Twenty-four hour rhythmicity of serum prolactin and median eminence and anterior pituitary content of dopamine (DA), serotonin (5HT), gamma-aminobutyric acid (GABA), taurine and somatostatin were examined in 2 months-old and 18-20 months-old Wistar male rats. The concentration of prolactin was higher in aged rats, with peaks in both groups of rats at the early phase of the activity span. Median eminence DA content of young rats attained its maximum at the middle of rest span and decreased as prolactin levels augmented while the lowest values of adenohypophysial DA were observed at the time of prolactin peak. DA rhythmicity disappeared in aged rats. GABA content of median eminence and adenohypophysis was lower in aged rats, with maximal values of median eminence GABA at light-dark transition in young rats and at the second half of activity span in aged rats. Serum prolactin correlated positively with median eminence GABA in young rats and negatively with pituitary GABA in young and aged rats. Median eminence somatostatin peaked at the beginning of the activity phase (young rats) or at the end of the rest phase (aged rats). Prolactin levels and somatostatin content correlated significantly in young rats only. Median eminence and pituitary 5HT and taurine content did not change with age. The results indicate disruption of prolactin regulatory mechanisms with aging in rats.
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