Behavioural tolerance to amphetamine and other psychostimulants: the case for considering behavioural mechanisms

Systematic review and exploratory meta-analysis of the efficacy, safety, and biological effects of psychostimulants and atomoxetine in patients with schizophrenia or schizoaffective disorder

OBJECTIVE

Our aim was to summarize the efficacy and safety of atomoxetine, amphetamines, and methylphenidate in schizophrenia.

METHODS

We undertook a systematic review, searching PubMed/Scopus/Clinicaltrials.gov for double-blind, randomized, placebo-controlled studies of psychostimulants or atomoxetine in schizophrenia published up to 1 January 2017. A meta-analysis of outcomes reported in two or more studies is presented.

RESULTS

We included 22 studies investigating therapeutic effects of stimulants (k=14) or measuring symptomatic worsening/relapse prediction after stimulant challenge (k=6). Six studies of these two groups plus one additional study investigated biological effects of psychostimulants or atomoxetine. No effect resulted from interventional studies on weight loss (k=1), smoking cessation (k=1), and positive symptoms (k=12), and no improvement was reported with atomoxetine (k=3) for negative symptoms, with equivocal findings for negative (k=6) and mood symptoms (k=2) with amphetamines. Attention, processing speed, working memory, problem solving, and executive functions, among others, showed from no to some improvement with atomoxetine (k=3) or amphetamines (k=6). Meta-analysis did not confirm any effect of stimulants in any symptom domain, including negative symptoms, apart from atomoxetine improving problem solving (k=2, standardized mean difference (SMD)=0.73, 95% CI=0.10-1.36, p=0.02, I2=0%), and trending toward significant improvement in executive functions with amphetamines (k=2, SMD=0.80, 95% CI=-1.68 to +0.08, p=0.08, I2=66%). In challenge studies, amphetamines (k=1) did not worsen symptoms, and methylphenidate (k=5) consistently worsened or predicted relapse. Biological effects of atomoxetine (k=1) and amphetamines (k=1) were cortical activation, without change in β-endorphin (k=1), improved response to antipsychotics after amphetamine challenge (k=2), and an increase of growth hormone-mediated psychosis with methylphenidate (k=2). No major side effects were reported (k=6).

CONCLUSIONS

No efficacy for stimulants or atomoxetine on negative symptoms is proven. Atomoxetine or amphetamines may improve cognitive symptoms, while methylphenidate should be avoided in patients with schizophrenia. Insufficient evidence is available to draw firm conclusions.

Drug Abuse as a Problem of Impaired Control: Current Approaches and Findings

The author presents a selective and integrative overview of cur-rent research examining acute effects of abused drugs on aspects of behavioral control in humans. The review discusses the concept of behavioral control and reviews research models that examine inhibitory influences as a central mechanism of control. Evidence for drug-induced impairment of inhibitory mechanisms is reviewed from current neuroanatomical, neuropharmacological, and behavioral perspectives. Model-based laboratory assessments of behavioral control are described in terms of their application to behavioral pharmacology and evidence for alcohol and psychostimulant effects on basic inhibitory mechanisms is reviewed. Drug effects on inhibitory mechanisms are also discussed in terms of their relevance to higher-order cognitive and behavioral functions, and in relation to impulsive behaviors, such as drug abuse. The review concludes by identifying important drug-related phenomenon (e.g., drug tolerance, cue reactivity) that might also be studied in behavioral control frameworks in future research.

The effect of chronic amphetamine treatment on cocaine-induced facilitation of intracranial self-stimulation in rats

RATIONALE

Chronic amphetamine treatment reduces cocaine self-administration in pre-clinical and clinical settings, and amphetamine has been proposed as a candidate medication for treatment of cocaine abuse.

OBJECTIVE

The objective of the present study was to investigate whether chronic amphetamine treatment can decrease abuse-related cocaine effects in an assay of intracranial self-stimulation (ICSS).

METHODS

Thirteen adult male Sprague-Dawley rats were equipped with intracranial electrodes targeting the medial forebrain bundle and trained to lever press for pulses of brain stimulation in a "frequency-rate" ICSS procedure. Cocaine (10 mg/kg) was administered before (day 0), during (days 7 and 14), and after (posttreatment days 1 and 3) 2 weeks of continuous treatment with either amphetamine (0.32 mg/kg/h, n = 7) or saline (n = 6) via osmotic pump.

RESULTS

Prior to treatment, cocaine facilitated ICSS in all rats. Saline treatment had no effect on baseline ICSS or cocaine-induced facilitation of ICSS at any time. Conversely, amphetamine produced a sustained though submaximal facilitation of baseline ICSS, and cocaine produced little additional facilitation of ICSS during amphetamine treatment. Termination of amphetamine treatment produced a depression of baseline ICSS and recovery of cocaine-induced facilitation of ICSS.

CONCLUSIONS

These data suggest that chronic amphetamine treatment blunts expression of abuse-related cocaine effects on ICSS in rats.

Tolerance to cocaine in brain stimulation reward following continuous cocaine infusions

This study examined tolerance to cocaine's threshold-lowering effect in brain stimulation reward (BSR) following continuous cocaine infusions and secondly, used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) to determine NO's involvement in the development of cocaine tolerance. Animals were continuously infused with saline or cocaine (30 mg/kg per day) via osmotic minipump for 14 days and injected daily with saline or L-NAME (30 mg/kg, i.p.) following BSR testing. Saline-treated animals continuously infused with saline showed stable BSR thresholds across the 14-day infusion period. Saline-treated animals continuously infused with cocaine showed markedly lowered BSR thresholds on Day 1 followed by a progressive increase in BSR thresholds across the infusion period - indicating the development of tolerance. L-NAME-treated animals continuously infused with cocaine showed stimulation thresholds that were not significantly different from saline-treated animals continuously infused with cocaine. A cocaine challenge injection (10 mg/kg, i.p.) administered 3 and again at 10 days following minipump removal revealed that saline-treated animals continuously infused with saline showed lowered BSR thresholds. Saline-treated animals continuously infused with cocaine displayed lowered BSR thresholds that were not significantly different from saline-infused animals. L-NAME treated animals continuously infused with cocaine showed higher BSR thresholds to a challenge 3 days following pump removal. However, stimulation thresholds for this group failed to reach statistical significance on both days (i.e., Days 3 and 10) following pump removal. Results showed that animals continuously infused with cocaine develop robust tolerance to cocaine's threshold-lowering effect during the 14-day infusion period. Tolerance to cocaine's threshold-lowering effect was short-lived and dissipated soon after minipump removal. L-NAME treatment failed to significantly alter the development of tolerance to cocaine's threshold-lowering suggesting that NO does not have a primary role in the development of cocaine tolerance.

Potential serotonin 5-HT(1A) and dopamine D(4) receptor modulation of the discriminative stimulus effects of amphetamine in rats

Activation of the dopaminergic system underlies the behavioral effects of (+)-amphetamine, and plays a major role in its discriminative stimulus properties. Although serotonin receptors modulate dopamine levels in the brain, and 5-HT(1A) and 5-HT(2) receptor agonists do not mimic (+)-amphetamine, pretreatment with 5-HT(2A/2C) agonists significantly potentiates the (+)-amphetamine cue. Further, 5-HT(2) antagonists do not modify the discriminative stimulus effect of (+)-amphetamine, but 5-HT(1A) antagonists have never been tested in (+)-amphetamine-trained rats. This study sought to characterize the effects of the 5-HT(1A) antagonist WAY 100635 on (+)-amphetamine-induced discriminative stimulus effects. Male Sprague-Dawley rats were trained in a two-lever, fixed ratio 50, food-reinforced task with (+)-amphetamine sulfate (1.0 mg/kg, i.p., 30 min pretreatment time) as the discriminative stimulus. Substitution and combination tests with WAY 100635 were then performed. WAY 100635 did not produce substitution in amphetamine-trained rats, but significantly increased behavioral disruption. In combination tests 0.4 and 5.4 mg/kg doses of WAY 100635 potentiated the amphetamine cue. We suggest that low doses of WAY 100635 potentiated the (+)-amphetamine cue by blockade of 5-HT(1A) receptors, but stimulation of the dopamine D(4) receptor by higher doses of WAY 100635 may be responsible for potentiation of amphetamine-induced behavioral sensitization. The high percentage of behavioral disruption in substitution tests might suggest that rats trained to discriminate (+)-amphetamine from saline show behavioral sensitization that is not detectable by the drug discrimination assay but may be expressed as hyperactivity and stereotypic behavior that disrupts operant behavior.

Tolerance to allopregnanolone with focus on the GABA-A receptor

Many studies have suggested a relationship between stress, sex steroids, and negative mental and mood changes in humans. The progesterone metabolite allopregnanolone is a potent endogenous ligand of the γ-amino butyric acid -A (GABA-A) receptor, and the most discussed neuroactive steroid. Variations in the levels of neuroactive steroids that influence the activity of the GABA-A receptor cause a vulnerability to mental and emotional pathology. There are physiological conditions in which allopregnanolone production increases acutely (e.g. stress) or chronically (e.g. menstrual cycle, pregnancy), thus exposing the GABA-A receptor to high and continuous allopregnanolone concentrations. In such conditions, tolerance to allopregnanolone may develop. We have shown that both acute and chronic tolerances can develop to the effects of allopregnanolone. Following the development of acute allopregnanolone tolerance, there is a decrease in the abundance of the GABA-A receptor α4 subunit and the expression of the α4 subunit mRNA in the ventral-posteriomedial nucleus of the thalamus. Little is known about the mechanism behind allopregnanolone tolerance and its effects on assembly of the GABA-A receptor composition. The exact mechanism of the allopregnanolone tolerance phenomena remains unclear. The purpose of this review is to summarize certain aspects of current knowledge concerning allopregnanolone tolerance and changes in the GABA-A receptors.

Influences on cocaine tolerance assessed under a multiple conjunctive schedule of reinforcement

Under multiple schedules of reinforcement, previous research has generally observed tolerance to the rate-decreasing effects of cocaine that has been dependent on schedule-parameter size in the context of fixed-ratio (FR) schedules, but not under the context of fixed-interval (FI) schedules of reinforcement. The current experiment examined the effects of cocaine on key-pecking responses of White Carneau pigeons maintained under a three-component multiple conjunctive FI (10 s, 30 s, & 120 s) FR (5 responses) schedule of food presentation. Dose-effect curves representing the effects of presession cocaine on responding were assessed in the context of (1) acute administration of cocaine (2) chronic administration of cocaine and (3) daily administration of saline. Chronic administration of cocaine generally resulted in tolerance to the response-rate decreasing effects of cocaine, and that tolerance was generally independent of relative FI value, as measured by changes in ED50 values. Daily administration of saline decreased ED50 values to those observed when cocaine was administered acutely. The results show that adding a FR requirement to FI schedules is not sufficient to produce schedule-parameter-specific tolerance. Tolerance to cocaine was generally independent of FI-parameter under the present conjunctive schedules, indicating that a ratio requirement, per se, is not sufficient for tolerance to be dependent on FI parameter.

Repeated inhalation of toluene by rats performing a signal detection task leads to behavioral tolerance on some performance measures

Previous work showed that trichloroethylene (TCE) impairs sustained attention as evidenced by a reduction in accuracy and elevation of response latencies in rats trained to perform a visual signal detection task (SDT). This work also showed that these effects abate during repeated exposures if rats inhale TCE while performing the SDT. The present experiment sought to determine whether toluene, another commonly-used solvent, would induce tolerance similarly if inhaled repeatedly during SDT testing. Sixteen male, Long-Evans rats were trained to perform the SDT. Upon completion of training, rats were divided into 2 groups. In Phase I, concentration-effect functions were determined for toluene (0, 1200, 1600, 2000, 2400 ppm) in both groups. Toluene reduced the proportion of correct responses [P(correct)], and increased response time (RT) and response failures. In Phase II, Group-Tol inhaled 1600 ppm toluene while Group-Air inhaled clean air during 11 daily SDT sessions. In Group-Tol the effect of toluene on P(correct) abated after 3 days, while RT remained elevated for the duration of the repeated exposures. In Phase III, toluene concentration-effect functions were re-determined for both groups. Group-Air remained impaired on all test measures, whereas for Group-Tol, toluene did not reduce P(correct), but continued to increase RT. These data confirm our previous hypothesis that animals can develop tolerance to chemical exposures that impair appetitively-motivated behaviors if that impairment leads to loss of reinforcement.

Transfer of learning to compensate for impairment by alcohol and visual degradation

RATIONALE

Past research on social drinkers shows that prior experience performing a task in a visually degraded environment results in the reduction of alcohol-induced impairment. It is possible that task experience under alcohol might similarly carry over to reduce impairment from visual degradation.

OBJECTIVES

The purpose of this study was to test the symmetry of the transfer of learning between two distinct sources of impairment, visual degradation and alcohol.

MATERIALS AND METHODS

Psychomotor impairment was measured by a pursuit rotor tracking task. Forty two participants were randomly assigned to one of six treatment groups. Two groups were tested under alcohol after having prior task experience performing with or without visual degradation. Two groups were tested under visual degradation after having prior task experience performing under active (0.65 g/kg) or inactive (placebo) doses of alcohol. The remaining two groups served as controls that tested the learning effects of repeating each active treatment.

RESULTS

Clear evidence for asymmetrical transfer of learning was observed. Prior task experience with visual degradation reduced the impairing effects of alcohol. By contrast, prior task experience under alcohol had no effect on impairment produced by visual degradation.

CONCLUSIONS

Evidence for differential transfer of learning to compensate for alcohol- and visual-degradation-induced impairment is of practical interest, given that the two disturbances commonly co-occur outside the laboratory. Reasons for the asymmetry are unclear, and likely involve differences in mechanisms by which each treatment impairs psychomotor function.

Associative and behavioral tolerance to the analgesic effects of nicotine in rats: tail-flick and paw-lick assays

RATIONALE

Theories of drug tolerance differentiate between associative and behavioral (instrumental) drug tolerance. However, there is little research comparing these two forms of drug tolerance beyond alcohol and morphine.

OBJECTIVE

We examined the time course development of associative and behavioral tolerance to the analgesic effects of nicotine.

METHODS AND RESULTS

Associative tolerance was investigated by giving independent groups of rats one, five, 15, ten or 20 administrations of nicotine either explicitly paired or unpaired with a distinctive context. Associative tolerance, assessed in the tail flick, developed more rapidly and reached greater magnitude when nicotine and distinctive context were explicitly paired than when they were unpaired. This effect was evidenced after the fifth conditioning session and was maintained through the tenth, 15th, and 20th sessions. Contextual tolerance, assessed in the hot plate, was first evident after ten sessions. However, this effect disappeared safter 15 and 20 sessions. A second study examined the acquisition of behavioral tolerance to the disruptive effects of nicotine on the hot-plate response. Animals that practiced the test response while drugged developed greater tolerance than animals receiving as much nicotine and hot-plate practice but with these two conditions explicitly unpaired. This effect was evident in two different environments but did not generalize to the tail-flick test.

CONCLUSIONS

The findings suggest that contextual tolerance to drug effects is test specific, with tail-flick responses depending on cue-associative tolerance processes and hot-plate responses requiring procedures that allow the animal to practice the test response while drugged.

Tolerance to amphetamine hypophagia: a real-time depiction of learning to suppress stereotyped movements in the rat

To analyze how tolerance develops to amphetamine-induced hypophagia, the authors recorded real-time licking responses in rats given chronic injections of the drug and access to milk for 30 min. Initially, licking was greatly reduced and occurred only late in the session. The acquisition of tolerance was characterized by a decrease in the latency to initiate licking, a gradual increase in the number of licks, and a reorganization of the temporal licking pattern such that licks were distributed throughout the session, interspersed with pauses. On post-tolerance dose-response tests, licking was directly proportional to drug dose in some rats. The results support the view that tolerance to amphetamine hypophagia involves a behavioral adaptation to the motor effects of the drug.

Effects of chronic d-amphetamine treatment on cocaine- and food-maintained responding under a second-order schedule in rhesus monkeys

Effective treatment of opioid dependence with methadone and of tobacco dependence with nicotine illustrates the potential usefulness of agonist medications for drug abuse treatment. The monoamine-releaser d-amphetamine is one drug under consideration as an agonist pharmacotherapy for cocaine dependence. To assess the concordance between preclinical evaluations and ongoing clinical trials, the present study examined the effects of chronic treatment with saline or d-amphetamine on cocaine- and food-maintained responding in rhesus monkeys. Cocaine injections and food pellets were available under a second-order schedule during alternating daily sessions of cocaine and food availability. d-Amphetamine (0.01-0.1 mg/kg per h i.v. for 7 consecutive days) dose-dependently decreased self-administration of a unit dose of cocaine (0.01 mg/kg per injection) at the peak of the cocaine self-administration dose-effect curve. d-Amphetamine (0.032-0.1 mg/kg per h for 7 days) also decreased self-administration of a broad range of cocaine doses (0.0032-0.1 mg/kg per injection) and produced rightward and downward shifts in the cocaine dose-effect curve. Food-maintained responding was usually decreased less than cocaine self-administration, and few signs of toxicity were noted. To evaluate the effects of a longer treatment regimen, d-amphetamine (0.1 mg/kg per h) was administered for 28 consecutive days. d-Amphetamine nearly eliminated self-administration of cocaine (0.01 mg/kg per injection) throughout this treatment, whereas food-maintained responding returned to baseline levels after approximately 9 days. These preclinical findings are concordant with recent clinical studies and suggest that chronic d-amphetamine may selectively decrease cocaine-taking behavior in rhesus monkeys, possibly by producing a selective decrease in the reinforcing effects of cocaine.

Characterizing tolerance to trichloroethylene (TCE): effects of repeated inhalation of TCE on performance of a signal detection task in rats

Previous work showed that rats develop tolerance to the acute behavioral effects of trichloroethylene (TCE) on signal detection if they inhale TCE while performing the task and that this tolerance depends more upon learning than upon changes in metabolism of TCE. The present study sought to characterize this tolerance by assessing signal detection in rats during three phases of TCE exposures. Tolerance was induced in Phase 1 (daily 1-h test sessions concurrent with TCE exposure), extinguished in Phase 2 (daily tests in air with intermittent probe tests in TCE), and reinduced in Phase 3. Original induction in Phase 1 required 2 weeks, whereas reinduction in Phase 3 required less than 1 week. Tolerance persisted for 2 (accuracy) or 8 weeks [response time] in Phase 2 and was resistant to changes in test conditions in Phase 3. The slow induction, gradual extinction, savings during reinduction and lack of disruption from altered test conditions suggest mediation by instrumental learning processes. These data and most other evidence for behavioral tolerance to solvents can be explained by solvent-induced loss of reinforcement.

Behavioral components of tolerance to repeated inhalation of trichloroethylene (TCE) in rats

The possibility that the acute neurotoxic effects of organic solvents change with repeated exposure will affect risk assessment of these pollutants. We observed previously that rats inhaling trichloroethylene (TCE) showed a progressive attenuation of impairment of signal detection behavior across several weeks of intermittent exposure, suggesting the development of tolerance. Here, we explored the development of tolerance to TCE during two weeks of daily exposures, and the degree to which learned behavioral modifications ("behavioral tolerance") could account for the effect. Adult Long-Evans rats were trained to perform a visual signal detection task (SDT) in which a press on one lever yielded food if a visual stimulus (a "signal") had occurred on that trial, and a press on a second lever produced food if no signal had been presented. In two experiments, with 2000 and 2400 ppm of TCE respectively, trained rats were divided into two groups (n = 8/group) with equivalent accuracy and then exposed to TCE in two-phase studies. In Phase 1, one group of rats received daily SDT tests paired with 70-min TCE exposures, followed by 70-min exposures to clean air after testing. The other group received daily SDT tests in clean air, followed by 70-min exposures to TCE (unpaired exposure and testing). All rats thus received the same number and daily sequence of exposures to TCE that differed only in the pairing with SDT testing. Both concentrations of TCE disrupted performance of the paired groups and this disruption abated over the 9 days of exposure. In Phase 2, the pairing of exposure and test conditions were reversed for the two groups. The groups that were shifted from unpaired to paired exposures (Unpaired-Paired groups) showed qualitatively similar patterns of deficit and recovery as did the rats whose tests were initially paired with TCE (Paired-Unpaired groups), indicating that task-specific learning was involved in the development of tolerance. Quantitative differences in the magnitude and duration of the effects of TCE in the two groups indicated that other factors, not specific to the SDT, also contributed to the development tolerance to TCE. Published by Elsevier Science Inc.

Loss of tolerance to amphetamine-induced hypophagia in rats: homeostatic readjustment vs. instrumental learning

According to the homeostatic model, the loss of tolerance to amphetamine-induced hypophagia requires a period of unrestricted feeding in the drug-free state, which transforms the compensatory response mediating tolerance ("hyperhunger") into a functional disturbance to homeostasis. In the absence of such a disturbance, tolerance should be retained. To test this prediction, rats tolerant to amphetamine's hypophagic effect were given a 4-week tolerance retention period during which milk intakes were restricted and deprivation levels held relatively constant. During this period the rats were assigned to one of the following drug treatment conditions: 1) saline injections both before and after daily milk tests (saline group); 2) saline injections before, and amphetamine injections after, daily milk tests (after group); 3) no injections and no milk tests (no-treatment group); or 4) amphetamine injections before, and saline injections after, milk tests (before group). Despite the restricted feeding regimen, both the saline and after groups lost tolerance. These results do not support the homeostatic model, but are consistent with the instrumental learning model, which views drinking milk in the undrugged state as analogous to receiving noncontingent reinforcement.

Tolerance and sensitization to the behavioral effects of cocaine in rats: relationship to benzodiazepine receptors

Tolerance and sensitization to the behavioral effects of cocaine were investigated in rats responding under a fixed-consecutive-number eight schedule of food reinforcement. The development of tolerance or sensitization was induced by delivering the drug either immediately before or after each behavioral session during chronic administration. Chronic cocaine administered before each session resulted in tolerance, as indicated by the shift to the right in the cocaine dose response curve. This tolerance was more likely to develop in the presence of an external discriminative stimulus. On the other hand, when cocaine was delivered after each session, the injections did not disrupt responding and sensitization or increased sensitivity rather than tolerance developed. This sensitization was more likely to occur when the external discriminative stimulus was not present. These data suggest that either tolerance or sensitization to the behavioral effects of cocaine can occur following the same number of chronic injections, with the effect dependent on the context under which the drug is delivered. Significant differences in benzodiazepine receptor binding measured autoradiographically using [3H]flumazenil were observed between rats that received cocaine before or after each session, suggesting that the development of tolerance and sensitization may be mediated through changes in benzodiazepine receptors in discrete brain regions.

Effects of postmethamphetamine treatment with restraint on ambulatory sensitization to methamphetamine in mice

The administration of methamphetamine (2 mg/kg SC) to mice was followed by acceleration of ambulation (locomotion) for 3 h, with the peak effect at 1/2-1 h. When mice were allowed free ambulation for 3 h in an activity cage of 20 cm in diameter after methamphetamine administration, repeated administrations of methamphetamine, four times at 3-day intervals, caused ambulatory sensitization; the count at the fourth administration being approximately 2.3 times as high as that at the first administration. Furthermore, the mice that were allowed ambulation in the activity cage during postmethamphetamine period of either 0-1/2, 0-1, 0-2, or 1/2-1 h (during the other periods, the mice were put in jars of 6 cm in diameter to restrict ambulation) demonstrated ambulatory sensitization to methamphetamine, and these levels were as same as that in the mice given methamphetamine with free ambulation for 3 h. However, the free ambulation during postmethamphetamine periods of 0-1/4 h and 1-3 h failed to produce strong sensitization to methamphetamine. These results suggest that a free ambulation for at least 1/2 h during the postmethamphetamine period of 0-1 h is important for induction of ambulatory sensitization to methamphetamine in mice.

Effects of interdose interval on ambulatory sensitization to methamphetamine, cocaine and morphine in mice

To determine the minimum interdose interval for induction of ambulatory sensitization to methamphetamine, cocaine and morphine, 3 sets of 5 groups of mice were treated with either methamphetamine (2 mg/kg s.c.), cocaine (10 mg/kg s.c.) or morphine (10 mg/kg s.c.) 3 times at intervals of 3, 6, 12, 24 or 48 h, and then 2 times at intervals of 3 days in all groups. During the first 3 administrations of both methamphetamine and cocaine, interdose intervals of 3-12 h did not produce a significant change in the ambulatory stimulation. However, 3 repeated administrations of morphine with interdose intervals of 3 and 6 h, but not 12 h, caused tolerance to the ambulatory stimulant effect. The administration of all drugs with interdose intervals of 24 and 48 h produced ambulatory sensitization. Furthermore, following the fourth and fifth administrations of each drug, all groups of mice demonstrated sensitization. These results indicate that an interdose interval of 24 h or longer is required for induction of ambulatory sensitization to methamphetamine, cocaine and morphine.

Effects of exposure regimen on changes in sensitivity to the effects of cocaine on schedule-controlled behavior in rhesus monkeys

It has been reported that conditions of drug exposure can influence changes in sensitivity to cocaine upon repeated administration. In the present experiment, the behavioral effects of prolonged exposure to continuous or intermittent infusion of cocaine were compared in rhesus monkeys responding under a multiple component fixed-ratio (FR) schedule of food presentation. In order to quantify changes in sensitivity to cocaine, cumulative dose-response functions for acute cocaine were determined using a multiple schedule comprised of six 10-minute components separated by time-out periods of 3 minutes. Initially, cocaine decreased responding in a dose-related manner. Continuous infusion of cocaine (4 mg/kg per day) for a period of 4 weeks resulted in a 2- to 4-fold shift to the right in the cocaine dose-response function, i.e., tolerance developed. In contrast, when the same daily dose of cocaine was injected intermittently (1.0 mg/kg per injection) four times/day to different monkeys, there was no change in the effects of cocaine on responding. The present results support the notion that the dosing regimen is an important determinant of changes in sensitivity to the behavioral effects of cocaine. Additionally, since sensitization failed to develop upon intermittent administration of cocaine, behavioral baseline may play a role in changes in sensitivity to cocaine.

Importance of post-drug environmental factors for induction of sensitization to the ambulation-increasing effects of methamphetamine and cocaine in mice

Mice given five repeated administrations of methamphetamine (MAP: 2 mg/kg SC) or cocaine (COC: 20 mg/kg SC) at 3-day intervals in a round tilting-type activity cage (20 cm in diameter) showed sensitization to the ambulation-increasing effect of each drug. The mean 3- or 2-h overall activity count at the fifth administration of MAP or COC, respectively, was 2.3-2.5 times higher than that at the first administration. Mice given MAP or COC 4 times in round spaces (15-30 cm in diameter), where the floor did not tilt, exhibited sensitization as strong as that demonstrated by mice given each drug in the activity cages, when the mice were given the fifth administration in the activity cages. In contrast, mice repeatedly given the drugs in spaces 4-9 cm in diameter never, and those in space 12 cm in diameter only partially, exhibited sensitization to MAP and COC. Furthermore, mice given MAP or COC 4 times in their home cages (25D x 20W x 15H cm, with ten mice in each cage) showed partial sensitization. Repeated administration of saline to mice in activity cages, in the spaces 4-30 cm in diameter, or in the home cages did not cause significant change in the sensitivity to either MAP or COC. These results suggest that repeated experience of the stimulant effect of drug and the resultant ambulation is required for induction of sensitization to MAP and COC in terms of ambulation in mice. It is also suggested that spaces larger than 12 cm in diameter, which correspond to 2-2.5 times as long as the body length without tail, and no interference from other mice are required for induction of strong sensitization to both MAP and COC.

Characteristics of behavioural stimulant effect of N-cyanomethylmethamphetamine, a main product of smoking methamphetamine mixed with tobacco: evaluation by ambulatory activity in mice

N-cyanomethylmethamphetamine is main product of smoking methamphetamine HCl (CAS code value: 537-46-2, molar mass: 185.70 g/mol) mixed with tobacco, and has a central stimulant action. The aims of this study were to assess in terms of ambulation in mice whether repeated administration of N-cyanomethylmethamphetamine HCl (molar mass: 211.70 g/mol) induced sensitization to its central stimulant effect, and whether there were cross-sensitization from N-cyanomethylmethamphetamine HCl to methamphetamine HCl, and from methamphetamine HCl to N-cyanomethylmethamphetamine HCl. N-cyanomethylmethamphetamine HCl 14.2 and 47.2 mumol/kg subcutaneously dose-dependently accelerated ambulation of mice. Five administrations at 3 day intervals caused sensitization, the activity counts in the fifth administration being approximately 2.6 and 1.6 times, higher respectively, than those in the first administration. Furthermore, the mice repeatedly given 14.2 and 47.2 mumol/kg N-cyanomethylmethamphetamine HCl exhibited cross-sensitization to methamphetamine HCl (10.8 mumol/kg subcutaneously). Whereas 1.4 and 4.7 mumol/kg N-cyanomethylmethamphetamine HCl did not induce significant acceleration of the ambulation throughout the 5 repeated administrations, nor cross-sensitization to methamphetamine HCl. On the other hand, the 5 repeated administrations of methamphetamine HCl (10.8 mumol/kg) also caused sensitization to its ambulation-increasing effect, and the activity count in the fifth administration attained approximately 2.0 times higher than that in the first administration. The mice given methamphetamine HCl demonstrated cross-sensitization to 1.42-14.2 mumol/kg N-cyanomethylmethamphetamine HCl. Haloperidol (CAS code value: 52-86-8, molar mass: 375.88 g/mol; 0.027-0.80 mumol/kg subcutaneously) dose-dependently reduced the ambulation-increasing effects of N-cyanomethylmethamphetamine HCl and methamphetamine HCl in the drug-naive, and N-cyanomethylmethamphetamine HCl- and methamphetamine HCl-sensitized mice. These dose-effect relationships were similar between groups. The present results suggest that N-cyanomethylmethamphetamine HCl has a behavioural stimulant effect almost similar to that of methamphetamine HCl.

Inhibitory effect of restraint on induction of behavioral sensitization to methamphetamine and cocaine in mice

Repeated intermittent (generally 3-day intervals) administrations of methamphetamine (MAP: 2 mg/kg, SC) or cocaine (COC: 20 mg/kg, SC) induced sensitization to the ambulation-increasing effect of individual drug in mice. The induction of MAP and COC sensitization was inhibited when restraint of the mouse (putting the mouse in a jar of 6 cm in diameter for 3 and 2 h after administration of MAP and COC, respectively) was started immediately after each drug administration. Furthermore, the induction of sensitizations to MAP and COC was significantly reduced when the restraint was started within 1/4 h and 1/6 h after the administration of MAP and COC, respectively, whereas the restraint starting thereafter did not affect the induction of sensitization. The three times repeated administrations of saline with or without restraint did not significantly change the sensitivities to MAP and COC. The ambulation-increasing effects of MAP and COC reached the peak at approximately 2/3 and 1/2 h, respectively, and persisted for 3 and 2 h after the administration. The present results suggest that, to completely induce sensitization to MAP and COC in terms of ambulation, the mice must freely move for at least half of the latency to their peak effects.

Dopamine D1 receptor antagonist SCH 23390 retards methamphetamine sensitization in both combined administration and early posttreatment schedules in mice

SCH 23390 [0.003-0.03 mg/kg, subcutaneously (SC)], a dopamine D1 receptor antagonist, dose-dependently inhibited the ambulation-stimulant effect of methamphetamine (MAP) (2 mg/kg, SC) in mice when two drugs were combined in repeated administrations at 3- to 4-day intervals, repeated five times. SCH 23390 (0.03 mg/kg), which was sufficient to abolish the acute effect of MAP completely throughout the repeated administrations, significantly inhibited the induction of MAP sensitization. Moreover, when the mice were posttreated with SCH 23390 (0.01 and 0.03 mg/kg) 3 h after each MAP administration, at which the ambulation-stimulant effect of MAP had almost disappeared, they showed a significant and dose-dependent retardation of the induction of MAP sensitization. However, the 24-h posttreatment with SCH 23390 had no such effect. The administration of SCH 23390 (0.003-0.03 mg/kg) alone in either the activity cage or the home cage, or saline in the activity cage with 3- or 24-h posttreatment with SCH 23390 (0.01 or 0.03 mg/kg) five times at 3- to 4-day intervals did not elicit any significant changes in MAP sensitivity. The present results indicate that an intense blockade of dopamine D1 receptors in the acute or subacute period after MAP administration causes retardation of MAP sensitization by means of ambulation in mice.

Tolerance to the anticonvulsant and ataxic effects of pentobarbital: effect of an ascending-dose regimen

We assessed the effect of an ascending-dose regimen on the development of tolerance to the anticonvulsant and ataxic effects of pentobarbital in four groups of amygdala-kindled rats. Each rat received 20 bidaily (one every 48 h) trials in which an intraperitoneal (IP) pentobarbital or vehicle injection was delivered 1 h before a convulsive amygdala stimulation. On each trial, the rats in the three pentobarbital groups received either a high dose (50 mg/kg), a low dose (10mg/kg), or ascending doses of pentobarbital that began at 10 mg/kg and increased to as high as 26 mg/kg by 1 mg/kg increments as tolerance developed to its anticonvulsant effect; the rats in the vehicle group received saline. The rats in the ascending-dose condition displayed significantly more tolerance to the anticonvulsant effect of pentobarbital than did the other rats; in contrast, the high-dose rats displayed more tolerance to the ataxic effect of pentobarbital than did the other rats. These findings extend previous reports of the facilitatory effect of ascending-dose regimens on the development of tolerance to the anticonvulsant effect of benzodiazepines, and show that the facilitatory effect of ascending-dose regimens does not extend to all drug effects.

Contingent drug tolerance: differential tolerance to the anticonvulsant, hypothermic, and ataxic effects of ethanol

The kindled-convulsion model of epilepsy was used to study contingent tolerance to ethanol's (1.5 g/kg; IP) anticonvulsant, hypothermic, and ataxic effects in adult male rats. In the present experiments, three groups of amygdala-kindled rats received a series of bidaily (one every 48 h) convulsive stimulations: one group received ethanol 1 h before each stimulation; one group received ethanol 1 h after each stimulation; and another group served as the saline control. Tolerance to ethanol's anticonvulsant effect (Experiments 1 and 2) was greatest in those rats that received ethanol before each convulsive stimulation; whereas, tolerance to ethanol's hypothermic (Experiments 1 and 2) and ataxic (Experiments 2) effects developed in both groups that received ethanol. These results were predicted on the basis of the drug-effect theory of drug tolerance: the theory that functional drug tolerance is an adaptation to the disruptive effects of drugs on concurrent patterns of neural activity, not to drug exposure per se.

Modification of cocaine sensitization by dopamine D1 and D2 receptor antagonists in terms of ambulation in mice

The progressive enhancement in the ambulation increase caused by the repeated five-time dosings of cocaine (10 mg/kg SC) at 3- to 4-day intervals was dose dependently reduced by simultaneous administration with the selective dopamine D1 and D2 receptor antagonists, SCH 23390; R(+)-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (0.01, 0.03, and 0.1 mg/kg SC) and YM-09151-2 (nemonapride); cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4- methylaminobenzamide (0.01, 0.03, and 0.1 mg/kg SC), respectively. However, the mice given cocaine with SCH 23390 (0.03 mg/kg) or YM-09151-2 (0.03 and 0.1 mg/kg) demonstrated significantly higher sensitivity than the mice given cocaine alone to challenge cocaine. Both 2-h and 24-h posttreatments with SCH 23390 (0.01-0.1 mg/kg) after each cocaine administration, at which the acute stimulant effect of cocaine had disappeared, significantly and dose dependently enhanced the cocaine sensitization. In contrast, 2-h, but not 24-h, posttreatment with YM-09151-2 (0.01-0.1 mg/kg), slightly retarded the induction of cocaine sensitization. The present results suggest that the blockade of dopamine D1 receptors is responsible for a significant enhancement in the cocaine sensitization, independent of the timings of its administration, whereas the blockade of dopamine D2 receptors elicits time-dependent alterations in the cocaine sensitization, a strong enhancement in the simultaneous administration schedule, but a slight retardation in the early posttreatment schedule.

Contrasting effects on methamphetamine sensitization of ceruletide, a cholecystokinin-like decapeptide, and haloperidol

Ceruletide, a cholecystokinin-like decapeptide, and haloperidol show neuroleptic actions through inhibition of dopamine release and blockade of dopamine receptors, respectively. In this study, the effects of both drugs on methamphetamine sensitization were assessed by means of ambulation in mice. The enhancement in ambulation increase caused by five repeated administrations of methamphetamine (2 mg/kg, SC) at 3- to 4-day intervals was dose-dependently reduced when it was administered simultaneously with ceruletide (0.01-0.1 mg/kg, SC) or haloperidol (0.03-0.3 mg/kg, SC). However, only haloperidol could inhibit the induction of methamphetamine sensitization as assessed by challenge with methamphetamine alone. Post-treatment with ceruletide (0.03 mg/kg) 3 h after each methamphetamine accelerated, whereas such post-treatment with ceruletide (0.1 mg/kg) or haloperidol (0.03-0.3 mg/kg) delayed, the induction of methamphetamine sensitization. On the other hand, mice given five pretreatments with ceruletide (0.01-0.1 mg/kg) or haloperidol (0.03-0.3 mg/kg) at 3- to 4-day intervals did not exhibit any significant change in the sensitivity to methamphetamine. The present results suggest that, in contrast to the dose-dependent inhibition of methamphetamine sensitization in the simultaneous administration and post-treatment schedules, although both drugs can antagonize the acute stimulant effect of methamphetamine.

Inhibition of methamphetamine sensitization by post-methamphetamine treatment with SCH 23390 or haloperidol

Methamphetamine (2 mg/kg SC) increased ambulation in mice for about 3 h, with a peak effect at around 40 min after the administration, and its repeated administration induced sensitization. Both SCH 23390 (0.03 mg/kg SC) and haloperidol (0.4 mg/kg SC), dopamine D1 and D2 receptor antagonists, respectively, completely inhibited not only the acute stimulant effect of methamphetamine but also its sensitization when repeated methamphetamine was repeatedly combined with either of these drugs. Moreover, treatment with SCH 23390 2-5 h or haloperidol 1-5 h after each methamphetamine administration significantly antagonized methamphetamine sensitization. The maximal inhibitory effect was observed in the schedules of 3-h post-methamphetamine treatment for both drugs. However, treatments with SCH 23390 or haloperidol at 0.5 h, 6 h and 24 h after methamphetamine had no such inhibitory effect. The mice treated with SCH 23390 or haloperidol after each saline administration (the control administration for methamphetamine) did not show significant change in the sensitivity to methamphetamine. These results suggest that methamphetamine has an effect on both dopamine D1 and D2 receptors for several hours even after cessation of its acute stimulant effect, and that such an effect is involved in the induction of sensitization to the stimulant effect of methamphetamine on ambulation in mice.

Dissipation of contingent tolerance to the anticonvulsant effect of diazepam: effect of the criterion response

The effect of convulsive stimulations on the dissipation of tolerance to the anticonvulsant effect of diazepam was investigated using the kindled-convulsion model. Amygdala-kindled rats were rendered tolerant to diazepam's anticonvulsant effect by 25 "bidaily" (one/48 h) diazepam injections (2.5 mg/kg), each followed 1 h later by a convulsive stimulation. They were then divided into nine groups for the tolerance-dissipation phase of the experiment. Of the nine groups, three received bidaily control handling for one trial, three trials, or seven trials; three received bidaily saline injections, each 1 h before a convulsive stimulation, for one, three, or seven trials; and three received bidaily diazepam injections, each 1 h after a convulsive stimulation, for one, three, or seven trials. Finally, each rat received a tolerance-retention test (i.e., a diazepam injection followed 1 h later by a convulsive stimulation) 48 h after its last tolerance-dissipation trial. The tolerance dissipated gradually but completely over the 4-, 8-, and 16-day test intervals in the rats that received a convulsive stimulation before each injection during the tolerance-dissipation phase, whether they were injected with saline or diazepam; in contrast, tolerance did not dissipate in the rats that received saline injections but no stimulations. Remarkably, the discontinuance of the bidaily diazepam injections, even for 16 days, was not sufficient to dissipate the tolerance that had developed to diazepam's anticonvulsant effect; nor was the continuation of the bidaily diazepam injections sufficient to keep tolerance from dissipating.(ABSTRACT TRUNCATED AT 250 WORDS)

Can posttreatment with the selective dopamine D2 antagonist, YM-09151-2, inhibit induction of methamphetamine sensitization? Evaluation by ambulatory activity in mice

Effects of YM-09151-2; cis-N-(1-benzyl-2-methyl pyrrolidin-3-yl)-5-chloro-2-methoxy-4-methylaminobenzamide (YM), a potent and selective dopamine D2 antagonist, on sensitization to methamphetamine (MAP) were investigated by means of ambulatory activity in mice. YM (0.003-0.03 mg/kg SC) reduced not only the acute ambulation-increasing effect of MAP (2 mg/kg SC) but also the induction of MAP sensitization when it was simultaneously administered with MAP in the repeated administration at 3-4 day intervals. Moreover, the post 3-h treatment with YM (0.01 and 0.03 mg/kg) following each MAP administration, at which time the acute ambulation-increasing effect of MAP almost disappeared, significantly and dose dependently inhibited the induction of MAP sensitization. The post 24-h treatment with YM did not show such effect. The present results suggest that blockade of the dopamine D2 receptors during postearly period following MAP administration is responsible for protecting the induction of MAP sensitization by means of ambulatory activity in mice.

Early post-treatment with haloperidol retards induction of methamphetamine sensitization in mice

The repeated administration of methamphetamine (2 mg/kg s.c.) at 3- to 4-day intervals induced sensitization to its ambulation-increasing effect in mice. When the simultaneous administration of methamphetamine with haloperidol (0.025, 0.1 and 0.4 mg/kg s.c.) was carried out, the acute ambulation-increasing effect as well as the development of sensitization were haloperidol-dose-dependently inhibited. Moreover, treatment with haloperidol (0.1 and 0.4 mg/kg) 3 h following each methamphetamine administration, which per se did not affect the acute ambulation increase caused by methamphetamine, could protect against the induction of methamphetamine sensitization in a dose-dependent manner, whereas treatment with haloperidol after 24 h did not show such protective action. The present results suggest that blockade of the dopamine receptors at an early period following each administration of methamphetamine may be responsible for the delay in development of methamphetamine sensitization as expressed by ambulatory activity of mice.

Effects of dopamine antagonism on methamphetamine sensitization: evaluation by ambulatory activity in mice

SCH 23390 (SCH: 0.001-0.03 mg/kg SC) and YM-09151-2 (YM: 0.001-0.03 mg/kg SC), the selective dopamine D1 and D2 antagonists, respectively, reduced dose-dependently the ambulation-increasing effect of methamphetamine (MAP: 2 mg/kg SC) in mice. The sensitization to MAP was inhibited when it was administered in combination with SCH (0.003-0.03 mg/kg) or YM (0.003-0.03 mg/kg) in the repeated administration regimen. The inhibitory action of YM on the MAP sensitization was more prominent than that of SCH. However, the repeated treatment with either SCH or YM could not ameliorate the established MAP sensitization. Rather, the repeated treatment with the highest dose of YM (0.03 mg/kg) increased the MAP sensitivity in both the MAP-sensitized and drug-naive mice. SCH had no such action. The present results suggest that the dopamine D2 receptors are more intimately involved than the dopamine D1 receptors in the increased sensitivity to MAP induced by the repeated treatment with MAP itself, behavioral sensitization, or dopamine antagonists, denervation supersensitivity.

Dopamine antagonists can inhibit methamphetamine sensitization, but not cocaine sensitization, when assessed by ambulatory activity in mice

The repeated subcutaneous administration of methamphetamine (2 mg kg-1) and cocaine (10 mg kg-1) at 3-4 day intervals induced sensitization to their ambulation-increasing effects in mice. Subcutaneous administration of SCH 23390 (R-(+)-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; 0.003-0.03 mg kg-1) and YM-09151-2 (cis-N-(1-benzyl-2-methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4- methylaminobenzamide; 0.003-0.03 mg kg-1), the selective dopamine D1 and D2 antagonists, respectively, reduced dose-dependently the acute ambulation-increasing effect of methamphetamine. The development of methamphetamine sensitization was inhibited when it was administered in combination with either SCH 23390 or YM-09151-2 in the repeated administration schedule. Although SCH 23390 (0.01-0.1 mg kg-1) and YM-09151-2 (0.01-0.1 mg kg-1) also reduced the ambulation-increasing effect of cocaine (10 mg kg-1), neither drug inhibited the cocaine sensitization. Mice given cocaine with SCH 23390 (0.03 mg kg-1) or YM-09151-2 (0.03 and 0.1 mg kg-1) showed higher sensitivity than those given cocaine alone. The present results suggest that, although both the dopamine D1 and D2 antagonists reduce the acute stimulant effects of both methamphetamine and cocaine, they are only effective for inhibition of the methamphetamine sensitization. Mechanisms other than the dopaminergic system appear to be involved in the cocaine sensitization.

Effects of haloperidol on the methamphetamine sensitization: assessment by ambulatory activity in mice

Haloperidol (HPD: 0.0063, 0.025, 0.1 and 0.4 mg/kg, s.c.) reduced dose-dependently the ambulation-increasing effect of methamphetamine (MAP: 2 mg/kg, s.c.) in mice. The repeated administration of MAP elicited a sensitization to its ambulation-increasing effect, and the development of sensitization was inhibited when MAP was administered in combination with HPD in the repeated administration schedule. However, any dose of HPD could not ameliorate the established MAP sensitization. Whereas, the mice experienced the repeated treatment with HPD 0.4 mg/kg showed an increase in the MAP sensitivity. The present results suggest that HPD, at comparatively higher doses, produces a denervation supersensitivity of postsynaptic dopamine receptors, and shows protective action on the development of MAP sensitization. However, it is also suggested that the established MAP sensitization is irreversible even after the treatment with HPD.

Cocaine levels in striatum and nucleus accumbens: augmentation following challenge injection in rats withdrawn from repeated cocaine administration

There is general agreement that the brain level of cocaine achieved in response to an intraperitoneal (i.p.) cocaine challenge is higher in animals that have been treated repeatedly with cocaine. However, whether this change in drug disposition persists following withdrawal from repeated treatment is controversial. Rats received either single or repeated injections of cocaine (10 mg/kg, i.p.), and levels of cocaine were measured in dorsal striatum and nucleus accumbens 20 min after the last injection. Cocaine levels were 60-70% higher in both brain regions of rats that had been treated with cocaine once daily for 7 days compared to rats that had received only a single injection. On the 7th day of withdrawal, similar elevations were observed after a challenge injection of cocaine in animals that had been treated repeatedly with cocaine but not in animals that had been treated repeatedly with saline. This persistent pharmacokinetic change should be taken into account in studies of cocaine-induced behavioral sensitization.

Effects of variation in chronic dose of cocaine on contingent tolerance as assessed in a milk-drinking task

Tolerance to the suppressive effects of cocaine on milk drinking by rats was studied using a contingent tolerance experimental design. Three separate groups (n = 6) of rats received 8.0, 16.0, or 32.0 mg/kg cocaine daily 15 min before a 15-min period of access to sweetened condensed milk for 20 days. Three additional groups of six rats each received the same chronic doses 15 min after access to milk. Milk, water, and food intake as well as body weight were measured daily. Tolerance effects were assessed by comparing initial acute dose-effect determinations with a probe dose-effect redetermination in which all rats again received doses of cocaine pre-session after having experienced the differential pre- or post-session chronic treatment. Behavioral tolerance on the milk intake measure was observed for the 8.0 mg/kg and 16.0 mg/kg doses, but not for the 32.0 mg/kg chronic treatment, even though the latter group exhibited evidence of tolerance in the water intake measure. Chronic treatment with 8.0 and 16.0 mg/kg produced different outcomes in that chronic exposure to 16.0 mg/kg in the presence of milk resulted in generalization of tolerance to both a lower (8.0 mg/kg) and a higher dose (32.0 mg/kg), but the group receiving 8.0 mg/kg did not exhibit generalization of tolerance to higher doses. Modest sensitization effects were observed in the rats treated post-session with either 8.0 or 16.0 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of the prophagic but not of the antidipsogenic effect of U-50, 488H after chronic amphetamine

Two groups of rats were treated with seven daily injections of either saline or d-amphetamine (3 mg/kg IP). On the 2 days following the last injection, rats were tested according to a counterbalanced experimental design, each animal receiving, immediately prior to the beginning of the dark phase, saline on one day and the highly selective kappa-opioid agonist trans- +/- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate [U-50,488H (U50)] on the other. A microcomputer-controlled data acquisition system was used for the structural analysis of the feeding and drinking responses to amphetamine and U50. U50 enhanced feeding and depressed drinking in the first hour. The increased food intake was probably the result of the effect of U50 on the development of satiation and duration of satiety. Chronic amphetamine potentiated the prophagic effect but not the antidipsogenic effect of U50. The structural analysis demonstrated that the characteristics of the prophagic effect of U50 were amplified but not changed.

Bidirectional contingent cross tolerance between the anticonvulsant effects of pentobarbital and ethanol

In Experiment 1, two groups of kindled rats received a pentobarbital injection (15 mg/kg, IP) and a convulsive amygdala stimulation once every 48 h. In one group, pentobarbital was injected 1 h before each stimulation; in the other, it was injected 1 h after each stimulation. Only the rats that received pentobarbital before each stimulation became tolerant to pentobarbital's anticonvulsant effect. Cross tolerance to the anticonvulsant effect of ethanol (1.5 g/kg, IP) was also found to be greater in the pentobarbital-before-stimulation rats. Experiment 2 was designed to assess the transfer of tolerance in the opposite direction, that is, from ethanol to pentobarbital, and the results mirrored those of Experiment 1: convulsive stimulation during the periods of ethanol exposure facilitated the development of tolerance to the anticonvulsant effect of ethanol and its transfer to pentobarbital. These results support the theory that functional drug tolerance and cross tolerance are adaptations to the effects of drugs on concurrent patterns of neural activity rather than to drug exposure per se.

Contingent tolerance to the disruptive effects of alcohol on the copulatory behavior of male rats

Sexually active male rats received five 30-min copulation tests with sexually receptive females, one every 4 days. One group of rats received alcohol (1 g/kg, IP) 45 min before, and an equivalent volume of saline 45 min after, each test; a second group received saline before and alcohol after each test; and a third, control group received saline both before and after. Four days after the last of the five tolerance-development trials, each rat received an injection of alcohol (1 g/kg, IP) 45 min before a copulation test so that the development of tolerance in the three groups could be compared. Tolerance to the disruptive effects of alcohol on mount, intromission, and ejaculation latencies, and on the duration of the postejaculatory interval was found to be significantly greater in the rats injected with alcohol before each copulation test than it was in the rats in the other two groups. These results constitute the first experimental evidence that tolerance develops to the disruptive effects of alcohol on male sexual behavior, and they support the theory that tolerance is an adaptive response to the disruptive effects of drugs on concurrent patterns of neural activity, rather than to drug exposure per se.

Contingent tolerance to the anticonvulsant effects of carbamazepine, diazepam, and sodium valproate in kindled rats

The effect of convulsive stimulation during periods of drug exposure on the development of tolerance to the anticonvulsant effects of carbamazepine (CBZ), diazepam (DZP), or sodium valproate (VPA) was studied in three similar experiments. In each experiment, amygdala-kindled rats were assigned to one of three groups: one group received a drug injection (CBZ, 70 mg/kg, IP; DZP, 2 mg/kg, IP; VPA, 250 mg/kg, gavage) 1 h before each of a series of 10 bidaily (one every 48 h) convulsive stimulations, a second group received the same dose of the drug 1 h after each of the 10 stimulations, and a third group served as a vehicle control. The drug tolerance test occurred in each experiment 48 h after the 10th tolerance-development trial; every rat received the appropriate dose of CBZ, DZP, or VPA 1 h before being stimulated. In each experiment, only the rats from the drug-before-stimulation group displayed a significant amount of tolerance to the drug's anticonvulsant effect. Thus the development of tolerance to the anticonvulsant effects of CBZ, DZP, and VPA was not an inevitable consequence of drug exposure; the development of tolerance was contingent upon the occurrence of convulsive stimulation during the periods of drug exposure. These results support the idea that functional drug tolerance is an adaptation to a drug's effects on ongoing patterns of neural activity, rather than to drug exposure per se.

Cellular and learned tolerance to pentobarbital ataxia

Pentobarbital was administered to 4 groups of rats: 1) intermittently before testing on the rotarod (RR) (experienced, EXP), 2) chronically (CHR) before testing on the RR (EXP), 3) intermittently (INT) after being tested on the RR (NONEXP), and 4) chronically (CHR) after being tested on the RR (NONEXP). On postchronic testing, Group 1 (INT/EXP) failed to show tolerance to the RR decrement, related to prechronic scores, while Group 3 (INT/NONEXP) actually showed an enhanced RR decrement. Group 2 (CHR/EXP) and Group 4 (CHR/NONEXP) both exhibited prominent tolerance to RR impairment at the postchronic test, with a nonsignificant trend for greater tolerance in Group 2. The lack of an expressed behavioral tolerance in INT/EXP rats and the enhanced RR decrement in INT/NONEXP subjects at the postchronic test was attributed to repeated use of a towel wrap restraint during the chronic treatment period. When the prechronic tests for INT/EXP animals were separated into the first 3 and last 3 days, pentobarbital impairment of RR during days 4-6 was significantly less than during days 1-3. This tolerance in INT/EXP rats was lost at the postchronic testing, while INT/NONEXP subjects had by then developed an enhanced RR impairment to pentobarbital. Following postchronic testing, chronic pentobarbital (CHR/EXP and CHR/NONEXP groups) and chronic vehicle (INT/EXP and INT/NONEXP groups) were discontinued for 9 days (withdrawal), after which an intermediate dose of the drug was tested on RR performance. Next, 9 days of extinction training involved vehicle injection daily before testing RR performance, after which the intermediate drug dose was again tested. INT/EXP and INT/NONEXP groups showed no change in RR impairment at the postwithdrawal and postextinction tests. However, in CHR/EXP rats pentobarbital tolerance was partly lost at the postwithdrawal test, with a significantly greater loss at the postextinction test.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic systemic administration of amphetamine increases food intake to morphine, but not to U50-488H, microinjected into the ventral tegmental area in rats

Chronic intermittent administration of amphetamine (AMPH) sensitizes rats to the stimulatory effects on feeding produced by systemic injections of either morphine (MORPH) or the kappa-opiate receptor agonist, U50,488H (U50). Both morphine and the putative kappa-receptor endogenous ligand, dynorphin, have been reported to stimulate feeding when administered into the ventral tegmental area (VTA). To evaluate whether the VTA is the site where AMPH produces sensitization to the feeding effects of opiates, rats were given daily IP injections of either saline or AMPH (3 mg/kg). The amount of powdered food ingested during the 5 h following the injections was measured. After 9 days of AMPH or saline administration, twice weekly tests were begun of the effects of either saline, MORPH (1-10 nmol) or U50 (10 pmol to 10 nmol) injected into the VTA; AMPH administration was continued on intervening days. MORPH produced a statistically significant greater increase in food intake in rats chronically treated with AMPH than in saline treated rats. No statistically significant effects were produced by U50. However, when U50 was administered systemically to the same animals, food intake increased, and the effect was significantly greater in the AMPH-pretreated group. Thus the sensitization to the feeding effects of MORPH and U50 produced by chronic AMPH administration appears to involve different systems; the mesolimbic dopamine system appears to mediate sensitization to the effects of the predominately mu-receptor agonist, MORPH, but not of the kappa-receptor agonist, U50.

Effects of fixed-ratio requirement on observed tolerance to decreased responding by clonidine

Responding of rats was maintained under either a 10- or a 40-response fixed-ratio schedule, and "local" rates of responding were 0.29-0.37 responses per sec for both schedules. Clonidine decreased responding for both schedules in a similar and dose-dependent manner, and the largest dose tested (0.3 mg/kg) completely suppressed behavior. When 0.1 mg/kg was administered immediately prior to 30 daily experimental sessions, FR10 responding recovered to control levels within 15 sessions, whereas FR40 responding recovered only to approximately 60% of control level at asymptote. These results continue to identify boundary conditions for the influence of reinforcer loss on tolerance development, and they emphasize the overriding influence of behavioral processes on observed tolerance to the behavioral effects of drugs.

The long-term effects of diazepam and pentylenetetrazol on the potentiated startle response

Stress desensitization is observed as a decrease in the disruptive effects of a stressor on behavior when the organism is repeatedly exposed to the stressor. For instrumental behavior, stress desensitization was also reported for rats preexposed to anxiogenic drugs; stress sensitization was reported for rats preexposed to an anxiolytic compound. The present study investigated whether similar effects are found in a noninstrumental task situation. First, rats received 12 daily injections of pentylenetetrazol (PTZ, 20 mg/kg, IP), diazepam (DZP, 5 mg/kg, IP) or saline. After each injection the effect of the drugs on the acoustic startle reflex was measured. No drugs were given during the remainder of the experiment. Eight days later rats were given 5 days of Pavlovian fear conditioning to establish a light as a shock signal. During the next 3 days, potentiation of the startle response by the light was measured. None of the drug treatments had an effect on startle potentiation, indicating that stress sensitivity is not affected by previous administration of PTZ and DZP in a noninstrumental task. An explanation for the different effects found for instrumental and noninstrumental tasks is suggested.

Haloperidol-induced catalepsy is modulated by the development of tolerance to amphetamine-induced anorexia

The effect of the development of tolerance to amphetamine-anorexia on both amphetamine-induced and haloperidol-induced motor effects was investigated. The animals in experiment 1 showed an acute anorexic reaction to 3 mg/kg amphetamine, whereas the rats in experiment 2 failed to meet the criterion level of acute anorexia. During initial training rats received 13 injections of saline or 3 mg/kg amphetamine intraperitoneally (i.p.) every other day. In both experiments, for one group each amphetamine injection was followed 20 min later by 30 min access to milk (CONT groups). In a second group, each amphetamine injection was followed 24 h later by 30 min access to milk (NONCON groups) and a third group received only saline injections and milk (controls). As expected, in experiment 1 originally anorexic animals in the CONT group developed tolerance, whereas the NONCON and control groups displayed no tolerance to amphetamine-anorexia. The NONCON group showed sig nificantly more stereotypy than either the CONT or control group. Furthermore, following 1.25 mg/kg haloperidol the CONT animals were less cataleptic than the NONCON and control groups which did not differ. In experiment 2, at the end of training the rats in all groups displayed no anorexia following amphetamine injection; they consumed an amount of milk equivalent to that normally consumed under no-drug conditions. Neither was there a difference in the amount of catalepsy between groups following injection of 1.25 mg/kg haloperidol.