The effects of fasting and appetite regulators on catecholamine and serotonin synthesis pathways in goldfish (Carassius auratus)

Monoamine neurotransmitters such as catecholamines [dopamine (DA), norepinephrine (NE) and epinephrine (E)] and serotonin have been shown to influence feeding in vertebrates. In order to better understand the role of monoamine neurotransmitters in the regulation of feeding in fish, we examined the effects of fasting on the brain and intestine gene expression of enzymes involved in their synthesis pathways (SPR: sepiapterin reductase; DHPR: dihydropteridine reductase; TH: tyrosine hydroxylase; TPH: tryptophan hydroxylase; AADC: aromatic l-amino acid decarboxylase; DBH: dopamine β-hydroxylase) in goldfish. In order possible interactions between the monoaminergic pathways and appetite-regulating hormones, we examined the effects of intraperitoneal injections of orexin, CCK and irisin on the brain and intestine gene expression of these enzymes. Fasting increased the expressions of SPR, TH, DBH, TPH1 and DHPR in the brain but did not affect the intestinal expressions of any of the enzymes examined, suggesting that nutritional status might affect the synthesis of monoamines in the central nervous system. CCK injections decreased feeding and increased SPR, TH, and TPH expressions in both brain and intestine. Orexin injections increased feeding and SPR and AADC expressions in the brain but did not affect the expressions of any of the enzymes in the intestine. Irisin injections decreased feeding and increased TPH2 and AADC brain expressions and TH and SPR intestinal expressions, and decreased TPH1 brain expression and AADC intestinal expression. Our results suggest that feeding/fasting and appetite-regulating hormones modulate in part the catecholamine and serotonin synthesis pathways in goldfish.