Intrinsic membrane characteristics distinguish two subsets of nociceptive modulatory neurons in the rat rostral ventromedial medulla. Aniko Zagon Xianwei Meng and Howard L. Fields. Departments of Physiology and Neurology, University of California, San Francisco, 513 Parnassus Avenue, San Francisco CA 94143 U.S.A.
APStracts 4:193N, 1997.
ABSTRACT
1. Pain modulating neurons of the rostroventromedial medulla (RVM) include three physiologically distinct classes of neurons in intact, anaesthetised animals: ON and OFF cells which change their activity prior to the onset of withdrawal reflexes and Neutral cells which have activity unrelated to withdrawal reflexes. A previous in vitro intracellular study demonstrated that the RVM contains two types of neurons that are distinguished by their action potential characteristics. The present, in vivo intracellular study examined whether these intracellularly recorded action potential characteristics are correlated with the physiological response properties of RVM neurons recorded in vivo. 2. RVM neurons exhibited two distinct types of action potentials in vivo. Fast spike neurons (n=30) had short duration action potentials (0.27 + 0.02 ms at half amplitude) and biphasic afterhyperpolarizations with a characteristic rapid overshooting spike repolarization. Slow spike neurons (n=25) had longer duration action potentials (0.44 + 0.02 ms at half amplitude) due to a slower spike repolarization rate and monophasic afterhyperpolarization. 3. ON and OFF cell classes included both fast and slow spike neurons. Fast spike ON and OFF neurons had an early onset response to noxious heat stimulation. Slow spike ON and OFF cells showed a delayed onset response to noxious heat. 4. Neutral cells (n=13) were all slow spike cells. Among the slow spike neurons, only Neutral cells had action potentials longer than 0.45 ms (n=9). 5. Fast and slow spike neurons were intermingled throughout the RVM. The majority of intracellularly labelled cells (n=15) had fusiform somata with 2-5 fine caliber primary dendrites and a predominantly mediolateral orientation of the long axis of their dendritic tree. All labelled fast spike ON cells (n=5) had large, multipolar somata with 4-9, large caliber primary dendrites. 6. The present study defines in vivo membrane and morphological characteristics of RVM neurons that correlate with physiological differences and may be used for identification of nociceptive modulatory RVM neurons in slice preparations. 

Received 14 April 1997; accepted in final form 15 August 1997.
APS Manuscript Number J304-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997