Naloxone prevents the facilitatory effect upon retention induced by adrenaline administration in rats

The possible involvement of endogenous opioid peptides in the development of the facilitatory effect of adrenaline on memory has been investigated. For this purpose post-training administration of adrenaline and/or naloxone was carried out in rats tested in an inhibitory avoidance paradigm and subjected or not to pre-training (extensive familiarization with the training situation prior to the acquisition trial). Adrenaline injected subcutaneously in a dose of 500 micrograms/kg facilitated retention performance in rats both subjected or not to pre-training. Naloxone administered SC (400 micrograms/rat) did not influence retention behaviour in rats subjected or not to pre-training, nor did ICV (0.80 ng/rat) administration. Interestingly, the opiate antagonist when injected SC (400 micrograms/rat) prevented the facilitatory effect exerted by adrenaline in pretrained as well as in not pre-trained rats. However, ICV administration of naloxone (0.80 microgram/rat) dit not influence the behavioral effects exerted by the bioamine. These data suggested a role of endogenous opioid peptides on the facilitatory effect of adrenaline on memory, possibly independent of novelty factors and thus of the brain beta-endorphin system. In addition, our results point to the periphery as the most likely site for such interaction.