A study of intracerebroventricular self-administration of leucine or methionine enkephalin by rats in response to intermittent electric shocks

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Abstract
Rats were taught to self-administer Leu-E (leucine-enkephalin) (10, 25 and 100 .mu.g/.mu.l) or Met-E (methionine-enkephalin) (0.5, 10 and 100 .mu.g/.mu.l) through a cannula implanted in the lateral cerebral ventricle (i.c.v.). Their self-injection behavior was studied before, during and the after nociceptive stimulation. In the course of the control period of the experiment, the rats rapidly learned lever pressing for self-injection of enkephalin but they did not increase their self-administration of Leu-E or Met-E during the nociceptive electrical stimulation period. Also, studied were the acute effect of i.c.v. enkephalin and morphine on tail-flick latency (s) and electrical threshold vocalization (mA). The analgesic effect of Leu-E and Met-E was of short duration (less than 2-6 min). The mean rise (i.e., analgesia) of the tail-flick threshold showed a significant difference after i.c.v. Met-E only. The acute i.c.v. effect of 20 or 30 .mu.g of morphine induced a long-lasting analgesia, > 40 min. These results show that Leu-E and Met-E are not rewarding during a nociceptive stimulus. This may be due to the short and inconstant analgesic action of i.c.v. enkephalins.