Alteration of thermoregulatory behavior in fish by 5-hydroxytryptamine

Guppies were exposed for 30 min to doses of 5-HT ranging from 20 to 120 mg/l, and then tested in a horizontal temperature gradient. Atlantic salmon injected with 2 microgram/g of 5-HT were similarly tested. The guppies exhibited a biphasic relationship between 5-HT dose and mean selected temperature, i.e., the lowest dose significantly decreased the selected temperature, while the highest dose significantly increased it. In the salmon, injection of 5-HT initially decreased the selected temperature by about 4 degrees C. With time, however, the selected temperature gradually increased until by three hours after injection it exceeded the control value by about 1.5 degrees C. This pattern of temperature change is similar to that observed with rabbits subjected to both systemic and central injection of 5-HT.