Novel benzodiazepine receptor ligands stimulate intake of hypertonic NaCl solution in rehydrating rats

Benzodiazepine effects on licking responses for sodium chloride solutions in water-deprived male rats

The aim of this study was to investigate, for the first time, the effects of a centrally active benzodiazepine receptor agonist, midazolam maleate, on the microstructure of licking responses for a range of sodium chloride (NaCl) solutions in mildly water-deprived male rats. Doses of midazolam were chosen (0.3-3.0 mg/kg. i.p.) which have been characterised in studies of licking responses for several different kinds of nutrients. NaCl concentrations (0.075 M-0.45 M) were chosen to cover a range of taste preferences and acceptability. A brief-contact testing session was employed to focus on the initial determinants of licking responses (i.e. taste palatability), and to minimise any contribution of post-ingestional effects. The results indicate the midazolam significantly increased the total number of licks recorded across all salt concentrations, but that it had no effect on the number of bouts of licking. Instead, midazolam specifically enhanced the mean duration of licking bouts, an effect that was most evident at the weaker but more acceptable NaCl concentrations (0.075 M and 0.15 M). In addition, midazolam diminished the intrabout rate of licking across all salt concentrations. These results confirm that benzodiazepines can exert a specific pattern of effects on the microstructure of licking for salt solutions. They are discussed in terms of the oropharyngeal stimulation controlling intake and the palatability or "liking" hypothesis for the effects of benzodiazepines on taste stimuli, and indicate that the hypothesis is applicable to salt solution ingestive behaviour.

Effects of ethanol, caffeine, and clorazepate on hypertonic NaCl solution intake in rats

Previous reports indicate that several anxiolytics enhance the intake of hypertonic saline in rehydrating rats. This experiment was conducted to determine the effects of repeated (5 sessions) injection (i.p.) of ethanol (0.4 or 0.8 g/kg), caffeine (20 or 40 mg/kg) or clorazepate (3 mg/kg) on the ingestion of hypertonic saline (1.8%) in water-deprived rats. Saline intake increased with the acute administration of both clorazepate and ethanol (two doses), but it decreased with caffeine (two doses). It seems that the increase or decrease of hypertonic saline ingestion following acute drug administration continues to correlate well with anxiolytic or anxiogenic actions. However, following repeated administration of caffeine and ethanol, the effects on saline intake were not maintained in a reliable manner.

Effects of selective 5-HT1 receptor agonists in water-deprived rats on salt intake in two-choice tests

Twenty-two-hour water-deprived rats were divided into two groups: The first was given access to 1.8% saline and water in a 30-min two-choice test; the second was given access to 0.9% saline and water in the same type of intake preference test. Animals were tested following administration of several selective 5-hydroxytryptamine1 (5-HT1) receptor agonists. The results indicated a clear-cut distinction between the effects of selective 5-HT1A receptor agonists, on the one hand, and putative 5-HT1B/1C agonists on the other. Ipsapirone, gepirone, and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) all showed evidence of increasing the consumption of 1.8% saline (less preferred to water) but had no effect on intake of the more preferred 0.9% saline. In contrast, 1-3-(chlorophenyl)piperazine (mCPP) and 1-(3-(trifluoromethyl)phenyl)piperazine (TFMPP) (5-HT1B/1C agonists) reduced intake of 1.8 and 0.9% saline in the two tests. One interpretation of these results is to assume that the 5-HT1A agonists act at inhibitory autoreceptors to diminish central serotonergic activity, while mCPP and TFMPP act postsynaptically to enhance serotonergic activity. The possibility is discussed that mCPP and TFMPP may act to increase the perceived salt concentration during drinking, whereas the 5-HT1A agonists may have the opposite effect.

"Anxiolytic" and "anxiogenic" benzodiazepines and beta-carbolines: effects on aggressive and social behavior in rats and squirrel monkeys

Ethopharmacological studies on the behavior of socially housed rats and squirrel monkeys were conducted to explore the role of the benzodiazepine GABAA-coupled ionophore receptor complex in aggressive and social interactions. Benzodiazepine receptor (BZR) antagonists, ZK 93426 (1-10 mg/kg) and flumazenil (3-10 mg/kg), the partial agonist, ZK 91296 (1-10 mg/kg) and the partial inverse agonists Ro 15-4513 (0.3-10 mg/kg), were administered to (1) squirrel monkeys prior to 1 h focal observations within established social groups or to (2) resident male rats before confrontations with a naive male intruder in their home cage for 5 min. Aggression was modified in a similar manner in both species, although squirrel monkeys were more sensitive to BZR challenges. Specifically, resident male rats showed dose dependent reductions in attack bites directed at intruder males that were significant at the highest dose of ZK 93426 (10 mg/kg). In squirrel monkeys, ZK 93426 (3 and 10 mg/kg) reduced aggressive grasps, threats and displays, as well as reducing the duration of being the target of aggression from untreated group members (1-10 mg/kg). The BZR partial agonist, ZK 91296 and the antagonist, flumazenil produced few effects on social behavior, low and high intensity aggression and motor activity in both species. Flumazenil (10-30 mg/kg) and ZK 91296 (10 mg/kg), but not ZK 93426, produced significant increases in foraging and feeding behaviors in squirrel monkeys. The hyperphagic effects of ZK 91296 and flumazenil, that are typical of BZR agonists compounds, were not observed in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel benzodiazepine receptor ligands: palatable food intake following zolpidem, CGS 17867A, or Ro23-0364, in the rat

The potent imidazopyridine hypnotic, zolpidem, binds to central benzodiazepine receptors and has predominantly sedative properties, as determine in animal models. In tests of palatable food consumption in nondeprived male rats, the present results indicate that zolpidem (0.3-3.0 mg/kg) had no effect on food intake. Its lack of effect contrasts sharply with other benzodiazepine agonists which strongly stimulate palatable food intake. Two other novel compounds, both of which bind to benzodiazepine receptors, and which have reduced propensity to induce sedative effects, increased palatable food consumption, although in differing ways. The imidazobenzodiazepine Ro23-0364 (0.3-10.0 mg/kg) dose-dependently increased feeding in the standard procedure, but failed to stimulate food intake in presatiated animals. The pyrazoloquinoline CGS 17867A (1.0-30.0 mg/kg) increased food intake in both test procedures, although the dose-effect relationship was nonmonotonic. Taken together, the data indicate a probable separation between hyperphagic and sedating effects of benzodiazepine receptor agonists. If zolpidem's sedative effect is linked to an action at a receptor subtype (benzodiazepine Type 1 or omega 1), then the hyperphagic effect of benzodiazepines may depend more on the alternative subtype.

Selective reduction by serotonergic agents of hypertonic saline consumption in rats: evidence for possible 5-HT1C receptor mediation

Previously, it has been shown that, in small doses, putative 5-HT1A receptor agonists selectively increase ingestion of hypertonic saline without affecting either water or isotonic saline intake. Evidence was obtained in the present series of experiments for selective reduction in hypertonic saline following the administration of a variety of serotonergic directly- and indirectly-acting agonists. Water, isotonic saline (0.9%), or hypertonic saline (1.8%) were made available to separate groups of water-deprived rats. The results indicated some selectivity with the 5-HT-uptake inhibitor and releaser d-fenfluramine and the 5-HT uptake inhibitor fluoxetine, and with the 5-HT agonists mCPP [1-3-chlorophenyl)piperazine] and MK 212 [6-chloro-2-(1-piperazinyl)pyrazine]. In each case, hypertonic intake was significantly suppressed. Distinct from these compounds were TFMPP [1-(3-(trifluoromethyl)phenyl)piperazine], RU 24969 [5-methoxy-3-(1,2,3,6-tetrahydropyridinyl)1H-indole], and quipazine. This second group either reduced fluid intake indiscriminately or reduced water and isotonic saline drinking. Selective reduction in the intake of hypertonic saline did not occur. Finally, peripheral-administration of 5-HT or the 5-HT1B agonist CGS 12066B [7-trifluoromethyl-4(4-methyl-1-piperazinyl-pyrolo) (1,2-a) 1:2 maleate], had no significant effect on fluid intake in any fluid condition. The results are discussed in terms of a possible serotonergic mechanism which may underlie inhibition of hypertonic salt drinking, and which involves mediation through a subtype of the 5-HT1 receptor.

Bi-directional changes in sham feeding in the rat produced by benzodiazepine receptor ligands

Both the real and sham intake of a 5% sucrose solution were increased by midazolam (3 mg/kg IP), a benzodiazepine agonist. In contrast, both the real and sham intakes of a 20% sucrose solution were reduced by Ro15-3505 (a benzodiazepine antagonist with weak inverse agonist properties) and by FG 7142 (a beta-carboline inverse agonist). These data demonstrate that drugs acting at benzodiazepine receptors can bi-directionally alter ingestional responses, probably by modulation of the oropharyngeal control of consumption.

Specific effect of putative 5-HT1A agonists, 8-OH-DPAT and gepirone, to increase hypertonic saline consumption in the rat: evidence against a general hyperdipsic action

Previous reports indicate that 5-HT1A agonists, in addition to benzodiazepines, increase the consumption of hypertonic saline in rehydrating rats. Experiment 1 investigated the effects of 8-OH-DPAT (10-100 micrograms/kg) and gepirone (0.1-3.0 mg/kg) on consumption of water and of saline over a range of concentrations (0.45%-2.8%) in a 30 min drinking test. The two 5-HT1A agonists dose-dependently increased ingestion of two hypertonic salt solutions, but produced little or no increase in the drinking of water, hypotonic or isotonic saline. Experiment 2 demonstrated that 8-OH-DPAT and gepirone did not enhance water consumption in animals given a water preload, or markedly increase drinking quinine-adulterated water. Taken together, the results indicate a selective dose-related effect of the two drugs to increase hypertonic saline drinking; they did not have a general hyperdipsic effect across all salt and water conditions, and they did not increase intake simply because of a low baseline level of consumption. Hence, 5-HT1A agonist act much more selectively than benzodiazepines in their effects on drinking responses.

The benzodiazepine partial agonists, Ro16-6028 and Ro17-1812, increase palatable food consumption in nondeprived rats

Two novel imidazobenzodiazepines, Ro16-6028 and Ro17-1812, have been described recently as partial agonists acting at benzodiazepine receptors. In a test of palatable food consumption using nondeprived rats, Ro16-6028 (0.01-10 mg/kg) and Ro17-1812 (0.01-10 mg/kg) were shown to produce dose-dependent increases in food intake. Ro16-6028 was more potent than Ro17-1812. Suriclone, midazolam, and the beta-carbolines ZK 93423 and ZK 91296 also significantly increased food intake. The maximum effects of Ro16-6028 and Ro17-1812 were at least equivalent to those obtained with full agonists acting at benzodiazepine sites. Neither Ro16-6028 nor Ro17-1812 reduced locomotion or rearing frequency in an open field test, although there was a reduction in grooming frequency. In contrast, the full agonist midazolam dose-dependently reduced all measures of general activity. The results indicate that some novel benzodiazepine partial agonists strongly stimulate food intake in the absence of side effects typical of the classical benzodiazepines.

Benzodiazepines and putative 5-HT1A agonists increase hypertonic saline consumption in rehydrating rats

Male rats were adapted to a 22 hr water-deprivation schedule, and to a 30 min test of hypertonic (1.8 or 2.7%) NaCl solution ingestion. A novel benzodiazepine, Ro23-0364, recently reported to have anxiolytic activity in rats and squirrel monkeys but to have limited potential to produce unwanted side effects, produced significant dose-related increases in hypertonic saline ingestion. Midazolam, a benzodiazepine full agonist, increased salt intake but the effect was offset at higher doses by the induction of sedation. Three putative 5-HT1A agonists, proposed as nonbenzodiazepine-related anxiolytics, were also tested: the highly selective 8-OH-DPAT, gepirone and ipsapirone (TVX Q 7821). In each case, occasions when hypertonic saline consumption was significantly increased were detected. At 300 micrograms/kg of 8-OH-DPAT and 10 mg/kg of gepirone, the appearance of a pronounced flattened body posture effectively interfered with drinking responses. It appears possible that a behavioural action shared by benzodiazepines and 5-HT1A agonists may be responsible for the increased hypertonic saline ingestion.

Partial agonists acting at benzodiazepine receptors can be differentiated in tests of ingestional behaviour

Several categories of compounds active at benzodiazepine receptors (BZR) in the brain have been distinguished: agonists, antagonists and the novel category of inverse agonist. In terms of their effects on ingestional responses (e.g., food, saline and water consumption), agonists increase levels of intake, inverse agonists reduce intake in some, if not all, tests, while antagonists block the effects of both agonists and inverse agonists. Attention is currently focussed upon a range of compounds which fall between full agonists and antagonists. These partial agonists are of particular interest since they act more selectively than full agonists, retaining effects in animal models of anxiolytic and anticonvulsant activity, for example, while largely lacking behaviourally-depressant effects. Recent data indicate that tests of ingestional behaviour distinguish between various BZR partial agonists. The benzodiazepines Ro23-0364, Ro16-6028 and Ro17-1812, as well as the beta-carboline ZK 91296, enhanced ingestional responses. The pyrazoloquinolines, CGS 9895 and CGS 9896, did not, but antagonized agonist-induced increases in ingestion.