Differences between young and aged rats in voiding frequency and detrusor muscle serotonergic contraction.

INTRODUCTION: The involvement of serotonin (5-HT) in increased lower urinary tract symptoms in aging is unclear. We sought to compare voiding function and 5-HT induced detrusor contraction between young and aged rats. METHODS: This study used young (2- to 3-month-old) and aged (26- to 30-month-old) male Fischer 344 rats. 1. Rats were housed in individual metabolic cages, and then the total volume of urination, volume per micturition, voiding frequency, and voiding interval were analyzed. 2. Using urinary bladder body strips, developed tension was recorded after cumulative addition of 5-HT (1-100 nM) in the absence or presence of tetrodotoxin (1 µM), and in the presence of tetrodotoxin with ketanserin (0.3-3 µM) or naftopidil (1 and 3 µM). We examined the effects of atropine, ketanserin, and naftopidil on electrical field stimulation (EFS)-induced contraction. RESULTS: 1. Compared to young rats, aged rats exhibited decreased voiding frequency and increased volume per micturition, but total volume of urination (normalized to body weight) did not differ. Moreover, voiding interval was significantly prolonged in aged rats during the active period. 2. In the presence of tetrodotoxin, pEC50 of 5-HT were significantly lower in aged rats than in young rats (P < 0.01), but the maximal response to 5-HT was not altered in the aged bladder. Ketanserin inhibited 5-HT-induced contraction in both groups, while suppression by naftopidil was relatively limited, especially in aged rats. EFS induced neurogenic contraction in a frequency-dependent manner. Atropine-resistant contraction was not inhibited by naftopidil, but was potentiated by ketanserin. CONCLUSIONS: Urination intervals were extended in aged rats, indicating that urination rhythm changed. In the senescent rat bladder, 5-HT induced detrusor contraction, but the effect of 5-HT and the naftopidil-sensitive contractile force were weaker than those in young rats. Additionally, 5-HT did not contribute to the increase in atropine-resistant EFS-induced contractions in aged rats.