Distribution of serotonin cells projecting to the pontomedullary reticular formation in the cat.

Using immunohistochemistry and retrograde transport techniques, this study demonstrates that serotonin (5-HT) cells in the dorsal tegmental gray of the pons at rostrocaudal from P1 to P6 levels are sources of bilateral serotonergic projections to the gigantocellular tegmental field of the medial pontine and medullary reticular formation in the cat. The cells observed were a relatively homogeneous population of small (30 microns) to medium-sized (40 microns) cells, oval, fusiform or spindle-shaped. Rostrally and caudally located 5-HT cells in the dorsal tegmental gray tended to project to the pontine and medullary reticular formation respectively. The dendrites of these cells spread in a plane perpendicular to the long axis of the brainstem. Thin 5-HT fibers with a series of fine varicosities (0.5-1.0 microns in diameter) surrounded reticular neurons, and some of these varicosities closely apposed the proximal dendrites and somata of the reticular neurons. Both large (50 microns) and small (20-30 microns) reticular neurons were uniformly innervated by 5-HT fibers. As revealed in the present study, the serotonergic system, together with the monoaminergic and cholinergic systems, seems to be involved in behavioral state regulation.