Electrical stimulation of the nucleus raphe magnus in the rat. Effects on 5-HT metabolism in the spinal cord.
The direct electrical stimulation (with biphasic pulses of 1 msec, 10 pulses/sec, 200 microA, for 30 min) of the nucleus raphe magnus in chloral hydrate anaesthesized rats produced a significant acceleration (+50%) of 5-HT synthesis in the spinal cord as revealed by the increased rate of 5-HTP accumulation occurring at this level after the blockade of central 5-HTP decarboxylase with benserazid. In contrast, no change was detected in 5-HT metabolism in the forebrain of stimulated rats. The acceleration of 5-HT synthesis was likely not due to an increased availability of tryptophan for the rate-limiting enzyme, tryptophan hydroxylase, since the concentration of this amino acid was changed neither in the spinal cord, nor in the forebrain of stimulated rats. The measurement of tryptophan hydroxylase activity in soluble extracts from the spinal cord of control and stimulated rats revealed that the acceleration in 5-HT synthesis produced by the electrical stimulation of the nucleus raphe magnus was not associated with a persisting activation of this enzyme. Although one cannot completely exclude that a short-lasting activation of tryptophan hydroxylase, no longer detectable in soluble extracts, has occurred in the spinal cord of stimulated rats, the present findings rather suggest that the rate of 5-HT synthesis can be controlled by factors other than only the concentration of tryptophan and the intrinsic activity of tryptophan hydroxylase in serotoninergic neurons. The demonstration of an acceleration of 5-HT synthesis in bulbospinal serotoninergic neurons under stimulating conditions close to those producing analgesia in rats further supports the role of these neuronal systems in the physiological mechanisms of pain control.
D006916:5-Hydroxytryptophan / Q000378:metabolism; D000818:Animals; D001933:Brain Stem / Q000502:physiology*; D002697:Chloral Hydrate / Q000494:pharmacology; D004558:Electric Stimulation; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D008297:Male; D011903:Raphe Nuclei / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism*; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism*
D011985:Receptors, Serotonin; D012701:Serotonin; D002697:Chloral Hydrate; D006897:Hydroxyindoleacetic Acid; D006916:5-Hydroxytryptophan