Parvalbumin-containing interneurons in the basolateral amygdala express high levels of the alpha1 subunit of the GABAA receptor

Neurosteroids: mechanistic considerations and clinical prospects

Like other classes of treatments described in this issue's section, neuroactive steroids have been studied for decades but have risen as a new class of rapid-acting, durable antidepressants with a distinct mechanism of action from previous antidepressant treatments and from other compounds covered in this issue. Neuroactive steroids are natural derivatives of progesterone but are proving effective as exogenous treatments. The best understood mechanism is that of positive allosteric modulation of GABAA receptors, where subunit selectivity may promote their profile of action. Mechanistically, there is some reason to think that neuroactive steroids may separate themselves from liabilities of other GABA modulators, although research is ongoing. It is also possible that intracellular targets, including inflammatory pathways, may be relevant to beneficial actions. Strengths and opportunities for further development include exploiting non-GABAergic targets, structural analogs, enzymatic production of natural steroids, precursor loading, and novel formulations. The molecular mechanisms of behavioral effects are not fully understood, but study of brain network states involved in emotional processing demonstrate a robust influence on affective states not evident with at least some other GABAergic drugs including benzodiazepines. Ongoing studies with neuroactive steroids will further elucidate the brain and behavioral effects of these compounds as well as likely underpinnings of disease.

Allopregnanolone Effects on Inhibition in Hippocampal Parvalbumin Interneurons

Allopregnanolone (AlloP) is a neurosteroid that potentiates ionotropic GABAergic (GABA_A) inhibition and is approved for treating postpartum depression in women. Although the antidepressant mechanism of AlloP is largely unknown, it could involve selective action at GABA_A receptors containing the δ subunit. Despite previous evidence for selective effects of AlloP on α4/δ-containing receptors of hippocampal dentate granule cells (DGCs), other recent results failed to demonstrate selectivity at these receptors (Lu et al., 2020). In contrast to DGCs, hippocampal fast-spiking parvalbumin (PV) interneurons express an unusual variant partnership of δ subunits with α1 subunits. Here, we hypothesized that native α1/δ receptors in hippocampal fast-spiking interneurons may provide a preferred substrate for AlloP. Contrary to the hypothesis, electrophysiology from genetically tagged PV interneurons in hippocampal slices from male mice showed that 100 nm AlloP promoted phasic inhibition by increasing the sIPSC decay, but tonic inhibition was not detectably altered. Co-application of AlloP with 5 μm GABA did augment tonic current, which was not primarily through δ-containing receptors. Furthermore, AlloP decreased the membrane resistance and the number of action potentials of DGCs, but the impact on PV interneurons was weaker than on DGCs. Thus, our results indicate that hippocampal PV interneurons possess low sensitivity to AlloP and suggest they are unlikely contributors to mood-altering effects of neurosteroids through GABA effects.

Expression of Calbindin, a Marker of Gamma-Aminobutyric Acid Neurons, Is Reduced in the Amygdala of Oestrogen Receptor β-Deficient Female Mice

Oestrogen receptor β (ERβ) knock-out female mice display increased anxiety and decreased threshold for synaptic plasticity induction in the basolateral amygdala. This may suggest that the γ-aminobutyric acid (GABA) inhibitory system is altered. Therefore, the immunoreactivity of main GABAergic markers-i.e., calbindin, parvalbumin, calretinin, somatostatin, α1 subunit-containing GABA_A receptor and vesicular GABA transporter-were compared in the six subregions (LA, BL, BM, ME, CE and CO) of the amygdala of adult female wild-type and ERβ knock-out mice using immunohistochemistry and quantitative methods. The influence of ERβ knock-out on neuronal loss and glia was also elucidated using pan-neuronal and astrocyte markers. The results show severe neuronal deficits in all main amygdala regions in ERβ knock-out mice accompanied by astroglia overexpression only in the medial, basomedial and cortical nuclei and a decrease in calbindin-expressing neurons (CB+) in the amygdala in ERβ knock-out mice compared with controls, while other markers of the GABAergic system remain unchanged. Concluding, the lack of ERβ led to failure in the structural integrity of the CB+ subpopulation, reducing interneuron firing and resulting in a disinhibitory effect over pyramidal function. This fear-promoting excitatory/inhibitory alteration may lead to the increased anxiety observed in these mice. The impact of neuronal deficits and astroglia overexpression on the amygdala functions remains unknown.

Effects of GABAA receptors in nucleus cuneiformis on the cannabinoid antinociception using the formalin test

RATIONALE

Nucleus cuneiformis (NC), a reticular nucleus of the midbrain, is a part of the descending pain modulatory system and therefore has an important role in pain perception.

OBJECTIVES

Considering the abundance of GABA_A and cannabinoid receptors in the NC and also the bidirectional roles for GABA in controlling nociception, the present study examined the effects of bilateral intra-NC microinjection of different doses of the GABA_A receptor agonist, muscimol, and the GABA_A receptor antagonist, bicuculline, on pain modulation using formalin test. We also assessed interaction between canabinergic and GABAergic systems in the NC during this test.

METHODS

Rats were exposed to intra-NC microinjection of bicuculline (50,100, and 200 ng/side) or muscimol (60, 120, and 240 ng/side) and then subjected to the formalin test. In another set of experiments, the effects of muscimol (60 ng/side) or bicuculline (50 ng/side) administration 5 min before a cannabinoid receptor agonist WIN 55,212-2 (5, 10, and 20 μg/side) microinjection into NC on the formalin test were evaluated.

RESULTS

Microinjection of bicuculline and muscimol into the NC decreased and increased pain responses, respectively, in a dose-dependent manner during both phases of the test. Microinjection of WIN 55,212-2 into the NC significantly reduced pain responses in a dose-dependent manner. Microinjection of bicuculline or muscimol in combination with WIN 55,212-2 into the NC respectively potentiated and attenuated WIN 55,212-2-induced antinociception in the formalin test.

CONCLUSIONS

This study shows that GABA in the NC is involved in pain modulation and suggests the existence of a GABA_A-mediated inhibitory system in the NC on pain control. Furthermore, it seems that the antinociceptive effect of WIN 55,212-2 in the formalin test is mediated partly by the activity of local GABA_A receptors in the NC.

Interneuronal δ-GABAA receptors regulate binge drinking and are necessary for the behavioral effects of early withdrawal

Extensive evidence points to a role for GABAergic signaling in the amygdala in mediating the effects of alcohol, including presynaptic changes in GABA release, suggesting effects on GABAergic neurons. However, the majority of studies focus solely on the effects of alcohol on principal neurons. Here we demonstrate that δ-GABA_ARs, which have been suggested to confer ethanol sensitivity, are expressed at a high density on parvalbumin (PV) interneurons in the basolateral amygdala (BLA). Thus, we hypothesized that δ-GABA_ARs on PV interneurons may represent both an initial pharmacological target for alcohol and a site for plasticity associated with the expression of various behavioral maladaptations during withdrawal from binge drinking. To investigate this, we used a mouse model of voluntary alcohol intake (Drinking-in-the-Dark-Multiple Scheduled Access) to induce escalating heavy binge drinking and anxiety-like behavior in mice. This pattern of intake was associated with increased δ protein expression on parvalbumin positive interneurons in both the BLA and hippocampus. Loss of δ-GABA_ARs specifically in PV interneurons (PV:δ^-/-) increased binge drinking behavior, reduced sensitivity to alcohol-induced motor incoordination, enhanced sensitivity to alcohol-induced hyperlocomotion and blocked the expression of withdrawal from binge drinking. This study is the first to demonstrate a role for δGABA_ARs specifically in PV-expressing interneurons in modulating binge alcohol intake and withdrawal-induced anxiety.

Parvalbumin, but not calretinin, neurons express high levels of α1-containing GABAA receptors, α7-containing nicotinic acetylcholine receptors and D2-dopamine receptors in the basolateral amygdala of the rat

The generation of emotional responses by the basolateral amygdala is largely determined by the balance of excitatory and inhibitory inputs to its principal neurons - the pyramidal cells. The activity of these neurons is tightly controlled by g-aminobutyric acid (GABA)ergic interneurons, especially by those expressing parvalbumin (PV) and calretinin (CR). Although it is known that GABAergic, cholinergic and dopaminergic fibres make synapses on PV and CR cells, knowledge of the various receptors which are used by these cells is still incomplete. Thus, the present study investigates whether neurons expressing PV or CR co-express specific GABA, acetylcholine and/or dopamine receptors in the basolateral amygdala of the rat. The results show that almost two-thirds of PV neurons co-express high concentrations of α1 subunit of GABA_A receptor, and more than half of them co-express high levels of α7 subunit of nicotinic acetylcholine receptor and/or D2-subtype of dopamine receptor. In contrast, a smaller percentage of CR neurons had detectable amounts of these receptors and at lower levels of abundance in most cases. In conclusion, the present results indicate that not only principal neurons but also GABAergic interneurons have specific receptors, which allow these cells to respond to the GABAergic, cholinergic and dopaminergic inputs coming to the basolateral amygdala of the rat. Since these cells receive intrinsic GABAergic inputs, they are strongly interconnected. Since they also receive extrinsic cholinergic and dopaminergic inputs, such stimulation may result in stimulus-driven feed-forward control of the principal neurons. The effects of such control may be either feed-forward inhibition of the principal neurons via α7 nicotinic acetylcholine receptors or disinhibition of these cells via D2-dopamine receptors.

MPTP-Induced Dopamine Depletion in Basolateral Amygdala via Decrease of D2R Activation Suppresses GABAA Receptors Expression and LTD Induction Leading to Anxiety-Like Behaviors

Anxiety disorders commonly occur in Parkinson’s disease. Using field potential recording and patch-clamp recording, we evaluated influence of MPTP-reduced dopaminergic afferent in basolateral amygdala (BLA), a main region for affective regulation, on excitatory–inhibitory circuits and synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slopes at external capsule-BLA synapses were increased in MPTP-mice with decreases in paired-pulse facilitation and long-term potentiation amplitude, which were corrected by bath-application of D2R agonist quinpirole or cannabinoid type 1 receptors agonist WIN55,212-2, but not D1R agonist SKF38393. Compared to single waveform fEPSP in control mice, a multi-spike waveform fEPSP was observed in MPTP-mice with prolongation of duration and an increase in paired-pulse inhibition, which were recovered by BLA-injection of quinpirole for 2 days rather than bath-application. Density of GABA-evoked current (I_GABA) in BLA principal neurons and GABA_AR-α2 subunit expression were reduced in MPTP-mice, which were recovered by administration of quinpirole. Decline of PKC phosphorylation in BLA of MPTP-mice was corrected by bath-application of quinpirole, but not SKF38393. In MPTP-mice, BLA-injection of quinpirole or PKC activator PMA could recover GABA_AR expression, which was sensitive to PKC inhibitor GF109203X. The impairment of long-term depression (LTD) in MPTP-mice was rescued by bath-application of GABA_AR agonist muscimol or BLA-injection of quinpirole and PMA. Finally, BLA-injection of muscimol, quinpirole or PMA relieved anxiety-like behaviors in MPTP-mice. The results indicate that the MPTP-induced dopamine depletion in BLA principal neurons through reducing D2R-mediated PKC phosphorylation suppresses GABA_AR expression and activity, which impairs GABA_AR-mediated inhibition and LTD induction leading to anxiety-like behaviors.

The neurons expressing calcium-binding proteins in the amygdala of the guinea pig: precisely designed interface for sex hormones

The generation of emotional responses by the amygdala is determined largely by the balance of excitatory and inhibitory inputs to its principal neurons. These responses are often sex-specific, and any imbalance in excitatory and/or inhibitory tones leads to serious psychiatric disorders which occur with different rates in men versus women. To investigate the neural basis of sex-specific processing in the amygdala, relationships between the neurons expressing calbindin (CB), parvalbumin (PV) and calretinin (CR), which form in the amygdala main subsets of γ-aminobutyric acid (GABA)-ergic inhibitory system, and neurons endowed with oestrogen alpha (ERα), oestrogen beta (ERβ) or androgen (AR) receptors were analysed using double immunohistochemistry in male and female guinea pig subjects. The results show that in various nuclei of the amygdala in both sexes small subsets of CB neurons and substantial proportions of PV neurons co-express ERβ, while many of the CR neurons co-express ERα. Both these oestrogen-sensitive populations are strictly separated as CB and PV neurons almost never co-express ERα, while CR cells are usually devoid of ERβ. In addition, in the medial nucleus and some other neighbouring regions, there are non-overlapping subpopulations of CB and CR neurons which co-express AR. In conclusion, the localization of ERα, ERβ or AR within subsets of GABAergic interneurons across diverse amygdaloid regions suggests that steroid hormones may exert a significant influence over local neuronal activity by directly modulating inhibitory tone. The control of inhibitory tone may be one of the mechanisms whereby oestrogen and androgen could modulate amygdala processing in a sex-specific manner. Another mechanism may be thorough steroid-sensitive projection neurons, which are most probably located in the medial and central nuclei.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

Predator Stress-Induced CRF Release Causes Enduring Sensitization of Basolateral Amygdala Norepinephrine Systems that Promote PTSD-Like Startle Abnormalities

The neurobiology of post-traumatic stress disorder (PTSD) remains unclear. Intense stress promotes PTSD, which has been associated with exaggerated startle and deficient sensorimotor gating. Here, we examined the long-term sequelae of a rodent model of traumatic stress (repeated predator exposure) on amygdala systems that modulate startle and prepulse inhibition (PPI), an operational measure of sensorimotor gating. We show in rodents that repeated psychogenic stress (predator) induces long-lasting sensitization of basolateral amygdala (BLA) noradrenergic (NE) receptors (α1) via a corticotropin-releasing factor receptor 1 (CRF-R1)-dependent mechanism, and that these CRF1 and NE α1 receptors are highly colocalized on presumptive excitatory output projection neurons of the BLA. A profile identical to that seen with predator exposure was produced in nonstressed rats by intra-BLA infusions of CRF (200 ng/0.5 μl), but not by repeated NE infusions (20 μg/0.5 μl). Infusions into the adjacent central nucleus of amygdala had no effect. Importantly, the predator stress- or CRF-induced sensitization of BLA manifested as heightened startle and PPI deficits in response to subsequent subthreshold NE system challenges (with intra-BLA infusions of 0.3 μg/0.5 μl NE), up to 1 month after stress. This profile of effects closely resembles aspects of PTSD. Hence, we reveal a discrete neural pathway mediating the enhancement of NE system function seen in PTSD, and we offer a model for characterizing potential new treatments that may work by modulating this BLA circuitry.

SIGNIFICANCE STATEMENT

The present findings reveal a novel and discrete neural substrate that could underlie certain core deficits (startle and prepulse inhibition) that are observed in post-traumatic stress disorder (PTSD). It is shown here that repeated exposure to a rodent model of traumatic stress (predator exposure) produces a long-lasting sensitization of basolateral amygdala noradrenergic substrates [via a corticotropin-releasing factor (CRF)-dependent mechanism] that regulate startle, which is exaggerated in PTSD. Moreover, it is demonstrated that the sensitized noradrenergic receptors colocalize with CRF1 receptors on output projection neurons of the basolateral amygdala. Hence, this stress-induced sensitization of noradrenergic receptors on basolateral nucleus efferents has wide-ranging implications for the numerous deleterious sequelae of trauma exposure that are seen in multiple psychiatric illnesses, including PTSD.

Structural and functional characterization of dendritic arbors and GABAergic synaptic inputs on interneurons and principal cells in the rat basolateral amygdala

The basolateral amygdala (BLA) is a complex brain region associated with processing emotional states, such as fear, anxiety, and stress. Some aspects of these emotional states are driven by the network activity of synaptic connections, derived from both local circuitry and projections to the BLA from other regions. Although the synaptic physiology and general morphological characteristics are known for many individual cell types within the BLA, the combination of morphological, electrophysiological, and distribution of neurochemical GABAergic synapses in a three-dimensional neuronal arbor has not been reported for single neurons from this region. The aim of this study was to assess differences in morphological characteristics of BLA principal cells and interneurons, quantify the distribution of GABAergic neurochemical synapses within the entire neuronal arbor of each cell type, and determine whether GABAergic synaptic density correlates with electrophysiological recordings of inhibitory postsynaptic currents. We show that BLA principal neurons form complex dendritic arborizations, with proximal dendrites having fewer spines but higher densities of neurochemical GABAergic synapses compared with distal dendrites. Furthermore, we found that BLA interneurons exhibited reduced dendritic arbor lengths and spine densities but had significantly higher densities of putative GABAergic synapses compared with principal cells, which was correlated with an increased frequency of spontaneous inhibitory postsynaptic currents. The quantification of GABAergic connectivity, in combination with morphological and electrophysiological measurements of the BLA cell types, is the first step toward a greater understanding of how fear and stress lead to changes in morphology, local connectivity, and/or synaptic reorganization of the BLA.

The effect of Midazolam and Propranolol on fear memory reconsolidation in ethanol-withdrawn rats: influence of d-cycloserine

BACKGROUND

Withdrawal from chronic ethanol facilitates the formation of contextual fear memory and delays the onset to extinction, with its retrieval promoting an increase in ethanol consumption. Consequently, manipulations aimed to reduce these aversive memories, may be beneficial in the treatment of alcohol discontinuation symptoms. Related to this, pharmacological memory reconsolidation blockade has received greater attention due to its therapeutic potential.

METHODS

Here, we examined the effect of post-reactivation amnestic treatments such as Midazolam (MDZ, 3 mg/kg i.p) and Propranolol (PROP, 5 mg/kg i.p) on contextual fear memory reconsolidation in ethanol- withdrawn (ETOH) rats. Next, we examined whether the activation of N-methyl-D-aspartate (NMDA) receptors induced by d-cycloserine (DCS, 5 mg/kg i.p., a NMDA partial agonist) before memory reactivation can facilitate the disruptive effect of PROP and MDZ on fear memory in ETOH rats.

RESULTS

We observed a resistance to the disruptive effect of both MDZ and PROP following memory reactivation. Although intra-basolateral amygdala (BLA; 1.25 ug/side) and systemic PROP administration attenuated fear memory in DCS pre-treated ETOH rats, DCS/MDZ treatment did not affect memory in these animals. Finally, a decrease of both total and surface protein expression of the α1 GABAA receptor (GABAA-R) subunit in BLA was found in the ETOH rats.

CONCLUSIONS

Ethanol withdrawal facilitated the formation of fear memory resistant to labilization post-reactivation. DCS administration promoted the disruptive effect of PROP on memory reconsolidation in ETOH rats. The resistance to MDZ's disruptive effect on fear memory reconsolidation may be, at least in part, associated with changes in the GABAA-R composition induced by chronic ethanol administration/withdrawal.

In vivo and in vitro analyses of amygdalar function reveal a role for copper

Mice with a single copy of the peptide amidating monooxygenase (Pam) gene (PAM(+/-)) are impaired in contextual and cued fear conditioning. These abnormalities coincide with deficient long-term potentiation (LTP) at excitatory thalamic afferent synapses onto pyramidal neurons in the lateral amygdala. Slice recordings from PAM(+/-) mice identified an increase in GABAergic tone (Gaier ED, Rodriguiz RM, Ma XM, Sivaramakrishnan S, Bousquet-Moore D, Wetsel WC, Eipper BA, Mains RE. J Neurosci 30: 13656-13669, 2010). Biochemical data indicate a tissue-specific deficit in Cu content in the amygdala; amygdalar expression of Atox-1 and Atp7a, essential for transport of Cu into the secretory pathway, is reduced in PAM(+/-) mice. When PAM(+/-) mice were fed a diet supplemented with Cu, the impairments in fear conditioning were reversed, and LTP was normalized in amygdala slice recordings. A role for endogenous Cu in amygdalar LTP was established by the inhibitory effect of a brief incubation of wild-type slices with bathocuproine disulfonate, a highly selective, cell-impermeant Cu chelator. Interestingly, bath-applied CuSO₄ had no effect on excitatory currents but reversibly potentiated the disynaptic inhibitory current. Bath-applied CuSO₄ was sufficient to potentiate wild-type amygdala afferent synapses. The ability of dietary Cu to affect signaling in pathways that govern fear-based behaviors supports an essential physiological role for Cu in amygdalar function at both the synaptic and behavioral levels. This work is relevant to neurological and psychiatric disorders in which disturbed Cu homeostasis could contribute to altered synaptic transmission, including Wilson's, Menkes, Alzheimer's, and prion-related diseases.

Peptidylglycine α-amidating monooxygenase heterozygosity alters brain copper handling with region specificity

Copper (Cu), an essential trace element present throughout the mammalian nervous system, is crucial for normal synaptic function. Neuronal handling of Cu is poorly understood. We studied the localization and expression of Atp7a, the major intracellular Cu transporter in the brain, and its relation to peptidylglycine α-amidating monooxygenase (PAM), an essential cuproenzyme and regulator of Cu homeostasis in neuroendocrine cells. Based on biochemical fractionation and immunostaining of dissociated neurons, Atp7a was enriched in post-synaptic vesicular fractions. Cu followed a similar pattern, with ~ 20% of total Cu in synaptosomes. A mouse model heterozygous for the Pam gene (PAM+/−) was selectively Cu deficient in the amygdala. As in cortex and hippocampus, Atp7a and PAM expression overlap in the amygdala, with highest expression in interneurons. Messenger RNA levels of Atox-1 and Atp7a, which deliver Cu to the secretory pathway, were reduced in the amygdala but not in the hippocampus in PAM+/− mice, GABAB receptor mRNA levels were similarly affected. Consistent with Cu deficiency, dopamine β-monooxygenase function was impaired as evidenced by elevated dopamine metabolites in the amygdala, but not in the hippocampus, of PAM+/− mice. These alterations in Cu delivery to the secretory pathway in the PAM+/− amygdala may contribute to the physiological and behavioral deficits observed. Atp7a, a Cu-transporting P-type ATPase, is localized to the trans-Golgi network and to vesicles distributed throughout the dendritic arbor. Tissue-specific alterations in Atp7a expression were found in mice heterozygous for peptidylglycine α-amidating monooxygenase (PAM), an essential neuropeptide-synthesizing cuproenzyme. Atp7a and PAM are highly expressed in amygdalar interneurons. Reduced amygdalar expression of Atox-1 and Atp7a in PAM heterozygous mice may lead to reduced synaptic Cu levels, contributing to the behavioral and neurochemical alterations seen in these mice.

Gephyrin plays a key role in BDNF-dependent regulation of amygdala surface GABAARs

Brain-derived neurotrophic factor (BDNF) is critically involved in synaptic plasticity and neurotransmission. Our lab has previously found that BDNF activation of neurotrophic tyrosine kinase, receptor, type 2 (TrkB) is required for fear memory formation and that GABAA receptor (GABAAR) subunits and the GABAA clustering protein gephyrin are dynamically regulated during fear memory consolidation. We hypothesize that TrkB-dependent internalization of GABAARs may partially underlie a transient period of amygdala hyperactivation during fear memory consolidation. We have previously reported that BDNF modulates GABAAR α1 subunit sequestration in cultured hippocampal and amygdala neurons by differential phosphorylation pathways. At present, no studies have investigated the regulation of gephyrin and GABAAR α1 subunits following BDNF activation in the amygdala. In this study, we confirm the association of GABAAR α1 and γ2 subunits with gephyrin on mouse amygdala neurons by coimmunoprecipitation and immunocytochemistry. We then demonstrate that rapid BDNF treatment, as well as suppression of gephyrin protein levels on amygdala neurons, induced sequestration of surface α1 subunits. Further, we find that rapid exposure of BDNF to primary amygdala cultures produced decreases in gephyrin levels, whereas longer exposure resulted in an eventual increase. While total α1 subunit levels remained unchanged, gephyrin was downregulated in whole cell homogenates, but enhanced in complexes with GABAARs. Our data with anisomycin suggest that BDNF may rapidly induce gephyrin protein degradation, with subsequent gephyrin synthesis occurring. Together, these findings suggest that gephyrin may be a key factor in BDNF-dependent GABAAR regulation in the amygdala. This work may inform future studies aimed at elucidating the pathways connecting BDNF, GABAA systems, gephyrin, and their role in underlying amygdala-dependent learning.

The parvalbumin-positive interneurons in the mouse dentate gyrus express GABAA receptor subunits α1, β2, and δ along their extrasynaptic cell membrane

Neuronal circuitries in the hippocampus are involved in navigation and memory and are controlled by major networks of GABAergic interneurons. Parvalbumin (PV)-expressing interneurons in the dentate gyrus (DG) are identified as fast-spiking cells, playing a crucial role in network oscillation and synchrony. The inhibitory modulation of these interneurons is thought to be mediated mainly through GABAA receptors, the major inhibitory neurotransmitter receptors in the brain. Here we show that all PV-positive interneurons in the granular/subgranular layer (GL/SGL) of the mouse DG express high levels of the GABAA receptor δ subunit. PV-containing interneurons in the hilus and the molecular layer, however, express the δ subunit to a lower extent. Only 8% of the somatostatin-containing interneurons express the δ subunit, whereas calbindin- or calretinin-containing interneurons in the DG seem not to express the GABAA receptor δ subunit at all. Hence, these cells receive a GABAergic control different from that of PV-containing interneurons in the GL/SGL. Experiments investigating a possible co-expression of GABAA receptor α1, α2, α3, α4, α5, β1, β2, β3, or γ2 subunits with PV and δ subunits indicated that α1 and β2 subunits are co-expressed with δ subunits along the extrasynaptic membranes of PV-interneurons. These results suggest a robust tonic GABAergic control of PV-containing interneurons in the GL/SGL of the DG via δ subunit-containing receptors. Our data are important for better understanding of the neuronal circuitries in the DG and the role of specific cell types under pathological conditions.

βCCT, an antagonist selective for α(1)GABA(A) receptors, reverses diazepam withdrawal-induced anxiety in rats

The abrupt discontinuation of prolonged benzodiazepine treatment elicits a withdrawal syndrome with increased anxiety as a major symptom. The neural mechanisms underlying benzodiazepine physical dependence are still insufficiently understood. Flumazenil, the non-selective antagonist of the benzodiazepine binding site of GABA(A) receptors was capable of preventing and reversing the increased anxiety during benzodiazepine withdrawal in animals and humans in some, but not all studies. On the other hand, a number of data suggest that GABA(A) receptors containing α(1) subunits are critically involved in processes developing during prolonged use of benzodiazepines, such are tolerance to sedative effects, liability to physical dependence and addiction. Hence, we investigated in the elevated plus maze the level of anxiety 24 h following 21 days of diazepam treatment and the influence of flumazenil or a preferential α(1)-subunit selective antagonist βCCt on diazepam withdrawal syndrome in rats. Abrupt cessation of protracted once-daily intraperitoneal administration of 2 mg/kg diazepam induced a withdrawal syndrome, measured by increased anxiety-like behavior in the elevated plus maze 24 h after treatment cessation. Acute challenge with either flumazenil (10mg/kg) or βCCt (1.25, 5 and 20 mg/kg) alleviated the diazepam withdrawal-induced anxiety. Moreover, both antagonists induced an anxiolytic-like response close, though not identical, to that seen with acute administration of diazepam. These findings imply that the mechanism by which antagonism at GABA(A) receptors may reverse the withdrawal-induced anxiety involves the α(1) subunit and prompt further studies aimed at linking the changes in behavior with possible adaptive changes in subunit expression and function of GABA(A) receptors.

In vivo knockdown of GAD67 in the amygdala disrupts fear extinction and the anxiolytic-like effect of diazepam in mice

In mammals, γ-aminobutyric acid (GABA) transmission in the amygdala is particularly important for controlling levels of fear and anxiety. Most GABA synthesis in the brain is catalyzed in inhibitory neurons from L-glutamic acid by the enzyme glutamic acid decarboxylase 67 (GAD67). In the current study, we sought to examine the acquisition and extinction of conditioned fear in mice with knocked down expression of the GABA synthesizing enzyme GAD67 in the amygdala using a lentiviral-based (LV) RNA interference strategy to locally induce loss-of-function. In vitro experiments revealed that our LV-siRNA-GAD67 construct diminished the expression of GAD67 as determined with western blot and fluorescent immunocytochemical analyses. In vivo experiments, in which male C57BL/6J mice received bilateral amygdala microinjections, revealed that LV-siRNA-GAD67 injections produce significant inhibition of endogenous GAD67 when compared with control injections. In contrast, no significant changes in GAD65 expression were detected in the amygdala, validating the specificity of LV knockdown. Behavioral experiments showed that LV knockdown of GAD67 results in a deficit in the extinction, but not the acquisition or retention, of fear as measured by conditioned freezing. GAD67 knockdown did not affect baseline locomotion or basal measures of anxiety as measured in open field apparatus. However, diminished GAD67 in the amygdala blunted the anxiolytic-like effect of diazepam (1.5 mg kg(-1)) as measured in the elevated plus maze. Together, these studies suggest that of GABAergic transmission in amygdala mediates the inhibition of conditioned fear and the anxiolytic-like effect of diazepam in adult mice.

Cell-type-specific recruitment of amygdala interneurons to hippocampal theta rhythm and noxious stimuli in vivo

Neuronal synchrony in the basolateral amygdala (BLA) is critical for emotional behavior. Coordinated theta-frequency oscillations between the BLA and the hippocampus and precisely timed integration of salient sensory stimuli in the BLA are involved in fear conditioning. We characterized GABAergic interneuron types of the BLA and determined their contribution to shaping these network activities. Using in vivo recordings in rats combined with the anatomical identification of neurons, we found that the firing of BLA interneurons associated with network activities was cell type specific. The firing of calbindin-positive interneurons targeting dendrites was precisely theta-modulated, but other cell types were heterogeneously modulated, including parvalbumin-positive basket cells. Salient sensory stimuli selectively triggered axo-axonic cells firing and inhibited firing of a disctinct projecting interneuron type. Thus, GABA is released onto BLA principal neurons in a time-, domain-, and sensory-specific manner. These specific synaptic actions likely cooperate to promote amygdalo-hippocampal synchrony involved in emotional memory formation.

Removal of GABA(A) receptor γ2 subunits from parvalbumin neurons causes wide-ranging behavioral alterations

We investigated the behavioral significance of fast synaptic inhibition by αβγ2-type GABA_A receptors on parvalbumin (Pv) cells. The GABA_A receptor γ2 subunit gene was selectively inactivated in Pv-positive neurons by Cre/loxP recombination. The resulting Pv-Δγ2 mice were relatively healthy in the first postnatal weeks; but then as Cre started to be expressed, the mice progressively developed wide-ranging phenotypic alterations including low body weight, motor deficits and tremor, decreased anxiety levels, decreased pain sensitivity and deficient prepulse inhibition of the acoustic startle reflex and impaired spatial learning. Nevertheless, the deletion was not lethal, and mice did not show increased mortality even after one year. Autoradiography with t-butylbicyclophosphoro[³⁵S]thionate suggested an increased amount of GABA_A receptors with only α and β subunits in central nervous system regions that contained high levels of parvalbumin neurons. Using BAC-transgenesis, we reduced some of the Pv-Δγ2 phenotype by selectively re-expressing the wild-type γ2 subunit back into some Pv cells (reticular thalamic neurons and cerebellar Pv-positive neurons). This produced less severe impairments of motor skills and spatial learning compared with Pv-Δγ2 mice, but all other deficits remained. Our results reveal the widespread significance of fast GABAergic inhibition onto Pv-positive neurons for diverse behavioral modalities, such as motor coordination, sensorimotor integration, emotional behavior and nociception.

Modulation of conditioned fear, fear-conditioned analgesia, and brain regional c-Fos expression following administration of muscimol into the rat basolateral amygdala

Evidence suggests that gamma-aminobutyric acid (GABA) signalling in the basolateral amygdala (BLA) is involved in pain, fear, and fear-conditioned analgesia (FCA). In this study, we investigated the effects of intra-BLA administration of the GABA(A) receptor agonist muscimol on the expression of conditioned-fear, formalin-evoked nociception, and fear-conditioned analgesia in rats, and the associated alterations in brain regional expression of the immediate early gene product and marker of neuronal activity, c-Fos. Formalin-evoked nociceptive behavior, conditioned-fear and fear-conditioned analgesia were apparent in animals receiving intra-BLA saline. Intra-BLA muscimol suppressed fear behavior and prevented fear-conditioned analgesia, but had no significant effect on the expression of formalin-evoked nociception. The suppression of fear behavior by intra-BLA muscimol was associated with increased c-Fos expression in the central nucleus of the amygdala (CeA) and throughout the periaqueductal grey (PAG). These intra-BLA muscimol-induced increases in c-Fos expression were abolished in rats receiving intraplantar formalin injection. These data suggest that alterations in neuronal activity in the CeA and PAG as a result of altered GABAergic signalling in the BLA may be involved in the behavioral expression of fear and associated analgesia. Furthermore, these alterations in neuronal activity are susceptible to modulation by formalin-evoked nociceptive input in a state-dependent manner.

PERSPECTIVE

The expression of learned fear and associated analgesia are under the control of GABA(A) receptors in the basolateral amygdala, through a mechanism which may involve altered neuronal activity in key components of the descending inhibitory pain pathway. The results enhance our understanding of the neural mechanisms subserving fear-pain interactions.

Fast oscillatory activity induced by kainate receptor activation in the rat basolateral amygdala in vitro

The basolateral amygdala (BLA) has a fundamental role in affective processing. In vivo studies have revealed rhythmic population activity of a similar type to that seen in the hippocampus and cortical areas during learning tasks. The amygdala contains densely interconnected networks of inhibitory interneurons similar to those responsible for fast network activity generation in the hippocampus and other cortical structures. Here we report that neuronal networks of the BLA in isolation generate persistent, gamma frequency (30–80 Hz) oscillations upon kainate receptor activation with kainic acid. We show that, like other cortical structures, BLA oscillations are completely dependent upon γ-aminobutyric acid (GABA)ergic inhibition. GABA_A receptor blockade abolished all oscillations, and the activity was also sensitive to the barbiturate, pentobarbital. Blockade of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors or N-methyl-d-aspartate (NMDA) receptors had no significant effect on gamma activity. However, the GluR5-containing kainate receptor-specific antagonist (S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxybenzyl) pyrimidine-2,4-dione (UBP302) abolished oscillations–evidence that glutamatergic receptor involvement is predominantly kainate receptor mediated. The mixed AMPA/kainate receptor antagonist 6-nitro-7-sulphamoylbenzo[f]quinoxalone-2,3-dione disodium (NBQX) abolished all oscillatory activity in 8/14 of slices tested. In the remaining slices, gamma frequency activity was abolished to reveal a low-amplitude, NMDA receptor-dependent, beta frequency (10–20 Hz) oscillation. Gamma oscillations are abolished by gap junction blockade. While these data show the BLA capable of generating gamma rhythms in common with other cortical areas studied to date, the network mechanisms appear to be different, suggesting a unique network structure underlies amygdala rhythmogenesis. Understanding how BLA networks produce synchronous activity is paramount to understanding how the BLA executes influence on important cognitive processes such as emotional learning.

Rapid brain-derived neurotrophic factor-dependent sequestration of amygdala and hippocampal GABA(A) receptors via different tyrosine receptor kinase B-mediated phosphorylation pathways

During the consolidation of fear memory, it has been shown that GABA(A) receptors (GABA(A)R) are rapidly downregulated in amygdala. This rapid decrease in GABA(A)R functioning may permit transient hyperexcitablity, contributing to cellular mechanisms of memory consolidation. Memory consolidation also requires brain-derived neurotrophic factor (BDNF) activation of tyrosine receptor kinase B (TrkB) receptors in the amygdala and hippocampus. We hypothesized that rapid internalization of GABA(A)Rα1 is mediated via TrkB activation of PKA and PKC-dependent processes. Primary neuronal cell cultures, from postnatal day 14-21 mouse amygdala and hippocampus, were analyzed with immunofluorescence using cell-surface, whole-cell permeabilization, and antibody internalization techniques, as well as with (3)H-muscimol binding assays. In both hippocampal and amygdala cultures, we found a >60% reduction in surface GABA(A)Rα1 within 5 min of BDNF treatment. Notably, the rapid decrease in surface GABA(A)Rα1 was confirmed biochemically using surface biotinylation assays followed by western blotting. This rapid effect was accompanied by TrkB phosphorylation and increased internal GABA(A)Rα1 immunofluorescence, and was blocked by k252a, a broad-spectrum tyrosine kinase antagonist. To further demonstrate TrkB specificity, we used previously characterized TrkB(F616A) mice, in which the highly selective TrkB-mutant specific antagonist, 1NMPP1, prevented the BDNF-dependent GABA(A)Rα1 internalization. In hippocampus, we found both PKA and PKC inhibition, using Rp-8-Br-cAMP and Calphostin C, respectively, blocked GABA(A)Rα1 internalization, whereas inhibition of MAPK (U0126) and PI3K (LY294002) did not prevent rapid internalization. By contrast in amygdala cultures, Rp-8-Br-cAMP had no effect. Together, these data suggest that rapid GABA(A)R internalization during memory consolidation is BDNF-TrkB dependent. Further, it appears that hippocampal GABA(A)R internalization is PKA and PKC dependent, while it may be primarily PKC dependent in amygdala, implying differential roles for TrkB-dependent kinase activation in BDNF-dependent memory formation.

Interneurons in the basolateral amygdala

The amygdala is a temporal lobe structure that is the center of emotion processing in the mammalian brain. Recent interest in the amygdala arises from its role in processing fear and the relationship of fear to human anxiety. The amygdaloid complex is divided into a number of subnuclei that have extensive intra and extra nuclear connections. In this review we discuss recent findings on the physiology and plasticity of inputs to interneurons in the basolateral amygdala, the primary input station. These interneurons are a heterogeneous group of cells that can be separated on immunohistochemical and electrophysiological grounds. Glutamatergic inputs to these interneurons form diverse types of excitatory synapses. This diversity is manifest in both the subunit composition of the underlying NMDA receptors as well as their ability to show plasticity. We discuss these differences and their relationship to fear learning. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Synaptic interactions underlying synchronized inhibition in the basal amygdala: evidence for existence of two types of projection cells

The basal amygdala (BA) plays a key role in mediating the facilitating effects of emotions on memory. Recent findings indicate that this function depends on the ability of BA neurons to generate coherent oscillatory activity, facilitating synaptic plasticity in target neurons. However, the mechanisms allowing BA neurons to synchronize their activity remain poorly understood. Here, we aimed to shed light on this question, focusing on a slow periodic inhibitory oscillation previously observed in the BA in vitro. Paired patch recordings showed that these large inhibitory postsynaptic potentials (IPSPs) occur almost synchronously in BA projection neurons, that they were typically not preceded by excitatory postsynaptic potentials (EPSPs), and that they had little or no correlate in neighboring amygdala nuclei or cortical fields. The initial phase of the IPSPs was associated with an increase in membrane potential fluctuations at 50-100 Hz. In keeping with this, the IPSPs seen in projection cells were correlated with repetitive firing at 50-100 Hz in presumed interneurons and they could be abolished by picrotoxin. However, the IPSPs were also sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione, implying that they arose from the interplay between glutamatergic and GABAergic BA neurons. In support of this idea, we identified a small subset of projection cells (15%), positively identified as such by retrograde labeling from BA projection sites, that began firing shortly before the IPSP onset and presumably drove interneuronal firing. These results add to a rapidly growing body of data indicating that the BA contains at least two distinct types of projection cells that vary in their relation with interneurons and extra-amygdala targets.

Multiple anxiogenic drugs recruit a parvalbumin-containing subpopulation of GABAergic interneurons in the basolateral amygdala

The basolateral amygdala is a nodal structure within a distributed and interconnected network that regulates anxiety states and anxiety-related behavior. Administration of multiple anxiogenic drugs increases cellular responses (i.e., increases c-Fos expression) in a subregion of the basolateral amygdala, but the neurochemical phenotypes of these cells are not known. The basolateral amygdala contains glutamatergic projection neurons and several populations of γ-aminobutyric acid-synthesizing (GABAergic) interneurons, including a population of parvalbumin (PV)-expressing GABAergic interneurons that co-express the excitatory 5-HT(2A) receptor. The role for these PV-expressing GABAergic interneurons in anxiety-states is unclear. In this experiment we examined the effects of multiple anxiogenic drugs including the 5-HT(2C/2A) receptor agonist m-chlorophenyl piperazine (mCPP), the adenosine receptor antagonist caffeine, the α(2)-adrenoreceptor antagonist yohimbine and the partial inverse agonist at the benzodiazepine allosteric site on the GABA(A) receptor, N-methyl-beta-carboline-3-carboxamide (FG-7142), on c-Fos expression in PV-immunoreactive (PV-ir) interneurons in subdivisions of the basolateral amygdala. All drugs with the exception of mCPP increased c-Fos expression in PV-ir neurons in the basolateral amygdaloid nucleus, anterior part (BLA). The numbers of c-Fos-immunoreactive (c-Fos-ir)/PV-ir GABAergic interneurons in the BLA were positively correlated with the numbers of c-Fos-ir serotonergic neurons in the mid-rostrocaudal dorsal raphe nucleus (DR) and with a measure of anxiety-related behavior. All four drugs increased c-Fos expression in non-PV-ir cells in most of the subdivisions of the basolateral amygdala that were sampled, compared with vehicle-injected controls. Together, these data suggest that the PV/5-HT(2A) receptor expressing GABAergic interneurons in the basolateral amygdala are part of a DR-basolateral amygdala neuronal circuit modulating anxiety-states and anxiety-related behavior.

Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear

The last 10 years have witnessed a surge of interest for the mechanisms underlying the acquisition and extinction of classically conditioned fear responses. In part, this results from the realization that abnormalities in fear learning mechanisms likely participate in the development and/or maintenance of human anxiety disorders. The simplicity and robustness of this learning paradigm, coupled with the fact that the underlying circuitry is evolutionarily well conserved, make it an ideal model to study the basic biology of memory and identify genetic factors and neuronal systems that regulate the normal and pathological expressions of learned fear. Critical advances have been made in determining how modified neuronal functions upon fear acquisition become stabilized during fear memory consolidation and how these processes are controlled in the course of fear memory extinction. With these advances came the realization that activity in remote neuronal networks must be coordinated for these events to take place. In this paper, we review these mechanisms of coordinated network activity and the molecular cascades leading to enduring fear memory, and allowing for their extinction. We will focus on Pavlovian fear conditioning as a model and the amygdala as a key component for the acquisition and extinction of fear responses.

The alpha1 subunit of the GABA(A) receptor modulates fear learning and plasticity in the lateral amygdala

Synaptic plasticity in the amygdala is essential for emotional learning. Fear conditioning, for example, depends on changes in excitatory transmission that occur following NMDA receptor activation and AMPA receptor modification in this region. The role of these and other glutamatergic mechanisms have been studied extensively in this circuit while relatively little is known about the contribution of inhibitory transmission. The current experiments addressed this issue by examining the role of the GABA(A) receptor subunit α1 in fear learning and plasticity. We first confirmed previous findings that the α1 subunit is highly expressed in the lateral nucleus of the amygdala. Consistent with this observation, genetic deletion of this subunit selectively enhanced plasticity in the lateral amygdala and increased auditory fear conditioning. Mice with selective deletion of α1 in excitatory cells did not exhibit enhanced learning. Finally, infusion of a α1 receptor antagonist into the lateral amygdala selectively impaired auditory fear learning. Together, these results suggest that inhibitory transmission mediated by α1-containing GABA(A) receptors plays a critical role in amygdala plasticity and fear learning.

Amygdalar GABAergic-rich neural grafts attenuate anxiety-like behavior in rats

Transplantation experiments have shown that neurologic deficits may be reversed by engrafting fresh tissue or engineered cells within dysfunctional neural circuitry. In experimental and clinical settings, this approach has provided insights into the pathology and treatment of neurologic diseases, primarily movement disorders. The present experiments were designed to investigate whether a similar strategy is feasible as a method to investigate, and perhaps repair, circuitry integral to emotional disorders. We focused on the amygdala, a macrostructure known to be involved in the expression of anxiety- and fear-related behaviors. GABAergic cell-rich suspensions were prepared from E17 rat lateral ganglionic eminence and engrafted bilaterally into the lateral and basolateral amygdaloid nuclei of young adult rats. After 6 weeks, increased numbers of GABAergic neurons were identified in the vicinity of the graft sites, and electron microscopy provided evidence for functional integration of transplanted cells. Rats with these grafts spent more time in the open arms of the elevated-plus maze, consistent with an anxioloytic-like phenotype. These rats were also less sensitive to the unconditioned anxiogenic effects of light on the acoustic startle response, although fear-potentiated startle was not affected, suggesting that the grafts produced an attenuation of unlearned fear but did not affect acquisition of conditioned fear. Our results raise the possibility that distinct components of emotion can be modulated by strategic neural engraftment.

Effect of gestational ethanol exposure on parvalbumin and calretinin expressing hippocampal neurons in a chick model of fetal alcohol syndrome

Fetal alcohol syndrome (FAS), a condition occurring in some children of mothers who have consumed alcohol during pregnancy, is characterized by physical deformities and learning and memory deficits. The chick hippocampus, whose functions are controlled by interneurons expressing calcium-binding proteins parvalbumin (PV) and calretinin (CR), is involved in learning and memory mechanisms. Effects on growth and development and hippocampal morphology were studied in chick embryos exposed to 5% and 10% ethanol volume/volume (vol/vol) for 2 or 8 days of gestation. There was a significant dose-dependent reduction (P<.05) in body weight and mean number per section of PV and CR expressing hippocampal neurons in ethanol-exposed chicks, without alterations in neuronal nuclear size or hippocampal volume, compared appropriate controls. Moreover, when chicks exposed to 5% ethanol for 2 and 8 days of gestation were compared, no significant differences were found in body parameters or neuronal counts. Similarly, exposure to 10% ethanol did not induce any significant changes in chicks exposed for 2 or 8 gestational days. Thus, these results suggest that gestational ethanol exposure induces a reduction in the mean number per section of PV and CR expressing hippocampal neurons, and could be a possible mechanism responsible for learning and memory disorders in FAS.

A deficit of brain dystrophin impairs specific amygdala GABAergic transmission and enhances defensive behaviour in mice

Duchenne muscular dystrophy (DMD) is accompanied by cognitive deficits and psychiatric symptoms. In the brain, dystrophin, the protein responsible for DMD, is localized to a subset of GABAergic synapses, but its role in brain function has not fully been addressed. Here, we report that defensive behaviour, a response to danger or a threat, is enhanced in dystrophin-deficient mdx mice. Mdx mice consistently showed potent defensive freezing responses to a brief restraint that never induced such responses in wild-type mice. Unconditioned and conditioned defensive responses to electrical footshock were also enhanced in mdx mice. No outstanding abnormality was evident in the performances of mdx mice in the elevated plus maze test, suggesting that the anxiety state is not altered in mdx mice. We found that, in mdx mice, dystrophin is expressed in the amygdala, and that, in the basolateral nucleus (BLA), the numbers of GABA(A) receptor alpha2 subunit clusters are reduced. In BLA pyramidal neurons, the frequency of norepinephrine-induced GABAergic inhibitory synaptic currents was reduced markedly in mdx mice. Morpholino oligonucleotide-induced expression of truncated dystrophin in the brains of mdx mice, but not in the muscle, ameliorated the abnormal freezing response to restraint. These results suggest that a deficit of brain dystrophin induces an alteration of amygdala local inhibitory neuronal circuits and enhancement of fear-motivated defensive behaviours in mice.

The GABAA receptor-mediated recurrent inhibition in ventral compared with dorsal CA1 hippocampal region is weaker, decays faster and lasts less

Hippocampal functions appear to be segregated along the dorso-ventral axis of the structure. Differences at the cellular and local neuronal network level may be involved in this functional segregation. In this study the characteristics of CA1 recurrent inhibition (RI) were measured and compared between dorsal (DH, n = 95) and ventral (VH, n = 60) hippocampal slices, using recordings of suprathreshold field potentials. RI strength was estimated as the percentile decrease of the population spike (PS) amplitude evoked with an orthodromic stimulus (at the Schaffer collaterals) when preceded by an antidromic stimulus (at the alveus). Varying the interpulse interval (IPI) between the two stimuli, we estimated RI duration. Alvear stimulation produced significant PS suppression in both VH and DH at every IPI tested, from 10 to 270 ms. Moreover, gradually more oblique DH (but not VH) slices displayed increasing RI, which at IPIs < or = 125 ms was reversibly abolished by the GABAA receptor antagonist picrotoxin (10 microM). The GABAA-mediated RI, measured under the blockade of GABAB receptors, was weaker, decayed faster and lasted less in VH compared to DH slices, regardless of the slice orientation. Specifically, in VH compared to DH, the PS suppression at 20 ms was 34.4 +/- 4.5% versus 69.9 +/- 6.5% (P < 0.001), the time constant of RI decay was 29 +/- 2.4 versus 87.5 +/- 13.6 ms (P < 0.01) and the duration was 50 versus 125 ms (P < 0.001). Thus, GABAA-mediated RI may control the CA1 excitatory output less effectively in VH compared to DH. The observed dorso-ventral differences in RI contribute to the longitudinal diversification of the structure and may underlie to some extent the region-specificity of hippocampal functions.

Neuronal localization of 5-HT type 2A receptor immunoreactivity in the rat basolateral amygdala

Although it is well established that there are alterations in type 2A 5-HT receptors (5-HT2ARs) in the basolateral nuclear complex of the amygdala (BLC) in several neuropsychiatric disorders, very little is known about the neuronal localization of these receptors in this brain region. Single-labeling and dual-labeling immunohistochemical techniques were utilized in the rat to address this question. Three different 5-HT2AR antibodies were used, each producing distinct but overlapping patterns of immunostaining. Two of three 5-HT2AR antibodies mainly stained pyramidal projection neurons in the BLC. The third antibody only stained pyramidal cells in the dorsolateral subdivision of the lateral amygdalar nucleus. With one of the antibodies, the most intensely stained neurons were a population of large nonpyramidal neurons whose morphology and distribution closely resembled those shown in previous studies to project to the mediodorsal thalamic nucleus (MD). This was confirmed in the present study using a technique that combined 5-HT2AR immunohistochemistry with fluorogold retrograde tract-tracing. Two of three 5-HT2AR antibodies stained large numbers of parvalbumin-containing interneurons in the BLC. One of these two antibodies also stained a subpopulation of somatostatin-containing neurons. None of the 5-HT2AR antibodies stained significant numbers of the other two main interneuronal subpopulations, the large cholecystokinin-positive neurons or the small interneurons that exhibit extensive colocalization of calretinin and cholecystokinin. Since each of the three antibodies was raised against a distinct immunizing antigen, they may recognize different conformations of 5-HT2AR in different neuronal domains. The expression of 5-HT2ARs in pyramidal cells and parvalbumin-positive interneurons in the BLC is consistent with the results of previous electrophysiological studies, and suggests that 5-HT may produce excitation of several neuronal populations in the BLC via 5-HT2ARs.

Networks of parvalbumin-positive interneurons in the basolateral amygdala

The amygdala is a temporal lobe structure that is required for processing emotional information. Polymodal sensory information enters the amygdala at the level of the basolateral amygdala (BLA) and undergoes local processing, after which the behavioral and autonomic responses that accompany emotions are initiated. Two main neuron types are present in the BLA, pyramidal-like principal neurons that use glutamate as their transmitter, and local circuit interneurons that use GABA as their transmitter. Although the properties of principal neurons are known in some detail, very little is known about the properties of BLA interneurons or the local circuits in which they are involved. Using mice in which EGFP (enhanced green fluorescent protein) is expressed under the control of the parvalbumin promoter, we characterized the properties of parvalbumin-positive interneurons in the BLA. By making recordings from interneuron-interneuron and interneuron-principal neuron pairs, we analyzed the intrinsic circuitry of the BLA. We show that parvalbumin-positive interneurons can be divided into four subtypes as defined by their firing properties. Interneurons are electrically coupled in subtype-specific networks and exhibit subtype-specific heterogeneities in their synaptic dynamics and patterns of connectivity. We propose that these properties allow networks of parvalbumin-expressing neurons to perform an array of information-processing tasks within the BLA.

Postnatal development of GABA and calbindin cells and fibers in the prefrontal cortex and basolateral amygdala of gerbils (Meriones unguiculatus)

The postnatal maturation of immunohistochemically stained gamma-amino-butyric acid (GABA) and calbindin (CB) cells and fibers were quantitatively examined in the prefrontal cortex (PFC) and the basolateral amygdala (BLA) of the Mongolian gerbil (Meriones unguiculatus). Animals of different ages, ranging from juvenile (postnatal day (PD)14, PD20, PD30), to adolescent (PD70), adult (PD180, PD540) and aged (PD720) were analyzed. Results reveal an increase in GABAergic fiber densities between PD14-20 in the PFC and the BLA with a concomitant decrease in cell density. After PD70 GABA fiber density slightly decreases again in the BLA, while there is a further slow but significant increase in the PFC between PD70 and PD540. Fibers immunoreactive for the calcium binding-protein CB, which is predominantly localized in particular GABAergic subpopulations, also accumulate between PD14 and PD20 in the PFC and BLA, while a concomitant decrease in cell density is only seen in the BLA. Both areas reveal a decrease of CB cells between PD30 and PD70, which parallels with a decrease of CB fibers in the PFC. However, there is no particular 'aging-effect' in the fiber or cell densities of GABA or CB in any of the investigated areas in old animals. In conclusion, we here demonstrate long-term dynamics in cell and fiber densities of the GABAergic system until late in development which might correspond to the prolonged maturation of other neuroanatomical and functional systems.

Systemic administration of the benzodiazepine receptor partial inverse agonist FG-7142 disrupts corticolimbic network interactions

The medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) coordinate various stress responses. Although the effects of stressors on mPFC and BLA activity have been previously examined, it remains unclear to what extent stressors affect functional interactions between these regions. In vivo electrophysiology in the anesthetized rat was used to examine mPFC and BLA activity simultaneously in response to FG-7142, a benzodiazepine receptor partial inverse agonist that mimics various stress responses, in an attempt to model the effects of stressors on corticolimbic functional connectivity. Extracellular unit and local field potential (LFP) recordings, using multielectrode arrays positioned in mPFC and BLA, were conducted under basal conditions and in response to systemic FG-7142 administration. This drug increased mPFC and BLA unit firing at the lowest dose tested, whereas higher doses of FG-7142 decreased various burst firing parameters in both regions. Moreover, LFP power was attenuated at lower (<1 Hz) and potentiated at higher frequencies in mPFC (1-12 Hz) and BLA (4-8 Hz). Interestingly, FG-7142 diminished synchronized unit firing, both within and between mPFC and BLA. Finally, FG-7142 decreased LFP synchronization between these regions. In a separate group of animals, pretreatment with the selective benzodiazepine receptor antagonist flumazenil blocked the changes in burst firing, LFP power and synchronized activity induced by FG-7142, confirming direct benzodiazepine receptor-mediated effects. These results indicate that FG-7142 disrupts corticolimbic network interactions via benzodiazepine receptor partial inverse agonism. Perturbation of mPFC-BLA functional connectivity induced by FG-7142 may provide a useful model of corticolimbic dysfunction induced by stressors.

A novel subpopulation of 5-HT type 3A receptor subunit immunoreactive interneurons in the rat basolateral amygdala

The amygdalar basolateral nuclear complex (BLC) has very high levels of the 5-HT type 3 receptor (5-HT(3)R). Previous studies have reported that 5-HT(3)R protein in the BLC is expressed in interneurons and that 5-HT(3)R mRNA is coexpressed with GABA and certain neuropeptides or calcium-binding proteins in these cells. However, there have been no detailed descriptions of the distribution of 5-HT(3)R+ neurons in the rat amygdala, and no quantitative studies of overlap of neurons expressing 5-HT(3)R protein with distinct interneuronal subpopulations in the BLC. The present investigation employed dual-labeling immunohistochemistry using antibodies to the 5-HT-3A receptor subunit (5-HT(3A)R) and specific interneuronal markers to address these questions. These studies revealed that there was a moderate density of nonpyramidal 5-HT(3A)R+ neurons in the BLC at all levels of the amygdala. In addition, immunostained cells were also seen in anterior portions of the cortical and medial nuclei. Although virtually all 5-HT(3A)R+ neurons in the BLC were GABA+, very few expressed neuropeptide or calcium-binding protein markers for individual subpopulations. The main interneuronal marker expressed by 5-HT(3A)R+ neurons was cholecystokinin (CCK), but only 8-16% of 5-HT(3)R+ neurons in the BLC, depending on the nucleus, were CCK+. Most of these CCK+/5-HT(3A)R+ double-labeled neurons appeared to belong to the subpopulation of large type L CCK+ interneurons. Very few 5-HT(3A)R+ neurons expressed calretinin, vasoactive intestinal peptide, or parvalbumin, and none expressed somatostatin or calbindin. Thus, the great majority of neurons expressing 5-HT(3A)R protein appear to constitute a previously unrecognized subpopulation of GABAergic interneurons in the BLC.

Increased GABAA-Receptor ?1-Subunit Expression in Hippocampal Dentate Gyrus after Early-life Status Epilepticus

PURPOSE

Previous studies in neonatal (postnatal day 10) and adult rats suggest that status epilepticus (SE) induces changes in the alpha1 subunit of the GABA(A) receptor (GABRA1) in dentate granule neurons (DGNs) that are age dependent and vary inversely with the likelihood of epilepsy development. In the present study, we examined GABRA1 expression after SE at postnatal day 20 (P20), an intermediate age when only a subset of SE-exposed animals develop epilepsy.

METHODS

SE was induced with lithium-pilocarpine or kainate at P20. Animals were video-EEG monitored after SE to determine the presence or absence of spontaneous seizures. GABRA1 mRNA and protein levels were determined 7 days or 3 months later in SE-exposed and control animals by using a combination of aRNA amplification, Western blotting, and immunohistochemistry techniques.

RESULTS

GABRA1 mRNA levels in DGNs of SE-exposed rats that did not become epileptic were higher than those in control rats, but were not different from DGNs in epileptic SE-exposed rats. GABRA1 protein levels in dentate gyrus were significantly increased in both epileptic and nonepileptic SE-exposed rats compared with controls. GABRA1 mRNA changes were region specific and did not occur in CA1 or CA3 areas of hippocampus. GABRA1 alterations were present by 1 week after P20 SE and were similar whether pilocarpine or kainate was used to induced SE.

CONCLUSIONS

P20 SE results in persistent increases in GABRA1 levels selectively in dentate gyrus. These changes preceded the onset of epilepsy, were not model specific, and occurred in both epileptic and nonepileptic animals.

Subpopulations of neurons expressing parvalbumin in the human amygdala

Amygdalar intrinsic inhibitory networks comprise several subpopulations of gamma-aminobutyric acidergic neurons, each characterized by distinct morphological features and clusters of functionally relevant neurochemical markers. In rodents, the calcium-binding proteins parvalbumin (PVB) and calbindin D28k (CB) are coexpressed in large subpopulations of amygdalar interneurons. PVB-immunoreactive (-IR) neurons have also been shown to be ensheathed by perineuronal nets (PNN), extracellular matrix envelopes believed to affect ionic homeostasis and synaptic plasticity. We tested the hypothesis that differential expression of these three markers may define distinct neuronal subpopulations within the human amygdala. Toward this end, triple-fluorescent labeling using antisera raised against PVB and CB as well as biotinylated Wisteria floribunda lectin for detection of PNN was combined with confocal microscopy. Among the 1,779 PVB-IR neurons counted, 18% also expressed CB, 31% were ensheathed in PNN, and 7% expressed both CB and PNN. Forty-four percent of PVB-IR neurons did not colocalize with either CB or PNN. The distribution of each of these neuronal subgroups showed substantial rostrocaudal gradients. Furthermore, distinct morphological features were found to characterize each neuronal subgroup. In particular, significant differences relative to the distribution and morphology were detected between PVB-IR neurons expressing CB and PVB-IR neurons wrapped in PNNs. These results indicate that amygdalar PVB-IR neurons can be subdivided into at least four different subgroups, each characterized by a specific neurochemical profile, morphological characteristics, and three-dimensional distribution. Such properties suggest that each of these neuronal subpopulations may play a specific role within the intrinsic circuitry of the amygdala.

Genes and neurons: molecular insights to fear and anxiety

Experimental animal models provide an important tool for the identification of inheritable components of fear and anxiety. 'Pavlovian' fear conditioning has been tremendously successful to characterize the neuronal circuitry and cellular mechanisms of the formation, consolidation and extinction of fear memories. Here we summarize recent progress that has led to the identification of gene products contributing to such experience-dependent changes in fear and anxiety and may guide the search for genetic factors involved in the development and treatment of human anxiety disorders.

Histological and cytological study of the bed nuclei of the stria terminalis in adult rat. II. Oval nucleus: Extrinsic inputs, cell types, neuropil, and neuronal modules

The oval nucleus (Ov) of the bed nuclei of the stria terminalis was studied in adult rats. The Ov is composed of 11 neuron types distributed into a shell and a core domain. The stria terminalis, internal capsule, ventral amygdaloid pathway, and medial forebrain bundle are the main sources of afferents to the neuropil of the Ov. The nucleus shell contains abundant intrinsic neurons possibly connected among themselves and with the core by centripetal axon collaterals. Series of intrinsic neurons in the shell, linked with both short-axon and projecting neurons in the core, suggest a centripetal control of projecting neurons. In situ hybidization for vesicular glutamate transporter (VGlu) and glutamic acid decarboxylase (GAD) show numerous GAD-synthesizing neurons and an absence of VGlu-synthesizing neurons. In the electron microscope, the neuropil of the Ov contains axospinous, axoshaft, axosomatic, mixed (i.e., chemical-electrical), and axoaxonic synapses, in order of frequency. Synaptic boutons in apposition with the initial segment, represent an additional axoaxonic interaction. Further neural synchronization of the Ov occurs via gap junctions between somata, soma-dendrite, and possibly by apposition between axon terminals. The putative inputs from the major tracts of the forebrain coupled with the cytological organization of the Ov make it one of the most complex structures of the mammalian brain.

Physiological and morphological characterization of parvalbumin-containing interneurons of the rat basolateral amygdala

The basolateral amygdala (BLA) is critical for the generation of emotional behavior and the formation of emotional memory. Understanding the neuronal mechanisms that contribute to emotional information processing in the BLA will ultimately require knowledge of the anatomy and physiology of its constituent neurons. Two major cell classes exist in the BLA, pyramidal projection neurons and nonpyramidal interneurons. Although the properties of projection neurons have been studied in detail, little is known about the properties of BLA interneurons. We have used whole-cell patch clamp recording techniques to examine the physiological properties of 48 visually identified putative interneurons from the rat anterior basolateral amygdalar nucleus. Here, we report that BLA interneurons can be differentiated into four electrophysiologically distinct subtypes based on their intrinsic membrane properties and their response to afferent synaptic input. Interneuron subtypes were named according to their characteristic firing pattern generated in response to transient depolarizing current injection and were grouped as follows: 1) burst-firing interneurons (n = 13), 2) regular-firing interneurons (n = 11), 3) fast-firing interneurons (n = 10), and 4) stutter-firing interneurons (n = 14). Post hoc histochemical visualization confirmed that all 48 recorded neurons had morphological properties consistent with their being local circuit interneurons. Moreover, by using triple immunofluorescence (for biocytin, calcium-binding proteins, and neuropeptides) in conjunction with patch clamp recording, we further demonstrated that over 60% of burst-firing and stutter-firing interneurons also expressed the calcium-binding protein parvalbumin (PV(+)). These data demonstrate that interneurons of the BLA show both physiological and neurochemical diversity. Moreover, we demonstrate that the burst- and stutter-firing patterns positively correlate with PV(+) immunoreactivity, suggesting that these neurons may represent functionally distinct subpopulations.

The dosage of the neuroD2 transcription factor regulates amygdala development and emotional learning

The amygdala is centrally involved in formation of emotional memory and response to fear or risk. We have demonstrated that the lateral and basolateral amygdala nuclei fail to form in neuroD2 null mice and neuroD2 heterozygotes have fewer neurons in this region. NeuroD2 heterozygous mice show profound deficits in emotional learning as assessed by fear conditioning. Unconditioned fear was also diminished in neuroD2 heterozygotes compared to wild-type controls. Several key molecular regulators of emotional learning were diminished in the brains of neuroD2 heterozygotes including Ulip1, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, and GABA(A) receptor. Thus, neuroD2 is essential for amygdala development and genes involved in amygdala function are altered in neuroD2-deficient mice.

Coupled networks of parvalbumin-immunoreactive interneurons in the rat basolateral amygdala

Recent studies indicate that the basolateral amygdala exhibits fast rhythmic oscillations during emotional arousal, but the neuronal mechanisms underlying this activity are not known. Similar oscillations in the cerebral cortex are generated by a network of parvalbumin (PV)-immunoreactive interneurons interconnected by chemical synapses and dendritic gap junctions. The present immunoelectron microscopic study revealed that the basolateral amygdalar nucleus (BLa) contains a network of parvalbumin-immunoreactive (PV+) interneurons interconnected by chemical synapses, dendritic gap junctions, and axonal gap junctions. Twenty percent of synapses onto PV+ neurons were formed by PV+ axon terminals. All of these PV+ synapses were symmetrical. PV+ perikarya exhibited the greatest incidence of PV+ synapses (30%), with lower percentages associated with PV+ dendrites (15%) and spines (25%). These synapses comprised half of all symmetrical synapses formed with PV+ cells. A total of 18 dendrodendritic gap junctions between PV+ neurons were observed, mostly involving secondary and more distal dendrites (0.5-1.0 microm thick). Dendritic gap junctions were often in close proximity to PV+ chemical synapses. Six gap junctions were observed between PV+ axon terminals. In most cases, one or both of these terminals formed synapses with the perikarya of principal neurons. This is the first study to describe dendritic gap junctions interconnecting PV+ interneurons in the basolateral amygdala. It also provides the first documentation of gap junctions between interneuronal axon terminals in the mammalian forebrain. These data provide the anatomical basis for a PV+ network that may play a role in the generation of rhythmic oscillations in the BLa during emotional arousal.

Effects of midazolam on the expression of conditioned taste aversion in rats

In conditioned taste aversion (CTA), the animals learn to avoid a taste substance (conditioned stimulus, CS) which was previously associated with visceral distress (unconditioned stimulus, US). The present study examined the effects of administration of midazolam (MDZ), a benzodiazepine agonist, after the acquisition of CTA on the expression of CTA. After ingestion of 0.5 M sucrose (CS) was paired with an intraperitoneal (i.p.) injection of 0.15 M LiCl (US), control rats showed strong CTA to the CS. However, a systemic injection of MDZ (1.5 mg/kg, i.p.) before the retention test prevented conditioned animals from rejecting the CS, but in the subsequent retention tests where the drug was not administrated, those animals again showed strong aversions to the CS. Aversive taste reactivity patterns to the intraorally infused sucrose and 0.3 M dl-alanine in the conditioned animals were also diminished by the similar injection of MDZ, but not by a serotonergic anxiolytic agent, buspirone (2.5 or 5.0 mg/kg, i.p.). General taste sensory deficit might not be induced by MDZ because the drug injection did not impair conditioned aversions to 0.2 M NaCl and 0.01 M HCl. Infusion of MDZ into the basolateral nucleus of the amygdala (BLA) also attenuated conditioned aversions to sucrose. These results suggest that systemic or intra-BLA administrations of MDZ impair the expression of CTA selectively to sweet-tasting substances, implying that a transient MDZ-induced CTA expression deficit is due to the enhancement of palatability of CSs with preferable tastes rather than general anxiolytic or amnesic effects of MDZ.