DIRECT PROJECTIONS FROM THE ANTERIOR PRETECTAL NUCLEUS TO THE VENTRALMEDULLA-OBLONGATA IN RATS

The anterior pretectal nucleus has recently been implicated in the des cending modulation of nociception. Electrical stimulation of the nucle us was found to reduce the nociceptive responses of deep dorsal horn n eurons and to inhibit spinally integrated withdrawal reflexes. It is b elieved that at least part of the descending inhibitory effects of the anterior pretectal nucleus are mediated by reticulospinal cells of th e ventrolateral medulla. The purpose of the present study was to trace the direct medullary projections of the anterior pretectal nucleus, t o describe their topographical organization and to reveal the chemical nature of some of their putative target cells. The connections were s tudied using anterograde tract-tracing with Phaseolus vulgaris leucoag glutinin. Direct projections from the anterior pretectal nucleus to th e ipsilateral rostral ventral medulla were found in all cases. A dense innervation of the dorsal inferior olive, the gigantocellular reticul ar nucleus pars ventralis and pars alpha and the ventral pontine retic ular nucleus was found from all aspects of the anterior pretectal nucl eus. Descending labelled terminals were also observed in the gigantoce llular reticular nucleus proper and, laterally, in the lateral paragig antocellular nucleus and in the region of the A5 noradrenergic cell gr oup. A relatively lower density of labelled terminals was noted in the medullary raphe nuclei and in the rostroventrolateral reticular nucle us. Following tract-tracer injections into five distinct subregions of the anterior pretectal nucleus, the topographical organization of the projection was examined and the relatively highest density and most w idespread projection was found to originate from the caudoventral part of the anterior pretectal nucleus. A combined tract-tracing and immun olabelling study revealed that some of the descending, labelled termin als were in close proximity of tyrosine hydroxylase-immunoreactive den drites in the C1 and A5 cell groups. Some labelled fibres were also no ted among the serotonin-immunoreactive cells in the lateral extension of the B3 cell population. The existence of direct projections to the ventral medulla and pens correlates well with physiological data which showed that the descending, antinociceptive effects of the anterior p retectal nucleus are relayed via the rostral ventrolateral medulla. Th e data are also in keeping with pharmacological studies that suggested the role of catecholaminergic cells in the mediation of these descend ing effects. It is proposed that the rostral ventral medullary project ions provide a path through which antinociceptive effects of the anter ior prerectal nucleus are mediated to the spinal cord.