Considerations on nonclinical approaches to modeling risk factors of suicidal ideation and behavior.

Given the serious nature of suicidal ideation and behavior (SIB) and the possibility of treatment-emergent SIB, pharmaceutical companies are now applying more proactive approaches in clinical trials and are considering the value of nonclinical models to predict SIB. The current review summarizes nonclinical approaches to modeling three common risk factors associated with SIB: aggression, impulsivity, and anhedonia. For each risk factor, a general description, advantages and disadvantages, species considerations, nonclinical to clinical translation, and pharmacological validation with respect to treatments associated with SIB are summarized. From this review, several gaps were identified that need to be addressed before use of these nonclinical models can be considered a viable option to predict the relative risk for SIB. Other future directions that may compliment these nonclinical approaches, including the use of selectively-bred or genetically-modified rodent models, transgenic models, gene expression profiling, and biomarker analysis, are discussed. This article was developed with the support of the DruSafe Leadership Group of the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ, www.iqconsortium.org).

Effects of the δ opioid agonist AZD2327 upon operant behaviors and assessment of its potential for abuse.

AZD2327 is a brain-penetrant agonist at δ opioid receptors which has antidepressant and anxiolytic properties in a wide array of animal models. As part of the preclinical safety pharmacology assessment, a number of studies were conducted in order to characterize its behavioral effects and its potential for abuse, in order to enable testing in humans. AZD2327 produced only modest effects when tested in a multiple fixed-ratio differential reinforcement of low rate schedule in rats, and did not enhance the rate-suppressing effects of ethanol in the procedure. In a suppressed responding test, AZD2327 only reduced rates of unpunished responding. In drug discrimination studies, AZD2327 produced partial or no generalization from known drugs of abuse. In primates trained to self-administer cocaine, substitution with AZD2327 did not result in appreciable self-administration of AZD2327, indicating that it does not behave as a positive reinforcer under the present conditions. Following termination of repeated administration of AZD2327, no signs of physical dependence (withdrawal) were noted. Overall, the data suggest that AZD2327 does not possess a high potential for abuse, and appears to have only subtle behavioral effects as measured by operant behaviors.

Preclinical pharmacology of AZD2327: a highly selective agonist of the δ-opioid receptor.

In the present article, we summarize the preclinical pharmacology of 4-{(R)-(3-aminophenyl)[4-(4-fluorobenzyl)-piperazin-1-yl]methyl}-N,N-diethylbenzamide (AZD2327), a highly potent and selective agonist of the δ-opioid receptor. AZD2327 binds with sub-nanomolar affinity to the human opioid receptor (K(i) = 0.49 and 0.75 nM at the C27 and F27 isoforms, respectively) and is highly selective (>1000-fold) over the human µ- and κ-opioid receptor subtypes as well as >130 other receptors and channels. In functional assays, AZD2327 shows full agonism at human δ-opioid receptors ([(35)S]GTPγ EC(50) = 24 and 9.2 nM at C27 and F27 isoforms, respectively) and also at the rat and mouse δ-opioid receptors. AZD2327 is active in a wide range of models predictive of anxiolytic activity, including a modified Geller-Seifter conflict test and social interaction test, as well as in antidepressant models, including learned helplessness. In animals implanted with microdialysis probes and then given an acute stressor by pairing electric shock delivery with a flashing light, there is an increase in norepinephrine release into the prefrontal cortex associated with this acute anxiety state. Both the benzodiazepine anxiolytic standard diazepam and AZD2327 blocked this norepinephrine release equally well, and there was no evidence of tolerance to these effects of AZD2327. Overall, these data support the role of the δ-opioid receptor in the regulation of mood, and data suggest that AZD2327 may possess unique antidepressant and anxiolytic activities that could make a novel contribution to the pharmacotherapy of psychiatric disorders.

Behavioral pharmacology of AR-A000002, a novel, selective 5-hydroxytryptamine(1B) antagonist.

The present review summarizes the behavioral pharmacology conducted to profile the anxiolytic and antidepressant potential of the selective 5-hydroxytryptamine (HT)(1B) antagonist (R)-N-[5-methyl-8-(4-methylpiperazin-1-yl)-1,2,3,4-tetrahydro-2-naphthyl]-4-morpholinobenzamide (AR-A000002). AR-A000002 functions as a 5-HT(1B) antagonist in vivo, which was shown by the antagonism of the discriminative stimulus effects in the guinea pig of the 5-HT(1B) agonist 3-(N-methylpyrrolidin-2R-ylmethyl)-5-(3-nitropyrid-2-ylamino)-lH-indole (CP135,807). Anxiolytic activity of AR-A000002 was demonstrated in the separation-induced vocalization paradigm in guinea pig pups, and in a suppressed responding procedure in pigeons and guinea pigs, but only a weak trend was noted in a suppressed responding procedure in squirrel monkeys. Antidepressant efficacy was shown in a number of paradigms. In pigeons and guinea pigs responding under a differential reinforcement of low rates schedule of reinforcement (DRL), AR-A000002 increased the number of reinforcers earned without altering the number of responses made. In guinea pigs trained under a response duration differentiation paradigm, AR-A000002 increased mean lever-press duration. Finally, AR-A000002 was shown to block escape failures in guinea pigs submitted to a learned helplessness paradigm. Taken together, these data suggest utility for 5-HT(1B) antagonists in the treatment of both anxiety and affective disorders.

T2-weighted MRI correlates with long-term histopathology, neurology scores, and skilled motor behavior in a rat stroke model.

The intraluminal suture model of transient middle cerebral artery occlusion (MCAO) in the Sprague Dawley strain of rats characteristically results in an inconsistently sized brain lesion. The purpose of the investigation reported here was to determine whether there were strong point-to-point correlations between the degree of cortical lesion size, as assessed in vivo using T2-weighted magnetic resonance imaging (MRI) and corresponding cortical lesion size using routine histopathological techniques. Moreover, we aimed to investigate if cortical lesion size as determined by these two modalities correlates with neurological and/or skilled motor deficits observed in individual animals. Baseline behavioral scores were obtained on the animals prior to receiving 60 min of transient MCAO. Following MCAO, animals were tested for 1-21 days for neurological deficits. T2-weighted MRIs of the cortex were taken at two and seven days post-MCAO. At 30 and 60 days the rats were retested for forelimb dexterity in the staircase test. Subsequently, the cortex was examined for histopathological damage. Indeed, there were highly significant correlations between lesion size determined by MRI and histopathology. The degree of cortical damage observed in the T2-weighted MRI, as well as the size of the histopathological lesions were, in turn, highly correlated with the degrees of deficiencies observed in the composite neurological assessments and with the deficits involving skilled use of the contralateral forepaw (damaged side).

Long-term deficits following cerebral hypoxia-ischemia in four-week-old rats: correspondence between behavioral, histological, and magnetic resonance imaging assessments.

We examined whether following a hypoxic-ischemic insult in young animals there are long-lasting functional deficits that correlate either to histological tissue damage or to potential compensatory plasticity changes. Four-week-old rats were subjected to an episode of cerebral hypoxia-ischemia (right carotid artery occlusion + 30 min of hypoxia) or a sham operation. In hypoxic-ischemic animals there were gross neurological deficits 1, 24, and 48 h postinsult with recovery by 1 week. Behavioral deficits were observed in both the acquisition and the performance of a response duration differentiation test and a fine motor control test (staircase test) 3 months after the hypoxia-ischemia. Functional magnetic resonance imaging studies demonstrated less activation in the sensory-motor cortex of hypoxic-ischemic animals in response to left vs right forepaw stimulation 4 months postinsult. Histological assessment delineated striatal, cortical, and hippocampal damage in the hypoxic-ischemic hemisphere and a reduction in cortical thickness, bilaterally. GFAP immunoreactivity was increased in injured striatum and cortex. Neurofilament heavy chain (NF200) immunoreactivity was normally most intense in white matter and decreased in areas of ipsilateral cortical damage. Synaptophysin immunoreactivity was reduced around areas of infarction and somewhat increased in adjacent undamaged striatum and in layer IV of parietal cortex. The histological damage or chronic degenerative changes could account for much of the variance in functional outcome detected with sensitive behavioral tests so that overall the compensatory or plasticity changes evident within the juvenile brain are rather modest.

Rat middle cerebral artery occlusion: correlations between histopathology, T2-weighted magnetic resonance imaging, and behavioral indices.

During attempts to develop the intraluminal suture model of transient middle cerebral artery occlusion (MCAO) in the Sprague Dawley strain of rats, we noticed a wide variability in lesion size seen with T2-weighed magnetic resonance imaging (MRI) or histopathology, as well as in scores for behavioral indices. It was our intent to examine the results of the study carefully and determine whether there were strong point-to-point correlations between the degree of lesion size determined from T2-weighted MRI or histopathology and intermediate or long-term neurologic/behavioral assessments. Baseline behavioral scores for forelimb dexterity (staircase test) were obtained on all animals in the period before receiving 60 minutes of transient MCAO. After MCAO, animals were tested at specified intervals from 1 to 21 days for composite neurologic deficits. T2-weighted MRI was taken at 2 and 7 days post-MCAO. At 30 and 60 days post-MCAO, animals were retested in the staircase test with subsequent histopathologic examination of the brains. Indeed, there were highly significant correlations between lesion size determined by MRI and histopathology. The damage observed in the T2-weighted MRI, as well as the size of the histopathologic lesions, were in turn highly correlated to deficiencies observed in the composite neurologic assessments, as well as to deficits at 30 and 60 days post-MCAO for skilled use of the contralateral forepaw (damaged side). In the latter test, the correlations were somewhat less significant for the ability of rats to reach for food with the ipsilateral forepaw (undamaged side).

Disruption of acquisition and performance of operant response-duration differentiation by unilateral nigrostriatal lesions.

Response duration differentiation (RDD), an operant schedule requiring fine motor timing and control, was assessed as a possible baseline for study of the long-term consequences of nigrostriatal lesions and as a possible baseline to test the therapeutic efficacy of candidate palliative, neuroprotective and neurorestorative drugs. Rats were subjected to unilateral 6-hydroxydopamine (6-OHDA) lesions of striatum, medial forebrain bundle (mfb), or were sham lesioned, and their ability to acquire the operant task was studied in a single overnight session. In a second set of studies, rats that had been well trained in the RDD task were sham lesioned or were given unilateral 6-OHDA lesions of the mfb, and behavior under this baseline was studied for more than 30 weeks. Lesions of both striatum and of mfb resulted in impaired acquisition of RDD responding, with the relatively greater effect by the mfb lesion. In rats previously trained under the RDD schedule, mfb lesions produced marked disruptions in RDD performance, which did not fully recover. L-DOPA administration decreased the variability of the response durations, primarily by decreasing the proportion of short-duration lever presses.

Antiparkinson potential of delta-opioid receptor agonists.

delta-Opioid receptors, present in very high concentrations in striatum and overlying cortex, are thought to be involved in a number of processes, including analgesia, mood, reward, modulation of neuronal excitability, and alterations in neurotransmitter release. Given the localization of the receptors in motor circuits in brain, we thought it of interest to study the antiparkinson potential of delta-opioid receptor agonists. Rats were given unilateral 6-hydroxydopamine lesions of the nigrostriatal tract, and following recovery, were tested for rotational activity. Tonazocine mesylate is a nonpeptide, partial delta-opioid receptor agonist with mu-receptor antagonist properties. Tonazocine (0.1-10 mg/kg) evoked a dose-related, ipsilateral rotation, consistent with augmentation of dopaminergic function on the unlesioned side. The rotation evoked by tonazocine was blocked by the selective delta-opioid receptor antagonist naltrindole, suggesting that the effect was mediated by delta-opioid receptors. The full delta-opioid receptor agonist (+)-4-¿9-alpha-R)-alpha-(2S,5RO-4-allyl-2, 5-dimethyl-1-piperaziny l)-3-methoxybenzyl-N,N-diethylbenzamide (SNC-80) produced both contralateral and ipsilateral rotation. Tonazocine additionally augmented the effects of L-3,4 dihydroxyphenylalanine (L-DOPA) on reserpine-induced suppression of motor activity. Binding affinities and efficacies of tonazocine and SNC-80 against mu-, kappa-, and delta-opioid receptors were also confirmed and compared to standards. These data suggest therapeutic potential of agents interacting with delta-opioid receptors, and indicate some differences in the activities of tonazocine and SNC-80.

Long-term functional end points following middle cerebral artery occlusion in the rat.

The purpose of the present study was to assess the magnitude and stability of a number of functional deficits in rats subjected to occlusion of the middle cerebral artery (MCAO). Three groups of rats, treated with 90-min, 120-min, or sham occlusion were used in functional studies for 22 weeks following surgery. The following tests were used: methamphetamine-induced rotation, the staircase test, acquisition of operant responding, running-wheel behavior, and performance of operant differential reinforcement of a low-rate responding (DRL) schedule of reinforcement. Histology performed at 23 weeks following infarct showed on average modest damage of a 19% reduction in hemispheric volume. Of the behavioral tests conducted, rotation, the staircase test, and the operant DRL were sensitive to ischemic damage, and were under some circumstances related to lesion size. These data show that long-term functional deficits following MCAO are demonstrable, and hence, assessment of long-term neuroprotection is feasible.

Behavioral effects of AR-R 15849, a highly selective CCK-A agonist.

The behavioral effects of AR-R 15849, a novel cholecystokinin agonist with high affinity and selectivity for the CCK-A receptor subtype, were examined. Initially, using an operant feeding paradigm to test for anorectic activity and specificity, acute administration of AR-R 15849 was found to alter the intake and pattern of feeding in a manner similar to prefeeding. Further, AR-R 15849 did not induce compensatory feeding as did CCK-8, and did not affect performance on running rates of responding, or motor activity on a running wheel, as did fenfluramine. In tests for subchronic anorectic activity, daily intraperitoneal injections of AR-R 15849 significantly reduced food intake in fasted rats over a 9-day test period with greater efficacy compared to its nonselective predecessor AR-R 14294 (formerly FPL 14294). The sustained decrease in food intake with AR-R 15849 treatment resulted in a significant reduction in body weight gain over 9 days. Finally, an experiment designed to determine the effect of caloric deprivation and subchronic drug exposure on the overall efficacy of AR-R 15849 indicated that pharmacological tolerance does not develop following subchronic treatment.

[S]-AR-R 15896AR-A novel anticonvulsant: acute safety, pharmacokinetic and pharmacodynamic properties.

A rational, chemical, synthetic effort to identify promising low-affinity uncompetitive N-methyl-D-aspartic acid receptor antagonists for use as antiepileptic drugs led to the discovery of AR-R 15035AR, or [RS]-alpha-phenyl-2-pyridine-ethanamine.2HCl. Chiral separation followed by intensive in vivo screening resulted in the selection of the [S] enantiomer, AR-R 15896AR, as the best compound for further preclinical development. AR-R 15896AR prevented tonic seizures in rodents for up to 6 to 8 h in response to maximal electroshock (MES), 4-aminopyridine, bicuculline, or strychnine, as well as characteristic seizures following injections of N-methyl-DL-aspartic or kainic acids. AR-R 15896AR was ineffective in two kindling models of epilepsy, did not produce tolerance to MES, and was devoid of proconvulsant and phencyclidine-like properties in mice and rats, respectively. Therapeutic indices for AR-R 15896AR were comparable to or exceeded those for standard anticonvulsants. Orally administered AR-R 15896AR rapidly entered the rat brain and was eliminated in parallel from the plasma and plasma-free compartment. A dose-response relationship between plasma and brain levels after p.o. or i.v. administration of AR-R 15896AR and protection against MES was highly correlative. The time course for loss of protection against MES mirrored the elimination of the compound from brain and plasma. The total brain concentration (25 microM) of drug at the ED50 value (approximately 3 mg/kg) for protection against MES seizures was consistent with the reported affinity of AR-R 15896AR at the N-methyl-D- aspartic acid binding site (IC50 value = 1.3 microM). The present findings demonstrated the attractiveness of AR-R 15896AR as a candidate for further development to treat epilepsy.

The low-affinity, use-dependent NMDA receptor antagonist AR-R 15896AR. An update of progress in stroke.

Use-dependent N-methyl-D-aspartate (NMDA) receptor antagonists protect neurons from the lethal consequences of excessive stimulation by excitatory amino acids. Clinical development of high-affinity compounds such as MK801 have been limited due to untoward side effects. Toward this end, the lower-affinity use-dependent NMDA antagonists have greater margins of safety and have advanced to clinical trials for stroke, epilepsy, head trauma and chronic neurodegenerative disorders. AR-R 15896AR is currently in Phase II trials for stroke and has been repeatedly demonstrated to afford neuroprotection in a variety of in vivo and in vitro models associated with ischemia/excitotoxic conditions.

Acute heat stress model of seizures in weanling rats: influence of prototypic anti-seizure compounds.

The present study tested the therapeutic potential for prototype anti-epilepsy drugs using an animal model of infantile febrile seizures. The model consisted of immersion of weanling rats (21 days old) in a 45 degrees C water bath for a maximum of 4 min (four exposures over a 2 week period) and observing for the progression to stage-5 seizures. All compounds were administered orally at the respective ED50 for prevention of seizures in the maximal electroshock (MES) test. Clonazepam effectively lowered the score for seizure grade, shortened the duration of seizures, as well as reduced the number of animals experiencing seizures during three of the four testing periods. MK801 reduced both the maximum seizure grade, and the number of animals experiencing seizures during sessions two and three. However, the dose of MK801 caused behavioral side effects. Valproate actively decreased seizure grade, while it modestly acted to attenuate seizure duration, extended the time to seizure onset, and reduced the number of animals experiencing seizures on testing day 1. Remacemide hydrochloride and phenobarbital were not effective. The method appears useful for evaluating the potential of agents to prevent acute febrile seizures.

Antidepressant-like effects of 5-hydroxytryptamine1A receptor agonists on operant responding under a response duration differentiation schedule.

A response duration differentiation schedule, where rats depress a lever for between 1.0 and 1.3 s to obtain a food reward, provides a useful measure for detecting antidepressant activity. It is known that 5-hydroxytryptamine1A (5-HT1A) receptor agonists exhibit antidepressant-like activity in multiple animal models of depression, however, compounds selective for this receptor have not been tested in this model to date. Thus, the present study sought to determine the effect of the full 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the partial 5-HT1A agonist, buspirone, on responding in the response duration differentiation task. The effects of these drugs were compared to the effects of the non-specific serotonergic agonist, lysergic acid diethylamide (LSD); the phenothiazine, chlorpromazine; the atypical antidepressant, trazodone; and the non-selective 5-HT1A antagonists, propranolol and alprenolol. It was found that propranolol, trazodone, and both the full (8-OH-DPAT) and partial (buspirone) 5-HT1A agonists produced increases in the mean response duration, which is typical of antidepressant activity. By contrast, with the exception of propranolol, compounds lacking antidepressant efficacy (e.g. chlorpromazine, LSD and alprenolol), failed to produce increases in mean response durations. Further, the effects of 8-OH-DPAT were inhibited by pretreatment with the 5-HT1A antagonist, (-)-alprenolol (3.0 and 30.0 mg/kg i.p.). The results of this study provide further support for the suggestion that 5-HT1A agonists may be useful for the treatment of clinical depression and that these effects are specifically mediated by 5-HT1A receptors.

Remacemide hydrochloride and ARL 15896AR lack abuse potential: additional differences from other uncompetitive NMDA antagonists.

This study was designed to determine the possible abuse liability and phencyclidine-like effects of the low-affinity uncompetitive N-methyl-D-aspartate (NMDA) antagonists remacemide hydrochloride [(+/-)-2-amino-N-(1-methyl-1,2-diphenylethyl)-acetamine hydrochloride] and ARL 15896AR [(+)-alpha-phenyl-2-pyridine-ethanamine dihydrochloride]. For the abuse-liability studies, in rats trained to self-administer cocaine intravenously (0.1 mg/kg/injection), doses of remacemide HCl, ARL 15896AR, phencyclidine, and saline were made available, and the number of injections self-administered was recorded. In different sets of rats, we assessed the ability of these drugs to induce phencyclidine-like stereotyped behavior. Doses of the compounds were expressed as multiples of the 50% effective dose (ED50), as determined from the maximal electroshock (MES) test by using either oral or intravenous administration. None of the remacemide hydrochloride or ARL 15896AR doses was self-administered at a level higher than that of the saline vehicle, unlike cocaine and phencyclidine, which were self-administered at high and moderate levels, respectively. Unlike that with remacemide hydrochloride and ARL 15896AR, oral administration of the high-affinity uncompetitive NMDA receptor-antagonists phencyclidine, ARL 16247 [N-(3-ethylphenyl)-N-methyl-N'-naphthylguanidine] and MK-801 engendered phencyclidine-like stereotypy at doses near their MES ED50 values. These data confirm the unusual safety of remacemide hydrochloride and ARL 15896AR and demonstrate that they do not possess reinforcing properties. As such, they are unlikely to present a drug-abuse problem in human beings.

sigma2 Site-mediated inhibition of electrically evoked guinea pig ileum longitudinal muscle/myenteric plexus contractions.

Functional and binding studies were performed in order to characterize the relative efficacy and affinity of a number of compounds that bind to sigma sites. The ability of sigma site ligands to inhibit electrically evoked contraction of the guinea pig ileum longitudinal muscle/myenteric plexus preparation was compared to the affinities of these compounds for sigma1 sites (assessed by displacement of [3H](+)-pentazocine) and sigma2 sites (assessed by displacement of [3H]1,3-di-o-tolylguanidine (DTG) in the presence of 5 microM dextromethorphan). It was shown that the rank order of potencies for suppression of electrically evoked contractions of guinea pig ileum perfectly matched the rank order of affinities of these compounds for the sigma2 binding site, while correlating poorly with the sigma1 binding site. In addition, no significant correlations were found between the efficacy of the tested compounds to inhibit contraction of the guinea pig ileum preparation and previously reported affinities for muscarinic, dopamine D2 or MK-801 binding sites. Thus, the present study represents the first functional bioassay selectively sensitive to agents interacting with the sigma2 receptor subtype binding site, and provides a means with which to further elucidate the functional role of sigma2 sites.

Drug effects on response-duration differentiation. IV: Effects of trimethyltin.

Trimethyltin (TMT) is a toxicological agent that produces damage in a number of limbic structures, resulting in concomitant disruptions of behavior. The purpose of the present study was to determine the utility of response-duration differentiation (RDD) responding as a behavioral baseline for studying the behavioral consequences of TMT administration. Under the RDD schedule, responses of a restricted duration (1-1.3 s) were reinforced, and disruption of this performance may represent effects upon fine motor control, timing behavior, or both. Two doses of TMT (4 mg/kg) were administered 1 week apart, and behavior under the schedule was studied daily for 6 weeks thereafter in a group of four rats. Additionally, the effects of diazepam (0.1-3mg/kg) administered prior to and following TMT administration were compared. TMT produced disruptions in accuracy of responding and increases in rates of responding in the weeks following its administration. Behavior had generally recovered by 6 weeks after the first TMT administration. Diazepam flattened the relative frequency distributions of response durations at lower doses in the TMT-treated rat. These data show that RDD responding is sensitive to the effects of TMT, and TMT treatment can result in alterations in the effects of diazepam.

Effects of anticonvulsants in a novel operant learning paradigm in rats: comparison of remacemide hydrochloride and FPL 15896AR to other anticonvulsant agents.

One of the primary undesired effects of anticonvulsant medication is an impairment in cognitive function, such as new learning ability. The purpose of the present study was to compare the effects of remacemide hydrochloride [(+/-)-2-amino-N-(1-methyl-1,2,-diphenylethyl)acetamide monohydrochloride] and FPL 15896AR [(+)-alpha-phenyl-2-pyridine-ethanamide] to a number of anticonvulsant agents on an operant acquisition baseline. Remacemide hydrochloride is currently in clinical trials for epilepsy and FPL 15896AR is under development. In the present procedure, fasted, experimentally naive rats were placed into operant chambers in which food pellets were initially available under a Fixed-Ratio 1 (FR1) schedule of food presentation, and as lever pressing progressed, the FR value incremented. All drugs were tested in multiples of three and ten times their respective ED50 values against maximal electroshock-induced seizure (MES) following p.o. administration. The drugs tested varied widely in their ability to disrupt acquisition of the lever-pressing task. Remacemide hydrochloride and a structurally related analog, FPL 15896AR, did not disrupt acquisition. Clonazepam, lamotrigine, MK-801, phenobarbital, felbamate, phenytoin, and carbamazepine increased the number of hours required to achieve FR3 (emit more than 100 responses) with respect to vehicle control performance. Of these, clonazepam, MK-801 and phenytoin produced robust enough disruption to result in significantly fewer reinforcers delivered over the 14-h operant session.

Drug effects on response duration differentiation. I: Differential effects of drugs of abuse.

Rats were trained to respond under a response duration differentiation schedule in which responses on a lever were reinforced if lever press durations were greater than or equal to 1.00 s but were also less than 1.30 s. Dose-effect curves were generated for cocaine, methamphetamine, pentobarbital, phencyclidine, delta-9-tetrahydrocanninabol (delta 9-THC), and morphine. All drugs produced dose-dependent decreases in accuracy (the percentage of total response durations that were reinforced); however, the degree to which changes in accuracy were accompanied by changes in response rates varied among drugs. Pentobarbital and morphine affected primarily longer (> 1.3 s) response durations, phencyclidine and delta 9-THC affected primarily shorter response durations, whereas cocaine and methamphetamine affected both shorter and longer response durations. High doses of methamphetamine and cocaine increased the dispersion of response duration distributions with increasing dose, whereas higher doses of pentobarbital, delta 9-THC and morphine did not increase dispersion of response duration distributions as much. These data show that behavior under this novel schedule is differentially sensitive to a number of pharmacologic manipulations, and that the schedule can provide a useful addition to the analysis of drug effects upon behavior.