DELTA-OPIOID RECEPTOR IMMUNOREACTIVITY - DISTRIBUTION IN BRAIN-STEM AND SPINAL GORD, AND RELATIONSHIP TO BIOGENIC-AMINES AND ENKEPHALIN

We have recently developed antisera which recognize epitopes of the cl oned delta-opioid receptor (DOR; Dado et al., 1993). In the present re port we have further characterized these antisera, and raised addition al antisera in rats. We used these antisera to determine the distribut ion of DOR-like immunoreactivity (-LI) in rat spinal cord and brainste m in relation to serotoninergic, noradrenergic, and enkephalinergic ne urons. We found DOR-LI in fibers and varicosities distributed througho ut the spinal cord gray matter, with highest densities in the superfic ial dorsal horn, in autonomic regions, around the central canal as wel l as in the ventral horn motor nuclei. In the brainstem a dense innerv ation of DOR-immunoreactive (-IR) fibers was found in several nuclei s uch as spinal trigeminal nuclei, midline raphe nuclei, parabrachial nu clei, periaqueductal gray matter (FAG), interpeduncular nucleus, and s ubstantia nigra. A group of DOR-positive cells was seen in the laterod orsal tegmental nucleus. In addition, a few DOR-IR cell bodies were de monstrated in the parabrachial nuclei, interpeduncular nucleus, FAG, a nd superior and inferior colliculi as well as around the central canal in the spinal cord. All DOR-positive cells showed a punctuate stainin g pattern within the cytoplasm of the cell body and in primary dendrit es. No plasma membrane staining of cells or dendrites could be demonst rated using the DOR antisera. Double-labeling experiments for DOR and 5-hydroxytryptamine (5HT, serotonin) revealed that some 5HT-IR neurons in the raphe complex were surrounded by DOR-IR fibers. In the spinal cord a high degree of coexistence was found between DOR and 5HT in ner ve fibers and varicosities in the neuropil around the motoneurons and in lamina V of the dorsal horn. In autonomic regions of the spinal cor d, a low degree of colocalization was seen between DOR and 5HT; in the superficial dorsal horn no coexistence was found. Tyrosine hydroxylas e (TH)-positive neurons in the brainstem (in the A5 area, locus coerul eus, and A7 area) were apposed by DOR-positive fibers. However, no coe xistence could be seen between DOR and TH in any part of the spinal co rd. A close relation, but no coexistence, was observed between DOR- an d enkephalin (ENK)-IR fibers in the spinal cord ventral horn; in the i ntermediolateral nucleus a low degree of colocalization was observed. Thus, a delta-opioid receptor may affect the activity of descending se rotoninergic and noradrenergic neurons by means of modulating the rele ase of neurotransmitters from afferents to these neurons. In addition, delta-opioid receptors appear poised to modulate the release of 5HT ( and coexisting peptides) from fibers in the spinal cord ventral horn a nd lamina V. However, these receptors appear unlikely to be importantl y involved in modulation of the release of catecholamines or enkephali n in the spinal cord, or in modulation of the release of 5HT in the su perficial dorsal horn. Finally, these data suggest that the antisera u sed preferentially localize a delta-opioid receptor which is targeted to the axonal compartment of neurons, and thus this receptor most like ly functions at a presynaptic site.