Development of neural receptors for serotonin in the murine bowel

During the ontogeny of the enteric nervous system, the varicosities of mature neurons contact dividing neural precursors, which persist in the developing murine gut for several weeks postnatally. This phenomenon has led to the hypothesis that the release of transmitter from mature neurons may influence the subsequent development of uncommitted neuroblasts. In order to test this hypothesis, it is necessary to know the timing of the expression of postsynaptic receptors for enteric neurotransmitters. Since serotoninergic neurons are among the earliest of enteric neurons to develop (appearing on day E12 of development in the mouse), the ontogeny of enteric neural serotonin receptors (5-HTR) was studied. These receptors have previously been characterized in adult guinea pigs and rabbits. In the current experiments, 5-HTR were identified in the adult murine bowel; their properties were compared with the 5-HTR of guinea pig and rabbits; their ontogeny was followed throughout the length of the developing mouse gut; and the properties of 5-HTR in the developing murine bowel were compared with those of the mature gut. The 5-HTR were assayed by measuring the binding of 3H-serotonin (3H-5-HT) to isolated enteric membranes by rapid filtration, and to frozen sections of bowel by radioautography. A single saturable, high affinity 3H-5-HT binding site was found in membranes from the adult mouse gut (KD = 3.9 +/- 0.5 nM; Bmax = 1.6 +/- 0.3 pmoles/mg protein). Binding of 3H-5-HT at this site was not antagonized by compounds known to be antagonists at receptors for other neurotransmitters or at the 5-HT1 or 5-HT2 class of CNS 5-HTR. Hydroxylation of the indole ring of analogues of serotonin was required for affinity at the enteric 3H-5-HT binding site. Binding of 3H-5-HT was inhibited by N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan dipeptide, a compound that has been demonstrated to antagonize those responses of myenteric neurons to serotonin that resemble slow excitatory postsynaptic potentials, but not by ICS 205-930 (Sandoz), a serotonin antagonist that does not block these responses. All of these properties of adult murine 3H-5-HT binding sites are virtually identical of those of guinea pigs and rabbits, which have previously been shown to be 5-HTR; therefore, murine enteric 3H-5-HT binding sites are probably 5-HTR as well.(ABSTRACT TRUNCATED AT 400 WORDS)