Preparing for the first breath: prenatal maturation of respiratory neural control

Abstract

By birth, the regulatory neural network responsible for respiratory control is capable of generating robust rhythm-driving ventilation that can adjust to homeostatic needs. The advent of in vitro models isolated from prenatal rodents has significantly advanced our understanding of these processes. In this topical review, we examine the development of medullary respiratory rhythm-generating centres and phrenic motoneurone-diaphragm properties during the prenatal period.

Figure 1. In vitro and in vivo recordings of fetal respiratory activity in rat

A, rectified and integrated suction electrode recordings of C4 ventral root activity from prenatal brainstem–spinal cord preparations. B, rectified and integrated suction electrode recordings made from the pre-BötC (PBC) and XII motoneurone pool of prenatal medullary slice preparations. Rhythmic respiratory discharge commenced at E17 and the frequency and amplitude of inspiratory bursting increased in an age-dependent manner in both types of in vitro preparations. C, ultrasound image of fetal rat used to measure the incidence of fetal breathing movements. Right panel shows graph depicting the age-dependent increase in FBMs from E16–E20. FBMs occurred as isolated single movements or as episodes of clustered movements lasting 40–180 s. Abbreviations: H, heart; D, diaphragm; S, stomach; SC, spinal cord.

Figure 2. Identification and targeting of pre-BötC neurones within the ventrolateral medulla of perinatal rats

A, an illustration in sagittal view of the primary structures in the ventrolateral medulla along the rostral (R)–caudal (C) axis. B, immunolabelling for NK1R (green) in E17 sagittal section of the ventrolateral medulla. The dotted circle demarcates the approximate area of the pre-BötC (calibration bar = 100 µm). C, photomicrograph of medullary slice preparation used for whole-cell recordings. D, IR-DIC image of the ventrolateral medulla with recording electrode positioned in the vicinity of the pre-BötC. E, neurone within the pre-BötC that has been fluorescently tagged via the internalization of TMR-conjugated SubP. F, whole-cell recording of membrane potential from a Tetramethylrhodamine (TMR)-SubP-positive neurone and simultaneous recording of integrated XII nerve inspiratory activity before and during block of rhythmic activity by bath application of CNQX. The neurone continues to burst after application of CNQX, consistent with it having pacemaker properties. Abbreviations: VII, facial nucleus; NAc, nucleus ambiguus, pars compacta; NAsc, nucleus ambiguus, semicompact NA, nucleus ambiguus; RTN, retrotrapezoid nucleus; pFRG, parafacial respiratory group; BötC, Bötzinger complex; pre-BötC, pre-Bötzinger complex; rVRG, rostral ventral respiratory group; LRN, lateral reticular nucleus.

Figure 3

Time line illustrating key events in the development of respiratory neuronal activity in fetal rats.