Glutamate Inhibition of the Adrenergic‐Stimulated Production of Melatonin in Rat Pineal Gland In Vitro

Abstract: The effect of l‐glutamate on the adrenergic‐stimulated release of melatonin in the rat pineal gland was examined using an in vitro perfusion system. l‐Glutamate by itself had no effect on melatonin secretion whereas l‐glutamate administered prior to (–)‐isoproterenol (β‐adrenergic agonist) and l‐phenylephrine (α‐adrenergic agonist) inhibited melatonin production by 42%. l‐Glutamate did not inhibit melatonin secretion when glands were stimulated with (–)‐isoproterenol alone. d‐Glutamate, as well as the l‐glutamate agonists kainate, N‐methyl‐d‐aspartate, quisqualate, and trans‐1‐aminocyclopentane‐1, 3‐dicarboxylic acid, had no effect on the (–)‐isoproterenol‐and l‐phenylephrine‐stimulated secretion of melatonin, which suggests that the inhibitory effects of glutamate are not mediated via any of the known glutamate receptor subtypes. The possibility that l‐glutamate may be converted to another neuroactive compound (GABA) prior to the addition of (–)‐isoproterenol and l‐phenylephrine is suggested by the observation that simultaneous administration of l‐glutamate with (–)‐isoproterenol and l‐phenylephrine did not inhibit melatonin production.