Characterization of the behavioral profile of the non-peptide CRF receptor antagonist CP-154,526 in anxiety models in rodents Comparison with diazepam and buspirone

The present series of experiments compared the behavioral effects of the novel non-peptide CRF antagonist CP-154,526 with those of diazepam and the 5-HT_1A receptor partial agonist buspirone in classical animal models of anxiety including conflict tests (punished lever pressing and punished drinking tests in rats) and exploratory models (elevated plus-maze test in rats, light/dark choice and free-exploration tests in mice), and a recently developed mouse defense test battery (MDTB) which has been validated for the screening of anxiolytic drugs. Results from both conflict procedures showed that diazepam (2.5–10 mg/kg, IP) produced clear anxiolytic-like effects, whereas buspirone (2.5 mg/kg, IP) displayed anticonflict activity in the punished drinking test only. CP-154,526 (0.6–20 mg/kg) was devoid of significant activity in both procedures. In the elevated plus-maze, diazepam (2 mg/kg, IP) produced significant effects on traditional (i.e. spatio-temporal) and ethologically derived (i.e. risk assessment and directed exploration) indices of anxiety. Buspirone (1–4 mg/kg, IP) reduced risk assessment activities only, and CP-154,526 (0.6–20  mg/kg, IP) did not modify the indices of anxiety in the elevated plus-maze. In the light/dark test, diazepam (2.5–5 mg/kg, IP) and CP-154,526 (10–40 mg/kg, IP) affected all behavioral indices of anxiety, while buspirone reduced risk assessment activities at the highest doses only (10 and 15 mg/kg, IP). In the free-exploration test, diazepam (1 mg/kg, IP) reduced avoidance responses towards novelty, as indicated by the increase in exploratory activity in a novel compartment and the decrease in risk assessment. CP-154,526 failed to affect the former behavior and weakly reduced the latter (5 and 20 mg/kg, IP). Buspirone (1.25–5 mg/kg, IP) was inactive in this test. Finally, in the MDTB, diazepam (0.5–3 mg/kg, IP) attenuated all defensive reactions of mice confronted with a rat stimulus (i.e. flight, risk assessment and defensive attack) or with a situation associated with this threat (i.e. contextual defense). Buspirone (1.25–5 mg/kg, IP) reduced defensive attack and contextual defense, while CP-154,526 (5–20 mg/kg, IP) affected all defensive behaviors, with the exception of one risk assessment measure. The finding that CP-154,526 displayed positive effects in mice but not in rats may be due to increased sensitivity to environmental stress of the strains used (i.e. BALB/c, Swiss) and/or to the fact that animals were exposed to unavoidable stress stimuli which may lead to a significant activation of the CRF system. Although in mice the anxiety-reducing potential of CP-154,526 is superior to that of the atypical anxiolytic buspirone, it is smaller in terms of the magnitude of the effects and the number of indices of anxiety affected than that of diazepam. Received: 7 September 1997/Final version: 26 November 1997     

Blockade of phencyclidine-induced hyperlocomotion by olanzapine, clozapine and serotonin receptor subtype selective antagonists in mice

In humans, phencyclidine (PCP) is known to produce a syndrome of behavioral effects which have many characteristics in common with schizophrenia. Therefore, antagonism of PCP effects might be evidence for antipsychotic efficacy of a compound. In the present studies, the effects of the D₂-like antagonist haloperidol, the mixed D₂-like/5-HT₂ antagonists olanzapine and clozapine, and a series of 5-HT receptor subtype selective antagonists on the hyperlocomotion produced by PCP were evaluated in mice. PCP (0.3–10 mg/kg) produced a dose-related increase in locomotor activity, with a peak effect at 3.0 mg/kg. The D₂-like antagonist haloperidol produced a dose-related decrease in locomotor activity when administered alone, and blocked the hyperactivity effects of PCP over the same dose-range (minimal effective dose, MED = 0.3 mg/kg for both effects). In contrast, olanzapine and clozapine reversed the hyperlocomotion effects of PCP at doses (MED = 0.03 and 0.3 mg/kg, respectively) approximately 30-and 10-fold, respectively, below those that decreased activity when administered alone (MED = 1.0 and 3.0 mg/kg, respectively). The selective 5-HT₂ antagonist LY53857 (0.3–3.0 mg/kg) administered alone had no effect on locomotor activity but reversed (MED = 0.1 mg/kg) the effects of PCP. Similarly, the selective 5-HT_2A/2C antagonist ritanserin (0.001–1.0 mg/kg) alone had no effect on locomotor activity, but reversed (MED = 0.01 mg/kg) the effects of PCP. The selective 5-HT_2A antagonists ketanserin (MED = 3.0 mg/kg) and MDL 100,907 (MED = 0.3 mg/kg) produced dose-related decreases in locomotor activity and ketanserin (MED = 0.1 mg/kg) and MDL 100,907 (MED = 0.003 mg/kg) reversed the effects of PCP. The selective 5-HT₃ antagonist zatosetron (0.01–10 mg/kg) and the selective 5-HT_1A antagonist WAY 100,635 (0.001–3 mg/kg) were without effects on spontaneous locomotor activity. Zatosetron reversed the effects of 3.0 mg/kg PCP at the nonselective dose of 10 mg/kg whereas WAY 100,635 (0.001–1 mg/kg) did not affect PCP-induced hyperlocomotion. The present results indicate that PCP increases locomotor activity, at least in part, due to actions at 5-HT_2A, but not 5-HT₃ or 5-HT_1A, receptors. Further, the present findings support the hypothesis that antagonism at 5-HT_2A receptors contributes to the in vivo actions of atypical antipsychotics such as olanzapine and clozapine. Received: 27 June 1996/Final version: 20 August 1996     

Anxiolytic-like effects of alpha-2-adrenoceptor agonists on conflict behavior in the rat: pre- versus postsynaptic receptor mechanisms.

The effects of acute pretest administration and chronic posttest administration of clonidine or the selective alpha 2-adrenoceptor agonist UK-14,304 on conflict behavior were investigated. In daily 10-min sessions, water-deprived rats were trained to drink water from a tube that was occasionally electrified (0.25 mA); electrification was signaled by a tone. Prior to treatment, subjects accepted 25-30 shocks/session (punished responding) and consumed approximately 12-15 ml/session (unpunished responding). Acute pretest administration of clonidine or UK-14,304 did not increase punished responding. In contrast, chronic posttest clonidine administration (40 micrograms/kg, IP, twice daily for 8 weeks) resulted in a robust and time-dependent increase in punished responding (60-70 shocks/session) relative to saline-treated controls. Moreover, the selective alpha 2-adrenoceptor agonist UK-14,304 also increased punished responding when administered chronically (1.0 mg/kg, BID). Administration of the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine HCl (DSP4, 65 mg/kg, IP) significantly decreased punished responding in control conditioned suppression of drinking sessions. The anticonflict effect associated with chronic posttest clonidine treatment was not altered by DSP4 pretreatment. These findings suggest that chronic posttest alpha 2-adrenoceptor agonist treatment produces an anticonflict effect independent of its actions at presynaptic alpha 2-adrenoceptors.     

The role of GABA-A and mitochondrial diazepam-binding inhibitor receptors on the effects of neurosteroids on food intake in mice

The present studies were undertaken to investigate the neuroactive steroidal modulation of feeding behavior and possible involvement of γ-aminobutyric acid type-A (GABA-A) and mitochondrial diazepam binding inhibitor (DBI) receptors (MDR) in food-deprived male mice. Allopregnanolone (0.5–2 mg/kg), a neurosteroid, progesterone (1–10 mg/kg), a neurosteroid precursor, and 4′-chlordiazepam (0.25–1  mg/kg), a specific high affinity MDR agonist, produced a dose-dependent hyperphagic effects. In contrast, neurosteroids pregnenolone sulfate (PS) (1–10 mg/kg) and dehydroepiandrosterone sulfate (DHEAS) (1–10 mg/ kg) produced a hypophagic effect, in a dose-dependent manner. The allopregnanolone-, progesterone- and 4′-chlordiazepam-induced hyperphagic effect was blocked by picrotoxin (1 mg/kg), a GABA-A chloride channel antagonist, but not by flumazenil (2 mg/kg), a benzodiazepine (BZD) antagonist. The 4′-chlordiazepam-induced hyperphagic effect was prevented by pretreatment with PK11195 (2 mg/kg), a selective partial MDR antagonist. The hypophagic effect of DHEAS (10 mg/kg) was reversed by dizocilpine (10 μg/kg), an NMDA receptor antagonist, but resistant to muscimol (0.1 mg/kg), a selective GABA-A receptor agonist. In contrast, the PS (10 mg/kg)-induced hypophagic response was resistant to dizocilpine, but sensitive to muscimol (0.1mg/kg). Both the sulfated neurosteroids PS and DHEAS also reversed the hyperphagic effect of allopregnanolone. In addition, the BZD agonist triazolam (0.05–0.25  mg/kg) also produced a flumazenil- and picrotoxin-sensitive hyperphagic effects, thereby suggesting the changes in feeding behavior by neurosteroids represent GABA-A receptor mediated hyperphagic action. Although the possible antistress or anxiolytic actions of neurosteroids may confound the hyperphagia, behavioral effects observed were specific to food because the mice were adopted to the test environment and diet, and of a possible variation between various neurosteroids in the extent to which antistress or anxiolytic effect produced at hyperphagic doses. The hyperphagic effects of progesterone and 4′-chlordiazepam resembled that of neurosteroid allopregnanolone. Therefore, the effect of progesterone may be imputed to its metabolism to allopregnanolone, while the 4′-chlordiazepam-induced hyperphagic response is related to its MDR-stimulated neurosteroidogenesis and subsequent modulation of GABA-A receptors. The hypophagic response following DHEAS may, at least partly, involve an NMDA receptor mechanism. However, PS-induced hypophagia may be mediated by GABA-A or other receptor systems. These data suggest a pivotal role for GABA-A and mitochondrial DBI receptors in the hyperphagic effects of neurosteroids and reinforces a role for endogenous neurosteroids in regulating feeding behavior. Future studies may lead to the development of neurosteroid-based anorectic/hyperphagic agents for therapeutic use. Received: 30 September 1997/Final version: 18 November 1997     

Dose-dependent noradrenergic and serotonergic properties of venlafaxine in animal models indicative of antidepressant activity

The present study was undertaken to investigate thoroughly the preclinical psychopharmacological profile of venlafaxine, testing a wide range of doses in animal models indicative of antidepressant-like effects. Venlafaxine was found to be active in mouse forced swimming test (at 8, 16, 32 and 64 mg/kg) and to increase spontaneous locomotor activity (at 16, 32 and 64 mg/kg). Venlafaxine antagonised apomorphine-induced (16 mg/kg) hypothermia (at 2, 4, 8, 16, 32 and 64 mg/kg). Pretreatment with PCPA significantly attenuated the anti-immobility effects of venlafaxine (8 and 16 mg/kg; P < 0.01) in the mouse forced swimming test. Venlafaxine at a dose of 32 mg/kg remained active, despite PCPA pretreatment. DSP-4 significantly attenuated the anti-immobility effects of venlafaxine (16 mg/kg; P < 0.05), whereas venlafaxine at 32 mg/kg remained active, despite DSP-4 pretreatment. Venlafaxine was active in the forced swimming test when administered at sub-effective doses in combination with (±) pindolol (venlafaxine: 1 and 2 mg/kg), RU 24969 (venlafaxine: 1, 2 and 4 mg/kg), 8-OH-DPAT (venlafaxine: 4 mg/kg), clonidine (venlafaxine: 1, 2 and 4 mg/kg), lithium (venlafaxine: 1, 2, and 4 mg/kg) and quinine (venlafaxine: 1 and 2 mg/kg). Prior administration with NAN-190 antagonised the anti-immobility effects of venlafaxine (8, 16 and 32 mg/kg). Interaction studies did not induce changes in locomotor activity. The results of the present study indicated that, at low doses, venlafaxine inhibited serotonin reuptake, while at higher doses it inhibited both serotonin and noradrenaline reuptake. Received: 19 May 1997/Final version: 1 December 1997     

Activation of acetylcholine receptors and 5-HT2 receptors have additive effects in the suppression of neocortical high-voltage spindles in aged rats

We investigated if activation of the muscarinic or nicotinic acetylcholine receptors and serotonin (5-hydroxytryptamine; 5-HT) subtype 2 receptors would have additive or synergistic effects on the suppression of thalamocortically generated rhythmic neocortical high-voltage spindles (HVSs) in aged rats. The 5-HT₂ receptor antagonist, ketanserin, at a moderate dose (5 mg/kg) prevented the ability of a muscarinic acetylcholine receptor agonist, (oxotremorine 0.1 mg/kg), and a nicotinic acetylcholine receptor agonist (nicotine 0.1 mg/kg), to decrease HVSs. At a higher dose (20 mg/kg), ketanserin completely blocked the decrease in HVSs produced by moderate doses of muscarinic acetylcholine receptor agonists (pilocarpine 1 mg/kg and oxotremorine 0.1 mg/kg), and by a high dose of nicotine (0.3 mg/kg), though not that produced by high doses of pilocarpine (3 mg/kg) and oxotremorine (0.9 mg/kg). The ability of a 5-HT₂ receptor agonist, (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.1–1.0 mg/kg), to suppress HVSs was non-significantly modulated by the nicotinic acetylcholine receptor antagonist, mecamylamine (1–15 mg/kg), and the muscarinic acetylcholine receptor antagonist, scopolamine (0.03–0.3 mg/kg). The effects of the drugs on behavioral activity could be separated from their effects on HVSs. The results suggest that activation of the muscarinic or nicotinic acetylcholine receptors plus 5-HT₂ receptors has additive effects in the suppression of thalamocortical oscillations in aged rats. Received: 2 November 1996 /Final version: 7 February 1997     

Comparison of the effects of venlafaxine, paroxetine and desipramine on the pupillary light reflex in man

Rationale: The time-course of the pupillary light reflex response is determined by the successive activation of the parasympathetic and sympathetic innervations of the iris, the latency and the amplitude reflecting parasympathetic and the recovery time mainly sympathetic activity. Objective: To compare the effects of single doses of three antidepressants (venlafaxine: serotonin/noradrenaline reuptake inhibitor, paroxetine: selective serotonin reuptake inhibitor, and desipramine: tricyclic antidepressant) on resting pupil diameter and the pupillary light reflex response. Methods: Fifteen healthy male volunteers participated in five weekly sessions, each of which was associated with one treatment (venlafaxine 75 mg or 150 mg, paroxetine 20 mg, desipramine 100 mg, or placebo) according to a double-blind, double-dummy, balanced, cross-over design. An infrared binocular television pupillometer was used for the recording of the resting pupil diameter and the pupillary light reflex in darkness, in previously dark-adapted eyes. Resting pupil diameter in darkness was recorded before and after treatment. The pupillary light reflex was elicited after treatment, with six light flashes (green, 565 nm peak wavelength) of 200 ms duration and of incremental illuminance (measured in the plane of the cornea): 3.0 × 10^–3, 8.5 × 10^–3, 2.5 × 10^–2, 7.0 × 10^–2, 0.18, 0.43 mW cm⁻². The parameters studied were: latency, amplitude and 75% recovery time. Results: Analyses of variance followed by post hoc tests (least significant difference test or Dunnett’s test; P < 0.05) revealed that both doses of venlafaxine produced a significant increase in resting pupil diameter, decrease in amplitude and shortening of the 75% recovery time of the light reflex response; venlafaxine 150 mg prolonged the latency, while the other treatments had no significant effects. Conclusions: The increase in resting pupil diameter could be indicative of parasympathetic inhibition and/or sympathetic activation. The shortening of the recovery time of the light reflex response is consistent with sympathetic potentiation resulting from noradrenaline uptake blockade in the iris. The prolongation of the latency and decrease of the amplitude of the light reflex response are indicative of a parasympatholytic effect of venlafaxine. However, as venlafaxine has negligible affinity for muscarinic cholinoceptors, this effect cannot be attributed to the blockade of cholinoceptors in the iris. A possible explanation for this finding is that it reflects a central rather than a peripheral effect of the drug: the blockade of noradrenaline uptake in the brain could lead to the potentiation of the noradrenergic inhibition of central parasympathetic (Edinger-Westphal) neurones. These results demonstrate the ability of therapeutically relevant single doses of venlafaxine to potentiate noradrenergic responses in man, consistent with the blockade of noradrenaline uptake. Received: 29 July 1998/Final version: 17 November 1998     

Pindolol does not act only on 5-HT1A receptors in augmenting antidepressant activity in the mouse forced swimming test

The present study was undertaken to identify the receptor subtypes involved in (±) pindolol’s ability to enhance the effects of antidepressant drugs in the mouse forced swimming test. Interaction studies were performed with S 15535 (presynaptic 5-HT_1A receptor agonist) and methiothepin (5-HT_1B autoreceptor antagonist) in an attempt to attenuate or potentiate antidepressant-like activity. (±) Pindolol was tested in combination with selective agonists and antagonists at 5-HT₁, 5-HT₂ and 5-HT₃ receptor subtypes. Pretreatment with S 15535 and methiothepin attenuated the activity of paroxetine, fluvoxamine and citalopram (32 mg/kg, IP; P < 0.01). (±) Pindolol (32 mg/kg, IP.) induced significant anti-immobility effects when tested in combination with 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU 24969) (1 mg/kg, IP; P < 0.05), 1-(2-methoxyphenyl)-4-[-(2-phthalimido) butyl]piperazine) (NAN 190) (0.5 mg/kg; P < 0.05) and ondansetron (0.00001 mg/kg, IP; P < 0.01). Pretreatment with NAN 190 (0.5 mg/kg, IP) potentiated the effects of RU 24969 (1 mg/kg, IP; P < 0.05) and (±) pindolol (32 mg/kg, IP; P < 0.05) in the forced swimming test, as did ondansetron (0.00001 mg/kg, IP). Significant additive effects were induced when RU 24969 (1 mg/kg, IP) was tested in combination with NAN 190 (0.5 mg/kg, IP; P < 0.05), (±) pindolol (32 mg/kg, IP; P < 0.05) and ondansetron (0.0000 mg/kg, IP; P < 0.05). 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (1 mg/kg, IP) or ketanserin (8 mg/kg, IP) did not induce significant antidepressant-like effects with any of the agonists/antagonists tested. The results of the present study suggest that pindolol is acting at presynaptic 5-HT_1B serotonergic receptors, in addition to the 5-HT_1A subtype, in augmenting the activity of antidepressants in the mouse forced swimming test. Received: 18 March 1997/Final version: 17 September 1997     

Effects of different classes of partial benzodiazepine agonists on punished and unpunished responding in pigeons

  Rationale: Although all of the benzodiazepines in use for the treatment of anxiety are presumably full agonists, it is conceivable that partial benzodiazepine agonists may also be clinically effective on the basis of their effects in preclinical models of anxiety. Objectives: To compare the anxiolytic-like effects of different pharmacological/chemical classes of partial benzodiazepine agonists in the pigeon conflict procedure. Methods: Anticonflict effects in pigeons whose responding was maintained under a multiple FR30 food:FR30 food+shock schedule were characterized by 1) the magnitude of punished responding or 2) the percentage of pigeons (n=5–7/dose) showing significant increases in punished responding. Results: The partial allosteric modulators bretazenil and imidazenil produced anticonflict effects comparable with or superior to those observed following administration of the relatively full agonist midazolam. In contrast, neither the β-carbolines CGS 9896, ZK 95962 and ZK 91296, nor the imidazopyridines, alpidem and zolpidem, produced anticonflict effects comparable to either bretazenil and imidazenil or the relatively full benzodiazepine agonist, midazolam, at the doses examined in this study. Conclusions: Although the β-carboline ZK 95962 produced some anticonflict effects, none of the other compounds had anxiolytic-like effects like those observed with midazolam, bretazenil or imidazenil. However, because bretazenil and imidazenil produced robust anticonflict activity, the results indicate that partial allosteric modulators could have anxiolytic effects similar to those produced by higher efficacy compounds. Altogether, the results indicate that partial benzodiazepine agonists differ in their ability to produce robust anticonflict effects in the pigeon. Received: 30 November 1998 / Final version: 5 February 1999     

Benzodiazepines enhance the consumption and palatability of alcohol in the rat

This study examined the effect of the benzodiazepine, midazolam, on the consumption and palatability of 6% ethanol in male Wistar rats. In the first experiment, it was found that midazolam (5 mg/kg) increased home cage ethanol drinking 0–2 h after administration. Another intake experiment, in which ethanol was infused directly into the oral cavity through an indwelling catheter, also showed that midazolam (10 mg/kg) stimulated alcohol ingestion. The affective response to intraoral infusions of ethanol (1 ml during 1 min) was subsequently monitored in benzodiazepine-treated rats. Taste reactivity testing showed that midazolam (5–10 mg/kg) significantly increased the occurrence of hedonic orofacial responses and suppressed the number of passive drippings. A similar response pattern was observed following administration of diazepam (5 mg/kg) and chlordiazepoxide (10 mg/kg), but not after exposure to cis(Z)flupentixol (10 mg/kg). Midazolam also increased the incidence of hedonic responses in alcohol-naive rats with no previous access to ethanol in the home cages. Hedonic responsiveness did not appear to diminish with repeated benzodiazepine exposure: the behaviour of rats given five midazolam injections (10 mg/kg every second day) was similar to that shown by rats with no benzodiazepine pre-exposure. The increased hedonic response to ethanol induced by midazolam was blocked by pretreatment with the benzodiazepine receptor antagonist flumazenil (10 mg/kg), the latter drug exerting no effects on its own. The present results suggest that benzodiazepines, by acting on GABA_A receptors, may facilitate ethanol intake by increasing ethanol’s taste hedonic properties. Received: 22 April 1997/Final version: 14 October 1997     

The pharmacology of impulsive behaviour in rats V: the effects of drugs on responding under a discrimination task using unreliable visual stimuli

Impulsivity is widely considered to be multifactorial. One factor, frequently termed “reflection-impulsivity”, refers to the need to give time to information analysis and reflection before making a response. In most reaction time tasks employed for non-human subjects, responding is expected immediately after a specific stimulus has been presented. In the present experiment, the need to wait before responding was made explicit by using a signal which increased in predictive value the longer the subject (a rat) waited before responding. First, the rats learned that a light signal indicated the availability of a food reinforcer if one of two levers was pressed. In the test procedure, on each trial, when the light was turned on it was only 50% likely to indicate the “correct” lever. After a brief interval it was turned off and on again, this time with a slightly higher probability (>50%) of indicating the correct lever. Over a period of a few seconds the probability that the light indicated the correct lever increased to almost 100%. Thus a quick response to the light would result in many errors, whereas a slow response could always result in food delivery. Once trained the rats were treated with a series of drugs: haloperidol (0.01–0.1 mg/kg), chlordiazepoxide (1–10 mg/kg), ethanol (100–1000 mg/kg), d-amphetamine (0.4–1.2 mg/kg), metergoline (0.3–3.0 mg/kg), imipramine, desipramine and clomipramine (all 1, 3 and 10 mg/kg), as well as being given additional food just prior to testing. Ethanol and clomipramine had no effects on behaviour in the dose range tested. Amphetamine did not affect accuracy and had no effect on reaction time. The other drugs all reduced the number of short reaction times and thereby increased overall accuracy. Additional feeding, chlordiazepoxide and haloperidol also increased selectively the accuracy of responses made with short reaction times. These results can be interpreted by supposing that desipramine and imipramine specifically reduced impulsivity whereas additional feeding, chlordiazepoxide, haloperidol increased the response criterion. The pharmacological specificity of the tricyclics suggests that stimulation principally of the noradrenergic system (via desipramine), but not necessarily the serotonergic (clomipramine) or dopaminergic (amphetamine) improves performance in this procedure. Received: 29 November 1997/Final version: 20 October 1998     

Cocaine decreases relative cerebral blood volume in humans: a dynamic susceptibility contrast magnetic resonance imaging study

Cocaine has substantial effects on cerebral hemodynamics which may partly underlie both its euphorigenic and toxic effects. Dynamic susceptibility contrast magnetic resonance imaging (DSC–MRI) was used to determine whether a dose-effect relationship could be detected between cocaine administration and cerebral blood volume reduction in human brain. Twenty-three healthy and neurologically normal adult males with a history of recreational cocaine use (3–40 lifetime exposures) participated. Subjects underwent DSC-MRI measurements of relative cerebral blood volume (rCBV) at baseline and 10min after IV double-blind placebo or cocaine (0.2 or 0.4 mg/kg) administration. Placebo administration resulted in superimposable rCBV curves with post-placebo CBV averaging 104 ± 4% (mean ± SE) of baseline, indicating no CBV change. Both cocaine doses induced CBV decreases which were statistically equivalent and post-cocaine CBV averaged 77 ± 4% of baseline (P < 0.002), when measured 10 min following drug administration. These data suggest that DSC-MRI can detect cocaine-induced CBV reductions indicative of vasoconstriction, and that it may be useful for evaluating treatments designed to reduce the cerebrovascular effects of cocaine. Received: 25 September 1997 / Final version: 30 December 1997     

On the role of noradrenaline in psychostimulant-induced psychomotor activity and sensitization

Rationale Psychostimulant drugs exert their behavioral effects primarily through enhancement of monoaminergic neurotransmission. Augmented dopamine activity is thought to play a critical role in the psychomotor stimulant effects of amphetamine and cocaine, as well as in the development of long-term behavioral sensitization evoked by repeated exposure to amphetamine. However, despite the fact that brain dopamine and noradrenaline systems are closely interconnected, the extent to which noradrenergic transmission contributes to these behavioral effects of psychostimulants is a relatively unexplored issue. Objectives By inhibiting noradrenergic neurotransmission with the α₂-adrenoceptor agonist clonidine, the α₁-antagonist prazosin and the β-antagonist propranolol, we investigated the involvement of noradrenaline neurotransmission in the psychomotor stimulant and long-term sensitizing effects of d-amphetamine and cocaine in rats. Methods Clonidine (0.003–0.1 mg/kg), prazosin (0.1–3.0 mg/kg) and propranolol (1.0–3.0 mg/kg) were administered prior to d-amphetamine (1.0 mg/kg), cocaine (15 mg/kg) or apomorphine (1.0 mg/kg) and psychomotor activity was measured. In separate studies, clonidine (0.03 mg/kg), prazosin (1.0 mg/kg) or propranolol (3.0 mg/kg) were co-administered with d-amphetamine (2.5 mg/kg) or cocaine (30 mg/kg) for 5 days, and locomotor sensitization was assessed 3 weeks post-treatment. Results The psychomotor stimulant effect of d-amphetamine, but not that of cocaine or apomorphine, was dose-dependently inhibited by clonidine and prazosin, and enhanced by propranolol. Clonidine, prazosin, and propranolol did not influence the induction of sensitization by amphetamine or cocaine. Conclusions Enhancement of synaptic noradrenaline concentrations contributes to the psychomotor stimulant effect of d-amphetamine, but not cocaine or apomorphine. In addition, noradrenergic neurotransmission is not critically involved in the induction of psychostimulant sensitization.     

A comparison of bd and tid dose regimens of quetiapine (Seroquel) in the treatment of schizophrenia

Quetiapine (Seroquel, ICI 204,636) is an atypical antipsychotic that is effective in the treatment of both positive and negative symptoms of schizophrenia, and has a low propensity to cause extrapyramidal symptoms. The compound has a relatively short plasma elimination half-life (approximately 7 h). However, since dopamine D₂ receptor occupancies correlate poorly with plasma concentrations of antipsychotics, plasma elimination half-life may not predict either duration of clinical effect or dosing frequency. Accordingly, the efficacy and tolerability of three dosing regimens (450 mg/day given in two or three divided doses daily, and 50 mg/day given twice daily) were compared in a 6-week, double-blind, randomized, multicentre, parallel-group study. The study recruited hospitalized men and women aged 18–65 years meeting DSM-IIIR criteria for acute exacerbation of chronic or subchronic schizophrenia. Six hundred and eighteen patients were randomly assigned to treatment with quetiapine 150 mg tid (n = 209), 225 mg bd (n = 200), or a comparator dose of 25 mg bd (n = 209). At day 42, the last day of randomized treatment and the primary timepoint for efficacy, quetiapine 450 mg/day was more effective than 50 mg/day: 225 mg bd was consistently superior to 25 mg bd in all measures of efficacy (total BPRS, P = 0.006; CGI severity, CGI improvement and SANS, P < 0.03), and 150 mg tid was statistically significantly superior to 25 mg bd with respect to BPRS total score (P = 0.05). The 225 mg bd and 150 mg tid groups were not significantly different from each other with respect to any efficacy measure. Quetiapine was generally well tolerated. Extrapyramidal symptom (EPS) adverse events were generally rare, and occurred with similar frequencies in the two 450 mg/day groups. Quetiapine was not associated with sustained increases in plasma prolactin at any dose. These data support the atypical profile developed from preclinical studies and show that quetiapine is an effective, well tolerated antipsychotic that can be given twice daily. Received: 16 May 1997/Final version: 13 October 1997     

Effects of selective dopamine D1- and D2-type receptor antagonists on the development of behavioral sensitization to 7-OH-DPAT

The primary objective of this study was to determine whether the development of behavioral sensitization to the putative dopamine D₃ receptor agonist 7-OH-DPAT could be prevented by either selective D₁-type or D₂-type dopamine receptor antagonists. In three experiments, male Wistar rats (250–350 g) were given seven to nine injections (at 48-h intervals) of 7-OH-DPAT (1.0 mg/kg, SC) or vehicle in combination with the D₂-type dopamine antagonist eticlopride (0.3 mg/kg, SC), the D₁-type dopamine antagonist SCH 23390 (0.1 or 0.2 mg/kg, SC), or vehicle. After the injections, the rats were tested for locomotor activity in photocell arenas for 2 h. In the first two experiments, after seven injections, all rats were tested for activity following vehicle injections to test for possible conditioning effects. In each experiment, after the last pre-exposure session, all rats were given a challenge injection of 7-OH-DPAT (1.0 mg/kg, SC) and tested for activity. Major findings were as follows: a) 7-OH-DPAT treatments produced a progressively greater increase in locomotor activity with repeated treatment; b) concurrent treatment with eticlopride or SCH 23390 (0.1 and 0.2 mg/kg) blocked the acute locomotor-activating effects of 7-OH-DPAT across days; c) eticlopride, but not SCH 23390, completely blocked the development of behavioral sensitization to 7-OH-DPAT. Although the low dose of SCH 23390 (0.1 mg/kg) produced a partial attenuation of sensitization, the higher dose (0.2 mg/kg) of SCH 23390 appeared to augment, rather than block, sensitization to 7-OH-DPAT; d) rats previously treated with SCH 23390 (0.2 mg/kg, but not 0.1 mg/kg) without 7-OH-DPAT displayed a hyperactive response to the 7-OH-DPAT challenge injection; and e) after vehicle injections, rats previously given 7-OH-DPAT, SCH 23390, or eticlopride either alone or in combination were more active than vehicle control rats. These findings suggest that the neurochemical mechanisms mediating the development of behavioral sensitization to 7-OH-DPAT may differ from those of other dopamine D₂-type agonists such as quinpirole or bromocriptine. Moreover, these results demonstrate that hyperactivity responses following vehicle injections in drug-pretreated animals do not necessarily reflect conditioning mechanisms. Received: 28 May 1997/Final version: 2 April 1998     

mCPP-induced anxiety in the light-dark box in rats – a new method for screening anxiolytic activity

The activity of anxiolytic and other drugs in a light-dark test situation was studied in rats treated with the anxiogenic compound m-chlorophenyl-piperazine (mCPP). mCPP 0.5 mg/kg significantly diminished the exploratory activity of the animals in the light compartment of the apparatus. Drugs to be tested against mCPP-induced anxiety when studied alone (not in combination with mCPP) did not significantly alter the activity of rats in the light-dark apparatus, except yohimbine, which reduced the movement time values in the lit area. 1,4-Benzodiazepines [diazepam (0.1–4 mg/kg) and chlordiazepoxide (2–8 mg/kg)], 5-HT_2A/2C antagonists [ritanserin (0.25–8 mg/kg) and deramciclane (0.5–8 g/kg)], the 5-HT₃ antagonist MDL-72222 (3 mg/kg) and ethanol (2–4 mg/kg) significantly reduced the effect of mCPP. A dose-dependent increase in the exploratory activity of mCPP-treated animals was found in the 2,3-benzodiazepine girisopam (2.5–5 mg/kg)-treated groups. Tofisopam, another 2,3-benzodiazepine molecule, also showed activity against mCPP, although its effect was not statistically significant. The 5-HT_1A partial agonist buspirone was also active in the dose range of 0.25–0.5 mg/kg, while the 5-HT_1A full agonist 8-OH-DPAT was the only drug with presumed anxiolytic activity that clearly lacked any effect in this model. Imipramine, amitriptyline, morphine, naloxone, haloperidol, clozapine, amphetamine, yohimbine, carbamazepine and diphenylhydantoin were not effective. We conclude that mCPP-induced anxiety in the light-dark box is a potent and useful method for screening and detecting anxiolytic activity of a wide range of compounds with various modes of action. Received: 2 September 1997 / Final version: 18 September 1997     

Preferential involvement of D3 versus D2 dopamine receptors in the effects of dopamine receptor ligands on oral ethanol self-administration in rats

There is evidence that dopamine transmission is involved in reinforcement processes and the present study investigated the relative involvement of D₃ versus D₂ dopamine receptors in the effects of dopamine ligands on the reinforcing action of ethanol. Rats were trained to self-administer ethanol (10% v/v) orally in a free-choice two-lever operant task using a saccharin-fading procedure. When preference in responding for ethanol over water had developed the rats were tested with several dopamine agonists and antagonists. Pretreatment with the non-selective dopamine agonist, apomorphine (0.01–0.1 mg/kg), the preferential D₂ agonist, bromocriptine (1–10 mg/kg) and the selective D₃ agonists, 7-OH-DPAT (0.003–0.1 mg/kg), PD 128907 (0.1–3 mg/kg), (+)3PPP (0.3–3 mg/kg), quinelorane (0.0001–0.003 mg/kg) and quinpirole (0.003–0.03 mg/kg), resulted in dose-dependent decreases in responding for ethanol. The relative potencies of the dopamine agonists to decrease ethanol self-administration were highly correlated with their published potencies to produce in vitro functional D₃ but not D₂ responses. Active doses could be considered as those selectively stimulating receptors involved in the control of dopamine release, suggesting that reduction of dopamine transmission was associated with a decrease in ethanol-reinforced responding. This conclusion was further supported by the finding that pretreatment with the D₂/D₃ dopamine antagonists, haloperidol (0.1–0.4 mg/kg) and tiapride (10–60 mg/ kg), decreased responding for ethanol at doses which have been shown previously to block dopamine transmission. Received: 25 January 1998/Final version: 24 April 1998     

Anxiolytic-like profiles of histamine H3 receptor agonists in animal models of anxiety: a comparative study with antidepressants and benzodiazepine anxiolytic

Rationale  Histamine H3 receptor functions as a presynaptic auto- and hetero-receptor on histaminergic and non-histaminergic neurons in the brain regulating the synaptic release of numerous neurotransmitters. Therefore, the ligands for this receptor have been proposed to be of therapeutic interest for the treatment of various neuropsychiatric disorders. At present, however, the psychopharmacological profiles of H3 ligands, particularly H3 agonists, have not been extensively studied. Objective  The present study investigated the anxiolytic-like profiles of H3-selective agonists in a variety of classical (benzodiazepine-sensitive) and atypical (antidepressant-effective) animal models of anxiety. Comparator drugs used were diazepam and both fluvoxamine and desipramine in the former and latter models, respectively. Results  H3 agonist R-α-methylhistamine and immepip were inactive in rat elevated plus maze test and Vogel type conflict test where diazepam (5 mg/kg) produced significant anxiolytic-like effects. Meanwhile, these H3 agonists (10–30 mg/kg) significantly reduced isolation-induced vocalizations in guinea pig pups and isolation-induced aggressive behavior in mouse resident–intruder test. Moreover, in rat conditioned fear stress test, R-α-methylhistamine (30 mg/kg) and immepip (10 mg/kg) significantly decreased freezing time, which were completely reversed by concomitant treatment with H3 antagonist, thioperamide (10 mg/kg). In contrast to the limited efficacy obtained with desipramine (30 mg/kg), fluvoxamine (20–60 mg/kg) exhibited anxiolytic-like effects in all the latter three atypical models. Conclusions  These data suggest that the H3 agonists may have anxiolytic-like effects similar to those of selective serotonin reuptake inhibitors but not benzodiazepine anxiolytics and represent a novel strategy for the treatment of some anxiety disorders in which selective serotonin reuptake inhibitors are prescribed.     

Discriminative stimulus effects of S(−)–methcathinone (CAT): a potent stimulant drug of abuse

Methcathinone (“CAT”) is a CNS stimulant that is a very significant drug of abuse in the former Soviet Union. It has also appeared on the clandestine market in the United States and has been recently classified as a Schedule I substance. In the present study, S(−)-methcathinone [S(−)-CAT, 0.50 mg/kg, IP] was employed as the training drug in a two-lever drug discrimination task in rats. Once established, the S(−)-CAT stimulus was shown to have a rapid onset to action (within 5 min) and a duration of effect of approximately 60–90 min. In tests of stimulus generalization (substitution), the S(−)-CAT (ED₅₀ = 0.11 mg/kg) stimulus generalized to S(+)-methamphetamine (ED₅₀ = 0.17 mg/kg), S(−)-cathinone (ED₅₀ =  0.19 mg/kg), S(+)-amphetamine (ED₅₀ = 0.23 mg/kg), aminorex (ED₅₀ = 0.27 mg/kg), (±)-CAT (ED₅₀ = 0.25 mg/kg), (±)-cathinone (ED₅₀ = 0.41 mg/kg), R(+)-CAT (ED₅₀ = 0.43 mg/kg), cis-4-methylaminorex (ED₅₀ = 0.49 mg/kg), methylphenidate (ED₅₀ = 0.83 mg/kg), and cocaine (ED₅₀ = 1.47 mg/kg). S(−)-CAT-stimulus generalization did not occur to fenfluramine, a structurally related nonstimulant anorectic. Lastly, haloperidol (AD₅₀ = 0.18 mg/kg), a dopamine receptor antagonist, potently antagonized the S(−)-CAT stimulus. It is concluded that S(−)-methcathinone is a very potent CNS stimulant, which appears to produce its stimulus effect, at least in part, via a dopaminergic mechanism. Received: 4 August 1997/Final version: 27 March 1998