Parameters of discharges of respiratory neurons of the ventrolateral medullary regions in early postnatal rats

In experiments on superfusedin situ semi-isolated medullo-spinal preparations (SIMSP) of newborn (1st day of life) and 4- to 5-day-old rats, we studied the parameters of extracellularly recorded spike activity of respiratory neurons of the ventrolateral medullary regions (VLMR). In SIMSP of 4- to 5-day-old rats, the frequency of discharges of pre-inspiratory, inspiratory, and expiratory neurons is shown to be significantly higher, while the dispersion of its values is considerably lower, as compared with the corresponding values for newborn animals. In the majority of pre-inspiratory and inspiratory neurons of SIMSP of newborn rats, irregular low-frequency discharges are usually generated within the interinspiration phase. The relative intensity of suppression of discharges of pre-inspiratory and expiratory neurons within an inspiration phase is much lower in SIMSP of newborn rats, as compared with that in 4- to 5-day-old preparations. The activity of most pre-inspiratory neurons manifests a trend toward transformation from a two-phase pattern in newborn rats (with two frequency peaks, pre- and post-inspiratory) to a monophasic pattern (with one pre-inspiratory frequency peak) typical of 4- to 5-day-old animals. The effects of electrical stimulation of the site of localization of pre-inspiratory neurons showed that in SIMSP of both age groups of rats an inspiratory response could be evoked in then. phrenicus only in the case when stimulation was applied within the second half of an interinspiratory phase. Therefore, it can be supposed that the respiratory network in newborn animals is to a considerable extent immature in the morphofunctional aspect. It seems probable that in early postnatal rats pre-inspiratory neurons are involved in the medullary mechanisms foron-off switching of the inspiratory and expiratory phases.Neirofiziologiya/Neurophysiology, Vol. 28, No. 4/5, pp. 207–217, July–October, 1996.