Roles of peripheral and central nicotinic receptors in the micturition reflex in rats

Purpose: We investigated the effects of nicotinic acetylcholine receptor activation in the bladder and central nervous system on the micturition reflex in urethane anesthetized rats. Materials and Methods: The effects of nicotinic acetylcholine receptor activation on bladder activity were examined during continuous infusion cystometrogram. Nicotine with or without the nicotinic acetylcholine receptor antagonist mecamylamine (Sigma Chemical Co., St. Louis, Missouri) was administered intravesically, intrathecally or intracerebroventricularly in normal or capsaicin pretreated rats. We also examined nicotine induced responses in dissociated bladder afferent neurons from L6 to S1 dorsal root ganglia that were sensitive to capsaicin using whole cell patch clamp recordings. Results: Intravesical nicotine (1 to 10 mM) significantly decreased intercontraction intervals in dose dependent fashion. This excitatory effect was abolished by co-application of mecamylamine (3 mM) as well as by capsaicin pretreatment. On patch clamp recordings 300 mu M nicotine evoked rapid inward currents that were antagonized by mecamylamine in capsaicin sensitive bladder afferent neurons. Intrathecal and intracerebroventricular administration of nicotine (10 fig) decreased and increase intercontraction intervals, respectively. Each effect was antagonized by mecamylamine (50 jig) administered intrathecally and intracerebroventricularly. The spinal excitatory effect was significantly inhibited by the N-methyl-D-aspartate receptor antagonist (+)-MK-801 hydrogen maleate (20 kg) given intrathecally or by capsaicin pretreatment, although the effects of capsaicin pretreatment were significantly smaller than those of (+)-MK-801 hydrogen maleate. Conclusions: These results indicate that nicotinic acetylcholine receptor activation in capsaicin sensitive C-fiber afferents in the bladder can induce detrusor overactivity. In the central nervous system nicotinic acetylcholine receptor activation in the spinal cord and brain has an excitatory and an inhibitory effect on the micturition reflex, respectively. In addition, the nicotine induced spinal excitatory effect may be mediated by the activation of glutamatergic mechanisms.