Effects of dose and time after administration of 4-isopropyl-2,6,7-trioxa-1-phosphabicyclo (2,2,2)octane-1-oxide (IPTBO) on cyclic nucleotide concentrations in mouse cerebellum

The concentrations of cyclic AMP and cyclic GMP in the mouse cerebellum after intracerebroventricular administration of a range of doses of IPTBO have been studied with particular interest in the temporal changes after injection. A non typical dose relationship was observed. After the lowest and non-convulsive dose used (0.06 μg/animal) cyclic AMP levels decreased and cyclic GMP levels increased within 1 min, but after higher doses cyclic AMP and cyclic GMP levels were both raised. At three different convulsive doses of IPTBO there were increased levels of cyclic AMP with time which were more apparent in convulsing animals. Raised levels of cyclic GMP however, were not so influenced by convulsions. The results suggest that (1) the immediate decrease in cyclic AMP and the immediate increase in cyclic GMP may play a part in the mechanism of action of IPTBO—possibly by triggering convulsions and (2) there is an increase in cyclic AMP in response to, or because of, the convulsions. It is concluded that time after treatment and time into convulsions are critical when studying cyclic nucleotide changes, particularly for cyclic AMP and that such factors may explain conflicting observations with respect to this nucleotide.     

Effects of γ-acetylenic GABA and γ-vinyl GABA on electrically-induced spinal cord convulsions and on spinal cord GABA concentration

The effects of γ-acetylenic GABA and γ-vinyl GABA on electrically-induced spinal cord convulsions were compared to the effects of these same drugs on spinal cord GABA concentration. The data show that the effects of these two compounds on seizure activity do not correlate either positively or negatively with changes in GABA concentration. Although both drugs produced marked increases in the amount of GABA in the spinal cord, their effects on spinal cord convulsions were qualitatively different and failed to correlate temporally with alterations in GABA concentration.     

Presynaptic kainate receptors that enhance the release of GABA on CA1 hippocampal interneurons

We report that kainate receptors are present on presynaptic GABAergic terminals contacting interneurons and that their activation increases GABA release. Application of kainate increased the frequency of miniature inhibitory postsynaptic currents recorded in CA1 interneurons. Local applications of glutamate but not of AMPA or NMDA also increased GABA quantal release. Application of kainate as well as synaptically released glutamate reduced the number of failures of GABAergic neurotransmission between interneurons. Thus, activation of presynaptic kainate receptors increases the probability of GABA release at interneuron-interneuron synapses. Glutamate may selectively control the communication between interneurons by increasing their mutual inhibition.     

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.     

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.     

GABA-mediated neurotransmission in the ventrolateral NTS plays a role in respiratory regulation in the rat

Our purpose was to determine whether endogenously released GABA in the ventrolateral nucleus of the solitary tract (vlNTS) of the rat influences respiration. Experiments were carried out in anesthetized, vagotomized and spontaneously breathing rats, and diaphragm electromyogram activity was measured while drugs affecting GABAergic neurotransmission were microinjected into the vlNTS and medial NTS (mNTS). Bilateral microinjection of nipecotic acid, 5 or 25 nmol, into the vlNTS (but not in the mNTS) produced dose-dependent increases in inspiratory duration (Ti) frequently culminating in apneustic breathing. Neither unilateral microinjection of bicuculline nor CGP-35348 (GABA(B) receptor antagonist) reversed this response; however, a combination of both GABA receptor antagonists effectively reversed apneustic breathing. Bilateral microinjection of either muscimol or baclofen into the vlNTS mimicked the effect of nipecotic acid. Microinjection of the bicuculline produced apnea, whereas microinjection of CGP-35348 produced a decrease in Ti and an increase in expiratory duration. Immunohistochemical analysis of the vlNTS region revealed GABA(A) receptors densely localized to processes, whereas GABA(B) immunoreactivity was localized to cell bodies. Our data indicate that GABA activity in the vlNTS is important for respiratory function.     

Effect of Inhibitors of GABA Aminotransferase on the Metabolism of GABA in Brain Tissue and Synaptosomal Fractions

Abstract: Five inhibitors of the GABA degrading enzyme GABA-aminotransferase (GABA-T), viz., gabaculine, γ-acetylenic GABA, γ-vinyl GABA, ethanolamine O -sulphate, and aminooxyacetic acid, as well as GABA itself and the antiepileptic sodium vdproate were administered to mice in doses equieffective to raise the electroconvulsive threshold by 30 V. The animals were killed at the time of maximal anticonvulsant effect of the respective drugs and GABA, GABA-T and glutamate decarboxylase (GAD) were determined in whole brain and synaptosomes, respectively. The synaptosomal fraction was prepared from brain by conventional ultracentrifugation procedures. All drugs studied brought about significant increases in both whole brain and synaptosomal GABA concentrations, and, except GABA itself, inhibited the activity of GABA-T. Furthermore, all drugs, except GABA and γ-acetylenic GABA, activated GAD in the synaptosomal fraction. This was most pronounced with ethanolamine O -sulphate, which induced a twofold activation of this enzyme but exerted only a weak inhibitory effect on GABA-T. The results suggest that activation of GAD is an important factor in the mechanism by which several inhibitors of GABA-T and also valproate increase GABA concentrations in nerve terminals, at least in the relatively non-toxic doses as used in this study.     

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.     

Effects of Lead In Vivo and In Vitro on GABAergic Neurochemistry

Abstract: Alterations in aspects of neurotransmission utilizing -γ-aminobutyric acid (GABA) are associated with in vivo exposure of rats to lead at doses that do not produce convulsions, but sensitize animals to convulsant agents. These effects are observed regionally and include: decreased GABA levels in cerebellum; increased activity of glutamate decarboxylase (GAD) in caudate; and decreased GABA release (both resting and K⁺-stimulated) in cortex, caudate, cerebellum and substantia nigra. Sodium-dependent uptake of GABA by synaptosomes of cerebellum, substantia nigra and caudate was also affected: in these regions, affinity (K_m) was increased and maximal velocity (V_max) was reduced. Sodium-independent binding of GABA to synaptic membranes was increased in cerebellum, but was observed only when tissue was Tritonized and prepared without freezing and washing. No effects on GAD or on GABA uptake, release, or binding were observed when lead was added to brain tissue in vitro in concentrations as high as 100 μM. The results suggest that lead may produce chronic inhibition of presynaptic GABAergic function, notably in the cerebellum, which is associated with supersensitivity of postsynaptic GABA receptors. Failure of lead to affect GABAergic function in vitro may indicate that these effects are secondary to another neurotoxic action of lead in the CNS or are consequent to a nonneuronal metabolic action of lead.     

Ethanol selectively potentiates GABA-mediated inhibition of single feline cortical neurons

Ethanol (alcohol) released from micropipettes by electro-osmosis (up to 10 nA from 0.3 M in 165 mM NaCl solution) potentiated the inhibition of firing of single cortical neurons produced by iontophoretically-applied pulses of γ-aminobutyric acid (GABA), whereas it had no effect or a mild antagonistic effect on the inhibition produced by pulses of glycine, and had an antagonistic effect on the inhibition produced by pulses of serotonin or dopamine. The potentiation of iontophoretically-applied GABA was also obtained by intravenously-applied ethanol (0.2–2 mg/kg). Furthermore, ethanol applied by electro-osmosis or intravenously in the same doses potentiated the inhibition of firing of single cortical neurons evoked by electrical stimulation of the surface of the cerebral cortex, which is believed to be mediated by endogenous GABA. These findings may have implications for alcoholism, since GABAergic neurotransmission is involved in the mechanism of action of anxiolytic drugs and anxiety is involved in the etiology of alcoholism.     

Effects of applying gamma-aminobutyric acid(B) drugs into the medial basal hypothalamus on basal luteinizing hormone concentrations and on luteinizing hormone surges in the female sheep

Prior investigations have shown that localized infusion by microdialysis of gamma-aminobutyric acid(B) (GABA(B)) agonists into the medial basal hypothalamus of male sheep rapidly increases GnRH and LH pulse amplitude. The objectives of these studies were to determine if infusion of GABA(B) agonists SKF 97541 or baclofen into the medial basal hypothalamus of female sheep would affect basal LH secretion and if infusion of a potent antagonist would alter expression of LH surges induced by injection of estrogen. Infusion of either SKF 97541 (10 or 40 microM) or baclofen (1 mM) into estrogen-treated ovariectomized ewes did not alter basal LH secretory patterns, whereas both drugs significantly elevated mean LH and LH pulse amplitude in ovariectomized ewes during the nonbreeding season. Infusion of the antagonist CGP 52432 (250 or 500 microM) did not affect expression of estrogen-induced LH surges in ovariectomized ewes. These observations support the concept that GABA(B) receptors in the medial basal hypothalamus regulate basal LH secretion but do not regulate the surge mode of LH secretion in the female sheep.     

Quantification of the GABA Shunt and the Importance of the GABA Shunt Versus the 2-Oxoglutarate Dehydrogenase Pathway in GABAergic Neurons

Abstract: We investigated the activity of the cerebral GABA shunt relative to the overall cerebral tricarboxylic acid (TCA) cycle and the importance of the GABA shunt versus 2-oxoglutarate dehydrogenase for the conversion of 2-oxoglutarate into succinate in GABAergic neurons. Awake mice were dosed with [1-¹³C]glucose, and brain extracts were analyzed by ¹³C NMR spectroscopy. The percent enrichments of GABA C-2 and glutamate C-4 were the same: 5.0 ± 1.6 and 5.1 ± 0.2%, respectively (mean ± SD). This, together with previous data, indicates that the flux through the GABA shunt relative to the overall cerebral TCA cycle flux equals the GABA/glutamate pool size ratio, which in the mouse is 17%. It has previously been shown that under the experimental conditions used in this study, the ¹³C labeling of aspartate from [1-¹³C]glucose specifically reflects the metabolic activity of GABAergic neurons. In the present study, the reduction in the formation of [¹³C]aspartate during inhibition of the GABA shunt by γ-vinyl-GABA indicated that not more than half the flux from 2-oxoglutarate to succinate in GABAergic neurons goes via the GABA shunt. Therefore, because fluxes through the GABA shunt and 2-oxoglutarate dehydrogenase in GABAergic neurons are approximately the same, the TCA cycle activity of GABAergic neurons could account for one-third of the overall cerebral TCA cycle activity in the mouse. Treatment with γ-vinyl-GABA, which increased GABA levels dramatically, caused changes in the ¹³C labeling of glutamate and glutamine, which indicated a reduction in the transfer of glutamate from neurons to glia, implying reduced glutamatergic neurotransmission. In the most severely affected animals these alterations were associated with convulsions.     

Alterations in CSF GABA levels and seizure susceptibility developing during repeated administration of pentetrazole in dogs. Effects of γ-acetylenic GABA,valproic acid and phenobarbital

Daily administration of convulsive doses of pentetrazole in dogs resulted in a decrease in the seizure threshold and development of increasingly severe clonic-tonic convulsions. Concomitantly, the concentration of γ-aminobutyric acid (GABA) in cisternal cerebrospinal fluid (CSF) was markedly reduced, whereas plasma GABA levels were not altered. When re-tested after a 3-week resting period, animals were found to have retained their increased seizure sensitivity and reduction in CSF GABA levels. γ-Acetylenic GABA and phenobarbital in doses antagonizing the establishment of increased convulsive sensitivity in response to repeated pentetrazole injections also counteracted the fall in CSF GABA. Valproic acid proved less effective to influence the convulsive response of continued pentetrazole administration. The data suggest that a functional deficit in the GABA system may underlie the persistent changes in seizure susceptibility observed.     

Time-course of malaoxon-induced alterations in brain regional inositol-1-phosphate levels in convulsing and nonconvulsing rats

The potential of a single dose of malaoxon (26.2 or 39.2 mg/kg i.p.) to produce convulsions and to increase cerebral levels of inositol-1-phosphate (Ins1P), an intermediate in phosphoinositide (PI) cycle, was followed for 1, 4, or 72 hr. The lower dose of malaoxon did not produce convulsions whereas the higher dose induced convulsions in 60% of the exposed rats. Malaoxon caused a dosedependent, at most 2-fold, increase in brain regional Ins1P levels in nonconvulsing rats as compared to controls. At the higher dose of malaoxon, in convulsing rats, the Ins1P-levels increased 4-fold above the control Ins1P-levels. In nonconvulsing rats, the Ins1P-levels reached their maximum 1–4 hr after the administration of malaoxon, whereas in convulsing rats the levels increased for 72 hr. The results suggest that PI-signalling is associated with convulsions produced by malaoxon.     

Altered response to GABAergic agents following electro and chemo convulsions in mice.

Effects of GABAergic agents and that of electroconvulsive shock (ECS) treatment were studied on bicuculline and picrotoxin (PTX)-induced convulsions in mice. Neither acute nor chronic ECS had any significant effect on bicuculline-induced convulsions, whereas the latency for PTX-induced convulsions was delayed by both acute and chronic ECS. Baclofen treatment delayed significantly the latency for PTX-induced convulsions in animals which were subjected to both acute and chronic ECS, whereas in bicuculline-induced convulsions, it shortened the latency of convulsions 24 hr after acute ECS. Progabide delayed the bicuculline-induced convulsions except in the case of 24 hr after acute ECS and PTX-induced convulsions except in the case of animals treated chronically with ECS. Fengabine showed no significant effect on bicuculline-induced convulsions. However, on PTX-induced convulsions, the latency was delayed in animals not subjected to ECS and in those subjected to chronic ECS. The possible explanations for the alterations in the effect of GABAergic agents following electro and chemo convulsions are (i) differences in the nature of antagonism by bicuculline and PTX, (ii) alterations in receptor sensitivity or number, and (iii) alterations in the levels of endogenous neurotransmitters, the latter two resulting as a result of acute or chronic ECS.     

Reduction of pentylenetetrazol-induced convulsions by the octadecaneuropeptide ODN

Intracerebroventricular injection of the octadecaneuropeptide ODN in mouse, at doses of 12.5-1000 ng, reduced the percentage of convulsing animals and increased the latency of convulsions elicited by pentylenetetrazol (50 mg/kg, intraperitoneal [i.p.]). ODN also reduced the percentage of mortality induced by pentylenetetrazol (100 mg/kg, i.p.). The COOH-terminal octapeptide fragment of ODN was approximately equally effective but acted more rapidly than ODN to reverse the convulsant effect of pentylenetetrazol. ODN (100 ng, intracerebroventricular [i.c.v.]) increased the convulsion latency and reduced the percentage of animals that convulsed after the administration of the inverse agonist of benzodiazepine receptors DMCM (13 mg/kg, i.p.), whereas the benzodiazepine receptor antagonist flumazenil (1 mg/kg, subcutaneously) abrogated the protective effect of ODN (100 ng, i.c.v.) on pentylenetetrazol-induced convulsions. ODN (100 ng, i.c.v.) also reduced the percentage of DBA/2J mice displaying audiogenic convulsions. In contrast, ODN did not reduce the percentage of mice displaying tonic or clonic convulsions when electrical interauricular stimulations were applied. It is concluded that ODN, or more likely a proteolytic fragment derived from ODN, reduces pentylenetetrazol-induced convulsions through activation of central-type benzodiazepine receptors.     

Effect of di-n-propylacetate and γ-acetylenic GABA on hyperbaric oxygen-induced seizures and GABA metabolism

The administration of -acetylenic GABA or di-n-propylacetate to mice delayed the onset of hyperbaric oxygen-induced seizures in the animals. The former compound caused large increases in brain GABA content and strong inhibition of glutamate decarboxylase activity, whereas the latter compound brough about only moderate increases in brain GABA level and had little or no effect on the enzyme activity. It is suggested that the GABA system is not involved in the anticonvulsant mechanism of -acetylenic GABA but may play a role in the action of di-n-propylacetate.     

Interaction between GABA agonists and the cholinergic muscarinic system in rat corpus striatum

Chronic treatment with γ-acetylenic GABA(GAG), a GABA transaminase inhibitor, causes an increase in striatal dopamine receptor binding and function in rat brain suggesting that extrapyramidal side effects may accompany the use of these agents. In the present investigation it was found that chronic administration of THIP, a direct acting GABA receptor agonist, induced a similar increase in dopamine receptor binding. In addition, co-administration of atropine, a cholinergic muscarinic antagonist, was found to completely prevent the GABA-induced dopamine receptor increase. Furthermore, high affinity choline uptake, a measure of cholinergic activity, in striatal synaptosomes is enhanced after the acute administration of either GAG or THIP. Taken together these results support the notion of an interaction between dopaminergic, cholinergic and GABA-ergic neurons in the extrapyramidal system and indicate that co-administration of an anticholinergic agent may be of benefit in preventing the extrapyramidal side effects which may accompany the use of GABAergic agonists.     

Effect of GABAergic substances on firing rate and thermal coefficient of hypothalamic neurons in the juvenile chicken

The goal of the study is to investigate the GABAergic action on firing rate (FR) and temperature coefficient (TC) on hypothalamic neurons in the juvenile chicken. Extracellular recordings were obtained from 37 warm-sensitive, 32 cold-sensitive and 56 temperature-insensitive neurons in brain slices to determine the effect of GABA(A)-receptor agonist muscimol, GABA(A)-receptor antagonist bicuculline, GABA(B)-receptor agonist baclofen and GABA(B)-receptor antagonist CGP 35348. Muscimol and baclofen in equimolar concentrations (1 microM) significantly inhibited FR of the neurons, regardless of their type of thermosensitivity. In contrast, bicuculline, as well as CGP 35348 (10 microM) increased FR of the majority of the neurons. The TC of most chick hypothalamic neurons could not be estimated during muscimol application because FR was completely inhibited. GABA(B)-receptor agonist specifically increased TC. This effect was restricted to cold-sensitive neurons, which were determined in a high number. The TC was significantly increased (p<0.05) by baclofen and significantly decreased (p<0.05) by CGP 35348. The effects of muscimol and baclofen on FR and TC were prevented by co-perfusion of the appropriate antagonists bicuculline and CGP 35348. The results suggest that the fundamental mechanisms of GABAergic influence on temperature sensitive and insensitive neurons in the chicken PO/AH are conserved during evolution of amniotes.     

Reduction of pentylenetetrazol-induced convulsions by the octadecaneuropeptide ODN

Intracerebroventricular injection of the octadecaneuropeptide ODN in mouse, at doses of 12.5-1000 ng, reduced the percentage of convulsing animals and increased the latency of convulsions elicited by pentylenetetrazol (50 mg/kg, intraperitoneal [i.p.]). ODN also reduced the percentage of mortality induced by pentylenetetrazol (100 mg/kg, i.p.). The COOH-terminal octapeptide fragment of ODN was approximately equally effective but acted more rapidly than ODN to reverse the convulsant effect of pentylenetetrazol. ODN (100 ng, intracerebroventricular [i.c.v.]) increased the convulsion latency and reduced the percentage of animals that convulsed after the administration of the inverse agonist of benzodiazepine receptors DMCM (13 mg/kg, i.p.), whereas the benzodiazepine receptor antagonist flumazenil (1 mg/kg, subcutaneously) abrogated the protective effect of ODN (100 ng, i.c.v.) on pentylenetetrazol-induced convulsions. ODN (100 ng, i.c.v.) also reduced the percentage of DBA/2J mice displaying audiogenic convulsions. In contrast, ODN did not reduce the percentage of mice displaying tonic or clonic convulsions when electrical interauricular stimulations were applied. It is concluded that ODN, or more likely a proteolytic fragment derived from ODN, reduces pentylenetetrazol-induced convulsions through activation of central-type benzodiazepine receptors.