Inhibition of the responses of cat dorsal horn neurons to noxious skin heating by stimulation in medial or lateral medullary reticular formation

Abstract

Responses of single lumbar dorsal horn units to noxious radiant heating (50 degrees C, 10 s) of glabrous footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% nitrous oxide. The heat-evoked responses of 37/40 units were reduced during electrical stimulation (100 ms trains, 100 Hz, 3/s, 25-600 microA) in the medullary nucleus raphe magnus (NRM) and/or in laterally adjacent regions of the medullary reticular formation (MRF). Inhibition was elicited by stimulation in widespread areas of the medulla, but with greatest efficacy at ventrolateral sites. The magnitude of inhibition increased with graded increases in medullary stimulation intensity. Mean current intensities at threshold for inhibition or to produce 50% inhibition were higher for NRM than for MRF sites. Units' responses to graded noxious heat stimuli increased linearly from threshold (42-43 degrees C) to 52 degrees C. During NRM (5 units) or ipsilateral MRF stimulation (7 units), responses were inhibited such that the mean temperature-response functions were shifted toward higher temperatures with increased thresholds (1.5 degrees and 1 degree C, respectively) and reduced slopes (to 60% of control). Contralateral MRF stimulation had a similar effect in 4 units. Inhibitory effects of NRM and MRF stimulation were reduced (by greater than 25%) or abolished in 4/6 and 5/12 units, respectively, following systemic administration of the serotonin antagonist methysergide. Inhibitory effects from NRM, ipsi- and contralateral MRF were reduced or abolished in 2/9, 4/8 and 6/9 cases, respectively, following systemic administration of the noradrenergic antagonist phentolamine. These results confirm and extend previous studies of medullospinal inhibition and the role of monoamines, and are discussed in terms of analgesic mechanisms.

Studies of brain structures involved in diffuse noxious inhibitory controls in the rat: the rostral ventromedial medulla.

“…In keeping with such results, electrical stimulation in Gi can also inhibit nociceptive activity in spinal and trigeminal neurones (Zorman et al 1981;Dostrovsky, Shah & Gray, 1983;Pretel, Guinan & Cartens, 1988;Yezierski, 1990). Finally, anatomical data are compatible with the participation of this structure in spino-bulbo-spinal loops since it receives afferents from, and projects to, the spinal cord (Basbaum, Clanton & Fields, 1978;Zemlan, Behbehani & Beckstead, 1984;Matsuyama, Ohta & Mori, 1988).…”

Response Properties of Nucleus Reticularis Lateralis Neurons After Electroacupuncture Stimulation in Rats

“…Histological studies showed that NRL neurons have complex input-output relations from-to various parts of the central nervous system (Janss and Gebhart, 1987, Pretel, at al., 1988, Murphy and Behbehani, 1993a, 1993b. It is thought that important descending analgesic mechanisms underlying acupuncture analgesia work through connections from NRL to NRM and from NRL to SC.…”