GABA and endocrine regulation—Relation to neurologic-psychiatric disorders

Alterations in a hypothalamic GABA system in the spontaneously hypertensive rat

The density (Bmax) of muscimol and clonazepam binding to hypothalamic membranes from spontaneously hypertensive rats (SHR) was reduced compared to age-matched Wistar Kyoto (WKY) animals in the period 80 - 120 days. There were no significant differences in dissociation constant (Kd) for either ligand at this time. At 30 - 36 days, prior to development of pronounced hypertension, there were no differences in Kd or Bmax for either ligand in SHR and WKY animals. There were also deficits in endogenous hypothalamic GABA concentrations in SHR at 75 and 120 days as compared to WKY. The hypothesis is advanced, that there may be a dysfunction of a hypothalamic GABA system in the SHR rat as hypertension develops.

Increased GABAergic activity inhibits alpha-fetoprotein mRNA expression and the proliferative activity of the HepG2 human hepatocellular carcinoma cell line

BACKGROUND/AIMS

Gamma aminobutyric acid (GABA) is a potent inhibitory neurotransmitter with growth regulatory properties. Recent data indicate that increased GABAergic activity inhibits hepatocyte proliferation in regenerating livers. In the present study, we aimed to investigate whether GABA inhibits the growth of malignant hepatocytes.

METHODS

Increasing concentrations of muscimol (0.05-50 microM), a specific GABA(A) receptor agonist, were added to HepG2 human hepatocellular carcinoma cells and alpha-fetoprotein (AFP) and albumin mRNA expression were determined for varying periods of time (maximum 24 h) thereafter. Cell proliferation was also documented after 48 h of exposure to muscimol.

RESULTS

Muscimol significantly (p<0.0001) decreased AFP mRNA expression (maximum decrease: 65% below baseline values) without affecting albumin mRNA expression. However, the effect on AFP mRNA was transient (maximum duration: 3-6 h) and not associated with changes in cell proliferation. Because preliminary data indicate that GABA(A) receptor activity is markedly downregulated in malignant hepatocytes, transfection studies were performed wherein HepG2 cells were cotransfected with GABA(A) receptor beta2 and beta2 subunit genes in a pCDM8 expression vector or vector alone followed by re-exposure to either muscimol (5 betaM) or saline. In this series of experiments, in addition to AFP mRNA inhibition being as extensive and more prolonged (maximum duration: 6-12 h) in muscimol-treated, GABA(A) receptor-transfected cells, proliferative activity was also significantly inhibited when compared to saline-treated GABA(A) receptor-transfected controls (p<0.01) and muscimol-treated cells transfected with vector alone (p<0.005).

CONCLUSION

The results of this study indicate that increased GABAergic activity inhibits AFP mRNA expression and cell proliferation in this malignant hepatocyte cell line.

Age-related development of gamma-aminobutyric acid (GABA)B receptor functions in various brain regions of spontaneously hypertensive rats

The age-related development of GABAB receptors and their coupling to adenylate cyclase were studied in the brains of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, the specific [3H]GABA binding to GABAB receptors showed a significant decrease not only in the posterior hypothalamus, midbrain, hippocampus and striatum of eleven-week-old SHR, which maintain a hypertensive state, but also in the posterior hypothalamus of four-week-old normotensive SHR. Similarly, the GABAB receptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity showed a decrease in the posterior hypothalamus of four-week-old SHR as well as in the posterior hypothalamus and striatum of eleven-week-old SHR. These results suggest that the functions of the GABAB receptor in the brain of SHR may be decreased independently from the occurrence of blood pressure elevation and that such changes may even be involved in the pathogenesis of SHR.

Baclofen, a gamma-aminobutyric acid B agonist, modifies hormonal secretion in pituitary cells from infantile female rats

Recent work from our laboratory has demonstrated that the activation of GABA B adenohypophyseal receptors by baclofen inhibits pituitary hormone secretion under basal (PRL) or stimulated conditions (PRL and LH) in adult female rats, suggesting a hypophyseal site of action in addition to the central site previously described. Since different patterns of hormone secretion are observed in infantile and adult rats, the purpose of the present study was to determine whether GABA B pituitary receptors were involved in endocrine responses at early stages of development. Pituitary cells of 12 day-old female rats were cultured in vitro and the effect of baclofen was determined in the presence or absence of stimulatory factors. Baclofen (1.10(-9), 1.10(-7) and 1.10(-5) M) did not alter basal LH or FSH secretion but significantly inhibited the LHRH induced gonadotropins release after 30 or 60 minutes of incubation (after 60 minutes of incubation LH (%): control: 100 +/- 5.6; BACL(1.10(-7)): 134.5 +/- 25.8; LHRH(1.10(-7)): 596.7 +/- 85.9; LHRH(1.10(-7))-BACL(1.10(-7)): 374.7 +/- 48.0; p<0.01. FSH (%): control: 100 +/- 6.5; BACL(1.10(-7): 103.7 +/- 6.5; LHRH(1.10(-7)): 283.9 +/- 29.3; LHRH(1.10(-7))-BACL(1.10(-7): 183.0 +/- 20.0; p<0.01). Baclofen did not significantly modify either basal or TRH-stimulated PRL or TSH secretion. These results show that baclofen has direct effects on the of adenohypophyseal cells of immature rats and such effects are different from those observed in adult rats, and depend on the stage of development of the neuroendocrine controls of each cellular type.

Pro- and anticonvulsant effects of stress: the role of neuroactive steroids

The present review deals with findings related to the contribution of pro- and anticonvulsant effects of "neuroactive" steroids and the role of the gamma-aminobutyric acid (GABA) receptor as a physiological target for naturally occurring steroids. Ways are discussed via which GABAergic neurotransmission can be enhanced or reduced following maneuvers that inflict stress. The duality of stress effects is emphasized in conjunction with different types of epileptogenesis (e.g., grand mal vs petit mal) that undergo dissimilar evolution. Among the issues covered are steroid-induced sedation and epileptogenicity, excitatory steroids, stress and epilepsy, GABA and respiratory functions, asymmetric brain injury, and psychopathology and stress.

Area-dependent changes in GABAergic function after acute and chronic cold stress

(1) The function of the gamma-aminobutyric acid (GABA)ergic system in certain areas of the rat brain was investigated after acute and chronic cold stress. (2) GABA concentration, [3H]GABA uptake and the activity of the synthesis enzyme glutamate decarboxylase (GAD) were measured. (3) Acute stress: (a) reduced GABA concentration in the corpus striatum (29%); (b) decreased GAD activity (under non-saturating substrate concentration) in the olfactory bulbs (24%); (c) diminished neuronal uptake of [3H]GABA in the frontal cerebral cortex (65%), hypothalamus (86%) and olfactory bulbs (82%). (4) Chronic stress: (a) reduced the endogenous levels of GABA in the frontal cerebral cortex (51%), hypothalamus (26%) and olfactory bulbs (15%); (b) decreased GAD activity in the corpus striatum (32%) and olfactory bulbs (34%); (c) decreased neuronal uptake of [3H]GABA in the hypothalamus (83%). (5) These findings suggest that compensatory changes may develop in the GABAergic system after chronic stress.

Significance of chloride channel activation in the gamma-aminobutyric acid induced growth hormone secretion in the neonatal rat pituitary

Growth hormone (GH) secretion of the neonatal pituitary is stimulated by tau-aminobutyric acid (GABA) (1,2). Since in most cases GABA is known to act by increasing postsynaptic membrane permeability to chloride ions we tested the importance of chloride channel activation in the GH stimulatory effect of GABA in the neonatal pituitary. In the absence of chloride in the superfusion medium GABA was without effect on GH secretion of the neonatal pituitaries and its effect was attenuated by chloride channel inhibitors. The effect of growth hormone releasing hormone (GHRH) on GH secretion was attenuated in the chloride-free media, but it was not affected by simultaneous administration of chloride channel blockers. The present study indicates that GH stimulatory effect of GABA in the neonatal pituitaries might involve chloride channel activation probably resulting in secondary activation of calcium channels.

The effect of gamma-aminobutyric acid on hepatic regenerative activity following partial hepatectomy in rats

BACKGROUND

gamma-Aminobutyric acid (GABA) is a potent inhibitory neurotransmitter with growth-regulatory properties. In fulminant hepatic failure, a condition in which hepatic regeneration may be impaired, systemic serum GABA concentrations are markedly elevated. The present study was designed to determine whether increased amounts of circulating GABA interfere with hepatic regenerative activity.

METHODS

Exogenous GABA or isotonic saline was administered to adult male rats (n = 6-12/group) 16 hours before partial hepatectomy and twice daily for 1-3 days thereafter. The hypertrophic and hyperplastic components of hepatic regeneration were determined by calculation of the restitution of liver mass, [14C]leucine incorporation into protein (protein synthesis), and [3H]thymidine incorporation into hepatic DNA (DNA synthesis).

RESULTS

Exogenous GABA impaired restitution of liver mass (GABA vs. controls, day 3: 76% +/- 7% vs. 90% +/- 9%, mean +/- SD) (P < 0.005) and the rate of protein synthesis (GABA vs. controls, day 1: 379 +/- 39 dpm/mg protein vs. 564 +/- 67 dpm/mg protein) (P < 0.01) without interfering with DNA synthesis. Supplemental administration of corticosterone and putrescine restored protein synthesis rates to normal in GABA-treated rats.

CONCLUSIONS

These results indicate that elevated serum GABA concentrations interfere with the hypertrophic component of hepatic regeneration following partial hepatectomy in rats.

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

GABA and enkephalin in the lateral septum of the guinea pig: light and electron microscopic evidence for interrelations

Tract tracing techniques combined with immunohistochemistry in rats and guinea pigs have demonstrated the existence of a hypothalamo-lateral septum enkephalinergic pathway. Numerous enkephalinergic nerve endings encompass cell bodies located in the lateral septum. The present immunocytochemical study, at light and electron microscopic levels, was undertaken in the guinea pig brain to determine whether the contacted perikarya contain gamma-aminobutyric acid (GABA). The antisera against GABA revealed the presence of immunoreactive cell bodies throughout the lateral septum. At the light microscopic level, most GABA neurons appeared round while others were oval with one or two emerging dendrites. Ultrastructurally, cell bodies displayed a moderate number of organelles and a pale nucleus with frequent indentations of the nuclear envelope. The precise relationship between GABA neurons and enkephalinergic terminals was examined by means of a double-immunostaining method showing that 60% of cell bodies receiving synaptic inputs from enkephalinergic afferents contained GABA. These results show that the hypothalamo-septal enkephalinergic pathway prominently innervates GABA-containing neurons and also provide anatomical basis suggesting a disinhibitory role for this enkephalinergic tract.

Substance P affects the GABAergic system in the hypothalamo-pituitary axis

In the present work we examined the effect of the neutralization of endogenous substance P by the administration of an anti-substance P serum (ASPS) on GABA concentration in the anterior pituitary in hyperprolactinemic conditions induced by 5-hydroxytryptophan or by grafting anterior pituitaries. ASPS reduced the increase in the anterior pituitary GABA concentration induced by hyperprolactinemia. In vitro experiments showed that substance P inhibited K(+)-evoked GABA efflux from hypothalamic fragments and decreased GABA concentration in the anterior pituitary but ASPS increased it. Our results demonstrate that substance P modifies hypothalamic GABA release and anterior pituitary GABA concentration and suggest that an interaction exists between substance P and GABA.

GABAergic mechanisms in morphine-induced hypothermia

Morphine produced a hypothermic effect in restrained rats which was antagonized by naloxone. This effect was completely inhibited by gamma-acetylenic-GABA, an inhibitor of GABA transaminase and by baclofen, a specific GABAB agonist. Pretreatment with diazepam, an agonist of the benzodiazepine receptor, partially inhibited morphine-induced hypothermia. Flumazenil, a benzodiazepine receptor blocker, potentiated the action of morphine on body temperature. A role of brain GABA in the thermoregulatory effects of morphine is proposed on the basis of these results.

Effects of serotonin on the hypothalamic-pituitary GABAergic system

The effects of serotonin (5-HT) and its precursor, 5-hydroxytryptophan (5-HTP) on the GABAergic system in the mediobasal hypothalamus (MBH) and the anterior pituitary were studied. The IP administration of 5-HTP produced a transient increase (only at 45 min after the injection) in glutamate decarboxylase activity (GAD) of MBH and in GABA concentration in anterior pituitary. Besides, 5-HTP administration increased the in vitro evoked GABA release from MBH. The administration of 5-HTP to hypophysectomized rats partially reversed the inhibitory effects of hypophysectomy on GABA concentration in MBH. We also examined the direct effect of 5-HT on some parameters on the hypothalamic GABAergic system. The presence of 5-HT in the incubation medium increased GAD activity and evoked GABA release from MBH. These observations indicate that the serotoninergic stimulation of the hypothalamic GABAergic system could be a direct effect which may, at least partially, be independent of the feedback mechanism induced by prolactin on the GABAergic neurons. The serotoninergic increase of prolactin secretion could be accomplished through stimulation of the hypothalamic GABAergic transmission.

Effects of inhibition of GABA catabolism by aminooxyacetic acid on enkephalinergic neurons--immunocytochemical and ultrastructural study in the enkephalinergic hypothalamoseptal tract of the guinea-pig

The immunocytochemical and ultrastructural features within [Met]enkephalin neurons of the guinea-pig hypothalamoseptal tract were investigated under chronic inhibition of GABAergic catabolism. This was achieved by raising the brain GABA concentration with aminooxyacetic acid which inhibits GABA-transaminase, the enzyme responsible for the catabolism of GABA. Guinea-pigs were injected intraperitoneally with 10 or 20 mg/kg per day aminooxyacetic acid for two, four or eight days and killed 16 h post-dose. Repeated injections of aminooxyacetic acid produced a great increase in immunoreactivity for GABA in nerve endings surrounding enkephalinergic perikarya in the magnocellular dorsal nucleus of the guinea-pig. Extensive immunocytochemical studies stressed the increase and redistribution of the immunoreaction for [Met]enkephalin in the perikarya of the magnocellular dorsal nucleus under such GABAergic activation. Quantitative and statistical analyses showed that administration of aminooxyacetic acid for eight days significantly increased the intensity of labelling within stimulated perikarya (P less than 0.001). A concomitant accumulation of immunopositive large granules in the enkephalinergic boutons of the lateral septum was observed. In the same way, ultrastructural changes in enkephalinergic cell bodies were analysed and reflected disturbances in the biosynthetic and digestive activities of enkephalinergic perikarya. We postulate that chronic inhibition of the GABAergic catabolism leads to modification in the metabolism of enkephalinergic neurons and to an inhibitory action of GABA on the [Met]enkephalin release from nerve endings. This study give morphological support to the complex functional interactions between GABA and opioid peptide transmitter system.

Evidence for evoked release of adenosine and glutamate from cultured cerebellar granule cells

Evoked release of [3H]-D-aspartate which labels the neurotransmitter glutamate pool in cultured cerebellar granule cells was compared with evoked release of adenosine from similar cultures. It was found that both adenosine and [3H]-D-aspartate could be released from the neurons in a calcium dependent manner after depolarization of the cells with either 10-100 microM glutamate or 50 mM KCl. Cultures of cerebellar granule cells treated with 50 microM kainate to eliminate GABAergic neurons behaved in the same way. This together with the observation that cultured astrocytes did not exhibit a calcium dependent, potassium stimulated adenosine release strongly suggest that cerebellar granule cells release adenosine in a neurotransmitter-like fashion together with glutamate which is the classical neurotransmitter of these neurons. Studies of the metabolism of adenosine showed that in the granule cells adenosine is rapidly metabolized to ATP, ADP, and AMP, but in spite of this, adenosine was found to be released preferential to ATP.

GABAergic drugs and lordosis behavior in the female rat

Agents modifying GABAergic neurotransmission were administered to ovariectomized rats treated with different doses of estradiol benzoate (EB) + progesterone (P) or with EB alone. Hormone treatments were designed to induce an intermediate level of receptivity in order to be able to observe both stimulatory and inhibitory effects on lordosis behavior. Both the GABAA receptor agonist THIP and the GABAB receptor agonist baclofen inhibited lordosis behavior at doses from 20 and 5 mg/kg, respectively. The GABA transaminase inhibitor gamma-acetylen GABA (GAG) and the GABA agonist 3-aminopropanesulfonic acid had no effects, even when high doses were administered. The GABAA receptor antagonist bicuculline had no effect by itself nor did it block the effects of THIP. It is therefore suggested that the GABAA receptor is of slight importance in the control of lordosis behavior. No evidence could be found supporting the hypothesis that an interaction between P and GABA is important for hormone-induced receptivity. It does not appear likely that motor disturbances are responsible for the inhibitory effects of baclofen and THIP. The exact mechanism by which these drugs inhibit lordosis behavior is not clear at present.

Stereospecific actions of baclofen on sociosexual behavior, locomotor activity and motor execution

The behavior effects of racemic baclofen and the R and S enantiomers were studied in order to determine whether the stereospecificity found in receptor binding studies also applies to the behavioral actions of the drug. Racemic and R-baclofen inhibited sexual behavior, locomotor activity and motor execution at relatively low doses while s-baclofen was completely inactive even when a dose 40 times higher than the minimum effective dose of R-baclofen was used. The R enantiomer seems to be twice as active as racemic baclofen. These data strongly suggest that the behavioral effects of baclofen are the result of an action at the GABA-B receptor. In order to differentiate the effects of baclofen on sexual interactions from those on nonspecific social interactions, the sociosexual behavior was observed with a castrated male or a receptive female as stimulus animal. R, S-baclofen had effects only upon sociosexual interaction with a receptive female. Moreover, the inhibitory effects of baclofen were restricted to behavioral items related to sexual interactions, primarily those constituting precopulatory behaviors. Since no effect was observed in social interactions with a castrated male, it is suggested that the inhibition of sociosexual behavior is not a consequence of impairment of motor execution. Rather it appears that baclofen has a specific inhibitory effect on behaviors associated with the initiation of copulatory activity. Therefore, once initiated, sexual behavior was not significantly modified by baclofen.

Effect of diazepam on brain neurotransmitters, plasma corticosterone, and the immune system of stressed rats

Rats were treated with injections of diazepam (1 or 10 mg/kg) and stressed by restraint lasting 3 hours. This was performed once or, in animals immunized with sheep erythrocytes, repeatedly for 4 consecutive days. After repeated stress and/or diazepam treatment, the levels of brain noradrenalin decreased in all treated groups. Although both treatments (stress and diazepam) diminished the 5-hydroxytryptamine (5-HT)/5-hydroxyindoleacetic acid (5-HIAA) ratio, treatment with either dose of diazepam prevented the stress-induced fall of this ratio. The activity of hypothalamic glutamate decarboxylase, the enzyme taking part in GABA synthesis, was affected neither by the acute nor by repeated stress and/or diazepam treatment. The levels of plasma corticosterone were enhanced in all stressed rats, with and without drug. This finding was in accordance with the enhanced weights of adrenal glands in repeatedly stressed rats. The tendency to a corticosterone rise after repeated treatment with diazepam, 10 mg/kg, coincided with the enhanced weights of adrenal glands in these animals. The plaque-forming cell (PFC) response was reduced in all stressed animals and in animals treated with diazepam, 10 mg/kg. Accordingly, high doses of diazepam given repeatedly to rats are immunosuppressive, achieving this effect presumably by an enhancement of glucocorticoid secretion. Neither the low nor the high doses of diazepam affect the stress-induced enhancement of hypothalamohypophysial-adrenal axis activity and consecutive immunosuppression.

Effect of early iron deficiency in rat on the gamma-aminobutyric acid shunt in brain

Early iron deficiency in rat does not affect the weight or the protein, DNA, and RNA content but results in a slight reduction in gamma-aminobutyric acid (GABA) (13%, p less than 0.01) and glutamic acid (20%, p less than 0.001) content of the brain. The activities of the two GABA shunt enzymes, glutamate dehydrogenase and GABA-transaminase, and of the NAD+-linked isocitrate dehydrogenase (ICDH) were inhibited whereas the glutamic acid decarboxylase, mitochondrial NADP+-linked ICDH, and succinic dehydrogenase activities remained unaltered in brain. On rehabilitation with the iron-supplemented diet for 1 week, these decreased enzyme activities in brain attained the corresponding control values. However, the hepatic nonheme iron content increased to about 80% of the control, after rehabilitation for 2 weeks. A prolonged iron deficiency resulting in decreased levels of glutamate and GABA may lead to endocrinological, neurological, and behavioral alterations.

GABA and circadian timekeeping: implications for manic-depression and sleep disorders

Circadian rhythms, evident in a wide variety of physiological and behavioural parameters, are under the control of central neural pacemakers, the best characterized of which is the suprachiasmatic nucleus of the hypothalamus. The neurophysiological mechanisms involved in central pacemaker function are unknown. Recent biochemical, pharmacological and behavioural evidence suggests that the inhibitory transmitter gamma-aminobutyric acid (GABA), present in the small interneurones of the suprachiasmatic nucleus, plays an important role in circadian timekeeping. This has enabled the formulation of strategies for treatment of patients with manic depressive illness and certain sleep disorders in which disorders of circadian timekeeping may be fundamental.

Baclofen, a GABA derivative, inhibits stress-induced prolactin release in the rat

The effect of baclofen, beta-(4-chlorophenyl)GABA, on prolactin secretion was investigated in rats under several experimental conditions. In adult male rats subjected either to immuobilization, ether, swimming or cold stress there was a rapid increase of serum prolactin levels; acute pretreatment with baclofen, 10 mg/kg i.p. inhibited the hormone response to all these stresses. The same blocking effect of the drug was observed in prepubertal male and female rats and in adult gonadectomized animals. In basal conditions, i.e. in undisturbed male rats, baclofen did not change the hormone titers significantly. Taken together our results indicate that baclofen blocks prolactin release when release of the hormone is dynamically stimulated by stress and this effect is relatively independent of the endocrine status of the rat.

Blockade of the LH surge and ovulation by GABA-T inhibitory drugs that increase brain GABA levels in rats

The present experiments were designed to evaluate the effects of GABA increase in the brain on the LH surge and ovulation in female rats. GABA-transaminase (GABA-T) inhibitory drugs were administered intraperitoneally in the early afternoon hours on the day before the expected ovulation. Gamma-acetylenic GABA (GAG) prevented the proestrus LH surge and ovulation in freely moving adult female rats. In a similar manner, GAG blunted the enhanced LH release found at 1800 h on the preovulatory day and the subsequent ovulation in PMSG-treated 32-day-old rats. LH secretion and ovulation in these animals was restored by administration of the selective GABA antagonist, bicuculline. The other GABA-T inhibitors, amino-oxyacetic acid and gamma-vinyl GABA, produced similar effects to GAG in the PMSG-treated rats. These results indicate that the increase of brain GABA levels during the preovulatory day alters LH release and prevents ovulation in rats.

Postnatal development of transmitter systems: sexual differentiation of the GABAergic system and effects of muscimol

The development and sex differences of the central nervous GABAergic system were examined by measuring GABA (gamma-aminobutyric acid) in discrete brain nuclei of the hypothalamus, as well as the nigrostriatal and the limbic systems of male and female rats on the day of birth and on days 5, 10, and 15. The highest concentrations were found in the hypothalamic and nigrostriatal nuclei; the lowest in the limbic system. Sex differences were observed only on day 10 in the medial preoptic area, with GABA being higher in males than in females; and also in the substantia nigra, where female GABA levels were higher than male. These results suggested an involvement of GABA in the sexual differentiation of the brain. As a control, concentrations of the GABA precursor glutamate were determined. No sex differences in glutamate concentrations were found in any brain region during the first 15 days postnatally. Since a GABA mimetic substance applied during the critical period of brain differentiation could disturb the development of the GABAergic system, the consequences of a perinatal treatment with the GABA agonist muscimol were investigated. Significant reduction of GABA concentrations by muscimol were observed in the hypothalamic and nigrostriatal systems at specific times postnatally. On day 5, GABA concentrations were diminished only in the medial preoptic area, then on day 10, in the anterior hypothalamus and the substantia nigra, and still later, on day 15, in the caudato putamen. In contrast, the effects of muscimol on glutamate concentrations could be observed over a longer postnatal period. Glutamate was already diminished on day 5 in 7 areas of the hypothalamic, nigrostriatal, and limbic systems.

Hypophysial GABA after ether stress, dexamethasone or inhibition of GABA catabolism

Ether stress (2 X 2 min within 15 min) and dexamethasone treatment (1 mg/kg IP; 1, 3 and 12 hours before sacrifice), the procedures supposed to increase the activity of glutamate decarboxylase (GAD) in the hypothalamus, fail to affect the concentration of GABA in the rat hypophysis. Five and/or ten minutes post-mortem an increased GABA level in the hypothalamus and cingulate cortex, and a decreased GABA concentration in the hypophysis was found. Three and four hours after the IP administration of 1-cycloserine (50 mg/kg) and 1-glutamic acid-gamma-hydrazide (160 mg/kg) respectively (both drugs are inhibitors of GABA catabolism) the concentration of GABA raised in all the regions examined. On the basis of studies in the whole gland it might be concluded that the concentration of GABA in the hypophysis depends more on GABA release from extrahypophysial tissue and GABA degradation in the hypophysis than on the extrahypophysial GABA synthesis. Also on the basis of post-mortem studies in the whole gland no indication for the appearance of GABA synthesis in hypophysis could be found.

Effect of GABA-T inhibitors on prolactin secretion in vitro

A study was made of the effect of GABA-transaminase (GABA-T) inhibitors on the in vitro release of prolactin by pituitaries of male and female rats. Aminooxyacetic acid (AOAA) and gamma-acetylenic GABA (GAG) added to the incubation medium decreased prolactin release from both male and female rat pituitaries. Additive effects on prolactin release were only observed when male rat pituitaries were incubated with AOAA plus GABA. Prolactin concentration in the pituitary gland was also decreased by AOAA in both sexes. The present results lend support to the idea of an inhibitory action of GABA on prolactin release by the pituitary gland.

gamma-Aminobutyric acid decreases levels of messenger ribonucleic acid encoding prolactin in the rat pituitary

The effect of gamma-aminobutyric acid (GABA) on levels of messenger ribonucleic acid (mRNA) encoding prolactin (PRL) was studied in cultured anterior pituitary cells, in vitro and in intact rats, in vivo. As quantitated by hybridization to a 32P-labeled rat PRL complementary deoxyribonucleic acid (cDNA) probe, levels of PRL mRNA in cultured pituitary cells were decreased by about 50% following 3 days exposure to 10(-5) M GABA. This effect was mimicked by muscimol (10(-6) M) and antagonized by bicuculline (10(-5) M). An increase of endogenous GABA levels in vivo effected by injection of GABA transaminase blockers (aminooxyacetic acid, 20 mg/kg, twice daily; vinyl GABA, 800 mg/kg) into rats resulted in a similar decrease in rat PRL mRNA levels in the adenohypophysis 3-4 days following commencement of the drug treatment. These findings suggest that GABA might inhibit PRL gene expression by a direct action on lactotrophs of the adenohypophysis.

gamma-Aminobutyric acid-ergic analgesia: implications for gamma-aminobutyric acid-ergic therapy for drug addictions

Recent in vivo and in vitro studies regarding the involvement of gamma-aminobutyric acid (GABA)-ergic systems in analgesia and in opiate dependence have been reviewed and analyzed. It seems evident that GABA-ergic systems play a role in mediating the effects of opiates and that some interplay might exist between endogenous opioid systems and GABA-ergic systems. Systemic administration of GABA-agonists, GABA uptake blockers and inhibitors of GABA-alpha-oxoglutarate transaminase (GABA-T) can produce potent analgesic actions in experimental animals and in man. Further study of such agents might lead to the development of new analgesics and/or new drugs for treating certain types of drug addiction.