The development of phasic and tonic inhibition in the rat visual cortex

Serotonergic Modulation of the Excitation/Inhibition Balance in the Visual Cortex

Serotonergic neurons constitute one of the main systems of neuromodulators, whose diffuse projections regulate the functions of the cerebral cortex. Serotonin (5-HT) is known to play a crucial role in the differential modulation of cortical activity related to behavioral contexts. Some features of the 5-HT signaling organization suggest its possible participation as a modulator of activity-dependent synaptic changes during the critical period of the primary visual cortex (V1). Cells of the serotonergic system are among the first neurons to differentiate and operate. During postnatal development, ramifications from raphe nuclei become massively distributed in the visual cortical area, remarkably increasing the availability of 5-HT for the regulation of excitatory and inhibitory synaptic activity. A substantial amount of evidence has demonstrated that synaptic plasticity at pyramidal neurons of the superficial layers of V1 critically depends on a fine regulation of the balance between excitation and inhibition (E/I). 5-HT could therefore play an important role in controlling this balance, providing the appropriate excitability conditions that favor synaptic modifications. In order to explore this possibility, the present work used in vitro intracellular electrophysiological recording techniques to study the effects of 5-HT on the E/I balance of V1 layer 2/3 neurons, during the critical period. Serotonergic action on the E/I balance has been analyzed on spontaneous activity, evoked synaptic responses, and long-term depression (LTD). Our results pointed out that the predominant action of 5-HT implies a reduction in the E/I balance. 5-HT promoted LTD at excitatory synapses while blocking it at inhibitory synaptic sites, thus shifting the Hebbian alterations of synaptic strength towards lower levels of E/I balance.

Reduction in hippocampal GABAergic transmission in a low birth weight rat model of depression

Prenatal stress is believed to increase the risk of developing neuropsychiatric disorders, including major depression. Adverse genetic and environmental impacts during early development, such as glucocorticoid hyper-exposure, can lead to changes in the foetal brain, linked to mental illnesses developed in later life. Dysfunction in the GABAergic inhibitory system is associated with depressive disorders. However, the pathophysiology of GABAergic signalling is poorly understood in mood disorders. Here, we investigated GABAergic neurotransmission in the low birth weight (LBW) rat model of depression. Pregnant rats, exposed to dexamethasone, a synthetic glucocorticoid, during the last week of gestation, yielded LBW offspring showing anxiety- and depressive-like behaviour in adulthood. Patch-clamp recordings from dentate gyrus granule cells in brain slices were used to examine phasic and tonic GABAA receptor-mediated currents. The transcriptional levels of selected genes associated with synaptic vesicle proteins and GABAergic neurotransmission were investigated. The frequency of spontaneous inhibitory postsynaptic currents (sIPSC) was similar in control and LBW rats. Using a paired-pulse protocol to stimulate GABAergic fibres impinging onto granule cells, we found indications of decreased probability of GABA release in LBW rats. However, tonic GABAergic currents and miniature IPSCs, reflecting quantal vesicle release, appeared normal. Additionally, we found elevated expression levels of two presynaptic proteins, Snap-25 and Scamp2, components of the vesicle release machinery. The results suggest that altered GABA release may be an essential feature in the depressive-like phenotype of LBW rats.

Comparison between an exact and a heuristic neural mass model with second-order synapses

Neural mass models (NMMs) are designed to reproduce the collective dynamics of neuronal populations. A common framework for NMMs assumes heuristically that the output firing rate of a neural population can be described by a static nonlinear transfer function (NMM1). However, a recent exact mean-field theory for quadratic integrate-and-fire (QIF) neurons challenges this view by showing that the mean firing rate is not a static function of the neuronal state but follows two coupled nonlinear differential equations (NMM2). Here we analyze and compare these two descriptions in the presence of second-order synaptic dynamics. First, we derive the mathematical equivalence between the two models in the infinitely slow synapse limit, i.e., we show that NMM1 is an approximation of NMM2 in this regime. Next, we evaluate the applicability of this limit in the context of realistic physiological parameter values by analyzing the dynamics of models with inhibitory or excitatory synapses. We show that NMM1 fails to reproduce important dynamical features of the exact model, such as the self-sustained oscillations of an inhibitory interneuron QIF network. Furthermore, in the exact model but not in the limit one, stimulation of a pyramidal cell population induces resonant oscillatory activity whose peak frequency and amplitude increase with the self-coupling gain and the external excitatory input. This may play a role in the enhanced response of densely connected networks to weak uniform inputs, such as the electric fields produced by noninvasive brain stimulation.

Developmental changes in GABAA tonic inhibition are compromised by multiple mechanisms in preadolescent dentate gyrus granule cells

The sustained tonic currents (I_tonic) generated by γ-aminobutyric acid A receptors (GABA_ARs) are implicated in diverse age-dependent brain functions. While various mechanisms regulating I_tonic in the hippocampus are known, their combined role in I_tonic regulation is not well understood in different age groups. In this study, we demonstrated that a developmental increase in GABA transporter (GAT) expression, combined with gradual decrease in GABA_AR α₅ subunit, resulted in various I_tonic in the dentate gyrus granule cells (DGGCs) of preadolescent rats. Both GAT-1 and GAT-3 expression gradually increased at infantile (P_6-8 and P_13-15) and juvenile (P_20-22 and P_27-29) stages, with stabilization observed thereafter in adolescents (P_34-36) and young adults (P_41-43). I_tonic facilitation of a selective GAT-1 blocker (NO-711) was significantly less at P_6-8 than after P_13-15. The facilitation of I_tonic by SNAP-5114, a GAT-3 inhibitor, was negligible in the absence of exogenous GABA at all tested ages. In contrast, I_tonic in the presence of a nonselective GAT blocker (nipecotic acid, NPA) gradually decreased with age during the preadolescent period, which was mimicked by I_tonic changes in the presence of exogenous GABA. I_tonic sensitivity to L-655,708, a GABA_AR α₅ subunit inverse agonist, gradually decreased during the preadolescent period in the presence of NPA or exogenous GABA. Finally, Western blot analysis showed that the expression of the GABA_AR α₅ subunit in the dentate gyrus gradually decreased with age. Collectively, our results suggested that the I_tonic regulation of altered GATs is under the final tune of GABA_AR α₅ subunit activation in DGGCs at different ages.

Assembly of functionally integrated human forebrain spheroids

The development of the nervous system involves a coordinated succession of events including the migration of GABAergic neurons from ventral to dorsal forebrain and their integration into cortical circuits. However, these interregional interactions have not yet been modelled with human cells. Here, we generate from human pluripotent cells three-dimensional spheroids resembling either the dorsal or ventral forebrain and containing cortical glutamatergic or GABAergic neurons. These subdomain-specific forebrain spheroids can be assembled to recapitulate the saltatory migration of interneurons similar to migration in fetal forebrain. Using this system, we find that in Timothy syndrome– a neurodevelopmental disorder that is caused by mutations in the Ca_V1.2 calcium channel, interneurons display abnormal migratory saltations. We also show that after migration, interneurons functionally integrate with glutamatergic neurons to form a microphysiological system. We anticipate that this approach will be useful for studying development and disease, and for deriving spheroids that resemble other brain regions to assemble circuits in vitro.

Brief Dark Exposure Reduces Tonic Inhibition in Visual Cortex

Tonic inhibition mediated by extrasynaptic GABA(A) receptors (GABARs) sensing ambient levels of GABA can profoundly alter the membrane input resistance to affect cellular excitability. Therefore, regulation of tonic inhibition is an attractive mechanism to control the levels of cortical firing. In cortical pyramidal cells, tonic inhibition is regulated by age and several neurotransmitters and is affected by stroke and epilepsy. However, the possible role of sensory experience has not been examined. Here, we report that a brief 2-day exposure to dark reduces by 1/3 the inhibitory tonic conductance recorded in layer II/III pyramidal cells of the mouse juvenile (postnatal day 12-27) visual cortex. In these cells, tonic inhibition is carried primarily by GABARs containing the δ subunit. Consistently, the dark exposure reduction in conductance was associated with a reduction in δ subunit levels, which were not affected in control frontal cortex. We propose that a deprivation-induced reduction in tonic inhibition might serve a homeostatic function by increasing the firing levels of cells in deprived cortical circuits.

GluN2B-containing N-methyl-D-aspartate receptors compensate for the inhibitory control of synaptic plasticity during the early critical period in the rat visual cortex

In the visual cortex, synaptic plasticity is very high during the early developmental stage known as the critical period and declines with development after the critical period. Changes in the properties of N-methyl-D-aspartate receptor (NMDAR) and γ-aminobutyric acid type A receptor (GABAA R) have been suggested to underlie the changes in the characteristics of plasticity. However, it is largely unknown how the changes in the two receptors interact to regulate synaptic plasticity. The present study investigates the changes in the properties of NMDAR and GABAA R from 3 to 5 weeks of age in layer 2/3 pyramidal neurons of the rat visual cortex. The impact of these changes on the characteristics of long-term potentiation (LTP) is also investigated. The amplitude and decay time constant of GABAA R-mediated currents increased during this period. However, the decay time constant of NMDAR-mediated currents decreased as a result of the decrease in the proportion of the GluN2B subunit-mediated component. Induction of NMDAR-dependent LTP at 3 weeks depended on the GluN2B subunit, but LTP at 5 weeks did not. Enhancement of GABAA R-mediated inhibition suppressed the induction of LTP only at 5 weeks. However, partial inhibition of the GluN2B subunit with a low concentration of ifenprodil allowed the GABAA R-mediated suppression of LTP at 3 weeks. These results suggest that changes in the properties of NMDAR- and GABAA R-mediated synaptic transmission interact to determine the characteristics of synaptic plasticity during the critical period in the visual cortex.

Phasic and Tonic Inhibition are Maintained Respectively by CaMKII and PKA in the Rat Visual Cortex

Phasic and tonic γ-aminobutyric acid_A (GABA_A) receptor-mediated inhibition critically regulate neuronal information processing. As these two inhibitory modalities have distinctive features in their receptor composition, subcellular localization of receptors, and the timing of receptor activation, it has been thought that they might exert distinct roles, if not completely separable, in the regulation of neuronal function. Inhibition should be maintained and regulated depending on changes in network activity, since maintenance of excitation-inhibition balance is essential for proper functioning of the nervous system. In the present study, we investigated how phasic and tonic inhibition are maintained and regulated by different signaling cascades. Inhibitory postsynaptic currents were measured as either electrically evoked events or spontaneous events to investigate regulation of phasic inhibition in layer 2/3 pyramidal neurons of the rat visual cortex. Tonic inhibition was assessed as changes in holding currents by the application of the GABA_A receptor blocker bicuculline. Basal tone of phasic inhibition was maintained by intracellular Ca²⁺ and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). However, maintenance of tonic inhibition relied on protein kinase A activity. Depolarization of membrane potential (5 min of 0 mV holding) potentiated phasic inhibition via Ca²⁺ and CaMKII but tonic inhibition was not affected. Thus, phasic and tonic inhibition seem to be independently maintained and regulated by different signaling cascades in the same cell. These results suggest that neuromodulatory signals might differentially regulate phasic and tonic inhibition in response to changes in brain states.

The impact of tonic GABAA receptor-mediated inhibition on neuronal excitability varies across brain region and cell type

The diversity of GABA_A receptor (GABA_AR) subunits and the numerous configurations during subunit assembly give rise to a variety of receptors with different functional properties. This heterogeneity results in variations in GABAergic conductances across numerous brain regions and cell types. Phasic inhibition is mediated by synaptically-localized receptors with a low affinity for GABA and results in a transient, rapidly desensitizing GABAergic conductance; whereas, tonic inhibition is mediated by extrasynaptic receptors with a high affinity for GABA and results in a persistent GABAergic conductance. The specific functions of tonic versus phasic GABAergic inhibition in different cell types and the impact on specific neural circuits are only beginning to be unraveled. Here we review the diversity in the magnitude of tonic GABAergic inhibition in various brain regions and cell types, and highlight the impact on neuronal excitability in different neuronal circuits. Further, we discuss the relevance of tonic inhibition in various physiological and pathological contexts as well as the potential of targeting these receptor subtypes for treatment of diseases, such as epilepsy.

Functional role of ambient GABA in refining neuronal circuits early in postnatal development

Early in development, γ-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the mature brain, depolarizes and excites targeted neurons by an outwardly directed flux of chloride, resulting from the peculiar balance between the cation-chloride importer NKCC1 and the extruder KCC2. The low expression of KCC2 at birth leads to accumulation of chloride inside the cell and to the equilibrium potential for chloride positive respect to the resting membrane potential. GABA exerts its action via synaptic and extrasynaptic GABAA receptors mediating phasic and tonic inhibition, respectively. Here, recent data on the contribution of "ambient" GABA to the refinement of neuronal circuits in the immature brain have been reviewed. In particular, we focus on the hippocampus, where, prior to the formation of conventional synapses, GABA released from growth cones and astrocytes in a calcium- and SNARE (soluble N-ethylmaleimide-sensitive-factor attachment protein receptor)-independent way, diffuses away to activate in a paracrine fashion extrasynaptic receptors localized on distal neurons. The transient increase in intracellular calcium following the depolarizing action of GABA leads to inhibition of DNA synthesis and cell proliferation. Tonic GABA exerts also a chemotropic action on cell migration. Later on, when synapses are formed, GABA spilled out from neighboring synapses, acting mainly on extrasynaptic α5, β2, β3, and γ containing GABAA receptor subunits, provides the membrane depolarization necessary for principal cells to reach the window where intrinsic bursts are generated. These are instrumental in triggering calcium transients associated with network-driven giant depolarizing potentials which act as coincident detector signals to enhance synaptic efficacy at emerging GABAergic and glutamatergic synapses.

Developmental Switch of the Serotonergic Role in the Induction of Synaptic Long-term Potentiation in the Rat Visual Cortex

Synaptic long-term potentiation (LTP) and long-term depression (LTD) have been studied as mechanisms of ocular dominance plasticity in the rat visual cortex. Serotonin (5-hydroxytryptamine, 5-HT) inhibits the induction of LTP and LTD during the critical period of the rat visual cortex (postnatal 3~5 weeks). However, in adult rats, the increase in 5-HT level in the brain by the administration of the selective serotonin reuptake inhibitor (SSRI) fluoxetine reinstates ocular dominance plasticity and LTP in the visual cortex. Here, we investigated the effect of 5-HT on the induction of LTP in the visual cortex obtained from 3- to 10-week-old rats. Field potentials in layer 2/3, evoked by the stimulation of underlying layer 4, was potentiated by theta-burst stimulation (TBS) in 3- and 5-week-old rats, then declined to the baseline level with aging to 10 weeks. Whereas 5-HT inhibited the induction of LTP in 5-week-old rats, it reinstated the induction of N-methyl-D-aspartate receptor (NMDA)-dependent LTP in 8- and 10-week-old rats. Moreover, the selective SSRI citalopram reinstated LTP. The potentiating effect of 5-HT at 8 weeks of age was mediated by the activation of 5-HT₂ receptors, but not by the activation of either 5-HT_1A or 5-HT₃ receptors. These results suggested that the effect of 5-HT on the induction of LTP switches from inhibitory in young rats to facilitatory in adult rats.