Neurochemical differentiation of rat enteric neurons during pre- and postnatal life

The presence of nitric oxide synthase (the enzyme which synthesizes nitric oxide), vasoactive intestinal peptide (VIP) and pituitary adenylyl-cyclase-activating peptide (PACAP) in the rat enteric nervous system between term (E18) and 90 days postpartum (P90) was investigated by immunohistochemistry. Neuronal maturity was ascertained by emanining for the presence of two novel neuron intermediate filament proteins: alpha-internexin, which is transiently expressed in developing neurons, and peripherin, which is expressed in differentiating as well as in mature neurons. The alimentary canal of the foetus at E18, unfed newborn, suckling, weaning and adult rats was studied. Throughout the rat gut the myenteric neurons were recognized by the time the foetal stage was reached and the submucous neurons by the suckling period. alpha-Internexin was present at E18; its levels increased during ageing and were markedly reduced at P30. Peripherin was first detected at birth; its levels increased with ageing and practically almost all of the neurons and nerve fibres were labelled from P30. Nitric oxide synthase was present at E18 and the number of labelled neurons gradually increased with ageing, while both VIP and PACAP could be detected by the end of the suckling period in an equal number of neurons to that in adulthood. These data indicate that in the rat gut the onset of the mature neuronal phenotype is not yet achieved at birth and that the neurochemical differentiation is accomplished during the first month of postnatal life.