Classical and 'new' neurotransmitters during development--some examples from control of respiration.

Classical and 'new' neurotransmitters appear in a certain sequence which seem to be similar in rat and man. Serotonin is one of the earliest transmitter which can be detected at a gestational age of 8 days in the rat. A couple of days later noradrenergic and dopaminergic fluorescence can be detected. The development of neurons with gamma-aminobutyric acid and acetylcholine lags behind the monoaminergic neurons. Endorphin is found in high concentrations at an early stage, while substance P, enkephalin and hypothalamic peptides like thyrotropin releasing hormone appear later in development. Inhibitory transmitters like GABA, somatostatin and endorphins reach their maximal concentrations in CNS during infancy, which might have some functional implications. The classical neurotransmitter noradrenaline might have certain unique functions during fetal and perinatal life. It seems to be important for the development of the cerebral cortex. It is released in high quantities at birth and might be of importance for the neonatal adaptation such as inducing arousal. The function of all the newly detected neuropeptides is far from elucidated even in adult life. Some of them seem to have important functions during perinatal life, while perhaps they occur in the adult organism only as evolutionary residues. For example endorphins seem to affect respiratory control in the fetus and the newborn in ways not seen in the adult. So called neuromodulators, for example adenosine, might also have particular functions during perinatal life.