Functional mapping of dorsal and median raphe 5-hydroxytryptamine pathways in forebrain of the rat using microdialysis.

Recent neurochemical studies of the properties of 5-hydroxytryptamine (5-HT) pathways arising from the dorsal raphe nucleus (DRN) and median raphe nucleus (MRN) have measured extracellular 5-HT in brain regions with reported preferential DRN or MRN 5-HT inputs. Here, we have tested whether electrical stimulation of the DRN and MRN releases 5-HT in rat forebrain regions in a pattern that fits the reported distribution of DRN/MRN pathways. The effect on extracellular 5-HT of electrical stimulation (5 Hz, 300 microA, 20 min) of the DRN, and then MRN, was determined in six regions of the anaesthetised rat. Stimulation of the DRN evoked a short-lasting but clear-cut release of 5-HT (+70-100%) in regions (frontal cortex, dorsal striatum, globus pallidus, and ventral hippocampus) reported to receive a 5-HT projection from the DRN. Regions receiving an MRN innervation (dorsal hippocampus, medial septum, and ventral hippocampus) released 5-HT (+70-100%) in response to MRN stimulation. Regions reported to receive a preferential DRN innervation (frontal cortex, dorsal striatum, and globus pallidus) did not respond to MRN stimulation. Of two regions (dorsal hippocampus and medial septum) reported to receive a preferential MRN innervation, one did not respond to DRN stimulation (dorsal hippocampus) although the other (medial septum) did. In summary, electrical stimulation of the DRN and MRN released 5-HT in a regionally specific pattern. With the exception of one region (medial septum), this pattern of release bears a strong relationship to the distribution of 5-HT projections from the DRN and MRN reported by anatomical studies. The combination of raphe stimulation with microdialysis may be a useful way to study the in vivo neurochemistry of DRN/MRN 5-HT pathways.