Day-night rhythms of shoaling behavior in goldfish: opioid and pineal involvement

Intracerebroventricular (ICV) administration of the opioid peptide, beta-endorphin (5.0 pg/g) to goldfish, Carassius auratus, significantly increased the cohesiveness and duration of shoaling ('bout' length) in shoals of five fish, as well as decreasing the latency of shoal formation in response to an external disturbance, while a higher dose of beta-endorphin (15 pg/g) decreased shoaling. There were day-night rhythms in shoaling and in the extent of the facilitatory effects of beta-endorphin (5.0 pg/g) on shoaling behavior, the fish displaying significantly greater shoaling responses in the day than during the night. The facilitatory effects of the low dose of beta-endorphin were blocked by systemic administration of naloxone (1.0 mg/kg), while ICV administrations of naloxone (1.0 pg/g) decreased daytime shoaling behavior. Removal of the pineal gland disrupted the day-night rhythm of shoaling, reducing daytime levels of shoaling. In addition, pinealectomy reduced the stimulatory effects of beta-endorphin (5.0 pg/g) on shoaling, and attenuated the day-night rhythms in the effects on beta-endorphin on shoaling. These results suggest that both opioid systems and the pineal gland influence shoaling behavior and the expression of its day-night rhythm in goldfish.