Physiological and anatomic evidence for functional subclasses of serotonergic raphe magnus cells

Serotonergic cells in the medullary nucleus raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) project to the spinal cord where they are likely to modulate nociceptive transmission. Previous studies hav e suggested that these cells are physiologically and anatomically heterogen eous. In the present investigation, we examined whether subclasses of serot onergic RM and NRMC cells can be delineated based on their response to a vi sceral stimulus, and whether any such subclasses are morphologically distin ct. Most RM and NRMC serotonergic cells tested (81 of 116) responded to ret raction of the descending aorta into a polyethylene tube (the snare stimulu s) with 57% of all cells tested excited and 13% inhibited. Responses of ser otonergic cells to the snare outlasted the stimulus, were not reflective of evoked cardiovascular changes, and were observed in sino-aortic deafferent ed rats, evidence that the snare stimulus does not influence serotonergic c ell discharge through activation of baroreceptors. Because serotonergic cel ls responsive to the snare were also responsive to mechanical brushing with in the retroperitoneum, the snare is likely to change serotonergic cell dis charge by means of the activation of mechanosensitive visceral afferents. I ntracellular labeling of physiologically characterized serotonergic RM and NRMC cells showed that cells that were responsive to the snare stimulus had simpler axonal collateralization patterns than cells that were unresponsiv e to the snare stimulus. This association between morphological and physiol ogical properties provides additional evidence that subpopulations of serot onergic cells exist and serve varied physiological functions. (C) 2001 Wile y-Liss, Inc.