Modulation of reflex pressor response to contraction and effect on substance P release by spinal 5-HT1A receptors

This study investigated whether activation of serotonin1A [5-hydroxytryptamine (5-HT)1A] receptors in the dorsal horn of the spinal cord attenuates the reflex pressor response to static contraction and passive muscle stretch. In addition, we determined if the attenuation of the response to contraction is mediated by inhibiting substance P (SP) release in the dorsal horn. Static contractions of the triceps surae muscle of chloralose-anesthetized cats were induced by stimulating the cut L7 and S1 ventral roots. Microdialysis (10 mM) of a selective 5-HT1A agonist [8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT)] into the L7 dorsal horn region produced a reversible attenuation of the reflex pressor response to a 1-min contraction (in mmHg: control = 36 +/- 3; 8-OH-DPAT = 17 +/- 3; recovery = 31 +/- 8; P = 0.013; n = 6) or passive stretch (in mmHg: control = 36 +/- 6; 8-OH-DPAT = 15 +/- 2; recovery = 32 +/- 6; P = 0.002; n = 6). However, a 5-HT1B agonist, 1-[3-(trifluoromethyl)-phenyl]piperazine, had no effect on the reflex pressor response. During 5-min contractions (n = 8), 8-OH-DPAT (10 mM) also blunted the pressor response but had no effect on the levels of SP-like immunoreactivity (in fmol/100 microliters: control = 0.492 +/- 0.026; 8-OH-DPAT = 0.501 +/- 0.034). These results suggest that activation of 5-HT1A receptors in the dorsal horn attenuates the reflex pressor response to contraction through a mechanism other than inhibition of SP release.