Topographic organization and neurochemical identity of dorsal raphe neurons that project to the trigeminal somatosensory pathway in the rat

The primary goals of this study were to: 1) examine the distribution of neu rons within the dorsal raphe (DR) nucleus that project to cortical and subc ortical sites along the trigeminal somatosensory pathway in rat; 2) determi ne the extent to which different regions within this ascending sensory syst em receive collateral projections from the same DR neuron; and 3) identify the putative transmitters contained within these DR projection neurons. Lon g-Evans hooded rats received pressure injections of various combinations of retrograde fluorescent tracers; into the whisker-related regions of the pr imary somatosensory cortex (barrel field cortex [BC]), ventral posterior me dial thalamus (VPM), and principal nucleus of the trigeminal complex (PrV). The distribution of retrogradely labeled neurons within the DR was examine d by fluorescence microscopy. The major finding was that cortically project ing neurons were located within the midline regions of the rostral portion of the DR, whereas cells projecting to subcortical trigeminal somatosensory structures were distributed bilaterally in the lateral wing regions of the DR as well as in the midline portions of the nucleus. Single neurons that send axon collaterals to multiple cortical and subcortical trigeminal somat osensory targets were observed in the dorsomedian and ventromedian regions of the DR. DR neurons that projected to cortical and subcortical sites cont ained serotonin but not tyrosine hydroxylase, the marker enzyme for catecho lamine transmitters. Taken together, these findings provide further evidenc e of neurochemical specificity and functional anatomical organization withi n the DR efferent projection system. (C) 2001 Wiley-Liss, Inc.