Micromolar substance P reduces spinal receptor binding for thyrotropin-releasing hormone — possible relevance to neuropeptide coexistence?

Abstract

Receptors for thyrotropin-releasing hormone (TRH) on membranes of rat spinal cord (SC) were labeled with [ 3H](3-Me-His 2)TRH ([ 3H]MeTRH) (dissociation constant = 3.6 ± 0.8 (6) nM). Substance P (SP) caused a concentration-dependent inhibition of TRH receptor binding in the micromolar range (43 ± 4 (6)% at 50 μM). Scatchard analyses of competition data revealed that 50 μM SP reduced TRH receptor number (40–66%, P < 0.05) with little or no effect on affinity. SP appeared more potent in reducing [ 3H]MeTRH binding to ventral cord membranes than those of dorsal or whole SC. A number of SP analogs also reduced TRH receptor binding in a dose-related manner but with different potencies. In contrast, various amino acids and serotonin (250 μM) produced little or no inhibition of [ 3H]MeTRH binding, and cholecystokinin, Leu- and Met-enkephalins, angiotensin II, LH-RH, bombesin and somatostatin were also markedly less potent than SP. Although it is unclear whether spinal TRH receptors are ever exposed to micromolar concentrations of SP in vivo, the reported colocalization of these neuropeptides in raphe efferents to the SC suggests that our findings may be of physiological relevance.