Abstract

The synthesis and uptake of 5-hydroxytryptaminc (5-HT) were studied in a preparation of isolated longtiudinal muscle with adherent myenteric plexus. dissected from guinea-pig ileum. 5-HT and catecholamines were demonstrated histochemically by formaldehyde-induced fluorescence. In some experiments, 5-HT was also shown by radioautography. In preparations from untreated animals only adrenergic nerve fibers could be detected histochemically although the preparations contained a small amount of 5-HT. Depletion of catecholamines by reserpine. α-methyl-p-tyrosine or α- methyl-m-tyrosine abolished fluorescence. If catecholamines were depleted by treatment with reserpine and/or α-methyl-p-tyrosine, and monoamine oxidase was subsequently inhibited (by any of three inhibitors), a pattern of widespread yellow fluorescence appeared in the myenteric plexus. Monoamine oxidase inhibition increased the concentration of 5-HT 4-fold. The appearance of this pattern was prevented by treatment of animals with the tryptophan hvdroxylase inhibitor, p-chlorophenylalanine. These results indicate that 5-HT is probably synthesized in the myentcric plexus. Administration of 5-hydroxytryptophan enhanced but did not qualitatively modify the pattern of widespread fluorescence. However, administration of l-dihydroxyphenylalanine restored the control pattern of fluorescence in adrenergic axons. When tissues were incubated in vitro with 5-HT (up to 10^-5 M). the amine was taken up into adrenergic axons but was also found in other elements of the myenteric plexus. In contrast, when tissues were incubated in vitro with norepinephrine (up to 10^-5 M), uptake appeared to be limited to normally adrenergic axons. Although adrenergic axons can take up 5-HT, uptake of the amine is also a characteristic of other unique elements of the myenteric plexus.