Vagal afferent fibers excite upper cervical neurons and inhibit activity of lumbar spinal cord neurons in the rat

Abstract

Effects of electrical stimulation of the cervical vagus nerve were determined in cervical or lumbar spinal neurons in 27 rats anesthetized with pentobarbital. Ipsilateral cervical vagus stimulation (ICVS) increased activity of 44 neurons in the C 1 segment. At the same stimulation parameters, contralateral cervical vagus stimulation (CCVS) either increased, decreased or did not affect activity of C 1 neurons that were excited by ICVS. For C 1 cells excited by both ICVS and CCVS, the mean latency for activation was significantly longer for CCVS than for ICVS, and ICVS produced a greater degree of excitation than CCVS. In segments C 2-C 6, 16 of 18 neurons were excited by ICVS and 2 were inhibited. However, CCVS did not excite the C 2-C 6 neurons but either inhibited or did not affect activity. In 6 cervical cells, a CCVS conditioning stimulus reduced the level of excitation by ICVS (test stimulus). Transection of the C 2 or C 3 dorsal roots did not significantly affect the excitatory vagal input to C 1 cells. Excitatory somatic receptive fields were classified for 60 cervical spinal cells that responded to vagal stimulation. Most (87%) cells were excited by noxious pinch; 29 were wide dynamic range (WDR) cells and 21 were high threshold cells. In contrast to upper cervical neurons, spinothalamic tract (STT) and spinal cells in lumbar segments were not excited by ICVS or CCVS at the stimulation parameters used in this study, but were primarily inhibited by vagal stimulation. Results of this study showed that a group of cells in upper cervical segments were excited by vagal afferents. This excitatory vagal input reaches the C 1 segment primarily via an ipsilateral, supraspinal route.