Dopamine-synthesis and tyrosine-hydroxylase are regulated by independent DA-receptor mediated mechanisms.

Intrastriatal injection of kainic acid, which destroys striatal post-synaptic structures, prevents the increase in TH cofactor affinity but not the in vivo stimulation of DA-synthesis produced by haloperidol. Destruction of cortico-striatal input by decortication fails to prevent the effect of haloperidol and of apomorphine on striatal DA-metabolism. The results are taken to indicate that post-synaptic DA-receptors control stable conformational changes of TH molecule while pre-synaptic DA-receptors control DA-synthesis in a manner independent of changes in TH activity persisting in vitro. No evidence could be obtained that pre-synaptic DA-receptors located on cortico-striatal axons play a major role in the control of DA-metabolism in vivo.     

The newly synthesized pool of dopamine determines the severity of methamphetamine-induced neurotoxicity

The neurotransmitter dopamine (DA) has long been implicated as a participant in the neurotoxicity caused by methamphetamine (METH), yet, its mechanism of action in this regard is not fully understood. Treatment of mice with the tyrosine hydroxylase (TH) inhibitor α-methyl- p -tyrosine (AMPT) lowers striatal cytoplasmic DA content by 55% and completely protects against METH-induced damage to DA nerve terminals. Reserpine, by disrupting vesicle amine storage, depletes striatal DA by more than 95% and accentuates METH-induced neurotoxicity. l -DOPA reverses the protective effect of AMPT against METH and enhances neurotoxicity in animals with intact TH. Inhibition of MAO-A by clorgyline increases pre-synaptic DA content and enhances METH striatal neurotoxicity. In all conditions of altered pre-synaptic DA homeostasis, increases or decreases in METH neurotoxicity paralleled changes in striatal microglial activation. Mice treated with AMPT, l -DOPA, or clorgyline + METH developed hyperthermia to the same extent as animals treated with METH alone, whereas mice treated with reserpine + METH were hypothermic, suggesting that the effects of alterations in cytoplasmic DA on METH neurotoxicity were not strictly mediated by changes in core body temperature. Taken together, the present data reinforce the notion that METH-induced release of DA from the newly synthesized pool of transmitter into the extracellular space plays an essential role in drug-induced striatal neurotoxicity and microglial activation. Subtle alterations in intracellular DA content can lead to significant enhancement of METH neurotoxicity. Our results also suggest that reactants derived from METH-induced oxidation of released DA may serve as neuronal signals that lead to microglial activation early in the neurotoxic process associated with METH.     

Differential regulation of dopamine D1 and D2 signaling by nicotine in neostriatal neurons

Nicotine, acting on nicotinic acetylcholine receptors (nAChRs) expressed at pre-synaptic dopaminergic terminals, has been shown to stimulate the release of dopamine in the neostriatum. However, the molecular consequences of pre-synaptic nAChR activation in post-synaptic neostriatal neurons are not clearly understood. Here, we investigated the effect of nAChR activation on dopaminergic signaling in medium spiny neurons by measuring phosphorylated DARPP-32 (dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa) at Thr34 (the PKA-site) in mouse neostriatal slices. Nicotine produced dose-dependent responses, with a low concentration (1 microm) causing a sustained decrease in DARPP-32 Thr34 phosphorylation and a high concentration (100 microm) causing a transient increase in DARPP-32 Thr34 phosphorylation. Depending on the concentration of nicotine, either dopamine D2 or D1 receptor signaling was predominantly activated. Nicotine at a low concentration (1 microm) activated dopamine D2 receptor signaling in striatopallidal/indirect pathway neurons, likely by activating alpha4beta2* nAChRs at dopaminergic terminals. Nicotine at a high concentration (100 microm) activated dopamine D1 receptor signaling in striatonigral/direct pathway neurons, likely by activating (i) alpha4beta2* nAChRs at dopaminergic terminals and (ii) alpha7 nAChRs at glutamatergic terminals, which, by stimulating the release of glutamate, activated NMDA/AMPA receptors at dopaminergic terminals. The differential effects of low and high nicotine concentrations on D2- and D1-dependent signaling pathways in striatal neurons may contribute to dose-dependent actions of this drug of abuse.     

Formation of p-tyramine from dopamine bound to synaptic receptors of the rat brain in vitro

Tyramine (TA) revealed earlier during the functioning of dopamine (DA)-receptors of the rat brain (after learning) in vivo was produced from dopamine bound by DA-receptors of the synaptic membranes in the system which was exposed to the influence of the microdischarge electroradialysis in vitro. It is shown that the formation of p-TA under these conditions depends on the period of the micro-discharge effect on the system, it is maximal at exposition of 30 s for I = 4.2 mA. In control solutions of standard DA and DA preincubated with the membranes of the cerebellum homogenate, without DA-receptors, p-TA was not revealed under these conditions. The results obtained confirm the supposition that p-TA is the product of the DA-receptors functioning in vivo.     

Inhibitory effects of dopamine on high affinity glutamate uptake from rat striatum

The role of dopamine (DA) input on the activity of glutamate neurons was investigated on rat striatal and cortical tissue using the measurement of sodium-dependent high affinity glutamate uptake (HAGU) as an index. Incubation of the tissue in the presence of DA, apomorphine or bromocriptine produced marked inhibition of 3H-glutamate uptake from rat striatal homogenates. No change occurred with samples from the frontal cortex. Dopaminergic inhibition of HAGU in striatal homogenates was shown to be reversed in the presence of haloperidol or domperidone which act by blocking dopaminergic receptor sites. These results are consistent with the existence of an inhibitory control of the neuronal activity of the glutamatergic neurons in the striatum by the nigro-striatal dopaminergic input. The effects could be due to the activation of D2-like DA receptors located at pre-synaptic levels on cortico-striatal glutamatergic nerve endings.     

Genetically Determined Measures of Striatal D2 Signaling Predict Prefrontal Activity during Working Memory Performance

Background

Variation of the gene coding for D2 receptors (DRD2) has been associated with risk for schizophrenia and with working memory deficits. A functional intronic SNP (rs1076560) predicts relative expression of the two D2 receptors isoforms, D2S (mainly pre-synaptic) and D2L (mainly post-synaptic). However, the effect of functional genetic variation of DRD2 on striatal dopamine D2 signaling and on its correlation with prefrontal activity during working memory in humans is not known.

Methods

Thirty-seven healthy subjects were genotyped for rs1076560 (G>T) and underwent SPECT with [¹²³I]IBZM (which binds primarily to post-synaptic D2 receptors) and with [¹²³I]FP-CIT (which binds to pre-synaptic dopamine transporters, whose activity and density is also regulated by pre-synaptic D2 receptors), as well as BOLD fMRI during N-Back working memory.

Results

Subjects carrying the T allele (previously associated with reduced D2S expression) had striatal reductions of [¹²³I]IBZM and of [¹²³I]FP-CIT binding. DRD2 genotype also differentially predicted the correlation between striatal dopamine D2 signaling (as identified with factor analysis of the two radiotracers) and activity of the prefrontal cortex during working memory as measured with BOLD fMRI, which was positive in GG subjects and negative in GT.

Conclusions

Our results demonstrate that this functional SNP within DRD2 predicts striatal binding of the two radiotracers to dopamine transporters and D2 receptors as well as the correlation between striatal D2 signaling with prefrontal cortex activity during performance of a working memory task. These data are consistent with the possibility that the balance of excitatory/inhibitory modulation of striatal neurons may also affect striatal outputs in relationship with prefrontal activity during working memory performance within the cortico-striatal-thalamic-cortical pathway.     

VMAT2 and dopamine neuron loss in a primate model of Parkinson's disease

We used positron emission tomography (PET) to measure the earliest change in dopaminergic synapses and glial cell markers in a chronic, low-dose MPTP non-human primate model of Parkinson's disease (PD). In vivo levels of dopamine transporters (DAT), vesicular monoamine transporter-type 2 (VMAT2), amphetamine-induced dopamine release (AMPH-DAR), D2-dopamine receptors (D2R) and translocator protein 18 kDa (TSPO) were measured longitudinally in the striatum of MPTP-treated animals. We report an early (2 months) decrease (46%) of striatal VMAT2 in asymptomatic MPTP animals that preceded changes in DAT, D2R, and AMPH-DAR and was associated with increased TSPO levels indicative of a glial response. Subsequent PET studies showed progressive loss of all pre-synaptic dopamine markers in the striatum with expression of parkinsonism. However, glial cell activation did not track disease progression. These findings indicate that decreased VMAT2 is a key pathogenic event that precedes nigrostriatal dopamine neuron degeneration. The loss of VMAT2 may result from an association with α-synuclein aggregation induced by oxidative stress. Disruption of dopamine sequestration by reducing VMAT2 is an early pathogenic event in the dopamine neuron degeneration that occurs in the MPTP non-human primate model of PD. Genetic or environmental factors that decrease VMAT2 function may be important determinants of PD.     

A Brownian model of glutamate diffusion in excitatory synapses of hippocampus

We simulated the diffusion of glutamate, following the release of a single vesicle from a pre-synaptic terminal, in the synaptic cleft by using a Brownian diffusion model based on Langevin equations. The synaptic concentration time course and the time course of quantal excitatory post-synaptic current have been analyzed. The results showed that they depend on the number of receptors located at post-synaptic membrane. Their time course are dependent both on the total number of the post-synaptic receptors and on the eccentricity of the pre-synaptic glutamate vesicle.     

Diverse pre- and post-synaptic expression of m1-m4 muscarinic receptor proteins in neurons and afferents in the rat neostriatum

We have utilized subtype specific antibodies to determine the cellular and subcellular distributions of the muscarinic acetylcholine receptor subtypes that are highly expressed in the rat striatum (m1-m4). Each receptor is expressed in distinct populations of striatal neurons in the relative proportions predicted by their mRNAs. They concentrate at post-synaptic sites and each of the four subtypes are also transported to pre-synaptic sites. m2 appears to be the only presynaptic autoreceptor in the striatum, but it is also localized in non-cholinergic terminals. These distinct pre- and post-synaptic localizations suggest that muscarinic receptor subtype diversity evolved to enable increasingly complex responses to acetylcholine release.     

Characterization of the striatal dopaminergic neurotransmission in MEN2B mice with elevated cerebral tissue dopamine

The Ret receptor tyrosine kinase is the common signaling receptor for the glial cell line-derived neurotrophic factor (GDNF) family ligands. The Met918Thr mutation leads to constitutive activation of Ret and is responsible for dominantly inherited cancer syndrome MEN2B. Previously, we found that the mice carrying the mutation (MEN2B mice) have profoundly increased tissue dopamine (DA) concentrations in the striatum as well as increased striatal levels of tyrosine hydroxylase (TH) and dopamine transporter. The aim of this study was to characterize the striatal dopaminergic neurotransmission in MEN2B mice and to clarify the mechanisms by which they compensate their over-production of DA. We found that tyrosine hydroxylase activity and DA synthesis are increased in MEN2B mice. Augmented effects of α-methyl-para-tyrosine (αMT, an inhibitor of TH) and tetrabenazine (VMAT2 blocker) on DA levels suggest that also storage of DA is increased in MEN2B mice. There was no difference in the basal extracellular DA concentrations or potassium-evoked DA release between the genotypes. The effects of cocaine and haloperidol were also similar between the genotypes as assessed by in vivo microdialysis. However, with in vivo voltammetry we found increase in stimulated DA release in MEN2B mice and detailed analysis of DA overflow showed that uptake of DA was also enhanced in MEN2B mice. Thus, our data show that enhanced synthesis of DA leading to increased storage and releasable pools in pre-synaptic terminals in MEN2B mice apparently also leads to increased DA release, which in turn is compensated by higher dopamine transporter activity.     

Evaluation of neurotoxicity of TIQ and MPTP and of parkinsonism-preventing effect of 1-MeTIQ by in vivo measurement of pre-synaptic dopamine transporters and post-synaptic dopamine D(2) receptors in the mouse striatum.

Parkinsonism-inducing neurotoxicity of 1,2,3,4-tetrahydroisoquinoline (TIQ), as contrasted to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and parkinsonism-preventing effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) have been investigated in mice by measuring their effects on the in vivo binding of radioligand to pre-synaptic dopamine transporters (DATs) or to dopamine D(2) receptors (D2R) in the striatum. A significant reduction of the ligand-DATs binding was found in the mice treated with MPTP, but not with TIQ, under the dosage inducing behavioral abnormality and loss of tyrosine hydroxylase-positive cells in the substantia nigra. A slight decrease in the ligand-DATs binding was observed in the mice given a larger dose of TIQ. Compensatory up-regulation in the post-synaptic D2Rs was found in the MPTP-treated mice. Pre-treatment with (S)-enantiomer, but not (R)-enantiomer, of 1-MeTIQ prevented the degeneration of DATs to some extent. We concluded that the TIQ-induced parkinsonism model is different from the MPTP-induced model as evaluated by the radioligand-DATs binding and that (S)-1-MeTIQ has a preventing effect for the degeneration of the DATs to a certain extent.     

Sympathetic histamine exerts different pre- and post-synaptic functions according to the frequencies of nerve stimulation in guinea pig vas deferens

Histamine (HA) was found to be present in the sympathetic nerve terminals of guinea pig hearts and vasa deferentia in our previous study; however, little is known about the functions of this neurogenic HA. In this study, we used guinea pig vasa deferentia to investigate the pre- and post-synaptic functions of HA evoked by different frequencies of sympathetic nerve stimulation. We found that sympathetic nerve stimulation could evoke HA release, which was independent to mast cell degranulator compound 48/80 and mast cell stabilizer cromolyn, but was highly sensitive to Na⁺ channel blocker tetrodotoxin and chemical sympathectomy with 6-hydroxydopamine. The neurogenically released HA evoked by 12.5 Hz of nerve stimulation activated only pre-synaptic H₃ receptors and mediated pre-synaptic inhibitory effects, while under 25 or 50 Hz stimulation condition, HA simultaneously activated both pre-synaptic H₃ receptors and post-synaptic H₁ receptors. However, the direct contractile responses evoked by sympathetic HA via H₁ receptors were observed at 50 Hz. HA release and HA-mediated contractile responses upon sympathetic nerve stimulation were significantly inhibited by pre-treatment of histidine decarboxylase inhibitor α-fluoromethylhistidine. Furthermore, application of exogenous HA could mimic these pre- and post-synaptic effects. Our findings indicate that HA in sympathetic neurons acts as a neurotransmitter and its functions vary from pre-synaptic inhibition, to post-synaptic facilitation, to direct post-synaptic contractile responses according to sympathetic nerve stimulation frequencies.     

Role of vesicle pools in action potential pattern-dependent dopamine overflow in rat striatum in vivo

Action potential (AP) patterns and dopamine (DA) release are known to correlate with rewarding behaviors, but how codes of AP bursts translate into DA release in vivo remains elusive. Here, a given AP pattern was defined by four codes, termed total AP number, frequency, number of AP bursts, and interburst time [N, f, b, i].. The 'burst effect' was calculated by the ratio (γ) of DA overflow by multiple bursts to that of a single burst when total AP number was fixed. By stimulating the medial forebrain bundle using AP codes at either physiological (20 Hz) or supraphysiological (80 Hz) frequencies, we found that DA was released from two kinetically distinct vesicle pools, the fast-releasable pool (FRP) and prolonged-releasable pool (PRP), in striatal dopaminergic terminals in vivo. We examined the effects of vesicle pools on AP-pattern dependent DA overflow and found, with given 'burst codes' [b=8, i=0.5 s], a large total AP number [N = 768, f = 80 Hz] produced a facilitating burst-effect (γ[b8/b1] = 126 ± 3%), while a small total AP number [N=96, 80 Hz] triggered a depressing-burst-effect (γ[b8/b1] = 29 ± 4%). Furthermore, we found that the PRP (but not the FRP) predominantly contributed to the facilitating-burst-effect and the FRP played an important role in the depressing-burst effect. Thus, our results suggest that striatal DA release captures pre-synaptic AP pattern information through different releasable pools.     

Effects of green tea polyphenols on dopamine uptake and on MPP+ -induced dopamine neuron injury

As antioxidants, polyphenols are considered to be potentially useful in preventing chronic diseases in man, including Parkinson's disease (PD), a disease involving dopamine (DA) neurons. Our studies have demonstrated that polyphenols extracted from green tea (GT) can inhibit the uptake of 3H-dopamine (3H-DA) and 1-methyl-4-phenylpyridinium (MPP(+)) by DA transporters (DAT) and partially protect embryonic rat mesencephalic dopaminergic (DAergic) neurons from MPP(+)-induced injury. The inhibitory effects of GT polyphenols on 3H-DA uptake were determined in DAT-pCDNA3-transfected Chinese Hamster Ovary (DAT-CHO) cells and in striatal synaptosomes of C57BL/6 mice in vitro and in vivo. The inhibitory effects on 3H-MPP(+) uptake were determined in primary cultures of embryonic rat mesencephalic DAergic cells. Inhibition of uptake for both 3H-DA and 3H-MPP(+) was dose-dependent in the presence of polyphenols. Incubation with 50 microM MPP(+) resulted in a significant loss of tyrosine-hydroxylase (TH)-positive cells in the primary embryonic mesencephalic cultures, while pretreatment with polyphenols (10 to 30 microg/ml) or mazindol (10 microM), a classical DAT inhibitor, significantly attenuated MPP(+)-induced loss of TH-positive cells. These results suggest that GT polyphenols have inhibitory effects on DAT, through which they block MPP(+) uptake and protect DAergic neurons against MPP(+)-induced injury.     

Acute ethanol suppresses glutamatergic neurotransmission through endocannabinoids in hippocampal neurons

Ethanol exposure during fetal development is a leading cause of long-term cognitive impairments. Studies suggest that ethanol exposure have deleterious effects on the hippocampus, a brain region that is important for learning and memory. Ethanol exerts its effects, in part, via alterations in glutamatergic neurotransmission, which is critical for the maturation of neuronal circuits during development. The current literature strongly supports the growing evidence that ethanol inhibits glutamate release in the neonatal CA1 hippocampal region. However, the exact molecular mechanism responsible for this effect is not well understood. In this study, we show that ethanol enhances endocannabinoid (EC) levels in cultured hippocampal neurons, possibly through calcium pathways. Acute ethanol depresses miniature post-synaptic current (mEPSC) frequencies without affecting their amplitude. This suggests that ethanol inhibits glutamate release. The CB1 receptors (CB1Rs) present on pre-synaptic neurons are not altered by acute ethanol. The CB1R antagonist SR 141716A reverses ethanol-induced depression of mEPSC frequency. Drugs that are known to enhance the in vivo function of ECs occlude ethanol effects on mEPSC frequency. Chelation of post-synaptic calcium by EGTA antagonizes ethanol-induced depression of mEPSC frequency. The activation of CB1R with the selective agonist WIN55,212-2 also suppresses the mEPSC frequency. This WIN55,212-2 effect is similar to the ethanol effects and is reversed by SR141716A. In addition, tetani-induced excitatory post-synaptic currents (EPSCs) are depressed by acute ethanol. SR141716A significantly reverses ethanol effects on evoked EPSC amplitude in a dual recording preparation. These observations, taken together, suggest the participation of ECs as retrograde messengers in the ethanol-induced depression of synaptic activities.     

Inhibitory Postsynaptic Currents Induced by Activation of Interneurons in Cultured Rat Hippocampal Neurons

Using the patch-clamp technique in the whole-cell configuration, we studied the characteristics of a series of action potentials (APs) induced by a 500-msec-long current pulse applied to a pre-synaptic unit, as well as the kinetic characteristics of post-synaptic currents (PSCs) evoked by the APs in a post-synaptic unit, in synaptically connected pairs of cultured hippocampal neurons. Presynaptic inhibitory units were identified as GABA-ergic interneurons; they were divided into two groups according to the size of the soma and the number of processes. The kinetic characteristics of PSCs, which were induced in the post-synaptic neuron by a series of the APs generated in the pre-synaptic cell, demonstrated a certain dependence on the morphological characteristics of these cells. In interneurons with large-sized somata, the kinetics of the currents were more fast, and the reversal potential was close to the equilibrium Cl potential. In interneurons with small-sized somata, currents were slower, and the reversal potential was shifted. We conclude that under conditions of culturing, a pre-synaptic cell not only directly provokes the development of PSC in a post-synaptic neuron and determines the amplitude of this current but also significantly influences the kinetics of this current. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 116–123, March–April, 2005.     

Striatal dopamine release is triggered by synchronized activity in cholinergic interneurons

Striatal dopamine plays key roles in our normal and pathological goal-directed actions. To understand dopamine function, much attention has focused on how midbrain dopamine neurons modulate their firing patterns. However, we identify a presynaptic mechanism that triggers dopamine release directly, bypassing activity in dopamine neurons. We paired electrophysiological recordings of striatal channelrhodopsin2-expressing cholinergic interneurons with simultaneous detection of dopamine release at carbon-fiber microelectrodes in striatal slices. We reveal that activation of cholinergic interneurons by light flashes that cause only single action potentials in neurons from a small population triggers dopamine release via activation of nicotinic receptors on dopamine axons. This event overrides ascending activity from dopamine neurons and, furthermore, is reproduced by activating ChR2-expressing thalamostriatal inputs, which synchronize cholinergic interneurons in vivo. These findings indicate that synchronized activity in cholinergic interneurons directly generates striatal dopamine signals whose functions will extend beyond those encoded by dopamine neuron activity.     

Dopamine release is impaired in a mouse model of DYT1 dystonia

Early onset torsion dystonia, the most common form of hereditary primary dystonia, is caused by a mutation in the TOR1A gene, which codes for the protein torsinA. This form of dystonia is referred to as DYT1. We have used a transgenic mouse model of DYT1 dystonia [human mutant-type (hMT)1 mice] to examine the effect of the mutant human torsinA protein on striatal dopaminergic function. Analysis of striatal tissue dopamine (DA) and metabolites using HPLC revealed no difference between hMT1 mice and their non-transgenic littermates. Pre-synaptic DA transporters were studied using in vitro autoradiography with [(3)H]mazindol, a ligand for the membrane DA transporter, and [(3)H]dihydrotetrabenazine, a ligand for the vesicular monoamine transporter. No difference in the density of striatal DA transporter or vesicular monoamine transporter binding sites was observed. Post-synaptic receptors were studied using [(3)H]SCH-23390, a ligand for D(1) class receptors, [(3)H]YM-09151-2 and a ligand for D(2) class receptors. There were again no differences in the density of striatal binding sites for these ligands. Using in vivo microdialysis in awake animals, we studied basal as well as amphetamine-stimulated striatal extracellular DA levels. Basal extracellular DA levels were similar, but the response to amphetamine was markedly attenuated in the hMT1 mice compared with their non-transgenic littermates (253 +/- 71% vs. 561 +/- 132%, p < 0.05, two-way anova). These observations suggest that the mutation in the torsinA protein responsible for DYT1 dystonia may interfere with transport or release of DA, but does not alter pre-synaptic transporters or post-synaptic DA receptors. The defect in DA release as observed may contribute to the abnormalities in motor learning as previously documented in this transgenic mouse model, and may contribute to the clinical symptoms of the human disorder.     

Gephyrin expression and clustering affects the size of glutamatergic synaptic contacts

We have recently shown that disrupting the expression and post-synaptic clustering of gephyrin in cultured hippocampal pyramidal cells, by either gephyrin RNAi (RNA interference) or over-expression of a dominant negative gephyrin-enhanced green fluorescent protein (EGFP) fusion protein, leads to decreased number of post-synaptic gephyrin and GABA_A receptor clusters and to reduced GABAergic innervation of these cells. On the other hand, increasing gephyrin expression led to a small increase in the number of gephyrin and GABA_A receptor clusters and to little or no effect on GABAergic innervation. We are now reporting that altering gephyrin expression and clustering affects the size but not the density of glutamatergic synaptic contacts. Knocking down gephyrin with gephyrin RNAi, or preventing gephyrin clustering by over-expression of the dominant negative gephyrin-enhanced green fluorescent protein fusion protein, leads to larger post-synaptic PSD-95 clusters and larger pre-synaptic glutamatergic terminals. On the other hand, over-expression of gephyrin leads to slightly smaller PSD-95 clusters and pre-synaptic glutamatergic terminals. The change in size of PSD-95 clusters were accompanied by a parallel change in the size of NR2-NMDA receptor clusters. It is concluded that the levels of expression and clustering of gephyrin, a protein that concentrates at the post-synaptic complex of the inhibitory synapses, not only has homotypic effects on GABAergic synaptic contacts, but also has heterotypic effects on glutamatergic synaptic contacts. We are proposing that gephyrin is a counterpart of the post-synaptic glutamatergic scaffold protein PSD-95 in regulating the number and/or size of the excitatory and inhibitory synaptic contacts.