Dramatic increase in lever-pressing activity in rats after training on the progressive ratio schedule of cocaine self-administration

Differential Effect of Fixed Ratio Magnitude on the Rate of Lever-Pressing and Interinjection Intervals of Cocaine Self-Administration in Rats

Background

Many features of self-administration behavior may be explained by reference to the properties of schedules of reinforcement. Schedules alter the probability of a behavior being reinforced and thereby increase, or decrease, the frequency of the behavior and fixed ratio (FR) magnitude reportedly alters the rate of responding to cocaine. A pharmacokinetic/pharmacodynamic interaction theory states that lever-pressing behavior is induced only when cocaine levels in the body are above the priming/remission threshold and below the satiety threshold-a range termed the compulsion zone. This theory successfully explains cocaine self-administration in rats on a progressive ratio and the FR1 schedule.

Objectives

To determine the effects of high FR magnitude on the rate of self-administration of cocaine and the rate of lever-pressing behavior when cocaine levels are within the compulsion zone.

Methods

Rats acquired cocaine self-administration on an FR1 schedule and then were switched to sessions that started with FR1 and then FR 5, 10, 20, or 50. An only FR1 session was run each week between FR1/FR50 sessions and then only FR1 sessions were conducted for several weeks.

Results

Interinjection intervals at a unit dose of 3 µmol/kg were regular at both FR1 and FR50 but were longer by the time required to complete the 50 presses. When responding by rats was maintained under an FR50 schedule of cocaine presentations, compared to baseline FR1 sessions, dramatic increases in the number of lever-presses were observed after access to cocaine was terminated, a previously unreported finding. However, lever-pressing occurred only when cocaine levels were in the compulsion zone, and this duration was unchanged. The increase in lever-pressing persisted for weeks. Interinjection intervals at FR1 were not altered after exposure to FR50.

Conclusions

Although previously considered key to understanding the regulation of cocaine self-administration behavior, FR magnitude simply increased interinjection intervals by the time required to complete 50 lever-presses. The dramatic increase in the rate of lever-pressing was caused by the high FR schedule rather than cocaine. The utility of the schedule-induced increase in the rate of lever-pressing is unclear. The compulsion zone theory provides a rational pharmacological basis for understanding cocaine self-administration behavior.

Effects of isolation housing stress and mouse strain on intravenous cocaine self-administration, sensory stimulus self-administration, and reward preference

Sensory stimuli are natural rewards in mice and humans. Consequently, preference for a drug reward relative to a sensory reward may be an endophenotype of addiction vulnerability. In this study, we developed a novel behavioral assay to quantify the preference for intravenous drug self-administration relative to sensory stimulus self-administration. We used founder strains of the BXD recombinant inbred mouse panel (C57BL/6J, DBA/2J) and a model of stress (isolation vs enriched housing) to assess genetic and epigenetic effects. Following 10 weeks of differential housing, all mice were tested under three reward conditions: sensory rewards available, cocaine rewards available, both rewards available. When a single reward was available (sensory stimuli or cocaine; delivered using distinct levers), DBA/2J mice self-administered significantly more rewards than C57BL/6J mice. When both rewards were available, DBA/2J mice exhibited a significant preference for cocaine relative to sensory stimuli; in contrast, C57BL/6J mice exhibited no preference. Housing condition influenced sensory stimulus self-administration and strain-dependently influenced inactive lever pressing when both rewards were available. Collectively, these data reveal strain effects, housing effects, or both on reward self-administration and preference. Most importantly, this study reveals that genetic mechanisms underlying preference for a drug reward relative to a nondrug reward can be dissected using the full BXD panel.

The progressive ratio and fixed ratio 1 schedules of cocaine self-administration in rats convey the same information

Progressive ratio (PR) schedules of drug delivery are used to determine the 'motivational' state of an animal and drug 'reinforcing efficacy'. This widely held interpretation is supported mainly by the observation that the PR breakpoint (BP) is proportional to the unit dose of self-administered drug. The compulsion zone theory of cocaine self-administration was applied to determine whether it can explain the pattern of lever-pressing behavior and cocaine injections under the PR schedule in rats. This theory states that cocaine induces lever pressing when levels are below the satiety threshold and above the priming/remission threshold. Rats were trained to self-administer cocaine on a fixed ratio FR1 schedule over a range of cocaine unit doses. Then they were switched to a PR schedule. Typical for the self-administration under a PR schedule, long post-injection pauses occurred when calculated cocaine levels were in the satiety zone. The compulsion zone theory interprets BP simply as the maximal number of responses which rats can perform after an injection while cocaine levels remain within the compulsion zone. The thresholds delineating the compulsion zone were very stable and independent of the self-administration schedule. PR and fixed ratio schedules convey the same pharmacokinetic/pharmacodynamic information, i.e., these two schedules are invariant.