In a previous study, we showed that analgesia induced by swimming stress (i.e. swimming in cold water, ١٧ C, for ١٠ minutes) is related to opium and non – opium – relevant mechanisms. The aim of our experiment is to study the mechanisms of analgesia (time for tail withdrawal test) stimulated by less stressful swimming (swimming in cold water, ١٧C, for ٤ minutes) in order to achieve an additional experimental evidence concerning the effect of adrenal cortex hormones induced by stress on this mechanism, and to discover some aspects of the effect mechanism in Wistar rats. Rats of control group showed, when being exposed to swimming stress,an analgesic feeling decreasing significantly and progressively until it disappears in the seventh day. However, control rats, which were exposed to morphine tolerance, showed no significant change in the threshold of pain feeling when exposed to related stress. On the other hand, no significant difference was reported in the progress of morphine tolerance stimulation (١٠mg/kg/day) in both control and identical groups regarding the number of injections and the average effect of morphine. In comparison with control rats, adrenalectomaized rats, when exposed to swimming stress,show analgesic feeling. This was reflected on the progress of the response, which was measured by latent time reduction of tail withdrawal to start value after ١٤ days of relevant stress. On the other hand, rats which had their adrenal glands removed and which were exposed to morphine tolerance stimulation, did not show significant change in the threshold of pain feeling when being exposed to swimming stress. In comparison with control rats, significant difference was reported in the progress of morphine tolerance stimulation among rats with removed adrenal glands. While a significant rise was reported in morphine doses and the average effect of morphine in adrenalectomized rats, a clear decrease in those doses and in the average effect of morphine was obtained among rats which had their adrenal glands removed and which are familiar with swimming stress. However, adrenalectomized rats, and those which were injected with adrenal gland extract (the extract of ٤٤,٦ mg tissue/kg/day) showed, when being exposed to swimming stress, similar response to that reported by control rats with respect to the progress of analgesic feeling or with respect to the progress of morphine tolerance stimulation, taking into account a relative significant decrease in the values of the latent time for tail withdrawal reported in both responses. These results show that analgesia induced by swimming stress occurs according to a response related to the decrease in the release of intrinsic opium. This decrease may be due to the blocking action of adrenal cortex hormones induced by stress on the release of the intrinsic opium, which may occur at glandular and nervous levels related to sedative organs.
Key Words: Swimming Stress – Analgesia – Morphine tolerance – Adrenal cortex hormones – Sedation