Pre- and postsynaptic distribution of mu, delta 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 mu, delta and kappa 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 mu, delta and kappa 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 mu, delta and kappa 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.