Increase of serotonin metabolism within the dorsal horn of the spinal cord during nucleus raphe magnus stimulation, as revealed by in vivo electrochemical detection.

Carbon fiber microelectrodes were used with the differential pulse voltammetry method for in vivo determination of indolamines within the extracellular space of the dorsal horn of the spinal cord or chloral hydrate-anesthetized rats. Under these conditions a peak of oxidation current which is characteristic of 5-hydroxyindoles is recorded at 280-300 mV. Stimulation of the nucleus raphé magnus (NRM) with stimulation parameters comparable to those used to elicit analgesia in freely moving animals produced marked alterations in the voltammograms: (1) stimulation of the NRM for 10 min induced an immediate and sustained increase in the peak amplitude; (2) post-effects of variable duration were observed; (3) the increase in the 5-hydroxyindolaminergic signal was significantly reduced during a second series of NRM stimulations indicating some degree of tolerance to central stimulation. The accuracy of these observations is strengthened by the fact that the basal 5 hydroxyindolaminergic signal is strongly depressed after pretreatment of the animal with p-chlorophenylalanine; in addition, under these conditions, NRM stimulation is totally inefficient. We suggest that these results reflect the in vivo modification of 5-HT metabolism. This represents the first evidence for an in vivo release of 5-HT during stimulation of brain stem areas which induces powerful analgesia in freely moving animals.