Combined opiate/adrenergic receptor blockade enhances squirrel monkey vocalization.

This study provides evidence for the interaction between opiate and noradrenergic neuronal systems on primate vocal behavior. The rate of twitters produced by adult squirrel monkeys was increased in an additive or synergistic manner by the combined administration of the alpha 2 adrenergic antagonist yohimbine and the opiate antagonist naloxone. Similar effects were not demonstrated on the isolation call. In addition, the drug combination led to an increase of autonomic symptoms. The anatomical localization of these findings and mechanism for the production of increased twitters requires further investigation.