Micturition in the unanesthetized rat: effects of intrathecal capsaicin, N-vanillylnonanamide, 6-hydroxydopamine and 5,6-dihydroxytryptamine.

Unanesthetized rats chronically implanted with vesical and intrathecal catheters were injected intrathecally (i.t.) with either capsaicin (CAP), N-vanillylnonanamide (VN), 6-hydroxydopamine (6-OHDA), or 5,6-dihydroxytryptamine (5,6-DHT). The volume-evoked micturition reflex was assessed by cystometrography before, and 2 h, 1 day and 7 days after injection. In control and vehicle-injected rats, the infusion of saline into the bladder resulted in a periodic contraction of the bladder with synergic sphincter relaxation. One day after i.t. CAP and VN (70 micrograms each), 50% and 30% of the animals displayed a blockade of the micturition reflex, respectively. In these animals, the infusion of saline resulted in a gradual rise in bladder pressure up to a plateau (overflow pressure) equivalent to the predrug bladder opening pressure. When the plateau was reached, continuous dribbling of urine with no bladder contractions was observed. Most of the affected rats displayed some micturition responses by day 7. Following i.t. injection of 5,6-DHT (20 micrograms) or 6-OHDA (20 micrograms), the micturition reflex displayed small but significant increases in bladder volume with no changes in pressure profile. Small primary afferents, sensitive to the neurotoxic effects of CAP and VN appear to play a major tonic role in the regulation of the micturition reflex in unanesthetized rats. Serotonergic and adrenergic descending pathways might play a role in the maintenance of resting bladder tone.