Effects of cervical spinal hemisection on dihydromorphine binding in brainstem and spinal cord in cat

Cats were sacrificed 1–3 weeks after cervical (C 1-C 2) hemisection and receptor binding experiments were carried out with 4.0 and 0.6 nM concentrations of [ 3H]dihydromorphine ([ 3H]DHM); these concentrations were shown by Scatchard analysis to represent the approximate K d values of high and low affinity dihydromorphine binding sites in brain homogenates. Unilateral cervical hemisection produced significant (P < 0.05), bilateral, reductions in the levels of [ 3H]DHM binding in the periaqueductal gray (PAG; 35–40%) and mesencephalic reticular formation (MRF; 47–51%), medial pons (40–56%) and medial medulla (30–37%). In paramedial pons and medulla, numerical reductions in [ 3H]DHM binding were observed (18 and 28%) which did not achieve statistical significance. In spinal cord, significant reductions were observed in the dorsal (45%) and ventral (29%) ipsilateral but not contralateral quadrants. We believe that these results in the brainstem and spinal cord reflect in part the loss of opiate binding on spinobulbar terminals and bulbospinal terminals, respectively, following orthograde degeneration. These observations support the hypothesis that the analgetic effects of opiates in the brainstem may in part be mediated by the direct inhibition of transmission through spinobulbar terminals.