CGS 8216, a novel anorectic agent, selectively reduces saccharin solution consumption in the rat

Bidirectional modulation of sweet and bitter taste by chlordiazepoxide and Ro 15-4513: lack of effect with GABA drugs

Five rats were trained to respond for 10% sucrose and 10% sucrose/0.006% quinine in an operant procedure. Both solutions were concurrently available on independent, variable-interval 5-s schedules of reinforcement. Rats reliably responded for both solutions throughout the sessions and made approximately 68% of their total daily responses for the sucrose solution. When injected prior to the sessions with 4 mg/kg of chlordiazepoxide, rats selectively increased quinine responding; injections of the benzodiazepine inverse agonist Ro 15-4513 (9 mg/kg) led to decreased quinine responding. The effects of both chlordiazepoxide and Ro 15-4513 were reversed by the benzodiazepine antagonist flumazenil. Presession injections of flumazenil, muscimol, baclofen, or picrotoxin all resulted in no changes in responding, or a decrease in responding for both solutions. These results are discussed in terms of a bidirectional modulation of sweet-bitter taste preference by drugs acting on the benzodiazepine receptor. Moreover, the data from these experiments suggest that any changes in the oral consumption of alcohol following administration of benzodiazepine drugs must be examined in light of their effects on taste palatability.

Ro 15-4513 selectively attenuates ethanol, but not sucrose, reinforced responding in a concurrent access procedure; comparison to other drugs

The experiments described in this report used a concurrent access procedure to study ethanol reinforcement. Rats were trained to lever press for a 10% sucrose solution and a 10% ethanol/10% sucrose mixture, and both reinforcers were available on variable-interval 5-s schedules. In baseline and vehicle injection sessions, the animals distributed their responding between both solutions. When injected with the partial inverse benzodiazepine agonist Ro 15-4513 (3, 9, and 18 mg/kg), responding for the ethanol solution decreased while responding for sucrose remained intact. Ethanol injections (0.5 and 1.0 g/kg) engendered a similar profile. Chlordiazepoxide led to an increase in ethanol mix responding at 2 mg/kg and a decrease in ethanol mix responding at higher doses; no dose affected sucrose responding. Morphine (0.5-16 mg/kg) decreased responding for both the ethanol mix and sucrose solutions, more or less simultaneously. Naloxone (0.125-20 mg/kg) selectively reduced ethanol mix responding at low doses, and decreased responding for both reinforcers at high doses. In another group of animals, isocaloric alternatives were concurrently available: 10% ethanol/0.25% saccharin versus 14% sucrose. Injections of Ro 15-4513 and chloridiazepoxide produced similar results as in the first group of rats: an increase in ethanol mix responding with low dose chlordizepoxide, and a decrease in ethanol mix responding with Ro 15-4513. However, naloxone injections did not selectively affect responding for either of the reinforcers when they were isocaloric. These results are discussed in terms of ethanol's neuropharmacological actions.

Food palatability and hunger modulated effects of CGS 9896 and CGS 8216 on food intake

The effect of food palatability and duration of food deprivation on the modulation of food intake by two benzodiazepine receptor (BDZR) ligands, CGS 9896 and CGS 8216, were investigated. Three diets differing in palatability (high, medium, or standard) and three different periods of food deprivation (0, 16, or 24 h) were used in all combinations to compare the effect of these variations on the observed modulation of food consumption by both BDZR ligands. Increasing diet palatability and/or food deprivation increased the baseline food consumption and reduced the sensitivity of the test to the detection of the hyperphagic effect of CGS 9896 but increased the sensitivity to detect the anorexic effect of CGS 8216. Only for the intermediate conditions of food deprivation (16 h) and for a standard or medium palatable diet were both significant hyperphagic effect of CGS 9896 and anorexic effect of CGS 8216 detected. Neither increased palatability nor hunger enhanced the modulation of feeding, indicating that neither "taste preference" nor "hunger" is the key factor in the mechanism of BDZR ligand-induced feeding response.

Benzodiazepines, appetite, and taste palatability

Benzodiazepine agonists stimulate feeding in animals. This paper reviews evidence which indicates that benzodiazepine-induced feeding is due to a specific enhancement of the perceived palatability of food and fluids, and is not a mere secondary consequence of anxiety reduction. In studies of the effect of benzodiazepines on affective reactions that are naturally elicited from rats by tastes, we have shown that (a) benzodiazepines enhance hedonic taste palatability in a receptor-specific fashion; (b) the relevant receptors and the minimal neural circuitry required to mediate benzodiazepine-induced palatability enhancement both exist complete in the decerebrate brain stem; and (c) even in normal brains, receptors in the brain stem, not forebrain, are the primary substrate for the benzodiazepine-induced enhancement of taste palatability. We conclude that a 'benzodiazepine-GABA' neural system in the brain stem constitutes an important component of the neural hierarchy responsible for taste pleasure. The reason why benzodiazepine tranquilizers have not been reported to enhance palatability for humans may be that the appropriate studies have not yet been done, that human doses are low, and that the brain stem palatability system is less responsive to commonly prescribed agonists that are anxiety/arousal benzodiazepine systems. Finally, in keeping with the purpose of the symposium in which this paper was originally presented, we discuss a number of issues regarding the measurement and interpretation of taste reactivity data.

The beta-carboline abecarnil, a novel agonist at central benzodiazepine receptors, influences saccharin and salt taste preferences in the rat

Abecarnil is a recently described beta-carboline which acts at central benzodiazepine receptors (BZR) and has anxioselective/anticonvulsant properties. While it may be classified provisionally as a partial agonist at BZR, there is also evidence that its pharmacological profile may be due to a selective action at BZR subtypes. The general aim of the present series of experiments was to investigate the effects of abecarnil on ingestional behaviour in the rat. The results indicated that abecarnil (0.3-10 mg/kg, i.p.) selectively increased the intake of preferred 0.05% sodium saccharin and 0.9% sodium chloride solutions in two-choice tests using water-deprived rats. These results confirm and extend previous work with the potent BZR agonist clonazepam. Moreover, abecarnil significantly increased the ingestion of sweetened mash and 3% sucrose solution in nondeprived animals. In general, these results indicate that abecarnil is effective in increasing ingestional responses, a characteristic it shares with classical agonists like diazepam. The results could be accounted for in terms of a partial agonist profile for abecarnil, but do not rule out the possibility of selective actions at BZR subtypes.

Effects of the imidazobenzodiazepine Ro 15-4513 on saccharin choice and acceptance, and on food intake, in the rat

The general aim of the present series of experiments was to investigate the effects of the imidazobenzodiazepine, Ro 15-4513, on ingestional behavior in the rat. The more specific aims were to test its effects on preference for sweet taste, to determine if it acts as a benzodiazepine-receptor inverse agonist, and if it selectively reduces sweetness preference. The results indicated that Ro 15-4513 (1.0-10 mg/kg, i.p.) abolished the preference for a 0.05% sodium saccharin solution in a two-choice test. Water intake in the same test was unaffected. Second, at 10 mg/kg, it suppressed saccharin ingestion in an acceptance test; this effect was completely reversed by the selective benzodiazepine antagonist, Ro 15-1788 (20 mg/kg). Third, Ro 15-4513 (1.0-10 mg/kg) reduced palatable food consumption in non-deprived rats, an effect which was also antagonized by Ro 15-1788. The results are consistent the bidirectional modulation of ingestional responses to palatable taste stimuli as a consequence of drug actions at benzodiazepine receptors. Furthermore, they emphasize that any reduction in consummatory responses produced by Ro 15-4513 is likely to reflect inverse agonist characteristics, as distinct from any putative ethanol antagonist property.

Chlordiazepoxide and valproate enhancement of saline drinking by nondeprived rats: effects of bicuculline, picrotoxin and Ro15-1788

Drinking of 0.85% saline by nondeprived rats was significantly enhanced by chlordiazepoxide (5 or 10 mg/kg) and by valproate (100 or 300 mg/kg), drug effects being strongest in the earlier parts of a 30-minute test. When given alone, both bicuculline and picrotoxin significantly reduced saline drinking at 2.5 mg/kg, but not 1.5 mg/kg. Administration of valproate at either dose or of chlordiazepoxide (10 mg/kg) completely prevented bicuculline action and 5 mg/kg chlordiazepoxide reduced it. Picrotoxin, however, largely prevented the actions of both chlordiazepoxide and valproate. The increase in saline drinking induced by valproate (300 mg/kg) was also blocked by RO15-1788 (10 or 25 mg/kg). These findings are discussed in the context of the three-site model of the GABA/benzodiazepine receptor complex. It is concluded that drugs acting at the benzodiazepine site or the chloride ion channel affect saline drinking, but that there is little evidence of an important functional role for the GABAa site at present.

The pyrazoloquinoline, CGS 8216, reduces sham feeding in the rat

The pyrazoloquinoline CGS 8216, a benzodiazepine receptor ligand, produced a dose-related (2.5-10.0 mg/kg, IP) attenuation of sham feeding a 30% sucrose solution by rats with open gastric fistulas. Ingestion was reduced by over 50% following the largest dose of CGS 8216 in a 60 min test. The initiation of sham feeding was not delayed by CGS 8216, but sham feeding was subsequently slowed over an extended test period. The suppressant effect of CGS 8216 was reversed by the specific benzodiazepine receptor antagonist Ro15-1788 (20 and 40 mg/kg, IP). Hence the effect of CGS 8216 on sham feeding may be mediated by benzodiazepine receptors, and is consistent therefore with the characterization of CGS 8216 as a benzodiazepine partial inverse agonist. In contrast to sham feeding, CGS 8216 (10.0 mg/kg, IP) did not affect sham drinking in 17 hr water-deprived rats. The results are discussed in relation to possible benzodiazepine receptor involvement in the neurochemical mediation of food palatability.

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.