Effects of dopamine and alpha-2 adrenoreceptor blockade on L-dopa and cholecystokinin-induced gastroprotection.

Dopamine and cholecystokinin have been colocalized in neurons and represent endogenous enteric neurotransmitters. Both peptides possess potent protective actions against gastric injury when given exogenously. This study was undertaken in conscious female rats to test the hypothesis that cholecystokinin may exert its protective actions via release of dopamine. Experiments were designed to ascertain whether L-dopa, a dopamine precursor, could prevent gastric injury with the same degree of efficacy as cholecystokinin and to determine what role alpha-2 adrenoreceptors and dopamine receptors play in mediating the protective actions of these peptides. Intraperitoneal administration of L-dopa (1 to 25 mg/kg) in a dose-dependent manner prevented the type of macroscopic injury to the acid-secreting portion of the stomach that is caused by 1 ml of orogastric acidified ethanol (150 mmol/L hydrochloric acid/50% ethanol), an effect corroborated by histologic examination. Administration of either the alpha-2 adreno-receptor antagonist yohimbine (0.1 to 1.0 mg/kg) or the dopamine receptor antagonist haloperidol (1 to 5 mg/kg) caused a partial reversal of L-dopa-induced protection but not the protective actions of subcutaneous cholecystokinin (100 microg/kg). Simultaneous administration of both receptor antagonists had an additive effect and completely reversed the protective actions of L-dopa. The dopamine precursor L-dopa was just as effective in maintaining the integrity of the gastric epithelium in the face of a damaging insult as the gut peptide cholecystokinin. However, the data indicate that L-dopa initiates its protective actions through activation of both alpha-2 adrenoreceptors and dopamine receptors, whereas the protective effects of cholecystokinin are elicited by means of a different mechanism.