Release probability-dependent scaling of the postsynaptic responses at single hippocampal GABAergic synapses

The probability of neurotransmitter release: variability and feedback control at single synapses

Information transfer at chemical synapses occurs when vesicles fuse with the plasma membrane and release neurotransmitter. This process is stochastic and its likelihood of occurrence is a crucial factor in the regulation of signal propagation in neuronal networks. The reliability of neurotransmitter release can be highly variable: experimental data from electrophysiological, molecular and imaging studies have demonstrated that synaptic terminals can individually set their neurotransmitter release probability dynamically through local feedback regulation. This local tuning of transmission has important implications for current models of single-neuron computation.

Synaptic cross talk between perisomatic-targeting interneuron classes expressing cholecystokinin and parvalbumin in hippocampus

Cholescystokinin (CCK)- or parvalbumin (PV)-containing interneurons are the major perisomatic-targeting interneurons in the cerebral cortex, including hippocampus, and are thought to form mutually exclusive networks. We used several techniques to test the alternative hypothesis that CCK and PV cells are coupled by chemical synapses. Triple immunofluorescence confocal microscopy revealed numerous axosomatic, axodendritic, and axoaxonic contacts stained for CCK, PV, and the presynaptic marker synaptophysin. The existence of mutual CCK and PV synapses was supported by dual EM immunolabeling. Paired whole-cell recordings detected unitary GABA(A)ergic synaptic transmission between identified CCK and PV cells, and single CCK cells could transiently inhibit action potential firing of synaptically coupled PV cells. We conclude that the major hippocampal perisomatic-targeting interneurons communicate synaptically. This communication should affect neuronal network activity, including neuronal oscillations, in which the CCK and PV cells have well established roles. The prevalence of CCK and PV networks in other brain regions suggests that internetwork interactions could be generally important.

Slow glycinergic transmission mediated by transmitter pooling

Most fast-acting neurotransmitters are rapidly cleared from synaptic regions. This feature isolates synaptic sites, rendering the timecourse of synaptic responses independent of the number of active synapses. We describe a striking exception at glycinergic synapses on granule cells of the rat dorsal cochlear nucleus. The duration of IPSCs was dependent on the number of presynaptic axons that were stimulated and on the number of vesicles released from each axon. Increasing stimulus number or frequency, or blocking glycine uptake, slowed synaptic decays, while a low-affinity competitive antagonist of GlyRs accelerated IPSC decay. These effects could be explained by unique features of GlyRs when activated by pooling of glycine across synapses. Functionally, increasing the number of IPSPs markedly lengthened the period of spike inhibition following cessation of presynaptic stimulation. Thus, temporal properties of inhibition can be controlled by activity levels in multiple presynaptic cells or by adjusting release probability at individual synapses.

Distinctive quantal properties of neurotransmission at excitatory and inhibitory autapses revealed using variance-mean analysis

Normal brain function depends on an interplay between glutamatergic and GABAergic synaptic transmission, yet questions remain about the biophysical differences between these two classes of synapse. By taking advantage of a simple culture system, we present here a detailed comparison of excitatory and inhibitory neurotransmission under identical conditions using the variance-mean (V-M) method of quantal analysis. First, we validate V-M analysis for glutamatergic autapses formed by isolated hippocampal pyramidal neurons in culture, confirming that the analysis accurately predicts the quantal amplitude (Q). We also show that V-M analysis is only weakly sensitive to intersite and intrasite quantal variance and to the known inhomogeneities in release probability (P(r)). Next, by repeating the experiments with GABAergic autapses, we confirm that V-M analysis provides an accurate account of inhibitory neurotransmission in this system. Mean P(r), provided by V-M analysis, shows a dependence on extracellular Ca(2+) concentration that is nearly identical for both excitatory and inhibitory autapses. Finally, the V-M method allows us to compare the locus of short-term synaptic plasticity at these connections. Glutamatergic autapses exhibit paired-pulse depression that depends mainly on changes in P(r), whereas depression at GABAergic autapses appears to depend primarily on changes in the number of release sites. We conclude that, apart from differences in the mechanisms of short-term plasticity, the basic quantal properties of excitatory and inhibitory connections in this hippocampal system are remarkably similar.

Contrasting the functional properties of GABAergic axon terminals with single and multiple synapses in the thalamus

Diverse sources of GABAergic inhibition are a major feature of cortical networks, but distinct inhibitory input systems have not been systematically characterized in the thalamus. Here, we contrasted the properties of two independent GABAergic pathways in the posterior thalamic nucleus of rat, one input from the reticular thalamic nucleus (nRT), and one "extrareticular" input from the anterior pretectal nucleus (APT). The vast majority of nRT-thalamic terminals formed single synapses per postsynaptic target and innervated thin distal dendrites of relay cells. In contrast, single APT-thalamic terminals formed synaptic contacts exclusively via multiple, closely spaced synapses on thick relay cell dendrites. Quantal analysis demonstrated that the two inputs displayed comparable quantal amplitudes, release probabilities, and multiple release sites. The morphological and physiological data together indicated multiple, single-site contacts for nRT and multisite contacts for APT axons. The contrasting synaptic arrangements of the two pathways were paralleled by different short-term plasticities. The multisite APT-thalamic pathway showed larger charge transfer during 50-100 Hz stimulation compared with the nRT pathway and a greater persistent inhibition accruing during stimulation trains. Our results demonstrate that the two inhibitory systems are morpho-functionally distinct and suggest and that multisite GABAergic terminals are tailored for maintained synaptic inhibition even at high presynaptic firing rates. These data explain the efficacy of extrareticular inhibition in timing relay cell activity in sensory and motor thalamic nuclei. Finally, based on the classic nomenclature and the difference between reticular and extrareticular terminals, we define a novel, multisite GABAergic terminal type (F3) in the thalamus.

Recurrent collateral connections of striatal medium spiny neurons are disrupted in models of Parkinson's disease

The principal neurons of the striatum, GABAergic medium spiny neurons (MSNs), are interconnected by local recurrent axon collateral synapses. Although critical to many striatal models, it is not clear whether these connections are random or whether they preferentially link functionally related groups of MSNs. To address this issue, dual whole patch-clamp recordings were made from striatal MSNs in brain slices taken from transgenic mice in which D(1) or D(2) dopamine receptor expression was reported with EGFP (enhanced green fluorescent protein). These studies revealed that unidirectional connections were common between both D(1) receptor-expressing MSN (D(1) MSN) pairs (26%) and D(2) receptor-expressing MSN (D(2) MSN) pairs (36%). D(2) MSNs also commonly formed synapses on D(1) MSNs (27% of pairs). Conversely, only 6% of the D(1) MSNs formed detectable connections with D(2) MSNs. Furthermore, synaptic connections formed by D(1) MSNs were weaker than those formed by D(2) MSNs, a difference that was attributable to fewer GABA(A) receptors at D(1) MSN synapses. The strength of detectable recurrent connections was dramatically reduced in Parkinson's disease models. The studies demonstrate that recurrent collateral connections between MSNs are not random but rather differentially couple D(1) and D(2) MSNs. Moreover, this recurrent collateral network appears to be disrupted in Parkinson's disease models, potentially contributing to pathological alterations in MSN activity patterns and psychomotor symptoms.

Molecular mechanisms of interaction between the neuroprotective substance riluzole and GABA(A)-receptors

The antiepileptic drug riluzole is used as a therapeutic agent in amyotrophic lateral sclerosis due to its neuroprotective effects. Besides presynaptic inhibition of GABAergic and preferentially glutamatergic transmission, it also potentiates postsynaptic GABA(A)-receptor function. We investigated the postsynaptic effects of riluzole on GABA(A)-receptor channels by use of the patch-clamp technique. Recombinant alpha1beta2gamma(2s) and alpha1beta2 GABA(A) receptors were expressed in HEK 293 cells by transient transfection. Pulses of GABA were applied in combination with different concentrations of riluzole to whole cell or outside-out patches with either alpha1beta2gamma(2s) or alpha1beta2 GABA(A)-receptor channels. Co-application of riluzole led to a slight decrease of absolute peak current amplitudes and steady-state currents in prolonged presence of GABA at saturating concentrations. In the presence of riluzole, enhancement of current amplitudes was observed with lower concentrations of GABA at alpha1beta2gamma(2s) receptors and to a lower extent also at alpha1beta2 receptors. Thus, the potentiating effect of riluzole was shown to be not abolished in the absence of the gamma(2s)-subunit. A further prominent effect of riluzole was a highly significant acceleration of the time course of current decay, most probably pointing to an open-channel block-like mechanism of action. As both receptor subtypes were affected similarly by the block, it could be concluded that the respective binding sites should be assumed within a region of high sequence homology like it is given for the channel-lining M2 domain of GABA(A)-receptor subunits. In conclusion, three different molecular mechanisms of interaction of the neuroprotective compound riluzole were observed at two different subtypes of GABA(A) receptor channels. The results further point to the impact of the inhibitory as well as the excitatory synaptic activity as a pharmacological target to counteract chronic excitotoxicity and reveal molecular mechanisms of action of the only one neuroprotective drug in current clinical use in patients suffering from amyotrophic lateral sclerosis.

Structural correlates of efficient GABAergic transmission in the basal ganglia-thalamus pathway

Giant inhibitory terminals with multiple synapses, the counterparts of excitatory "detonator" or "driver" terminals, have not been described in the forebrain. Using three-dimensional reconstructions of electron microscopic images, we quantitatively characterize a GABAergic pathway that establishes synaptic contacts exclusively via multiple synapses. Axon terminals of the nigrothalamic pathway formed, on average, 8.5 synapses on large-diameter dendrites and somata of relay cells in the ventromedial nucleus of the rat thalamus. All synapses of a given terminal converged on a single postsynaptic element. The vast majority of the synapses established by a single terminal were not separated by astrocytic processes. Nigrothalamic terminals in the macaque monkey showed the same ultrastructural features both in qualitative and quantitative terms (the median number of synapse per target was also 8.5). The individual synapses were closely spaced in both species. The nearest-neighbor synaptic distances were 169 nm in the rat and 178 nm in the monkey. The average number of synapses within 0.75 microm from any given synapse was 3.8 in the rat and 3.5 in the monkey. The arrangement of synapses described in this study creates favorable conditions for intersynaptic spillover of GABA among the multiple synapses of a single bouton, which can result in larger charge transfer. This could explain faithful and efficient GABAergic signal transmission in the nigrothalamic pathway in the healthy condition and during Parkinson's disease. In addition, our structural data suggest that the rodent nigrothalamic pathway can be a valid model of the primate condition, when the mechanism of GABAergic transmission is studied.

Impaired GABAergic inhibition in the visual cortex of brain-derived neurotrophic factor heterozygous knockout mice

Brain derived neurotrophic factor (BDNF) promotes the formation, maturation and stabilization of inhibitory synapses in the central nervous system. In addition, BDNF has been suggested to regulate the critical period for ocular dominance plasticity in the visual system. Here we further evaluated the role of BDNF in the visual cortex by studying the GABAergic synaptic transmission under conditions of chronically reduced levels of BDNF. Whole-cell patch-clamp recordings were performed from pyramidal neurons located in layers II/III of visual cortical slices in heterozygous BDNF knockout mice (BDNF (+/-)) and their wild-type littermates at the age of 21-25 days. The BDNF (+/-) mice showed a decreased frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs) as well as a reduced amplitude and prolonged decay time constant of evoked IPSCs. Further analyses indicated an impaired presynaptic GABAergic function in BDNF (+/-) mice, as shown by the decreased release probability, steady-state release and synchronous release of GABA. However, the number of functional release sites remained unchanged. In line with these observations, an impaired glutamate-driven GABA release was observed in BDNF (+/-) mice. Furthermore, the overall balance in the strength of cortical excitation to inhibition shifted towards a decreased inhibition. Finally, the reversal potential for chloride-mediated evoked IPSCs was not affected. These findings suggested that chronically reduced levels of BDNF strongly impair the GABAergic inhibitory function in visual cortex by altering postsynaptic properties and by reducing presynaptic GABA release as well as the overall strength of inhibition onto pyramidal neurons within the cortical network. These impairments of inhibitory function are compatible with a rather immature status of the GABAergic system in BDNF (+/-) mice, which supports the hypothesis that the level of expression for BDNF critically affects maturation and function of the GABAergic inhibition.

Perisomatic inhibition

Recent evidence supports the hypothesis of a functional dichotomy of perisomatic inhibition in the cerebral cortex: the parvalbumin- and cholecystokinin-containing basket cells that are specialized to control rhythm (as a clockwork) and "mood" (as a fine-tuning device), respectively, of network oscillations. Pathology extends this conclusion further, as the former is implicated in epilepsy and the latter in anxiety. The well-balanced cooperation of the two inhibitory systems is required for the normal network operations underlying the cognitive functions of the cerebral cortex.

Multiple effects of bisphenol A, an endocrine disrupter, on GABAA receptors in acutely dissociated rat CA3 pyramidal neurons

Bisphenol A (BPA), an endocrine disrupter, is contained in cans, polycarbonate bottles and some dental sealants. While the toxicological effects of BPA on the endocrine system have been extensively studied, its action on the central nervous system is poorly understood. Herein, we report the effects of BPA on GABA-induced currents (I(GABA)), using a conventional whole-cell patch clamp technique from acutely isolated rat CA3 pyramidal neurons. By itself, BPA concentration-dependently elicited the membrane current, which was significantly blocked by bicuculline, a selective GABA(A) receptor antagonist. BPA potentiated the peak I(GABA) induced by lower concentrations of GABA (<10 microM) in a concentration-dependent manner. The extent of BPA-induced potentiation of I(GABA) was significantly reduced by either diazepam or ethanol, allosteric modulators of GABA(A) receptors. BPA, however, inhibited the peak I(GABA) induced by higher concentrations of GABA (>30 microM), and accelerated the desensitization rate of I(GABA). BPA also greatly inhibited the steady state I(GABA) induced by higher concentrations of GABA (>30 microM) in a noncompetitive manner. In addition, BPA affected synaptic GABA(A) receptors as it decreased the amplitude of GABAergic miniature inhibitory postsynaptic currents in a concentration-dependent manner. Considering its complex modulatory effects on GABA(A) receptors, BPA might have potential toxicological effects on the central nervous system.

Quantitative ultrastructural differences between local and medial septal GABAergic axon terminals in the rat hippocampus

Functionally distinct subsets of hippocampal inhibitory neurons exhibit large differences in the frequency, pattern and short-term plasticity of GABA release from their terminals. Heterogeneity is also evident in the ultrastructural features of GABAergic axon terminals examined in the electron microscope, but it is not known if or how this corresponds to interneuron subtypes. We investigated the feasibility of separating morphologically distinct clusters of terminal types, using the approach of measuring several ultrastructural parameters of GABAergic terminals in the CA1 area of the rat hippocampus. Septo-hippocampal axon terminals were anterogradely labeled by biotinylated dextran amine and visualized by pre-embedding immunogold staining to delineate one homogeneous terminal population. Long series (100-150) of ultrathin sections were cut from stratum oriens and stratum radiatum of the CA1 area, and GABAergic terminals were identified by post-embedding immunogold staining. Stereologically unbiased samples of the total GABAergic axon terminal population and a random sample of the septal axon terminals were reconstructed in 3D, and several of their parameters were measured (e.g. bouton volume, synapse surface, volume occupied by vesicles, mitochondria volume). Septal terminals demonstrated significantly larger mean values for most parameters than the total population of local GABAergic terminals. There was no significant difference between terminals reconstructed in the basal and apical dendritic regions of pyramidal cells, neither for the septal nor for the local population. Importantly, almost all parameters were highly correlated, precluding the possibility of clustering the local terminals into non-overlapping subsets. Factor and cluster analysis confirmed these findings. Our results suggest that similarly to excitatory terminals, inhibitory terminals follow an "ultrastructural size principle," and that the terminals of different interneuron subtypes cannot be distinguished by ultrastructure alone.

Dopaminergic modulation of short-term synaptic plasticity at striatal inhibitory synapses

Circuit properties, such as the selection of motor synergies, have been posited as relevant tasks for the recurrent inhibitory synapses between spiny projection neurons of the neostriatum, a nucleus of the basal ganglia participating in procedural learning and voluntary motor control. Here we show how the dopaminergic system regulates short-term plasticity (STP) in these synapses. STP is thought to endow neuronal circuits with computational powers such as gain control, filtering, and the emergence of transitory net states. But little is known about STP regulation. Employing unitary and population synaptic recordings, we observed that activation of dopamine receptors can modulate STP between spiny neurons. A D(1)-class agonist enhances, whereas a D(2)-class agonist decreases, short-term depression most probably by synaptic redistribution. Presynaptic receptors appear to be responsible for this modulation. In contrast, STP between fast-spiking interneurons and spiny projection neurons is largely unregulated despite expressing presynaptic receptors. Thus, the present experiments provide an explanation for dopamine actions at the circuit level: the control of STP between lateral connections of output neurons and the reorganization of the balance between different forms of inhibitory transmission. Theoretically, D(1) receptors would promote a sensitive, responsive state for temporal precision (dynamic component), whereas D(2) receptors would sense background activity (static component).