Responsiveness of tuberoinfundibular dopaminergic neurons to 5-hydroxytryptophan

Serotonin is known to stimulate prolactin secretion by decreasing tyrosine hydroxylase (TH) activity in the tuberoinfundibular dopaminergic (TIDA) neurons. However, the effects of aging on the responsiveness of TIDA neurons to serotonin are not known. An effective way to increase serotonergic activity is to administer 5-hydroxytryptophan (5-HTP), a serotonin precursor. The present study was done to investigate the effects of 5-HTP on TIDA neuronal activity in aging animals. Middle-aged (10–12 mo), old (18–20 mo), and very old (22–24 mo) female Sprague-Dawley rats were bilaterally ovariectomized. Ten days later, they were injected iv with 50 mg/kg body wt of 5-HTP or the vehicle for 5-HTP (PBS-HCl). Twenty minutes later, m-hydroxybenzylhydrazine (NSD), a DOPA decarboxylase inhibitor, was administered. Ten minutes later, the animals were killed, and tyrosine hydroxylase (TH) activity was determined by measuring l-DOPA accumulation in the stalk median eminence by HPLC-EC. In all three groups, administration of 5-HTP increased serum prolactin levels significantly. In control middle-aged rats, TH activity (l-DOPA pg/μg protein) was 33.0 ± 5.6. Treatment with 5-HTP decreased TH activity by 60%. Similarly, 5-HTP treatment decreased TH activity by 52 and 56% in 18- to 20- and 22- to 24-mo-old rats, respectively, compared to the control rats. The magnitudes of the 5-HTP-induced decreases in TH activities in middle-aged, old, and very old rats were not different from each other. These results indicate that TIDA neuronal responsiveness to serotonin does not change with age and that 5-HTP is capable of stimulating PRL release even in very old rats.