PRESYNAPTIC AND POSTSYNAPTIC DISTRIBUTION OF MU-OPIOID, DELTA-OPIOID AND KAPPA-OPIOID RECEPTORS IN THE SUPERFICIAL LAYERS OF THE CERVICAL DORSAL HORN OF THE RAT SPINAL-CORD

Highly selective tritiated ligands and quantitative autoradiography have been used to study μ, δ and κ binding sites in the dorsal horn of the rat spinal cord. We have measured the proportions of the 3 main types of opioid binding sites in the superficial layers of the cervical dorsal horn (laminae I and II). The proportions of μ, δ and κ sites were 70 ± 4%, 23 ± 2% and 7 ± 1%, respectively, over the whole C4-T2 extent. Similar percentages were encountered at the level of each individual segment from C4 to T2. Eight days after a unilateral dorsal rhizotomy C4-T2, dramatic decreases were seen on the ipsilateral side to the lesion by comparison to the intact side. In the C7 segment, these decreases were 76 ± 1%, 61 ± 1% and 53 ± 3% for μ, δ and κ binding sites, respectively. The C7 segment can be considered as completely deafferented, so we attribute the residual values to postsynaptic binding whereas the decrease can be attributed to a loss of the presynaptic sites. These results are discussed with respect to the contribution of pre- and postsynaptic depressive effects of opiates on the transmission of noxious messages at the level of the dorsal horn. © 1990.