Nucleus-specific differences in GABA(A)-receptor-mediated inhibition are enhanced during thalamic development

From circuits to behavior: Amygdala dysfunction in fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by a repeat expansion mutation in the promotor region of the FMR1 gene resulting in transcriptional silencing and loss of function of fragile X messenger ribonucleoprotein 1 protein (FMRP). FMRP has a well-defined role in the early development of the brain. Thus, loss of the FMRP has well-known consequences for normal cellular and synaptic development leading to a variety of neuropsychiatric disorders including an increased prevalence of amygdala-based disorders. Despite our detailed understanding of the pathophysiology of FXS, the precise cellular and circuit-level underpinnings of amygdala-based disorders is incompletely understood. In this review, we discuss the development of the amygdala, the role of neuromodulation in the critical period plasticity, and recent advances in our understanding of how synaptic and circuit-level changes in the basolateral amygdala contribute to the behavioral manifestations seen in FXS.

Disrupted inhibitory plasticity and homeostasis in Fragile X syndrome

Fragile X Syndrome (FXS) is a neurodevelopmental disorder instigated by the absence of a key translation regulating protein, Fragile X Mental Retardation Protein (FMRP). The loss of FMRP in the CNS leads to abnormal synaptic development, disruption of critical periods of plasticity, and an overall deficiency in proper sensory circuit coding leading to hyperexcitable sensory networks. However, little is known about how this hyperexcitable environment affects inhibitory synaptic plasticity. Here, we show that in vivo layer 2/3 of the primary somatosensory cortex of the Fmr1 KO mouse exhibits basal hyperexcitability and an increase in neuronal firing rate suppression during whisker activation. This aligns with our in vitro data that indicate an increase in GABAergic spontaneous activity, a faulty mGluR-mediated inhibitory input and impaired inhibitory plasticity processes. Specifically, we find that mGluR activation sensitivity is overall diminished in the Fmr1 KO mouse leading to both a decreased spontaneous inhibitory postsynaptic input to principal cells and a disrupted form of inhibitory long-term depression (I-LTD). These data suggest an adaptive mechanism that acts to homeostatically counterbalance the cortical hyperexcitability observed in FXS.

Leptin alters somatosensory thalamic networks by decreasing gaba release from reticular thalamic nucleus and action potential frequency at ventrobasal neurons

Leptin is an adipose-derived hormone that controls appetite and energy expenditure. Leptin receptors are expressed on extra-hypothalamic ventrobasal (VB) and reticular thalamic (RTN) nuclei from embryonic stages. Here, we studied the effects of pressure-puff, local application of leptin on both synaptic transmission and action potential properties of thalamic neurons in thalamocortical slices. We used whole-cell patch-clamp recordings of thalamocortical VB neurons from wild-type (WT) and leptin-deficient obese (ob/ob) mice. We observed differences in VB neurons action potentials and synaptic currents kinetics when comparing WT vs. ob/ob. Leptin reduced GABA release onto VB neurons throughout the activation of a JAK2-dependent pathway, without affecting excitatory glutamate transmission. We observed a rapid and reversible reduction by leptin of the number of action potentials of VB neurons via the activation of large conductance Ca²⁺-dependent potassium channels. These leptin effects were observed in thalamocortical slices from up to 5-week-old WT but not in leptin-deficient obese mice. Results described here suggest the existence of a leptin-mediated trophic modulation of thalamocortical excitability during postnatal development. These findings could contribute to a better understanding of leptin within the thalamocortical system and sleep deficits in obesity.

Electrical synapses and the development of inhibitory circuits in the thalamus

KEY POINTS

The thalamus is a structure critical for information processing and transfer to the cortex. Thalamic reticular neurons are inhibitory cells interconnected by electrical synapses, most of which require the gap junction protein connexin36 (Cx36). We investigated whether electrical synapses play a role in the maturation of thalamic networks by studying neurons in mice with and without Cx36. When Cx36 was deleted, inhibitory synapses were more numerous, although both divergent inhibitory connectivity and dendritic complexity were reduced. Surprisingly, we observed non-Cx36-dependent electrical synapses with unusual biophysical properties interconnecting some reticular neurons in mice lacking Cx36. The results of the present study suggest an important role for Cx36-dependent electrical synapses in the development of thalamic circuits.

ABSTRACT

Neurons within the mature thalamic reticular nucleus (TRN) powerfully inhibit ventrobasal (VB) thalamic relay neurons via GABAergic synapses. TRN neurons are also coupled to one another by electrical synapses that depend strongly on the gap junction protein connexin36 (Cx36). Electrical synapses in the TRN precede the postnatal development of TRN-to-VB inhibition. We investigated how the deletion of Cx36 affects the maturation of TRN and VB neurons, electrical coupling and GABAergic synapses by studying wild-type (WT) and Cx36 knockout (KO) mice. The incidence and strength of electrical coupling in TRN was sharply reduced, but not abolished, in KO mice. Surprisingly, electrical synapses between Cx36-KO neurons had faster voltage-dependent decay kinetics and conductance asymmetry (rectification) than did electrical synapses between WT neurons. The properties of TRN-mediated inhibition in VB also depended on the Cx36 genotype. Deletion of Cx36 increased the frequency and shifted the amplitude distributions of miniature IPSCs, whereas the paired-pulse ratio of evoked IPSCs was unaffected, suggesting that the absence of Cx36 led to an increase in GABAergic synaptic contacts. VB neurons from Cx36-KO mice also tended to have simpler dendritic trees and fewer divergent inputs from the TRN compared to WT cells. The findings obtained in the present study suggest that proper development of thalamic inhibitory circuitry, neuronal morphology, TRN cell function and electrical coupling requires Cx36. In the absence of Cx36, some TRN neurons express asymmetric electrical coupling mediated by other unidentified connexin subtypes.

Early postnatal switch in GABAA receptor α-subunits in the reticular thalamic nucleus

The GABAergic neurons of the thalamic reticular nucleus (nRt) provide the primary source of inhibition within the thalamus. Using physiology, pharmacology, and immunohistochemistry in mice, we characterized postsynaptic developmental changes in these inhibitory projection neurons. First, at postnatal days 3-5 (P3-5), inhibitory postsynaptic currents (IPSCs) decayed very slowly, followed by a biphasic developmental progression, becoming faster at P6-8 and then slower again at P9-11 before stabilizing in a mature form around P12. Second, the pharmacological profile of GABA(A) receptor (GABA(A)R)-mediated IPSCs differed between neonatal and mature nRt neurons, and this was accompanied by reciprocal changes in α3 (late) and α5 (early) subunit expression in nRt. Zolpidem, selective for α1- and α3-containing GABA(A)Rs, augmented only mature IPSCs, whereas clonazepam enhanced IPSCs at all stages. This effect was blocked by the α5-specific inverse agonist L-655,708, but only in immature neurons. In α3(H126R) mice, in which α3-subunits were mutated to become benzodiazepine insensitive, IPSCs were enhanced compared with those in wild-type animals in early development. Third, tonic GABA(A)R activation in nRt is age dependent and more prominent in immature neurons, which correlates with early expression of α5-containing GABA(A)Rs. Thus neonatal nRt neurons show relatively high expression of α5-subunits, which contributes to both slow synaptic and tonic extrasynaptic inhibition. The postnatal switch in GABA(A)R subunits from α5 to α3 could facilitate spontaneous network activity in nRt that occurs at this developmental time point and which is proposed to play a role in early circuit development.

Deficient tonic GABAergic conductance and synaptic balance in the fragile X syndrome amygdala

Fragile X syndrome (FXS) is the leading cause of inherited intellectual disability. Comorbidities of FXS such as autism are increasingly linked to imbalances in excitation and inhibition (E/I) as well as dysfunction in GABAergic transmission in a number of brain regions including the amygdala. However, the link between E/I imbalance and GABAergic transmission deficits in the FXS amygdala is poorly understood. Here we reveal that normal tonic GABAA receptor-mediated neurotransmission in principal neurons (PNs) of the basolateral amygdala (BLA) is comprised of both δ- and α5-subunit-containing GABAA receptors. Furthermore, tonic GABAergic capacity is reduced in these neurons in the Fmr1 knockout (KO) mouse model of FXS (1.5-fold total, 3-fold δ-subunit, and 2-fold α5-subunit mediated) as indicated by application of gabazine (50 μM), 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP, 1 μM), and α5ia (1.5 μM) in whole cell patch-clamp recordings. Moreover, α5-containing tonic GABAA receptors appear to preferentially modulate nonsomatic compartments of BLA PNs. Examination of evoked feedforward synaptic transmission in these cells surprisingly revealed no differences in overall synaptic conductance or E/I balance between wild-type (WT) and Fmr1 KO mice. Instead, we observed altered feedforward kinetics in Fmr1 KO PNs that supports a subtle yet significant decrease in E/I balance at the peak of excitatory conductance. Blockade of α5-subunit-containing GABAA receptors replicated this condition in WT PNs. Therefore, our data suggest that tonic GABAA receptor-mediated neurotransmission can modulate synaptic E/I balance and timing established by feedforward inhibition and thus may represent a therapeutic target to enhance amygdala function in FXS.

Optogenetic activation of nigral inhibitory inputs to motor thalamus in the mouse reveals classic inhibition with little potential for rebound activation

The inhibitory output from the internal pallidum and substantia nigra to the thalamus forms an important link in the transmission of basal ganglia processing to cortex. Two hypotheses consider either inhibition of thalamic activity or thalamic excitation via post-inhibitory rebound burst firing as the functional mode of this link. We used optogenetics to characterize the synaptic properties of nigral input to motor thalamus in adult mouse brain slices, and to determine in what conditions the nigral inhibition of motor thalamus is transmitted via inhibition or rebound firing. Our results are more consistent with graded inhibition of spiking for conditions expected in normal awake animals, because inhibitory potentials from nigral input were generally not sufficient to elicit rebound spikes when the thalamic neurons were actively firing. However, with bursty or fast trains of nigral input low-threshold rebound spike bursts could be triggered for low levels of excitation. This may form the basis of pathological burst generation and transmission in parkinsonian conditions.

Two functional inhibitory circuits are comprised of a heterogeneous population of fast-spiking cortical interneurons

Cortical fast spiking (FS) interneurons possess autaptic, synaptic, and electrical synapses that serve to mediate a fast, coordinated response to their postsynaptic targets. While FS interneurons are known to participate in numerous and diverse actions, functional subgroupings within this multi-functional interneuron class remain to be identified. In the present study, we examined parvalbumin-positive FS interneurons in layer 4 of the primary somatosensory (barrel) cortex - a brain region well-known for specialized inhibitory function. Here we show that FS interneurons fall into two broad categories identified by the onset of the first action potential in a depolarizing train as: "delayed firing FS interneurons (FSD) and early onset firing FS interneurons (FSE). Subtle variations in action potential firing reveal six subtypes within these two categories: delayed non-accommodating (FSD-NAC), delayed stuttering (FSD-STUT), early onset stuttering (FSE-STUT), early onset-late spiking (FSE-LS), early onset early-spiking (FSE-ES), and early onset accommodating (FSE-AC). Using biophysical criteria previously employed to distinguish neuronal cell types, the FSD and FSE categories exhibit several shared biophysical and synaptic properties that coincide with the notion of specificity of inhibitory function within the cortical FS interneuron class.

Homeostatic responses fail to correct defective amygdala inhibitory circuit maturation in fragile X syndrome

Fragile X syndrome (FXS) is a debilitating neurodevelopmental disorder thought to arise from disrupted synaptic communication in several key brain regions, including the amygdala, a central processing center for information with emotional and social relevance. Recent studies reveal defects in both excitatory and inhibitory neurotransmission in mature amygdala circuits in Fmr1(-/y) mutants, the animal model of FXS. However, whether these defects are the result of altered synaptic development or simply faulty mature circuits remains unknown. Using a combination of electrophysiological and genetic approaches, we show the development of both presynaptic and postsynaptic components of inhibitory neurotransmission in the FXS amygdala is dynamically altered during critical stages of neural circuit formation. Surprisingly, we observe that there is a homeostatic correction of defective inhibition, which, despite transiently restoring inhibitory synaptic efficacy to levels at or beyond those of control, ultimately fails to be maintained. Using inhibitory interneuron-specific conditional knock-out and rescue mice, we further reveal that fragile X mental retardation protein function in amygdala inhibitory microcircuits can be segregated into distinct presynaptic and postsynaptic components. Collectively, these studies reveal a previously unrecognized complexity of disrupted neuronal development in FXS and therefore have direct implications for establishing novel temporal and region-specific targeted therapies to ameliorate core amygdala-based behavioral symptoms.

Endogenous positive allosteric modulation of GABA(A) receptors by diazepam binding inhibitor

Benzodiazepines (BZs) allosterically modulate γ-aminobutyric acid type-A receptors (GABAARs) to increase inhibitory synaptic strength. Diazepam binding inhibitor (DBI) protein is a BZ site ligand expressed endogenously in the brain, but functional evidence for BZ-mimicking positive modulatory actions has been elusive. We demonstrate an endogenous potentiation of GABAergic synaptic transmission and responses to GABA uncaging in the thalamic reticular nucleus (nRT) that is absent in both nm1054 mice, in which the Dbi gene is deleted, and mice in which BZ binding to α3 subunit-containing GABAARs is disrupted. Viral transduction of DBI into nRT is sufficient to rescue the endogenous potentiation of GABAergic transmission in nm1054 mice. Both mutations enhance thalamocortical spike-and-wave discharges characteristic of absence epilepsy. Together, these results indicate that DBI mediates endogenous nucleus-specific BZ-mimicking ("endozepine") roles to modulate nRT function and suppress thalamocortical oscillations. Enhanced DBI signaling might serve as a therapy for epilepsy and other neurological disorders.

Postnatal development of synaptic properties of the GABAergic projection from the inferior colliculus to the auditory thalamus

The development of auditory temporal processing is important for processing complex sounds as well as for acquiring reading and language skills. Neuronal properties and sound processing change dramatically in auditory cortex neurons after the onset of hearing. However, the development of the auditory thalamus or medial geniculate body (MGB) has not been well studied over this critical time window. Since synaptic inhibition has been shown to be crucial for auditory temporal processing, this study examined the development of a feedforward, GABAergic connection to the MGB from the inferior colliculus (IC), which is also the source of sensory glutamatergic inputs to the MGB. IC-MGB inhibition was studied using whole cell patch-clamp recordings from rat brain slices in current-clamp and voltage-clamp modes at three age groups: a prehearing group [postnatal day (P)7-P9], an immediate posthearing group (P15-P17), and a juvenile group (P22-P32) whose neuronal properties are largely mature. Membrane properties matured substantially across the ages studied. GABAA and GABAB inhibitory postsynaptic potentials were present at all ages and were similar in amplitude. Inhibitory postsynaptic potentials became faster to single shocks, showed less depression to train stimuli at 5 and 10 Hz, and were overall more efficacious in controlling excitability with age. Overall, IC-MGB inhibition becomes faster and more precise during a time period of rapid changes across the auditory system due to the codevelopment of membrane properties and synaptic properties.

Setting the time course of inhibitory synaptic currents by mixing multiple GABA(A) receptor α subunit isoforms

The kinetics of IPSCs influence many neuronal processes, such as the frequencies of oscillations and the duration of shunting inhibition. The subunit composition of recombinant GABA(A) receptors (GABA(A)Rs) strongly affects the deactivation kinetics of GABA-evoked currents. However, for GABAergic synapses, the relationship between subunit composition and IPSC decay is less clear. Here we addressed this by combining whole-cell recordings of miniature IPSCs (mIPSCs) and quantitative immunolocalization of synaptic GABA(A)R subunits. In cerebellar stellate, thalamic relay, and main olfactory bulb (MOB) deep short-axon cells of Wistar rats, the only synaptic α subunit was α1, and zolpidem-sensitive mIPSCs had weighted decay time constants (τ(w)) of 4-6 ms. Nucleus reticularis thalami neurons expressed only α3 as the synaptic α subunit and exhibited slow (τ(w) = 28 ms), zolpidem-insensitive mIPSCs. By contrast, MOB external tufted cells contained two α subunit types (α1 and α3) at their synapses. Quantitative analysis of multiple immunolabeled images revealed small within-cell, but large between-cell, variability in synaptic α1/α3 ratios. This corresponded to large cell-to-cell variability in the decay (τ(w) = 3-30 ms) and zolpidem sensitivity of mIPSCs. Currents evoked by rapid application of GABA to patches excised from HEK cells expressing different mixtures of α1 and α3 subunits displayed highly variable deactivation times that correlated with the α1/α3 cDNA ratio. Our results demonstrate that diversity in the decay of IPSCs can be generated by varying the expression of different GABA(A)R subunits that alone confer different decay kinetics, allowing the time course of inhibition to be tuned to individual cellular requirements.

Neonatal exposure to antiepileptic drugs disrupts striatal synaptic development

OBJECTIVE

Drug exposure during critical periods of brain development may adversely affect nervous system function, posing a challenge for treating infants. This is of particular concern for treating neonatal seizures, as early life exposure to drugs such as phenobarbital is associated with adverse neurological outcomes in patients and induction of neuronal apoptosis in animal models. The functional significance of the preclinical neurotoxicity has been questioned due to the absence of evidence for functional impairment associated with drug-induced developmental apoptosis.

METHODS

We used patch-clamp recordings to examine functional synaptic maturation in striatal medium spiny neurons from neonatal rats exposed to antiepileptic drugs with proapoptotic action (phenobarbital, phenytoin, lamotrigine) and without proapoptotic action (levetiracetam). Phenobarbital-exposed rats were also assessed for reversal learning at weaning.

RESULTS

Recordings from control animals revealed increased inhibitory and excitatory synaptic connectivity between postnatal day (P)10 and P18. This maturation was absent in rats exposed at P7 to a single dose of phenobarbital, phenytoin, or lamotrigine. Additionally, phenobarbital exposure impaired striatal-mediated behavior on P25. Neuroprotective pretreatment with melatonin, which prevents drug-induced neurodevelopmental apoptosis, prevented the drug-induced disruption in maturation. Levetiracetam was found not to disrupt synaptic development.

INTERPRETATION

Our results provide the first evidence that exposure to antiepileptic drugs during a sensitive postnatal period impairs physiological maturation of synapses in neurons that survive the initial drug insult. These findings suggest a mechanism by which early life exposure to antiepileptic drugs can impact cognitive and behavioral outcomes, underscoring the need to identify therapies that control seizures without compromising synaptic maturation.

Fragile X syndrome: the GABAergic system and circuit dysfunction

Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, sensory hypersensitivity, and high incidences of autism spectrum disorders and epilepsy. These phenotypes are suggestive of defects in neural circuit development and imbalances in excitatory glutamatergic and inhibitory GABAergic neurotransmission. While alterations in excitatory synapse function and plasticity are well-established in Fmr1 knockout (KO) mouse models of FXS, a number of recent electrophysiological and molecular studies now identify prominent defects in inhibitory GABAergic transmission in behaviorally relevant forebrain regions such as the amygdala, cortex, and hippocampus. In this review, we summarize evidence for GABAergic system dysfunction in FXS patients and Fmr1 KO mouse models alike. We then discuss some of the known developmental roles of GABAergic signaling, as well as the development and refinement of GABAergic synapses as a framework for understanding potential causes of mature circuit dysfunction. Finally, we highlight the GABAergic system as a relevant target for the treatment of FXS.

Impact of synaptic neurotransmitter concentration time course on the kinetics and pharmacological modulation of inhibitory synaptic currents

The time course of synaptic currents is a crucial determinant of rapid signaling between neurons. Traditionally, the mechanisms underlying the shape of synaptic signals are classified as pre- and post-synaptic. Over the last two decades, an extensive body of evidence indicated that synaptic signals are critically shaped by the neurotransmitter time course which encompasses several phenomena including pre- and post-synaptic ones. The agonist transient depends on neurotransmitter release mechanisms, diffusion within the synaptic cleft, spill-over to the extra-synaptic space, uptake, and binding to post-synaptic receptors. Most estimates indicate that the neurotransmitter transient is very brief, lasting between one hundred up to several hundreds of microseconds, implying that post-synaptic activation is characterized by a high degree of non-equilibrium. Moreover, pharmacological studies provide evidence that the kinetics of agonist transient plays a crucial role in setting the susceptibility of synaptic currents to modulation by a variety of compounds of physiological or clinical relevance. More recently, the role of the neurotransmitter time course has been emphasized by studies carried out on brain slice models that revealed a striking, cell-dependent variability of synaptic agonist waveforms ranging from rapid pulses to slow volume transmission. In the present paper we review the advances on studies addressing the impact of synaptic neurotransmitter transient on kinetics and pharmacological modulation of synaptic currents at inhibitory synapses.

Aberrant GABA(A) receptor-mediated inhibition in cortico-thalamic networks of succinic semialdehyde dehydrogenase deficient mice

Aberrant γ-aminobutyric acid type A (GABA_A) receptor-mediated inhibition in cortico-thalamic networks remains an attractive mechanism for typical absence seizure genesis. Using the whole-cell patch clamp technique we examined ‘phasic’ and ‘tonic’ GABA_A inhibition in thalamocortical neurons of somatosensory (ventrobasal, VB) thalamus, nucleus reticularis thalami (NRT) neurons, and layer 5/6 pyramidal neurons of the somatosensory (barrel) cortex of succinic semialdehyde dehydrogenase (SSADH) knock-out (SSADH^−/−) mice that replicate human SSADH deficiency and exhibit typical absence seizures. We found increased sIPSC frequency in both VB and NRT neurons and larger sIPSC amplitude in VB neurons of SSADH^−/− mice compared to wild-type animals, demonstrating an increase in total phasic inhibition in thalamus of SSADH^−/− mice. mIPSCs in both VB and NRT neurons were no different between genotypes, although there remained a trend toward more events in SSADH^−/− mice. In cortical layer 5/6 pyramidal neurons, sIPSCs were fewer but larger in SSADH^−/− mice, a feature retained by mIPSCs. Tonic currents were larger in both thalamocortical neurons and layer 5/6 pyramidal neurons from SSADH^−/− mice compared to WTs. These data show that enhanced, rather than compromised, GABA_A receptor-mediated inhibition occurs in cortico-thalamic networks of SSADH^−/− mice. In agreement with previous studies, GABA_A receptor-mediated inhibitory gain-of-function may be a common feature in models of typical absence seizures, and could be of pathological importance in patients with SSADH deficiency.

Differential short-term plasticity at convergent inhibitory synapses to the substantia nigra pars reticulata

Inhibitory projections from the striatum and globus pallidus converge onto GABAergic projection neurons of the substantia nigra pars reticulata (SNr). Based on existing structural and functional evidence, these pathways are likely to differentially regulate the firing of SNr neurons. We sought to investigate the functional differences in inhibitory striatonigral and pallidonigral traffic using whole-cell voltage clamp in brain slices with these pathways preserved. We found that striatonigral IPSCs exhibited a high degree of paired-pulse facilitation. We tracked this facilitation over development and found the facilitation as the animal aged, but stabilized by postnatal day 17 (P17), with a paired pulse ratio of 2. We also found that the recovery from facilitation accelerated over development, again, reaching a stable phenotype by P17. In contrast, pallidonigral synapses show paired-pulse depression, and this depression could be solely explained by presynaptic changes. The mean paired-pulse ratio of 0.67 did not change over development, but the recovery from depression slowed over development. Pallidonigral IPSCs were significantly faster than striatonigral IPSCs when measured at the soma. Finally, under current clamp, prolonged bursts of striatal IPSPs were able to consistently silence the pacemaker activity of nigral neurons, whereas pallidal inputs depressed, allowing nigral neurons to reinstate firing. These findings highlight the importance of differential dynamics of neurotransmitter release in regulating the circuit behavior of the basal ganglia.

Defective GABAergic neurotransmission and pharmacological rescue of neuronal hyperexcitability in the amygdala in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder characterized by variable cognitive impairment and behavioral disturbances such as exaggerated fear, anxiety and gaze avoidance. Consistent with this, findings from human brain imaging studies suggest dysfunction of the amygdala. Underlying alterations in amygdala synaptic function in the Fmr1 knock-out (KO) mouse model of FXS, however, remain largely unexplored. Utilizing a combination of approaches, we uncover profound alterations in inhibitory neurotransmission in the amygdala of Fmr1 KO mice. We demonstrate a dramatic reduction in the frequency and amplitude of phasic IPSCs, tonic inhibitory currents, as well as in the number of inhibitory synapses in Fmr1 KO mice. Furthermore, we observe significant alterations in GABA availability, both intracellularly and at the synaptic cleft. Together, these findings identify abnormalities in basal and action potential-dependent inhibitory neurotransmission. Additionally, we reveal a significant neuronal hyperexcitability in principal neurons of the amygdala in Fmr1 KO mice, which is strikingly rescued by pharmacological augmentation of tonic inhibitory tone using the GABA agonist gaboxadol (THIP). Thus, our study reveals relevant inhibitory synaptic abnormalities in the amygdala in the Fmr1 KO brain and supports the notion that pharmacological approaches targeting the GABAergic system may be a viable therapeutic approach toward correcting amygdala-based symptoms in FXS.

Early enriched environment promotes neonatal GABAergic neurotransmission and accelerates synapse maturation

Environmental stimulation is critical for brain development. Here, we report that natural stimulation through enriched environment (EE) rearing during the first 2 weeks of mouse postnatal development promotes GABAergic neurotransmission and accelerates maturation of GABAergic and glutamatergic synapses. Using whole-cell recordings from CA1 pyramidal neurons in acute hippocampal slices, we found that EE-reared mice exhibited higher amplitude of miniature GABAergic postsynaptic currents (mGPSCs) at 1 week of postnatal development, as well as accelerated transition of GABA action from excitation to inhibition, compared with mice reared under standard housing conditions. This enhanced GABAergic synaptic transmission persisted until the end of the second postnatal week, when GABA mostly acts as an inhibitory neurotransmitter. Consistent with these electrophysiological results, we observed elevated levels of GABA(A) receptors and the K(+)-Cl(-) cotransporter KCC2. Similarly, increased levels of excitatory synaptic components, including NMDA and AMPA receptors and the scaffolding protein PSD95, were detected in synaptosomal fractions from the forebrain/hippocampus of EE-reared mice during the first two postnatal weeks. Functional increase in glutamatergic synaptic transmission, as measured by increased amplitude of miniature and spontaneous EPSCs, was also detected during the second postnatal week. Together, these results demonstrate that early environmental stimulation through EE rearing enhances early postnatal GABAergic neurotransmission, which is known to play important trophic functions in many aspects of neural development.

Modulation and function of the autaptic connections of layer V fast spiking interneurons in the rat neocortex

Neocortical fast-spiking (FS) basket cells form dense autaptic connections that provide inhibitory GABAergic feedback after each action potential. It has been suggested that these autaptic connections are used because synaptic communication is sensitive to neuromodulation, unlike the voltage-sensitive potassium channels in FS cells. Here we show that layer V FS interneurons form autaptic connections that are largely perisomatic, and without perturbing intracellular Cl(-) homeostasis, that perisomatic GABAergic currents have a reversal potential of 78 +/- 4 mV. Using variance-mean analysis, we demonstrate that autaptic connections have a mean of 14 release sites (range 4-26) with a quantal amplitude of 101 +/- 16 pA and a probability of release of 0.64 (V(command) = 70 mV, [Ca(2+)](o) = 2 mM, [Mg(2+)](o) = 1 mM). We found that autaptic GABA release is sensitive to GABA(B) and muscarinic acetylcholine receptors, but not a range of other classical neuromodulators. Our results indicate that GABA transporters do not regulate FS interneuron autapses, yet autaptically released GABA does not act at GABA(B) or extrasynaptic GABA(A) receptors. This research confirms that the autaptic connections of FS cells are indeed susceptible to modulation, though only via specific GABAergic and cholinergic mechanisms.

MeCP2 is required for normal development of GABAergic circuits in the thalamus

Methyl-CpG binding protein 2 (MeCP2) is highly expressed in neurons in the vertebrate brain, and mutations of the gene encoding MeCP2 cause the neurodevelopmental disorder Rett syndrome. This study examines the role of MeCP2 in the development and function of thalamic GABAergic circuits. Whole cell recordings were carried out in excitatory neurons of the ventrobasal complex (VB) of the thalamus and in inhibitory neurons of the reticular thalamic nucleus (RTN) in acute brain slices from mice aged P6 through P23. At P14-P16, the number of quantal GABAergic events was decreased in VB neurons but increased in RTN neurons of Mecp2-null mice, without any change in the amplitude or kinetics of quantal events. There was no difference between mutant and wild-type mice in paired-pulse ratios of evoked GABAergic responses in the VB or the RTN. On the other hand, unitary responses evoked by minimal stimulation were decreased in the VB but increased in the RTN of mutants. Similar changes in the frequency of quantal events were observed at P21-P23 in both the VB and RTN. At P6, however, quantal GABAergic transmission was altered only in the VB not the RTN. Immunostaining of vesicular GABA transporter showed opposite changes in the number of GABAergic synaptic terminals in the VB and RTN of Mecp2-null mice at P18-P20. The loss of MeCP2 had no significant effect on intrinsic properties of RTN neurons recorded at P15-P17. Our findings suggest that MeCP2 differentially regulates the development of GABAergic synapses in excitatory and inhibitory neurons in the thalamus.

Early development of the thalamic inhibitory feedback loop in the primary somatosensory system of the newborn mice

Spontaneous neuronal activity plays an important role during the final development of the brain circuits and the formation of the primary sensory maps. In young rats, spindle bursts have been recorded in the primary somatosensory cortex. They are correlated with spontaneous muscle twitches and occur before active whisking. They bear similarities with the spindles recorded in adult brain that occur during early stages of sleep and rely on a thalamic feedback loop between the glutamatergic nucleus ventroposterior medialis (nVPM) and the GABAergic nucleus reticularis thalami (nRT). However, whether a functional nVPM-nRT loop exists in newborn rodents is unknown. We studied the reciprocal synaptic connections between nVPM and nRT in thalamic acute slices from mice from birth [postnatal day 0 (P0)] until P9. We first demonstrated that nVPM-to-nRT EPSCs could be distinguished from corticothalamic EPSCs by their inhibition by 5-HT attributable to the transient expression of functional presynaptic serotonin 1B receptors. The nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs were both detected the first day after birth; their amplitude near 2 nS was relatively stable until P5. At P6-P7, there was a rapid and simultaneous increase of both nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs that reached 8 and 9 nS, respectively. Our results show that the thalamic synapses implicated in spindle activity are functional shortly after birth, suggesting that they could already generate spindles during the first postnatal week. Our results also suggest an inhibitory action of 5-HT on the spindle bursts of the newborn mice.

Long-term sensory deprivation selectively rearranges functional inhibitory circuits in mouse barrel cortex

Long-term whisker removal alters the balance of excitation and inhibition in rodent barrel cortex, yet little is known about the contributions of individual cells and synapses in this process. We studied synaptic inhibition in four major types of neurons in live tangential slices that isolate layer 4 in the posteromedial barrel subfield. Voltage-clamp recordings of layer 4 neurons reveal that fast decay of synaptic inhibition requires alpha1-containing GABA(A) receptors. After 7 weeks of deprivation, we found that GABA(A)-receptor-mediated inhibitory postsynaptic currents (IPSCs) in the inhibitory low-threshold-spiking (LTS) cell recorded in deprived barrels exhibited faster decay kinetics and larger amplitudes in whisker-deprived barrels than those in nondeprived barrels in age-matched controls. This was not observed in other cell types. Additionally, IPSCs recorded in LTS cells from deprived barrels show a marked increase in zolpidem sensitivity. To determine if the faster IPSC decay in LTS cells from deprived barrels indicates an increase in alpha1 subunit functionality, we deprived alpha1(H101R) mutant mice with zolpidem-insensitive alpha1-containing GABA(A) receptors. In these mice and matched wild-type controls, IPSC decay kinetics in LTS cells were faster after whisker removal; however, the deprivation-induced sensitivity to zolpidem was reduced in alpha1(H101R) mice. These data illustrate a change of synaptic inhibition in LTS cells via an increase in alpha1-subunit-mediated function. Because alpha1 subunits are commonly associated with circuit-specific plasticity in sensory cortex, this switch in LTS cell synaptic inhibition may signal necessary circuit changes required for plastic adjustments in sensory-deprived cortex.

Isoflurane-sensitive presynaptic R-type calcium channels contribute to inhibitory synaptic transmission in the rat thalamus

Because inhibitory synaptic transmission is a major mechanism of general anesthesia, we examined the effects of isoflurane on properties of GABAergic inhibitory currents in the reticular thalamic nucleus (nRT) in brain slices. The evoked IPSCs (eIPSCs) and spontaneous miniature synaptic currents (mIPSCs) of visualized nRT cells in young and adult rats were recorded. Consistent with postsynaptic effects on GABA(A) receptors, isoflurane prolonged the decay-time constants of both eIPSCs and mIPCSs. Surprisingly, isoflurane completely inhibited the amplitude of eIPSCs at clinically relevant concentrations (IC(50) of 240+/-20 microm), increased the paired-pulse ratio, and decreased the frequency of mIPSCs, indicating that presynaptic mechanisms may also contribute to the effects of isoflurane on IPSCs. The overall effect of isoflurane on eIPSCs in nRT cells was a decrease of net charge-transfer across the postsynaptic membrane. The application of 100 microm nickel (Ni(2+)) and the more specific R-type Ca(2+) channel blocker SNX-482 (0.5 microm) decreased eIPSC amplitudes, increased the paired-pulse ratio, and attenuated isoflurane-induced inhibition of eIPSCs. In addition, isoflurane potently blocked currents in recombinant human Ca(V)2.3 (alpha1E) channels with an IC(50) of 206 +/- 22 mum. Importantly, in vivo electroencephalographic (EEG) recordings in adult Ca(V)2.3 knock-out mice demonstrated alterations in isoflurane-induced burst-suppression activity. Because the thalamus has a key function in processing sensory information, sleep, and cognition, modulation of its GABAergic tone by presynaptic R-type Ca(2+) channels may contribute to the clinical effects of general anesthesia.

The modulation of synaptic GABA(A) receptors in the thalamus by eszopiclone and zolpidem

Eszopiclone (Lunesta; Sepracor, Marlborough, MA) and zolpidem [N,N,6-trimethyl-2-(4-methylphenyl)-imidazo(1,2-a)pyridine-3-acetamide] are among the most commonly prescribed hypnotics in use in the United States. The thalamus plays a pivotal role in sleep regulation and rhythmicity. Two distinct subtypes of synaptic GABA(A) receptors (GABA(A)-Rs), alpha(1)beta(2)gamma(2) and alpha(3)beta(3)gamma(2), are expressed in thalamocortical relay neurons and in interneurons of the RTN (reticular thalamic nucleus), respectively. Thalamocortical neurons also express extrasynaptic GABA(A)-Rs composed of alpha(4)beta(2)delta subunits. In this study, we compared the effects of eszopiclone and zolpidem on miniature inhibitory postsynaptic currents (IPSCs), spontaneous IPSCs, and tonic inhibition in the mouse thalamus. Eszopiclone (0.1-1 microM) slowed the decay phase of IPSCs recorded from RTN neurons, whereas zolpidem was less effective and increased the decay time constant only at > or = 0.3 microM. IPSCs of RTN neurons were more sensitive to eszopiclone than zolpidem at all concentrations tested. On the other hand, IPSCs of relay neurons in the ventrobasal nucleus (VB) were more sensitive to zolpidem than eszopiclone. Zolpidem (0.1-1 microM) prolonged the decay of IPSCs from VB neurons, whereas eszopiclone increased the decay time constant only at > or = 0.3 microM. Neither of these two hypnotics affected tonic inhibition in relay neurons. Our results demonstrate that eszopiclone has greater efficacy at synaptic GABA(A)-Rs of RTN neurons than in relay neurons, whereas zolpidem exerts bigger effects on relay neurons than RTN neurons. This distinct pattern of activity on thalamic neurons may contribute to some of the observed differences in the clinical effects of these two hypnotics.

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.

Isoflurane modulates excitability in the mouse thalamus via GABA-dependent and GABA-independent mechanisms

GABAergic neurons in the reticular thalamic nucleus (RTN) synapse onto thalamocortical neurons in the ventrobasal (VB) thalamus, and this reticulo-thalamocortical pathway is considered an anatomic target for general anesthetic-induced unconsciousness. A mutant mouse was engineered to harbor two amino acid substitutions (S270H, L277A) in the GABA(A) receptor (GABA(A)-R) alpha1 subunit; this mutation abolished sensitivity to the volatile anesthetic isoflurane in recombinant GABA(A)-Rs, and reduced in vivo sensitivity to isoflurane in the loss-of-righting-reflex assay. We examined the effects of the double mutation on GABA(A)-R-mediated synaptic currents and isoflurane sensitivity by recording from thalamic neurons in brain slices. The double mutation accelerated the decay, and decreased the (1/2) width of, evoked inhibitory postsynaptic currents (eIPSCs) in VB neurons and attenuated isoflurane-induced prolongation of the eIPSC. The hypnotic zolpidem, a selective modulator of GABA(A)-Rs containing the alpha1 subunit, prolonged eIPSC duration regardless of genotype, indicating that mutant mice incorporate alpha1 subunit-containing GABA(A)-Rs into synapses. In RTN neurons, which lack the alpha1 subunit, eIPSC duration was longer than in VB, regardless of genotype. Isoflurane reduced the efficacy of GABAergic transmission from RTN to VB, independent of genotype, suggesting a presynaptic action in RTN neurons. Consistent with this observation, isoflurane inhibited both tonic action potential and rebound burst firing in the presence of GABA(A)-R blockade. The suppressed excitability in RTN neurons is likely mediated by isoflurane-enhanced Ba(2+)-sensitive, but 4-aminopyridine-insenstive, potassium conductances. We conclude that isoflurane enhances inhibition of thalamic neurons in VB via GABA(A)-R-dependent, but in RTN via GABA(A)-R-independent, mechanisms.

Taurine is a potent activator of extrasynaptic GABA(A) receptors in the thalamus

Taurine is one of the most abundant free amino acids in the brain. In a number of studies, taurine has been reported to activate glycine receptors (Gly-Rs) at moderate concentrations (> or = 100 microM), and to be a weak agonist at GABA(A) receptors (GABA(A)-Rs), which are usually activated at high concentrations (> or = 1 mM). In this study, we show that taurine reduced the excitability of thalamocortical relay neurons and activated both extrasynaptic GABA(A)-Rs and Gly-Rs in neurons in the mouse ventrobasal (VB) thalamus. Low concentrations of taurine (10-100 microM) decreased neuronal input resistance and firing frequency, and elicited a steady outward current under voltage clamp, but had no effects on fast inhibitory synaptic currents. Currents elicited by 50 microM taurine were abolished by gabazine, insensitive to midazolam, and partially blocked by 20 microM Zn2+, consistent with the pharmacological properties of extrasynaptic GABA(A)-Rs (alpha4beta2delta subtype) involved in tonic inhibition in the thalamus. Tonic inhibition was enhanced by an inhibitor of taurine transport, suggesting that taurine can act as an endogenous activator of these receptors. Taurine-evoked currents were absent in relay neurons from GABA(A)-R alpha4 subunit knock-out mice. The amplitude of the taurine current was larger in neurons from adult mice than juvenile mice. Taurine was a more potent agonist at recombinant alpha4beta2delta GABA(A)-Rs than at alpha1beta2gamma2 GABA(A)-Rs. We conclude that physiological concentrations of taurine can inhibit VB neurons via activation of extrasynaptic GABA(A)-Rs and that taurine may function as an endogenous regulator of excitability and network activity in the thalamus.

Developmental maturation of synaptic and extrasynaptic GABAA receptors in mouse thalamic ventrobasal neurones

Thalamic ventrobasal (VB) relay neurones express multiple GABA(A) receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA(A) receptors of developing VB neurones and investigate the role of the alpha(1) subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and alpha(1)(0/0) mice and mice engineered to express diazepam-insensitive receptors (alpha(1H101R), alpha(2H101R)). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the alpha(4) and delta subunit occurred independently of the alpha(1) subunit. Subsequently (> or = P15), synaptic alpha(2) subunit/gephyrin clusters of WT VB neurones were replaced by those containing the alpha(1) subunit. Surprisingly, in alpha(1)(0/0) VB neurones the frequency of mIPSCs decreased between P12 and P27, because the alpha(2) subunit also disappeared from these cells. The loss of synaptic GABA(A) receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA(A) receptors in VB follows a developmental programme independent of the alpha(1) subunit. Changes to synaptic GABA(A) receptor function and the increased expression of extrasynaptic GABA(A) receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

Development of thalamocortical response transformations in the rat whisker-barrel system

Extracellular single-unit recordings were used to characterize responses of thalamic barreloid and cortical barrel neurons to controlled whisker deflections in 2, 3-, and 4-wk-old and adult rats in vivo under fentanyl analgesia. Results indicate that response properties of thalamic and cortical neurons diverge during development. Responses to deflection onsets and offsets among thalamic neurons mature in parallel, whereas among cortical neurons responses to deflection offsets become disproportionately smaller with age. Thalamic neuron receptive fields become more multiwhisker, whereas those of cortical neurons become more single-whisker. Thalamic neurons develop a higher degree of angular selectivity, whereas that of cortical neurons remains constant. In the temporal domain, response latencies decrease both in thalamic and cortical neurons, but the maturation time-course differs between the two populations. Response latencies of thalamic cells decrease primarily between 2 and 3 wk of life, whereas response latencies of cortical neurons decrease in two distinct steps--the first between 2 and 3 wk of life and the second between the fourth postnatal week and adulthood. Although the first step likely reflects similar subcortical changes, the second phase likely corresponds to developmental myelination of thalamocortical fibers. Divergent development of thalamic and cortical response properties indicates that thalamocortical circuits in the whisker-to-barrel pathway undergo protracted maturation after 2 wk of life and provides a potential substrate for experience-dependent plasticity during this time.

GABA affinity shapes IPSCs in thalamic nuclei

Precise neural inhibition in thalamocortical circuits is required for the generation of sleep spindles and suppression of hypersynchrony associated with epileptiform activity. Accordingly, the time course of GABA(A) receptor-mediated IPSC events is an important parameter influencing the strength of inhibitory signaling. In the thalamus, two distinct types of IPSC kinetics are observed: thalamocortical relay neurons in the ventrobasal nucleus (VB) exhibit a fast decaying IPSC, whereas neurons in the adjacent reticular nucleus (RTN) display a long-lasting, slowly decaying IPSC. Here, we used patch-clamp electrophysiology and computational modeling to elucidate the basis for IPSC kinetic heterogeneity in the thalamus. Rapid application of GABA to excised membrane patches revealed that decay kinetics were attributable to intrinsic differences in GABA(A) receptor deactivation. Examination of desensitization and gating properties revealed these to be similar in VB and RTN, with the notable lack of fast and long-lasting desensitized states in both cell types. Computational simulations demonstrate that slow GABA binding and unbinding rates could reproduce the characteristic long-lasting IPSCs in RTN cells. These results indicate that within thalamic circuits, a powerful diversity of inhibitory function can result from simple differences in underlying GABA(A) receptor affinity.

GABAergic currents in RT and VB thalamic nuclei follow kinetic pattern of alpha3- and alpha1-subunit-containing GABAA receptors

Inhibitory postsynaptic currents (IPSCs) of the thalamic reticular (RT) nucleus are dramatically slower than in the neighboring ventrobasal (VB) neurons. It has been suggested that α3-subunit-containing receptors underlie slow IPSCs in RT neurons, while rapid synaptic currents in the VB nucleus are due to γ-aminobutyric acid A receptors (GABA_ARs), including the α1-subunit. In our recent study [Barberis et al. (2007) Eur. J. Neurosci., 25, 2726–2740] we have found that profound differences in kinetics of currents mediated by α3β2γ2 and α1β2γ3 receptors resulted from distinct binding and desensitization properties. However, a direct comparison between kinetics of neuronal GABA_ARs from RT and VB neurons and α3- and α1-subunit-containing receptors has not been made. For this purpose, current responses to ultrafast GABA applications were recorded from patches excised from neurons in VB and RT areas. Deactivation kinetics determined for RT and VB neurons closely resembled that in currents mediated by α3β2γ2 and α1β2γ2 receptors. In RT neurons, currents elicited by non-saturating [GABA] had a remarkably slow onset, a hallmark of α3-subunit-containing receptors. In VB and RT neurons, single-channel currents elicited by brief GABA pulses had similar characteristics to those of α1β2γ2 and α3β2γ2 receptors. However, in stationary conditions, similarity between single-channel currents in neurons and respective recombinant receptors was less apparent. We propose that the non-stationary kinetics of GABAergic currents in VB and RT nuclei mimic that of currents mediated by α1- and α3-subunit-containing receptors. The dissimilarity between stationary kinetics of neuronal and recombinant receptors probably reflects differences between GABA_ARs mediating phasic and tonic currents in these neurons.

Reticular nucleus-specific changes in alpha3 subunit protein at GABA synapses in genetically epilepsy-prone rats

Differential composition of GABA(A) receptor (GABA(A)R) subunits underlies the variability of fast inhibitory synaptic transmission; alteration of specific GABA(A)R subunits in localized brain regions may contribute to abnormal brain states such as absence epilepsy. We combined immunocytochemistry and high-resolution ImmunoGold electron microscopy to study cellular and subcellular localization of GABA(A)R alpha1, alpha3, and beta2/beta3 subunits in ventral posterior nucleus (VP) and reticular nucleus (RTN) of control rats and WAG/Rij rats, a genetic model of absence epilepsy. In control rats, alpha1 subunits were prominent at inhibitory synapses in VP and much less prominent in RTN; in contrast, the alpha3 subunit was highly evident at inhibitory synapses in RTN. beta2/beta3 subunits were evenly distributed at inhibitory synapses in both VP and RTN. ImmunoGold particles representing all subunits were concentrated at postsynaptic densities with no extrasynaptic localization. Calculated mean number of particles for alpha1 subunit per postsynaptic density in nonepileptic VP was 6.1 +/- 3.7, for alpha3 subunit in RTN it was 6.6 +/- 3.4, and for beta2/beta3 subunits in VP and RTN the mean numbers were 3.7 +/- 1.3 and 3.5 +/- 1.2, respectively. In WAG/Rij rats, there was a specific loss of alpha3 subunit immunoreactivity at inhibitory synapses in RTN, without reduction in alpha3 subunit mRNA or significant change in immunostaining for other markers of RTN cell identity such as GABA or parvalbumin. alpha3 immunostaining in cortex was unchanged. Subtle, localized changes in GABA(A)R expression acting at highly specific points in the interconnected thalamocortical network lie at the heart of idiopathic generalized epilepsy.

Diversity of GABA(A) receptor synaptic currents on individual pyramidal cortical neurons

Miniature GABA(A) receptor-mediated inhibitory postsynaptic currents (mIPSCs) in cortical pyramidal neurons have previously been categorized into two types: small amplitude mIPSCs with a mono-exponential deactivation (mono-mIPSCs) and relatively larger mIPSCs with bi-exponential deactivation (bi-mIPSCs). The aim of this study was to determine if the GABA(A) channels that underlie these mIPSCSs are molecularly distinct. We found, using non-stationary noise analysis, that the difference in their amplitude could be not accounted for by their single channel conductance (both were 40 pS). Next, using alpha subunit selective GABA(A) receptor modulators, we examined the identity of the alpha subunits that may be expressed in the synapses that give rise to these mIPSCs. Zolpidem (100 and 500 nM, alpha1 selective) affected the deactivation of a subset of the mono-mIPSCs, indicating that alpha1 subunits are not highly expressed in these synapses. However, zolpidem (100 nM) prolonged the deactivation of all bi-mIPSCs, indicating a high abundance of alpha1 subunits in these synapses. SB-205384 (alpha3 selective) had no effect on the mono-mIPSCs but the bi-mIPSCs were prolonged. Furosemide (alpha4 selective) reduced the amplitude of only the mono-mIPSCs. L655,708 (alpha5 selective) reduced the amplitude of both populations and shortened the duration of the mono-mIPSCs. Finally, we found that the neuroactive steroid pregesterone sulphate reduced the amplitude of both mIPSC types. These results provide pharmacological evidence that synapses on cortical pyramidal neurons are molecularly distinct. The purpose of these different types of synapses may be to provide different inhibitory timing patterns on these cells.

Nucleus-specific abnormalities of GABAergic synaptic transmission in a genetic model of absence seizures

Human and experimental studies indicate that molecular genetic changes in GABA(A) receptors may underlie the expression of spike-and-waves discharges (SWDs) occurring during absence seizures. However, the full spectrum of the genetic defects underlying these seizures has only been partially elucidated, the expression and functional profiles of putative abnormal protein(s) within the thalamocortical network are undefined, and the pathophysiological mechanism(s) by which these proteins would lead to absence paroxysms are poorly understood. Here we investigated GABA(A) inhibitory postsynaptic currents (IPSCs) in key thalamocortical areas, i.e., the somatosensory cortex, ventrobasal thalamus (VB) and nucleus reticularis thalami (NRT), in preseizure genetic absence epilepsy rats from Strasbourg (GAERS), a well-established genetic model of typical absence seizures that shows no additional neurological abnormalities, and compared their properties to age-matched non-epileptic controls (NECs). Miniature GABA(A) IPSCs of VB and cortical layers II/III neurons were similar in GAERS and NEC, whereas in GAERS NRT neurons they had 25% larger amplitude, 40% faster decay. In addition, baclofen was significantly less effective in decreasing the frequency of NRT mIPSCs in GAERS than in NEC, whereas no difference was observed for cortical and VB mIPSCS between the two strains. Paired-pulse depression was 45% smaller in GAERS NRT, but not in VB, and was insensitive to GABA(B) antagonists. These results point to subtle, nucleus-specific, GABA(A) receptor abnormalities underlying SWDs of typical absence seizures rather than a full block of these receptors across the whole thalamocortical network, and their occurrence prior to seizure onset suggests that they might be of epileptogenic significance.

Distinct electrical and chemical connectivity maps in the thalamic reticular nucleus: potential roles in synchronization and sensation

GABAergic neurons of the thalamic reticular nucleus (nRt) provide thalamocortical relay neurons with feedback inhibition that influences sensory processing and thalamocortical rhythm generation. Mutual interactions between reticular neurons coordinate oscillatory activities developed within the network during normal sleep and in absence epilepsy, but the chemical versus electrical nature of these connections and their functional influence remain controversial. Here, we investigated the incidence and spatial extent of intra-nRt connectivity in vitro in horizontal and coronal thalamic slices from rat. Laser scanning photostimulation activated presynaptic nRt cells during patch-clamp recordings of postsynaptic neurons. Photolysis of caged glutamate evoked GABAergic IPSCs and/or depolarizing events (spikelets, mediated via electrical coupling) in a large proportion of neurons, thus indicating connectivity with presynaptic cell(s). Synaptic inputs were organized along the major axis of the nucleus in the same orientation as, but commonly exceeding the extent of, dendritic arborization of the postsynaptic neuron. In the anteroposterior (horizontal) plane, chemical connectivity had higher incidence (60% of recorded neurons vs 40% in vertical plane) and longer spatial extent, whereas in the dorsoventral (vertical) plane, electrical coupling dominated (47% incidence vs 37% in horizontal plane) and was more widely distributed. These data demonstrate that both electrical and chemical synapses are prominent within nRt and suggest different roles for the two types of connections. We thus propose that, along the vertical plane, electrical connectivity will promote coordinated rhythmic activity of sleep and/or thalamocortical epilepsy, whereas along the horizontal plane, chemical connectivity will oppose widespread thalamocortical synchronization and modulate sensory throughput.

GABAA receptor-mediated tonic inhibition in thalamic neurons

Tonic GABAA receptor-mediated inhibition is typically generated by delta subunit-containing extrasynaptic receptors. Because the delta subunit is highly expressed in the thalamus, we tested whether thalamocortical (TC) neurons of the dorsal lateral geniculate nucleus (dLGN) and ventrobasal complex exhibit tonic inhibition. Focal application of gabazine (GBZ) (50 microM) revealed the presence of a 20 pA tonic current in 75 and 63% of TC neurons from both nuclei, respectively. No tonic current was observed in GABAergic neurons of the nucleus reticularis thalami (NRT). Bath application of 1 microM GABA increased tonic current amplitude to approximately 70 pA in 100% of TC neurons, but it was still not observed in NRT neurons. In dLGN TC neurons, the tonic current was sensitive to low concentrations of the delta subunit-specific receptor agonists allotetrahydrodeoxycorticosterone (100 nM) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (100 nM) but insensitive to the benzodiazepine flurazepam (5 microM). Bath application of low concentrations of GBZ (25-200 nM) preferentially blocked the tonic current, whereas phasic synaptic inhibition was primarily maintained. Under intracellular current-clamp conditions, the preferential block of the tonic current with GBZ led to a small depolarization and increase in input resistance. Using extracellular single-unit recordings, block of the tonic current caused the cessation of low-threshold burst firing and promoted tonic firing. Enhancement of the tonic current by THIP hyperpolarized TC neurons and promoted burst firing. Thus, tonic current in TC neurons generates an inhibitory tone. Its modulation contributes to the shift between different firing modes, promotes the transition between different behavioral states, and predisposes to absence seizures.

Extrasynaptic GABAA receptors of thalamocortical neurons: a molecular target for hypnotics

Among hypnotic agents that enhance GABAA receptor function, etomidate is unusual because it is selective for beta2/beta3 compared with beta1 subunit-containing GABAA receptors. Mice incorporating an etomidate-insensitive beta2 subunit (beta(2N265S)) revealed that beta2 subunit-containing receptors mediate the enhancement of slow-wave activity (SWA) by etomidate, are required for the sedative, and contribute to the hypnotic actions of this anesthetic. Although the anatomical location of the beta2-containing receptors that mediate these actions is unknown, the thalamus is implicated. We have characterized GABAA receptor-mediated neurotransmission in thalamic nucleus reticularis (nRT) and ventrobasalis complex (VB) neurons of wild-type, beta2(-/-), and beta(2N265S) mice. VB but not nRT neurons exhibit a large GABA-mediated tonic conductance that contributes approximately 80% of the total GABAA receptor-mediated transmission. Consequently, although etomidate enhances inhibition in both neuronal types, the effect of this anesthetic on the tonic conductance of VB neurons is dominant. The GABA-enhancing actions of etomidate in VB but not nRT neurons are greatly suppressed by the beta(2N265S) mutation. The hypnotic THIP (Gaboxadol) induces SWA and at low, clinically relevant concentrations (30 nM to 3 microM) increases the tonic conductance of VB neurons, with no effect on VB or nRT miniature IPSCs (mIPSCs) or on the holding current of nRT neurons. Zolpidem, which has no effect on SWA, prolongs VB mIPSCs but is ineffective on the phasic and tonic conductance of nRT and VB neurons, respectively. Collectively, these findings suggest that enhancement of extrasynaptic inhibition in the thalamus may contribute to the distinct sleep EEG profiles of etomidate and THIP compared with zolpidem.

Fast IPSCs in rat thalamic reticular nucleus require the GABAA receptor beta1 subunit

Synchrony within the thalamocortical system is regulated in part by intranuclear synaptic inhibition within the reticular nucleus (RTN). Inhibitory postsynaptic currents (IPSCs) in RTN neurons are largely characterized by slow decay kinetics that result in powerful and prolonged suppression of spikes. Here we show that some individual RTN neurons are characterized by highly variable mixtures of fast, slow and mixed IPSCs. Heterogeneity arose largely through differences in the contribution of an initial decay component (tau(D) approximately 10 ms) which was insensitive to loreclezole, suggesting involvement of the GABA(A) receptor beta(1) subunit. Single-cell RT-PCR revealed the presence of beta(1) subunit mRNA only in those neurons whose IPSCs were dominated by a rapid and prominent initial decay phase. These data show that brief, beta(1)-dependent, loreclezole-insensitive IPSCs are present in a subpopulation of RTN neurons, and suggest that striking differences in IPSC heterogeneity within single neurons can result from of the presence or absence of a single GABA(A) receptor subunit.

An Extrasynaptic GABAA Receptor Mediates Tonic Inhibition in Thalamic VB Neurons

Whole cell patch-clamp recordings were obtained from thalamic ventrobasal (VB) and reticular (RTN) neurons in mouse brain slices. A bicuculline-sensitive tonic current was observed in VB, but not in RTN, neurons; this current was increased by the GABAA receptor agonist 4,5,6,7-tetrahydroisothiazolo-[5,4-c]pyridine-3-ol (THIP; 0.1 μM) and decreased by Zn2+ (50 μM) but was unaffected by zolpidem (0.3 μM) or midazolam (0.2 μM). The pharmacological profile of the tonic current is consistent with its generation by activation of GABAA receptors that do not contain the α1 or γ2 subunits. GABAA receptors expressed in HEK 293 cells that contained α4β2δ subunits showed higher sensitivity to THIP (gaboxadol) and GABA than did receptors made up from α1β2δ, α4β2γ2s, or α1β2γ2s subunits. Western blot analysis revealed that there is little, if any, α3 or α5 subunit protein in VB. In addition, co-immunoprecipitation studies showed that antibodies to the δ subunit could precipitate α4, but not α1 subunit protein. Confocal microscopy of thalamic neurons grown in culture confirmed that α4 and δ subunits are extensively co-localized with one another and are found predominantly, but not exclusively, at extrasynaptic sites. We conclude that thalamic VB neurons express extrasynaptic GABAA receptors that are highly sensitive to GABA and THIP and that these receptors are most likely made up of α4β2δ subunits. In view of the critical role of thalamic neurons in the generation of oscillatory activity associated with sleep, these receptors may represent a principal site of action for the novel hypnotic agent gaboxadol.

Neuroactive steroids and inhibitory neurotransmission: mechanisms of action and physiological relevance

Dysfunction of GABA(A) receptor-mediated inhibition is implicated in a number of neurological and psychiatric conditions including epilepsy and affective disorders. Some of these conditions have been associated with abnormal levels of certain endogenously occurring neurosteroids, which potently and selectively enhance the function of the brain's major inhibitory receptor, the GABA(A) receptor. Consistent with their ability to enhance neuronal inhibition, such steroids exhibit in animals and humans anxiolytic, anticonvulsant and anesthetic actions. Neurosteroids, exemplified by the potent progesterone metabolite, 5alpha-pregnan-3alpha-ol-20-one can be synthesized de novo in the CNS both in neurones and glia in levels sufficient to modulate GABA(A) receptor function. Neurosteroid levels are not static, but are subject to dynamic fluctuations, for example during stress, or the later stages of pregnancy. These observations suggest that these endogenous modulators may refine the function of the brain's major inhibitory receptor and thus, play an important physiological and pathophysiological role. However, given the ubiquitous expression of GABA(A) receptors throughout the mammalian CNS, changes in neurosteroid levels should be widely experienced, causing a generalized enhancement of neuronal inhibition. Such a non-specific action would seem incompatible with a physiological role. However, neurosteroid action is both brain region and neurone selective. This specificity results from a variety of molecular mechanisms including receptor subunit composition, local steroid metabolism and phosphorylation. This paper will evaluate the relative contribution these mechanisms play in defining the interaction of neurosteroids with synaptic and extra-synaptic GABA(A) receptors.

Propofol suppresses synaptic responsiveness of somatosensory relay neurons to excitatory input by potentiating GABA(A) receptor chloride channels

Propofol is a widely used intravenous general anesthetic. Propofol-induced unconsciousness in humans is associated with inhibition of thalamic activity evoked by somatosensory stimuli. However, the cellular mechanisms underlying the effects of propofol in thalamic circuits are largely unknown. We investigated the influence of propofol on synaptic responsiveness of thalamocortical relay neurons in the ventrobasal complex (VB) to excitatory input in mouse brain slices, using both current- and voltage-clamp recording techniques. Excitatory responses including EPSP temporal summation and action potential firing were evoked in VB neurons by electrical stimulation of corticothalamic fibers or pharmacological activation of glutamate receptors. Propofol (0.6 - 3 microM) suppressed temporal summation and spike firing in a concentration-dependent manner. The thalamocortical suppression was accompanied by a marked decrease in both EPSP amplitude and input resistance, indicating that a shunting mechanism was involved. The propofol-mediated thalamocortical suppression could be blocked by a GABAA receptor antagonist or chloride channel blocker, suggesting that postsynaptic GABAA receptors in VB neurons were involved in the shunting inhibition. GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked in VB neurons by electrical stimulation of the reticular thalamic nucleus. Propofol markedly increased amplitude, decay time, and charge transfer of GABAA IPSCs. The results demonstrated that shunting inhibition of thalamic somatosensory relay neurons by propofol at clinically relevant concentrations is primarily mediated through the potentiation of the GABAA receptor chloride channel-mediated conductance, and such inhibition may contribute to the impaired thalamic responses to sensory stimuli seen during propofol-induced anesthesia.

Single-column thalamocortical network model exhibiting gamma oscillations, sleep spindles, and epileptogenic bursts

To better understand population phenomena in thalamocortical neuronal ensembles, we have constructed a preliminary network model with 3,560 multicompartment neurons (containing soma, branching dendrites, and a portion of axon). Types of neurons included superficial pyramids (with regular spiking [RS] and fast rhythmic bursting [FRB] firing behaviors); RS spiny stellates; fast spiking (FS) interneurons, with basket-type and axoaxonic types of connectivity, and located in superficial and deep cortical layers; low threshold spiking (LTS) interneurons, which contacted principal cell dendrites; deep pyramids, which could have RS or intrinsic bursting (IB) firing behaviors, and endowed either with nontufted apical dendrites or with long tufted apical dendrites; thalamocortical relay (TCR) cells; and nucleus reticularis (nRT) cells. To the extent possible, both electrophysiology and synaptic connectivity were based on published data, although many arbitrary choices were necessary. In addition to synaptic connectivity (by AMPA/kainate, NMDA, and GABA(A) receptors), we also included electrical coupling between dendrites of interneurons, nRT cells, and TCR cells, and--in various combinations--electrical coupling between the proximal axons of certain cortical principal neurons. Our network model replicates several observed population phenomena, including 1) persistent gamma oscillations; 2) thalamocortical sleep spindles; 3) series of synchronized population bursts, resembling electrographic seizures; 4) isolated double population bursts with superimposed very fast oscillations (>100 Hz, "VFO"); 5) spike-wave, polyspike-wave, and fast runs (about 10 Hz). We show that epileptiform bursts, including double and multiple bursts, containing VFO occur in rat auditory cortex in vitro, in the presence of kainate, when both GABA(A) and GABA(B) receptors are blocked. Electrical coupling between axons appears necessary (as reported previously) for persistent gamma and additionally plays a role in the detailed shaping of epileptogenic events. The degree of recurrent synaptic excitation between spiny stellate cells, and their tendency to fire throughout multiple bursts, also appears critical in shaping epileptogenic events.

Selective GABA-receptor actions of amobarbital on thalamic neurons

1. We studied amobarbital's effects on membrane properties and currents, and electrically evoked inhibitory postsynaptic currents (IPSCs) mediated by gamma-aminobutyric acid (GABA) in rat thalamic slices. Using concentration-response relationships, we compared amobarbital's effects in nociceptive nuclei and non-nociceptive nucleus reticularis thalami (nRT). 2. Amobarbital decreased input resistance by activating GABA(A) receptors. Amobarbital produced a larger decrease in ventrobasal than nRT neurons. 3. Amobarbital depressed burst and tonic firing. Depression of burst firing was more effective, particularly in ventrobasal and intralaminar neurons. Depression was reversed by GABA(A) antagonists, and surmountable by increasing current injection, implicating a receptor-mediated shunt mechanism. 4. Amobarbital did not affect the tetrodotoxin-isolated low threshold Ca(2+) spike during GABA(A) blockade. Amobarbital reduced excitability without altering outward leak, or hyperpolarisation-activated inward currents. 5. Amobarbital increased mean conductance and burst duration of single GABA(A) channels. Consistent with this, amobarbital increased amplitude and decay time of IPSCs with distinct EC(50)s, implicating actions at two GABA(A) receptor sites. 6. Activation of GABA(A) receptors by low concentrations, fast IPSC amplitude modulation, and failure to affect intrinsic currents distinguished amobarbital's mechanism of action from previously characterised barbiturates. The selective actions of amobarbital on GABA(A) receptor may have relevance in explaining anaesthetic and analgesic uses.

Major differences in inhibitory synaptic transmission onto two neocortical interneuron subclasses

Locally projecting GABAergic interneurons are the major providers of inhibition in the neocortex and play a crucial role in several brain functions. Neocortical interneurons are connected via electrical and chemical synapses that may be crucial in modulating complex network oscillations. We investigated the properties of spontaneous and evoked IPSCs in two morphologically and physiologically identified interneuron subtypes, the fast-spiking (FS) and low threshold-spiking (LTS) cells in layer V of rodent sensorimotor cortex. We found that IPSCs recorded in FS cells were several orders of magnitude more frequent, larger in amplitude, and had faster kinetics than IPSCs recorded in LTS cells. GABA(A) receptor alpha- and beta-subunit selective modulators, zolpidem and loreclezole, had different effects on IPSCs in FS and LTS interneurons, suggesting differential expression of GABA(A) receptor subunit subtypes. These pharmacological data indicated that the alpha1 subunit subtype is poorly expressed by LTS cells but makes a large contribution to GABA(A) receptors on FS cells. This was confirmed by experiments performed in genetically modified mice in which the alpha1 subunit had been made insensitive to benzodiazepine-like agonists. These results suggest that differences in IPSC waveform are likely attributable to distinctive expression of GABA(A) receptor subunits in FS and LTS cells. The particular properties of GABAergic input on different interneuronal subtypes might have important consequences for generation and pacing of cortical rhythms underlying several brain functions. Moreover, selective pharmacological manipulation of distinct inhibitory circuits might allow regulation of pyramidal cell activities under specific physiological and pathophysiological conditions.

Corticothalamic inhibition in the thalamic reticular nucleus

Mutual inhibition between the GABAergic cells of the thalamic reticular nucleus (RTN) is important in regulating oscillations in the thalamocortical network, promoting those in the spindle range of frequencies over those at lower frequencies. Excitatory inputs to the RTN from the cerebral cortex are numerically large and particularly powerful in inducing spindles. However, the extent to which corticothalamic influences can engage the inhibitory network of the RTN has not been fully explored. Focal electrical stimulation of layer VI in the barrel cortex of the mouse thalamocortical slice in vitro resulted in prominent di- or polysynaptic inhibitory postsynaptic currents (IPSCs) in RTN cells under the experimental conditions used. The majority of cortically induced responses consisted of mixed PSCs in which the inhibitory component predominated or of large IPSCs alone, implying inhibition of neighboring cells by other, cortically excited RTN cells. Within the mixed PSCs, fixed and variable latency components could commonly be identified. IPSCs could be blocked by application of ionotropic glutamate receptor antagonists or of GABA(A) receptor antagonists, also indicating their dependence on corticothalamic excitation triggering disynaptic or polysynaptic inhibition. Spontaneous GABA(A) receptor-dependent IPSCs were routinely observed in the RTN and, taken together with the results of cortical stimulation, indicate the existence of a substantial network of intrareticular inhibitory connections that can be effectively recruited by the corticothalamic system. These results suggest activation of cortical excitatory inputs triggers the propagation of inhibitory currents within the RTN and support the view that activation of the RTN from the somatosensory cortex, although focused by the topography of the corticothalamic projection, is capable of disynaptically engaging the whole inhibitory network of the RTN, by local and probably by reentrant GABA(A) receptor-based synapses, thus spreading the corticothalamic influence throughout the RTN.

GABA receptors containing Rdl subunits mediate fast inhibitory synaptic transmission in Drosophila neurons

GABAergic inhibition in Drosophila, as in other insects and mammals, is important for regulation of activity in the CNS. However, the functional properties of synaptic GABA receptors in Drosophila have not been described. Here, we report that spontaneous GABAergic postsynaptic currents (sPSCs) in cultured embryonic Drosophila neurons are mediated by picrotoxin-sensitive chloride-conducting receptors. A rapid increase in spontaneous firing in response to bath application of picrotoxin demonstrates that these GABA receptors mediate inhibition in the neuronal networks formed in culture. Many of the spontaneous GABAergic synaptic currents are sodium action potential independent [miniature IPSCs (mIPSCs)] but are regulated by external calcium levels. The large variation in mIPSC frequency, amplitude, and kinetics properties between neurons suggests heterogeneity in GABA receptor number, location, and/or subtype. A decrease in the mean mIPSC decay time constant between 2 and 5 d, in the absence of a correlated change in rise time, demonstrates that the functional properties of the synaptic GABA receptors are regulated during maturation in vitro. Finally, neurons from the GABA receptor subunit mutant Rdl exhibit reduced sensitivity to picrotoxin blockade of the mIPSCs and resistance to picrotoxin-induced increases in spontaneous firing frequency. This demonstrates that Rdl containing GABA receptors play a direct role in mediating synaptic inhibition in Drosophila neural circuits formed in culture.

Divergent GABA(A) receptor-mediated synaptic transmission in genetically seizure-prone and seizure-resistant rats

Recent evidence suggests that abnormal expression of GABA(A) receptors may underlie epileptogenesis. We observed previously that rats selectively bred to be seizure-prone naturally overexpressed, as adults, GABA alpha subunits (alpha2, alpha3, and alpha5) seen at birth, whereas those selected to be seizure-resistant overexpressed the adult, alpha1 subunit. In this experiment, we gathered GABA miniature IPSCs (mIPSCs) from these strains and correlated their attributes with the subunit expression profile of each strain compared with a normal control strain. The mIPSCs were collected from both cortical pyramidal and nonpyramidal neurons. In seizure-prone rats, mIPSCs were smaller and decayed more slowly than in normal rats, which in turn were smaller and slower than in seizure-resistant rats. A detailed analysis of individual mIPSCs revealed two kinds of postsynaptic responses (those with monoexponential vs biexponential decay) that were differentially altered in the three strains. The properties of monoexponentially decaying mIPSCs did not differ between pyramidal and nonpyramidal neurons within a strain but differed between strains. In contrast, an interaction was observed between cell morphology and strain for biexponentially decaying mIPSCs. Here, the mIPSCs of pyramidal neurons in the seizure-resistant rats formed a distinct subpopulation compared with the seizure-prone rats; yet in the latter rats, it was the mIPSCs of the nonpyramidal neurons that were unique. Given these differences, we were surprised to find that the total inhibitory charge transfer between the strains was similar. This suggests that the timing of inhibition, particularly slow inhibitory neurotransmission between nonpyramidal neurons, may be a contributing factor in seizure genesis.