DETECTION OF THE 5-HT1A RECEPTOR AND 5-HT1A RECEPTOR MESSENGER-RNA IN THE RAT BOWEL AND PANCREAS - COMPARISON WITH 5-HT1P RECEPTORS

We tested the hypothesis that the rat bowel and pancreas contain 5-HT1A receptors. H-3-8-hydroxy-2-(di-n-propylamino)tetralin (H-3-8-OH-DPAT) was used as a radioligand. Binding of H-3-8-OH-DPAT to membranes derived from the myenteric plexus and the pancreas was investigated by rapid filtration. Alternatively, radioautography was employed to locate H-3-8-OH-DPAT binding sites in frozen sections of unfixed bowel or pancreas. An excess of 5-HT (10 muM) was used to define nonspecific binding. Saturable, high affinity binding of H-3-8-OH-DPAT to enteric (K(d) = 2.8 +/- 1.1 nM; B(max) = 83.8 +/- 4.3 fmol/mg protein) and pancreatic (K(d) = 6.6 +/- 1.3 nM; B(max) = 44 +/- 2.2 fmol/mg protein) membranes was found. The binding of H-3-8-OH-DPAT to enteric and pancreatic membranes was inhibited by 8-OH-DPAT, NAN-190, and spiperone. In contrast, the binding of H-3-8-OH-DPAT to enteric and pancreatic membranes was not inhibited by 5-carboxyamidotryptamine, or by a variety of compounds known to bind to other subtypes of 5-HT receptor. Digoxigenin-labeled oligonucleotides were found to detect mRNA encoding the 5-HT1A receptor in a subset of neurons in myenteric and submucosal ganglia. In contrast, 5-HT1A mRNA was not found in the pancreas. Radioautography revealed that the highest density of H-3-8-OH-DPAT binding sites was found in the stomach. These sites were especially numerous in the lamina propria adjacent to gastric glands, and in myenteric ganglia. Pancreatic 5-HT1A receptors were located on nerves, lymphoid tissue (especially the capsule of nodes), and on cells scattered in the pancreatic parenchyma. The concentration of H-3-8-OH-DPAT binding sites in the rat bowel and pancreas was less than that of H-3-5-HT binding sites; however, the distribution of H-3-8-OH-DPAT binding sites was similar to that of sites that bind H-3-5-HT. It is concluded that the rat gut and its extension in the pancreas contains 5-HT1A receptors. Many, if not all, of the nerve cells and processes that express 5-HT1A receptors express 5-HT1P receptors as well. The function of these receptors in the physiology of the entero-pancreatic innervation remains to be determined.