Developmental and injury induced plasticity in the micturition reflex pathway.

The storage and periodic elimination of urine are dependent upon neural circuits in the brain and spinal cord that co-ordinate the activity of the urinary bladder, the urethra and the striated urethral sphincter. This study utilized anatomical, electrophysiological and pharmacological techniques to examine: (1) the organization of the parasympathetic excitatory reflex mechanisms that control the urinary bladder of the rat and the cat; and (2) the changes in these reflexes during postnatal development and after spinal cord injury. In normal adult cats and rats, the parasympathetic excitatory input to the bladder is dependent upon a spinobulbospinal reflex pathway that is activated by myelinated (Adelta) bladder afferents and that passes through an integrative center (the pontine micturition center, PMC) in the rostral brain stem. Transneuronal tracing studies using pseudorabies virus as well as physiological methods have revealed that the PMC is located in close proximity to the locus coeruleus. Single unit recordings indicate that neurons in the PMC respond to afferent input from the bladder and are excited prior to or during reflex bladder contractions. Glutamic acid is the major excitatory transmitter in the micturition reflex pathway. Glutamatergic transmission which is mediated by AMPA/kainate and NMDA receptors can be modulated by a variety of other transmitters. In neonatal animals, a spinal micturition reflex is activated by somatic afferent fibers from the perigenital region. This reflex is suppressed during postnatal development, but can be unmasked in adult animals following spinal cord injury. Spinal injury also causes the emergence of a spinal bladder-to-bladder reflex which in the cat is activated by capsaicin-sensitive C-fiber bladder afferents. Patch clamp studies in spinal cord slice preparations indicate that developmental and spinal cord injury induced plasticity in sacral parasympathetic reflex pathways is due in part to alterations in glutamatergic excitatory transmission between interneurons and preganglionic neurons. Changes in the electrical properties of bladder afferent pathways may also contribute to the reorganization of bladder reflexes in paraplegic animals.