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1 Howard Florey Institute of Experimental Physiology and Medicine, Parkville, Victoria, Australia
2 Howard Florey Institute of Experimental Physiology and Medicine, Parkville, Victoria, Australia; Department of Anatomy & Cell Biology, University of Melbourne, Parkville, Victoria, Australia
* To whom correspondence should be addressed. E-mail: r.mcallen{at}hfi.unimelb.edu.au.
Anatomical studies indicate that sympathetic preganglionic neurons receive inputs from several brainstem cell groups, but the functional significance of this organization for vasomotor control is not known. We studied the roles of two brainstem premotor cell groups, the medullary raphe and the rostral ventrolateral medulla (RVLM), in determining the activity of sympathetic vasomotor supply to the tail of urethane-anaesthetized, artificially ventilated rats. Chemical inactivation of either RVLM (bilaterally) or raphe cells by microinjecting glycine (120-200 nl, 0.5 M) or muscimol (40-160 nl, 2.1-8 mM) was sufficient to inhibit ongoing tail sympathetic fiber activity and to block its normally strong response to mild cooling via the trunk skin (reducing rectal temperature from 38.5 to 37°C). After bilateral RVLM inactivation, tail sympathetic fibers could still be excited by chemical stimulation of raphe neurons (l-glutamate, 120 nl, 50mM) and strong cooling (rectal temperature ~33°C) caused a low level of ongoing activity. After chemical inhibition of raphe neurons, however, neither strong cooling nor chemical stimulation of RVLM neurons activated tail sympathetic fibers. Electrical stimulation of the RVLM elicited tail sympathetic fiber volleys before and after local anaesthesia of the raphe (150-500 nl of 5% tetracaine), demonstrating the existence of an independent descending excitatory pathway from the RVLM. The data show that neurons in both the medullary raphe and the RVLM, acting together, provide the essential drive to support vasomotor tone to the tail. Inputs from these two premotor nuclei interact in a mutually facilitatory manner to determine tonic, and cold-induced, tail sympathetic activity.
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