Electrical stimulation of the subdiaphragmatic vagus in rats: inhibition of heat-evoked responses of spinal dorsal horn neurons and central substrates mediating inhibition of the nociceptive tail flick reflex.

Electrical stimulation of the subdiaphragmatic branch of the vagus nerve (SDVAS) inhibits the nociceptive tail flick (TF) reflex. The present experiments examined (1) the central substrates mediating SDVAS-produced inhibition of the TF reflex and (2) the effects of SDVAS on either background or noxious heat-evoked responses of spinal dorsal horn neurons. Microinjections of ibotenic acid in the ipsilateral nucleus tractus solitarius (NTS), nucleus raphe magnus (NRM), or bilateral locus coeruleus/subcoeruleus (LC/SC) significantly increased the intensity of SDVAS required to inhibit the TF reflex in lightly anesthetized rats. In studies of class-2 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 17/25 (68%) units, facilitation of 4/25 (16%) units, and no effect on 4/25 (16%) units. In studies of class-3 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of 8/9 (89%) units. Noxious heat-evoked responses of 1/9 (11%) unit were facilitated by SDVAS. In general, the background activity of either class-2 or class-3 units was not significantly affected by SDVAS. SDVAS produced a significant rightward, parallel shift in the stimulus response function (SRF) of class-2 neurons to noxious, graded heat stimuli ranging from 40 to 52 degrees C, while SDVAS produced a significant increase in the threshold and a significant reduction in the slope of the SRF of class-3 neurons. These data indicate that SDVAS generally inhibits noxious heat-evoked responses of lumbosacral spinal dorsal horn neurons in the rat, but does not significantly affect background activity of the same neurons. Furthermore, the inhibition of the TF reflex produced by SDVAS depends on central relays in the ipsilateral NTS, NRM, and bilateral LC/SC.