Gut motility in goldfish (Carassius auratus)role of dopamine

Abstract

Dopamine (DA) is recognized as a neurotransmitter of the enteric nervous system, regulating digestive processes and intestinal motility in mammals. However, there is no such evidence to date in fish gut. A previous report shows that DA is acetylated in the intestine of goldfish (Carassius auratus) in vitro, suggesting possible functions of DA at this location. Therefore, the aim of the present study was to investigate the possible role of DA on gut motility in goldfish, and deep into the receptors and the intracellular signaling pathways involved. We used an in vitro organ bath system, coupled to an isometric force transducer for studying DA effect on gut motility. Conventional PCRs were carried out to identify the distribution pattern of DA receptor subtypes in different layers of the intestinal bulb. A biphasic effect of DA was observed in vitro inducing gut contraction at low concentration (1μM), and relaxations at higher concentrations (10 and 100 μM). To identify the possible receptors involved in the DA relaxation effect, we used a specific antagonist for D1 (R(+)SCH23390) and D2 (domperidone) dopaminergic receptors, and antagonists for α- and β- adrenergic receptors (yohimbine and propranolol, respectively). The relaxation induced by DA was totally blocked by the D1 receptor antagonist. Moreover, we found that D1a1, D1b, D1c1, D2a, and D3 receptor subtypes are differently distributed in the muscular-serosal and mucosa-submucosal layers. The relaxing effect of DA was not blocked by the inhibitors of the following enzymes: nitric oxide synthase (Nω- Nitro-L-arginine methyl ester, L-NAME), adenylyl cyclase (2’, 3’, dideoxyadenosine, DDA) and guanylyl cyclase (1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one, ODQ). Finally, this DA effect seems to be on muscle cell directly, since it is insensitive to a voltage-dependent Na+ channel blocker (tetrodotoxine). These results show the first evidence of DA as a neurotransmitter in the enteric nervous system that regulates gut motility in fish via specific receptors.