Anticonflict and discriminative stimulus effects in the pigeon of a new methoxy-chroman 5-HT1A agonist, (+)S 20244 and its enantiomers (+)S 20499 and (−)S 20500

The present experiments examined the behavioral and receptor binding characteristics of new 5-HT_1A methoxy-chroman derivatives in procedures known to be sensitive to the activity of 5-HT_1A compounds. Key peck responding of pigeons was maintained by a 30-response fixed-ratio schedule of food delivery. In studies involving punished responding, every 30th response during one keylight stimulus also produced shock (conflict procedure). In drug discrimination studies, pigeons were trained to discriminate injections of the 5-HT_1A agonist 8-OH-DPAT (0.3 mg/kg) from saline. Three forms of the methoxy-chroman compounds were tested: the enantiomers (+)S 20499 (0.01–3.0 mg/kg) and (–) S 20500 (0.3–5.6 mg/kg), as well as the racemic mixture (+)S 20244 (0.03–5.6 mg/kg). (+)S 20499 was approximately 10-fold more potent than (–)S 20500 in producing maximal increases in punished responding. (+)S 20244 was comparable in potency to (–)S 20500 in producing maximal increases in punished responding, but increases also occurred at much lower doses with (+)S 20244 and the magnitude of the effect with (–)S 20500 was less than that of the two other compounds. While increases in punished responding were observed with all three drugs at doses that did not affect unpunished responding, the highest doses of all drugs decreased unpunished responding. All compounds substituted for 8-OH-DPAT in the drug discrimination procedure, suggestive of 5-HT_1A agonist activity. (+)S 20499 was approximately 30-fold more potent than (–)S 20500 in substituting for 8-OH-DPAT and 3-fold more potent than the racemate. All three compounds bound with high affinity to pigeon cerebrum receptor sites labelled by [³H]8-OH-DPAT. As in behavioral studies, (+)S 20499 was approximately 10-fold more potent than (–)S 20500 in displacing [³H]8-OH-DPAT (IC₅₀=2.79 versus 20.3 nM). These studies demonstrate that the enantiomers of this compound, as well as the racemic mixture, are effective 5-HT_1A compounds and that (+)S 20499 in particular is likely to be a clinically effective anxiolytic and/or antidepressant.     

Pharmacological characterization of stress-induced hyperthermia in DBA/2 mice using metabotropic and ionotropic glutamate receptor ligands

Rationale Accumulating evidence suggests that drugs acting on the glutamatergic system may represent promising novel therapeutic targets for the treatment of anxiety disorders. The stress-induced hyperthermia paradigm has been used widely to model some of the physiological symptoms associated with anxiety disorders and has produced results that are predictive of clinical efficacy. We have modified this paradigm to measure the autonomic consequences of stress induced by the fear of predation in mice.Objective To evaluate the efficacy of several classes of metabotropic and ionotropic glutamate receptor ligands, as well as known anxiolytics and psychotropic comparators, in attenuating predatory-stress-induced hyperthermia.Methods Male DBA/2 mice were implanted with radiotelemetric transmitters in the peritoneal cavity to measure stress-related increases in core body temperature, following placement in a novel cage containing soiled rat shavings.Results Clinically active compounds such as chlordiazepoxide (5–10 mg/kg), alprazolam (0.3–3 mg/kg), and buspirone (10–30 mg/kg) exhibited an anxiolytic profile. Assessment of glutamatergic agents indicated that the mGlu1 receptor antagonist LY456236 (10–30 mg/kg), mGlu5 receptor antagonist MPEP (10–30 mg/kg), mGlu2/3 receptor agonist LY354740 (3–10 mg/kg), mGlu2 receptor potentiator LY566332 (30 and 100 mg/kg), mGlu8 receptor agonist (S)-3,4-dicarboxyphenylglycine (30–60 mg/kg), competitive NMDA receptor antagonist LY235959 (1 mg/kg), AMPA receptor antagonist GYKI-52466 (10–20 mg/kg), and glycine transporter-1 (GlyT-1) inhibitor ALX-5407 (3–10 mg/kg) dose-dependently attenuated stress-induced hyperthermia. The AMPA receptor potentiator LY451646, iGlu5 kainate receptor antagonist LY382884, glycine_B receptor partial agonist d-cycloserine, and GlyT-1 inhibitor ORG-24461 were ineffective in this model.Conclusion Select metabotropic and ionotropic glutamate receptor ligands exhibited an anxiolytic profile, as measured by the attenuation of stress-induced hyperthermia, and may represent viable targets for the development of pharmacological treatments for anxiety-related disorders.     

A comparison of drug-seeking behavior maintained by d-amphetamine, l-deprenyl (selegiline), and d-deprenyl under a second-order schedule in squirrel monkeys

l-Deprenyl (selegiline) is used in the treatment of Parkinson’s disease and has been proposed as an aid for cigarette smoking cessation and a treatment for psychostimulant abuse. l-Deprenyl is metabolized in the body to l-methamphetamine and l-amphetamine, suggesting that it may have abuse potential. The current study assessed whether l-deprenyl or its isomer would maintain drug-seeking behavior on a second-order schedule and whether l-deprenyl would alter drug-seeking behavior maintained by d-amphetamine if given as a pretreatment. Squirrel monkeys learned to respond on a second-order schedule of reinforcement, where every tenth response was followed by a brief light flash, and the first brief light flash after 30 min was paired with intravenous (i.v.) injection of d-amphetamine (0.56 mg/kg), administered over a 2-min period at the end of the session. When responding was stable, saline or different i.v. doses of d-amphetamine (0.3–1.0 mg/kg), l-deprenyl (0.1–10.0 mg/kg), and d-deprenyl (0.1–3.0 mg/kg) were substituted for 10 days each. Subsequently, monkeys were pretreated with 0.3 or 1.0 mg/kg l-deprenyl intramuscularly 30 min prior to d-amphetamine baseline sessions. d-Amphetamine maintained high rates of drug-seeking behavior on the second-order schedule. d-Deprenyl maintained high rates of drug-seeking behavior similar to d-amphetamine. l-Deprenyl maintained lower rates of responding that were not significantly above saline substitution levels. Pretreatment with l-deprenyl failed to alter drug-seeking behavior maintained by d-amphetamine. These results indicate that d-deprenyl, but not l-deprenyl, may have abuse potential. Under conditions where drug-seeking and drug-taking behaviors are actively maintained by d-amphetamine, l-deprenyl, at doses that specifically inhibit type B monoamine oxidase, may not be effective as a treatment.     

Amphetamine increases schedule-induced drinking reduced by negative punishment procedures

Rationale. d-Amphetamine has been reported to increase schedule-induced drinking punished by lick-dependent signalled delays in food delivery. This might reflect a drug-behaviour interaction dependent on the type of punisher, because no such effect has been found when drinking was reduced by lick-contingent electric shocks. However, the anti-punishment effect of amphetamine could be mediated by other behavioural processes, such as a loss of discriminative control or an increase in the value of delayed reinforcers. Objectives. To test the effects of d-amphetamine on the acquisition and maintenance of schedule-induced drinking reduced by unsignalled delays in food delivery. Methods. Rats received 10-s unsignalled delays initiated by each lick after polydipsia was induced by a fixed-time 30-s food reinforcement schedule or from the outset of the experiment. Yoked-control rats received these same delays but independently of their own behaviour. d-Amphetamine (0.1–3.0 mg/kg) was then tested IP. Results. d-Amphetamine dose-dependently increased and then decreased punished schedule-induced drinking. The drug led to dose-dependent reductions when the delays were not contingent or when they were applied from the outset of training. Conclusions. These results support the contention that d-amphetamine has an increasing effect on schedule-induced drinking that has been previously reduced by a negative punishment procedure. This effect cannot be attributed to other potentially involved processes, and therefore support the idea that drug effects on punished behaviour depend on punishment being delays in food or shock deliveries. Electronic Publication     

Behavioural profiles in the mouse defence test battery suggest anxiolytic potential of 5-HT1A receptor antagonists

  Rationale: Compounds varying in selectivity as 5-HT_1A receptor antagonists have recently been reported to produce anxiolytic-like effects comparable to those of benzodiazepines in the mouse elevated plus-maze procedure. Objective: In view of the potential clinical significance of these findings, the present experiments compared the behavioural effects of diazepam (0.5–3.0 mg/kg) with those of several non-selective 5-HT_1A receptor antagonists [NAN-190, 0.1–3.0 mg/kg, MM-77, 0.03–1.0 mg/kg, (S)-UH-301, 0.3–3.0 mg/kg and pindobind-5-HT_1A, 0.03–1.0 mg/kg], and three selective 5-HT_1A receptor antagonists (WAY100635, 0.01–3.0 mg/kg, p-MPPI, 0.1–3.0 mg/kg and SL88.0338, 0.3–3.0 mg/kg) in the mouse defence test battery (MDTB). Methods: In this well-validated anxiolytic screening test, Swiss mice are directly confronted with a natural threat (a rat) as well as situations associated with this threat. Primary measures taken during and after rat confrontation were flight, risk assessment (RA), defensive threat/attack and escape attempts. Results: Diazepam significantly decreased flight reactions after the rat was introduced into the runway, reduced RA activities of mice chased by the rat, increased RA responses displayed when subjects were constrained in a straight alley and reduced defensive upright postures and biting upon forced contact. All the selective 5-HT_1A receptor antagonists and NAN-190 also reduced flight, RA in the chase test, and defensive threat and attack behaviours. (S)-UH-301 and pindobind-5-HT_1A reduced RA in the chase test, but only partially modified defensive threat and attack. Unlike the other drugs tested, MM-77 produced significant effects only at doses which also markedly reduced spontaneous locomotor activity, suggesting a behaviourally non-specific action. In contrast to diazepam, the 5-HT_1A receptor ligands failed to affect RA in the straight alley test. Following removal of the rat from the test area, only diazepam and (S)-UH-301 reduced escape behaviour (contextual defence) at doses which did not decrease locomotion. Overall, the present findings indicate that except for one RA behaviour and escape responses, the 5-HT_1A receptor ligands studied modified the same defensive behaviours as diazepam, suggesting potential therapeutic efficacy in the management of anxiety disorders. However, the magnitude of the effects of the 5-HT_1A compounds on defence was generally smaller than that of the benzodiazepine. Conclusion: As all of the 5-HT_1A compounds tested in this series share antagonistic activity in models of postsynaptic 5-HT_1A receptor function, it is proposed that this action accounts for their effects on defence. Received: 29 June 1998 / Final version: 16 December 1998     

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.     

Potential anxiolytic properties of 8-hydroxy-2-(Di-N-propylamino)tetralin,a selective serotonin1A receptor agonist

The effects of a selective serotonin_1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), were studied in two animal models of anxiety. Peripherally injected 8-OH-DPAT in doses ranging from 0.125 to 2.0 mg/kg did not increase black-white transitions (BWT) and black square entries (BSE) in a two-compartment exploratory test or punished responding in a test of conditioned suppression of drinking. With 2.0 mg/kg 8-OH-DPAT BSE and unpunished responding were reduced. In an investigation of the drinking time of water-deprived rats, naive or habituated to the test environment, 1.0 and 2.0 mg/kg 8-OH-DPAT increased the drinking time of naive rats but 2.0 mg/kg 8-OH-DPAT reduced that of habituated animals. In animals deprived of water for 48 h or subjected to immobilization stress for 2 h, 1.0 mg/kg 8-OH-DPAT increased BWT and BSE values in the two-compartment exploratory test. Infusions of 5 g/0.5 l 8-OH-DPAT in the nucleus raphe medianus increased BWT and BSE values in the exploratory test and punished responding in the test of conditioned suppression of drinking, whereas the same dose of 8-OH-DPAT injected in the nucleus raphe dorsalis had no effect on punished but suppressed unpunished responding. The effects of 8-OH-DPAT are only detectable in the appropriate experimental conditions. When injected systemically, the effects are evident when a state of arousal of the animals contributes to the overall behavioural output. 8-OH-DPAT shows effects comparable to those of established anxiolytics such as benzodiazepines and barbiturates when it is injected in the nucleus raphe medianus, but not in the dorsalis. The data support the hypothesis that brain serotonin is involved in the mechanisms mediating behavioural suppression in the presence of aversive stimuli.     

Distress vocalizations in maternally separated mouse pups: modulation via 5-HT1A, 5-HT1B and GABAA receptors

Rationale: Young rodents emit ultrasonic vocalizations (USVs) when separated from their dams and littermates. Pharmacological agents that act on GABA_A and/or 5-HT receptors and that alleviate anxiety in humans reduce the emission of these calls. Objectives: 1) to investigate specific 5-HT₁ receptor subtypes that modulate maternal separation-induced USVs in mice; 2) to assess the behavioral specificity of these effects; and 3) to compare 5-HT₁ agonists with a positive neurosteroid modulator of the GABA_A receptor complex. Methods: Seven-day old CFW mouse pups were isolated from their littermates and placed onto a 20°C surface for 4 min. USVs between 30 and 80 kHz, grid crossing, and rectal temperature were measured in separate groups of mouse pups following subcutaneous administration of 5-HT_1A and 5-HT_1B receptor agonists and antagonists, the neurosteroid allopregnanolone, or the benzodiazepine midazolam. Results: The 5-HT_1A agonists (+)8-OH-DPAT (0.01–0.1 mg/kg) and flesinoxan (0.3–1.0 mg/kg), the selective 5-HT_1B agonist CP-94,253 (0.03–30.0 mg/kg), and the mixed 5-HT_1B/2C receptor agonist TFMPP (0.1–10.0 mg/kg) dose-dependently reduced USVs. These effects were reversed by the 5-HT_1A receptor antagonist WAY 100,635 (0.1 mg/kg) or the 5-HT_1B/D receptor antagonist GR 127935 (0.1 mg/kg). The effects of TFMPP were biphasic; low doses (i.e. 0.01 and 0.03 mg/kg) increased the rate of vocalization. Midazolam and allopregnanolone also reduced USVs. The highest doses of flesinoxan, (+)8-OH-DPAT, and allopregnanolone suppressed locomotion, whereas CP-94,253, TFMPP, and midazolam stimulated motor activity. Conclusions: These experiments confirm that agonists at the 5-HT₁ receptors and a positive allosteric modulator of the GABA_A receptor complex decrease maternal separation-induced USVs in mice, with 5-HT_1B manipulations dissociating the effects on vocalizations from sedative effects. Received: 22 September 1999 / Final version: 14 December 1999     

In vitro and in vivo studies in rats with LY293558 suggest AMPA/kainate receptor blockade as a novel potential mechanism for the therapeutic treatment of anxiety disorders

Rationale Although convergent evidence exists for a role of glutamate in the regulation of anxiety, the involvement of specific glutamate receptor subtypes has yet to be defined. Objective To evaluate the potential for blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptors to produce anxioltyic-like effects with the AMPA/GLU_K5 (kainate) antagonist (3S,4aR,6R,8aR)-6-[2-(1(2)H-tetrazole-5yl)ethyl]decahydroisoquinoline-3carboxylic acid (LY293558) Materials and methods Punished responding of rats was used to determine the efficacy of LY293558. Other in vivo and in vitro studies further characterized the specificity of LY293558 for AMPA/kainate receptors. Results LY293558 had a rank order of potency of GLU_K5 ≥ GLU_K5/6 ≈ GLU_A2i ≈ GLU_K2/5 ≈ GLU_A1i ≈ GLU_A2o ≈ GLU_A3i ≈ GLU_A1o ≥ GLU_A3o ≥ GLU_A4i ≈ GLU_A4o and >100 μM affinity for rat cortical GABA_A receptors. Comparison of the blockade of AMPA- vs N-methyl-d-aspartate (NMDA)-induced inward currents demonstrated that LY293558 was five-fold more potent as an antagonist at AMPA vs NMDA receptors in vitro. In keeping with the low affinity of LY293558 for NMDA receptors, LY293558 was not effective in preventing NMDA-induced seizures in mice. LY293558 increased punished responding, a preclinical predictor of anxiolytic efficacy, at a dose that decreased unpunished responding (10 mg/kg, i.p.). Chlordiazepoxide produced comparable increases in both punished and unpunished responding. The NMDA antagonist dizocilpine [(+)-MK-801] also increased both punished and unpunished responding. Conclusions These data along with those in the literature suggest that AMPA and/or kainate receptor blockade may be an important component to producing anxiolytic-like effects and may therefore be a target for compounds with efficacy in the therapeutic treatment of anxiety disorders.     

The effects of systemic,intrastriatal, and intrapallidal injections of caffeine and systemic injections of A<Subscript>2A</Subscript> and A<Subscript>1</Subscript> antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

Rationale and objectives  Given that adenosine A_2A antagonists appear to be therapeutic in several animal models of Parkinson’s disease (PD), we examined the extent to which caffeine and selective A_2A and A₁ antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat. Materials and methods  Following unilateral injections of 12 μg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill. Results  The MFB lesions decreased contralateral stepping by 74–83%, and 8 mg/kg 3,4-dihydroxy-l-phenylalanine (L-DOPA) increased contralateral stepping by 25–26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GP_E; 20–40 μg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A_2A antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A_2A antagonist 8-cyclopentyltheophylline (3–7 mg/kg) and A₁ agonist N ⁶-cyclopentyladenosine (0.03–0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects. Conclusions  In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A_2A, but not an A₁, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A_2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GP_E are critical sites of therapeutic action. An erratum to this article can be found at     

The development of addiction to d-amphetamine in an animal model: same principles as for alcohol and opiate

We have established a rat model that reflects the course of development of alcohol and opiate addiction. The present study with d-amphetamine aimed to define general principles in the development of an addiction. Male rats had a continuous free choice between d-amphetamine solutions (100, 200 and 400 mg/l) and water for 47 weeks. An initial intake of high doses of d-amphetamine during the first weeks of drug choice was followed by an individually stable pattern of drug consumption of moderate drug doses. During this period of controlled consumption (from week 10 to week 40), the voluntary intake of d-amphetamine depended on individual factors (dominant rats: 0.37 ± 0.02 mg/kg per day, subordinate rats: 0.57 ± 0.05 mg/kg per day) and environmental variables (group housing: 0.21 ± 0.02 mg/kg per day, single housing: 0.41 ± 0.03 mg/kg per day). Beginning with week 41, voluntary d-amphetamine consumption progressively increased (1.9 ± 0.2 mg/kg per day in week 47), although the experimental conditions remained unchanged. Drug intake during a retest (free choice as before) after 6 months of drug deprivation revealed that the rats had persistently lost their control over drug intake and were no longer able to adjust drug taking to internal and external conditions. These addicted rats took very high drug doses, even when all d-amphetamine solutions but not water were adulterated with bitter tasting quinine (6.6 ± 0.6 mg/kg per day; age-matched controls: 0.37 ± 0.04 mg/kg per day). Forced intake of d-amphetamine for 47 weeks (7.1 ± 0.3 mg/kg per day) via the drinking fluid caused physical dependence (hyperreactivity during withdrawal) but did not lead to drug addiction (voluntary intake in the retest with adulteration: 0.42 ± 0.04 mg/kg per day). Both the temporal development and the prerequisites of psychostimulant addiction were in principle the same as for alcohol and opiates. Received: 3 April 1998/Final version: 26 August 1998     

Comparative effects of novel 5-HT1A receptor ligands, LY293284, LY315712 and LY297996, on plus-maze anxiety in mice

In contrast to the variable efficacy of 5-HT_1A receptor full and partial agonists in animal models of anxiety, recent findings in our laboratory have revealed remarkably consistent anxiolytic-like effects for 5-HT_1A receptor antagonists in the murine elevated plus-maze paradigm. In the present study, ethological techniques were used directly to compare the plus-maze profiles of three novel ligands varying in intrinsic efficacy at 5-HT_1A receptors: LY293284 (full agonist; 0.01–0.3 mg/ kg), LY315712 (partial agonist; 0.3–3.0 mg/kg), and LY297996 (antagonist; 0.03–10.0 mg/kg). At the lowest dose tested, LY293284 tended to enhance several indices of anxiety, whereas higher doses suppressed all active behaviours. Although few behavioural effects were observed with LY315712 under present test conditions, a selective reduction in risk assessment was apparent at 1.0–3.0 mg/kg. In contrast to these profiles, LY297996 (3.0–10.0 mg/kg) produced robust anxiolytic-like effects on conventional and ethological parameters but, importantly, did not alter general activity levels. These results provide further support for the anxiolytic potential of 5-HT_1A receptor antagonists and are discussed in relation to possible factors underlying inter-laboratory variation in the effects of these agents in animal models of anxiety. Received: 5 May 1997 /Final version: 4 February 1998     

Pharmacological analysis of the heterogeneous discriminative stimulus effects of ethanol in rats using a three-choice ethanol-dizocilpine-water discrimination

The present study used a three-choice operant drug discrimination procedure to determine if NMDA-mediated discriminative stimulus effects could be separated from other stimulus effects of 2.0 g/kg ethanol. Adult male Long-Evans rats (n = 7) were trained to discriminate dizocilpine (0.17 mg/kg; IG) from ethanol (2.0 g/kg; IG) from water (4.7 ml; IG) using food reinforcement. Substitution tests were conducted following administration of the GABA_A positive modulators allopregnanolone (5.6–30.0 mg/kg; IP), diazepam (0.3–10.0 mg/kg; IP) and pentobarbital (1.0–21.0 mg/kg; IP), the non-competitive NMDA antagonist phencyclidine (0.3–10.0 mg/kg; IP), the 5-HT₁ agonists TFMPP (0.3–5.6 mg/kg; IP) and RU 24969 (0.3–3.0 mg/kg; IP), and isopropanol (0.10–1.25 g/kg; IP). Allopregnanolone, diazepam and pentobarbital substituted completely (>80%) for ethanol. Isopropanol partially (77%) substituted for ethanol. Phencyclidine substituted completely for dizocilpine. RU 24969 and TFMPP did not completely substitute for either training drug, although RU 24969 partially (62%) substituted for ethanol. Successful training of this three-choice discrimination indicates that the discriminative stimulus effects of 0.17 mg/kg dizocilpine were separable from those of 2.0 g/kg ethanol. The finding that attenuation of NMDA-mediated effects of ethanol occurred without altering significantly GABA_A- and 5-HT₁-mediated effects suggests that the NMDA component may be independent of other discriminative stimulus effects of 2.0 g/kg ethanol. Received: 18 November 1997 / Final version: 10 February 1998     

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     

The influence of nicotine pretreatment on mesoaccumbens dopamine overflow and locomotor responses to D-amphetamine

Pretreatment with psychostimulant drugs causes sensitisation of their effects on locomotor activity and dopamine (DA) overflow in the nucleus accumbens (NAcc) and there is evidence for similarities in the mechanisms involved. This study used in vivo microdialysis in conscious freely moving rats to investigate the extent to which pretreatment with nicotine causes sensitisation to D-amphetamine. Pretreatment with nicotine (0.4 mg/kg SC daily for 5 days) caused sensitisation of the locomotor responses to D-amphetamine (0.1–0.5 mg/kg SC) but not cocaine (15 mg/kg IP). Nicotine pretreatment did not influence the increase in DA overflow into dialysis probes, located in the core of the NAcc, evoked by systemic injections of D-amphetamine or cocaine (15 mg/kg IP) but decreased the overflow evoked by the administration of D-amphetamine (1 × 10^–6M) through the dialysis probe. The results provide further evidence for a dissociation between the expression of sensitised locomotor responses to psychostimulant drugs and sensitisation of their stimulatory effects on DA overflow in the core of the NAcc. The results suggest that the sensitisation of the effects of nicotine on DA overflow in this subdivision of the NAcc may be pharmacologically specific to nicotinic drugs. Received: 23 July 1997/Final version: 19 March 1998     

Lack of central 5-hydroxytryptamine influence on the anticonflict activity of diazepam

This study examined the effects of various drug treatments (IP injections) proposed to modify central 5-hydroxytryptamine (5-HT) activity on a conditioned suppression of drinking behavior in water-deprived rats. The subjects were trained to drink their daily water requirement during a 10-min session. Intermittent tone periods of 7 s were then introduced, the last 5 s of which the drinking tube was electrified. The animals gradually suppressed tube contacts during the tone to a low constant level within 2 weeks of training. Diazepam increased punished responding dramatically. The 5-HT antagonists methysergide (1–18 mg/kg), cyproheptadine (1–18 mg/kg), metergoline (0,25–2.0 mg/kg) and cinanserin (10–100 mg/kg) failed to induce large, reliable increases in punished responding. When a low dose of diazepam was combined with 5-HT antagonists, only one treatment, methysergide at 3 mg/kg, potentiated the anticonflict activity of diazepam. Acute or chronic treatment with PCPA increased behavior suppressed by punishment, but this effect was weak, brief, and poorly related to the depletion of brain 5-HT. LSD (0.3–100 g/kg) administered 1,10, or 30 min before the test was ineffective in overcoming suppression by punishment. Mescaline (6–30 mg/kg) had no significant effect on punished responding. 5-HTP (18 mg/kg) decreased the number of shocks accepted, but not after pretreating with carbidopa. Pretreatment with carbidopa plus 5-HTP potentiated the anticonflict effect of diazepam. The 5-HT agonist mCPP (0.25–2.0 mg/kg) enhanced suppression due to punishment, but only in doses that interfered with unpunished responding. The 5-HT-releasing agent fenfluramine (0.25–1.0 mg/kg) did not affect this behavior. Amitriptyline pretreatment in a dose not affecting unpunished behavior (5.6 mg/kg) potentiated the diazepam-induced increase in punished responding. These results are difficult to reconcile with the proposal that suppression of behavior consequent to punishment is related to brain 5-HT activity.     

Modulation of the ethanol-like discriminative stimulus effects of diazepam and phencyclidine by L-type voltage-gated calcium-channel ligands in rats

Rationale: Administration of voltage-gated calcium-channel (VGCC) modulators with ethanol can result in enhancement or attenuation of some behavioral effects of ethanol, including its discriminative stimulus effects. Objectives: The present study used a drug- discrimination paradigm to characterize modulation of the ethanol-like discriminative stimulus effects of a γ-amino-butyric acid (GABA)_A and N-methyl-d-aspartate (NMDA) ligand by administration of VGCC ligands. Methods: Two groups of adult male Long-Evans rats were trained to discriminate either 1.0 g/kg ethanol (n=8) or 2.0 g/kg ethanol (n=9) from water under a fixed-ratio (FR) 20 schedule of food presentation. Following training, ethanol substitution tests were conducted with cumulative doses of the GABA_A-positive modulator diazepam (0.3–10 mg/kg, i.p.) (DZP) and the uncompetitive NMDA antagonist phencyclidine (0.3–5.6 mg/kg, i.p.) (PCP). Next, a single dose of the VGCC antagonist nimodipine, nifedipine, isradipine, or the VGCC agonist (–)-BAY k 8644 (0.3 mg/kg, i.p.) was administered prior to a cumulative DZP or PCP dose–response determination. Results: None of the VGCC modulators produced robust or consistent alterations in the ethanol-like discriminative stimulus effects of DZP in animals trained with either 1.0 g/kg or 2.0 g/kg ethanol. However, the ethanol-like discriminative stimulus effects of PCP were significantly enhanced in the presence of the VGCC antagonists and attenuated in the presence of the agonist in animals trained with 2.0 g/kg ethanol. Conclusions: Overall, these data show that VGCC modulation is not a robust component of ethanol-like discriminative stimulus effects of DZP in animals trained with 1.0 g/kg or 2.0 g/kg ethanol. However, the ethanol-like effects of PCP, particularly at higher training doses, appear to be modulated by dihydropyridine-sensitive VGCCs. Received: 26 August 1999 / Final version: 22 October 1999     

Pharmacological analysis of the effects of benzodiazepines on punished schedule-induced polydipsia in rats

Food-deprived Wistar rats were exposed to a fixed-time 60-s food delivery schedule until they developed schedule-induced polydipsia. Every fifth lick was then followed by an electric shock during two, signalled, 5-min periods, which ran concurrently with the food delivery schedule. Shock intensities were adjusted to reduce licking to 60-70% of the unpunished licking rates. The benzodiazepine full agonists, diazepam (0.3-3.0 mg/kg), chlordiazepoxide (0.3-10.0 mg/kg), oxazepam (0.3-3.0 mg/kg) and the benzodiazepine partial agonist, RU-32698 (3.0-17.0 mg/kg), led to increases in punished responding at intermediate doses and decreases at the highest doses tested. All benzodiazepine agonists brought about dose-dependent decreases in unpunished schedule-induced polydipsia, with doses required to reduce drinking proving higher than doses required to increase punished schedule-induced polydipsia. The antipunishment effect of 0.3 mg/kg of diazepam was dose-dependently antagonized by flumazenil and the benzodiazepine inverse agonist, RU-34000. Flumazenil effects, however, could reflect actions of flumazenil as a partial inverse agonist at GABAA receptors. RU-32698 at 10.0 mg/kg further facilitated the rate-increasing effect of 0.3 mg/kg of diazepam, but at 17.0 mg/kg partially blocked such antipunishment effect. Overall, the present results extend the similarities of the effects of benzodiazepine compounds on adjunctive and operant patterns of behaviour by showing similar interactions within the benzodiazepine receptor complex.     

Depletion of brain norepinephrine: differential influence on anxiolyic treatment effects

Rationale: Previous studies have demonstrated that anxiolytic-like anticonflict effects can be produced by either (1) acute administration of traditional anti-anxiety compounds (benzodiazepines or barbiturates) or (2) chronic administration of tricyclic (TCA) or monoamine oxidase inhibitor (MAOI) antidepressants. Objective: The present study determined the effects of noradrenergic neuronal depletion on these distinct anticonflict treatments. Methods: After 3 weeks of training in a repeated measures drink suppression conflict paradigm, water-restricted rats consumed 11–14 ml water/session (unpunished responding) and accepted 25–40 shocks/session (punished responding) during control (i.e., non-drug) 10-min test sessions. The noradrenergic neurotoxin DSP4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride; 65 mg/kg, IP] or its vehicle (saline; 1 ml/kg) was administered after 3 weeks of conflict testing. Conflict behavior was then evaluated for 8 weeks post-treatment. In separate groups of DSP4- and vehicle-pretreated subjects, the effects of acute administration (10-min pretreatment) of phenobarbital (5–40 mg/kg) or alprazolam (0.3–2.5 mg/kg) were determined. In a third experiment, the effects of chronic treatment with the TCA desipramine (DMI; 5 mg/kg, twice daily for 8 weeks) or the non-selective MAOI phenelzine (4.0 mg/kg, twice daily for 8 weeks) on conflict behavior were determined in additional groups of DSP4- or vehicle-pretreated subjects. Results: DSP4 treatment produced a modest yet statistically significant decrease in punished responding (i.e., anxiogenic-like effect) relative to vehicle controls. DSP4 pretreatment did not alter the anticonflict effects of acute alprazolam or phenobarbital treatment. In contrast, DSP4 treatment completely abolished the anticonflict effects produced by chronic DMI or chronic phenelzine treatment. Conclusions: Thus, noradrenergic neuronal integrity appears to be required for the anxiolytic-like effects of chronic antidepressant treatment, but not for the anxiolytic-like effects of acute treatment with barbiturates and benzodiazepines. Received: 6 August 1998/Final version: 16 October 1998     

Effects of dopamine D1 and D2 receptor blockade on MK-801-induced hyperlocomotion in rats

Blocking glutamatergic transmission at the N-methyl-d-aspartate (NMDA) receptor complex with MK-801 (0.15–0.5 mg/kg, IP) was found to induce a robust, dose-dependent increase in locomotor activity. This behavioural activation was similar in intensity to that observed afterd-amphetamine (1 mg/kg, SC). The locomotor stimulation induced by MK-801 at 0.3 mg/kg was significantly inhibited by the D₂ dopamine receptor antagonist raclopride (0.1–0.3 mg/kg, SC) and by the D₁ receptor antagonist SCH 23390 (0.04 mg/kg, SC). The locomotor activity induced by a higher dose of MK-801 (0.5 mg/kg) was reduced by higher doses of raclopride or SCH 23390 administered alone (0.3 and 0.08 mg/kg, respectively), and was inhibited by simultaneous administration of ineffective doses. Raclopride significantly reducedd-amphetamine-induced locomotor activity at a dose (0.2 mg/kg) that also blocked the effects of a low dose of MK-801. In contrast, SCH 23390 blocked the effects ofd-amphetamine at a dose (i.e. 0.01 mg/kg) lower than that needed to block MK-801. These results suggest that the dopaminergic system may in part mediate the locomotor effects induced by the NMDA antagonist, MK-801, in rats. However, the locomotor activity induced by MK-801 appears to be less sensitive to dopaminergic receptor blockade than that induced byd-amphetamine, suggesting that the underlying mechanisms, although similar, are not identical.