Abstract

Respiration represents an unusual motor activity with respect to its development. As newly born mammals enter the world, their limb movements are not coordinated; time and experience are required for effective performance to be achieved. Yet the rhythm of respiration is of necessity functionally perfected and unfailing at birth. Inspiratory and expiratory motor neurons are already able to fire at appropriate rates, under the command of rhythmically active neurons in the medulla. In this review, we discuss refinements of control present in the newborn opossum, particularly with respect to mechanisms that allow adaptation of respiration to changes in the level of activity or in the outside environment. Our own studies have been aimed at analyzing respiration at the earliest stages, and at establishing the way in which important variables influence inspiration and expiration. To this end, we have used the central nervous system (CNS) of a neonatal opossum, isolated in its entirety and maintained in culture. Although the opossum is unable to walk and highly immature at birth, its respiration is regular and unfailing. The isolated CNS survives, undergoes development, and maintains its neural activity and fine structure in vitro. Moreover, fictive respiration persists for over a day or longer at rates similar to those of the intact pup. The effects of altered pH, of increased temperature, and of drugs known to alter respiratory rhythm in intact animals can be measured directly, by electrical recordings made from medullary neurons or ventral roots. As in a slice, fluids of different composition can be applied focally, through micropipettes to the surface of the ventral medulla, or diffusely to the brainstem, With highly localized application of procaine hydrochloride (2%) to selected areas of the ventral medulla, the respiratory rhythm is reduced or abolished. As in adult mammals, both the rate and the amplitude of respiration simultaneously increase in response to lowered pH (6.5-7.1) or to topical application of 1.0 muM carbachol. Conversely, as expected, the rate and amplitude decrease in response to increased pH (pH 7.5-7.7), or 100 muM scopolamine. Two characteristic features of the control of respiration in the neonatal opossum are evident from such tests. First, changes in rate are achieved by changes in the duration of the expiratory phase of respiration. This result suggests that the timing of the respiratory cycle in the neonatal opossum is controlled by an expiratory instead of an inspiratory off-switch. Second, the rate and the amplitude of the respiratory excursions can be controlled independently, depending on the stimulus. For example, an increase in temperature increases the rate of fictive respiration without changing its amplitude, whereas noradrenaline decreases the rate while increasing the amplitude. Thus, changes of timing and amplitude need not go hand in hand. The opossum CNS offers a favorable preparation for the analysis of neural mechanisms that generate and modulate a motor rhythm, as the animal develops from embryonic to adult stages. (C) 2001 Elsevier Science Inc.