or Medical Research Volume 27. No. 3, pp. 291-297, 1996 Printed in Mexico Original Article - Influence of 5Hydro::ytryptamine on the Pressor Responses Induced by Norepinephrine and Electrical Sympathetic Stimulation in the Pithed Rat 1 JOSE A. TERRON,* ARACELI SANCHEZ-LOPEZ,* EDUARDO RAMIREZ- SAN JUAN,** CARLOS CASTILLO,*** and CARLOS M. VILLALON* * Seccidn de TerapPdca fipeririten~al. Depar-rarneriro de Fnrrnacologia y Toxicologia, CINVEgA V, I.P.N., Mixico. D.F. ** Deparlanrenro de Fisiologia, Esc~reln Nacionol tle Ciencim Bioldgicm clel I.P.N. Mhxico. D.F. *** Deparmn~enro de Fisiologia y Farn~acologia. Escuela Slrpenor de Meclicinn del I.P.N., MP.rico, D.F. Received for publication July 19, 1995; accepted March 5, 1996 (95193). Abstract Serotonin (5-hydroxytryptamine; 5- HT) modifies contrast, 5 - HT significantly and dose-dependently the responses to several vasoconstrictor stimuli inhibited the increases in diastolic blood pressure - prejunctionally and/orpostjunctionally.Thepresent but not those in heart rate - produced by stimulation study analyzed the efTects of 5- HT on the pressor of the appropriate spinal segments. These effects of responses induced by norepinephrine (NE) or 5- HT were more pronounced on the responses to electrical sympathetic stinlulation in pithed rats. lower frequencies of stimulation. It is suggested that Responses to intravenous (i.v.) NE (0.03 - 3pgkg) or 5 - HT inhibits the electrically induced pressor electrical stinlulation at increasing frequencies responses by a prejunctional mechanism which (0.1 - 3 Hz) were evaluated before and during would lead toa reductionofneurotransn~itter release continuous i.v. infusions of physiological saline (0.01 from the sympathetic nerves supplying the systemic mVmin) or 5 - HT (1 - 10 pgkg x min). The effects of vasculature. The selective stimulation of this 5 - HT on the tachycardic responses to NE and inhibitory mechanism might represent a new sympathetic stimulation were studied in parallel. approach for the development of novel The increases in diastolic blood pressure and heart antihypertensiveagents.(Arcl~ Med Res1996;27:291) rate produced by NE were not modified by 5-tlT. In KEY WORDS: Blood pressure; Electrical sympathetic stimulation; 5Hydroxytryptamine;Prejunctional inhibition; Rat. Introduction responses produced by norepinephrine (NE) and electrical stimulation of sympathetic nerves in many experimental It has long been recognized that serotonin (5- setups. Thus, amplification has been reported for the hydroxytryptarnine; 5 - HT) modifies the vasoconstrictor former (1- 9) and a~nplification andlor inhibition for the Correspondence to: latter (9- 15). Dr. JosC A. Terr6n. Secci6n de Teraphtica Experimental, Depto. de The fact that the effects of 5-HT* which Farmacologia y Toxicologia, CINVESTAV. I.P.N. Apdo. Postal al~~occurwithnon-adrenc~gicagon~stssuchash~shne, 22026, 14000 MCxico, D.F. FAX: (525) 675-9168. angiotensin 11. prostaglandin FZa and endothelin- 1 1SupportedbyagrantfromConsejoNacionaldeCiencinyTecnologia, (3,6,16), are observed with concentrations of 5-HT Mtxico. similar to those found in human plasma (4,17), and that