Inhibitory control of the urinary bladder in the neonatal rat in vitro spinal cord-bladder preparation

Urinary bladder activity of the neonatal rat is tonically inhibited by neural input from the spinal cord passing through axons in the pelvic nerve. The present study was undertaken to examine the organization of this inhibitory mechanism using in vitro spinal cord-bladder preparations of neonatal rats in which the lumbosacral dorsal roots (DRs) or ventral roots (VRs) were transected. Isovolumetric bladder contractions occurring spontaneously or induced by electrical stimulation of the bladder wall (ES-BW) were measured. In DR transected (DRT) preparations, removal of the spinal cord significantly enhanced (50-59%) the amplitude of spontaneous and ES-BW-evoked bladder contractions; whereas in VR transected (VRT) preparations removal of the spinal cord produced only a small enhancement (6.7-12%). However, in VRT preparations, electrical stimulation of the dorsal roots reduced the amplitude of spontaneous contractions, an effect blocked by a nicotinic ganglionic blocking agent, hexamethonium. In DRT preparations, MK-801 enhanced the amplitude of spontaneous and ES-BW-evoked contractions. These results demonstrate that bladder activity of the neonatal rat is tonically inhibited by input from the lumbosacral spinal cord via parasympathetic pathways in the pelvic nerve. The inhibitory outflow is not dependent upon afferent input to the cord but is facilitated by NMDA glutamatergic transmission in the spinal cord. Antidromic activation of afferent axons also appears to induce inhibition in the bladder via a mechanism involving nicotinic cholinergic receptors. These findings suggest that spinal and peripheral inhibitory mechanisms may play an important role in controlling voiding in the neonatal rat.