Muscle pain activates a direct projection from ventrolateral periaqueductal gray to rostral ventrolateral medulla in rats

Activation of the ventrolateral periaqueductal gray (vIPAG) evokes a reacti on of quiescence, immobility, hypotension and bradycardia. Pain of deep som atic or visceral origin also often triggers a reaction of quiescence, immob ility, hypotension and bradycardia and further, evokes a selective increase in immediate-early-gene (c-Fos) expression within the vIPAG. Vasodepressio n evoked from the vIPAG is thought to be mediated by an inhibition of presy mpathetic neurons within the rostral ventrolateral medulla (RVLM). In this study the prior injection of retrograde tracer into the RVLM was combined w ith the use of Fos expression as a marker of neuronal activation, to determ ine if deep (muscle) pain-evoked vasodepression could be mediated by a dire ct vIPAG-RVLM pathway. It was revealed that intramuscular injection of form alin, in the anaesthetised rat, evoked a significant increase in Fos expres sion within the caudal vIPAG, and that approximately 25% of the Fos-immunor eactive neurons projected to the RVLM. (C) 2000 Published by Elsevier Scien ce Ireland Ltd.