Modulation of the GABA autoreceptor by benzodiazepine receptor ligands

Modulation of the GABAA-like autoreceptor by barbiturates but not by steroids

In slices of cerebral cortex from rat preloaded with [3H]GABA, muscimol produced a concentration-related inhibition of K(+)-evoked release of tritium with a pIC25 value of 7.80 +/- 0.39. Dimethylbarbituric acid (10 and 100 microM) and pentobarbitone (100 microM) significantly increased this value to 8.31 +/- 0.09, 9.91 +/- 0.21 and 8.50 +/- 0.21, respectively, whereas the steroid ligands alphaxalone (1 microM) and 5 beta-pregnane-3 alpha-ol-20-one (10 nM) had no significant effect. The 5 beta-pregnane-3 alpha-ol-20-one and 5 beta-pregnane-3,20-dione produced a concentration-related increase in K(+)-evoked release of tritium alone. These data suggest that the GABAA-like autoreceptor may be modulated by barbiturates but not by steroids and thus may be different from the postsynaptic GABAA receptor.

Desensitization of GABAB receptors and antagonism by CGP 35348, prevent bicuculline- and picrotoxin-induced antinociception

The effect of the GABAA antagonists, bicuculline and picrotoxin, in the hot plate and writhing tests in mice and the paw-pressure test in rats was assessed. Subconvulsant doses of bicuculline (1.3-4 mumol kg-1, s.c.) or picrotoxin (0.8-2.5 mumol kg-1, s.c.) induced a dose-related increase in latency of licking in the hot plate test in mice, whereas subconvulsant doses of strychnine and thiosemicarbazide (0.9 and 6 mg kg-1, s.c. respectively), did not modify the threshold to thermal stimuli in mice. The effects of bicuculline and picrotoxin were not modified by naloxone (3 mg kg-1, i.p., a dose which inhibited the antinociceptive effect of morphine) or by atropine (5 mg kg-1, i.p., a dose which prevented oxotremorine-induced antinociception) but were antagonized by the GABAB antagonist CGP 35348 (2.5 micrograms, i.c.v., a dose which prevented (+/-)baclofen-induced antinociception). Mice, rendered tolerant to baclofen-induced antinociception by twice daily injection of increasing doses of baclofen (5-18 mg kg-1, s.c.), were unresponsive to the antinociceptive effects of bicuculline and picrotoxin but still responded to morphine. Bicuculline and picrotoxin, in the same range of doses which affected the three models of antinociception used, inhibited pentobarbital-induced hypnosis. Large doses of bicuculline and picrotoxin (4 and 2.5 mumol kg-1, s.c. respectively), reduced locomotor activity and impaired rota-rod performance in mice. The changes in response to noxious stimuli, induced by bicuculline and picrotoxin, are interpreted as an antinociceptive effect. It is then suggested that this effect might depend on an indirect activation of GABAB receptors through release of GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic benzodiazepine treatment of rats induces reduction of paired-pulse inhibition in CA1 region of in vitro hippocampus

Paired-pulse inhibition was studied extracellularly in in vitro hippocampal slices from rats sacrificed 48 h or 7 days after 1 week flurazepam (FZP) treatment. Population spikes and field excitatory postsynaptic potentials (EPSPs) were recorded with NaCl-containing glass micropipettes in the stratum pyramidale and stratum radiatum, respectively, of the CA1 region. Conditioning pulses were delivered by stimulating Shaffer collaterals (orthodromic) or the alveus (antidromic). Orthodromic test pulses were delivered with interpulse intervals of 10-200 ms. There was a significant reduction in paired-pulse inhibition in slices from treated vs control rats in both the orthodromic-orthodromic and antidromic-orthodromic paradigms. Reduced inhibition was evident 48 h, but not 7 days, after the end of FZP treatment. Furthermore, there was a significant prolongation of the half decay time of the field EPSP, without a significant change in the initial slope or maximum amplitude. The results may suggest an impairment of endogenous gamma-aminobutyric acid function in the hippocampus after chronic benzodiazepine (BZ) treatment and may provide a basis for a mechanism of BZ tolerance.

GABA turnover in the brain of rat lines developed for differential ethanol-induced motor impairment

The role of GABAergic neurons in the differential sensitivity to ethanol between the AT (Alcohol Tolerant) and ANT (Alcohol Nontolerant) rat lines developed for low and high degree of motor impairment from ethanol, was studied by comparing the effect of ethanol (2 or 4 g/kg, IP) on GABA turnover in different regions of the brain in these rat lines. GABA turnover was estimated from the accumulation of GABA after inhibition of GABA aminotransferase with aminooxyacetic acid (AOAA, 50 mg/kg, IP) given 10 min after administration of ethanol. The rats were killed two hours after the AOAA treatment with focused microwaves. The concentrations of GABA, aspartate, glutamate, glutamine and taurine were analyzed with HPLC. The saline-treated ANT rats were found to have a higher concentration of GABA in the striatum and a higher rate of GABA accumulation in the cerebellum than the AT rats. Ethanol suppressed the accumulation of GABA in both lines, but the suppression was significantly greater in the AT rats than in the ANT rats. In specific regions, this line difference was significant in the cerebral cortex and cerebellum with the higher ethanol dose. No line differences were found in the brain or tail blood ethanol concentration. AOAA increased the concentration of glutamine, decreased that of aspartate and glutamate, and did not modify that of taurine. The AOAA-induced changes in the concentrations of these amino acids were, however, minor relative to those found in the concentrations of GABA. The results that GABAergic mechanisms are involved in the differential sensitivity to the motor-impairing effects of ethanol between the AT and ANT rats.