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 <1.5 μm. δ-Opioid receptor immunoreactivity was affiliated with multivesicular bodies in dually labeled perikarya, whereas δ-opioid receptor immunoperoxidase labeling appeared as isolated clusters within both singly and dually labeled dendrites. The majority (∼83% of 338) of δ-opioid receptor-immunoreactive profiles were axons and axon terminals. δ-Opioid receptor-immunoreactive terminals averaged 0.75 μm in diameter, contained numerous large dense-core vesicles and usually formed appositions or asymmetric (excitatory-type) synapses with their targets. The majority (>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 <1.5 μm. The prominent localization of δ-opioid receptors to dense-core vesicle-rich presynaptic profiles suggests that δ-opioid receptor activation by endogenous or exogenous agonists may modulate neuropeptide release. Furthermore, the presence of δ-opioid receptors on axon terminals that form excitatory-type synapses with PNMT-immunoreactive dendrites suggests that δ-opioid receptor ligands may modulate afferent activity to C1 adrenergic neurons. The observation that some PNMT-immunoreactive neurons contain δ-opioid receptor immunoreactivity associated with multivesicular bodies and other intracellular organelles suggests that some C1 adrenergic neurons may present, endocytose and/or recycle δ-opioid receptors.