mu-Opioid receptors often colocalize with the substance P receptor (NK1) in the trigeminal dorsal horn

Substance P (SP) is a peptide that is present in unmyelinated primary affer ents to the dorsal horn and is released in response to painful or noxious s timuli. Opiates active at the mu-opiate receptor (MOR) produce antinocicept ion, in part, through modulation of responses to SP. MOR ligands may either inhibit the release of SP or reduce the excitatory responses of second-ord er neurons to SP. We examined potential functional sites for interactions b etween SP and MOR with dual electron microscopic immunocytochemical localiz ation of the SP receptor (NK1) and MOR in rat trigeminal dorsal horn. We al so examined the relationship between SP-containing profiles and NK1-bearing profiles. We found that 56% of SP-immunoreactive terminals contact NK1 den drites, whereas 34% of NK1-immunoreactive dendrites receive SP afferents. T his result indicates that there is not a significant mismatch between sites of SP release and available NK1 receptors, although receptive neurons may contain receptors at sites distant from the peptide release site. With rega rd to opioid receptors, we found that many MOR-immunoreactive dendrites als o contain NK1 (32%), whereas a smaller proportion of NK1-immunoreactive den drites contain MOR (17%). Few NK1 dendrites (2%) were contacted by MOR-immu noreactive afferents. These results provide the first direct evidence that MORs are on the same neurons as NK1 receptors, suggesting that MOR ligands directly modulate SP-induced nociceptive responses primarily at postsynapti c sites, rather than through inhibition of SP release from primary afferent s. This colocalization of NK1 and MORs has significant implications for the development of pain therapies targeted at these nociceptive neurons.