Isoniazid-induced convulsions in rats: effects of Ro 15-1788 and beta-CCE

The behavioral pharmacology of zolpidem: evidence for the functional significance of α1-containing GABA(A) receptors

RATIONALE

Zolpidem is a positive allosteric modulator of γ-aminobutyric acid (GABA) with preferential binding affinity and efficacy for α1-subunit containing GABA(A) receptors (α1-GABA(A)Rs). Over the last three decades, a variety of animal models and experimental procedures have been used in an attempt to relate the behavioral profile of zolpidem and classic benzodiazepines (BZs) to their interaction with α1-GABA(A)Rs.

OBJECTIVES

This paper reviews the results of rodent and non-human primate studies that have evaluated the effects of zolpidem on motor behaviors, anxiety, memory, food and fluid intake, and electroencephalogram (EEG) sleep patterns. Also included are studies that examined zolpidem's discriminative, reinforcing, and anticonvulsant effects as well as behavioral signs of tolerance and withdrawal.

RESULTS

The literature reviewed indicates that α1-GABA(A)Rs play a principle role in mediating the hypothermic, ataxic-like, locomotor- and memory-impairing effects of zolpidem and BZs. Evidence also suggests that α1-GABA(A)Rs play partial roles in the hypnotic, EEG sleep, anticonvulsant effects, and anxiolytic-like of zolpidem and diazepam. These studies also indicate that α1-GABA(A)Rs play a more prominent role in mediating the discriminative stimulus, reinforcing, hyperphagic, and withdrawal effects of zolpidem and BZs in primates than in rodents.

CONCLUSIONS

The psychopharmacological data from both rodents and non-human primates suggest that zolpidem has a unique pharmacological profile when compared with classic BZs. The literature reviewed here provides an important framework for studying the role of different GABA(A)R subtypes in the behavioral effects of BZ-type drugs and helps guide the development of new pharmaceutical agents for disorders currently treated with BZ-type drugs.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

Sarmazenil-precipitated withdrawal: a reliable method for assessing dependence liability of benzodiazepine receptor ligands

The benzodiazepine receptor partial inverse agonist sarmazenil exhibits in vivo proconvulsive, but not convulsant, effects in different paradigms in rodents. Intravenous sarmazenil challenge given at several fixed intervals following the termination of repeated treatment with a markedly sedative dose of diazepam in squirrel monkeys was effective in precipitating withdrawal signs, but had no comparable effects in vehicle-treated controls. The precipitated withdrawal reaction was not only robust, but it was consistently observed in all of the diazepam-treated monkeys. Thus, the use of sarmazenil challenge in the precipitated withdrawal paradigm provides a reliable method for assessing the development of physical dependence during repeated treatment with benzodiazepine receptor agonists.

Evidence that background GABAergic tone and possible ligand release may alter the action of the 'neutral' benzodiazepine-receptor antagonist, Ro 15-1788, in hypothalamic self-stimulation

Upward or downward shifts in the level of brain GABAergictransmission have been held to be necessary and sufficient to promote release of endogenous ligands ('endocoids') for the benzodiazepine (BZD) recognition site. To investigate this possibility, variable-interval self-stimulation performance was used to monitor 'intrinsic' benzodiazepine-like and anti- benzodiazepine activity by the 'neutral' benzodiazepine-receptor antagonist, Ro 15-1788 (flumazenil) (10 or 30 mg/kg intraperitoneally). Rats were pretreated with either a GABA synthesis blocking agent (isoniazid, 130 mg/kg subcutaneously), or with a GABA agonist (progabide, 30 or 100 mg/kg intraperitoneally). The lower dose of Ro 15-1788 (10 mg/kg), without pretreatment, did not affect self-stimulation; higher doses (30 mg/kg) caused a brief (<20 min) depression. Isoniazid (130 mg/kg) depressed self-stimulation, but did not modify the activity of Ro 15-1788. In rats pretreated with progabide (100 mg/kg), low doses of Ro 15- 1788 (10 mg/kg) that were previously without effect now caused a sharp fall in responding. These findings can be interpreted as showing that even low doses of Ro 15-1788 may affect self-stimulation under certain conditions, and that they do so by competing with an endogenous ligand for the benzodiazepine site, released by upward shifts in GABAergic activity. Alternative explanations in terms of altered receptor function seem less feasible. The results imply that the action of the endogenous ligand would not resemble that of a typical benzodiazepine, but that of an inverse agonist (that is, proconflict and proconvulsant); this conclusion agrees with recent biochemical evidence.

Proconflict and electrocorticographic effects of drugs modulating GABAergic neurotransmission

Proconflict and electrocorticographic effects of drugs acting on the benzodiazepine (BDZ)/GABA/chloride-ionophore receptor complex were studied in rats in an attempt to correlate their anxiogenic and epileptogenic activities. Evidence for proconflict activity was assessed by means of an operant conflict procedure based on the simultaneous reward and punishment of a conditioned task, while epileptogenic properties were assessed by monitoring the electrocorticogram (ECoG) of free-moving rats. Pentylenetetrazole and picrotoxin, which act through a site on the chloride channel, and the benzodiazepine (BDZ) inverse agonist FG 7142 showed epileptogenic alterations in the ECoG at doses, respectively, 8, 2 and 3 times higher than those eliciting a significant proconflict effect. For the partial inverse agonist CGS 8216, a ratio of about 60 was found while the BDZ antagonist Ro 15-1788 showed neither epileptogenic nor proconflict activity, except at the highest tested dose for the latter effect (40 mg X kg-1 PO). Inhibition of GABA transmission may mediate both anxiogenic and epileptogenic actions, and a link between these properties may exist as a continuous spectrum of negative intrinsic efficacy at the central BDZ/GABA/chloride-ionophore receptor complex.

Intrinsic actions of the benzodiazepine receptor antagonist Ro 15-1788

The imidazodiazepine Ro 15-1788 is a benzodiazepine receptor antagonist that was initially reported to be lacking in intrinsic activity in a variety of test situations in which benzodiazepine-like effects can be identified. However, many recent studies have shown that this compound does indeed have intrinsic activity in a variety of behavioural, neurological, electrophysiological and biochemical preparations in both animals and man. The purpose of the present review is firstly to describe these intrinsic actions, and secondly to consider to what extent these intrinsic actions of Ro 15-1788 have implications for current concepts of the functioning of the benzodiazepine receptor.