Mode of action of the neurotransmitter 5‐hydroxytryptamine in stimulating ovarian maturation in the red swamp crayfish, Procambarus clarkii: An in vivo and in vitro study

Ovarian explants from the red swamp crayfish, Procambarus clarkii, were incubated with 5‐hydroxytryptamine (5‐HT) and muscle, and with 5‐HT in combination with eyestalk tissue, brains, or thoracic ganglia. In vitro, 5‐HT without brain or thoracic ganglia in the incubation medium had no significant effect on the ovarian explants. In contrast, in vitro brains and thoracic ganglia induced ovarian maturation, indicating the presence of a gonad‐stimulating hormone (GSH) in these tissues. However, this in vitro stimulatory effect was significantly greater when 5‐HT was present in the incubation medium with the brains and thoracic ganglia. These results provide in vitro evidence in support of the hypothesis that the previously observed in vivo stimulatory action of 5‐HT on the ovaries is an indirect one, 5‐HT acting to stimulate release of a GSH.