Effects of the training dose on generalization of morphine stimulus to clonidine

5-HT1a Receptor Involvement in Temporal Memory and the Response to Temporal Ambiguity

It has previously been demonstrated that rats trained on the peak-interval procedure to associate two different cues with two different fixed interval schedules will generate a scalar peak function at an intermediate time when presented with the compound cue. This response pattern has been interpreted as resulting from the simultaneous retrieval of different temporal memories, and a consequential averaging process to resolve the ambiguity. In the present set of studies, we investigated the role that serotonin 1a receptors play in this process. In Experiment 1, rats were trained on a peak-interval procedure to associate the interoceptive states induced by saline and the 5-HT1a agonist, 8-OH-DPAT, with a 5 s or 20 s fixed-interval schedule signaled by the same tone cue (counter-balanced). While peak functions following administration of saline were centered at the appropriate time (5 s or 20 s), peak functions following administration of the agonist were centered around 7 s, irrespective of the reinforced time during training, suggesting agonist-induced disruption in selective temporal memory retrieval, resulting in increased ambiguity regarding the appropriate time at which to respond. In Experiment 2, rats were trained in a peak-interval procedure to associate a tone cue with a 10 s fixed interval and a light cue with a 20 s fixed interval. Administration of the 5-HT1a antagonist, WAY-100635, had no impact on timing when single cues were presented, but altered the intermediate, scalar, response to the stimulus compound, suggesting antagonist-induced disruption in the processes used to deal with temporal memory ambiguity. Together, these data suggest that manipulations of 5HT transmission at the 5-HT1a receptor cause changes in the temporal pattern of responding that are consistent with alterations in temporal memory processes and responses to temporal ambiguity.

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Drug discrimination under a concurrent fixed-interval fixed-interval schedule

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a concurrent fixed-interval (FI) FI schedule of food presentation on which, after pentobarbital administration, responses on one key were reinforced with food under an FI 60-s component and responses on the other key were reinforced under an FI 240-s component. After saline administration, the schedule contingencies on the two keys were reversed. After both pentobarbital and saline, pigeons responded more frequently on the key on which responses had been programmed to produce the reinforcer under the FI 60 component of the concurrent schedule. The schedule was changed to concurrent FI 150 FI 150 s for drug-substitution tests. In each bird, increasing doses of pentobarbital, ethanol, and chlordiazepoxide produced increases in the proportion of responses on the key on which responses had been reinforced under the FI 60 component after pentobarbital administration during training sessions. The proportion of responses on that key was slightly lower for ethanol than for chlordiazepoxide and pentobarbital. At a dose of pentobarbital higher than the training dose, responding decreased on the key that had been reinforced under the FI 60 component during training sessions. Phencyclidine produced less responding on the key programmed under the FI 60-s component than did pentobarbital. Methamphetamine produced responding primarily on the key on which responses had been reinforced under the FI 60-s component after saline administration.

Drug discrimination is a continuous rather than a quantal process following training on a VI-TO schedule of reinforcement

Debate continues as to whether drug discrimination in animals is an inherently quantal or continuous process. This issue is important in determining the appropriate interpretation of results from drug discrimination studies designed to assess the nature of drug-induced interoceptive cues. The quantal approach holds that subjects perceive a drug cue in an all-or-none manner, while the continuous view proposes that when appropriate training and testing procedures are used, subjects can discriminate along a continuum of interoceptive cues. Data consistent with the quantal view have consistently been generated by animals trained to respond on schedules of reinforcement having an FR component. Since quantal responding is a characteristic of these schedules, results from drug discrimination studies using training schedules with FR components are of little value in empirically determining whether drug discrimination reflects a quantal or continuous process. Use of variable schedules of reinforcement might be more appropriate because the pattern of responding generated does not preclude results consistent with either of the competing views. Data from the following studies that trained subjects using VI schedules with a concurrent TO for incorrect lever responding were analyzed: Barrett et al. (1982): L-5-hydroxytryptophan versus saline; Smith (1990): diazepam versus pentylenetetrazol; Barrett et al. (1992): amphetamine versus haloperidol; Barrett and Steranka (1983): amphetamine versus haloperidol. In every case, when experimental conditions produced a group mean intermediate to that for the training drugs, the distribution of scores for individual animals was normally rather than bimodally distributed.

Effects of schedule of reinforcement on a pentobarbital discrimination in rats

The purpose of this study was to determine the effects of the schedule of reinforcement on a pentobarbital discrimination in rats. Five rats were trained to discriminate 10 mg/kg pentobarbital from saline under a multiple fixed-interval 180-s fixed-ratio 20 schedule of reinforcement. During both saline and pentobarbital training sessions, subjects emitted a higher percentage of correct responses under the fixed-ratio component as compared to the fixed-interval component of the multiple schedule. Determination of the pentobarbital dose-response curve under the fixed-ratio component resulted in a steep curve characterized by responding on the saline lever at low doses and on the drug lever at higher doses. Under the fixed-interval component, a graded dose-effect curve was produced, with considerable responding on both levers after intermediate doses of pentobarbital. The administration of phencyclidine and MK-801 resulted in an intermediate level of drug-lever responding for some subjects. Administration of d-amphetamine resulted in saline (nondrug) appropriate responding. The results of this study demonstrate that the schedule of reinforcement is a determinant of drug stimulus control, just as it is a determinant of other drug effects.

Tolerance to the analgesic, but not discriminative stimulus effects of morphine after brief social defeat in rats

One of the most prominent consequences of defeat in a social confrontation is a long-lasting tolerance-like insensitivity to the analgesic effects of opiates, even when only small short-lived changes in nociception are detectable during the acute social stress. The present experiments examined (1) which kinds of social experiences lead to morphine tolerance, (2) whether or not the morphine tolerance in defeat-experienced rats extends from the analgesic effects to the discriminative stimulus and rate-decreasing effects of morphine, and (3) how long morphine tolerance lasts after a defeat experience. After five brief social confrontations including attack and threat by a resident rat leading to submission or defeat of the intruder, the latter exhibits marked tolerance to the analgesic effects of morphine, but not to the discriminative stimulus or behaviorally suppressive effects. Changes in social housing did not alter morphine's behavioral effects. Tolerance to the analgesic morphine effects was detected for 2 months after the defeat experience, whereas the discriminative stimulus and rate-decreasing effects were closely similar to those before defeat. This pattern was seen in animals for whom discriminative stimulus training with morphine was suspended after defeat as well as in those for whom it continued. In additional defeated and non-defeated animals, morphine's effects on the acoustic startle reflex was assessed. In contrast to the tail flick reflex to a noxious heat stimulus, the acoustic startle response remained unaffected by defeat experience or by morphine (up to 30 mg/kg). The long-lasting and large degree of tolerance after brief social defeat experiences appears to be limited to the analgesic effects of morphine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of prior saline-morphine discrimination by pigeons on three-way discrimination including two morphine doses

The discriminative stimulus properties of morphine sulfate (MS) and their alteration by naltrexone (NTX) and d-amphetamine (AMP) challenges were examined in a quantitative dose 1, dose 2, and saline (SAL) drug discrimination task utilizing 1.8 mg/kg MS, 10 mg/kg MS, and SAL as discriminative stimuli under a fixed-ratio 30 schedule of food-maintained behavior in two groups of White Carneaux pigeons. Group A (Gp A) (n = 6) subjects (Ss) were initially experimentally- and drug-naive, whereas group B Ss (n = 4) were originally trained in a two-choice MS versus SAL discrimination task, and had a long behavioral and drug history. Significant differences were found in (1) number of sessions to criterion (STC) (group B greater than group A); (2) group A Ss generalized both NTX and AMP to SAL, whereas group B Ss generalized AMP to the low dose (1.8 mg/kg) MS stimulus; and (3) in drug interaction test sessions, the high dose MS stimulus (10 mg/kg) in group A was unmodified by a range of challenge AMP doses (0.32 to 3.2 mg/kg). In contrast, group B Ss exhibited a shift to the low dose or SAL-appropriate keys when the same high dose MS stimulus was challenged by moderate doses of AMP. Group A and group B were similar in their pattern and distribution of responses when tested with various doses of MS, and also when challenge tests of the high dose MS stimulus were made with NTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for perceptual masking of the discriminative morphine stimulus

Morphine-amphetamine and morphine-naltrexone interactions were examined in three groups of White Carneaux pigeons (n = 3), which were trained in a two-choice drug discrimination procedure under a FR-30 schedule of food reinforcement using 3.2 mg/kg morphine and saline as discriminative stimuli. Once stimulus control was acquired by these initial training stimuli, the training doses of morphine were gradually changed to 1.0 mg/kg for group A and to 10 mg/kg for group C. The three groups differed in the minimum dose required for stimulus control and the drugs to which the training stimulus generalized. Stimulus generalization to amphetamine was inversely related to training dose. Amphetamine potentiated the discriminative stimulus properties of morphine. Naltrexone blocked the discriminative stimulus properties of morphine to varying degrees, which appeared to be limited by the training dose and the rate-suppressing effects of naltrexone when administered alone. Challenging the morphine stimulus with amphetamine resulted in a qualitatively similar blockade. This blockade was a direct function of the morphine training dose. It is argued that MS-AMP interactions result in perceptual masking of the MS stimulus, which can be differentiated from pharmacological antagonism by NTX. Two other challenge drugs, ketamine and sodium pentobarbital, did not alter stimulus control by morphine.

Behavioral performance effects of antihypertensive drugs: human and animal studies

Antihypertensive drug treatments have been reported in clinical investigations to produce adverse effects to a degree that causes hypertensive patients to discontinue medication. Many of the debilitating effects reported by patients appear to be of central nervous system origin, such as sedation, fatigue, memory loss and sensorimotor disturbances. Human and animal laboratory studies in the past two decades have been characterizing the psychotropic effects of antihypertensive medications with use of a wide range of behavioral techniques. Antihypertensive drug classes covered in this review are beta-adrenergic blocking agents, alpha-adrenergic agonists, diuretics, angiotensin-converting enzyme inhibitors and calcium channel blockers. While findings in animal studies show generally greater behavioral impairments after administration of alpha-adrenergic agonists in comparison with other drug classes, the few laboratory studies conducted with hypertensive subjects present a confusing picture. A need for further laboratory research with hypertensive subjects and, study of antihypertensive drug combinations is discussed.