Are adrenergic neurons subject to a serotoninergic influence in the nucleus tractus solitarii? A morphological and biochemical study in the rat.

The possible relationships between adrenaline-synthesizing neurons and serotoninergic afferent fibers in the nucleus tractus solitarii of the rat were investigated both morphologically and biochemically. Adrenergic elements (cell bodies, dendrites and nerve endings) were detected simultaneously with serotoninergic axonal varicosities in the same electron-microscopic sections by means of combined phenylethanolamine-N-methyltransferase immunocytochemistry and [3H]serotonin-uptake radioautography. Among some 500 serotoninergic varicosities scanned in the areas of significant overlap between the 2 types of labeling, only 3 were directly apposed to an adrenergic process, identified as a dendrite in each case. No synaptic membrane differentiations were seen at these occasional sites of contact. Destruction of the serotonin input by 5,7-dihydroxytryptamine had no significant effect on the tyrosine hydroxylase dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase enzymatic activities in the C2 adrenergic region, but induced 22% and 38% increases of tyrosine hydroxylase and dopamine-beta-hydroxylase activities, respectively, in the neighboring A2 noradrenergic area. Taken together, these results suggest that serotoninergic and adrenergic neurons do not significantly interact in the nucleus tractus solitarii; this implies that the possible catecholaminergic relays for the action of serotonin in autonomic regulation at this level could consist of noradrenergic neurons rather than of their adrenergic counterparts.