Circadian patterns of serotonin (5-HT) accumulation after pargyline treatment in the rat hypothalamus during development

The purpose of this study was to determine if developmental changes in serotonin (5-HT) synthesis occur during maturation of the 5-HT circadian rhythm in the rat hypothalamus of 10- to 35-day-old rats. Pargyline (75 mg/kg, s.c.), a monoamine oxidase (MAO) inhibitor, was administered 30 min prior to decapitation. Hypothalamic 5-HT concentrations after pargyline treatment were measured at six time points over a 24-hr interval in male and female rat pups at 10, 20, 30 and 35 days of age. Daily fluctuations in hypothalamic 5-HT accumulation were detectable as early as 10 days of age in rats of both sexes. Since the influence of MAO on hypothalamic 5-HT concentrations was eliminated by pargyline treatment, the findings suggest that changes in tryptophan hydroxylase activity contribute significantly to daily and age-related patterns of 5-HT accumulation in the rat hypothalamus during development. Circadian rhythms in hypothalamic norepinephrine (NE) and dopamine (DA) also occurred during development; however, sex differences were not detected for these biogenic amines. Since catecholamine catabolism is not exclusively mediated by MAO, our findings are insufficient to conclude whether or not sex differences actually exist in patterns of these monoamines.