Discriminative stimulus properties of CL218872 and chlordiazepoxide in the rat

Zaleplon and triazolam: drug discrimination, plasma levels, and self-administration in baboons

Zaleplon is a chemically novel hypnotic that preferentially binds alpha(1)-subunit containing subtypes of the alphabetagamma configuration of the gamma-aminobutyric acid (GABA)(A) receptor. Zaleplon and the non-subtype-selective hypnotic triazolam occasioned 100% drug-appropriate responding in baboons trained to discriminate lorazepam or pentobarbital from vehicle. Flumazenil shifted the zaleplon generalization gradient at least five-fold to the right. A plasma elimination half-life of 6-8 h for oral 10 mg/kg zaleplon and 0.32 mg/kg triazolam was paralleled by discriminative control for 7 h. Zaleplon maintained self-injection greater than vehicle, as did comparison doses of the similarly selective hypnotic zolpidem and triazolam. Concurrent food-maintained responding increased during self-injection of all three drugs. Preferential binding at this alpha(1)-containing GABA(A) subtype did not diminish the benzodiazepine (Bzs)-like behavioral effects of zaleplon.

Discriminative stimulus effects of drugs acting at GABA(A) receptors: differential profiles and receptor selectivity

The GABA(A) receptor complex contains a number of binding sites at which a variety of psychotropic drugs, including benzodiazepines, barbiturates, and some neurosteroids, act to potentiate or inhibit the effect of the transmitter. Many studies have reported that these drugs can produce discriminative stimulus actions, but the cueing effects of compounds acting at different sites to enhance the effects of GABA are not identical. The discriminative stimulus effects of benzodiazepines have been analyzed in detail, and there is also a great deal of information available on the effects of nonbenzodiazepine compounds acting at BZ(omega) recognition sites, which form part of the GABA(A) receptor complex. Of particular interest are compounds with selectivity for the BZ1(omega1) receptor subtype including zolpidem, zaleplon, and CI 218,872. BZ1(omega1)-selective drugs substitute for the discriminative stimulus produced by chlordiazepoxide only partially and at sedative doses. This is consistent with the view that sedative effects of BZ(omega) receptor agonists are mediated by the BZ1(omega1) receptor subtype, whereas the discriminative stimulus produced by chlordiazepoxide may be produced by activity at the BZ2(omega2) subtype. Analysis of this hypothesis is complicated by the variety of levels of intrinsic activity shown by different drugs.

Recent developments in the behavioral pharmacology of benzodiazepine (omega) receptors: evidence for the functional significance of receptor subtypes

Recent research in molecular biology has demonstrated the complexity of GABAA receptors and shown that benzodiazepine (BZ-omega) receptor subtypes have a structural reality. It is therefore appropriate to ask whether the different pharmacological effects produced by benzodiazepines (anticonvulsant activity, anxiety reduction, motor incoordination, learning deficits, characteristic discriminative stimulus effects, tolerance and dependence) are associated with activity at different receptor subtypes. The present paper reviews the literature dealing with the behavioral effects of novel BZ (omega) receptor ligands relevant to the question of the functional significance of the BZ1 (omega 1) and BZ2 (omega 2) receptor subtypes. The only drugs currently available with a considerable degree of selectivity are alpidem and zolpidem. These compounds have relatively high affinity for GABAA receptors containing the alpha 1 subunit (corresponding to the BZ1 (omega 1) subtype) and very low affinity for receptors with the alpha 5 subunit (corresponding to one type of BZ2 (omega 2) receptor). Pharmacological effects observed with these, and other, less selective compounds allow several tentative conclusions to be drawn: (a) Little is known of the role of subtype selectivity in anxiolytic or amnestic effects but compounds with low intrinsic activity may reduce anxiety without giving rise to sedation or motor incoordination and BZ1 (omega 1) selective drugs appear to disrupt memory only at sedative doses; (b) Selectivity for BZ1 (omega 1) receptors may be associated with sleep-inducing activity but not with motor incoordination, suggesting that BZ2 (omega 2) receptors may be of particular importance in mechanisms of muscle relaxation; (c) The discriminative stimulus effects of different BZ (omega) receptor ligands are not identical and differences may be related to receptor selectivity; (d) Compounds with BZ1 (omega 1) selectivity and compounds with low intrinsic activity produce little or no tolerance and dependence. A wider range of selective compounds will be necessary to investigate these factors in detail and many different pharmacological profiles can be expected from drugs with selectivity and different levels of intrinsic activity.

Discriminative stimulus effects of alpidem, a new imidazopyridine anxiolytic

Alpidem in an imidazopyridine derivative which binds selectively to the omega 1 (BZ1) receptor subtype. It is active in some, but not all, behavioural tests sensitive to benzodiazepine anxiolytics and has clinical anti-anxiety effects. However, in a previous study, it was shown that alpidem did not substitute for chlordiazepoxide in rats trained to discriminate this benzodiazepine. The present experiments were carried out to investigate the discriminative stimulus properties of alpidem in greater detail. In the first experiment rats learned to discriminate a dose of 10 mg/kg alpidem from saline. Acquisition of the discrimination was long and performance unstable. Chlordiazepoxide, clorazepate and zolpidem substituted only partially for alpidem but the effects of the training dose of alpidem were blocked by 10 mg/kg flumazenil. The second experiment established stimulus control more rapidly to a dose of 30 mg/kg alpidem. Alpidem induced dose-related stimulus control, and dose-related and complete substitution for alpidem was produced by zolpidem, abecarnil, CL 218,872, triazolam and suriclone. Partial substitution occurred with chlordiazepoxide, clorazepate and pentobarbital. In most cases, high levels of substitution were produced only by doses which greatly reduced response rates even though the training dose of alpidem produced only modest decreases in rates. Ethanol, buspirone and bretazenil produced very little substitution for alpidem and both flumazenil and bretazenil antagonised the effects of alpidem. In two further experiments alpidem was found to substitute for the stimulus produced by zolpidem (2 mg/kg) but not for that produced by ethanol (1.5 g/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of a new anxiolytic, DN-2327, in rats

In an operant learning lever-pressing procedure on an FR10 schedule of milk reinforcement, male Wistar rats were trained to discriminate between saline and 3 mg/kg IP DN-2327, a new anxiolytic which acts on benzodiazepine receptors, 3 mg/kg IP diazepam or 15 mg/kg IP pentylenetetrazol (PTZ). More than 80% appropriate lever responding was established after 27, 38 and 44 daily training sessions with DN-2327, diazepam and PTZ, respectively, as the training drug. Although rats trained with DN-2327 dose-dependently generalized to various doses of DN-2327 and diazepam, the cue of DN-2327 was more potent than that of diazepam: ED50 values of DN-2327 and diazepam for stimulus generalization were 0.30 and 0.66 mg/kg, respectively. These animals partially generalized to pentobarbital (1-10 mg/kg) but did not generalize to buspirone (0.1-10 mg/kg). Rats trained with diazepam dose-dependently generalized to various doses of DN-2327, diazepam and pentobarbital with ED50 values of 0.51, 0.47 and 4.5 mg/kg, respectively, but did not generalize to buspirone. In rats trained with PTZ, DN-2327 and diazepam antagonized the discriminative stimulus produced by 15 mg/kg PTZ in a dose-dependent manner with ED50 values of 0.27 and 0.83 mg/kg, respectively, but buspirone neither antagonized nor was able to substitute for the PTZ-induced stimulus. The cue of DN-2327 was antagonized by flumazenil dose-dependently as was that of diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of omega (BZ) receptor ligands: correlation with in vivo inhibition of [3H]-flumazenil binding in different regions of the rat central nervous system

Rats can be trained to discriminate benzodiazepines (BZ) from vehicle and there is considerable evidence that the stimulus effects of these drugs are mediated by activity at omega (BZ) modulatory sites of the GABAA receptor complex. A number of recent studies, however, have indicated that differences may exist between the discriminative stimulus effects of benzodiazepines and those of certain non-benzodiazepine ligands for the omega (BZ) receptors (e.g. zolpidem, abecarnil). As it is known that several subtypes of omega (BZ) sites are found in the central nervous system, and that drugs such as zolpidem have selectivity for certain subtypes, it is possible that differential stimulus effects may be associated with receptor selectivity. In the present study, correlations were calculated between the potencies of nine compounds with affinity for omega receptors (diazepam, lorazepam, triazolam, clonazepam, alprazolam, zopiclone, suriclone, CL 218, 872 and zolpidem) to substitute for chlordiazepoxide in rats trained to discriminate a dose (5 mg/kg) of this benzodiazepine and the ability of the same compounds to inhibit the binding of [3H]-flumazenil from different structures in the rat central nervous system in vivo. The correlations obtained were: cerebellum 0.46, cortex 0.39, striatum 0.78 (P < 0.05), hippocampus 0.79 (P < 0.05) and spinal cord 0.95 (P < 0.001). These different structures are known to contain different relative concentrations of omega 1 (BZ1) and omega 2 (BZ2) sites with the spinal cord containing the greatest (80%) and cerebellum the lowest (5%) concentration of omega 2 (BZ2) sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus properties of RU 33965, a benzodiazepine receptor weak partial inverse agonist

Rats were trained to discriminate the low-efficacy benzodiazepine receptor inverse agonist RU 33965 from vehicle in a two-lever discrimination task on a fixed ratio (FR) 20 schedule. Consistent discrimination was obtained at 0.5 mg/kg PO RU 33965. Both leptazol and stronger inverse agonists (FG7142, S-135, RU 34000) substituted for the cue. The weak inverse agonists/antagonists RU 33094, RU 34030, Ro 15-1788, and ZK 93426 also substituted for the cue with the latter two compounds being particularly potent. The agonist and partial agonists diazepam, RU 33203, and RU 39419 did not substitute for the RU 33965 cue but RU 39419 antagonised it. The full agonists diazepam and loprazolam only consistently antagonised the cue when given IP 5 min pretest. These data suggest that the RU 33965 cue results from its weak inverse agonist activity at benzodiazepine receptors, but kinetic factors must be considered when interpreting drug effects in discrimination studies.

A review of recently-developed ligands for neuronal benzodiazepine receptors and their pharmacological activities

There has recently been a large increase in the number of new benzodiazepine receptor ligands, some with benzodiazepine structures, but many with other chemical structures. The pharmacological activities of these ligands have been interpreted on the basis of a continuum of efficacy ranging from full agonist through different degrees of partial agonism to antagonist and through partial to full inverse agonists. Inconsistencies with this hypothesis are considered in terms of alternative hypotheses, particularly the existence of functionally separable receptor subtypes. The potential of partial agonists as non-sedative anxiolytic agents with reduced potential of dependence and of weak partial inverse agonists as pro-cognitive agents is discussed. A pharmacophore for benzodiazepine receptors is proposed and supporting evidence presented.