Midazolam-induced increase in NaCl solution ingestion: differential effect of the benzodiazepine antagonists Ro 15-1788 and CGS 8216

Activation of serotonergic 5-HT1A receptors in the lateral parabrachial nucleus increases NaCl intake

Previous studies using non-specific serotonergic agonists and antagonists have shown the importance of serotonergic inhibitory mechanisms in the lateral parabrachial nucleus (LPBN) for controlling sodium and water intake. In the present study, we investigated whether the serotonergic 5-HT(1A) receptor subtype in the LPBN participates in this control. Male Holtzman rats had cannulas implanted bilaterally into the LPBN. Bilateral injections of the 5-HT(1A) receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.1, 1.25, and 2.5 microg/0.2 microl), into the LPBN enhanced 0.3 M NaCl and water intake of rats injected subcutaneously with the diuretic furosemide (10 mg/kg bw) and a low dose of the angiotensin-converting enzyme inhibitor, captopril (5 mg/kg bw). The increase in NaCl intake produced by 8-OH-DPAT injections was reduced in dose-related manner by pre-treating the LPBN with the selective 5-HT(1A) serotonergic antagonist, WAY-100635 (WAY, 1 and 2 microg/0.2 microl). In contrast, WAY did not affect water intake produced by 8-OH-DPAT. WAY-100635 injected alone into the LPBN had no effect on NaCl ingestion. Injections of 8-OH-DAPT (0.1 microg/0.2 microl) into the LPBN also increased 0.3 M NaCl intake induced by 24-h sodium depletion (furosemide, 20 mg/kg bw plus 24 h of sodium-free diet). Serotonin (5-HT, 20 mug/0.2 mul) injected alone or combined with 8-OH-DPAT into the LPBN reduced 24-h sodium depletion-induced 0.3 M NaCl intake. Therefore, the activation of serotonergic 5-HT(1A) receptors in the LPBN increases stimulated hypertonic NaCl and water intake, while 5-HT injections into the LPBN reduce NaCl intake and prevent the effects of serotonergic 5-HT(1A) receptor activation.

Schedule-induced polydipsic consumption of hypertonic NaCl solutions: effects of chlordiazepoxide

Rats were exposed to daily, 3-h sessions of schedule-induced polydipsia (SIP) in which either water, hypertonic NaCl solution (1.5% or 2.2%), or concurrent water and 1.5% NaCl were available. Each condition was in effect for several, consecutive weeks. Presession subcutaneous injections of chlordiazepoxide (CDZP) produced dose-related increases in the polydipsic ingestion of both NaCl solutions but had smaller and less certain effects on water consumption. Under the concurrent-fluid presentation condition, CDZP primarily increased NaCl solution consumption. Conditions generating SIP may function to attenuate what might be viewed as the punishing effects of ingesting highly hypertonic NaCl solutions, thereby permitting the chronic self-administration of large, daily amounts. Like many benzodiazepines, CDZP can attenuate the effects of punishment and thereby also increase NaCl solution ingestion. The two sources of punishment attenuation may be additive, with both differentially effecting greater increases in hypertonic NaCl ingestion, compared to water, when both fluids are presented concurrently.

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.

Effect of diazepam on cortical 5-HT release and behaviour in the guinea-pig on exposure to the elevated plus maze

Previous studies have used the elevated plus maze to test for "anxiolytic" drugs in rats. The present study demonstrates that guinea-pigs handled daily from birth exhibit similar behaviour to rats on the plus maze. Pretreatment with diazepam (1.0 mg/kg) significantly increased the time the animals spent in the open arms and amount of entries into the open arms. Using intra-cortical microdialysis on exposure of the guinea-pig to the elevated plus maze resulted in increased extracellular 5-HT in the frontal cortex. Diazepam reduced, but not significantly, the increase in extracellular 5-HT and produced an "anxiolytic" profile of behaviour. Pretreatment with the benzodiazepine antagonist flumazenil (10.0 mg/kg) fully antagonised the behavioural effects of diazepam. Flumazenil also reduced the effect of diazepam on the increase in extracellular 5-HT observed on exposure of the guinea-pig to the elevated plus maze. Flumazenil alone decreased basal extracellular cortical 5-HT but had no effect on behaviour in the elevated plus maze. The results show that an increase in extracellular 5-HT occurs in the guinea-pig exposed to aversive conditions. While it remains to be determined whether the "anxiolytic" effects of diazepam in the guinea-pig are causally associated with decreased extracellular 5-HT, it is of interest that the selective benzodiazepine antagonist also prevented the increase in basal extracellular 5-HT produced by the exposure to the elevated plus maze but had no effect on behaviour. Results indicate that there is no simple relationship between inhibition of 5-HT release and the "anxiolytic" action of benzodiazepines.

Decreased reactivity to anxiolytics caused by early protein malnutrition in rats

In order to investigate whether early malnutrition causes lasting changes in the reactivity to anxiolytic drugs, rat dams during lactation (21 days) and pups after weaning until the 49th day of life were fed on 8% casein diet (M rats), while their well-nourished controls received 25% casein (W rats). From day 50 on all animals ate the same balanced diet. Experiments started on the 91st day. Rats deprived for 22 hours drank water containing either 1.8% or 2.7% sodium chloride for 30 min in a test chamber, total intake being measured. Dose-effect curves for diazepam (0.5-5.0 mg/kg, IP), as well as for the nonbenzodiazepine anxiolytics ipsapirone (0.5-5.0 mg/kg), ritanserin (0.05-1.0 mg/kg) and isamoltane (2.5-20.0 mg/kg) were determined in M as well as in W rats. Diazepam and ipsapirone dose-dependently released drinking suppressed by either salt concentration in W rats, but caused little or no effect in M rats. Ritanserin and isamoltane were ineffective in both groups. These and previously reported results show that early protein malnutrition markedly reduces anticonflict effects of anxiolytics, indicating long-lasting impairment of neuronal systems underlying emotional behavior.

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.

Anxiolytic effect of caffeine and caffeine-clonazepam interaction: evaluation by NaCl solution intake

The administration of drugs with anxiolytic action to rehydrating rats augments the intake of 1.5% NaCl solution. In order to clarify the status of caffeine as an anxiolytic agent and its possible interaction with a benzodiazepine having high potency and efficacy in this regard, caffeine (0.78-100 mg/kg) alone and caffeine (0.78-50 mg/kg) plus clonazepam (0.05 or 0.50 mg/kg) injections (IP) were administered to rehydrating rats prior to 1-hr sessions during which they drank 1.5% NaCl solution. When given alone, caffeine, within a particular dose range, and clonazepam at both doses, augmented NaCl solution intake, but when administered in combination, caffeine antagonized the effects of clonazepam.

Sodium appetite: species and strain differences and role of renin-angiotensin-aldosterone system

The characteristics of the appetite for NaCl in humans differ in some aspects from those in other species. The mechanisms of appetite for NaCl have been studied in detail in two species, rats and sheep. We review the treatments known to induce an appetite for NaCl in rats, with special reference to differences among strains in their spontaneous preference for NaCl solution. The current view of the mechanism is critically appraised, with particular emphasis on the role of angiotensin II, mineralocorticoids, cerebroventricular sodium transport, and the relation between preference for NaCl and the concentration of sodium in saliva. The appetite for NaCl in rodents other than rats is considered next, and reveals that mice, hamsters and gerbils are reluctant to ingest NaCl either spontaneously or after treatment with several of the natriorexigenic stimuli that are effective in rats. The characteristics of the appetite for NaCl in non-rodent species, including sheep, rabbit, dog, and non-human primates, are then described. We discuss some of the possible differences in mechanism that might account for this behavioral diversity among species.

The role of central- and peripheral-type benzodiazepine receptors in anxiolytic-agent augmentation of NaCl solution intake: effects of clonazepam and Ro 5-4864

Two 1,4 benzodiazepines bind preferentially to the central- and peripheral-type benzodiazepine receptor in the brain, clonazepam and Ro 5-4864, respectively. They were administered to rats to determine if the relation between known anxiolytic action and efficacy in augmenting NaCl solution ingestion in rehydrating rats would remain the case for these prototypic agents. Clonazepam (0.062-32.0 mg/kg, PO) was highly potent and efficacious and increased 1.5% NaCl solution intake in a dose-related fashion. Water intake could also be increased, but to a relatively minor degree. Ro 5-4864 (4-8 mg/kg, IP) did not affect 1.5% NaCl solution ingestion, nor did this dose range suppress the augmenting effect of clonazepam (0.5-2.0 mg/kg, PO) on the solution intake. Since clonazepam does, and Ro 5-4864 does not, possess punishment-attenuation properties in other tests, drug augmentation of NaCl solution ingestion by rehydrating rats continues to correlate well with known anxiolytic action.

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.

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.

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.

Characteristics of reserpine-induced suppression of NaCl solution intake in rats

Effects of single and repeated administration of reserpine on time-limited drinking of a hypertonic (1.5% w/w) NaCl solution were investigated in rats to assess whether this drug possesses anxiolytic action. Rats adapted to a 23-hr water-deprivation schedule with a free-feeding regimen were allowed a daily 1-hr water rehydration session. In the single-administration experiment, reserpine (0.1, 0.2 and 0.4 mg/kg, IP) was administered to rats at 15 min or 23 hr before a drinking session, where the fluid available was 1.5% NaCl solution. Drug was administered every 7th day. In the repeated-administration experiment, reserpine (0.1 mg/kg/day) was injected daily for 10 days 15 min before each drinking session. The fluid available was water on the first 9 days and NaCl solution on the 10th day. Reserpine suppressed NaCl solution intake when it was singly administered at 15 min before the rehydration, whereas no significant change in the fluid intake occurred when it was administered 23 hr before drinking, even though rats showed ptosis in response to 0.2 and 0.4 mg/kg doses. Tolerance developed to the suppressing effect of repeated administration of reserpine on fluid intake, although ptosis and sedation continued and body weights decreased. Tolerance was almost complete after 11 days. The results suggest that reserpine does not have an anxiolytic effect.

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

Experiments were conducted to determine the degree of generality of previous findings that anxiolytics increased the ingestion of hypertonic saline in rehydrating rats. Further, potential differential effects amongst recently described benzodiazepine receptor partial agonists were explored. Finally, the hypothesis that benzodiazepine receptor partial inverse agonists would decrease the ingestion of hypertonic NaCl solution was tested. Results indicated that full agonists (midazolam, ZK 93423, zopiclone) produced substantial dose-related increases in hypertonic saline consumption. The putative 5-HT1A agonist, buspirone, produced only a dose-dependent decrease in saline intake. Partial agonists fell into two distinct categories: ZK 91296, CL 218,872 and two novel benzodiazepines, Ro16-6028 and Ro17-1812, also increased saline ingestion. In contrast, two pyrazoloquinolines, CGS 9896 and CGS 9895, had no significant effect on intake. Two compounds, CGS 8216 and FG 7142, described as benzodiazepine partial inverse agonists, did not significantly affect consumption of the hypertonic saline.

Differential effects of benzodiazepine receptor ligands on isotonic saline and water consumption in water-deprived rats

Water-deprived male rats were adapted to a 30 min test of water or saline drinking in a single-bottle acceptance test. The potent benzodiazepine agonist, clonazepam, produced significant increases in both water and saline consumption. Increases in the consumption of both were also obtained with the non-benzodiazepine agonist, zopiclone (a cyclopyrrolone), but not with the pyrazoloquinoline agonist, CGS 9896. Hence, some, but not all, benzodiazepine receptor agonists enhance drinking responses. The benzodiazepine receptor antagonists, Ro15-1788 and CGS 8216, had no significant effect on the intake of either isotonic saline or water. In contrast, the beta-carboline FG 7142, which has been described as an inverse agonist acting at benzodiazepine receptors, reduced both saline and water drinking at 10 and 20 mg/kg. Although the baseline level of saline drinking was considerably higher than that of water, there was no general indication that any drug effect on consumption interacted with the type of fluid in the drinking test. However, in the case of agonist-induced increases in consumption, peak effects occurred at different doses; they were lower for saline- than for water-drinking.

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.

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.

Benzodiazepine receptor-mediated effect of CGS 8216 on milk consumption in the non-deprived rat

The pyrazoloquinoline CGS 8216, which binds with high affinity to central benzodiazepine recognition sites, produced a highly significant reduction in the consumption of familiar, sweetened milk by non-deprived male rats, when administered in a dose of 20 mg/kg, IP. The anorectic effect was present during the first 5 min period of a 20-min drinking test, and remained in evidence throughout the remainder of the test. The benzodiazepine receptor antagonist Ro15-1788, administered 15 min before the consumption test, produced a dose-related (10-40 mg/kg, IP) reversal of the anorectic effect of CGS 8216, during the first 10 min of the test. Injection of Ro15-1788 alone had no significant effect on milk ingestion. This experiment shows that the reduction in the consumption of a palatable liquid food by CGS 8216 can be attributed to an action at benzodiazepine receptors. The result is consistent with the characterization of CGS 8216 as a weak benzodiazepine receptor inverse agonist.

Convulsant component of a depressant benzodiazepine

The convulsant influence of high doses of diazepam, in the presence of the benzodiazepine receptor antagonist Ro 15-1788, was studied in rats. Animals were implanted with permanent cortical screw electrodes for EEG recording. EEG spiking and accompanying clonic activity was observed in rats receiving greater than or equal to 200 mg/kg diazepam, followed 10 minutes later by Ro 15-1788 (20 mg/kg). Pentylenetetrazole and picrotoxin seizure thresholds, measured during constant rate iv infusion, were significantly lowered by pretreatment with diazepam (250 mg/kg) and Ro 15-1788 (20 mg/kg) administered 30 and 20 minutes, respectively, before seizure threshold measurement. It is proposed that this convulsive activity of diazepam is mediated through the picrotoxinin receptor.

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.

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

Chlordiazepoxide (1.25-20.0 mg/kg i.p.) was administered to non-food-deprived male rats given 30 min access to a highly palatable, familiar diet, and produced a potent stimulation of food consumption. At the maximum dose effect, the rats consumed about 24 g food in the 30 min test. The benzodiazepine receptor antagonist, Ro15-1788 (2.5-40.0 mg/kg i.p.) had no effect on food intake when given alone, but did dose-dependently attenuate chlordiazepoxide's hyperphagic effect. The antagonist CGS 8216 (5.0-20.0 mg/kg i.p.) completely abolished the hyperphagic effect, and in doses of 10.0 and 20.0 mg/kg produced significant suppression of feeding when administered by itself. ZK 93 426, in doses (0.625-10.0 mg/kg i.p.) which have previously been shown to antagonize the discriminative cue of chlordiazepoxide produced no significant change in chlordiazepoxide's hyperphagic effect. These data point to an interesting and important distinction between ZK 93426 and the other two benzodiazepine receptor antagonists when given in combination with chlordiazepoxide in the palatable food consumption test.

Benzodiazepine receptor ligands and the consumption of a highly palatable diet in non-deprived male rats

Non-deprived rats were familiarized with a highly palatable diet until baseline consumption in a 60-min daily access period had stabilised. The benzodiazepine receptor agonist midazolam (1.25-10.0 mg/kg, IP) produced a large, dose-related increase in food consumption during the first 30 min of access. It also produced significant, short-term hyperphagia in animals which had been partially pre-satiated on the diet before drug administration, an effect which was reversible by the benzodiazepine receptor antagonist Ro15-1788. Administered alone, Ro15-1788 (1.25-10.0 mg/kg, IP) had no intrinsic activity in the food consumption test. In contrast, CGS 8216 (2.5-40.0 mg/kg, IP) produced a marked dose-related suppression of food intake. This anorectic effect was shared by two benzodiazepine receptor inverse agonists, FG 7142 and DMCM, which also produced dose-dependent reductions in consumption. The effects on feeding produced by FG 7142 (20 mg/kg, IP) and DMCM (1.25 mg/kg, IP) were reversed by either Ro15-1788 (2.5 and 5.0 mg/kg) or midazolam (5.0 and 10.0 mg/kg). A matched anorectic effect produced by CGS 8216 (40 mg/kg) was not, however, reversed by either Ro15-1788 or midazolam. This suggests that at a high dose CGS 8216 may act by a mechanism different from that of the two inverse agonists. The feeding test described in the report proved sensitive to both hyperphagic and anorectic effects of drugs active at benzodiazepine receptors, pointing to a possible bi-directional control of palatable food consumption.