Postnatal changes in intrinsic optical responses to peripheral nerve stimulation in the in vivo rat spinal cord

We previously applied an intrinsic optical imaging technique to the in vivo rat spinal cord, and showed its feasibility for analyzing neural functions. Three- to 7-week-old postnatal and adult (>10-week-old) rats were anesthetized and laminectomy was performed between C5 and Th1 to expose the dorsal surface of the cervical spinal cord. Optical reflectance changes in response to simultaneous stimulation of the median and ulnar nerves were recorded from the spinal cord at a depth corresponding to the dorsal horn (substantia gelatinosa), using a differential video acquisition system. The shape and extent of the area of the evoked optical signals varied during postnatal development. In 3- to 4-week-old rats, the response area was small and ellipsoidal, and constrictions between spinal segments were not clear. As postnatal ages proceeded, the area expanded in a rostrocaudal direction, and the constrictions became clear. In adult rats, the response area exhibited a segmental pattern. At all ages, the optical responses showed trial-to-trial variations. A semiquantitative analysis of the variations in one spinal segment showed that the variations in a mediolateral direction appeared to decrease slightly with development, although statistical differences were insignificant between ages. The dynamical changes in optical response patterns were discussed in relation to postnatal maturation of neural functions in the spinal cord.