Changes in mechanoreceptive field properties of trigeminal somatosensory brainstem neurons induced by stimulation of nucleus raphe magnus in cats.

Experiments were carried out on adult anesthetized cats in which the effects of nucleus raphe magnus (NRM) conditioning stimulation (20 ms) were tested on the responses evoked by orofacial stimuli in single brainstem neurons of trigeminal (V) subnucleus oralis. The NRM stimulation induced inhibition of the responses of 57 of 77 low-threshold mechanoreceptive (LTM) neurons and the one wide-dynamic range (WDR) neuron tested. The duration of the neuronal inhibition ranged from 300-600 ms and the mean threshold for inhibition ranged from 47.8 +/- 4.8 to 102.7 +/- 15 microA depending on the orofacial stimulation site (skin or tooth pulp) and form (mechanical or electrical) of cutaneous stimuli used to evoke neuronal responses. In 20 LTM neurons showing NRM-induced inhibition that were specifically examined for the effects of NRM stimulation on the mechanoreceptive field, one population (n = 11) showed shrinkage (mean 55 +/- 4.4% from control area) of the mechanoreceptive field while the remaining neurons (n = 9) showed no change in mechanoreceptive field size during NRM stimulation. The former group of neurons were also distinguished from the latter neurons by their significantly larger mechanoreceptive field and the activation of the majority of them by electrical stimuli applied outside their mechanoreceptive field. The responses of these neurons evoked by low-threshold inputs from the edge of the mechanoreceptive field were more sensitive to NRM conditioning stimulation than responses evoked from the mechanoreceptive field center, as judged by threshold, magnitude and duration of the NRM-induced inhibition. These findings underscore the sensitivity of LTM neurons to NRM influences. They also reveal a particular population of oralis neurons which have a differential sensitivity of low-threshold inputs evoked from the edge compared to the center of the mechanoreceptive field.