A prejunctional mechanism in the midbrain periaqueductal gray inhibition of vagal bradycardia in rats. Nosaka, Shoichiro, Koji Inui, Sumio Murase, and Keiko Murata. Departments of Physiology and Psychiatry, Mie University School of Medicine, Tsu, Mie 514, Japan
APStracts 2:0259R, 1995.
Stimulation of the dorsal part of the midbrain periaque-ductal gray matter (dPAG) inhibits baroreflex vagal bradycardia (BVB) due to a central mechanism. Here we report that the dPAG suppresses vagal bradycardia also by a peripheral mechanism. In chloralose-urethane anesthetized, [beta]-blocked rats, the cervical vagus nerve was cut and the distal cut end electrically stimulated to induce vagal bradycardia (VIB). Sustained electrical stimulation of the dPAG attenuated VIB in a duration dependent manner, but did not affect bradycardia induced by intravenous acetylcholine (AIB). The dPAG inhibition of VIB was abolished by C1 transection. Intravenous norepinephrine (NE) reduced VIB but did not affect AIB. Both the dPAG and NE inhibitions of VIB, were largely attenuated during intravenous prazosin, a selective a1 antagonist. In contrast, BVB provoked by aortic depressor nerve stimulation was remarkably inhibited by a shortly preceding dPAG stimulation but the inhibition was not affected by C1 transection. Prazosin treatment reduced the inhibition definitely but only moderately. In conclu-sion, the dPAG has a potential ability to suppress vagal bradycardia by prejunctionally inhibiting acetylcholine release from cardiac vagus nerve terminals via a1 receptors. However, dPAG stimulation precedently suppresses BVB largely at a central site, leaving a limited fraction of vagal outflow to be inhibited by a prejunctional mechanism operating with long latency. 

Received 8 August 1994; accepted in final form 28 August 1995.
APS Manuscript Number R434-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.