Diencephalic connections of the raphé nuclei of the rat brainstem: an anatomical study with reference to the somatosensory system.

The present study was undertaken to analyse in detail the connections of the various raphé nuclei with thalamic structures. Micropipettes filled with an aqueous solution of wheat-germ agglutinin conjugated to horseradish peroxidase were used to produce small iontophoretic deposits restricted to the various raphé nuclei in male Sprague Dawley albino rats. Tetramethyl benzidine was used as a chromogen to reveal both fiber terminals anterogradely labelled and retrogradely filled neurons. A detailed discussion of the possible cases of artefactual labelling using this technique is given. The present study confirms the results obtained previously in the cat that indicate that the various raphé nuclei project to different areas of the diencephalon. Related to the somatosensory system, the B3 area (nucleus raphé magnus) projects to the nucleus submedius and anterior intralaminar nuclei known to receive spinothalamic inputs, but not to the ventrobasal complex. The distribution of afferents from this nucleus suggests an innervation primarily of thalamic structures involved in the somatosensory system. The nucleus raphé medianus projects to the ventrobasal complex and the nucleus submedius , but the fact that its projections are widespread, including all thalamic sensory "relay" nuclei and the entire nucleus reticularis thalami, suggests that it could participate in a "nonspecific" system of control of different sensory modalities. The nucleus raphé dorsalis generally does not project to the thalamic nuclei believed to be involved in the somatosensory system.