Chlordiazepoxide-induced hyperphagia in non-food-deprived rats: effects of Ro15-1788, CGS 8216 and ZK 93 426

A comparison of dehydroepiandrosterone and 7-keto dehydroepiandrosterone with other drugs that modulate ethanol intake in rats responding under a multiple schedule

Dehydroepiandrosterone (DHEA), 7-keto DHEA, and several comparison drugs (ethanol, chlordiazepoxide, rauwolscine, and RO15-4513) were administered to male rats responding under a multiple schedule of food and ethanol presentation to determine their selectivity for decreasing ethanol-maintained responding. DHEA and 7-keto DHEA significantly decreased both ethanol-maintained and food-maintained responding, compared with the control, while also decreasing the blood ethanol concentration (BEC). Acute ethanol administration also decreased responding for both food and ethanol; however, ethanol-maintained responding was more potently decreased than food-maintained responding. BEC remained relatively stable after increasing ethanol doses. Among the other drugs tested, RO15-4513 was the most selective for decreasing ethanol-maintained responding compared with food-maintained responding, and it decreased BECs as ethanol-maintained responding decreased. The largest dose of rauwolscine significantly decreased responding for food, whereas it did not affect ethanol-maintained responding compared with the control. Low to intermediate doses of rauwolscine produced small, nonsignificant increases in ethanol-maintained responding and BECs. Chlordiazepoxide produced significant decreases in food-maintained responding and the dose of ethanol presented, but only at the highest dose tested. Although DHEA and 7-keto DHEA did not decrease ethanol-maintained responding as selectively as ethanol or RO15-4513 under the multiple schedule, these neurosteroids may be valuable pharmacological tools in the development of new treatments for alcohol abuse and dependence.

FG 7142 specifically reduces meal size and the rate and regularity of sustained feeding in female rats: evidence that benzodiazepine inverse agonists reduce food palatability

Benzodiazepine receptor inverse agonists reduce food intake in males, but their actions in females, in whom stress-related eating disorders are more common, as well as their behavioral mode of action remain unclear. The consummatory effects of benzodiazepine receptor ligands have alternately been hypothesized to reflect changes in the hedonic evaluation of food or secondary effects of anxiety-related or cognitive properties. To test the anorectic mode of action of benzodiazepine inverse agonists, the effects of FG 7142 on feeding microstructure were studied in nondeprived female Wistar rats (n=32). Microstructure analysis used a novel meal definition that recognizes prandial drinking. On pharmacologically synchronized diestrus I, rats were pretreated (-30 min dark onset) with the benzodiazepine partial inverse agonist FG 7142 (i.p. 0, 3.75, 7.5, 15 mg/kg) in a between-subjects design. FG 7142 delayed the onset of (16-541%), decreased the amount eaten (36-52%) and drunk (63-87%), and reduced the time spent drinking (59-87%) within the first nocturnal meal. Dose-dependent incremental anorexia continued 6 h into the dark cycle, whereas FG 7142 did not suppress the quantity, duration or rate of drinking past the first meal. Treated rats ate smaller meals (17-42%) of normal duration. This reflected that FG 7142 slowed feeding within meals (9-38%) by decreasing the regularity and maintenance of feeding from pellet-to-pellet. FG 7142 did not influence postprandial satiety; meal frequency and inter-meal intervals were unaffected. FG 7142 anorexia was blocked by the benzodiazepine receptor antagonist flumazenil in a 2:1 molar ratio (n=17 rats). The very early, nonspecific (+10 min), but not subsequent (2.5, 4.5 h) feeding-specific phase, of FG 7142 anorexia was mirrored by anxiogenic-like behavior in FG 7142-treated (7.5 mg/kg) female rats (n=48) in the elevated plus-maze. Thus, benzodiazepine receptor inverse agonists preferentially lessen the maintenance of feeding in female rats, effects opposite to those of palatable food.

Palatability-dependent appetite and benzodiazepines: new directions from the pharmacology of GABA(A) receptor subtypes

This paper updates an early review on benzodiazepine-enhanced food intake, published in the first issue of Appetite, and describes the considerable advances since then in the pharmacology of benzodiazepines, their sites and mechanisms of action, and in understanding the psychological processes leading to the increase in food consumption. A great diversity of benzodiazepine receptor ligands have been developed, many of which affect food intake. Agonists can be divided into full agonists (which produce the full spectrum of benzodiazepine effects) and partial agonists (which are more selective in their effects). In addition, inverse agonists have been identified, with high affinity for benzodiazepine receptors but having negative efficacy: these drugs exhibit anorectic properties. Benzodiazepine receptors are part of GABA(A) receptor complexes, and ligands thereby modulate inhibitory neurotransmission in the brain. Molecular approaches have identified a palette of receptor subunits from which GABA(A) receptors are assembled. In all likelihood, benzodiazepine-induced hyperphagia is mediated by the alpha2/alpha3 subtype not the alpha1 subtype. Novel alpha2/alpha3 selective compounds will test this hypothesis. A probable site of action in the caudal brainstem for benzodiazepines is the parabrachial nucleus. Behavioural evidence strongly indicates that a primary action of benzodiazepines is to enhance the positive hedonic evaluation (palatability) of tastes and foodstuffs. This generates the increased food intake and instrumental responding for food rewards. Therapeutic applications may derive from the actions of benzodiazepine agonists and inverse agonists on food procurement and ingestion.

The octadecaneuropeptide [diazepam-binding inhibitor (33-50)] exerts potent anorexigenic effects in rodents

The effects of intracerebroventricular administration of the octadecaneuropeptide ODN on food intake have been investigated in rat and mouse. In rats deprived of food from 9:00 a.m. to 7:00 p.m., i.c.v. injection of ODN (30 to 100 ng) provoked a dose-dependent reduction of food consumption during the following 12-h nocturnal period. At a dose of 100 ng, ODN almost completely suppressed food intake. Treatment of rats with diazepam (2 mg/kg s.c.; 15 min before ODN administration) did not affect the anorexigenic response evoked by 100 ng ODN. Continuous i.c.v. infusion of ODN (10 ng/h during 15 days) using osmotic minipumps, significantly reduced food intake during the 2nd, 3rd and 4th days of treatment. The decrease in food consumption was associated with a significant reduction in body weight, which persisted during the 15-day duration of the experiment. In mice deprived of food for 18 h, i.c.v. administration of a low dose of ODN (5 ng) significantly reduced food intake. Treatment of mice with diazepam (1 mg/kg s.c.; 10 min before ODN administration) did not prevent the inhibitory effect of ODN (100 ng) on food intake. The C-terminal octapeptide fragment of ODN mimicked the anorexigenic effect of the intact peptide. Taken together, the present data demonstrate that i.c.v. injection of ODN causes, in both rat and mouse, a long-lasting anorexigenic effect that is not mediated through central-type benzodiazepine receptors. The biologically active region of ODN appears to be located in the C-terminal domain of the peptide.

The hedonic impact and intake of food are increased by midazolam microinjection in the parabrachial nucleus

Benzodiazepines have been reported to induce eating when administered into the brainstem of rats (either the fourth ventricle or the parabrachial nucleus). Benzodiazepines in the brainstem also have been reported to enhance the hedonic impact of taste, as measured by hedonic/aversive taste reactivity patterns, when administered to the fourth ventricle. The present study examined whether the parabrachial nucleus in particular is a brainstem site of the benzodiazepine-produced enhancement of eating and palatability. Food intake (cereal mash) was measured after brainstem microinjections of midazolam or vehicle (0.0, 7.5, and 15.0 microg) into the parabrachial nucleus, the nucleus of the solitary tract, the pedunculopontine tegmental nucleus, or the fourth ventricle (60 microg). We used the taste reactivity paradigm to measure hedonic/aversive affective reactions elicited from rats by oral infusions of a bittersweet solution (7% sucrose-0.01% quinine). Positive hedonic reactions and negative aversive reactions to sucrose-quinine were also measured after microinjections of midazolam (0.0, 7.5, and 15 microg) into the parabrachial nucleus. Midazolam increased food intake and selectively enhanced positive hedonic taste reactivity patterns to the bittersweet solution when microinjections were delivered to the parabrachial nucleus. When administered to the other brainstem sites at the same doses, however, midazolam had no effect. We therefore conclude that the parabrachial nucleus can mediate the benzodiazepine-induced enhancement of the hedonic impact of taste as well as mediating the enhancement of eating behavior.

Benzodiazepine receptor antagonists modulate the actions of ethanol in alcohol-preferring and -nonpreferring rats

The pyrazoloquinoline CGS 8216 (2-phenylpyrazolo-[4,3-c]-quinolin-3 (5H)-one, 0.05-2 mg/kg) and the beta-carboline ZK 93426 (ethyl-5-isopropyl-4-methyl-beta-carboline-3-carboxylate, 1-10 mg/kg) benzodiazepine receptor antagonists were evaluated for their capacity to modulate the behavioral actions of ethanol in alcohol preferring and -nonpreferring rats. When alcohol-preferring rats were presented with a two-bottle choice test between ethanol (10% v/v) and a saccharin (0.0125% g/v) solution, both antagonists dose-dependently reduced intake of ethanol by 35-92% of control levels on day 1 at the initial 15 min interval of the 4 h limited access. Saccharin drinking was suppressed only with the highest doses. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) unmasked the anxiolytic effects of a hypnotic ethanol dose (1.5 g/kg ethanol) on the plus maze test in alcohol-preferring rats, but potentiated the ethanol-induced suppression in alcohol-nonpreferring rats. CGS 8216 (0.25 mg/kg) and ZK 93426 (4 mg/kg) attenuated the ethanol (0.5 and 1.5 g/kg)-induced suppression in the open field in alcohol-nonpreferring rats; however, CGS 8216 potentiated the depressant effects of the lower ethanol dose (0.5 g/kg) in alcohol-preferring rats. These findings provide evidence that benzodiazepine receptor antagonists may differentially modulate the behavioral actions of ethanol in alcohol-preferring and-nonpreferring rats. It is possible that the qualitative pharmacodynamic differences seen in the present study may be related to selective breeding for alcohol preference. The findings indicate the potential for development of receptor specific ligands devoid of toxic effects which may be useful in the treatment of alcohol abuse and alcoholism.

Food intake in Prader-Willi syndrome and controls with obesity after administration of a benzodiazepine receptor agonist

Benzodiazepine receptor (BZR) agonists, used extensively for their anxiolytic effects, have been shown to increase food intake in many mammalian species. Little information, however, is available on the effects of BZR agonists on feeding behaviors of humans. Food intake was evaluated in a 60-minute free-feeding standardized test after the acute administration of the BZR agonist chlordiazepoxide (CDP, Librium; 5 mg or 20 mg) or placebo. Subjects were 12 individuals with the Prader-Willi syndrome (PWS), a disorder characterized by extreme hyperphagia and morbid obesity, and 11 controls with obesity. PWS subjects showed the characteristic hyperphagia associated with the appetite disorder, consuming more than six times as many sandwiches as controls with obesity. Results revealed no significant effect of either dose of CDP on the food intake of either group. Serum assays revealed that dose-dependent, clinically effective levels of CDP and active metabolites were achieved. These results suggest that acute administration of the BZR agonist CDP, at the therapeutic levels used, may not increase food intake in populations with obesity. However, the chronic effects of CDP on appetite in human populations still need to be explored.

Hyperphagia induced by direct administration of midazolam into the parabrachial nucleus of the rat

Benzodiazepine receptor agonists increase food intake in many different species, yet there has been little investigation of the central site of actions of these drugs on ingestive behaviour. In the present experiments, direct administration of the benzodiazepine receptor agonist midazolam (3-30 micrograms/microliter) into the parabrachial nucleus of the pons significantly increased the consumption of a wet mash diet and a 3% sucrose solution in adult non-deprived rats. The hyperphagic response was blocked by pre-treatment with the selective benzodiazepine receptor antagonist flumazenil. Injection of midazolam into the parabrachial nucleus had no effect on locomotor activity, despite the fact that in the same animals an increase in mash intake was observed following intra-parabrachial midazolam. These data suggest that benzodiazepine receptors located in the parabrachial nucleus may be an important site of action for the effects of benzodiazepines specifically on ingestive behaviour.

Increased food intake following injection of the benzodiazepine receptor agonist midazolam into the IVth ventricle

Despite a prolonged period of research with benzodiazepines, the central site(s) of action for the hyperphagic effects of these compounds remains to be determined. The aim of the present studies was to examine the effect of direct administration of the benzodiazepine receptor agonist midazolam into the IVth ventricle on ingestive behavior in nondeprived rats. In Experiment 1, microinjection of midazolam (3 and 30 micrograms/microliter) into the IVth ventricle was sufficient to increase consumption of a palatable mash. In Experiment 2, the hyperphagic effect was blocked by systemic administration of the selective benzodiazepine receptor antagonist flumazenil (20 mg/kg). The results indicate that a brainstem site of action may be important for the effects of benzodiazepine receptor agonists on ingestive behavior.

Nitrous oxide induces feeding in the nondeprived rat that is antagonized by naltrexone

Three experiments investigated a possible effect of nitrous oxide (N2O) on food intake in nondeprived male hooded rats in independent groups designs. Experiment 1 demonstrated a concentration-related increase in intake with increasing level of nitrous oxide (10-40% N2O), reaching statistical significance at 20% N2O when compared to room air controls (p < 0.05). In experiment 2, pretreatment with 10 and 20 mg/kg of the benzodiazepine antagonist, flumazenil, failed to significantly attenuate 30% N2O-induced hyperphagia. In Experiment 3, pretreatment with the opioid antagonist, naltrexone, effectively antagonized 30% N2O-induced hyperphagia. Pronounced attenuation (to 59% of 30% N2O-induced intake level over a 1 h period) at the lowest dose of naltrexone (0.1 mg/kg, p < 0.01) compared to vehicle level resulted in a shallow dose-response curve across the dose range tested (0.1-10.0 mg/kg). These results suggest that an endogenous opioid mechanism is prominently involved in the N2O-induced ingestive response.

"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)

Motor performance decrement by midazolam: antagonism by Ro 15-1788 and CGS 8216

Rats were trained in a fine motor control performance that required operation by a paw of a force transducer so that it remained between upper and lower limits of a force band for a continuous 1.5-sec period to deliver each food pellet. Acute doses of midazolam (0.75-3.0 mg/kg, SC) impaired indices of motor performance in a graded, dose-related fashion. When administered alone, Ro 15-1788 (0.1-5.0 mg/kg, SC) had no effect on motor behavior while CGS 8216 (0.5 and 1.0 mg/kg, IP) alone had small effects. In general, the motor performance decrements produced by midazolam were antagonized in a dose-related fashion by both Ro 15-1788 and CGS 8216.

Interaction of cholecystokinin and diazepam: effects on brain monoamines

An antagonism between cholecystokinin (CCK) peptides and benzodiazepines (BZD) has been described in various paradigms. We sought to determine whether CCK and BZD are also antagonistic in their effects on brain neurotransmitter levels in the rat. No effect on the noradrenergic system was induced in any brain area by CCK 8 S and diazepam alone or in combination. Administered alone, sulfated CCK octapeptide (CCK 8 S) (5 micrograms/kg ip) and diazepam (5 mg/kg ip) were found to decrease DOPAC levels in the cortex and to induce 5-hydroxy-tryptamine accumulation in the hippocampus. When administered together, these variations were no longer observed. However, a slight tendency by each substance to decrease 3-methoxy-tyramine levels in the striatum, became significant when given in association. The differences in CCK-BZD interactions observed in the striatum, cortex and hippocampus suggest that different mechanisms of action are involved. The addition of the effects occurring in the striatum might involve a GABA-ergic mechanism.

Effect of diazepam on behaviour and associated changes in ascorbate concentration in rat brain areas: striatum, n. accumbens and hippocampus

The effect of diazepam on spontaneous and tail-pinch-induced behaviour was monitored together with the measurement of extracellular ascorbate using constant potential voltammetry with carbon paste electrodes. Diazepam (3 mg/kg) was followed by eating during the 1st hour after administration in non-food-deprived rats and a reduction in the behaviour triggered by a mild tail-pinch 90 min after drug administration. There was no change in ascorbate concentration in parallel with the spontaneous eating; however, the brisk increase in ascorbate concentration in striatum, nucleus accumbens and hippocampus, which accompanies the tail-pinch, was decreased in size and duration after diazepam. This effect was blocked by the central benzodiazepine receptor antagonist Ro15 1788 (5 mg/kg).

Appetite-modulating drugs in dwarf goats, with special emphasis on benzodiazepine-induced hyperphagia and its antagonism by flumazenil and RO 15-3505

In dwarf goats fasted for 2 h, i.v. administration of the benzodiazepine (BZ) agonists diazepam (60 micrograms/kg), brotizolam (2 and 4 micrograms/kg) and climazolam (100 micrograms/kg) induced hyperphagic effects, whereas i.v. injections of the BZ-antagonist flumazenil (R degrees 15-1788; 0.5 mg/kg), the anthelmintic ivermectin (0.1 mg/kg), the 5-HT2 antagonist ritanserine (0.1 mg/kg), ACTH (10 micrograms/kg) and prednisolone (1 mg/kg) were inactive in a 30-min feeding test. Both the BZ-antagonist R degrees 15-3505 (greater than or equal to 0.1 mg/kg) and the opiate receptor antagonist naloxone (0.1 mg/kg) had anorectic effects in dwarf goats given 30 min access to a palatable pelleted concentrate. The hyperphagic effects of climazolam and brotizolam were not antagonized by flumazenil, whereas similar doses of this drug completely reversed muscle incoordination and ataxia induced by much higher doses of these BZ-agonists. In the combination experiments with naloxone and BZ-agonists, naloxone antagonized the hyperphagic effects of both diazepam and brotizolam. Similarly, in the diazepam-R degrees 15-3505 study, there was a significant effect of diazepam and a significant inhibition of this effect by R degrees 15-3505 (50 micrograms/kg). In the diazepam-ivermectin combination experiment no evidence for drug potentiation was found. These results and the mode of action of the above mentioned drugs are discussed in relation to feeding behaviour.

Effects of diazepam, FG 7142, and RO 15-1788 on schedule-induced polydipsia and the temporal control of behavior

Although benzodiazepine agonists and inverse agonists have opposite effects on drinking elicited by water deprivation, there is much less information about the effects of these drugs on nonhomeostatic drinking. In this experiment the effects of diazepam (0.3-5.0 mg/kg), a benzodiazepine receptor agonist, and FG 7142 (1.0-9.0 mg/kg), an inverse agonist, were determined on drinking elicited by a FT-60 schedule of food delivery (SIP). Both diazepam and FG 7142 dose-dependently reduced SIP, measured as either licking or volume consumed. In addition, diazepam reduced panel pressing for food, decreased locomotor activity, and changed the time course of each behavior. In contrast, FG 7142 reduced schedule-induced drinking without significantly altering other behaviors. The antagonist RO 15-1788, when given in combination with these drugs, only partially restored the reductions in licking produced by diazepam, but was much more effective in reversing the effects of FG 7142 at doses of the antagonist that failed by themselves to affect responding. The opposite pattern of effects was seen on the volume of water consumed. These effects are discussed in terms of the behavioral and pharmacological specificity of these drugs.

Effects of Ro 15-1788 and CGS 8216 in diazepam-dependent baboons

Baboons received continuous intragastric infusions of diazepam (20 mg/kg per day) for one or more months. While diazepam treatment continued, baboons received intragastric doses of Ro 15-1788 (0.032-32.0 mg/kg) or CGS 8216 (1.0-100.0 mg/kg) at intervals of two or more weeks. Baboons were observed following administration of these antagonists for the presence of precipitated withdrawal signs. The following results were obtained: (1) both Ro 15-1788 and CGS 8216 produced signs of precipitated withdrawal in the baboon; (2) a more severe overall withdrawal syndrome was precipitated with Ro 15-1788 than with CGS 8216 at testable doses; (3) Ro 15-1788 produced dose-related increases in the overall severity of withdrawal, while CGS 8216 did not produce a clear dose-related increase in the overall severity of withdrawal; (4) dose-effect curves for Ro 15-1788 for certain signs (e.g. limb-tremor) were monotonicly increasing, while for other signs dose-effect curves plateaued at lower doses of Ro 15-1788 (e.g. retching and vomiting) or were an inverted U-shape (e.g. scratching). CGS 8216 precipitated withdrawal signs were less clearly dose-dependent; (5) onset of Ro 15-1788 precipitated withdrawal signs were rapid (5-15 min) and reliable, while the onset of CGS 8216 precipitated withdrawal signs were generally slower (approximately 30 min) and more variable; (6) at doses of Ro 15-1788 and CGS 8216 that produced equal levels of vomiting and retching, Ro 15-1788 produced more limb-tremor than CGS 8216. These studies indicate that Ro 15-1788 and CGS 8216 may produce quantitatively and qualitatively different precipitated withdrawal syndromes.

Bidirectional changes in the consumption of food produced by beta-carbolines

beta-Carboline derivatives provide examples of benzodiazepine receptor ligands which span the range: full agonist-partial agonist-antagonist-partial inverse agonist-full inverse agonist. Taken together, the effects of these compounds illustrate two important principles: firstly, the bidirectionality of effects which can be achieved using benzodiazepine receptor ligands; secondly, the selectivity of effects which are produced by partial agonists. Applied to the study of feeding processes, these principles imply that both hyperphagic and anorectic effects can be generated by actions of selected ligands at benzodiazepine receptors. Furthermore, they suggest that a hyperphagic effect may occur in the absence of side-effects (e.g., sedation, muscle-relaxation), which are characteristic of classical benzodiazepines. Experimental data in support of these predictions are presented. A microstructural approach to feeding behavior indicated that a benzodiazepine receptor agonist and an inverse agonist extend and abbreviate, respectively, the duration of individual bouts of eating. Preference for a saccharin solution was attenuated by the beta-carboline inverse agonist, FG 7142, but rejection of a quinine solution was not increased. Adrenalectomy had no effect on the anorectic effect of inverse agonists.

A benzodiazepine receptor inverse agonist inhibits stress-induced ulcer formation

The effects of a benzodiazepine receptor inverse agonist (FG 7142) on gastric ulcer formation were studied in restrained rats. FG 7142 (10-50 mg/kg) reduced in a dose-dependent fashion both the number and cumulative length of gastric ulcers elicited by restraint for 2 hr at 4 degrees C, but did not affect ulcer formation in unrestrained animals maintained in this environment. FG 7142 also reduced gastric ulcer formation in restrained rats maintained at 22 degrees C for 5 hr. The ability of FG 7142 to reduce restraint-stress induced gastric ulcer formation was blocked by the benzodiazepine receptor antagonist ZK 93426 and the beta-adrenoceptor antagonist propranolol. These findings suggest that FG 7142 produces a benzodiazepine-receptor mediated reduction in gastric ulcer formation, which may result from its ability to increase activity of the sympathetic nervous system.

Differential effects of CGS 8216 and naltrexone on ingestional behaviour

Effects of the pyrazoloquinoline CGS 8216 (a partial benzodiazepine receptor inverse agonist) and the opiate antagonist, naltrexone, were compared in several tests of ingestion in non-deprived and deprived male rats. Both naltrexone (0.1-10.0 mg/kg, SC) and CGS 8216 (1.25-10.0 mg/kg, IP) significantly reduced the consumption of a highly palatable saccharin-glucose solution by non-deprived rats. Both compounds were also effective in reducing, dose-dependently, the intake of palatable sweet or oily mash by non-deprived animals. Hence, naltrexone and CGS 8216 attenuated palatability-induced ingestional responses, and sweet taste was not necessary for this effect to occur. The two drugs also reduced the intake of the saccharin-glucose solution in food-deprived rats, but their effects diverged in water-deprived animals. CGS 8216 had relatively little effect in the thirsty animals, whereas the effect of naltrexone was enhanced. This difference was underscored in a final test of deprivation-induced consumption of water. Naltrexone reduced the drinking, but CGS 8216 had no effect. Taken together, these data indicate that CGS 8216 was more selective in its effects on 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.

Chlordiazepoxide-induced selection of saccharin-flavoured food in the food-deprived rat

Following a period of food-deprivation, adult male rats were given a choice between three sources of food in a 15 min test. All three consisted of a mash of powdered food and water; saccharin and quinine were added to two of them, respectively. Chlordiazepoxide (5 and 10 mg/kg, IP) significantly reduced the latency to begin eating, and also increased the overall level of food consumption. Its hyperphagic effect was due to a selective increase in ingestion of the saccharin-flavoured food, and was not due to a general increase in food consumption across all three food sources. Analysis of the micro-structure of feeding indicated that the increase in food intake depended upon an increase in the duration of individual bouts. Other feeding parameters were not significantly affected by chlordiazepoxide. These data suggest the CDP treatments affect feeding behaviour selectively, both in relation to the taste characteristics of the food, and also with regard to alterations in the micro-structure of the feeding responses.

The benzodiazepine antagonist CGS 8216 decreases both shocked and unshocked drinking in rats

Previous studies of the benzodiazepine antagonist CGS 8216 have reported that this compound may enhance the punishment-induced suppression of behaviour. In order to investigate this phenomenon further, water-deprived rats were trained to drink from a water spout during a multiple schedule with shocked and unshocked components. During the shocked components a very mild electric footshock was presented after every 20th lick. The shock slightly reduced the rate of licking during these components below that which occurred during periods without shock, although this effect decreased during the experiment. CGS 8216 (0.3-10 mg/kg) produced a dose-related reduction in licking during both schedule components. The overall volumes of water consumed were reduced by CGS 8216 as was the number of licks during the first, unshocked schedule component, before shock was applied, showing that the effect on unshocked licking was not due to a generalisation of suppression between periods with or without shock. In contrast to CGS 8216, a dose of 10 mg/kg pentylenetetrazol selectively reduced shocked licking. In a second group of rats which drank under identical conditions but without shock, CGS 8216 again reduced water intake. These results show that CGS 8216 can reduce water intake in rats regardless of whether drinking results in shock presentation.

Beta-carbolines characterized as benzodiazepine receptor agonists and inverse agonists produce bi-directional changes in palatable food consumption

Drugs which bind to specific benzodiazepine recognition sites fall into three categories: agonists, antagonists, and inverse agonists. A set of biochemical parameters is available which distinguishes between the three. In addition, actions of the drugs result in physiological and behavioural effects which are distinguishable. beta-Carboline derivatives provide a group of compounds which show high affinity for the benzodiazepine sites, and which contains examples belonging to each of the three categories. Evidence is reviewed which shows that beta-carboline benzodiazepine receptor agonists (ZK 93423, ZK 91296) produce increases in the consumption of a palatable diet by non-deprived rats, that beta-carboline inverse agonists (FG 7142, DMCM) produce an anorectic effect, and that the beta-carboline ZK 93426 acts as a benzodiazepine receptor antagonist. The results support the proposal of bi-directional control of feeding responses through the action of drugs at a common benzodiazepine receptor. Furthermore, benzodiazepine receptor inverse agonists provide a novel class of anorectic agents. Evidence is also reviewed which is suggestive of the modulation of food-related reward by drug actions at benzodiazepine receptors.

Investigation of the actions of the benzodiazepine antagonists Ro 15-1788 and CGS 8216 using the schedule-controlled behavior of rats

Ro 15-1788 and CGS 8216 antagonise many of the pharmacological effects of benzodiazepines but both of these compounds have also been shown to exert behavioral effects when administered alone. In the present study the effects of Ro 15-1788 and CGS 8216, alone and in combination with diazepam and with the benzodiazepine receptor ligand zolpidem, were investigated. Diazepam and zolpidem produced dose-related decreases in rates of food-reinforced lever-pressing maintained by a fixed-ratio (FR 10) schedule. CGS 8216 also reduced response rates although Ro 15-1788, at several doses, produced small, but statistically significant, increases in responding. When the diazepam and zolpidem dose-response curves were re-established in the presence of a dose of Ro 15-1788 or CGS 8216 the depressant effects of the higher doses were antagonised. However, neither diazepam nor zolpidem blocked the rate reducing effect of CGS 8216 which may not therefore be due to an action at benzodiazepine receptors.

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

The pyrazoloquinoline CGS 8216, a high-affinity ligand for benzodiazepine recognition sites, significantly reduced the consumption of a preferred 0.05% sodium saccharin solution in a 30 min two-bottle test. A highly significant effect was detected at 5.0 mg/kg, IP and at higher doses. The consumption of water and 0.6% saline, in two-bottle tests, or of quinine solution and water, in a forced-choice test, was not reliably affected by CGS 8216. The results point to a sensitive and selective intrinsic effect of CGS 8216 on ingestional responses in the rat.

Midazolam-induced hyperphagia and FG 7142-induced anorexia: behavioural characteristics in the rat

Non-deprived male rats, familiarized with a highly palatable diet, were treated with 0.3-10.0 mg/kg of the imidazobenzodiazepine midazolam. The increases in consumption of the food observed at larger doses of midazolam were due to increases in the duration of feeding, but not in the rate of eating. These, in turn, were due to increases in the duration of eating bouts, but not in their frequency. The beta-carboline FG 7142, a partial benzodiazepine receptor inverse agonist, reduced the consumption of the diet when it was injected at 10.0 and 15.0 mg/kg (IP). The overall duration of feeding was not affected at these doses in the 30 min test, but rate of eating was reduced. However, during the first 5 min interval of the test, when feeding behaviour was most motivated, FG 7142 did significantly reduce the duration of feeding. The effect depended upon a reduction in the duration of eating bouts, but not upon any change in their frequency. Hence, midazolam and FG 7142 had opposite effects on the duration of bouts of feeding. Both midazolam and FG 7142 reduced the frequencies of concurrent grooming, locomotor activity, and rearing in the test of palatable food consumption. Possible explanations for these effects are briefly considered.

Hyperphagic and anorectic effects of beta-carbolines in a palatable food consumption test: comparisons with triazolam and quazepam

The triazolobenzodiazepine triazolam (0.1-1.0 mg/kg i.p.) and quazepam (0.3-30.0 mg/kg i.p.) were administered to non-food-deprived rats which had been partially-satiated on a palatable diet. In a subsequent 30 min feeding test, both compounds produced a significant increase in the level of food consumption. While triazolam had a dose-related effect and produced a 151.5% increase in the level of food intake, quazepam exerted only a partial effect, achieving a 73.9% increase in food intake at 3.0 mg/kg but no additional increase in food intake at higher doses. The two beta-carbolines, ZK 93423 (0.1-3.0 mg/kg i.p.) and ZK 91296 (1.0-30.0 mg/kg i.p.), a full agonist and a partial agonist at benzodiazepine receptors respectively, also produced significant increases in food consumption under the same experimental conditions. ZK 93423 had effects which were similar to those of triazolam, ZK 91296 had effects similar to quazepam. The beta-carboline benzodiazepine inverse agonist FG 7142 (10.0 mg/kg i.p.) had an anorectic effect in non-food-deprived rats given 30 min access to the highly palatable diet. This effect was reversed by the beta-carboline benzodiazepine receptor antagonist ZK 93426 in a dose-dependent manner. These results emphasize that within the series of beta-carboline ligands for benzodiazepine receptors, their characterization in terms of agonists, antagonists and inverse agonists has validity with respect to the behavioural response of palatable food consumption in non-food-deprived rats.

Benzodiazepine-induced hyperphagia: stereospecificity and antagonism by pyrazoloquinolines, CGS 9895 and CGS 9896

Non-deprived male rats were familiarized with a highly palatable diet until baseline consumption in a 30-min daily access period had stabilised. Stereospecificity of the hyperphagic effect of benzodiazepine receptor agonists was demonstrated using two enantiomers, the (S)-enantiomer being Ro11-3128 (methylclonazepam) and the (R)-enantiomer, Ro11-3624. The benzodiazepine receptor antagonists, Ro15-1788 and CGS 8216, reversed the hyperphagic effect of Ro11-3128. These data confirm the mediation of the hyperphagic effect of benzodiazepines by specific receptors. In further experiments, the effects of the pyrazoloquinolines CGS 9895 and CGS 9896 were examined both alone and also in combination with clonazepam. In doses of 1.25-10.0 mg/kg, neither CGS 9895 nor CGS 9896, when given alone, had a significant effect on the consumption of the palatable diet. Both, however, dose-dependently antagonised the hyperphagic effect of clonazepam. In a test of palatable food consumption, therefore, both compounds can be characterised as benzodiazepine antagonists.

The anorectic effect of FG 7142, a partial inverse agonist at benzodiazepine recognition sites, is reversed by CGS 8216 and clonazepam but not food deprivation

The benzodiazepine partial inverse agonist N'-methyl-beta-carboline-3-carboxamide (FG 7142; 5.0 and 10.0 mg/kg, i.p.) produced a dose-dependent reduction in the consumption of a familiar, highly palatable diet by non-food-deprived male rats. At dose levels which exhibited no significant intrinsic effects, the benzodiazepine receptor antagonist 2-phenylpyrazolo-[4,3-c]-quinoline-3(5H)-one (CGS 8216; 1.25-5.0 mg/kg, i.p.) reversed the anorectic effect of FG 7142. When clonazepam and FG 7142 were given in combination, mutual cancelling of their opposite effects occurred. These results are consistent with an action of FG 7142 at benzodiazepine recognition sites to reduce the level of palatable food consumption, and imply that a bidirectional control of food intake via benzodiazepine recognition sites can be achieved. The anorectic effect of FG 7142 was not reversed by 24-h food deprivation, indicating a possible separation from the effects of hunger mechanisms.

Bidirectional control of palatable food consumption through a common benzodiazepine receptor: theory and evidence

A classical approach to the control of food consumption has been to assume separate mechanisms for the arousal to eat, on the one hand, and the satiation of feeding responses, on the other. The present paper is concerned with a single, and a comparatively simple, neuronal mechanism which is endowed with properties to allow the complete determination of the level of feeding, from hyperphagia to anorexia. The model for the control of feeding, which is presented here, draws attention to the benzodiazepine receptor found distributed through the brain, and present in certain hypothalamic nuclei. Recent evidence which characterizes the receptor is reviewed, and the various categories of benzodiazepine receptor ligands are described. Pharmacological data, collected in a palatable food consumption model using non-food-deprived rats, demonstrate that benzodiazepine receptor agonists produce hyperphagia, benzodiazepine receptor inverse agonists produce anorexia, and benzodiazepine receptor antagonists block both effects. Hence, bidirectional control of food intake can be achieved through differential ligand action at a common set of receptors. Speculatively, these data can be extended, if it is assumed that two endogenous ligands exist in the brain which act like benzodiazepine agonist and inverse agonist, respectively. Evidence for the presence in hypothalamic nuclei of endogenous ligands of the latter kind is discussed. Benzodiazepine withdrawal-induced anorexia is also described, and is taken as evidence for the part played by feeding mechanisms in the development of benzodiazepine physical dependence.