Evidence for an interaction of opioid and noradrenergic locus coeruleus systems in the regulation of environmental stimulus-directed behavior.

The roles of opioid and noradrenergic (NE) dorsal bundle systems in an animal's response to particular environmental stimuli were examined in a multicompartment exploration chamber. The opiate antagonist naloxone (NAL) produced a stereo-specific increase in environmental stimulus interaction, while the opiate agonist morphine induced a NAL-reversible reduction in stimulus contact. Thus, a specific opioid role in environmental stimulus-directed behavior is indicated. Several reports suggest inhibitory actions of opioids on NE locus coeruleus (LC) activity. Consistent with these reports, we observed that 6-hydroxydopamine (6-OHDA) lesions of the NE dorsal bundles of the LC produced a morphine-like reduction in stimulus contact. Naloxone, which increased stimulus interaction in sham-lesioned rats, had no effect in dorsal bundle lesioned animals. These findings indicate an interaction between opioid and LC systems in the regulation of environmental stimulus-directed behavior.