EFFECTS OF VAGAL AFFERENT NERVE-STIMULATION ON NOXIOUS HEAT-EVOKED FOS-LIKE IMMUNOREACTIVITY IN THE RAT LUMBAR SPINAL-CORD

Electrophysiological and behavioral studies have described modulation of nociception by vagal afferent fibers. The objectives of this study were to 1) use Fos-like immunoreactivity as a marker for neuronal acti vity to examine populations of neurons in the spinal cord that are act ivated by a noxious heat stimulus, 2) determine whether heat-evoked Fo s-like immunoreactivity can be modulated by vagal afferent stimulation , and 3) determine whether vagally-mediated effects on heat-evoked Fos -like immunoreactivity can be blocked by intrathecally administered se rotoninergic receptor and alpha-adrenergic receptor antagonists.Neuron s demonstrating Fos-like immunoreactivity were located in the ipsilate ral superficial and deep dorsal horn laminae extending from the caudal L(3) through the rostral L(6) region of the spinal cord. Stimulation of the right cervical vagus nerve attenuated significantly (42%) heat- evoked Fos-like immunoreactivity in the superficial laminae. The reduc tion in Fos-like immunoreactivity by vagal stimulation was abolished b y intrathecal administration of methysergide, a nonselective serotonin ergic receptor antagonist, but not by phentolamine, a nonselective alp ha-adrenoceptor antagonist. These results suggest that vagal afferent modulation of spinal nociceptive transmission is mediated, at least in part, by serotonin receptors. (C) 1994 Wiley-Liss, Inc.