Characterization of Multiple [³H]5–Hydroxytryptamine Binding Sites in Rat Spinal Cord Tissue

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.

Meta‐chlorophenylpiperazine attenuates formalin‐induced nociceptive responses through 5‐HT1/2 receptors in both normal and diabetic mice

“…Although Fasmer et al (1986) showed that 8-OH-DPAT 1 mg kg-' s.c. (8-hydroxy-2-(di-n-propylamino)-tetralin; a 5-HTIA receptor agonist) elicited hypoalgesia in the acute phase of the formalin test, a non-specific effect might be responsible for the weak activity of 8-OH-DPAT compared with its strong affinity to the 5-HTlA receptor (Middlemiss & Fozard, 1983). Whereas it has been demonstrated that intrathecal administration of m-CPP (selective for 5-HTlB receptors over 5-HTlA receptors, Murphy & Zemlan, 1990) induces antinociception (Eide et al, 1990;Eide, 1992), and oral administration of the drug dose-dependently inhibited both phases of the formalin-induced nociceptive response, several lines of evidence suggest that there are multiple 5-HT1 binding sites in the spinal cord, and about 35% are specific for 5-HTIB, but the density of 5-HT2 receptors is very low in the spinal cord (Leysen et al, 1982;Monroe & Smith, 1983;Marlier et al, 1991). 5-HT2 binding sites have been demonstrated in the cortex (Hoyer et al, 1986;Molineaux et al, 1989), therefore activation of spinal 5-HTlB receptors and supraspinal 5-HT2 receptors might be involved in the antinociception.…”

Cardiovascular effects of intrathecal administration of substance P in the rat: interactions with serotonergic mechanisms

“…5-hydroxytryptamine binding sites primarily of the 5-HTl-type are clearly present on preganglionic sympathetic neurons in the spinal cord (Monroe and Smith, 1983;Leysen etal., 1982;Mitchell and Riley, 1985). Serotonin by an activation of a serotonin receptor located on the preganglionic sympathetic neurones can induce a blood pressor response.…”

Pharmacological Studies on Lappaconitine: Possible Interaction with Endogenous Noradrenergic and Serotonergic Pathways to Induce Antinociception.

“…Currently available data suggest that the primary type of 5-HT receptor in the rat spi nal cord is the 5-HT1 receptor. No measurable specific binding of the 5-HT2-selective anta gonist 3H-ketanserin was observed in the spi nal cord (24,25). Recently, Zemlan et al (26) have reported that 5-HT1 and 5-HT2 receptors are located in the spinal cord and cortex, re spectively, and that these receptors participate in the increase or inhibition of pain and supra spinal narcotic analgesia.…”