Detection of the 5‐HT1Areceptor and 5‐HT1Areceptor mRNA in the rat bowel and pancreas: Comparison with 5‐HT1Preceptors

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Abstract
We tested the hypothesis that the rat bowel and pancreas contain 5‐HT1A receptors. 3H‐8‐hydroxy‐2‐(di‐n‐propylamino) tetralin (3H‐8‐OH‐DPAT) was used as a radioligand. Binding of 3H‐8‐OH‐DPAT to membranes derived from the myenteric plexus and the pancreas was investigated by rapid filtration. Alternatively, radioautography was employed to locate 3H‐8‐OH‐DPAT binding sites in frozen sections of unfixed bowel or pancreas. An excess of 5‐HT (10 μM) was used to define nonspecific binding. Saturable, high affinity binding of 3H‐8‐OH‐DPAT to enteric (Kd= 2.8 ± 1.1 nM; Bmax =83.8 ± 4.3 fmol/mgproteim) and pancreatic (Kd = 6.6 ± 1.3 nM; Bmax = 44 ± 2.2 fmol/mg protein) membranes was found. The binding of 3H‐8‐OH‐DPAT to enteric and pancreatic membranes was inhibited by 8‐OH‐DPAT, NAN‐190, and spiperone. In contrast, the binding of 3H‐8‐OH‐DPAT to enteric and pancreatic membranes was not inhibited by 5‐carboxyamidotryptamine, or by avariety 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 3H‐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‐HT1Areceptors were located on nerves, lymphoid tissue (especially the capsule of nodes), and on cells scattered in the pancreatic parenchyma. The concentration of 3H‐8‐OH‐DPAT binding sites in the rat bowel and pancreas was less than that of 3H‐5‐HT binding sites; however, the distribution of 3H‐8‐OH‐DPAT binding sites was similar to that of sites that bind 3H‐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.