Effects of Intravenous Serotonin on the Excitability of Spinomesencephalic Neurons

Chung, Byung Sun; Jeong, Han Seong; Park, Jong Seong; Park, Sah Hoon

Abstract

The present study was carried out to examine the effect of intravenous serotonin on the excitability of spinomesencephalic tract(SMT) neuron and to investigate the functional role of vagal afferents in this effect. Extracellular single unit recordings were made from spinal neurons antidromically activated by stimulation of midbrain. Recording sites were in upper cervical(C1-C3, n=41) and lumbar(L3-L6, n=19) spinal cord. Intravenous serotonin(5-100 microg/kg) changed the spontaneous firing rates of upper cervical(21/41) and lumbar(9/19) SMT neurons. Three different responses of SMT neurons to i.v. serotonin were observed: 1) excitation, 2) inhibition, and 3) biphasic responses. Distributions of stimulation and recording sites were not significantly different between serotonin-responsive and nonresponsive groups and/or between excitatory and inhibitory response groups. Excitatory responses were observed in 11/21 of cervical and 2/9 of lumbar SMT neurons. Inhibitory responses were observed 2/21 of cervical and 6/9 lumbar SMT neurons. SMT responses to i.v. serotonin were similar to those to electrical stimulation of cervical vagus nerve, and decreased or abolished by severing bilateral cervical vagi. Cardiac vagal afferents revealed the facilitated activity by i.v. serotonin. These result suggest that i.v. serotonin could modulate the excitability of SMT neurons partly through the activation of cardiac vagal afferent fibers.