ALL-TRANS-RETINOIC AND 9-CIS-RETINOIC ACID ENHANCE THE CHOLINERGIC PROPERTIES OF A MURINE SEPTAL CELL-LINE - EVIDENCE THAT THE EFFECTS ARE MEDIATED BY ACTIVATION OF RETINOIC ACID RECEPTOR-ALPHA

We investigated the effects of retinoids on the cholinergic properties of a murine septal cell line, SN56. Treatment of the cells with all-trans-retinol (vitamin A), all-trans-retinal, all-trans-retinoic acid (t-RA), 9-cis-retinoic acid (9c-RA), or 13-cis-retinoic acid caused time-and concentration-dependent increases in choline acetyltransferase activity (up to 3.4-fold) and in intracellular acetylcholine levels (up to 2.5-fold, with respective EC(50) values of 68, 50, 18, 15, and 56 nM). Furthermore, treatment with either t-RA or 9c-RA at 1 mu M for 48 h resulted in an increase in the expression of choline acetyltransferase mRNA by threefold that of controls. These data and the presence of putative retinoic acid response elements in the 5' region of the murine choline acetyltransferase gene indicate that retinoids stimulate choline acetyltransferase transcription in murine cholinergic neurons. No additivity or synergism was observed between the effects of t-RA and 9c-RA on any of these cholinergic properties of SN56 cells, suggesting a common mechanism of action of the two retinoids. However, a combined treatment with t-RA and forskolin, which activates adenylate cyclase, resulted in an additive increase in acetylcholine content. Using an antagonist selective for the retinoic acid receptor-alpha subtype, Ro 41-5253, we found that the effects of t-RA and 9c-RA on acetylcholine levels were abolished. An agonist selective for retinoic acid receptor-alpha, Ro 40-6055, increased acetylcholine levels to a similar extent as t-RA and 9c-RA, and this effect was blocked by the antagonist. Our results suggest that retinoids modulate the cholinergic phenotype of septal neurons by activation of retinoic acid receptor-alpha.