Role of intrathecal tachykinins for micturition in unanaesthetized rats with and without bladder outlet obstruction.

1. The effects on micturition of RP 67,580, a selective NK1 receptor antagonist, and SR 48,968, a highly, potent antagonist at NK2 receptor sites, given intrathecally (i.t.) or intra-arterially (i.a.) near the bladder, were investigated in unanaesthetized rats with and without bladder outlet obstruction. 2. In normal rats, RP 67,580, given i.t. in doses of 2 and 20 nmol per rat, decreased micturition pressure, but did not change other cystometric parameters. After 20 nmol of RP 67,580, dribbling incontinence due to retention was observed in 1 out of 7 animals. This effect was reversible. I.t. RP 67,580 in a dose of 2 nmol, had no effect on hyperactivity induced by intravesically instilled capsaicin. 3. In animals with bladder hypertrophy secondary to outflow obstruction, RP 67,580, given i.t. in a dose of 2 nmol per rat, decreased the micturition pressure, but had no effect on other cystometric parameters. After 20 nmol, dribbling incontinence due to retention was observed in 5 out of 7 animals. 4. RP 67,580, given i.a. in a dose of 4 nmol, had little effect on the cystometric parameters investigated, both in normal animals and rats with bladder hypertrophy. 5. SR 48,968, given i.t. in doses of 2 and 20 nmol per rat, had no clear-cut effects on the micturition pattern in normal rats, or rats with bladder hypertrophy. However, the drug reduced capsaicin-induced bladder hyperactivity. When given i.a. in a dose of 4 nmol, SR 48,968 had no effect on cystometric parameters in normal rats or rats with bladder hypertrophy. 6. The effects of both RP 67,580 and SR 48,968 were stereoselective, their enantiomers (RP 68,651 and SR 48,965) being inactive.7. These results thus suggest that at the spinal level there is a tachykinin involvement (via NK,receptors) in the micturition reflex induced by bladder filling, both in normal rats, and, more clearly, in animals with bladder hypertrophy secondary to outflow obstruction. The bladder response to filling was not influenced by blockade of vesical NKI and NK2 receptors. On the other hand, the bladder hyperactivity evoked by intravesical capsaicin seems to involve NK2 receptors both at the bladder and spinal levels.