Adaptation of the GABAA-receptor complex in rat brain during chronic elevation of GABA by ethanolamine O-sulphate

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.

Action of delta-9-tetrahydrocannabinol on GABA(A) receptor-mediated responses in a grease-gap recording preparation of the rat hippocampal slice

We have investigated the effects of delta-9-tetrahydrocannabinol (delta-9-THC) on gamma-aminobutyric acid (GABA) receptor-mediated responses in a grease-gap recording preparation of the rat hippocampus. GABA, and the selective GABA(A) receptor agonist muscimol, evoked depolarizing responses with EC50 values of 8.5 mM and 17.0 microM, respectively. Responses to both of these agonists were selectively reduced by the non-competitive GABA(A) antagonist picrotoxin (5 microM), but were unaffected by the GABA(B) antagonist 2-hydroxysaclofen (500 microM). Responses evoked by the selective GABA(B) receptor agonist baclofen were not sufficiently large to analyse. The GABA uptake inhibitor, nipecotic acid (500 microM), potentiated responses to GABA, but not to muscimol. Similarly, 10-1000 nM delta-9-THC had no significant effect on the response to muscimol, whereas 1000 nM delta-9-THC significantly increased the response to GABA. Since GABA is the substrate of an avid uptake system, but muscimol is not, the results are consistent with the suggestion that delta-9-THC inhibits the uptake of GABA in the hippocampus.

Effects of single and multiple increasing doses of vigabatrin on brain GABA metabolism and correlation with vigabatrin plasma concentration

The effects of increasing (50-1600 mg/kg/day) doses of vigabatrin (GVG) both as single doses and after 8 or 28 days of treatment have been studied in 19 groups of 10 adult Wistar rats. The parameters studied were brain gamma-aminobutyric acid-transaminase (GABA-T) activity, GABA concentration and L-glutamate decarboxylase (GAD) activity. Single increasing doses of GVG progressively inhibited GABA-T activity, but a residual activity of about 40% was observed with the highest doses. GABA concentration increased in a dose-dependent manner but a ceiling was not reached. GAD activity was slightly inhibited at low doses and stimulated at high ones. When treatment was continued for 8 days, more marked effects of GVG on GABA-T and GABA, a more severe toxicity and higher GVG plasma concentrations were observed. GAD was inhibited instead of stimulated by high GVG doses. After 28 days of treatment the effects of GVG on GABA-T and GABA were similar to those after 8 days. However, toxic effects decreased and lower GVG plasma concentrations were found.

IN CONCLUSION

(a) the more marked brain GABAergic effects observed after 8 days of treatment with GVG may explain the greater anticonvulsant effects observed by others in animals, and (b) GVG plasma concentrations correlate well with changes in brain GABA-T and GABA, and may partly explain changes in the effects of GVG related to the length of treatment.

Functional changes in GABAA receptor stimulation during the oestrous cycle of the rat

1. Slices of rat cuneate nucleus were used to study whether or not gonadal steroids influence the gamma-aminobutyric acid (GABA) system in vivo. Females in different stages of the oestrous cycle as well as steroid-treated (oestrogen, progesterone or both) ovariectomized animals were used. 2. Functional changes in the GABAA receptors were assayed using the effects of potentiators (benzodiazepine, barbiturate) and antagonists (picrotoxin) on the muscimol control dose-response curves. 3. The potentiating effect of the benzodiazepine, flurazepam was unchanged during the oestrous cycle, and the hormone treatments did not alter this effect. 4. During oestrus, an increase was seen in the potentiating effect of the barbiturate (pentobarbitone). This suggests a synergistic effect between barbiturates and gonadal steroids. Progesterone treatment also increased the effect of pentobarbitone. 5. The antagonistic action of picrotoxin was unaffected during the oestrous cycle. However, progesterone (or progesterone and oestrogen) treatment reduced the potency of picrotoxin. 6. This study supports the idea that endogenous steroids (presumably progesterone) affect the GABAA receptors during the oestrous cycle by a mechanism associated with the barbiturate site of the GABAA receptor complex.

Effects of bilateral clamping of carotid arteries on hippocampal kindling in rats

Moderate reduction of cerebral blood flow by bilateral clamping of carotid arteries (BCCA) in pentobarbital anaesthetized Wistar rats induces decreased PO2 and temperature values in vulnerable brain structures such as hippocampus and frontal cortex during the acute phase of clamping. Up to two weeks chronic increased GABA contents in hippocampus, substantia nigra and frontal cortex could be observed. During this time the development of hippocampal kindling was difficult. These results demonstrated an acute and chronic influence of GABA on most vulnerable brain structures which, however, do not lead to ischemic impacts as histological analyses demonstrate.

Decreased susceptibility to seizures induced by bicuculline after transient bilateral clamping of the carotid arteries in rats

Rats were exposed for 24 min to bilateral clamping of the common carotid arteries (BCCA) in pentobarbital anaesthesia. 14 days later the animals were subjected to subcutaneous injection of (+)-bicuculline (3 or 4 mg/kg). A significantly decreased susceptibility to bicuculline-induced seizures could be observed in BCCA treated rats compared with sham operated controls. It is suggested that BCCA treatment protects animals against status epilepticus and lethal toxicity produced by bicuculline. Electrographic recordings of the BCCA animals revealed no ictal activity within 1 h after bicuculline injection. An analysis of the GABA content showed a significant increase in the hippocampus (HPC), frontal cortex (FCX), parietal cortex and substantia nigra in BCCA animals compared with controls. It is therefore possible that an increase in GABA content postsynaptically counteracts the GABAA antagonistic effect of bicuculline in BCCA animals thus preventing the normal seizure inducing effect of this substance.

Behavioural and biochemical effects of muscimol in ethanolamine O-sulphate-treated rats

A behavioural model, accompanied by biochemical analysis, was used to study the effects of the GABA-A receptor agonist muscimol on untreated and ethanolamine O- sulphate (EOS)-treated male rats. The most pronounced effect of muscimol was a lengthening of the duration of investigating behaviour. Pretreatment with EOS for 29 days potentiated this effect. Muscimol decreased the accumulation of GABA in the substantia nigra after 8 days but not after 30 days of EOS treatment, indicating that receptor adaptation might have occurred. We conclude that muscimol influences investigating behaviour and that chronic elevated brain GABA levels potentiate this effect.

The behavioural effects of chronic elevation of rat brain GABA by ethanolamine O-sulphate

This study examined the functional effects of long-term elevated brain GABA concentrations. The increase in GABA was achieved by use of the GABA-aminotransferase (GABA-T) inhibitor, ethanolamine O-sulphate (EOS). Forms of exploratory behaviour pre viously demonstrated to be sensitive to GABAergic receptor stimulation were measured over a 4 week period of EOS treatment. The most pronounced behavioural effect was an increased duration of investigating behaviour following 2-21 days of EOS treatment. This effect was accompanied by an accumulation of GABA in the substantia nigra and the corpus striatum. After 28-30 days of EOS treatment the GABA concentration was still significantly elevated, whereas the investigating behaviour was no longer different from controls.