Projections from dorsal raphe nucleus to the periaqueductal grey matter: studies in slices of rat midbrain maintained in vitro.

In coronal slices of rat midbrain localised injections of FluoroGold or DiO into the dorsolateral periaqueductal grey matter (PAG) labelled cells retrogradely in the dorsal half of the dorsal raphe nucleus (DRN) and the ipsilateral ventrolateral PAG. Low intensity electrical stimulation in the dorsolateral PAG (22.9 +/- 1.9 microA) evoked antidromic responses in neurones recorded intracellularly in the dorsal subnucleus of the DRN. Antidromic responses could also be evoked in neurones in the ventral DRN and its wings but only at much higher currents (40.9 +/- 2.5 microA) which likely spread to activate axons in the ventrolateral PAG that originated from perikarya in the ventral DRN. The findings are discussed in relation to modulation of the excitability of the aversive system in the dorsolateral PAG by the DRN.