GABAA receptors mediate the changes produced by stress on GABA function and locomotor activity

Understanding the mechanism of action and clinical effects of neuroactive steroids and GABAergic compounds in major depressive disorder

The pathophysiology of major depressive disorder (MDD) is thought to result from impaired connectivity between key brain networks. Gamma-aminobutyric acid (GABA) is the key inhibitory neurotransmitter in the brain, working primarily via GABAA receptors, with an important role in virtually all physiologic functions in the brain. Some neuroactive steroids (NASs) are positive allosteric modulators (PAMs) of GABAA receptors and potentiate phasic and tonic inhibitory responses via activation of synaptic and extrasynaptic GABAA receptors, respectively. This review first discusses preclinical and clinical data that support the association of depression with diverse defects in the GABAergic system of neurotransmission. Decreased levels of GABA and NASs have been observed in adults with depression compared with healthy controls, while treatment with antidepressants normalized the altered levels of GABA and NASs. Second, as there has been intense interest in treatment approaches for depression that target dysregulated GABAergic neurotransmission, we discuss NASs approved or currently in clinical development for the treatment of depression. Brexanolone, an intravenous NAS and a GABAA receptor PAM, is approved by the U.S. Food and Drug Administration for the treatment of postpartum depression (PPD) in patients 15 years and older. Other NASs include zuranolone, an investigational oral GABAA receptor PAM, and PH10, which acts on nasal chemosensory receptors; clinical data to date have shown improvement in depressive symptoms with these investigational NASs in adults with MDD or PPD. Finally, the review discusses how NAS GABAA receptor PAMs may potentially address the unmet need for novel and effective treatments with rapid and sustained antidepressant effects in patients with MDD.

Anxiety, Mental Stress, and Sudden Cardiac Arrest: Epidemiology, Possible Mechanisms and Future Research

Sudden cardiac arrest (SCA) is a leading cause of mortality and morbidity in affluent societies, which underscores the need to identify persons at risk. The etiology of SCA is however complex, with predisposing and precipitating factors interacting. Although anxiety and mental stress have been linked to SCA for decades, their precise role and impact remain unclear and the biological underpinnings are insufficiently understood. In this paper, we systematically reviewed various types of observational studies (total n = 20) examining the association between anxiety or mental stress and SCA. Multiple methodological considerations challenged the summarizing and interpretation of the findings. For anxiety, the overall picture suggests that it predisposes for SCA in physically healthy populations (unadjusted OR = 2.44; 95% CI: 1.06-5.59; n = 3). However, in populations at risk for SCA (n = 4), associations were heterogeneous but not significant. Anxiety may partly predispose to SCA by contributing to other risk factors such as cardiovascular disease and diabetes mellitus via mechanisms such as unhealthy lifestyle and metabolic abnormalities. Mental stress appears to precipitate SCA, presumably by more directly impacting on the cardiac ion channels that control the heart's electrical properties. This may lead to ventricular fibrillation, the arrhythmia that underlies SCA. To advance this field of research, experimental studies that unravel the underlying biological mechanisms are deemed important, and most easily designed for mental stress as a precipitating factor because of the short timeframe. These proof-of-concept studies should examine the whole pathway from the brain to the autonomic nervous system, and eventually to cardiac ion channels. Ultimately, such studies may facilitate the identification of persons at risk and the development of novel preventive strategies.

Endothelial cell-derived GABA signaling modulates neuronal migration and postnatal behavior

The cerebral cortex is essential for integration and processing of information that is required for most behaviors. The exquisitely precise laminar organization of the cerebral cortex arises during embryonic development when neurons migrate successively from ventricular zones to coalesce into specific cortical layers. While radial glia act as guide rails for projection neuron migration, pre-formed vascular networks provide support and guidance cues for GABAergic interneuron migration. This study provides novel conceptual and mechanistic insights into this paradigm of vascular-neuronal interactions, revealing new mechanisms of GABA and its receptor-mediated signaling via embryonic forebrain endothelial cells. With the use of two new endothelial cell specific conditional mouse models of the GABA pathway (Gabrb3^ΔTie2-Cre and Vgat^ΔTie2-Cre), we show that partial or complete loss of GABA release from endothelial cells during embryogenesis results in vascular defects and impairs long-distance migration and positioning of cortical interneurons. The downstream effects of perturbed endothelial cell-derived GABA signaling are critical, leading to lasting changes to cortical circuits and persistent behavioral deficits. Furthermore, we illustrate new mechanisms of activation of GABA signaling in forebrain endothelial cells that promotes their migration, angiogenesis and acquisition of blood-brain barrier properties. Our findings uncover and elucidate a novel endothelial GABA signaling pathway in the CNS that is distinct from the classical neuronal GABA signaling pathway and shed new light on the etiology and pathophysiology of neuropsychiatric diseases, such as autism spectrum disorders, epilepsy, anxiety, depression and schizophrenia.

Acute stress effects on GABA and glutamate levels in the prefrontal cortex: A 7T 1H magnetic resonance spectroscopy study

There is ample evidence that the inhibitory GABA and the excitatory glutamate system are essential for an adequate response to stress. Both GABAergic and glutamatergic brain circuits modulate hypothalamus-pituitary-adrenal (HPA)-axis activity, and stress in turn affects glutamate and GABA levels in the rodent brain. However, studies examining stress-induced GABA and glutamate levels in the human brain are scarce. Therefore, we investigated the influence of acute psychosocial stress (using the Trier Social Stress Test) on glutamate and GABA levels in the medial prefrontal cortex of 29 healthy male individuals using 7 Tesla proton magnetic resonance spectroscopy. In vivo GABA and glutamate levels were measured before and 30 min after exposure to either the stress or the control condition. We found no associations between psychosocial stress or cortisol stress reactivity and changes over time in medial prefrontal glutamate and GABA levels. GABA and glutamate levels over time were significantly correlated in the control condition but not in the stress condition, suggesting that very subtle differential effects of stress on GABA and glutamate across individuals may occur. However, overall, acute psychosocial stress does not appear to affect in vivo medial prefrontal GABA and glutamate levels, at least this is not detectable with current practice ¹H-MRS.

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Psychosocial stress did not alter glutamate and GABA levels in the medial prefrontal cortex in healthy male individuals.

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Moreover, cortisol stress reactivity was not associated with medial prefrontal glutamate and GABA level change over time.

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Together, acute stress does not seem to affect in vivo medial prefrontal 7T MRI GABA and glutamate levels in humans.

Neurochemical Changes Associated with Stress-Induced Sleep Disturbance in Rats: In Vivo and In Vitro Measurements

The goal of this study was to quantitatively assess the changes in the cerebral neurochemical profile and to identify those factors that contribute to the alteration of endogenous biomolecules when rats are subjected to stress-induced sleep disturbance. We exposed Sprague-Dawley rats (controls: n = 9; stress-induced sleep perturbation rats: n = 11) to a psychological stressor (cage exchange method) to achieve stress-induced sleep perturbation. In vivo magnetic resonance imaging assessments were carried out using a high-resolution 9.4 T system. For in vivo neurochemical analysis, a single voxel was localized in the right dorsal hippocampal region, and in vivo spectra were quantified for 17 cerebral neurochemical signals. Rats were sacrificed upon completion of the magnetic resonance spectroscopy protocol, and whole-brain tissue was harvested from twenty subjects. The dopamine and serotonin signals were obtained by performing in vitro liquid chromatography-tandem mass spectrometry on the harvested tissue. In the right dorsal hippocampal region, the gamma-aminobutyric-acid (GABA) and glutamine (Gln) concentrations were significantly higher in the sleep-perturbed rats than in the sham controls. The ratios of Gln/Glu (glutamate), Gln/tCr (total-creatine), and GABA/Glu were also significantly higher in the sleep-perturbed group, while serotonin concentrations were significantly lower in the sleep-perturbed rats. Pearson correlation results among individual rat data indicate that concentrations of dopamine (DA) and serotonin (5-HT) were significantly higher in SSP rats. A larger correlation coefficient was also observed for the SSP rats. Analysis of the correlation between the in vivo and in vitro signals indicated that the concentrations of Gln, 5-HT, and DA exhibited a significant negative correlation in the SSP rat data but not in that of control rats. The authors propose that the altered and correlated GABA, Gln, 5-HT, and DA concentrations/ratios could be considered key markers of neurological function in animal models of stress-induced sleep perturbation.

Inhibition of the cortex using transcranial magnetic stimulation in psychiatric populations: current and future directions

Several lines of evidence suggest that deficits in γ-aminobutyric acid (GABA) inhibitory neurotransmission are implicated in the pathophysiology of schizophrenia, bipolar disorder, major depressive disorder and obsessive-compulsive disorder. Cortical inhibition refers to a neurophysiological process, whereby GABA inhibitory interneurons selectively attenuate pyramidal neurons. Transcranial magnetic stimulation (TMS) represents a noninvasive technique to measure cortical inhibition, excitability and plasticity in the cortex. These measures were traditionally specific to the motor cortex, which is an important limitation when nonmotor neurophysiological processes are of primary interest. Recently, TMS has been combined with electro encephalography (EEG) to derive such measurements directly from the cortex. This review focuses on neurophysiological studies related to inhibitory and excitatory TMS paradigms, linking dysfunctional GABAergic neurotransmission to disease states. We review evidence that suggests cortical inhibition deficits among psychiatric populations and demonstrate how each disorder has a specific neurophysiological response to treatment. We conclude by discussing the future directions of TMS combined with EEG, demonstrating the potential to identify biological markers of neuropsychiatric disorders.

Amphetamine differentially modifies the expression of monoaminergic and GABAergic synaptic boutons and processes in lateral habenula, dorsal and ventral hippocampal formation

The habenular complex is thought to be associated with cognitive functions and indirectly connected with the hippocampal formation (HF). Thus the responses of the monoaminergic and GABAergic neurons were examined in both structures to the psychostimulant, amphetamine (Amph). Immunocytochemical analysis was performed on brain sections prepared from adult mice treated with a single or multiple (2 doses/day, 7 doses in total) injections of saline or Amph, 5mg/kg. The synaptic boutons were verified by immuno-electron microscopy specific for parvalbumin (PV), glutamic acid decarboxylase(67) (GAD(67)), aromatic amino acid decarboxylase (AADC) or dopamine-β-hydroxylase (DBH). In the lateral part of the lateral habenula (LHb), at 4h post-acute Amph, the densities of PV-positive boutons/processes and DBH-boutons were decreased by approximate 75% and 72% respectively, compared with corresponding saline-controls; however, at 4h post-repeated Amph exposure, PV was increased by 244%, and DBH unaltered. In the dorsal HF (DHF), at 4h post-repeated Amph exposure, GAD(67)-boutons and PV resembled controls in CA1 and CA3 pyramidal cell layers, whereas in the granule cell layer of dentate gyrus (DG), PV was increased by 112%, and GAD(67) unchanged. As shown by biochemical methods, at 4h post-repeated Amph, the decreased level of DHF GABA probably correlates with the immunocytochemical changes. In the ventral HF (VHF), at 4h post-repeated Amph treatment, PV and the enzymes of CA1 and DG were unaltered, while CA3 PV was decreased by 63%, and AADC-boutons increased 55%. Double immuno-electron microscopy revealed synaptic contacts between PV and GAD(67) containing presynaptic or postsynaptic elements, and between PV or GAD(67) and DBH or AADC. This ultrastructural evidence may support the functional significance of the Amph-induced differential changes, which could reflect Amph toxicity and distinct characteristics of the LHb, DHF and VHF.

Discovering imaging endophenotypes for major depression

Psychiatry research lacks an in-depth understanding of mood disorders phenotypes, leading to limited success of genetics studies of major depressive disorder (MDD). The dramatic progress in safe and affordable magnetic resonance-based imaging methods has the potential to identify subtle abnormalities of neural structures, connectivity and function in mood disordered subjects. This review paper presents strategies to improve the phenotypic definition of MDD by proposing imaging endophenotypes derived from magnetic resonance spectroscopy measures, such as cortical gamma-amino butyric acid (GABA) and glutamate/glutamine concentrations, and from measures of resting-state activity and functional connectivity. The proposed endophenotypes are discussed regarding specificity, mood state-independence, heritability, familiarity, clinical relevance and possible associations with candidate genes. By improving phenotypic definitions, the discovery of new imaging endophenotypes will increase the power of candidate gene and genome-wide associations studies. It will also help to develop and evaluate novel therapeutic treatments and enable clinicians to apply individually tailored therapeutic approaches. Finally, improvements of the phenotypic definition of MDD based on neuroimaging measures will contribute to a new classification system of mood disorders based on etiology and pathophysiology.

Evidence for GABAergic inhibitory deficits in major depressive disorder

Converging evidence suggests that deficits in gamma-aminobutyric acid (GABA) functioning are implicated in the pathophysiology of major depressive disorder (MDD). This is highlighted by research investigating cortical inhibition (CI), a process whereby GABAergic interneurons selectively attenuate pyramidal neurons. Transcranial magnetic stimulation (TMS) paradigms evaluate this marker of neuronal inhibitory activity in the cortex. This review will examine the neuroanatomic and neurophysiological evidence from neuroimaging, molecular, treatment, and TMS studies linking dysfunctional GABAergic neurotransmission to MDD.

Effect of acute psychological stress on prefrontal GABA concentration determined by proton magnetic resonance spectroscopy

OBJECTIVE

Impaired function of the central gamma-aminobutyric acid (GABA) system, which provides the brain's major inhibitory pathways, is thought to play an important role in the pathophysiology of anxiety disorders. The effect of acute psychological stress on the human GABA-ergic system is still unknown, however. The purpose of this study was to determine the effect of acute stress on prefrontal GABA levels.

METHOD

A recently developed noninvasive magnetic resonance spectroscopy method was used to measure changes in the GABA concentration of the prefrontal cortex in 10 healthy human subjects during a threat-of-shock condition and during a safe condition (two sessions on different days). The main outcome measure was the mean GABA concentration within a 3×3×2-cm(3) voxel selected from the medial prefrontal cortex.

RESULTS

Prefrontal GABA decreased by approximately 18% in the threat-of-shock condition relative to the safe condition. This reduction was specific to GABA, since the concentrations of N-acetyl-aspartate, choline-containing compounds, and glutamate/glutamine levels obtained in the same spectra did not change significantly.

CONCLUSIONS

This result appeared compatible with evidence from preclinical studies in rodents, which showed rapid presynaptic down-regulation of GABA-ergic neurotransmission in response to acute psychological stress. The molecular mechanism and functional significance of this reduced inhibitory effect of acute psychological stress in relation to impaired GABA-ergic function in anxiety disorders merit further investigation.

Differential alterations in the relations among GABAergic, catecholaminergic and calcium binding protein expression in the olfactory bulb of amphetamine-administered mouse

This study explores cellular responses of distinct layers of the main olfactory bulb (OB) to amphetamine (Amph), by examining the expression of glutamic acid decarboxylase67 (GAD67), calcium binding proteins (CaBP) and tyrosine hydroxylase (TH). Immunocytochemical analysis was performed on OB sections prepared from adult mice at 0.5 h or 4 h after receiving one intraperitoneal injection or multiple (2 doses/day, 7 doses in total) injections of saline or Amph, 5 mg/kg. In the glomerular layer, though the expression of TH and GAD67 was unaltered by the single Amph injection, at 0.5 h post-repeated Amph exposure the levels of TH-immunopositive somata and processes/punctates, and GAD67-somata/punctates were increased by 48-147%, compared with respective saline controls. By contrast, at 4 h post-repeated Amph GAD67 levels were lower than saline, and TH similar to saline. For the repetitively saline-injected groups, TH and GAD67 levels were higher at 4h than 0.5 h, suggesting an injection-associated stress response. Double staining revealed that at 0.5h post-repeated Amph exposure, the percentage of TH-soma number that expressed GAD67 was raised to 46%, compared with 30% of the corresponding saline, and thus implies an activation of dopaminergic neurons to become GABAergic. In the external plexiform layer, the numbers of CaBP, parvalbumin or calretinin-somata were increased at 0.5 h/4 h or 4 h post-acute Amph injection; double staining disclosed that at 4 h post-acute Amph, 66% or 47% of GAD67-somata contained parvalbumin or calretinin, being greater than 43% or 28% of the saline. In the granule somata, Amph probably inhibits expression of GAD67 by decreasing phosphorylation of CREB (pCREB). The up-regulation of CaBPs, GAD67 and TH at 0.5/4 h post-acute or 0.5 h post-repeated Amph could implicate protective roles and synaptic plasticity against Amph, whereas decreases of GAD67 and pCREB at 4 h post-repeated Amph may indicate toxicity of Amph.

Sex differences in neurotransmission parameters in response to repeated mild restraint stress exposures in intact male, female and ovariectomised female rats

The present study determined whether a repeated mild restraint stress exposure would differentially alter neuronal activity in male and female rats to gain insights into neurobiological substrates involved in sex differences in stress-induced behavioural responses. In our first set of experiments, we used Western blot analysis to determine whether alterations in several synaptic proteins were elicited by the repeated stress treatment. We found bidirectional changes in synaptophysin levels in female cerebral cortex and hippocampus that diverged between intact and ovariectomised females. There were persistent elevations in spinophilin levels in the male, but not female, hippocampus following the repeated mild restraint stress exposure. By contrast, levels of the NMDA receptor scaffolding protein, PSD-95, were altered only in intact female cerebral cortex and ovariectomised female hippocampus. We next used immunohistochemical evaluation of Fos expression as a marker for neuronal activation. We found significant increases in Fos immunoreactivity in all sex conditions across multiple brain regions in response to the repeated mild stress. Fos protein induction was greatest in the frontal cortex, piriform cortex and amygdala, with the degree of induction varying by sex condition. Fos induction was dramatically higher in amygdala and piriform cortex only in intact females following repeated stress compared to a single restraint stress exposure, suggestive of sensitisation rather than habituation. By contrast, the frontal cortex of intact and ovariectomised females showed habituation to the repeated stressor. Males displayed modest sensitisation in both the frontal cortex and dentate gyrus with no changes in other brain areas. Taken together, these findings show that exposure to a mild repeated stress results in sex differences in synaptic adaptations and patterns of brain activation that likely contribute to observed sex differences in stress-induced behaviours. This approach provides valuable insights into interactions between the hormonal milieu and responses to a repeated mild stress, and further supports the importance of considering hormonal status in treatment of stress-related disorders.

Evidence for impaired cortical inhibition in patients with unipolar major depression

BACKGROUND

Several lines of evidence suggest that central cortical inhibitory mechanisms, especially associated with gamma-aminobutyric acid (GABA) neurotransmission, may play a role in the pathophysiology of major depression. Transcranial magnetic stimulation is a useful tool for investigating central cortical inhibitory mechanisms associated with GABAergic neurotransmission in psychiatric and neurological disorders.

METHODS

By means of transcranial magnetic stimulation, different parameters of cortical excitability, including motor threshold, the cortical silent period, and intracortical inhibition/facilitation, were investigated in 20 medication-free depressed patients and 20 age- and gender-matched healthy volunteers.

RESULTS

Silent period and intracortical inhibition were reduced in depressed patients, consistent with a reduced GABAergic tone. Moreover, patients showed a significant hemispheric asymmetry in motor threshold.

CONCLUSIONS

This study provides evidence of reduced GABAergic tone and motor threshold asymmetry in patients with major depression.

Repetitive transcranial magnetic stimulation of the dorsolateral prefrontal cortex and cortical excitability in patients with major depressive disorder

Repetitive transcranial magnetic stimulation (rTMS) of the dorsolateral prefrontal cortex is a relatively non-invasive technique with putative therapeutic effects in major depression. However, the exact neurophysiological basis of these effects needs further clarification. Therefore, we studied the impact of ten daily sessions of left, dorsolateral prefrontal rTMS on motor cortical excitability, as revealed by transcranial magnetic stimulation-elicited motor-evoked potentials in 30 patients. As compared to the non-responders, responders (33%) showed changes in parameters pointing towards a reduced cortical excitability. These results suggest that repetitive transcranial magnetic stimulation of the dorsolateral, prefrontal cortex may have inhibitory effects on motor cortical neuronal excitability in patients with major depressive disorder. Furthermore, measurement of motor cortical excitability may be a useful tool for investigating and monitoring inhibitory brain effects of antidepressant stimulation techniques like rTMS.

Differential effects of mild repeated restraint stress on behaviors and GABA(A) receptors in male and female rats

We previously reported that the very mild stress of individual housing influenced seizure risk and gamma-amino butyric acid (GABA(A)) receptor activity differentially between male and female rats. The aim of the present set of studies was to assess sex differences in behavioral responses to a more pronounced type of stressor, repeated restraint stress. We also wanted to determine the role of GABA(A) receptors in effects of this stressor. Our data suggest that repeated restraint stress afforded short-term protection against seizure induction in both male and female rats. Moreover, this protection was more persistent in female than male rats. This stress paradigm also elicited a reduction in general activity in male rats, whereas female rats displayed prolonged increased activity following the repeated restraint stress exposure. However, there were limited effects on anxiety-like behaviors, as determined by time spent in the open arms on the elevated plus maze. Sex differences in stress-induced increases in plasma corticosterone levels were observed, which generally correlated with sex differences in behavioral measures. There were no significant effects of the repeated restraint stress exposure on benzodiazepine/GABA(A) receptor density or affinity nor on receptor function. Taken together, these findings provide additional evidence to support the important influences of sex in responding to stress and highlight the need to consider this context when addressing the role of stress in health issues for women and men.

GABA and glutamate systems as therapeutic targets in depression and mood disorders

Advances made in diverse areas of neuroscience suggest that neurotransmitter systems, additional to the monoaminergic, contribute to the pathophysiology of mood disorders. This ever accruing body of preclinical and clinical research is providing increased recognition of the contribution made by amino acid neurotransmitters to the neurobiology of mood disorders. This review examines evidence supporting the role of GABA and glutamate in these processes and explores the potential to target these systems in the development of novel compounds; the viability of these agents for treatment-related co-morbidities will also be considered.

Clinical studies implementing glutamate neurotransmission in mood disorders

Emerging evidence suggests that the amino acid neurotransmitter systems are associated with the pathophysiology and treatment of mood disorders. Recent advances in the areas of molecular neurobiology, pharmacology, and magnetic resonance spectroscopy (MRS) now provide better tools to probe the function of the amino acid neurotransmitter systems and are affording us the opportunity to better investigate the relationship of these systems to mood disorders. Here we review the available literature in the field and suggest a possible pathophysiological model that may account for the many of the findings.

Mutation screen of the glutamate decarboxylase-67 gene and haplotype association to unipolar depression

Abnormally low concentrations of brain gamma-aminobutyric acid (GABA) have been reported in unipolar depression. Almost all of the brain GABA is synthesized by glutamate decarboxylase (GAD) enzymes (GAD67 and GAD65). These enzymes, therefore, play a central role in brain GABA homeostasis. We screened all the 17 exons of the GAD67 gene for mutations using single strand conformation polymorphism (SSCP) or denaturing high pressure liquid chromatography (dHPLC) in a sample of 43 individuals diagnosed with major unipolar depression or other disorders with putative GABAergic dysfunction. We identified eight novel variants (five synonymous base substitutions, two insertion/deletions and one tandem repeat). Three relatively common (minor allele frequency >20%) single nucleotide polymorphisms (SNPs), located in the 5' non-coding region (exon 0), intron 8, and the 3' non-coding region (exon 16) of the gene, were genotyped in 103 European-American (EA) subjects with depression and 125 EA psychiatrically screened controls. Linkage disequilibrium (LD) and haplotype frequencies were estimated using the 3LOCUS program. Significant LD was observed between the intron 8 SNP and the exon 16 SNP and between the exon 0 SNP and the exon 16 SNP. Three common GAD67 haplotypes were observed in this population, which accounted for >90% of the possible GAD67 three-locus haplotypes. Comparison of SNP and haplotype frequencies between individuals with depression and controls revealed no differences. These results demonstrate a significant within-gene LD for GAD67 in the EA population and begin to establish a haplotype map for this gene. Furthermore, these results suggest that common genetic variation within the GAD67 gene does not play a major role in the predisposition to unipolar depression.

Involvement of adrenal medulla grafts in the open field behavior

Immunohistochemical and behavioral techniques were used to study the effects of adrenal medulla grafts, implanted in striatum after bilateral kainic acid (KA) lesions of this structure, on the open field behavior of mice. KA-induced behavioral changes in leaning, grooming and locomotor activity of the open field test were significantly improved after grafting of the adrenal medulla, and in some respects, fully restored. Immunohistochemical identification showed that grafts contained neuron-like cells with a tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and enkephalin-like immunostainings. A likely interpretation of this complex pattern of results is that adrenal medullary grafts may restore the deficits of GABAergic neurons which in turn reverse the abnormalities in emotionality and locomotion. Neurobiologically, these behavioral improvements probably involve GABAergic and catecholaminergic factors of adrenal medulla grafts, although other neuroactive substances, such as acetylcholine and enkephalins, cannot be excluded.

Sensitivity to glucocorticoid-mediated fast-feedback regulation of the hypothalamic-pituitary-adrenal axis is dependent upon stressor specific neurocircuitry

Fos-protein immunoreactivity (Fos-IR) was used to identify neurocircuits potentially participating in the regulation of hypothalamic-pituitary-adrenal (HPA) axis sensitivity to glucocorticoid-mediated fast-feedback in rats exposed to the physical stressor, hemorrhage, or the psychological stressor, airpuff startle. Marked regional brain differences in the Fos-IR expression were observed in response to these stressors. Specifically, after hemorrhage, nuclear Fos-IR increased in the nucleus of the solitary tract and other brainstem regions known to regulate hemodynamic processes including the supraoptic nucleus, and the magnocellular division of hypothalamic paraventricular nucleus (PVN). In contrast, after airpuff startle Fos-IR increased in the dorsomedial and lateral hypothalamus as well as in the lateral septum. Thus, activation of brainstem neurocircuits predominated after hemorrhage whereas activation of forebrain neurocircuits predominated after airpuff startle. In other regions, the magnitude of stressor-induced Fos-IR expression varied in a region-specific manner. When stressor exposure was preceded by administration of corticosterone to achieve levels within the physiological range after stressors, HPA axis responses were suppressed in response to the airpuff startle but not to either a small or moderate hemorrhage.

IN CONCLUSION

(1) fast-feedback mediated inhibition of HPA axis activity is critically dependent upon stressor modality; (2) this apparent selectivity is reflected by differences in the nature of the neurocircuitry mediating these stressors. It is suggested that determination of the central actions of glucocorticoids in mediating fast-feedback regulation of the HPA axis requires evaluation of the interactions between activated glucocorticoid receptors and intracellular signaling cascades evoked by convergent neuronal input.

Inhibition of the LH Surge by Restraint Stress in Cyclic Rats: Studies on the Role of GABAA and GABAB Receptors

There is evidence that stress can alter the activity in the brain of gamma-aminobutyricacid (GABA), a neurotransmitter that has been implicated in the regulation of LH secretion. In the present study the role of GABA in the restraint stress-induced inhibition of the LH surge was investigated in the intact cyclic rat. Intracerebroventricular (icv) administration of the GABAA receptor agonist muscimol (0.1, 0.5 or 1 µg) 5 min before the presumed onset of the pro-oestrous LH surge (at 0900 h) caused a dose dependent suppression of the surge. A single dose of the GABAB receptor agonist baclofen (1 µg; icv) injected at 0855 h postponed the onset of the LH surge, and repeated injections at 0855 and 1130 h suppressed the surge. These data indicate that GABA-ergic activity in the brain can inhibit the LH surge in the cyclic rat via GABAA and GABAB receptors. Pro-oestrous rats were subjected to 5 hrs of restraint starting at 0855 h. Pretreatment with the GABAA receptor antagonist bicuculine (1 µg; icv) at 0840, 0940 and 1040 h or pretreatment with the GABAB receptor antagonist phaclofen (10 µg; icv) at 0840 h were ineffective in preventing the restraint-induced inhibition of the LH surge. The results suggest that GABAA and GABAB receptors are not involved in the inhibitory effect of restraint stress on the LH surge.

Behaviour laterality in male rats: influence of practice and stress

During recent decades, the existence of cerebral and behavioural asymmetries has been manifested in many different animal species. In this work, the asymmetric spatial preference recorded in the water T-maze has been analysed. Fifty-five male rats were tested in this maze for 8 consecutive days. Spatial preference, the number of times each arm was chosen, and the percentages of animals that showed right, left, or no spatial preference were calculated. The results show the evolution of this asymmetric behaviour. Most of the animals tested preferred the right arm (populational laterality). Recently, the influence of factors such as stress and practice in the process of acquiring this behaviour has been investigated. We used 40 male rats divided into two groups with two different stress levels: water temperature at 21 degrees C and 14 degrees C. Our results, based on these two criteria, hypothesize that both variables, stress and practice, are involved in the acquisition of spatial preference behaviour.