CAPSAICIN-INDUCED BLADDER HYPERACTIVITY IN NORMAL CONSCIOUS RATS

Capsaicin, instilled intravesically in normal, unanesthetized rats ind uced a concentration-dependent bladder hyperactivity, which could be a bolished by hexamethonium, given intraarterially near the bladder, or by morphine administered intrathecally. The effect was reversible and could be repeated. The NK-2 receptor selective antagonist SR 48,968 an d the nonselective NK receptor antagonist spantide, given intra-arteri ally near the bladder, which by themselves, in the concentrations used , did not affect cystometric parameters, both counteracted the capsaic in-induced hyperactivity, whereas the NK-1 receptor selective antagoni st RP 67,580 failed to do so. Blockade of tachykinin receptors in the urinary bladder does not seem to produce changes of the micturition re flex associated with bladder filling in the conscious rat. However, ta chykinins released from capsaicin-sensitive nerves by various stimuli may, through stimulation of NK-2 receptors, lower the threshold for in itiation of the micturition reflex. In the rat, intravesical capsaicin may be a suitable model for studies of afferent activity caused by st imuli releasing peptides from sensory nerves in the bladder, thereby p rovoking bladder hyperactivity.