C1 adrenergic neurons are contacted by presynaptic profiles containing DELTA-opioid receptor immunoreactivity

Ligands of the δ-opioid receptor tonically influence sympathetic outflow. Some of the actions of δ-opioid receptor agonists may be mediated through C1 adrenergic neurons in the rostral ventrolateral medulla. The goal of this study was to determine whether C1 adrenergic neurons or their afferents contain δ-opioid receptors. Single sections through the rostral ventrolateral medulla were labeled for δ-opioid receptor using the immunoperoxidase method and the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT) using the immunogold method, and examined at the light and electron microscopic level. Few (∼5% of 903) profiles dually labeled for PNMT and δ-opioid receptor were detected; most of these were dendrites with diameters 50% of 250) of δ-opioid receptor-immunoreactive axons and axon terminals contacted PNMT-immunoreactive profiles. Most of the contacts formed by δ-opioid receptor-immunoreactive profiles (∼75% of 132) were on single-labeled PNMT-immunoreactive dendrites with diameters