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

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Abstract
Ground 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 Symbol (68%) units, facilitation of Symbol (16%) units, and no effect on Symbol (16%) units. In studies of class-3 spinal dorsal horn neurons, SDVAS produced significant intensity-dependent inhibition of noxious heat-evoked responses of Symbol (89%) units. Noxious heat-evoked responses of Symbol (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°C, while SDVAS produced a significant increase in the threshold and a significant reduction in the slope of the SRF of class-3 neurons. JOURNAL/jpain/04.03/00006396-199212000-00014/figure1-14/v/2021-01-12T064328Z/r/image-png JOURNAL/jpain/04.03/00006396-199212000-00014/figure2-14/v/2021-01-12T064328Z/r/image-png JOURNAL/jpain/04.03/00006396-199212000-00014/figure3-14/v/2021-01-12T064328Z/r/image-png JOURNAL/jpain/04.03/00006396-199212000-00014/figure4-14/v/2021-01-12T064328Z/r/image-png JOURNAL/jpain/04.03/00006396-199212000-00014/figure5-14/v/2021-01-12T064328Z/r/image-png 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. ∗Correspondence to: Dr. C.L. Thurston, Dept. of Biomedical Sciences, University of South Alabama, Mobile, AL 36688, USA. Tel.: (205) 934-9649; FAX: (205) 460-7821. 1Current address: Dept. of Biomedical Sciences, University of South Alabama, Mobile, AL 36688, USA. (Received 1 May 1992; revised 10 August 1992; accepted 13 August 1992.) © Lippincott-Raven Publishers....