Recurrent inhibition of the bladder C fibre reflex in the cat and its response to naloxone

Recurrent inhibition of the bladder C fibre reflex was studied in adult female cats anaesthetized with alpha-chloralose. Test reflexes were evoked by electrical stimulation of bladder A delta and C afferents in the right pelvic nerve and were recorded from the proximal end of a small ipsilateral pelvic nerve branch, transected close to the bladder. Such test reflexes were consistently depressed by repetitive electrical stimulation of the contralateral bladder pelvic nerve (20 Hz, 20 s) at intensities sufficient to recruit axons of bladder preganglionic neurones. The inhibition could be evoked after transection of the left dorsal roots S1-S4 and the sympathetic supply to the bladder but was abolished by transection of the pelvic nerve central to the site of stimulation. Hence, it most likely involved central recurrent collaterals of antidromically activated bladder preganglionic neurones. The reflex suppression was quite considerable - maximal C fibre reflexes were reduced to a group mean of 25% (+/- 9% confidence interval) of their control size. The effect had a slow onset, requiring a few seconds of conditioning stimulation to be revealed, and was very long lasting (minutes). Naloxone (0.01-0.5 mg kg(-1) i.v.) abolished the recurrent inhibition of both the C fibre and A delta bladder reflexes, while inhibition from afferents in the dorsal clitoris nerve remained unchanged. It is concluded that the segmental bladder C fibre reflex and the spino-ponto-spinal A delta micturition reflex are both targets of recurrent inhibition from bladder parasympathetic preganglionic neurones and that the effect involves an enkephalinergic mechanism.