| Abstract No.: | B-C2106 |
| Country: | Canada |
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| Title: | GENERAL LOCOMOTION, WHEEL RUNNING AND MORPHINE IN MALE RATS
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| Authors/Affiliations: | 1 Ali Gheidi*; 1 Clint Inkster; 1 Roelof Eikelboom;
1 Wilfrid Lauier University, Waterloo, ON, Canada
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| Content: | Objectives: Given ad lib wheel access, rats will voluntarily begin running at low levels and steadily increase their running to thousands of wheel turns per day (Looy & Eikelboom, 1989). Rats will lever press to gain access to wheels (Iversen, 1993), and prefer an environment paired with the aftereffect of wheel running (Lett, et al, 2002). Wheel running has thus been postulated as an animal model of non-drug addiction (Lattanzio and Eikelboom, 2000). In the acquisition phase of drug self administration (SA), individual differences in consumption are observed with low doses of stimulants (Piazza, et al, 1989). These differences in consumption are positively correlated to motor responses in a novel environment. Thus the first aim of the present study was to determine if individual differences in novel environment responsiveness predicted subsequent levels of wheel running. The opioid system has been implicated in wheel running (Lett, et al, 2001), raising the possibility that repeated activation of endorphins by wheel running may induce opiate tolerance. Thus, our second aim was to determine if a history of wheel running alters morphine’s effect on locomotor activity.
Materials and Methods. Experiment 1. 36 male Sprague Dawley rats were exposed to a novel environment for 2 hours and locomotion recorded. The next day, 24 rats were given ad lib access to Nalgene running wheels for 24 days, while 12 remained in their home cages. On the final days of wheel exposure, rats were taken from the wheel or home cages, injected with either saline or 10 mg/kg i.p. morphine (counterbalanced over two days) and exposed to the same open field for 2 hours. Experiment 2. 32 rats were treated in the same way as in the first experiment, except 8 of the rats served as home cage controls while 24 had wheel access. Unlike the first experiment, after the final day of wheel exposure all rats were placed in their home cages for 7 days before morphine/saline testing.
Results. While in both experiments there was considerable variability in wheel running and locomotion, the locomotion in a novel environment did not predict initial wheel running, maintenance running, or how long it took animals to increase their running to half of their maximum level. However, in both experiments animals that had prior wheel access showed a smaller morphine induced hypoactivity in the first 30 min in the open field compared to non-wheel controls. The degree of morphine tolerance was not correlated to the amount of running behaviour.
Conclusions. Unlike with drug SA, locomotor activity in a novel environment does not predict wheel running behaviour. However, wheel running seems to activate endorphins, as animals with a prior wheel history show a decrease in morphine induced locomotor suppression.
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