PROJECTIONS FROM NUCLEUS RAPHE OBSCURUS TO THE PERIAQUEDUCTAL GRAY-MATTER IN THE RAT

Retrograde transport of rhodamine- or coumarin-labelled latex microsph eres was used to investigate projections from nucleus raphe obscurus ( NRO) to the periaqueductal grey matter (FAG) in rats. Few labelled neu rones (3.5 +/- 1.2; mean +/- S.E.M.) were seen in NRO after injections of microspheres into the dorsolateral and lateral FAG (n = 11) but af ter injections into the ventrolateral sector (n = 9), significant numb ers (42.6 +/- 8.1) were present, particularly in the rostral third of NRO (66% of labelled cells). The results suggest that the projection f rom NRO to the FAG is restricted to the ventrolateral sector. Any infl uence of NRO on the dorsal FAG must therefore be mediated indirectly.