Monoamines suppressed phosphorylation of SNAP-25 in mouse brain

Abstract

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transporters (SERTs) clear the amine from extracellular spaces in the CNS and periphery as a mechanism for transmitter inactivation and recycling. Although it is known that SERTs are preferentially expressed on basolateral domains in transfected epithelial cells, details of the transporter's membrane localizationin vivoare lacking. 5HT and 5HT receptors have been identified in the rodent adrenal gland. Using SERT antagonist autoradiography, we establish the presence of antidepressant-sensitive transport sites in the rat adrenal medulla. Immunofluorescence experiments using antibodies specific for the SERT COOH and NH2termini, for 5HT, or for catecholamine biosynthetic enzymes suggest that SERT mediates intracellular 5HT accumulation by epinephrine-secreting chromaffin cells. Using confocal microscopy, we establish that SERT expression is nonuniformly distributed along the plasma membrane of chromaffin cells. Notably, SERT immunoreactivity is largely absent from plasma membranes bordering smooth muscle that surrounds vascular sinusoids. Rather, SERT is highly expressed in membranes adjoining other chromaffin cells, consistent with a role for 5HT and SERT in autocrine or paracrine control of chromaffin cell physiology. SNAP-25, a t-SNARE protein implicated in neurotransmitter release, was found to colocalize with SERT. In contrast, Na,K ATPase and NCAM are uniformly distributed along the entire perimeter of chromaffin cell membranes. These findings underscore a role for 5HT and SERT in adrenal physiology, reveal unrecognized polarity of chromaffin cell plasma membranes, and warrant a consideration of common targeting mechanisms localizing amine transporters near release sites.