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Volume 17, Number 9, Issue of May 1, 1997 pp. 3285-3292
Copyright ©1997 Society for NeuroscienceSEROTONERGIC Pontomedullary Neurons Are Not Activated by Antinociceptive Stimulation in the Periaqueductal Gray
Received Dec. 9, 1996; revised Jan. 22, 1997; accepted Feb. 11, 1997.
Keming Gao, Yoo-Hang Hugh Kim, and Peggy Mason Department of Pharmacological and Physiological Sciences and the Committee on Neurobiology, University of Chicago, Chicago, Illinois 60637
The antinociceptive and cardiovascular effects of midbrain periaqueductal gray (PAG) stimulation are mediated through a relay in the pontomedullary raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC). To test whether the neurons important in mediating PAG-evoked effects are SEROTONERGIC, the responses of pontomedullary SEROTONERGIC-LIKE cells to PAG stimulation were tested. SEROTONERGIC-LIKE neurons (n = 21) were recorded extracellularly in halothane-anesthetized Sprague Dawley rats. Serotonergic-like neurons were distinguished by their slow and steady background discharge. Two neurons that were physiologically characterized as SEROTONERGIC-LIKE were intracellularly labeled and processed for serotonin immunoreactivity; both cells tested contained immunoreactive serotonin. Train stimulation of sites within the midbrain PAG, at intensities of
50 µA, suppressed the tail withdrawal from noxious heat and evoked changes in blood pressure and heart rate. No SEROTONERGIC-LIKE cells were activated by single-pulse or short-train (two to five pulses) stimulation of the PAG at antinociceptive intensities. In most cases, SEROTONERGIC-LIKE cells were unaffected by long-train stimulation (5-6 sec) of the PAG, which produced antinociception and cardiovascular changes. In contrast, >50% of the cells in two nonserotonergic-like cell classes were activated at short latency by such PAG stimulation. In conclusion, monosynaptic excitation of SEROTONERGIC cells in RM/NRMC is unlikely to be necessary for the nociceptive and autonomic modulatory effects of PAG stimulation.
Key words: raphe magnus; serotonin; monoamine; pain modulation; autonomic modulation; antinociception
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