Rates of 5-hydroxytryptamine synthesis in various brain tissues can be estimated from the linear increase in 5-hydroxytryptophan levels following inhibition of 5-hydroxytryptophan decarboxylation with RO4-4602 or NSD-1015. In addition, NSD-1015 can prevent 5-hydroxytryptamine oxidative-deamination via monoamine oxidase inhibition, leading to linear decreases in a major metabolite of this amine, 5-hydroxyindole acetic acid. In the rat pineal gland we demonstrated similar increases in 5-hydroxytryptophan levels after nocturnal or diurnal injection of RO4-4602 (100 mg · kg-1) or NSD-1015 (200 mg · kg-1). Similar decreases in 5-hydroxyindole acetic acid were also observed after nocturnal or diurnal injection of NSD-1015 or pargyline (an inhibitor of monoamine oxidase) (75 mg · kg-1). 5-Hydroxytryptamine levels increased after nocturnal pargyline injection but remained constant after diurnal pargyline administration. 5-Hydroxytryptamine levels exhibited little change following nocturnal injection of NSD-1015 but decreased linearly after diurnal injection of NSD-1015. We suggest that (1) rat pineal 5-hydroxytryptamine synthesis is increased nocturnally, (2) metabolic utilization, primarily by oxidative-deamination, of 5-hydroxytryptamine is increased diurnally and (3) basal levels of pineal 5-hydroxytryptamine may be stored within adrenergic nerve endings which innervate the pinealocytes responsible for synthesizing this amine, thus "protecting" or otherwise making unavailable this pool of 5-hydroxytryptamine for metabolic utilization.
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