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GABA distribution in a pain-modulating zone of trigeminal subnucleus interpolaris.

A recent model for control of spinal and medullary nociceptive neurons (Basbaum and Fields, 1984) incorporates a gamma-aminobutyric acid-ergic (GABA-ergic) cell into this circuitry and indicates that such elements could act as one substrate for presynaptic inhibition of primary afferents. This concept is supported by a variety of pharmacological and electrophysiological studies. We therefore examined the distribution of GABA-ergic activity in trigeminal subnucleus interpolaris (Vi) by focusing on the types of cells, together with dendritic and synaptic profiles, that are immunocytochemically labeled with an antiserum against glutamic acid decarboxylase (GAD). GAD occurred throughout Vi but was most concentrated in the ventrolateral quadrant and interstitial nucleus. It was localized to groups of small neurons with two to three primary dendrites, and within numerous punctate profiles suggestive of synaptic elements. Electron microscopy revealed labeled dendrites, some of which were postsynaptic to scalloped terminals of presumptive primary afferents. Other labeled dendritic elements, which were quite variable in size, engaged both GAD-labeled and unlabeled synapses. Most GAD synapses displayed clear round vesicles and formed contacts with unlabeled perikarya and a variety of dendritic processes. Numerous GAD-positive synapses were also incorporated into axoaxonic clusters, in which the GAD element was presynaptic to scalloped terminals. Others engaged in serial arrays with other unlabeled terminals, which, in turn, were presynaptic to dendrites. Occasionally, GAD synapses formed contacts with GAD-positive dendrites. These data show that GABA is localized to a variety of neuronal elements in ventrolateral Vi and the interstitial nucleus. These occur in spatial arrangements providing an anatomical substrate for postsynaptic modulation of activity in this area. GABA terminals also appear to be involved in a presynaptic inhibitory mechanism, which may, in some instances, affect transmission in primary afferents.

Animals↗

Effects of LHRH on avoidance conditioning in normally cycling and ovariectomized female rats.

Several studies have demonstrated that the peptide LHRH can modify behavior in the male rat. Peripheral and intracerebral infusions of LHRH impair the acquisition of conditioned avoidance responses (CARs) and increase some spontaneous motor behaviors, such as head shaking and grooming. The present study was undertaken to detect the effects of LHRH on the acquisition of CARs and spontaneous motility in normally cycling and ovariectomized (OVX) Sprague-Dawley female rats. Normally cycling females were separated in four groups, according to the stage of the estrous cycle. Ovariectomized female rats were pretreated, 48 h before the experiment, with estradiol benzoate (10 microg/kg) or corn oil. LHRH (6.25, 25, or 50 microg/kg) was subcutaneously injected and the behavioral tests began 1 h after. Low doses of LHRH stimulated the acquisition of CARs during proestrus, estrus, and metestrus, whereas higher doses impaired conditioning in all the four stages of the cycle. High doses of LHRH impaired acquisition in OVX rats treated with oil and potentiated the depressant effects of EB on this behavior. The effects of LHRH on spontaneous motor activity were either stimulatory or inhibitory, according to the hormonal status and the dose administered. High doses of LHRH decreased motor responses in the diestrous rat. All the doses of LHRH increased the number of headshakes during proestrus, estrus, and metestrus, while the other motor responses were scarcely or not affected by LHRH in these stages. In OVX rats LHRH increased rearing, head shaking, and grooming behavior. These results support a role of LHRH in the modulation of conditioned and spontaneous behavior. They could provide an explanation to the behavioral changes observed across the estrous cycle and those observed after EB priming in OVX rats.

Animals↗

Zinc-containing neurons are distinct from GABAergic neurons in the telencephalon of the rat.

We used a double-labeling protocol that combined the silver amplification of endogenous zinc with routine immunocytochemistry to determine if telencephalic neurons that exhibit GABA-, calbindin- or parvalbumin-like immunoreactivity give rise to zinc-containing boutons. We did not observe telencephalic neurons double-labeled for zinc and GABA or parvalbumin. Zinc and strong calbindin immunoreactivity were colocalized in hippocampal CA1 pyramidal cells and dentate granule cells. Other strongly calbindin-immunoreactive neurons of the telencephalon were never double-labeled. We conclude that GABAergic cells do not contain histochemically reactive zinc and, therefore, are unlikely to use this pool of zinc as a neuromodulator. This observation does not support an in vivo significance of the modulation of GABA receptors by zinc such as has been observed in vitro. In CA1 of the hippocampus, we observed the histochemical label for zinc in all visibly calbindin-immunoreactive pyramidal cells and vice versa. Thus, two markers define a subpopulation of hippocampal pyramidal cells.

Animals↗

The ventral surface of the medulla in the rat: pharmacologic and autoradiographic localization of GABA-induced cardiovascular effects.

Experiments were done to evaluate a rat model for studying the cardiovascular effects of pharmacological manipulations of the ventral surface of the medulla. GABAergic drugs were used because of their well-characterized actions at the ventral surface of the medulla in the cat. GABA and muscimol, applied to the exposed ventral surface with filter paper pledgets, produced dose-dependent decreases in heart rate (HR) and mean arterial pressure (MAP) which were reversed with bicuculline but not with strychnine. Bicuculline alone raised HR and MAP. The GABA- or bicuculline-induced cardiovascular effects were mediated primarily by inhibition of sympathetic outflow. The most sensitive site was localized to an intermediate area on the ventral surface of the medulla, between the trapezoid body and exits of the hypoglossal nerves and just lateral to the pyramids. Topical application of [3H]GABA to the intermediate area resulted in labeling that was concentrated at the site of application, and which penetrated the parenchyma 1 mm dorsally. The heaviest labeling was found primarily in the ventral halves of the lateral paragigantocellular nuclei. No tritium was detected in peripheral blood. These data provide evidence for a neuronal system at the ventral medullary surface of the rat which influences sympathetic outflow and is modulated by GABA.

Animals↗

Inhibitory effect of GABA on cerebrovascular sympathetic neurotransmission.

The possibility that gamma-aminobutyric acid (GABA) could modulate sympathetic neurotransmission in the cerebrovascular bed of the goat has been investigated by means of 3 experimental approaches: measurement of cerebral blood flow in the anesthetized animal, recording of isometric tension in isolated cerebral arteries, and measurement of tritium efflux from cerebral arteries preloaded with [3H]noradrenaline. Electrical stimulation of cervical sympathetic nerve produced reductions in cerebral blood flow which were significantly diminished during continuous infusion of GABA (20-40 micrograms/min) into the internal maxillary artery. Picrotoxin (3 mg) did not change the inhibitory effect of GABA. Exogenously administered noradrenaline (1-9 micrograms) and tyramine (50-500 micrograms) reduced cerebral blood flow as well, but this effect was unchanged by GABA infusion. Transmural electrical stimulation elicited frequency-dependent contractile responses in isolated cerebral arteries which were significantly blocked when GABA was present, at a dose (10(-4) M) which did not modify the contractile response to exogenous noradrenaline (10(-8)-10(-4) M). Moreover, GABA (10(-5)-10(-4) M) inhibited transmural electrical stimulation-evoked tritium efflux from arteries preloaded with [3H]noradrenaline. These results show that GABA inhibits adrenergic neurotransmission in cerebral arteries by a mechanism involving inhibition of transmitter release. Probably, specific presynaptic GABA-B receptors mediate this inhibitory effect.

Adrenergic Fibers↗

Calbindin D28k-containing neurons in the paratrigeminal nucleus receive convergent nociceptive information and project to nucleus of the solitary tract in rat.

The paratrigeminal nucleus (PTN) receives orofacial somatic and visceral afferent fibers and contains many calbindin-D28k neurons (CB-containing neurons) that project to nucleus of the solitary tract (NTS). In the present study, retrograde and transganglionic tracing methods combined with immunofluorescence histochemistry and confocal laser scanning microscopy were used. After Fluoro-gold (FG) injection into the unilateral NTS, 74.4% FG-labeled neurons of ipsilateral PTN were double-labeled with CB. Furthermore, 41.0% and 32.5% FG/CB double-labeled neurons co-existed with Fos induced by nociceptive stimulation of the lips and the upper alimentary tract, respectively. In the PTN unilateral to FG injection site, 26.6% CB-LI neurons were double-labeled with PAG, 61.5% and 79.0% CB/PAG double-labeled neurons were triple-labeled with FG and Fos, and 22.9% FG/CB double-labeled neurons were triple-labeled with PAG, 84.3% FG/PAG double-labeled neurons expressed Fos induced by the upper alimentary tract stimulation. In the intact animals, 62.8% CB-LI neurons and 88.3% PAG-LI neurons co-existed with GABA(B)R, respectively. In addition, some terminals from the inferior alveolar nerve (IAN) were closely apposed to CB/Fos double-labeled or CB single-labeled neurons. These results suggested that CB-containing neurons in the PTN receive the nociceptive information converge from the orofacial area and visceral organs, and comprising the glutamatergic excitatory transmission pathway from the PTN to the NTS. This pathway might be modulated by GABA via the GABA(B) receptor.

Animals↗

Injections of baclofen into the ventral medial prefrontal cortex block the initiation, but not the expression, of cocaine sensitization in rats.

RATIONALE: Increased excitatory output from the medial prefrontal cortex (mPFC) is thought to play a key role in the development of sensitization to cocaine. Gamma-aminobutyric acid (GABA) inhibits this excitatory output. OBJECTIVES: The present studies were designed to determine the effects of intra-mPFC injections of the GABA(B) agonist baclofen on cocaine-induced motor activity and on the development of sensitization to cocaine. METHODS: Rats received bilateral cannula implants above the ventral mPFC. Initial studies examined the dose-response effects of injection of baclofen (0.05-0.5 nmol/side) into the mPFC on the acute motor-stimulant response to cocaine (15 mg/kg, i.p.). Additional studies determined whether coadministration of intra-mPFC baclofen (0.5 nmol/side) and systemic cocaine (15 mg/kg, i.p.) could alter the initiation and/or expression of cocaine-induced behavioral sensitization. RESULTS: Intra-mPFC baclofen dose-dependently blocked cocaine-induced motor activity. In sensitization studies, intra-mPFC baclofen was able to prevent the initiation, but not the expression of cocaine-induced sensitization. CONCLUSIONS: The data suggest that the ability of GABA to modulate excitatory output from the mPFC may be attenuated in animals sensitized to cocaine.

Animals↗

Prevalence of the GABAA receptor assemblies containing alpha1-subunit in the rat cerebellum and cerebral cortex as determined by immunoprecipitation: lack of modulation by chronic ethanol administration.

The anti-alpha1 antibody elicited higher immunoprecipitation (%) values of the [3H]flunitrazepam and [3H]muscimol binding activity in the rat cerebellum vs. cerebral cortex, whereas immunoprecipitation values for [3H]Ro 15-4513 and [3H]zolpidem were comparable in these brain regions. Chronic ethanol administration neither changed the radioligand binding to the immunoprecipitated pellet nor the percentage immunoprecip-itation values, thereby indicating that chronic ethanol did not result in down-regulation of the GABAA receptor assemblies containing alpha1-subunit.

Affinity Labels↗

GABA actions on the excitability of cultured CNS neurons.

The membrane mechanisms associated with Cl- conductance activated by GABA in spinal and hippocampal neurons cultured from the embryonic mouse and rat have been studied with voltage- and patch-clamp techniques. The elementary mechanism underlying the conductance response is predominantly all-or-none with both similar and different kinetics in different assays. beta-Alanine and glycine also alter conductance via a similar mechanism, but the electrical properties associated with each transmitter are unique. Clinically important drugs modulate GABA-mediated responses by altering the kinetics of ion channel activity. Inhibitory synaptic currents whose time constant of decay coincides with the common, slower phase of channel kinetics in pharmacological experiments are altered in amplitude and/or time course by the same drugs. The results strongly suggest that the synaptic events reflect the activation of 1700 channels whose open-time distribution describes the time constant of decay.

Animals↗

GABA, not glutamate, controls the activity of substantia nigra reticulata neurons in awake, unrestrained rats.

Substantia nigra pars reticulata (SNr) receives both GABAergic and glutamatergic (GLU) inputs that are believed to act together to regulate neuronal activity in this structure. To examine the role of these inputs, single-unit recording was coupled with iontophoresis of GLU and GABA in rats under two conditions: awake, unrestrained and under chloral hydrate anesthesia. Although GABA potently inhibited SNr cells in both conditions, freely moving rats showed lower sensitivity than anesthetized animals. Likewise, GLU effectively induced excitations in most SNr neurons in anesthetized animals but was much less effective in awake, unrestrained animals in terms of both the number of sensitive cells and the magnitude of GLU-induced excitation. These findings, along with consistent excitations induced by bicuculline in awake, unrestrained rats, suggest that modulation of GABA inhibitory input, not the opposing actions of GLU and GABA, is the primary factor that regulates the activity state of SNr neurons.

Action Potentials↗

GABA(A), but not NMDA, receptors modulate in vivo NO-mediated cGMP synthesis in the rat cerebral cortex.

We have investigated the functional relationships between NMDA receptors and the NOS/sGC system in the rat pre-frontal cortex in vivo by microdialysis. cGMP basal levels were sensitive to NOS or sGC inhibitors (L-NARG or ODQ) or NO donors (SNAP) when enzymatic breakdown was blocked by the phosphodiesterase inhibitor IBMX, indicating that basal cGMP production derives, at least in part, from the NOS/sGC pathway activity and that the pre-frontal cortex possesses a very efficient degradation system for cGMP. The glutamate receptor agonist NMDA did not alter extracellular cGMP either in absence or presence of IBMX. cGMP was not augmented when NMDA was co-infused with the NOS substrate L-arginine, the glycine site agonist d-serine or the glutamate receptor agonist AMPA. Interestingly, the selective GABA(A) receptor antagonist bicuculline enhanced cGMP production, revealing that the cortical NOS/sGC system is tonically inhibited by endogenous GABA. However, in the presence of bicuculline, NMDA did not increase extracellular cGMP. In the presence of bicuculline, blockade of 5-HT1/2 receptors, known to inhibit the NMDA/NOS/sGC pathway, with the antagonist methiothepin did not unmask cGMP elevations by NMDA. Thus, it would seem that NMDA receptors do not regulate cortical NOS/sGC activity that, on the other hand, is modulated by endogenous GABA acting at GABA(A) receptors.

Animals↗

Pharmacological modulation of behavioral and neuronal correlates of repetition priming.

In this experiment we address the pharmacological modulation of repetition priming, a basic form of learning, using event-related functional magnetic resonance imaging. We measured brain activity in a word-stem completion paradigm in which, before study, volunteers were given either placebo, lorazepam (2 mg orally), or scopolamine (0.4 mg, i.v.). Relative to placebo, both drugs attenuated the behavioral expression of priming. Repetition was associated with a decreased neuronal response in left extrastriate, left middle frontal, and left inferior frontal cortices in the placebo group. Both drugs abolished these "repetition suppression" effects. By showing a concurrence of behavioral and neuronal modulations, the results suggest that GABAergic and cholinergic systems influence the neuronal plasticity necessary for repetition priming.

Adolescent↗

Dopamine-antagonistic, anticholinergic, and GABAergic effects on declarative and procedural memory functions.

Declarative and procedural memory functions are related to dissociable neuroanatomic substrates. In the present study differential effects of pharmacologically induced changes in dopaminergic, GABAergic, and cholinergic activity in the brain on declarative (object and face recognition, immediate and delayed word recall) and procedural memory processes (compensatory tracking) were investigated. In a double-blind design, either 3 mg of haloperidol, 11 mg of midazolam, 1 mg of scopolamine, or placebo were administered to 80 healthy volunteers randomly assigned to one of the four drug conditions. Although all three drugs produced a detrimental effect on immediate and delayed word recall, recall performance was substantially more impaired by the benzodiazepine midazolam than by either haloperidol or scopolamine. While recognition of faces was affected by neither of the drugs, performance on object recognition was significantly decreased by midazolam as compared to placebo. Procedural learning was markedly impaired by all drugs but, again, the observed effect was most pronounced with midazolam. Additional analyses of measures of subjective activation, cortical arousal, and psychomotor performance argued against the assumption that the observed memory-impairing effects were secondary to drug-induced sedation. The overall pattern of results revealed that memory processes are much more susceptible to changes in GABAergic than in dopaminergic or cholinergic neurotransmitter activity. Furthermore, the present findings point to the conclusion that the modulating effects of dopaminergic, GABAergic, and cholinergic neurotransmitter systems on declarative and procedural memory functions are less specific than suggested by neuropsychological studies in patients.

Acetylcholine↗

Neurotransmitter modulation of glucocorticoid receptor mRNA levels in the rat hippocampus.

Glucocorticoids in the hippocampus mediate adaptive responses elicited by stressful stimuli. In this study we investigated glucocorticoid receptor gene expression in the rat hippocampus following acute stress. A significant decrease in glucocorticoid receptor mRNA levels was observed in the hippocampus less than 1 h after the onset of stress. This decrease was inhibited by administering either MK-801, diazepam or propranolol prior to exposure to stress. The effect of diazepam on the stress-induced decrease in hippocampal glucocorticoid receptor mRNA was reversed by Ro-15-1788, suggesting that it is mediated by central benzodiazepine receptors, i.e. GABA-A. These results indicate that NMDA, GABA-A and beta-adrenergic receptors are involved in the mechanism of the stress-induced decrease in glucocorticoid receptor mRNA levels in the rat hippocampus.

Adrenergic beta-Antagonists↗

Modulation of basal and stress-induced release of acetylcholine and dopamine in rat brain by abecarnil and imidazenil, two anxioselective gamma-aminobutyric acidA receptor modulators.

The effects of imidazenil (6-(2-bromophenyl)-8-fluoro-4-H-imidazo[1-5-a][1-4]benzodiazepine-3- carboxamide) and abecarnil (isopropyl-6-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate), new partial and selective benzodiazepine recognition site agonists, respectively, on basal and stress-induced hippocampal acetylcholine and cortical dopamine release were determined with the microdialysis technique in freely moving rats. The actions of these new anxioselective and anticonvulsant drugs were compared with those of diazepam and midazolam, two classical benzodiazepine full agonists. Abecarnil (0.05-1 mg/kg i.p.), imidazenil (0.05-1 mg/kg i.p.), diazepam (2.5-10 mg/kg i.p.) and midazolam (2.5-10 mg/kg i.p.) inhibited basal hippocampal acetylcholine release in a dose-dependent manner. In contrast, whereas diazepam and midazolam significantly decreased dopamine release in the prefrontal cortex, abecarnil and imidazenil had no effect on basal dopamine output. The effects of these drugs on both acetylcholine and dopamine release were antagonized by the benzodiazepine receptor antagonist flumazenil (1 mg/kg i.p.). Foot-shock stress (0.2 mA for 500 msec/sec) delivered for 8 min induced a rapid and marked (+75%) increase in hippocampal acetylcholine output that persisted for approximately 40 min. Foot-shock stress also increased dopamine release in the cerebral cortex; the effect was maximal (+90%) after 20 min and persisted for approximately 30 min. Prior administration of abecarnil or imidazenil at a dose (0.05 mg/kg) that did not significantly affect the basal release of either acetylcholine or dopamine completely prevented the effect of stress on the output of these neurotransmitters, an effect mimicked by higher doses of diazepam (2.5 mg/kg) and midazolam (2.5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗

GABA antagonists differentiate between recombinant GABAA/benzodiazepine receptor subtypes.

Seventeen rat GABAA receptor subtypes were transiently expressed in the human embryonic kidney 293 cell line from alpha 1, alpha 2, alpha 3, alpha 5, or alpha 6 variants with any of the three beta subunits and gamma 2S or gamma 3. We obtained fingerprints in the form of subtype characteristic concentration-response curves of 35S-TBPS binding to GABA and the GABAA antagonists SR 95531 and bicuculline. alpha 3 beta 3 gamma 2S/3 and alpha 5 beta 3 gamma 2S/3 containing receptors effectively recognized 35S-TBPS but not when beta 3 was replaced by the beta 1 or beta 2 subunit. This indicates a specific interaction of alpha and beta variants to form high-affinity 35S-TBPS binding sites. At low levels GABA allosterically increased 35S-TBPS binding to all receptors with the concentration and magnitude depending on the subunit combination. Exchange of the beta variant did not alter the concentration-response curves for alpha 1 and alpha 6 containing receptors but did so for alpha 2 containing receptors. alpha 2 beta 3 gamma 3 receptors displayed strong GABA-induced stimulation of 35S-TBPS binding, whereas binding to alpha 2 beta 3 gamma 3 receptors was marginally increased. SR 95531 and bicuculline decreased 35S-TBPS binding to all gamma 3 containing receptors. In addition, bicuculline was effective on alpha 1 beta x gamma 2 receptors. SR 95531 was threefold more potent than bicuculline in reversing GABA-induced modulation of 35S-TBPS binding in most receptor types, but was 30-fold more potent on alpha 2 beta 1 gamma 3 and alpha 6 beta 1 gamma 2S receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

The neuroactive steroid allopregnanolone suppresses hypothalamic gonadotropin-releasing hormone release through a mechanism mediated by the gamma-aminobutyric acidA receptor.

The central nervous system (CNS) is able to synthesize and/or metabolize steroid hormones. These neuroactive steroids are capable of modulating several brain functions and, among these, they seem to regulate the hypothalamic-pituitary-gonadal (HPG) axis. Indeed, recent observations have shown that 5 alpha-pregnane-3 alpha-ol-20-one (allopregnanolone), one of the most abundant naturally occurring neuroactive steroids, suppresses ovulation and sexual behaviour when administered within the CNS. The present study was undertaken to evaluate the effects of allopregnanolone and its inactive stereoisomer, 5 alpha-pregnane-3 beta-ol-20-one, upon the release of gonadotropin-releasing hormone (GnRH) from individually-incubated hemihypothalami. Allopregnanolone suppressed GnRH release in a concentration-dependent manner with maximal activity in the nanomolar range, a range at which this neurosteroid is capable of playing a biological action. The specificity of allopregnanolone suppression of GnRH release was provided by the lack of effect of its known inactive stereoisomer. To evaluate the involvement of gamma-aminobutyric acidA (GABAA) receptor, we examined the effects of two neurosteroids with GABA-antagonistic properties, pregnanolone sulfate (PREG-S) and dehydroepiandrosterone sulfate (DHEAS), and of bicuculline, a selective antagonist of the GABA binding site on the GABAA receptor, on allopregnanolone (10 nM)-suppressed GnRH release. Both PREG-S and bicuculline overcame the inhibitory effects of allopregnanolone on GnRH release, whereas DHEAS did not. To substantiate the involvement of the GABAA receptor further, we tested the effects of muscimol, a selective agonist for this receptor, which suppressed GnRH release. In conclusion, allopregnanolone suppressed hypothalamic GnRH release in vitro and this effect appeared to be mediated by an interaction with the GABAA receptor. We speculate that the inhibitory effect of allopregnanolone on the HPG axis may also be caused by its ability to suppress hypothalamic GnRH release.

Animals↗

Regulation of the GABA receptor complex by a phosphorylation mechanism.

GABA- modulin , a regulatory component of the GABA/benzodiazepine receptor complex, is phosphorylated by cyclic-AMP-, Ca/calmodulin-, and Ca/phospholipid-dependent protein kinases at distinct sites in the molecule. Phosphorylation of GM by the cyclic-AMP-dependent process results in a complete loss of GM inhibitory activity on specific 3H-GABA binding to synaptic membrane recognition sites. The effect of the Ca2+-dependent phosphorylation, dependent on CaM or PS, of GM is presently unknown but may involve a synergistic or antagonistic action on the cyclic-AMP-dependent phosphorylation. Alternatively, the Ca2+-dependent protein kinases may regulate another function of GM, perhaps its postulated role as a coupler of the GABA/benzodiazepine recognition sites. The results of these experiments strongly implicate a role for protein phosphorylation in the regulation or modulation of GABA receptor function, and such a mechanism may be extrapolated to other neurotransmitter receptor complexes.

Animals↗