Intracerebroventricular self-injection of beta-endorphin by rats in response to intermittent electrical shocks

Rats were taught to self-administer beta-endorphin (0.1 microgram or 0.5 microgram/microliter) or NaCl 9% by pressing a lever that activated a pump connected with a cannula implanted in the lateral cerebral ventricle (icv). The pump delivered 1 microliter at each lever pressing. After a training period of 2 weeks, the self-injection behaviour was studied before, during and after nociceptive stimulation. In response to a nociceptive stimulation, 4 rats increased their self-injection of beta-endorphin at 0.5 microgram/microliter per injection. This effect is specific for beta-endorphin since under identical conditions 6 rats did not increase the injection of isotonic NaCl saline solution. Another 6 rats did not increase their self-injection during nociceptive electrical stimulation and the post-nociceptive period when the dose of beta-endorphin was 0.1 microgram/microliter per injection. However these animals self-administered significantly higher levels of beta-endorphin during the pre-control period. Also studied were the acute effects of 10 micrograms of icv beta-endorphin on tail-flick latency in sec. The acute administration of 10 micrograms of beta-endorphin induced a long-lasting analgesia. The results show that the rats increased beta-endorphin self-administration during the pre-control period when the dose was 0.1 microgram/microliter and during the nociceptive stimulation period when the dose was 0.5 microgram/microliter. In the former case the self-administration had the profile of a voluntary doping drug intake, in the latter case the profile was that of an antalgic self-medication.