Maturation of kitten ventral medullary surface activity during pressor challenges

We used large-array optical recording procedures to examine maturation of regional neural activity within the ventral medullary surface (VMS) of anesthetized kittens during pharmacologically induced blood pressure elevation. Under sodium pentobarbital anesthesia, the VMS was exposed in 10, 20 and 30- to 45-day-old kittens and in adult cats. Arterial pressure, costal diaphragmatic EMG, and ECG were continuously monitored. An imaging camera, composed of a charge-coupled device and a coherent bundle of optic fibers, was positioned over the VMS. Light at 660 nm illuminated the neural tissue, and was collected by the probe. Resulting light-scatter images were acquired at 2-second intervals during a baseline period, and following intravenous administration of phenylephrine at 10, 20 and 40 micrograms/kg. Sixty to seventy-five images within each epoch were averaged, and subtracted from baseline. Regional differences within the image were determined by ANOVA procedures (alpha = 0.05). Phenylephrine elicited dose-dependent elevations of blood pressure accompanied by decreased diaphragmatic EMG activity which were less profound in younger animals. With maturation, responsiveness of respiratory patterning to the pressor response increased. In contrast to adult cats, 10-day kittens increased VMS neural activity in a dose-dependent fashion with pressor stimulation. A progressive transition to adult response patterns was observed with increasing postnatal age, and was established in over half of the kittens by 30-45 days. We conclude that phenylephrine-induced baroreceptor stimulation elicits divergent VMS activity responses in developing and mature animals. Such a developmental pattern may reflect immature function of central and/or peripheral baroreflexes.