No abstract

Peripheral neural 5-hydroxytryptamine (5-HT) receptors are different from both classes 5-HT1 and 5-HT2, which have been described from studies of 5-HT receptors in the brain. Recently, it has been shown that, as in the CNS, there is more than a single type of neural receptor for 5-HT in the enteric nervous system. One of these, called 5-HT1P, has a high affinity for 3H-5-HT, initiates a long-lasting depolarization of enteric neurons associated with an increase in membrane resistance, and is the physiological receptor through which enteric serotoninergic neurons mediate slow EPSPs. The other receptor, called 5-HT3 (5-HT2P), does not bind 3H-5-HT with high affinity, and initiates a brief depolarization of enteric neurons with decreased input resistance, but a physiological action of 5-HT mediated by these receptors has not yet been identified. Hydroxylated indalpines have been found to be agonists at 5-HT1P receptors. We have now examined 5-HT1P receptors using 5-hydroxyindalpine (5-OHIP) as a probe. The action of 5-OHIP on enteric neurons was determined electrophysiologically and compared with that of 5-HT; the binding of 3H-5-OHIP to isolated enteric membranes was studied by rapid filtration, and to frozen sections of tissue by radioautography. 3H-5-OHIP binding was compared with that of 3H-5-HT. 5-OHIP, like 5-HT, induced a triphasic response in most enteric neurons: an initial short-lived depolarization, during which input resistance fell, followed by recovery, and then a long-lasting depolarization, during which the input resistance increased. 5-OHIP bound saturably, reversibly, and with high affinity to enteric membranes (Kd = 7.6 +/- 0.7 nM; Bmax = 76 +/- 14 fmol/mg protein). Binding of 3H-5-OHIP was not inhibited by agents that bind to alpha- or beta-adrenoceptors, nicotinic or muscarinic receptors, histamine H1 or H2 receptors, or 5-HT1(A,B,C, or D), 5-HT2, or 5-HT3 receptors, but was displaced by substances, such as hydroxylated indoles and a dipeptide of 5-hydroxytryptophan (5-HTP-DP), that antagonize the binding of 3H-5-HT to enteric membranes or tissue sections. It is concluded that 5-OHIP is an agonist at peripheral neural 5-HT1P receptors and can be used to label these receptors selectively outside the brain. Radioautographs demonstrated enteric 5-HT1P receptors in the lamina propria of the intestinal mucosa and in the submucosal and myenteric plexuses. Extraenteric 5-HT1P receptors were also found in the skin and heart. It is suggested that 5-HT1P receptors may be found on subtypes of primary afferent nerve fibers.

A large number of human and animal studies have challenged the hypothesis that cystic duct obstruction by gallstones causes cholecystitis. These studies suggest that lithogenic bile that can deliver high cholesterol concentrations to the gallbladder wall causes hypomotility and creates a permissive environment that allows normal concentrations of hydrophobic bile salts to inflame the mucosa and impair muscle function inhibiting gallbladder emptying. High concentrations of cholesterol increase its diffusion rates through the gallbladder wall where they are incorporated into the sarcolemmae of muscle cells by caveolin proteins. High caveolar cholesterol levels inhibit tyrosine-induced phosphorylation of caveolin proteins required to transfer receptor-G protein complexes into recycling endosomes. The sequestration of these receptor-G protein complexes in the caveolae results in fewer receptors recycling to the sarcolemmae to be available for agonist binding. Lower internalization and recycling of CCK-1 and other receptors involved in muscle contraction explain gallbladder hypomotility. PGE2 receptors involved in cytoprotection are similarly affected. Cells with a defective cytoprotection failed to inactivate free radicals induced by normal concentrations of hydrophobic bile salts resulting in chronic inflammation that may lead to acute inflammation. Ursodeoxycholic acid salts (URSO) block these bile salts effects thereby preventing the generation of free radicals in muscle cells in vitro and development of cholecystitis in the ligated common bile duct in guinea pigs in vivo. Treatment with URSO improves muscle contraction and reduces the oxidative stress in patients with symptomatic cholesterol gallstones by lowering cholesterol concentrations and blocking the effects of hydrophobic bile salts on gallbladder tissues.