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1 Neurobiology, University of Chicago, Chicago, Illinois, United States
2 Committee on Neurobiology, University of Chicago, Chicago, Illinois, United States
* To whom correspondence should be addressed. E-mail: p-mason{at}uchicago.edu.
Serotonergic cells are located in a restricted number of brainstem nuclei, send projections to virtually all parts of the central nervous system, and are critical to normal brain function. They discharge tonically at a rate modulated by sleep/wake cycle and, in the case of medullary serotonergic cells in raphe magnus and the adjacent reticular formation (RM), are excited by cold challenge. Yet, beyond behavioral state and cold, endogenous factors that influence serotonergic cell discharge remain largely mysterious. The present study in the anesthetized rat investigated predictors of serotonergic RM cell discharge by testing whether cell discharge correlated to three rhythms observed in blood pressure recordings that averaged >30 minutes in length. A very slow frequency rhythm with a period of minutes, a respiratory rhythm, and a cardiac rhythm were derived from the blood pressure recording. Cross correlations between each of the derived rhythms and cell activity revealed that the discharge of 38 of the 40 serotonergic cells studied was significantly correlated to the very slow and/or respiratory rhythms. Very few serotonergic cells discharged in relation to the cardiac cycle and those that did, did so weakly. The correlations between serotonergic cell discharge and the slow and respiratory rhythms cannot arise from baroreceptive input. Instead we hypothesize that they are by-products of on-going adjustments to homeostatic functions that happen to alter blood pressure. Thus, serotonergic RM cells integrate information about multiple homeostatic activities and challenges and can consequently modulate spinal processes according to the most pressing need of the organism.
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