A comparison between chlordiazepoxide and CL 218,872--a synthetic nonbenzodiazepine ligand for benzodiazepine receptors on spontaneous locomotor activity in rats

Lateral habenular norepinephrine contributes to states of arousal and anxiety in male rats

Recent research has identified the lateral habenula (LHb) as a brain region playing an important role in the production of stressful and anxiogenic states. Additionally, norepinephrine (NE) has long been known to be involved in arousal, stress and anxiety, and NE projections to the LHb have been identified emanating from the locus coeruleus (LC). The current research was devised to test the hypothesis that NE release within the LHb contributes to the occurrence of anxiogenic behaviors. Male rats were implanted with bilateral guide cannula aimed at the LHb and subsequently treated with intracranial (IC) infusions of the selective α2 adrenergic autoreceptor agonist, dexmedetomidine (DEX) (0, 0.5, 1.0 μg/side), prior to assessment of ambulatory and anxiogenic behavior in tests of spontaneous locomotion, open field behavior, and acoustic startle-response. Results demonstrated that DEX administration significantly reduced the overall locomotor behavior of subjects at both doses indicating that infusion of even small doses of this α2 agonist into the LHb can have profound effects on the subjects' general levels of alertness and activity. DEX was also found to attenuate anxiety as evidenced by a reduction in the magnitude of a startle-response to an acoustic 110 dB stimulus. Taken together, these results identify a role for NE release within the LHb in both arousal and anxiety.

Are benzodiazepines really anxiolytic? Evidence from a 3D maze spatial navigation task

The effects of diazepam and chlordiazepoxide were assessed in a 3D maze which is a modification of an 8-arm radial maze. Each arm of the maze is attached to a bridge radiating from a central platform. Animals exposed for the first time to the maze do not venture beyond the line that separate a bridge from an arm. The prime criteria set for an anxiolytic effect is whether mice would increase the frequency of entries onto arms and increase arm/bridge entries ratio. C57 mice readily cross the line on first exposure and make more than 8 arm visits onto arms on second exposure, while other strains (CD-1 and Balb/c) hold back and rarely cross the line on first exposure and require more sessions to make more than 8 arm entries. An anxiolytic drug is expected to encourage intermediate (CD-1) and high (Balb/c) anxiety mice to adventure onto the arms of the maze and make more visits to the arms to comparable levels seen with low anxiety c57 mice. In the present report, administration of different doses of diazepam (0.625, 1.25, 2.5 and 5 mg kg(-1) i.p.) and chlordiazepoxide (5, 10 and 15 mg kg(-1) i.p.) did not reduce anxiety in animals, with the lowest dose of diazepam increasing motor activity in Balb/c and increasing anxiety in c57 mice while the highest doses of both diazepam (2.5 and 5 mg kg(-1) i.p.) and chlordiazepoxide (15 mg kg(-1) i.p.) induced mild sedation. Our results raise some concerns about the methodological foundations in the current assessment of anxiety and anxiolytic compounds both in animal and human studies.

Comparison of the pharmacological profiles of the hypnotic drugs, zaleplon and zolpidem

The BZ1 (omega 1)-selective compound, zolpidem, is a clinically effective hypnotic drug with a pharmacological profile which differs from those of benzodiazepine anxiolytics and hypnotics. Zaleplon (CL 284,846) has recently been described as a hypnotic agent which also has BZ1 (omega 1) receptor selectivity. The pharmacological effects of zolpidem and zaleplon were therefore compared in mice and rats. Both drugs blocked tonic convulsions induced in mice by pentylenetetrazole and electroconvulsive shock and clonic convulsions induced by isoniazid. Zaleplon was more potent than zolpidem but the maximal effect of zolpidem for increasing the latency to isoniazid-induced convulsions was greater than that of zaleplon. Little tolerance developed to the anticonvulsant effect of zaleplon against isoniazid-induced seizures following twice daily administration of 10 or 30 mg/kg for 10 days. Both compounds reduced locomotor activity and produced motor deficits in the rotarod and loaded grid tests in mice. However, while zaleplon produced all three effects at similar doses, zolpidem showed the greatest potency for reducing locomotion. Zaleplon and zolpidem also decreased locomotion and produced a rotarod deficit in rats. Again, the difference between the doses giving rise to these two effects was greater for zolpidem than for zaleplon. In a drug discrimination procedure using rats trained to discriminate a dose (5 mg/kg) of chlordiazepoxide, zaleplon produced partial substitution for chlordiazepoxide at doses which greatly reduced response rates. These results show that zaleplon and zolpidem have similar pharmacological profiles, presumably related to their BZ1 (omega 1) receptor selectivity. However, the difference between doses producing motor deficits (rotarod, loaded grid) and those giving rise to other effects (anticonvulsant, decreased locomotion) was greater for zolpidem than for zaleplon. This difference may be related to a greater in vivo intrinsic activity of zolpidem as indicated by the different efficacies of the two drugs to antagonise isoniazid-induced convulsions.

Effects of chronic treatment with triazolam on operant responding in rats

The aims of the present study were to investigate the effect of the benzodiazepine antagonist, flumazenil, on operant responding in rats treated chronically with the short-acting hypnotic triazolam and to study the consequence of chronic triazolam treatment on the time course of effects of triazolam and zolpidem. Zolpidem is an imidazopyridine with a pharmacological and behavioral profile that differs from that of the benzodiazepine hypnotics. Rats were treated with saline or triazolam (1 or 3 mg/kg) twice daily for 5 days and were tested daily 1, 3, 5.5 or 7.5 h after injection. In addition, on the 5th day of chronic treatment all rats were injected with flumazenil (10 mg/kg) 10 min before session. The time course of effects of triazolam and zolpidem was determined after cessation of repeated saline or triazolam treatment. Tolerance to the depressant effect of 1 mg/kg of triazolam developed during long-term administration. Flumazenil decreased operant responding in rats pretreated with triazolam. The effect was statistically significant when rats had received 1 mg/kg of triazolam 3 h before the session or 3 mg/kg of triazolam 3, 5.5 or 7.5 h before the session. After cessation of chronic treatment, rats pretreated chronically with 3 mg/kg of triazolam displayed decreased sensitivity to triazolam and to 10 mg/kg but not 3 mg/kg of zolpidem. The present results indicate that chronic treatment with triazolam induces tolerance to the rate-decreasing effect of the drug and dependence as measured by flumazenil-induced disruption of operant responding. The limited degree of cross-tolerance between zolpidem and triazolam may suggest that their pharmacological mechanisms of action are distinct.

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

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

Tolerance, cross-tolerance and dependence measured by operant responding in rats treated with triazolam via osmotic pumps

Previous research has found that drugs with affinity for omega (benzodiazepine) sites differ in their abilities to produce tolerance and dependence. The present study therefore investigated the effects of ligands of omega (BZ) sites in rats that had been rendered tolerant to a benzodiazepine. Two experiments were carried out in separate groups of rats. Behavioral changes induced by chronic infusion of triazolam (3 mg/kg/day, SC, for 14 days) via osmotic pumps were studied in animals trained on a fixed ratio 10 schedule of food presentation. Control animals were implanted with pumps containing the vehicle. Test drugs were administered IP using cumulative dosing. In one experiment triazolam decreased response rates on days 1, 2 and 3 after implantation of the pumps and tolerance developed to this depressant effect. In the other experiment, vehicle and triazolam treated rats differed in their responding during chronic infusion but differences were not statistically significant on any particular day. Flumazenil (3.0-30 mg/kg) greatly decreased rates of responding on day 11 in triazolam treated rats. This effect may represent a precipitated withdrawal syndrome. However, no withdrawal effects on operant performance were observed upon pump removal. Chronic infusion of triazolam did not affect the sensitivity of rats to alpidem on day 11 (10-100 mg/kg) whereas it abolished the stimulant effect of bretazenil (0.1-1.0 mg/kg). Chronic triazolam treatment produced tolerance to the depressant effects of triazolam (1.0-3.0 mg/kg), lorazepam (0.3-3.0 mg/kg) and zopiclone (10 mg/kg) but no tolerance to those of CL 218,872 (3.0-30 mg/kg) and zolpidem (0.3-3.0 mg/kg) when tested 3-14 days after pump removal.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ritanserin and chlordiazepoxide on sleep-wakefulness alterations in rats following chronic cocaine treatment

The effects of ritanserin, a 5-hydroxytryptamine-2 (5-HT2) receptor antagonist, and chlordiazepoxide, a benzodiazepine agonist, on sleep-wakefulness disturbances in rats after acute administration of cocaine and after discontinuation of chronic cocaine treatment were examined. Intraperitoneal (IP) injection of chlordiazepoxide (10 mg/kg) but not ritanserin (0.63 mg/kg) prevented the increase of wakefulness (W) and the reduction of light slow wave sleep (SWS1) and deep slow wave sleep (SWS2) induced by an acute injection of cocaine (20 mg/kg IP). Daily injection of cocaine (20 mg/kg for 5 days, then 30 mg/kg for 5 days IP) at the onset of the light phase elicited an increase of W and a concomitant decrease of SWS1, SWS2 and paradoxical sleep (PS) in the light phase, followed by a rebound in SWS2 and PS in the subsequent dark phase. Following cocaine discontinuation, the circadian distribution of sleep-wakefulness states remained disturbed in saline-treated rats for at least 5 days. Both ritanserin (0.63 mg/kg IP/day) and chlordiazepoxide (10 mg/kg IP/day) reduced the alteration in the distribution of W and SWS2 throughout the light-dark cycle from the first day of administration on, but failed to prevent PS alterations. The mechanisms by which both compounds exert their effect are probably quite different. For chlordiazepoxide sedative and sleep-inducing properties probably play a major role. In contrast, for ritanserin SWS2-increasing properties and its ability to reverse preference for drugs of abuse without inducing aversion might be key factors.

Like diazepam, CL 218,872, a selective ligand for the benzodiazepine omega 1 receptor subtype, impairs place learning in the Morris water maze

The sedative, anxiolytic, and amnesic effects of diazepam were compared to those of CL 218,872, a triazolopyridazine that has a preferential affinity for the benzodiazepine omega 1 receptor subtype. Spontaneous locomotion was assessed using a running wheel, anxiety was assessed using an open-field divided into central and peripheral areas (thigmotaxis), and amnesia was assessed using the Morris water maze. It was found that CL 218,872, like diazepam, depressed spontaneous locomotion, reduced anxiety, and impaired place learning in a dose-dependent manner. Flumazenil, a benzodiazepine receptor antagonist with a similar affinity for both omega 1 and omega 2 subtypes, reversed all of the effects of diazepam and antagonized the anxiolytic and amnesic effects, and some but not all of the sedative effects of CL 218,872. These results suggest that the selective activation of the omega 1 receptor subtype by CL 218,872 is sufficient to produce sedation, anxiolysis, and amnesia in a manner similar to that produced by the coactivation of both the omega 1 and omega 2 receptor subtypes with diazepam.

Involvement of the "peripheral" benzodiazepine receptor type (omega 3) in the tolerance to the electroencephalographic effects of benzodiazepines in rats: comparison of diazepam and clonazepam

Rapid tolerance to the sedative effect of large doses of diazepam (10 mg/kg IV), but not of large doses of clonazepam (2 mg/kg IV) occurs in rats after 5 days of treatment on a once-a-day regimen. Electroencephalographic (EEG) studies show that such behavioral tolerance is associated with a decreased induction of spindle bursts and with an increased induction of 20-30 Hz waves (beta-like activity). Administration of clonazepam plus the agonist of the "peripheral" benzodiazepine receptor type (omega 3) Ro 5-4864 (4 mg/kg IV) for 5 days induces signs of behavioral and EEG tolerance to sedative effects of the benzodiazepine agonist. In animals treated for 5 days with diazepam plus the omega 3 antagonist PK 11195 (5 mg/kg IV), no signs of EEG and behavioral tolerance are observed. These results suggest that omega 3 type activation influences the development of rapid tolerance to the sedative effect of diazepam in rats.

The interaction between zolpidem and beta-CMC: a clue to the identification of receptor sites involved in the sedative effect of zolpidem

The interaction of beta-CMC, an amino beta-carboline recently described as a selective antagonist of the sedative effect of diazepam, with zolpidem, an imidazopyridine hypnotic, which like beta-CMC binds preferentially to the omega 1 (BZ-1) site of the GABA benzodiazepine chloride channel receptor complex, was investigated. In mice, beta-CMC antagonized the effect of zolpidem against isoniazid-induced convulsions without affecting its activity against convulsions induced by pentylenetetrazole or electroshock. beta-CMC also antagonized the decrease in locomotor activity and the impairment in muscle strength provoked by zolpidem. In rats trained to discriminate zolpidem, beta-CMC antagonized both the interoceptive stimulus and the decrease in the rate of lever pressing produced by zolpidem. This selective antagonism, inhibition of effects of zolpidem exerted by low doses (locomotor activity and isoniazid-induced convulsions) as well as effects produced by high doses (muscle strength) but not those provoked by intermediate doses (pentylenetetrazole and electroshock-induced convulsions), could not be explained by a receptor occupancy hypothesis. These results suggest that the anticonvulsant and sedative effects of zolpidem do not involve the same receptor subtype and that the hypnoselective properties of zolpidem may be linked to its selectivity for the omega 1 (BZ-1) site of the GABAA receptor.

Effects of non-sedative anxiolytic drugs on responses to GABA and on diazepam-induced enhancement of these responses on mouse neurones in cell culture

1. Intracellular microelectrode recording techniques were performed on mouse spinal cord and cerebral hemisphere neurones grown in primary dissociated cell culture. The effects of several anxiolytics applied by local pressure ejection on responses to gamma-aminobutyric acid (GABA) evoked by iontophoresis were investigated. Responses to GABA were depolarizing since intracellular chloride ion concentration was increased by injection from potassium chloride (3M)-filled recording micropipettes and neurones were held at large negative membrane potentials (-70 to -90 mV). The agents studied were six 'non-sedative anxiolytics', CL 218,872 (3-methyl-6-(3-trifluoromethyl-phenyl)1,2,4-triazolo(4,3-b) pyridazine), PK 8165 (2-phenyl-4-(2-(4-piperidinyl)ethyl)-quinoline), PK 9084 (2-phenyl-4-(2-(3-piperidinyl)ethyl)-quinoline), CGS 9896 (2-(4-chlorophenyl)-2,5-dihydropyrazolo(4,3-c)quinoline-3(3H)-one) , ZK 91296 (ethyl 5-benzyloxy-4-methoxymethyl-beta-carboline-3-beta-carboxylate), buspirone (8-4-[4-(2-pyrimidinyl)-1-piperazinyl]butyl-8-azaspiro[4.5]decane- 7,9- dione), and two sedative anxiolytics, diazepam and zopiclone [( 6-(5-chloro-2-pyridyl)-6,7-dihydro-7-oxo-5H-pyrrolo[3,4-b]pyrazin- 5- yl]4-methyl-1-piperazinecarboxylate). 2. Direct effects on responses to GABA were studied for all drugs applied in varying concentrations. For the drugs which significantly altered responses to GABA, the effects of the benzodiazepine receptor antagonists Ro 15-1788 (ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo(1,5a)-(1,4)benzodi azepine - 3-carboxylate) and CGS 8216 (2-phenylpyrazolo(4,3-c)-quinolin-3(5H)-one) were evaluated. For the drugs devoid of significant direct effect on responses to GABA, the influence on diazepam-induced enhancement of responses to GABA was evaluated. 3. Diazepam, zopiclone and CL 218,872 concentration-dependently and reversibly enhanced responses to GABA. Maximal enhancement was 82% for diazepam (500 nM), 64% for zopiclone (10 microM) and 20% for CL 218,872 (10 microM). PK 8165 effects varied with concentration, enhancing responses to GABA (up to 18%) at nM concentrations and reducing responses to GABA (up to 90%) at microM concentrations. CGS 9896, ZK 9126, PK 9084 and buspirone, in concentrations ranging from 1 nM to 10 microM, lacked significant direct effects on responses to GABA. 4. The enhancing effects of diazepam, zopiclone, CL 218,872 and PK 8165 were antagonized by Ro 15-1788. However, the reducing effect on responses to GABA of PK 8165 at microM concentrations was not antagonized by CGS 8216. CGS 9896 and ZK 91296 concentration-dependently blocked the diazepam-induced enhancement of responses to GABA. However, PK 9084 and buspirone did not antagonize the diazepam-induced enhancement of responses to GABA. 5. These results indicate that diazepam and zopiclone may be full agonists, CL 218,872 and PK 8165 are partial agonists, and CGS 9896 and ZK 91296 are pure antagonists at benzodiazepine receptors. On the other hand, PK 9084 and buspirone do not interact with benzodiazepine receptors.

Investigation of the development of tolerance to the actions of zolpidem and midazolam

It is well established that tolerance can develop very rapidly to the behaviour-suppressing effects of benzodiazepines. In previous studies, however, the depressant action of zolpidem, a novel hypnotic acting at the benzodiazepine receptor, on operant behaviour in rats was maintained after many injections. An experiment was carried out, therefore, to compare the effects of acute and chronic administration of zolpidem and midazolam. Rats were trained to press a lever in standard operant test chambers to obtain 45 mg food pellets on an FR 10 schedule. Dose-response curves were then established, before, immediately after and 4 weeks after the daily administration of midazolam (3.0 mg/kg s.c.) or zolpidem (1.0 mg/kg) for 10 days. In confirmation of previous work, marked tolerance developed to the response-rate-decreasing effect of midazolam, the dose-response curve being shifted to the right by a factor of 6 and also flattened. No significant dissipation of this tolerance occurred during a period of 4-6 weeks. In contrast, repeated administration of zolpidem produced only a small degree of tolerance, the dose-response curve being shifted by a factor of two. There was little evidence for cross tolerance between the two drugs, zolpidem-treated rats being sensitive to a dose of midazolam and midazolam-treated rats sensitive to a dose of zolpidem. Although the explanation for the development of tolerance to midazolam is unknown, these results suggest that the mechanisms of action of midazolam and zolpidem in reducing response rates are different.

Sleep and benzodiazepine receptor sub-types

CL 218,872, which preferentially binds to benzodiazepine (BZ) type 1 receptors, and flurazepam, which is thought to bind to both types 1 and 2, were given alone and in combination to rats. CL 218,872 had little effect on sleep latency, but significantly increased total sleep time. As expected, flurazepam both shortened sleep latency and prolonged total sleep. Pretreatment with CL 218,872 had no effect on these alterations in sleep induced by flurazepam. The implications for possible significance of sub-typing of BZ receptors are discussed.

Effects of agents which interact with central benzodiazepine binding sites on stress-induced ultrasounds in rat pups

Compounds of different chemical classes, with affinity for central benzodiazepine receptors, were investigated for effects on handling-induced ultrasonic cries in rat pups. Diazepam and clobazam inhibited ultrasounds and impaired motor performance at higher doses. Suriclone showed a similar profile to diazepam but premazepam clearly separated ultrasound inhibition from motor impairment. ZK 91296, CGS 9896, RU 39419 and RU 43028 inhibited ultrasounds with a lower maximal response but induced little or no motor incoordination. CL 218872 inconsistently inhibited sounds with no motor impairment and PK 8165 and PK 9084 had no consistent effects. Benzodiazepine antagonists weakly inhibited (RU 40410) or did not affect (Ro 15-1788 or CGS 8216) ultrasounds alone but fully (Ro 15-1788 or CGS 8216) or partially (RU 40410) antagonised the effects of benzodiazepines. Inverse agonists FG 7142, methyl-beta-carboline-3-carboxylate (BCM) and DMCM tended to increase ultrasounds alone, particularly when less stressful handling stimuli were used, and antagonised benzodiazepines. This procedure detects the differing behavioural effects of benzodiazepines receptor ligands and is proposed as a simple model of anxiety.

Lack of cross-tolerance in mice between the stimulatory and depressant actions of novel anxiolytics in the holeboard

CL 218,872, tracazolate and tofisopam are compounds that are believed to act at the GABA-benzodiazepine (BDZ) receptor complex in the CNS and that have anxiolytic properties in animals or in man. Doses of each drug were selected to elevate, or to depress, exploratory head-dipping and locomotor activity in the holeboard in mice, and the development of tolerance to these effects was investigated. As previously found with benzodiazepines, tolerance did not develop to the stimulant effects of low doses of these compounds after 10 days pretreatment with either a low (stimulant) or a high (depressant) dose of the same compound. When animals were pretreated with a low (stimulant) dose, tolerance did not develop to the depressant effects of a high test dose. Tolerance was observed only to the depressant effects of a high dose of a drug, and only when animals were also pretreated with a high dose. The results are compared with those obtained with benzodiazepines, and the ability of current theories of tolerance to account for the results is discussed.

A 7-phenyl substituted triazolopyridazine has inverse agonist activity at the benzodiazepine receptor site

To investigate further the structural requirements for benzodiazepine (BZD) receptor ligands, we synthesized SR 95195, [7-phenyl-3-methyl-1,2,4-triazolo-(4,3-b) pyridazine], a positional isomer of the 6-phenyl-triazolo-pyridazines, which were the first non-BZD derivatives to exhibit high affinity for the BZD receptor and BZD-like activity in vivo. In vitro, SR 95195 displaced specifically bound [3H]-flunitrazepam from rat cerebellar and hippocampal membranes with respective IC50 values of 4 and 8 microM. In vivo, SR 95195 lacked BZD-like activity. At high doses SR 95195 induced clonic seizures in mice (threshold convulsant dose: 150 mg kg-1; CD50: 160 mg kg-1 i.p.) which were antagonized by Ro 15-1788. At non-convulsant doses (25 mg kg-1 i.p. and 100 mg kg-1 i.p.) SR 95195 significantly decreased punished responding in an operant conflict procedure in the rat, suggesting SR 95195 has intrinsic anxiogenic activity. SR 95195, in mice, reversed the anticonvulsant and myorelaxant actions of diazepam 3 mg kg-1, orally (respective ED50 values: 45 mg kg-1 i.p. and 44 mg kg-1 i.p.). In an operant-conflict test in rats, SR 95195 at non-anxiogenic doses, antagonized the disinhibitory action of diazepam 4 mg kg-1, i.p. (ED50: 8.6 mg kg-1, i.p.), but not that of pentobarbitone 15 mg kg-1, i.p. It is concluded that SR 95195 has the pharmacological profile of an inverse BZD agonist and that displacing the phenyl from the 6- to the 7-position in the triazolopyridazine series causes a shift from agonist to inverse agonist type activity at the BZD receptor site.

Comparison of the effects of chlordiazepoxide and CL 218,872 on serum corticosterone concentrations in rats

Fifteen minute exposure to a novel environment plus 120 dB sound stimulation produced a three-fold increase in serum corticosterone concentrations in rats. A low dose of intraperitoneally (IP) administered chlordiazepoxide (CDP) (5 mg/kg) attenuated this response, whereas a higher dose (20 mg/kg) elevated corticosterone concentrations in rats not subjected to sound stress. Parallel results were obtained after intracerebroventricular (ICV) drug administration, with a low dose of CDP (5 micrograms) reducing the sound stress response and higher doses (25 and 50 micrograms) increasing corticosterone concentrations in unstressed animals. Thus, despite the presence of benzodiazepine (BDZ) receptors at every level of the hypothalamic-pituitary-adrenocortical axis, it appears that BDZs alter the activity of this system via an interaction with BDZ receptors in brain. CL 218,872 (2.5-20 mg/kg), a novel non-BDZ anxiolytic compound, did not attenuate the corticosterone elevation produced by sound stimulation, and also failed to alter baseline corticosterone concentrations in unstressed animals. The fact that CL 218,872 is a selective agonist for brain Type I BDZ receptors suggests that BDZs are not influencing corticosterone secretion through an interaction with this BDZ receptor subtype. Furthermore, these results indicate that stress (as measured by pituitary-adrenocortical activation) can be dissociated from anxiety (as measured by conflict paradigms), thus challenging the validity of the corticosteroid stress test as a screening procedure for anxiolytic activity.

A comparison between chlordiazepoxide and CL 218,872, a synthetic non-benzodiazepine ligand for benzodiazepine receptors, on serotonin and catecholamine turnover in brain

Serotonin (5-HT) and catecholamine (CA) turnover rates in midbrain-hindbrain were investigated in rats treated with the benzodiazepine chlordiazepoxide (CDP) or with the anxiolytic triazolopyridazine CL 218,872. A single injection of CDP (2.5-20 mg/kg) decreased 5-HT but not CA turnover rates. Seven daily CDP injections decreased the turnover rates of both 5-HT and CA. In contrast, neither acute nor chronic administration of CL 218,872 (2.5-20 mg/kg) altered baseline turnover rates of 5-HT or CA. Since CL 218,872 has a selective affinity for Type I benzodiazepine receptors, these results suggest that stimulation of Type I receptors is not sufficient to decrease 5-HT or CA turnover rates in midbrain-hindbrain. More importantly, these results question the long-held belief that benzodiazepines exert their anxiolytic and depressant effects through reductions in 5-HT and CA turnover respectively.