Behavioural effects of novel benzodiazepine (omega) receptor agonists and partial agonists: increases in punished responding and antagonism of the pentylenetetrazole cue.

In recent years a number of novel compounds have been described with affinity and specificity for BZ (omega) receptors. While some of these agents appear to act, like benzodiazepines themselves, as full agonists at different receptor subtypes (e.g. suriclone), several non-selective partial agonists (e.g. bretazenil) have been described, as have a number of BZ(1) (omega(1)) selective drugs (e.g. zolpidem). Previous work has reported a number of differences between the behavioural effects of some of these drugs and those of benzodiazepines; however, very few studies have attempted systematic comparisons of a large number of drugs in different procedures. In the present study a wide range of BZ (omega) receptor ligands was studied using two behavioural methods in rats: unpunished and punished food-reinforced operant responding and the discriminative stimulus effects of pentylenetetrazole. Punished operant responding showed increases with the benzodiazepines, chlordiazepoxide and clorazepate, the non-benzodiazepines, saripidem, CL 273,547 and F 2692 (limited effect at a single dose) and the partial agonists bretazenil and Ro 19-8022, but the BZ(1) selective agents, alpidem, abecarnil and CL 284,846, did not increase rates of punished operant responding. Rates of unpunished responding were decreased by higher doses of all drugs except bretazenil and Ro 19-8022. Dose-related antagonism of the pentylenetetrazole (18mg/kg) discriminative stimulus was produced by several benzodiazepines, by the partial agonists bretazenil, Ro 19-8022 and divaplon, and by suriclone, saripidem and CL 273,547. The BZ(1) (omega(1)) selective drugs abecarnil, CL 284,846, zolpidem, CL 218,872 and alpidem were also active in blocking pentylenetetrazole but produced only partial antagonism which was not clearly dose-related. The results show that novel BZ (omega) receptor ligands do not always produce a behavioural profile identical to that shown by benzodiazepines. In particular, BZ(1) (omega(1)) selective drugs do not give rise to clear increases in punished operant responding and have only limited efficacy in blocking the pentylenetetrazole cue. These effects may be due to the marked propensity of BZ(1) (omega(1)) selective drugs to decrease operant response rates.