Blockade of endogenous opiates reduces activity in the rat.

Naloxone (2 mg/kg, SC) was found to result in a substantial and significant reduction in general activity levels in the rat (90--120 days old). This effect was seen both under baseline conditions and after stress manipulations which would be expected to result in elevated levels of endogenous opiate peptides. Thus, under baseline conditions general activity was reduced to less than half of the saline control value thirty min after injection. Similarly, a reduction was seen after stress induced by a 30 min swim. While naloxone may have some non-opiate effects, these results support the view that endogenous opiate systems may play an important activational role in behavioral regulation, under baseline conditions and conditions of stress.