Agonist and antagonist effects of benzodiazepines on motor performance: influence of intrinsic efficacy and task difficulty

Benzodiazepine-induced spatial learning deficits in rats are regulated by the degree of modulation of α1 GABA(A) receptors

Despite significant advances in understanding the role of benzodiazepine (BZ)-sensitive populations of GABAA receptors, containing the α1, α2, α3 or α5 subunit, factual substrates of BZ-induced learning and memory deficits are not yet fully elucidated. It was shown that α1-subunit affinity-selective antagonist β-CCt almost completely abolished spatial learning deficits induced by diazepam (DZP) in the Morris water maze. We examined a novel, highly (105 fold) α1-subunit selective ligand-WYS8 (0.2, 1 and 10 mg/kg), on its own and in combination with the non-selective agonist DZP (2 mg/kg) or β-CCt (5 mg/kg) in the water maze in rats. The in vitro efficacy study revealed that WYS8 acts as α1-subtype selective weak partial positive modulator (40% potentiation at 100nM). Measurement of concentrations of WYS8 and DZP in rat serum and brain tissues suggested that they did not substantially cross-influence the respective disposition. In the water maze, DZP impaired spatial learning (acquisition trials) and memory (probe trial). WYS8 caused no effect per se, did not affect the overall influence of DZP on the water-maze performance and was devoid of any activity in this task when combined with β-CCt. Nonetheless, an additional analysis of the latency to reach the platform and the total distance swam suggested that WYS8 addition attenuated the run-down of the spatial impairment induced by DZP at the end of acquisition trials. These results demonstrate a clear difference in the influence of an α1 subtype-selective antagonist and a partial agonist on the effects of DZP on the water-maze acquisition.

The motor-impairing effects of GABA(A) and GABA(B) agonists in gamma-hydroxybutyrate (GHB)-treated rats: cross-tolerance to baclofen but not flunitrazepam

gamma-Hydroxybutyrate (GHB) is believed to function as a neurotransmitter in the mammalian brain by binding to a GHB-specific binding site. In addition, GHB may also indirectly enhance the neuroinhibitory actions of gamma-aminobutyric acid (GABA) by converting to GABA at neuronal synapses. The purpose of the present study was to examine the effects of representative GABA(A) and GABA(B) receptor agonists in rats treated chronically with GHB. Using a rotorod apparatus, the motor-impairing effects of GHB, the indirect GABA(A) receptor agonist, flunitrazepam, and the direct GABA(B) receptor agonist, baclofen, were examined before, during and after chronic treatment with 1000 mg/kg GHB, b.i.d. Prior to chronic treatment, all three drugs produced dose-dependent decreases in motor performance at low (8 rpm) and high (32 rpm) rotational speeds. Chronic treatment with GHB significantly decreased the potency of baclofen at both speeds, but did not alter the potency of either GHB or flunitrazepam. Following termination of chronic treatment, the potency of baclofen increased significantly at both speeds and returned to that observed prior to chronic treatment. These data indicate that chronic treatment with GHB confers tolerance to a GABA(B) receptor agonist under conditions in which tolerance is not conferred to a GABA(A) receptor agonist. These findings are consistent with the in vivo behavioral profile of GHB, which reveals a greater role for GABA(B) receptors than for GABA(A) receptors in its behavioral effects.

Comparison of the behavioral effects of bretazenil and flumazenil in triazolam-dependent and non-dependent baboons

Behavioral effects of the benzodiazepine receptor partial agonist bretazenil were compared with those of the benzodiazepine receptor antagonist flumazenil under conditions in which three baboons received continuous intragastric (i.g.) infusion of vehicle and then continuous i.g. infusion of triazolam (1.0 mg/kg/day). In each condition, acute doses of flumazenil (0.01-3.2 mg/kg) and bretazenil (0.01-10.0 mg/kg) were administered every 2 weeks (beginning after 30 days of treatment in the triazolam-dependent condition). Food pellets were available during daily 20-h sessions. Following test injections, 60-min behavioral observations were conducted followed by a fine motor assessment. During chronic vehicle administration, neither drug produced changes in observed behaviors. Bretazenil increased pellets earned and time to complete the fine-motor task (10.0 mg/kg dose). During chronic triazolam dosing, both bretazenil and flumazenil precipitated benzodiazepine withdrawal syndromes, characterized by vomiting, tremors/jerks, and a decrease in pellets earned. Thus, bretazenil can function as an antagonist under conditions of benzodiazepine physical dependence.