AGE-RELATED-CHANGES IN MELATONIN MODULATION OF SYMPATHETIC NEUROTRANSMISSION

An age-related reduction in the maximal acetylcholine (ACh)-induced co ntraction in the prostatic portion of rat vas deferens during the day was observed. This contraction is due to the release of norepinephrine and adenosine triphosphate from sympathetic nerve terminals. Male Wis tar rats (4 and 24 months-old) were housed on a light/dark cycle (12:0 0 h/12:00 h, lights on at 6:00). The diurnal variation of ACh-induced contraction was determined on animals sacrificed every 3 h during the day. Aging reduced the maximal contraction and shortened the length of the nocturnal response. In both young and old rats, the response to A Ch increased at 21:00, but it remained elevated until 12:00 in young r ats and until 24:00 in old rats. Both nocturnal administration of mela tonin for 2 consecutive days (9 mg/kg/day, s.c.) and melatonin incubat ion (100 pg/ml, 4 h) of the vas deferens from old rats sacrificed at 1 5:00 h significantly potentiated the ACh-induced contraction. However, this potentiation was smaller than that observed in young rats. The r esults presented here show the loss of maximal contractile response to ACh with age and a qualitative change in the rhythm characteristics o f this phenomenon. Moreover, the age-related decrease in the ACh-induc ed contraction in the prostatic portion of the rat vas deferens is dep endent not only on a reduction in the plasma nocturnal increase of mel atonin concentration but also on a lower responsiveness of short sympa thetic neurons to melatonin.