Dopamine receptors in human gastrointestinal mucosa

Abstract

Dopamine is a putative enteric neurotransmitter that has been implicated in exocrine secretory and motility functions of the gastrointestinal tract of several mammalian species including man. This study was designed to determine the presence of dopamine binding sites in human gastric and duodenal mucosa and to describe certain biochemical characteristics of these enteric receptor sites. The binding assay was performed in triplicate with tissue homogenates obtained from healthy volunteers of both sexes using 3H-dopamine as a ligand. The extent of nonspecific binding was determined in the presence of a 100-fold excess of unlabeled dopamine. Scatchard analysis performed with increasing concentrations of 3H-dopamine (20–500 nM) revealed a single class of saturable dopamine binding sites in gastric and duodenal mucosa. Binding parameters obtained from the regression lines of the Scatchard plots of gastric mucosa of males were B max = 73.4±4.0 pmoles/mg protein; K D = 154±20 nM and B max = 95±13.6 pmoles/mg protein and K D = 826±200 nM in females. In duodenal mucosa of males these parameters were B max = 63.9±15.9 pmoles/protein; K D = 235±53 nM and B max = 83.2±19.4 pmoles/mg proteinK D = 568±104 nM in females. The results of this report demonstrate the presence of specific dopamine receptors in human gastric and duodenal mucosa. These biochemical data suggest that molecular abnormalities of these receptor sites may be operative in the pathogenesis of important gastrointestinal disorders. The view that the autonomic innervation of the mammalian gastrointestinal tract is provided by parasympathetic (cholinergic) excitatory and sympathetic (adrenergic) inhibitory fibers has been expanded by the findings that these fibers contain not only acetylcholine and norepirephrine, but also several other aminergic and peptidergic neurotransmitters (1,2). Immunohistochemical studies have shown, for example, that the vagus nerve (in addition to acetylcholine) contains important amounts of serotonin, norepinephrine, gastrin, somatostatin, enkephalin, substance P and vasoactive-intestinal peptide (3). These findings suggest that the neural control of the gut is considerably more complex than originally thought, and raise the possibility that exocrine secretory and motility functions of the gut may be regulated by multiple enteric neurotransmitters. Recent evidence suggests that dopamine may be an important transmitter in enteric synapses, as well as a precursor molecule in the biosynthesis of norepinephrine in peripheral adrenergic terminals (4). Although, specific immunohistochemical demonstration of tyrosine hydroxylase in gastric tissue has not been provided, significant amounts of dopamine have been found in the gastrointestinal mucosa and muscle of several mammalian species (5) and in human gastric juice (6,7). In addition, it is also important to indicate that DOPA decarboxylase activity has been detected in a specific enterochromaffin-like cell system in the gastric mucosa (8). There is also evidence that dopamine levels in gastric mucosa but not in muscle are regulated independently of either parasympathetic or sympathetic control (9). Furthermore, neurophysiological studies indicate that dopamine, like norepinephrine, exerts an inhibitory effect on cholinergic neurotransmission by including hyperpolarization of the post-synaptic membrane (10). Inhibitory actions of dopamine and dopamine-related compounds on acid secretion (11–14), gastric emptying (15), antral activity (16), and intragastric pressure have been demonstrated in rats, dogs and man (17). Specific high-affinity dopamine receptors and different molecular species of these receptor sites have been extensively studied in rat and human brain tissue (18,19). Similar, but less extensive studies, have been performed with gastrointestinal tissue. Szabo et al. (20) provided an initial description of dopamine receptors in rat stomach and duodenum, and suggested that they may be involved in the pathogenesis of experimental duodenal ulceration. More recently, we have provided a detailed biochemical characterization of specific receptor sites for dopamine in rat gastric mucosa and muscle and suggested that these molecular entities may play a critical role in the onset of stress ulcers (21). This body of evidence indicates that dopamine and dopamine receptors may play an important role in gastrointestinal physiology and suggest that similar dopamine receptors may also be present in human gastrointestinal tissue. The study described below was specifically designed to determine the presence of dopamine binding sites in human gastric and duodenal mucosa.