Tolerance to ethanol's effects on operant performance in rats: role of number and pattern of intoxicated practice opportunities

Acute tolerance to rate-decreasing effects of single doses of ethanol

Acute tolerance occurs when behavioral impairment is greater at a given blood ethanol concentration (BAC) on the ascending versus descending limb of the BAC-time curve following administration of a single dose of ethanol, however studies utilizing learned behaviors have not been widely reported. We assessed acute tolerance to single doses of ethanol in five Lewis rats responding under a fixed-ratio (FR8) schedule of food presentation. Response rates for food during 1-min components (ending 2, 4, 11, 18, 33, and 57 min after ethanol administration) were determined, and BAC was measured immediately after each component using a rat breathalyzer. Ethanol (0.4, 0.6, 0.8, and 1.2 g/kg, i.p.) produced dose-related decreases in responding for food that tended to recover over time for all but the highest dose tested. Similarly, dose-related increases in BAC were also observed. Using either an analysis that expressed impairment per unit BAC on the ascending limb versus the descending limb (by assessing the area under the curve (AUC) for behavior and BAC on each limb), the slope of the function that relates the behavioral effect to BAC (each expressed as percent maximum effect), or a variant of the Mellanby method (hysteresis), acute tolerance was observed following a dose of 0.4 g/kg ethanol. Though behavior appeared to recover on the descending limb following higher doses (especially 0.6 and 0.8 g/kg), acute tolerance to these doses was not present.

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.

Effect of duration and pattern of chronic ethanol exposure on tolerance to the discriminative stimulus effects of ethanol in C57BL/6J mice

This study was conducted to examine whether amount and/or pattern (intermittent or continuous) of chronic ethanol exposure subsequently alters sensitivity to the discriminative stimulus effects of ethanol. Adult male C57BL/6J mice were trained to discriminate between 1.5 g/kg ethanol and saline in a two-lever food-reinforced operant procedure. Once ethanol discrimination was successfully acquired, generalization testing was conducted using a cumulative dosing procedure to generate a baseline dose-response function (0-2.5 g/kg ethanol). Discrimination training was then suspended while mice received chronic ethanol vapor or air exposure in inhalation chambers. The total amount of ethanol exposure was systematically increased, but it was delivered in an intermittent or continuous manner. At 24 or 16 h after inhalation treatment, ethanol discriminability was reassessed using the same generalization testing procedures. Results indicated that discrimination performance in control (air-exposed) mice was similar to baseline. However, sensitivity to the discriminative cue of ethanol following chronic ethanol treatment was reduced (as evidenced by rightward shifts in the dose-response functions and increased ED(50) values). The magnitude of this tolerance effect increased as a function of the number of chronic ethanol exposures as well as the total duration of ethanol exposure. In addition, tolerance was more robust when generalization testing was conducted earlier (16 versus 24 h) after chronic ethanol treatment was halted (2- to 3-fold increase in ED(50) values). These results may have important clinical implications, because blunted sensitivity to the discriminative cue of ethanol may contribute to enhanced ethanol self-administration behavior observed in these mice following similar chronic ethanol treatment.

Effects of MDMA exposure on the conditioned place preference produced by other drugs of abuse

RATIONALE

MDMA is a serotonergic neurotoxin but few pre-clinical studies have found long-term behavioural consequences. As human users of MDMA are polydrug users, it is important to investigate whether the behavioural effects of other drugs are modulated by prior exposure to MDMA.

OBJECTIVE

. This study investigated whether pretreatment with a multiple high dose regimen of MDMA altered the rewarding effects of other drugs of abuse.

METHODS

Adult male Lister Hooded rats ( n=10/group) were pretreated with 10 mg/kg MDMA or 1 ml/kg saline vehicle IP every 2 h for 6 h. Fourteen days later, conditioned place preference (CPP) to d-amphetamine (3 mg/kg), cocaine (20 mg/kg), ethanol (2.0 g/kg), heroin (0.5 mg/kg), or MDMA (10 mg/kg) was assessed.

RESULTS

In general, MDMA pretreatment had no effect on drug reward or habituation to the place conditioning apparatus. However, in contrast to saline pretreated rats, those animals receiving MDMA failed to show CPP after ethanol.

CONCLUSION

MDMA pretreatment reduced the rewarding properties of ethanol. This finding may represent a functional consequence of MDMA-induced neurotoxicity. By extrapolation, human users of MDMA may be exposed to an increase in risks associated with alcohol abuse.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

Peripheral opioid receptors may mediate a portion of the aversive and depressant effect of EtOH: CPP and locomotor activity

Previous investigators have reported that peripheral opioid receptors (located in the gut) may produce aversive effects when activated. Opioid receptors can be activated by endogenous opioids or by-products of ethanol (EtOH) metabolism [e.g., tetrahydroisoquinolines (TIQs)]; both are stimulated following EtOH consumption. Therefore, we tested the hypothesis that a portion of the aversive or depressant effects of EtOH may be mediated through, or modulated by, peripheral opioid receptors. Conditioned place preference (CPP) and locomotor activity were the dependent variables. Prior to EtOH gavage, we antagonized the peripheral opioid receptors with methylnaltrexone (MNTX), an opioid antagonist that does not easily pass through the blood-brain barrier. The effects of EtOH were found to be dose dependent: 1.5 g/kg was hedonically neutral but depressed locomotor activity; 2.25 g/kg EtOH was aversive and also depressed locomotor activity. MNTX (32 mg/kg) treatment was rewarding and stimulated motor activity (especially during the first conditioning session). When combined, 1.5 g/kg EtOH tended to enhance the rewarding effects of MNTX whereas MNTX blocked the aversive effects of 2.25 g/kg EtOH. During the first conditioning session EtOH attenuated the motor stimulant effects of MNTX whereas MNTX antagonized the motor depressant effects of EtOH; there was little effect of MNTX on EtOH-induced motor depression during subsequent conditioning sessions. Pretreatment with various doses of MNTX (0.1, 1.0, 10.0, 32.0 mg/kg) of rats receiving 1.5 g/kg EtOH indicated the effects of MNTX were dose dependent. Drug-induced locomotor activity and time spent in the conditioned compartment were positively correlated, suggesting that both behaviors were homologous. The data suggest that peripheral opioid receptors participate in mediating or modulating a portion of the behavioral effects of EtOH.

Appetitive operant behavior and free-feeding in rats exposed to acute stress

This study investigated whether appetitive operant food reinforcement or free-feeding behavior in rats, food-restricted to 85% of body weight, was disrupted by exposures to 3 h of restraint stress or by 3 h of restraint plus water immersion stress (RWI). Rats were trained under a 3-cycle 10-min time-out-10-min time-in fixed-ratio 15 (FR15) schedule of food reinforcement. Free-feeding was measured in a 3-cycle 10-min food jar-out-10-min food jar-in test conducted in the operant chambers. Three hours of restraint stress did not significantly affect response rate or food reinforcement in the operant FR15 task or in the free-feeding condition. In contrast, 3 h of RWI completely abolished operant food reinforcement and suppressed response rate, whereas free-feeding was significantly reduced but not abolished in food-restricted, hungry rats. We conclude that acute restraint stress in food-restricted, hungry rats does not affect their appetite or motor ability to lever-press for food nor did it affect their ability to feed in a free-feeding situation. In contrast, RWI stress may have induced a motor impairment, or some other aspect of motivation independent of hunger, that disrupted their performance in the operant FR15 task.

Influence of thiamine on the behavioral sensitivity to ethanol

Changes in sensitivity to ethanol's rate-decreasing effects on operant performance were examined in control rats and cohorts that received diet-induced or diet+pyrithiamine-induced thiamine deficiency. Seven groups of male Sprague-Dawley rats (12 rats/group) were trained in a 5-cycle lever-press operant task under a fixed-ratio 30 schedule of food reinforcement. Once trained to maintain consistent operant performance across all 5 cycles, each rat was tested with various doses of ethanol injected at the beginning of each time-out cycle. Each group of rats demonstrated equivalent saline baseline operant performance and ED50 for ethanol's rate-suppressing effects. Training sessions were suspended and rats received either a short- (9 days) or long-term (5-week) exposure to regular rat chow diet or thiamine-deficient diet, and received either saline or pyrithiamine injections in a 2 x 2 design. Three additional control groups were maintained on a regular rat chow diet and received supplemental injections of either thiamine+pyrithiamine injections, thiamine+saline injections, or saline+pyrithiamine injections. The controlled diet phase continued until the development of overt signs of thiamine deficiency, at which time thiamine supplements were administered for 4 days. In phase 3, all rats were retrained in the operant task and a second ethanol dose-effect function was generated. A history of thiamine deficiency and recovery failed to shift the behavioral dose-effect functions significantly for ethanol and their associated blood alcohol curves. Most interestingly, significant behavioral sensitization to ethanol's rate suppressant effects was demonstrated in the two control groups of rats receiving regular rat chow diet in combination with supplemental injections of thiamine and either saline or pyrithiamine.(ABSTRACT TRUNCATED AT 250 WORDS)