Autoradiographic localization of the GABA-A-receptor agonist (3H)-muscimol in the rat intestinal musculature.

Radioreceptor-binding assay and autoradiography were used to study the pharmacological profile and the anatomical localization of GABA-A-receptor sites in sections of rat duodenum, jejunum and ileum. (3H)-Muscimol, used as a ligand, was bound by sections of the intestinal portions investigated in a manner consistent with the labeling of GABA-A-receptor sites. The dissociation constant (Kd) was about 12.5 nmol/l in the three different intestinal portions. The maximum density of binding sites (Bmax) was highest in the duodenum (118.9 +/- 7.4 fmol/mg tissue followed, in descending order, by the jejunum (105.8 +/- 6.3 fmol/mg tissue) and the ileum (67.8 +/- 5.9 fmol/mg tissue). Light microscope autoradiography revealed a dense accumulation of specific silver grains within intestinal smooth muscle. In the duodenum (3H)-muscimol-binding sites were rather homogeneously distributed both in circular and longitudinal smooth muscle. In the jejunum the density of silver grains was similar to that seen in the duodenum in the circular musculature and lower in the longitudinal musculature. The ileum displayed the lowest accumulation of (3H)-muscimol-binding sites, with no significant differences in the density of silver grains between the two muscular layers. The possible significance of the GABA-A-receptor sites observed in the intestinal musculature is discussed.