POLARIZED EXPRESSION OF THE ANTIDEPRESSANT-SENSITIVE SEROTONIN TRANSPORTER IN EPINEPHRINE-SYNTHESIZING CHROMAFFIN CELLS OF THE RAT ADRENAL-GLAND

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transpor ters (SERTs) clear the amine from extracellular spaces in the CNS and periphery as a mechanism for transmitter inactivation and recycling. A lthough it is known that SERTs are preferentially expressed on basolat eral domains in transfected epithelial cells, details of the transport er's membrane localization in vivo are lacking. 5HT and 5HT receptors have been identified in the rodent adrenal gland. Using SERT antagonis t autoradiography, we establish the presence of antidepressant-sensiti ve transport sites in the rat adrenal medulla. Immunofluorescence expe riments using antibodies specific for the SERT COOH and NH, termini, f or 5HT, or for catecholamine biosynthetic enzymes suggest that SERT me diates intracellular 5HT accumulation by epinephrine-secreting chromaf fin cells. Using confocal microscopy, we establish that SERT expressio n is nonuniformly distributed along the plasma membrane of chromaffin cells. Notably SERT immunoreactivity is largely absent from plasma mem branes bordering smooth muscle that surrounds vascular sinusoids. Rath er, SERT is highly expressed in membranes adjoining other chromaffin c ells, consistent with a role for 5HT and SERT in autocrine or paracrin e control of chromaffin cell physiology. SNAP-25, a t-SNARE protein im plicated in neurotransmitter release, was found to colocalize with SER T. In contrast, Na,K ATPase and NCAM are uniformly distributed along t he entire perimeter of chromaffin cell membranes. These findings under score a role for 5HT and SERT in adrenal physiology, reveal unrecogniz ed polarity of chromaffin cell plasma membranes, and warrant a conside ration of common targeting mechanisms localizing amine transporters ne ar release sites.