Neurochemicals and respiratory control during development.

During ontogeny, the central nervous system undergoes neuronal growth, regression, and remodeling. The development of neurotransmitter and modulator systems is a plastic process with individual temporal characteristics for each system. These characteristics include the synthesis, degradation, or uptake of neurochemicals and, largely independently, the appearance of their receptors. Message transmission during ontogeny is compounded by the variable development of these systems and by the coexistence and cofunction among these chemicals. Nine neurochemical systems are discussed: adenosine, gamma-aminobutyric acid, opioids, prostaglandins, serotonin, progesterone, substance P, thyrotropin-releasing hormone, and the catecholamines. The possible role of each of these in natural perinatal respiratory control is evaluated according to predetermined criteria. These include the presence of a substance system in respiratory-related regions, physiologically appropriate changes in its concentration in these regions, elicitation of respiratory effects by agonists and antagonists, and abolition with an antagonist of the effect of a substance during its presumed activation by a physiological process. It is suggested that excessive levels of suppressant neuromodulators or an imbalance among neurochemicals can partly explain the special features of respiratory control in the perinatal period.