The 5-HT_1A receptor antagonist WAY-100635 (WAY i.c.v. & i.t.) suppresses micturition in anaesthetised rats (Secker et al, 2003). Further, repeated administration of WAY was significantly less effective than the initial dose at suppressing micturition (Secker et al, 2003). The current study investigates the ability of the micturition reflex to, seemingly, become tolerant to the effects of WAY. This was carried out by studying the effect of chronic administration of WAY and the structurally different 5-HT_1A receptor antagonist robalzotan (NAD-299; Johansson et al., 1997) on the micturition reflex in conscious and anaesthetised female rats.

Thirty two female Sprague Dawley rats (250-350g) were implanted subcutaneously, under isoflurane anaesthesia with osmotic pumps (model 1002 ALZET; 14 day administration, 0.25 µl h^-1) connected to the jugular vein for constant i.v. administration of WAY or NAD to give a free plasma concentration of 10 and 2.5 nM, respectively (which have previously been shown to attenuate micturition frequency, unpublished data) or their vehicles (saline and acidified saline). These animals were placed in metabolic cages on the 4, 6, 8,11,13,15 & 18^th days after implantation so measurements could be made of total volume voided (ml), volume per void (ml) and frequency of voiding over a 3h period. All rats were pretreated with furosemide (2 mg kg^-1, i.p.) and a saline load (20 ml kg^-1, i.p.) prior to placing them in these cage. 4 rats from each group on day 11 were terminally anaesthetized (urethane 1.2 g kg^-1; i.p.) to obtain a more accurate measurements of inter-void frequency and test the effect WAY & NAD (given i.v. over 30 min to give a final plasma concentration of 10 and 2.5 nM). This was also carried out on day 20. Drug levels were confirmed at the end of the experiments. Changes in conscious parameters and the anaesthetized inter-void intervals were compared to vehicle control by ANOVA and unpaired Student's t-test, respectively (P<0.05 considered significant).

Neither WAY nor NAD had significant effects on any of the variables except the frequency on day 6 (-58 ± 6 and 56 ± 28% of control, mean ± s.e.mean) in conscious rats. In the anaesthetised animals on both days 11 and 20 there was no significant difference in the inter-void interval in chronic WAY (17.6 ± 24.2 & -33.4 ± 7.9%) and NAD (23.9 ± 21.2 & 105.3 ± 52.1%) treated animals when compared to vehicle control. A subsequent infusion of WAY caused a significant increase in the time to first void in vehicle pre-treated animals (308.3 ± 49s to 564.8 ± 83s day 11 and 535.6 ± 104s to 935.9 ± 188s for day 20), however, it had no effect in WAY pre-dosed animals (362.7 ± 75s to 502.4 ± 164s and 343.5 ± 40s to 450.0 ± 123.6s respectively). Similarly NAD was ineffective in NAD pretreated rats but effective in vehicle treated rats.

These data indicate that tolerance develops to the effects of both WAY and NAD and that it is not compound specific but related to their ability to interfere with 5-HT_1A receptors.

Johansson, L. et al. (1997) J. Pharmacol. Exp. Ther. 283, 216-225.
Secker, A.G. et al. (2003) pa2online.org/Vol1Issue2abst020P.html

AGG is a MRC CASE student with Pfizer Ltd.