NITRIC-OXIDE SYNTHASE EXPRESSION IN ENTERIC NEURONS DURING DEVELOPMENT IN THE PIG DUODENUM

The expression of the constitutive neural isoform of nitric oxide synt hase (bNOS) is dynamic and thus forms an ideal parameter to evaluate w hether development and region affect the enteric nervous system. By ap plying NADPH-diaphorase histochemistry on wholemount preparations of t he myenteric and submucosal plexuses and by using the 'unbiased counti ng frame', a qualitative and quantitative description of bNOS-expressi on in enteric neurons in the pig duodenum in various developmental sta ge and region was obtained. Examinations were carried out on the oral and aboral duodenum of fetal pigs from the second half of gestation, o f 1-2-day-old pigs and of 6-8-week-old pigs. In the pig duodenum, thre e enteric plexuses were readily distinguished: the inner submucous, th e outer submucous and the my enteric plexuses. All three plexuses alre ady harboured, to different degrees, bNOS-expressing neurons at midges tation. Although the enteric nervous system was present at midgestatio n, the enteric neurons had not yet reached their adult phenotype and m orphology. During gestation, the number of inner submucous bNOS-expres sing neurons increased approximately 50-fold, whereas after birth that number fell to about 10% of the prenatal value. During further postna tal development it returned to prenatal values. In addition, the numbe r of bNOS-expressing myenteric neurons doubled postnatally. These chan ges favour a role for NO in mediating the development of enteric neuro ns and point to a greater necessity for inhibitory innervation in the adult pig as compared with the fetal pig. Furthermore, the number of b NOS-expressing outer submucosal and myenteric neurons was significantl y higher in the oral duodenal segment compared with the aboral duodena l segment. This regional difference suggests that the oral duodenal se gment is more prominently involved in the regulation of NO-mediated ga strointestinal processes than the aboral one. The developmentally and regionally dependent bNOS-expression can be explained by shifts and di fferences in the balanced system of hormones, presynaptic input and ta rget-derived signals that affects neurotransmitter expression.