Involvement of endogenous enkephalins in the mouse 'behavioral despair' test

In the mouse 'behavioral despair' test the immobility time was shortened by [D-Ala2,Met5]enkephalin at doses which did not modify locomotor activity. Similarly, the inhibitors of the enzymes degrading enkephalins, thiorphan and/or bestatin were effective. Their effect was antagonized by naloxone. We conclude that endogenous enkephalins are implicated in the 'behavioral despair' test.