularly in parvalbumin (PV)-positive/fast-spiking GABAergic interneurons [19, 20]. Presumably, these events drive the functional maturation of PV interneurons in the PFC and its contribution to sustaining local E-I balance [12].

In addition, several neuromodulators like dopamine, acetylcholine, and endocannabinoids have the ability to modify the strength of both glutamatergic and GABAergic synapses in the PFC, providing a nuanced layer of regulation that is also age-dependent (reviewed in [14]). Accordingly, disruption of these systems in the PFC during adolescence also decreases local GABAergic transmission [10, 21] and alters the E-I balance of prefrontal output. More recently, our lab has found that the age-dependent PFC modulation of behavior is likely due to the late-adolescent maturation of the ventral hippocampal recruitment of prefrontal GABAergic transmission [22].

The number of excitatory synapses largely outnumbers those of inhibitory synapses, even after adolescence when there is a loss of mostly excitatory, axospinous synapses and dendritic spines [23–25]. However, it is worth noting that the distribution of excitatory and inhibitory terminals in pyramidal neurons is not uniform: inhibitory synapses seem to be more concentrated at the soma and proximal dendrites, whereas the density of excitatory synapses is increased in the tufts or terminal segments of dendrites [26]. Thus, each segment seemingly has its own E-I ratio that can be modified to alter dendritic integration and output signal of each individual neuron [26]. This is an especially important consideration for the aforementioned neuromodulators and thalamic afferents, which typically innervate layer I in the PFC where dendritic tufts terminate.

The E-I balance in humans

The overproduction and subsequent loss of spines observed in rodents and non-human primates during adolescence is also recapitulated in the human PFC extending until early adulthood [27]. This late pruning of synapses and dendrites is likely to underlie the decrease in cortical thickness that occurs in frontal areas during development [2]. This is accompanied by an “stabilization” of remaining connections, which may mediate the changes in global E-I ratio observed during adolescence. Accordingly, several markers associated with glutamatergic and GABAergic synapses have been analyzed in human post-mortem tissue. Among those, complexins CX1 and CX2, presynaptic proteins expressed in inhibitory and excitatory synapses, respectively, change during postnatal development. CX1 increases until young adulthood whereas CX2 reaches a plateau during childhood, resulting in a reduced CX2/CX1 ratio in young adults [28]. Together with the above changes at the level of spines, it suggests that both excitatory and inhibitory inputs surviving the period of massive pruning strengthen their connections with post-synaptic targets. Conversely, the presynaptic transporters for glutamate and GABAA, vGlut and vGAT, respectively, do not appear to change substantially during adolescence [29], even when markers for several GABAergic interneurons are changing extensively [14, 30]. Thus, based on anatomical evidence and the expression of GABAergic and glutamatergic-associated proteins, there is a relative increase in the number of inhibitory synapses during the transition to adulthood. This may constitute the structural basis of the increased GABAergic function observed during adolescence.

Nonetheless, the link between individual neuronal E/I ratios and brain activity in humans remains obscure. Currently available methodologies used to map brain function in humans provide information over large areas in stark contrast to the high resolution, cell-type specific recordings of GABAergic and glutamatergic synapses from which E-I ratios are derived. Functional MRI has been valuable in mapping dynamic network activity changes during adolescence either in resting or task-dependent states [31], yet it fails to capture the functionality of excitatory and inhibitory synapses. Among human studies is the default-mode network, a set of brain structures with high correlated activity in the absence of goal-directed behaviors that becomes more functionally integrated in adults compared to children [32, 33], suggesting increased connectivity with long-range connections. On the other hand, goal-directed tasks in which the PFC is involved result in enhanced correlated activity with neighboring structures (e.g. anterior cingulate) [34]. In general, both resting-state and task-based studies suggest that during PFC maturation there is an increased connectivity with local and distal structures. Thus, it is conceivable that the developmental gain of GABAergic function in the PFC is a consequence of increased activity of long-range afferents, which in turn opens a window of communication with neighboring cortical areas through which information can be disseminated and relayed to the next processing hub.

Outstanding Questions

Several observations in pre-clinical and clinical models point towards the E-I balance being disrupted in developmental disorders such as schizophrenia and autism. It is possible that multiple neurotransmitter systems contribute to fine-tuning the E-I balance in the PFC during adolescence. The synaptic contribution of each afferent/input to PFC output computation remains to be studied.

Similarly, while it is known that disruption of prefrontal E-I balance affects PFC-dependent behaviors, the mechanisms by which individual neuronal E-I ratios mediate cortical information processing are unknown. A balanced E-I ratio may increase cross-cortical firing that subserves recurrent activity to increase the receptivity of the PFC and associated cortical areas to modulation by a common afferent [4].

What is the relevant correlate of the E/I ratio in humans? The correlates of neuronal E-I ratio in humans remain incompletely defined. Better methods to analyze the correspondence of local field potentials or EEG to individual pyramidal neuron E-I ratios need to be implemented [35]. This needs to be further integrated into knowledge of how networks are changing to subserve the increase in inter-regional connectivity observed during the transition to adulthood [33].

Conclusions

The E-I ratio of PFC output neurons is refined during adolescence by an increase in GABAergic function. As such, adolescence constitutes a period of exquisite sensitivity to environmental disruption that can have long-lasting consequences in PFC output function. This susceptibility stems from an apparent arrest in the development of local prefrontal GABAergic network that reaches its maturation late in adolescence. Multiple layers of neuromodulators aid in the maturation of PFC GABAergic function and interruption of these processes can exert near-identical disruptions in the E-I balance. Understanding the underlying mechanisms contributing to the maturation of prefrontal excitatory and inhibitory synapses and its impact on local network function is necessary to reveal how developmental disorders where the PFC is involved emerge during adolescence.

Highlights.

The prefrontal excitatory-inhibitory (E-I) balance is actively changing during adolescence

A gain of GABA function during late adolescence dictates the prefrontal E-I balance

Reduced GABA function up to adolescence can permanently alter prefrontal E-I ratio