OPTICAL IMAGING OF THE VENTRAL MEDULLARY SURFACE OF DEVELOPING KITTENS DURING VENTILATORY CHALLENGES

We used large-array optical recording procedures to examine maturation of regional neural activity within the ventral medullary surface (VMS ) of anaesthetized kittens during graded hypercapnic and hypoxic chall enges. The VMS was exposed through a ventral surgical approach in 10, 20, 30, and 45-day-old kittens and in adult cats under sodium pentobar bital anaesthesia. Arterial pressure, costal diaphragmatic EMG, and EC G were continuously monitored. A coherent image conduit with 12 mu fib re resolution was attached to a charge-coupled-device camera and posit ioned over the VMS. Reflected 660 nm light was digitized continuously at 2-s intervals during a baseline period, hyperoxic hypercapnia, (3, 5, and 10% CO2 in O-2), and poikylocapnic hypoxia (6%, 9%, and 12% O-2 in N-2), and recovery. Sixty to seventy-five images within each epoch were averaged, and subtracted from baseline. Regional differences wit hin the image were determined by ANOVA procedures (alpha=0.05). During hypercapnia, an overall decrease in neural activity (increase in scat tered light) occurred, which was marginally age-dependent. By 30 days, regional bidirectional reflectance changes in response to CO2 emerged in a small proportion of animals, and were similar to adult responses . Hypoxia induced a dose- and age-dependent decrease in overall scatte red light. Transient ''on'' and ''off'' responses were common under bo th ventilatory stimuli. In 20-30-day kittens, marked rebound responses in reflectance accompanied cessation of hypoxic stimuli; such pattern s were absent at other ages. At 30 days, a caudal-rostral bidirectiona lity in response to mild hypoxia (12% O-2) began to emerge in a subset of animals. We conclude that dose-dependent responses to ventilatory stimuli occur in the VMS at all post-natal ages of the kitten; however , in hypoxia, the magnitude of the overall reflectance changes is dimi nished relative to adult patterns. Rebound responses to hypoxia are pr esent at particular ages, and older kittens begin to show a topographi cal organization of neural activation.