FATIGUING ISOMETRIC CONTRACTION OF HINDLIMB MUSCLES RESULTS IN THE RELEASE OF IMMUNOREACTIVE NEUROKININS FROM SITES IN THE ROSTRAL MEDULLA IN THE ANESTHETIZED CAT

Antibody-coated microprobes were used to determine whether immunoreact ive neurokinins (irNK) were released from sites in the brainstem durin g fatiguing isometric contractions of the triceps surae muscles in cat s anesthetized with alpha-chloralose. Contractions were generated by s timulating the tibial nerve using a microprocessor-controlled stimulat or. Microprobes were inserted into the periaqueductal grey (P 0.5-1.0 mm) or the medullary brainstem (either 3.0 or 3.5 mm rostral to the ob ex) prior to, during and following fatiguing contractions. No release of irNK was detected from the periaqueductal grey as a result of fatig uing isometric contractions. When probes were inserted 3.0 mm rostral to the obex, a basal release of irNK was detected from the medulla but this was inhibited during isometric contractions from sites correspon ding to the lateral tegmental field. When probes were inserted into th e more rostral site in the medulla (3.5 mm rostral to the obex), irNK were released in response to contractions from sites corresponding to lateral reticular nucleus, ventral regions of the nucleus tractus soli tarius and the medial vestibular nucleus. No irNK were released from t his site (3.5 mm rostral to obex) either during passive leg flexing, d uring nerve stimulation following gallamine injection and muscle paral ysis or during stimulation of the central end of the cut tibial nerve. These results demonstrate that neurokinins are released from discrete sites in the medulla in response to fatiguing muscle contractions and suggest that tachykinin neurons may be a component of the pathways re gulating blood pressure during ergoreceptor activation.