Characterization of multiple [3H]5-hydroxytryptamine binding sites in rat spinal cord tissue.

Abstract

Abstract: High‐affinity [3H]5‐hydroxytryptamine ([3H]5‐HT) binding in the rat spinal cord is similar to that demonstrated in the frontal cortex. [3H]5‐HT binds with nearly the same affinity to sites in both tissues. Furthermore, similar patterns of displacement of [3H]5–HT were seen in both tissues, with either spiperone or LSD as the unlabeled ligand. This high‐affinity binding appears to be to multiple sites, since displacement studies using 2 nM [3H]5–HT result in Hill coefficients less than unity for spiperone, LSD, and quipazine [Hill coefficients (nH): 0.44, 0.39, 0.40, respectively]. These sites apparently have an equal affinity for [3H]5‐HT, since unlabeled 5‐HT did not discriminate between them. Thus, the high‐affinity [3H]5‐HT binding in the spinal cord may be analogous to that observed in the frontal cortex, where two populations of sites have previously been described (5‐HTIA, 5‐HTIB). In addition to the multiple high‐affinity spinal cord binding sites, a low‐affinity [3H]5‐HT binding component was also identified. A curvilinear Scatchard plot results from saturation studies using [3H]5‐HT (0.5–100 nM) in the spinal cord. The plot can be resolved into sites having apparent dissociation constants of 1.4 nM and 57.8 nM for the high‐and low‐affinity components, respectively. Additional support for a change in affinity characteristics at higher radioligand concentrations comes from the displacement of 30 nM [3H]5‐HT by the unlabeled ligand. A nonparallel shift in the dissociation curve was seen, resulting in a Hill coefficient less than unity (0.32). None of the specifically bound [3H]5‐HT in the spinal cord is associated with the 5‐HT uptake carrier, since fluoxetine, an inhibitor of 5‐HT uptake, does not alter binding characteristics. In addition, a 5‐HT binding site analogous to the site designated 5‐HT, was not apparent in the spinal cord. Ketanse‐rin and cyproheptadine, drugs that are highly selective for 5‐HT, sites, did not displace [3H]5‐HT from spinal tissue, and [3H]spiperone, a radioligand that binds with high affinity to 5‐HT2 sites, did not exhibit saturable binding in the tissue. Thus, the 5‐HT2 binding site reported in other regions of the central nervous system, and the serotonin uptake carrier do not appear to contribute to the multiple binding sites demonstrated in the spinal cord.