| Abstract: | 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 activity to examine populations of neurons in the spinal cord that are activated by a noxious heat stimulus, (2) determine whether heat-evoked Fos-like immunoreactivity can be modulated by vagal afferent stimulation, and (3) determine whether vagallymediated effects on heat-evoked Fos-like immunoreactivity can be blocked by intrathecally administered serotoninergic receptor and α-adrenergic receptor antagonists. Neurons demonstratiring Fos-like immunoreactivity were located in the ipsilateral superficial and deep dorsal horn laminae extending from the caudal L3 through the rostral L6 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 reduction in Fos-like immunoreactivity by vagal stimulation was abolished by intrathecal administration of methysergide, a nonselective serotoninergic receptor antagonist, but not by phentolamine, a nonselective α-adrenoceptdr antagonist. These results suggest that vagal afferent modulation of spinal nociceptive transmission is mediated, at least in part, by serotonin receptors. © 1994 Wiley-Liss, Inc. |
