GABA receptors inhibited by benzodiazepines mediate fast inhibitory transmission in the central amygdala

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

The Direction of response selectivity between conspecific and heterospecific auditory stimuli varies with response metric

Species recognition is an essential behavioral outcome of social discrimination, flocking, mobbing, mating, and/or parental care. In songbirds, auditory species recognition cues are processed through specialized forebrain circuits dedicated to acoustic discrimination. Here we addressed the direction of behavioral and neural metrics of zebra finches' (Taeniopygia guttata) responses to acoustic cues of unfamiliar conspecifics vs. heterospecifics. Behaviorally, vocal response rates were greater for conspecific male zebra finch songs over heterospecific Pin-tailed Whydah (Vidua macroura) songs, which paralleled greater multiunit spike rates in the auditory forebrain in response to the same type of conspecific over heterospecific auditory stimuli. In contrast, forebrain activation levels were reversed to species-specific song playbacks during two functional magnetic resonance imaging experiments: we detected consistently greater responses to whydah songs over finch songs and did so independently of whether subjects had been co-housed or not with heterospecifics. These results imply that the directionality of behavioral and neural response selectivity metrics are not always consistent and appear to be experience-independent in this set of stimulus-and-subject experimental paradigms.

Diazepam limits microglia-mediated neuronal remodeling in the prefrontal cortex and associated behavioral consequences following chronic unpredictable stress

Chronic stress induces neuronal atrophy and synaptic loss in the medial prefrontal cortex (PFC), and this leads to behavioral and cognitive impairments. Our recent findings indicate that microglia contribute to structural remodeling of neurons via increased colony-stimulating factor (CSF)-1 in the medial PFC. Other work shows that chronic stress induces aberrant neuronal activity in the medial PFC, and that neuronal hyperactivity increases CSF1 signaling and alters microglia function. Thus, the present studies were designed to examine the role of neuronal activity in stress-induced CSF1 signaling and microglia-mediated neuronal remodeling in the medial PFC. Additional analyses probed stress effects on the dorsal hippocampus (HPC), basolateral amygdala (BLA), and somatosensory cortex (SSCTX). Mice were exposed to chronic unpredictable stress (CUS) or handled intermittently as controls, and received daily injection of vehicle or diazepam (1 mg/kg). As anticipated, diazepam attenuated CUS-induced behavioral despair and cognitive impairments. Further studies showed that diazepam normalized Csf1 and C3 mRNA in the PFC, and prevented increases in Csf1r and Cd11b in frontal cortex microglia following CUS. Stress had no effect on neuroimmune gene expression in the HPC. Confocal imaging in Thy1-GFP(M) mice demonstrated that diazepam limited microglial engulfment of neuronal elements and blocked CUS-induced dendritic spine loss in the medial PFC. Altogether, these findings indicate that modulation of chronic stress-induced neuronal activity limits microglia-mediated neuronal remodeling in the medial PFC, and subsequent behavioral and cognitive consequences.

Amygdala: Neuroanatomical and Morphophysiological Features in Terms of Neurological and Neurodegenerative Diseases

The amygdala is one of the most discussed structures of the brain. Correlations between its level of activity, size, biochemical organization, and various pathologies are the subject of many studies, and can serve as a marker of existing or future disease. It is hypothesized that the amygdala is not just a structural unit, but includes many other regions in the brain. In this review, we present the updated neuroanatomical and physiological aspects of the amygdala, discussing its involvement in neurodegenerative and neurological diseases. The amygdala plays an important role in the processing of input signals and behavioral synthesis. Lesions in the amygdala have been shown to cause neurological disfunction of ranging severity. Abnormality in the amygdala leads to conditions such as depression, anxiety, autism, and also promotes biochemical and physiological imbalance. The amygdala collects pathological proteins, and this fact can be considered to play a big role in the progression and diagnosis of many degenerative diseases, such as Alzheimer’s disease, chronic traumatic encephalopathy, Lewy body diseases, and hippocampal sclerosis. The amygdala has shown to play a crucial role as a central communication system in the brain, therefore understanding its neuroanatomical and physiological features can open a channel for targeted therapy of neurodegenerative diseases.

Genetic and pharmacological inactivation of astroglial connexin 43 differentially influences the acute response of antidepressant and anxiolytic drugs

AIM

Astroglial connexins (Cxs) 30 and 43 are engaged in gap junction and hemichannel activities. Evidence suggests that these functional entities contribute to regulating neurotransmission, thereby influencing brain functions. In particular, preclinical and clinical findings highlight a role of Cx43 in animal models of depression. However, the role of these proteins in response to currently available psychotropic drugs is still unknown.

METHODS

To investigate this, we evaluated the behavioural effects of the genetic and pharmacological inactivation of Cx43 on the antidepressant- and anxiolytic-like activities of the selective serotonin reuptake inhibitor fluoxetine and the benzodiazepine diazepam, respectively.

RESULTS

A single administration of fluoxetine (18 mg/kg; i.p.) produced a higher increase in hippocampal extracellular serotonin levels, and a greater antidepressant-like effect in the tail suspension test in Cx43 knock-down (KD) mice bred on a C57BL/6 background compared to their wild-type littermates. Similarly, in outbred Swiss wild-type mice, the intra-hippocampal injection of a shRNA-Cx43 or the acute systemic injection of the Cxs inhibitor carbenoxolone (CBX: 10 mg/kg; i.p.) potentiated the antidepressant-like effects of fluoxetine. Evaluating the effects of such strategies on diazepam (0.5 mg/kg; i.p.), the results indicate that Cx43 KD mice or wild-types injected with a shRNA-Cx43 in the amygdala, but not in the hippocampus, attenuated the anxiolytic-like effects of this benzodiazepine in the elevated plus maze. The chronic systemic administration of CBX mimicked the latter observations.

CONCLUSION

Collectively, these data pave the way to the development of potentiating strategies in the field of psychiatry based on the modulation of astroglial Cx43.

Perinatal compromise contributes to programming of GABAergic and glutamatergic systems leading to long-term effects on offspring behaviour

Extensive evidence now shows that adversity during the perinatal period is a significant risk factor for the development of neurodevelopmental disorders long after the causative event. Despite stemming from a variety of causes, perinatal compromise appears to have similar effects on the developing brain, thereby resulting in behavioural disorders of a similar nature. These behavioural disorders occur in a sex-dependent manner, with males affected more by externalising behaviours such as attention deficit hyperactivity disorder (ADHD) and females by internalising behaviours such as anxiety. Regardless of the causative event or the sex of the offspring, these disorders may begin in childhood or adolescence but extend into adulthood. A mechanism by which adverse events in the perinatal period impact later in life behaviour has been shown to be the changing epigenetic landscape. Methylation of the GAD1/GAD67 gene, which encodes the key glutamate-to-GABA-synthesising enzyme glutamate decarboxylase 1, resulting in increased levels of glutamate, is one epigenetic mechanism that may account for a tendency towards excitation in disorders such as ADHD. Exposure of the fetus or the neonate to high levels of cortisol may be the mediator between perinatal compromise and poor behavioural outcomes because evidence suggests that increased glucocorticoid exposure triggers widespread changes in the epigenetic landscape. This review summarises the current evidence and recent literature about the impact of various perinatal insults on the epigenome and the common mechanisms that may explain the similarity of behavioural outcomes occurring following diverse perinatal compromise.

Reduced Neurosteroid Exposure Following Preterm Birth and Its' Contribution to Neurological Impairment: A Novel Avenue for Preventative Therapies

Children born preterm are at an increased risk of developing cognitive problems and neuro-behavioral disorders such as attention deficit hyperactivity disorder (ADHD) and anxiety. Whilst neonates born at all gestational ages, even at term, can experience poor cognitive outcomes due to birth-complications such as birth asphyxia, it is becoming widely known that children born preterm in particular are at significant risk for learning difficulties with an increased utilization of special education resources, when compared to their healthy term-born peers. Additionally, those born preterm have evidence of altered cerebral myelination with reductions in white matter volumes of the frontal cortex, hippocampus and cerebellum evident on magnetic resonance imaging (MRI). This disruption to myelination may underlie some of the pathophysiology of preterm-associated brain injury. Compared to a fetus of the same post-conceptional age, the preterm newborn loses access to in utero factors that support and promote healthy brain development. Furthermore, the preterm ex utero environment is hostile to the developing brain with a myriad of environmental, biochemical and excitotoxic stressors. Allopregnanolone is a key neuroprotective fetal neurosteroid which has promyelinating effects in the developing brain. Preterm birth leads to an abrupt loss of the protective effects of allopregnanolone, with a dramatic drop in allopregnanolone concentrations in the preterm neonatal brain compared to the fetal brain. This occurs in conjunction with reduced myelination of the hippocampus, subcortical white matter and cerebellum; thus, damage to neurons, astrocytes and especially oligodendrocytes of the developing nervous system can occur in the vulnerable developmental window prior to term as a consequence reduced allopregnanolone. In an effort to prevent preterm-associated brain injury a number of therapies have been considered, but to date, other than antenatal magnesium sulfate and corticosteroid therapy, none have become part of standard clinical care for vulnerable infants. Therefore, there remains an urgent need for improved therapeutic options to prevent brain injury in preterm neonates. The actions of the placentally derived neurosteroid allopregnanolone on GABAA receptor signaling has a major role in late gestation neurodevelopment. The early loss of this intrauterine neurotrophic support following preterm birth may be pivotal to development of neurodevelopmental morbidity. Thus, restoring the in utero neurosteroid environment for preterm neonates may represent a new and clinically feasible treatment option for promoting better trajectories of myelination and brain development, and therefore reducing neurodevelopmental disorders in children born preterm.

Stress-Induced Functional Alterations in Amygdala: Implications for Neuropsychiatric Diseases

The amygdala plays a major role in the processing of physiologic and behavioral responses to stress and is characterized by gamma-aminobutyric acid (GABA)-mediated high inhibitory tone under resting state. Human and animal studies showed that stress lead to a hyperactivity of amygdala, which was accompanied by the removal of inhibitory control. However, the contribution of hyperactivity of amygdala to stress-induced neuropsychiatric diseases, such as anxiety and mood disorders, is still dubious. In this review, we will summarize stress-induced various structural and functional alterations in amygdala, including the GABA receptors expression, GABAergic transmission and synaptic plasticity. It may provide new insight on the neuropathologic and neurophysiological mechanisms of neuropsychiatric diseases.

Sodium and water intake are not affected by GABAC receptor activation in the lateral parabrachial nucleus of sodium-depleted rats

The activation of GABAergic receptors, GABAA and GABAB, in the lateral parabrachial nucleus (LPBN) increases water and sodium intake in satiated and fluid-depleted rats. The present study investigated the presence of the GABAC receptor in the LPBN, its involvement in water and sodium intake, and its effects on cardiovascular parameters during the acute fluid depletion induced by furosemide combined with captopril (Furo/Cap). One group of male Wistar rats (290-300g) with bilateral stainless steel LPBN cannulas was used to test the effects of a GABAC receptor agonist and antagonist on the fluid intake and cardiovascular parameters. We investigated the effects of bilateral LPBN injections of trans-4-aminocrotonic acid (TACA) on the intake of water and 0.3M NaCl induced by acute fluid depletion (subcutaneous injection of Furo/Cap). c-Fos expression increased (P<0.05), suggesting LPBN neuronal activation. The injection of different doses of TACA (0.5, 2.0 and 160 nmol) in the LPBN did not change the sodium or water intake in Furo/Cap-treated rats (P>0.05). Treatment with the GABAC receptor antagonist (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid sulfate (ZAPA, 10nmol) or with ZAPA (10nmol) plus TACA (160nmol) did not change the sodium or water intake compared with that for vehicle (saline) (P>0.05). Bilateral injections of the GABAC agonist in the LPBN of Furo/Cap-treated rats did not affect the mean arterial pressure (MAP) or heart rate (HR). The GABAC receptor expression in the LPBN was confirmed by the presence of a 50kDa band. Although LPBN neurons might express GABAC receptors, their activation produced no change in water and sodium intake or in the cardiovascular parameters in the acute fluid depletion rats. Therefore, the GABAC receptors in the LPBN might not interfere with fluid and blood pressure regulation.

Failing as doorman and disc jockey at the same time: Amygdalar dysfunction in Parkinson's disease

In Braak's model of ascending degeneration in Parkinson's disease (PD), involvement of the amygdala occurs simultaneously with substantia nigra degeneration. However, the clinical manifestations of amygdalar involvement in PD have not been fully delineated. Considered a multitask manager, the amygdala is a densely connected "hub," coordinating and integrating tasks ranging from prompt, multisensorial emotion recognition to adequate emotional responses and emotional tuning of memories. Although phylogenetically predisposed to handle fear, the amygdala handles both aversive and positive emotional inputs. In PD, neuropathological and in vivo studies suggest primarily amygdalar hypofunction. However, as dopamine acts as an inverted U-shaped amygdalar modulator, medication-induced hyperactivity of the amygdala can occur. We propose that amygdalar (network) dysfunction contributes to reduced recognition of negative emotional face expressions, impaired theory of mind, reactive hypomimia, and impaired decision making. Similarly, impulse control disorders in predisposed individuals, hallucinations, anxiety, and panic attacks may be related to amygdalar dysfunction. When available, we discuss amygdala-independent trigger mechanisms of these symptoms. Although dopaminergic agents have mostly an activation effect on amygdalar function, adaptive and compensatory network changes may occur as well, but these have not been sufficiently explored. In conclusion, our model of amygdalar involvement brings together several elements of Parkinson's disease phenomenology heretofore left unexplained and provides a framework for testable hypotheses in patients during life and in autopsy analyses.

Artemisia capillaris Thunberg Produces Sedative-Hypnotic Effects in Mice, Which are Probably Mediated Through Potentiation of the GABAA Receptor

The Artemisia group of plants has long been used as a traditional remedy for various conditions. The present study assessed the sleep-promoting (sedative-hypnotic) effects of Artemisia capillaris Thunberg (A. capillaris), and elucidated a possible mechanism behind its effect. ICR mice were given A. capillaris extract (oral) at different dosages (50, 100, 200, 300, or 400 mg/kg), distilled water (oral; control), or diazepam (intraperitoneal; reference drug). One hour after administration, locomotion (open-field test) and motor coordination (rota-rod test) were assessed. The extract's effect on pentobarbital-induced sleep was also evaluated. Additionally, electroencephalographic (EEG) recordings were measured in rats. To evaluate a possible mechanism behind its effects, changes in chloride ( Cl -) ion influx were measured in human neuroblastoma cells. As compared to the control group, mice treated with A. capillaris demonstrated significantly decreased locomotor activity and impaired motor balance and coordination. The extract also shortened the onset and lengthened the duration of sleep induced by pentobarbital sodium. These effects were comparable to that induced by diazepam. Furthermore, A. capillaris-treated rats showed increased delta and decreased alpha EEG waves; an electroencephalographic pattern indicative of relaxation or sedation. In neuroblastoma cells, the extract dose-dependently increased Cl - ion influx, which was blocked by co-administration of bicuculline, a GABAA receptor competitive antagonist, suggesting that its effects are mediated through the GABAA receptor– Cl - ion channel complex. Altogether, the results of the present study demonstrate that A. capillaris possesses potent sedative-hypnotic effects, which are probably mediated through potentiation of the GABAA receptor– Cl - ion channel complex.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Evaluation of anxiolytic activity of compound Valeriana jatamansi Jones in mice

Background

Compound Valeriana jatamansi Jones is a formula for treating anxiety-related diseases in the clinic, which is composed of Valeriana jatamansi Rhizoma et Radix, Ziziphi Spinosae Semen, Albiziae Cortex and Junci Medulla. The purpose of this study was to explore the anxiolytic properties of this compound in mice.

Methods

Male ICR mice were treated with compound Valerianae Jatamansi Jones (1.2 g/kg, 2.4 g/kg, 4.8 g/kg), saline, diazepam (2 mg/kg) orally for 10 days and then exposed to elevated maze-plus (EPM) and light–dark box (LDB). The effects of the compound on spontaneous activity were evaluated by locomotor activity test. We further investigated the mechanism of action underlying the anxiolytic-like effect of compound by pre-treating animals with antagonists of benzodiazepine (flumazenil, 3mg/kg) prior to evaluation using EPM and LDB.

Results

Compound Valerianae Jatamansi Jones (2.4, 4.8 g/kg, p.o.) significantly increased entries (P<0.05) into and time spent (P<0.05) on the open arms of the EPM, and number of transitions (P<0.05) and time spent (P<0.05) in the light compartment of the LDB. However, the anxiolytic-like effects of compound were significantly reduced by pre-treatment with flumazenil (P>0.05). In addition, compound Valerianae Jatamansi Jones treatment didn’t affect the spontaneous activity in mice (P> 0.05).

Conclusions

The present study supports the hypothesis that compound Valeriana jatamansi Jones exert anxiolytic action but no sedative effects in mice and that this effect might be mediated by benzodiazepine receptors.

Expression of GABAρ receptors in the neostriatum: localization in aspiny, medium spiny neurons and GFAP-positive cells

GABAergic transmission in the neostriatum plays a central role in motor coordination, in which a plethora of GABA-A receptor subunits combine to modulate neural inhibition. GABAρ receptors were originally described in the mammalian retina. These receptors possess special electrophysiological and pharmacological properties, forming a characteristic class of ionotropic receptors. In previous studies, we suggested that GABAρ receptors are expressed in the neostriatum, and in this report we show that they are indeed present in all the calretinin-positive interneurons of the neostriatum. In addition, they are located in calbindin-positive interneurons and projection neurons that express the dopamine D(2) receptor. GABAρ receptors were also located in 30% of the glial fibrillary acidic protein-positive cells, and may therefore also contribute to gliotransmission. Quantitative reverse transcription-PCR suggested that the mRNAs of this receptor do not express as much as in the retina, and that GABAρ2 is more abundant than GABAρ1. Electrophysiological recordings in brain slices provided evidence of neurons expressing a cis-4-aminocrotonic acid-activated, 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid-sensitive ionotropic GABA receptor, indicating the presence of functional GABAρ receptors in the neostriatum. Finally, electron-microscopy and immunogold located the receptors mainly in perisynaptic as well as in extrasynaptic sites. All these observations reinforce the importance of GABAρ receptors in the neostriatum and contribute to the diversity of inhibitory regulation in this area.

The GABA(A)ρ receptors in hippocampal spontaneous activity and their distribution in hippocampus, amygdala and visual cortex

A bicuculline-resistant and TPMPA-sensitive GABAergic component was identified in hippocampal neurons in culture and in acute isolated brain slices. In both preparations, total GABAergic activity showed two inactivation kinetics: fast and slow. RT-PCR, in situ hybridization (ISH) and immunohistochemistry detected expression of GABAρ subunits. Immunogold and electron microscopy indicated that the receptors are mostly extrasynaptic. In addition, by RT-PCR and immunofluorescence we found GABAρ present in amygdala and visual cortex.

GABA(A) ρ receptor mechanisms in the rat amygdala and its role in the modulation of fear and anxiety

RATIONALE

Accumulating evidence for the presence of GABA(A) ρ receptors within the amygdala which differ from other members of the GABA(A) receptor family in both subunit composition and functional properties has been recently obtained.

OBJECTIVES

This work was conducted to study whether GABA(A) ρ receptors may have a putative role in the amygdaloid modulation of fear and anxiety.

RESULTS

It was found that the bilateral intra-amygdaloid administration (6-240 pmol/side) of (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid, a selective GABA(A) ρ receptor antagonist, reduced dose-dependently the exploration of the open arms of the elevated plus-maze without affecting locomotion and increased the plasma levels of corticosterone. In contrast, bicuculline in the dose range used (1.8-60 pmol/side) induced seizures, but had no effects on the exploration of the maze.

CONCLUSIONS

It is suggested that GABA(A) ρ receptors may have a role in the amygdaloid modulation of fear and anxiety.

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.

Differential expression of glycine receptor subunits in the rat basolateral and central amygdala

The amygdalar complex is a limbic structure that plays a key role in emotional processing and fear conditioning. Although inhibitory transmission in the amygdala is predominately GABA-ergic, neurons of the amygdala are also known to express glycine receptors. The subtype and function of these glycine receptors within the synaptic circuits of the amygdala are unknown. In this study, we have investigated the relative expression of the four major glycine receptor subunits (alpha1-3 and beta) in the rat basolateral (BLA) and central amygdala (CeA), using real-time PCR and protein biochemistry. We demonstrate that alpha1, alpha2, alpha 3, and beta subunits are all expressed in the BLA and CeA with alpha2 being the predominant alpha-subunit in both nuclei. Electrophysiological recordings from BLA and CeA neurons in acute brain slices indicated that differences in relative expression of these subunits were correlated with the pharmacological properties of native glycine receptors expressed on these neurons. We conclude that glycine receptors assembled in BLA neurons are largely alpha 1 beta-containing heteromultimers whereas receptors assembled in neurons of the central amygdala are primarily alpha 2 beta-, alpha 3 beta- or alpha 1 beta-containing heteromultimers, with a minor component of alpha2 or alpha 3 homomeric receptors also expressed.

Effects of acute and repeated restraint stress on GABA efflux in the rat basolateral and central amygdala

Stress can precipitate onset of multiple mood-related disorders, including depression. Examination of the neural basis of this phenomenon has highlighted the amygdala as a key component. Alterations in amygdalar activity and structure accompany various mood-related disorders, and interestingly, amygdalar morphology and behavior can be altered in animals subjected to repeated stress. Gamma-aminobutyric acid (GABA) transmission in the amygdala represents an important means by which information flow, activity, and function can be controlled; therefore, we determined the effects of acute and repeated restraint stress (RRS) on GABA efflux in the basolateral and central amygdalar complexes. In vivo microdialysis revealed that acute restraint stress increased GABA efflux in the basolateral amygdala, whereas central amygdala efflux remained unchanged. Animals subjected to prior repeated stress displayed no acute stress-mediated increases in GABA efflux in the basolateral amygdala, an event accompanied by no changes in basal GABA concentrations. Conversely, repeated restraint stress had no effect on GABA efflux or basal GABA levels in the CeA. Collectively, these data demonstrate that acute stress elicits unique and region-specific increases in GABA efflux in the rat amygdala, and that prior repeated stress differentially modifies this response.

GABAA receptors containing gamma1 subunits contribute to inhibitory transmission in the central amygdala

gamma-Aminobutyric acid (GABA) is the primary inhibitory transmitter in the mammalian brain. This inhibition is mediated by type A (GABA(A)) receptors that are pentameric proteins assembled from 14 different subunits. Although inhibitory synaptic transmission has been studied in the amygdala, the subunit composition of receptors present at different synapses is not well understood. In this study we examined the subunit composition of GABA(A) receptors at synapses in the basolateral and central amygdala. Using receptors expressed in HEK293 cells we first determined the pharmacology of receptors of different subunit compositions. We then used this pharmacological profile to test the properties of receptors present at synapses in the central and basolateral amygdala. These results show that the GABA(A) receptor subunits are differentially distributed in the amygdala. Our data indicate that in the basolateral amygdala, GABAergic synapses are likely composed of receptors that contain alpha2betaxgamma2 subunits. In the central amygdala receptors at the medial input, carrying afferents from the bed nucleus of the stria terminalis contain similar receptors, whereas in the lateral input GABA receptors likely contain gamma1 subunits. These inputs arise from the intercalated cells masses, thought to be responsible for mediating extinction of conditioned fear, raising the possibility of new targets for the treatment of anxiety-related disorders.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Lack of feedback inhibition on rat basolateral amygdala following stress or withdrawal from sedative-hypnotic drugs

Previous research has demonstrated that suppression of inhibition in projection neurons of the basolateral complex of the amygdala (BLA) represents an essential mechanism underlying the emergence of negative emotional responses, including exaggerated fear and anxiety. The present work evaluates inhibitory postsynaptic potentials (IPSPs) in pyramidal projection neurons of the BLA in rats subjected to either diazepam or ethanol withdrawal or uncontrollable stress. These are experimental paradigms conducive to a negative emotional state. In slices containing the BLA, IPSPs were studied using whole-cell patch clamp. In control animals, a small IPSP was evoked by sub-threshold stimulation of the external capsule. When an action potential (AP) was evoked by supra-threshold stimuli, IPSPs were considerably larger; these IPSPs were sensitive to blockade of GABA(A) receptors by picrotoxin. However, IPSPs were clearly reduced in diazepam- or ethanol-withdrawn and in stressed rats. Firing of an AP by a depolarizing pulse applied through the patch pipette consistently evoked an inhibitory postsynaptic current (IPSC) in the pyramidal neurons of control animals from all three experimental models; these IPSCs were mediated by GABA(A) receptor activation and were blocked after suppression of glutamatergic transmission. In contrast, no IPSCs were observed in slices from diazepam- or ethanol-withdrawn or stressed animals, although the depolarizing pulse regularly evoked an AP in pyramidal neurons. It is concluded that, in withdrawn or stressed rats, GABAergic disinhibition occurs due to attenuation or suppression of feedback inhibition.

Involvement of 5-HT1A and GABAA receptors in the anxiolytic-like effects of Cinnamomum cassia in mice

An elevated plus maze (EPM) test was used to determine if the 5-HT1A, GABAA, and benzodiazepine receptors play a role in the anxiolytic-like effects of a 50% EtOH extract of Cinnamomum cassia (C. cassia) in mice. A single treatment with C. cassia (750 mg/kg, p.o.) significantly increased the number of entries into and the time spent in the open arms of the EPM compared with the controls. A repeated treatment with C. cassia (100 mg/kg, 5 days, p.o.) significantly increased the time spent in the open arms of the EPM. Moreover, WAY 100635, (+)-bicuculline, and flumazenil blocked the effect of C. cassia. However, there were no changes in the locomotor activity and horizontal wire test observed in any group compared with the controls. Taken together, these results show that C. cassia has no adverse effects, such as myorelaxant effects, and might be an effective anxiolytic agent by regulating the serotonergic and GABAergic system.

Neurosteroid modulation of GABAergic neurotransmission in the central amygdala: a role for NMDA receptors

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone or ALLO) positively modulates GABA(A) receptors, an action that may contribute to the anxiolytic effects of ALLO. Recent evidence suggests that ALLO's anxiolytic effects appear to be mediated by the amygdala, a key neural structure for emotional and cognitive behaviors. However, little is known regarding ALLO effects on amygdala physiology. We therefore explored ALLO effects on GABA neurotransmission in the central nucleus (Ce) of the amygdala, a major output nucleus involved in fear and anxiety. We recorded evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) in Ce neurons using whole-cell patch clamp techniques. We observed that ALLO significantly reduced the amplitude of evoked GABA(A) receptor-mediated IPSCs. However, the effect of ALLO was occluded by the NMDA receptor antagonist D-APV. D-APV alone also reduced evoked IPSCs in Ce neurons. These results suggest that ALLO-induced reduction of GABAergic transmission in Ce appears to depend on neural network activity, possibly involving an NMDA receptor-mediated mechanism. These ALLO effects on GABAergic transmission in the central amygdala may play a role in mediating its anxiolytic actions.

Effect of beta-lactotensin on acute stress and fear memory

beta-Lactotensin (beta-LT) is a bioactive peptide derived from bovine milk beta-lactoglobulin and is a natural ligand for neurotensin receptors. We examined the effect of beta-LT on restraint stress and fear memory in mice. Mice subjected to acute restraint stress exhibited a decreased number of head-dips and increased head-dip latency compared to non-stressed controls in the hole-board test, reflecting increased stress-induced behaviors. However, prior administration of beta-LT improved the behaviors caused by stress. The anti-stress effect of beta-LT was blocked by levocabastine, a neurotensin receptor subtype 2 (NTR2) antagonist. In the fear-conditioning test, the duration of freezing responses by cued fear conditioning was significantly reduced in mice administered beta-LT compared with control mice. These results suggest that beta-LT has an anti-stress effect and promotes the extinction of fear memory, which may be mediated by NTR2.

Autoradiographic localization and binding study of benzodiazepines receptor sites in carp brain (Cyprinus carpio L.)

This study demonstrates, for the first time, by both autoradiography and binding assay that [3H]Ro 15-1788 binds to carp brain with a high degree of anatomical selectivity. Saturation binding of the radioligand was determined in seven anatomically defined regions and suggested the presence of one class of binding sites (Type I-lke). In general, there was a good correlation between the autoradiographic and the binding data. By far, the optic tectum and the vagal, facial, and glossopharyngeal lobes showed the majority of [3H]Ro 15-1788 binding sites. Low to negative concentration of binding sites was detected in the cerebellum. The location of [3H]Ro 15-1788 binding sites in particular brain regions, indicates that benzodiazepine receptors could be associated with pathways involved in the control of basic central functions as spatial learning acquisition and retention, and feeding behaviour.

Ethanol modulation of GABAergic transmission: the view from the slice

For almost three decades now, the GABAergic synapse has been the focus of intense study for its putative role in mediating many of the behavioral consequences associated with acute and chronic ethanol exposure. Although it was initially thought that ethanol interacted solely with the postsynaptic GABAA receptors that mediate the majority of fast synaptic inhibition in the mammalian central nervous system (CNS), a number of recent studies have identified novel pre- and postsynaptic mechanisms that may contribute to the acute and long-term effects of ethanol on GABAergic synaptic inhibition. These mechanisms appear to differ in a brain region specific manner and may also be influenced by a variety of endogenous neuromodulatory factors. This article provides a focused review of recent evidence, primarily from in vitro brain slice electrophysiological studies, that offers new insight into the mechanisms through which acute and chronic ethanol exposures modulate the activity of GABAergic synapses. The implications of these new mechanistic insights to our understanding of the behavioral and cognitive effects of ethanol are also discussed.

Amygdaloid GABA, not glutamate neurotransmission or mRNA transcription controls footshock-associated fear arousal in the acoustic startle paradigm

In Pavlovian conditioning the fear-evoking properties of the aversive unconditioned stimulus are represented by the conditioned stimulus. A major challenge for theories of classical fear conditioning has been to understand how associations are formed between a conditioned stimulus and unconditioned stimulus. Although the cellular mechanisms in the amygdala that underlie fear learning have received considerable attention relatively little is known about the neural substrates underlying unconditioned stimulus-associated fear. In the present study we examined the role of GABA(A), N-methyl-D-aspartic acid and non-N-methyl-D-aspartic acid receptors, and protein synthesis inhibition on the immediate fear arousal produced by footshock as measured by the shock sensitization of acoustic startle. Laboratory rats showed shock-enhanced startle after infusion into the basolateral amygdala of the N-methyl-D-aspartic acid receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (5.0 microg), the non-N-methyl-D-aspartic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione disodium (5.0 microg) and the protein synthesis inhibitor anisomycin (80.0 microg). We concluded that fear arousal provoked by footshock is not mediated by glutamate neurotransmission in the amygdala and does not involve de novo protein synthesis. Bilateral infusion into the basolateral amygdala of the GABA(A) receptor agonist muscimol in doses ranging from 0.001-0.5 microg reliably blocked the shock sensitization of acoustic startle responding. None of the muscimol doses altered shock reactivity amplitudes indicating the normal perception of footshock. The muscimol results were interpreted to suggest that decreased GABA neurotransmission in the amygdala may be essential for the neural causation of fear that is acquired and expressed by conditioned stimuli.

Benzodiazepine involvement in LTP of the GABA-ergic IPSC in rat hippocampal CA1 neurons

Benzodiazepine binding sites are present on gamma-aminobutyric acid (GABA) receptors in hippocampal neurons. Diazepam is known to potentiate the amplitude and prolong the decay of GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs). In this study, benzodiazepine involvement in long-term potentiation (LTP) of the IPSC was examined. Whole-cell recordings of IPSCs were made from rat hippocampal CA1 neurons in a slice preparation. LTP was induced by a tetanic stimulation in the stratum radiatum (2 trains of 100 Hz for 1 s, 20 s inter-train interval) while pharmacologically blocking ionotropic glutamate receptors. During LTP, the amplitude of the IPSCs was potentiated in the majority of neurons with the IPSC decay and shape unaffected. Diazepam (5 microM) potentiated the IPSC amplitude and prolonged the decay when applied before, but not during, LTP. In neurons in which LTP could not be induced by a tetanic stimulation, diazepam did not increase the amplitude of the pre-tetanic IPSC. Flumazenil, at a concentration (10 microM) that blocked the enhancement of the IPSC by applied diazepam, had no effect on the IPSC amplitude when applied before LTP induction but significantly decreased the IPSC when applied during LTP maintenance. The antagonist, when applied during the tetanic stimulation, did not block LTP, suggesting that benzodiazepine receptors do not participate in LTP induction. These results indicate that the maintenance of LTP of the IPSC involves (a) the release of endogenous benzodiazepine agonist(s) and/or (b) the participation of benzodiazepine binding sites on subsynaptic GABA(A) receptors.

Effects of the mGluR8 agonist (S)-3,4-DCPG in the lateral amygdala on acquisition/expression of fear-potentiated startle, synaptic transmission, and plasticity

The lateral amygdala plays an important role in emotional learning. Previous studies found that amygdaloid plasticity processes involve the activation of metabotropic glutamate receptors. In the present study we examined the effect of the highly specific mGluR8 agonist (S)-3,4-DCPG on conditioned fear in vivo measuring fear-potentiated startle. Both, acquisition and expression of conditioned fear were dose-dependently inhibited by (S)-3,4-DCPG injections into the amygdala. Since synaptic long-term potentiation in the lateral amygdala has been correlated with the acquisition of conditioned fear in rats, the effect of (S)-3,4-DCPG in vitro on synaptic transmission, short- and long-term plasticity in the lateral amygdala was evaluated in parallel. Patch clamp recordings in rat brain slices revealed that (S)-3,4-DCPG strongly attenuated synaptic transmission from sensory afferents. The lack of detectable effects on postsynaptic neurons and altered short-term plasticity indicate that (S)-3,4-DCPG acts at presynaptic sites. Long-term potentiation of thalamic afferent fiber synapses induced by a pairing protocol was slightly attenuated in the presence of (S)-3,4-DCPG, but long-term potentiation by tetanic afferent stimulation was inhibited. We conclude that mGluR8 activation is not specifically involved in long-term plasticity processes but that it rather provides a powerful inhibitory control of synaptic transmission within the lateral amygdala, with the ability to reduce activity in such a way that the expression and the acquisition of learned fear become strongly impaired in vivo.

Differential expression of GABAA receptor subunits in the distinct nuclei of the rat amygdala

Detailed knowledge of the anatomical distribution of different GABA(A) receptor subunits is crucial for understanding the physiological actions of GABA in individual brain areas and for developing drugs acting through the individual GABA receptor subtypes. Since the amygdala is a key brain structure in the processing of emotional information with distinct functions in each nucleus, GABA(A) receptors in the amygdala are an important target of treatment for emotional disorders. In this study, we analyzed by quantitative RT-PCR the expression levels of all GABA(A) receptor subunits in distinct nuclei of the amygdala, the central (Ce) and the lateral/basolateral (LA/BLA) amygdala. We found the strongest expression of the gamma(2) subunit mRNA in both the Ce and LA/BLA, modest expressions of alpha(1), alpha(2) and alpha(3) mRNAs in the LA/BLA and alpha(2) and gamma(1) mRNAs in the Ce, and weak expressions of alpha(6), rho(2) and rho(3) mRNAs in both regions. We further revealed the significantly different expressions of alpha(1), alpha(3), alpha(5), gamma(1), gamma(2), delta, epsilon and theta subunit mRNAs in the Ce and LA/BLA. Differences in the expression levels of GABA(A) receptor subunits suggest different sensitivity to a variety of drugs including benzodiazepines and anesthetics in amygdala nuclei with distinct functions.

Inhibition of the central extended amygdala by loud noise and restraint stress

It is well established that the central nucleus of the amygdala (CEA) is involved in responses to stress, fear and anxiety. Many studies have used c-fos expression to map the brain's response to processive stress, but curiously the CEA generally is not highly activated. We have previously shown that exposure to a novel vs. home environment reduces amphetamine-induced activation of the lateral CEA (CEAl) and the oval nucleus of the bed nucleus of the stria terminalis (BSTov). This is consistent with the idea that processive stress inhibits neurons in these nuclei. We have tested this hypothesis by exposing rats to noise, at a range of intensities from non-stressful to stressful, or to restraint conditions, immediately after a remote injection of amphetamine, 2 mg/kg i.p., or interleukin-1beta (IL-1beta) 0.5 microg/kg i.p. (used to obtain a level of c-fos mRNA against which to measure inhibition). In keeping with our hypothesis, amphetamine- or IL-1beta-induced c-fos and zif-268 mRNA were significantly decreased in the CEAl and BSTov under conditions of loud noise or restraint stress compared with control conditions. This inhibition does not require a stress-induced rise in corticosterone because data were similar in animals that had been adrenalectomized with a low-dose corticosterone replacement. As both the CEAl and BSTov are highly gamma-aminobutyric acid (GABA) -ergic and project to the medial CEA (CEAm), their inhibition potentially causes an increased input to the CEAm. As the CEAm is a major output nucleus of the amygdala, this could have important consequences within the neural circuitry controlling responses to processive stress.

Heterogeneous GABAA receptor subunit expression in pediatric epilepsy patients

The gamma-amino-butyric acid type A receptors (GABAAR) are a heteropentameric receptor complex, composed of 16 possible subunits in various combinations, forming a ligand-gated ion channel. Subunit composition is the primary determinant of GABAAR physiology and pharmacology. Here we have measured mRNA levels for 16 GABAAR subunits in isolated dentate granule neurons (DGN) from eight pediatric patients undergoing resective surgery for intractable epilepsy. We found tightly correlated expression of a subset of GABAAR subunit mRNAs within a single DGN (alpha1, gamma1, and gamma2; alpha4, alpha5, and beta2; alpha4 and beta3). Analysis of inter-patient variability (ANOVA) of eleven highly expressed GABAAR subunit mRNAs found seven of the subunits varied between patients, as did whole cell GABAAR currents. Due to inter-patient differences, there is heterogeneity in DGN GABAAR subunit mRNA and physiology within pediatric epilepsy patients. Patient-specific GABAAR expression might contribute to variability in anti-epileptic drug efficacy, side-effect profiles, and seizure susceptibility.

Repeated stress alters caffeine action on motor coordination in C57Bl6/J male mice

This study was aimed to evaluate the effects of stress on caffeine action on motor coordination in mice. For 6 consecutive days, the mice were subjected to three different stressors. Saline or caffeine (30, 60 or 120 mg kg(-1)) was i.p. administered after the last stressful experience, then the animals were behaviorally tested in the holeboard. Their stumbling frequency was compared to that of unstressed mice injected with either saline or caffeine. (1) There was a strong trend for stress to impair motor coordination. (2) In unstressed mice, caffeine induced a linear dose-dependent increase of stumbling frequency. (3) Stress decreased the stumbling frequency induced by the highest dose of caffeine. The results are discussed in terms of interaction of stress and caffeine on dopaminergic and GABAergic systems.

Differential actions of diazepam and zolpidem in basolateral and central amygdala nuclei

Benzodiazepines are among the most widely prescribed therapeutic agents, having anxiolytic, anticonvulsant, sedative/hypnotic, and amnestic properties (Mehta and Ticku, Brain Res. Rev. 29 (1999) 196). Recent research indicates that these disparate actions are dissociable (Nature 401 (1999) 796; Science 290 (2000) 131; Kralic et al., Neuropharmacology 43 (2002) 685). Behavioral studies indicate that the amygdala plays a critical role in the anxiolytic effect of benzodiazepines (Nagy et al., Neuropharmacology 18 (1979) 573; The amygdala: anxiety and benzodiazepines. The Amygdala: a Functional Analysis. p. 195). However, the neuronal substrates of this anxiolytic effect remain unclear. Our study characterizes the physiological response to acute application of the benzodiazepine diazepam and the non-benzodiazepine sedative zolpidem using whole cell patch recording in two discrete amygdala subnuclei. We found that acute application of diazepam enhances GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) with equal potency in the basolateral (BL) and central (Ce) amygdala subnuclei. However, zolpidem enhanced IPSCs with similar potency only in the BL, and was effective in the Ce only at high concentrations. This finding is in agreement with histochemical data regarding the localization of GABA(A) receptor isoforms in the amygdala (J. Comp. Neurol. 359 (1995) 154; Brain Res. 964 (2003) 91) and suggests that anxiolytic effects of allosteric modulators of the GABA(A) receptor may be further dissociated from their hypnotic/sedative effects.

Pre- and postsynaptic contribution of GABAC receptors to GABAergic synaptic transmission in rat collicular slices and cultures

The mammalian superior colliculus (SC) is reported to contain gamma-aminobutyric acid (GABA)C receptors (GABACRs) at high concentration. However, their role in GABAergic synaptic transmission is not yet known. The aim of the present study was: (i) to clarify whether GABACRs are activated by endogenous GABA; and (ii), to determine whether GABACRs play a role in inhibitory synaptic transmission. Experiments were performed on acute horizontal slices from the postnatal rat SC or on collicular neurons in dissociated cell culture. In both preparations, bicuculline-resistant current responses to exogenous GABA and currents elicited by cis-4-aminocrotonic acid (CACA) were blocked by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA), a GABACR antagonist. The CACA-induced currents exhibited a linear current-voltage relationship and reversed at the Cl- equilibrium potential. These results indicate that functional GABACRs are present in the somato-dendritic membrane of collicular neurons. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the whole-cell patch clamp technique. TPMPA significantly decreased mIPSC amplitudes in slices, but not in cultured neurons. As TPMPA decreased also the coefficient of variation of mIPSCs, we suggest that somatodendritic GABACRs are located extrasynaptically but can be involved in the generation of IPSCs if GABA diffusion is constrained. In cultures, individual connections were activated by focal electrical stimulation of single neurons, and evoked inhibitory postsynaptic currents (eIPSCs) were recorded. Paired-pulse stimulation revealed that TPMPA significantly decreased the paired-pulse ratio at short (50 ms) interstimulus intervals, and this effect was inversely dependent on the amplitude of the first eIPSC. We conclude that presynaptic GABACRs are activated by endogenous GABA and can alleviate the short-term depression resulting from a preceding episode of GABA release. Thus, in GABAergic synapses of the SC GABACRs are involved in pre- and postsynaptic functions and may therefore contribute to the activity-dependent adjustment of GABAergic inhibition.

The amygdaloid complex: anatomy and physiology

A converging body of literature over the last 50 years has implicated the amygdala in assigning emotional significance or value to sensory information. In particular, the amygdala has been shown to be an essential component of the circuitry underlying fear-related responses. Disorders in the processing of fear-related information are likely to be the underlying cause of some anxiety disorders in humans such as posttraumatic stress. The amygdaloid complex is a group of more than 10 nuclei that are located in the midtemporal lobe. These nuclei can be distinguished both on cytoarchitectonic and connectional grounds. Anatomical tract tracing studies have shown that these nuclei have extensive intranuclear and internuclear connections. The afferent and efferent connections of the amygdala have also been mapped in detail, showing that the amygdaloid complex has extensive connections with cortical and subcortical regions. Analysis of fear conditioning in rats has suggested that long-term synaptic plasticity of inputs to the amygdala underlies the acquisition and perhaps storage of the fear memory. In agreement with this proposal, synaptic plasticity has been demonstrated at synapses in the amygdala in both in vitro and in vivo studies. In this review, we examine the anatomical and physiological substrates proposed to underlie amygdala function.

The pharmacology of spontaneously open alpha 1 beta 3 epsilon GABA A receptor-ionophores

Human alpha(1)beta(3) epsilon GABA(A) receptors were expressed in Xenopus oocytes and examined using the conventional two-electrode voltage-clamp technique and compared to alpha(1)beta(3)gamma(2) receptors. The effects of several GABA(A) agonists were studied, and the allosteric modulation of the channel by a number of GABAergic modulators investigated. The presence of the epsilon subunit increased the potency and efficacy of direct activation by partial GABA(A) agonists (piperidine-4-sulphonic acid and thio-4-PIOL), pentobarbital and neuro-steroids. Direct activation by 3-hydroxylated neurosteroids was restricted to 3alpha epimers, while chirality at C5 was indifferent. The 3beta-sulfate esters of pregnenolone and dehydroepiandrosterone inhibited the spontaneous currents with efficacies higher, while bicuculline methiodide and SR 95531 did so lower than picrotoxin and TBPS. Furosemide, fipronil, triphenylcyanoborate and Zn(2+) blocked the spontaneous currents of alpha(1)beta(3) epsilon receptors with different efficacies. Flunitrazepam and 4'-chlorodiazepam inhibited the spontaneous currents with micromolar potencies. In conclusion, spontaneously active alpha(1)beta(3) epsilon GABA(A) receptors can be potentiated and blocked by GABAergic agents within a broad range of efficacy.

Cells in midline thalamus, central amygdala, and nucleus accumbens responding specifically to antipsychotic drugs

RATIONALE

Determining brain regions in which neuroleptic drugs of different types produce similar effects, especially where these effects are not shared with drugs lacking antipsychotic efficacy, provides evidence as to how and where the clinical effects of neuroleptic drugs are mediated.

OBJECTIVE

For this study, the pattern of expression of the protein Fos, a marker of cellular activation, was compared after administration of the typical neuroleptic haloperidol, the antipsychotic drug clozapine, and the atypical neuroleptic olanzapine, as well as the sedative drug diphenhydramine and the anxiolytic lorazepam.

METHODS

Animals (Sprague-Dawley rats, three per cohort) received intraperitoneal injections of haloperidol (1 mg/kg), clozapine (20 mg/kg), olanzapine (5 mg/kg), diphenhydramine hydrochloride (1 mg/kg), lorazepam (5 mg/kg) or vehicle (2% lactic acid, 1 ml/kg). Two hours after drug administration, animals were killed. Patterns of activated cells were observed by immunohistochemistry for Fos-like antibodies in regions previously suggested as responding to all antipsychotic drugs, including nucleus accumbens, central amygdala, and central medial thalamus. Cells staining for Fos were counted by semi-automated methods. RESULTS. A very similar pattern and number of Fos positive cells in nucleus accumbens, central amygdala, and central medial thalamus followed administration of each antipsychotic drug. The numbers of apparently activated cells were much greater following antipsychotic drug administration than after vehicle, with differences between each drug and vehicle being highly statistically significant in each region. Lorazepam produced apparent activation of cells of the central amygdala similar in degree and location but not identical in distribution to that of antipsychotic drugs. Diphenhydramine produced no apparent activation of cells in any of the sites tested.

CONCLUSION

Typical and atypical antipsychotic drugs shared a distinctive pattern of robust activation of cells in nucleus accumbens, central medial thalamus, and central amygdala. Antipsychotic drug-induced activation of amygdala was shared by lorazepam, but activation of thalamus and nucleus accumbens was much greater following antipsychotic drugs than following lorazepam. The pattern of activated cells may be relevant to the therapeutic actions of antipsychotic drugs.

Excitatory synaptic transmission in the lateral and central amygdala

The amygdala plays a major role in the acquisition and expression of fear conditioning. NMDA receptor-dependent synaptic plasticity within the basolateral amygdala has been proposed to underlie the acquisition and possible storage of fear memories. Here the properties of fast glutamatergic transmission in the lateral and central nuclei of the amygdala are presented. In the lateral amygdala, two types of neurons, interneurons and projection neurons, could be distinguished by their different firing properties. Glutamatergic inputs to interneurons activated AMPA receptors with inwardly rectifying current-voltage relations (I-Vs), whereas inputs to projection neurons activated receptors that had linear I-Vs, indicating that receptors on interneurons lack GluR2 subunits. Inputs to projection neurons formed dual component synapses with both AMPA and NMDA components, whereas at inputs to interneurons, the contribution of NMDA receptors was very small. Neurons in the central amygdala received dual component glutamatergic inputs that activated AMPA receptors with linear I-Vs. NMDA receptor-mediated EPSCs had slow decay time constants in the central nucleus. Application of NR2B selective blockers ifenprodil or CP-101,606 blocked NMDA EPSCs by 70% in the central nucleus, but only by 30% in the lateral nucleus. These data show that the distribution of glutamatergic receptors on amygdalar neurons is not uniform. In the lateral amygdala, interneurons and pyramidal neurons express AMPA receptors with different subunit compositions. Synapses in the central nucleus activate NMDA receptors that contain NR1 and NR2B subunits, whereas synapses in the lateral nucleus contain receptors with both NR2A and NR2B subunits.

Effects of benzodiazepines on frog ERG

Effect of flurazepam (water-soluble benzodiazepine) on the amplitude and time course of ERG waves was investigated in superfused frog eyecups (Rana ridibunda). Flurazepam (50 and 100 microM) had inhibitory effect on the b- and d-wave amplitude, which was not accompanied with significant changes in their implicit time. Flurazepam potentiated the depressant effect of GABA (2.5 and 5 mM) on the b- and d-wave amplitude. The inhibitory effect of flurazepam was not blocked by 50 microM bicuculline (BCC), (GABA(A) antagonist), although the blocker markedly potentiated the b- and d-wave amplitude. The suppressive effect of flurazepam on the b- but not d-wave amplitude was blocked by 100 microM BCC. Our results indicate existence of functional benzodiazepine regulatory sites on GABA(A) receptors in distal frog retina.

Contextual inhibitory gating of impulse traffic in the intra-amygdaloid network

New data on the organization of the intra-amygdaloid circuit is reviewed, beginning with the basolateral (BL) complex, the main input station of the amygdala for sensory afferents, and concluding with the central (CE) nucleus, an important source of projections to brain-stem structures mediating fear responses. The BL complex is endowed with a highly divergent system of intrinsic glutamatergic connections. Yet, BL projection cells have unusually low firing rates. This apparent contradiction is explained by the presence of powerful inhibitory pressures in the BL amygdala: (1) interneurons that generate large-amplitude inhibitory synaptic potentials and (2) projection cells that express a Ca(2+)-dependent K(+) current that can be activated by subthreshold synaptic inputs. Likewise, excitatory projections from the BL amygdala to the CE nucleus are controlled by clusters of GABAergic neurons, termed the intercalated (ITC) cell masses. In response to BL inputs, ITC cells generate feedforward inhibition in CE neurons. However, ITC neurons exhibit properties that allow them to modify the amount of inhibition they generate depending on the distribution of BL activity in space and time. Indeed, ITC cell masses can inhibit each other via lateromedial connections. Moreover, they express an unusual K(+) conductance that modifies their response to BL inputs depending on their recent firing history. Thus, inhibitory mechanisms of the amygdala allow for flexible, context-dependent gating of BL impulses to the CE nucleus.

Relative picrotoxin insensitivity distinguishes ionotropic GABA receptor-mediated IPSCs in hippocampal interneurons

Inhibitory GABAergic signalling in the hippocampus plays an important role in synchronizing principal cells and regulating the excitability of this seizure-prone structure. Distinct mechanisms modulate release from GABAergic terminals in the hippocampus, depending on whether the postsynaptic partner is an interneuron or a principal cell. Here, we report that postsynaptic ionotropic GABA receptors in principal cells and interneurons also show a striking pharmacological difference. The broad-spectrum antagonist picrotoxin (PTX) was less potent at blocking IPSCs evoked in stratum radiatum interneurons than in pyramidal neurons in the CA1 region. GABA-evoked currents in membrane patches from interneurons showed a smaller mean unitary conductance than in patches from pyramidal neurons. Because retinal GABA(C) receptors show decreased picrotoxin sensitivity and conductance, we examined the effect of the GABA(C) receptor agonist cis-aminocrotonic acid (CACA). Although this agent evoked picrotoxin-resistant currents in interneurons, these were enhanced by the GABA(A) allosteric modulator pentobarbital. Moreover, both picrotoxin-resistant IPSCs and CACA-evoked currents were blocked by the GABA(A) receptor-selective antagonist bicuculline. The presence of relatively picrotoxin-resistant GABA(A) receptors in interneurons provides a potential target for agents to modulate the activity of sub-populations of hippocampal neurons.

Glutamate-induced gamma oscillations in the dentate gyrus of rat hippocampal slices

In this paper we show that gamma oscillations can be elicited by brief (< or = 200 ms) local applications of glutamate in the dentate gyrus of rat hippocampal slices. Dentate gamma oscillations show an initial peak frequency of approximately 70 Hz and last for up to 4 min. The network activity involves functional GABA(A) receptors as it is drastically reduced by GABA(A) receptor antagonists. The oscillations can be observed in the whole dentate gyrus-CA3-network and are coherent between the dentate gyrus and area CA3 for variable periods. Thus, long-lasting gamma oscillations can be experimentally induced in the dentate gyrus and are propagated into the hippocampus proper.

Role of NMDA, non-NMDA, and GABA receptors in signal propagation in the amygdala formation

Although the synaptic physiology of the amygdala has been studied with single neuron recordings, the properties of the networks between the various nuclei have resisted characterization because of the limitations of field recording in a neuronally diffuse structure. We addressed this issue in the rat amygdala complex in vitro by using a photodiode array coupled with a voltage-sensitive dye. Low-intensity single pulse stimulation of the lateral amygdala nucleus produced a complex multi-phasic potential. This signal propagated to the basolateral nucleus and the amygdalostriatal transition zone but not to the central nucleus. The local potential, which depended on both synaptic responses and activation of voltage-dependent ion channels, was reduced in amplitude by the non-N-methyl-D-aspartate (non-NMDA) glutamate receptor antagonist 6,7-dinitroquinoxaline (DNQX) and reduced to a lesser extent by the NMDA glutamate receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV). We next characterized the less complex signals that propagated to more distal regions with or without the addition of the GABA receptor antagonist bicuculline (BIC). BIC alone greatly increased the signal propagation and permitted activation of previously silent areas within the amygdala. DNQX blocked signal propagation to amygdala regions outside of La, even in the presence of BIC, whereas D-APV had minimal effects on these distal signals. These data represent several novel findings: the characterization of the multi-component potential near the site of stimulation, the gating of signal propagation within the amygdala by GABAergic inhibition, the critical role of non-NMDA receptor-mediated depolarization in signal propagation, and the lack of a role for NMDA receptors in maintaining propagation.

Pathway-specific targeting of GABA(A) receptor subtypes to somatic and dendritic synapses in the central amygdala

Neurons in the central amygdala express two distinct types of ionotropic GABA receptor. One is the classical GABA(A) receptor that is blocked by low concentrations of bicuculline and positively modulated by benzodiazepines. The other is a novel type of ionotropic GABA receptor that is less sensitive to bicuculline but blocked by the GABA(C) receptor antagonist (1,2,5,6-tetrohydropyridine-4-yl) methylphosphinic acid (TPMPA) and by benzodiazepines. In this study, we examine the distribution of these two receptor types. Recordings of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) showed a wide variation in amplitude. Most events had amplitudes of < 50 pA, but a small minority had amplitudes >100 pA. Large-amplitude events also had rise times faster than small-amplitude events. Large-amplitude events were fully blocked by 10 microM bicuculline but unaffected by TPMPA. Small amplitude events were partially blocked by both bicuculline and TPMPA. Focal application of hypertonic sucrose to the soma evoked large-amplitude mIPSCs, whereas focal dendritic application of sucrose evoked small-amplitude mIPSCs. Thus inhibitory synapses on the dendrites of neurons in the central amygdala express both types of GABA receptor, but somatic synapses expressed purely GABA(A) receptors. Minimal stimulation revealed that inhibitory inputs arising from the laterally located intercalated cells innervate dendritic synapses, whereas inhibitory inputs of medial origin innervated somatic inhibitory synapses. These results show that different types of ionotropic GABA receptors are targeted to spatially and functionally distinct synapses. Thus benzodiazepines will have different modulatory effects on different inhibitory pathways in the central amygdala.

Heterogeneity of functional GABA(A) receptors in rat dentate gyrus neurons revealed by a change in response to drugs during the whole-cell current time-course

We examined if the drug sensitivity of GABA(A) receptors in dentate gyrus granule neurons changed during the whole-cell current time-course. Effects of drugs on currents evoked immediately (the peak current) upon drug application and currents remaining about two seconds later (semi-plateau current) were compared. The apparent affinity for GABA (EC(50)) of the peak and the semi-plateau current were 14 and 4 microM, respectively. Bicuculline inhibited 50% of the peak and the semi-plateau current (IC(50)) at 7 and 36 microM, respectively, while 100 microM was required for full inhibition of the 100 microM GABA-evoked current. Zinc inhibited about 50% of the peak current with an IC(50) value of 94 microM whereas biphasic, but complete inhibition of the semi-plateau current was recorded with IC(50) values of 3 and 558 microM. The decay phase of the 100 microM GABA-evoked current was fitted by a fast (tau(1), 100-300 ms) and a slow (tau(2), 1-2 s) time-constants in all cells. The relative current amplitude associated with the fast (A1) and the slow (A2) component varied. The A1 current amplitude appeared more sensitive to bicuculline than the A2 current while the opposite was true for zinc. The results are consistent with heterogenous population of functional GABA(A) receptors in the dentate gyrus granule neurons.