Interaction Between Photoperiod and Gonadal Feedback on cck Expressions in Three-spined Stickleback, Gasterosteus aculeatus

Abstract

Cholecystokinin (CCK) is a peptide hormone that plays crucial roles not only in the digestive system but also in the central nervous system as a neurotransmitter. Recent research indicate that it is involved in the reproductive endocrine system, inducing the release of follicle-stimulating hormone (FSH) in medaka, Oryzias latipes. The present study tested whether cck expression is involved in the control of maturation in three-spined stickleback (Gasterosteus aculeatus). To that end, castrated and sham-operated males were exposed to short-day (8L:16D) or long-day (16L:8D) photoperiods for 3 or 30 days. Pituitary fsh beta was strongly elevated in castrated fish, particularly under short-day conditions, whereas lh beta increased only in sham-operated fish after 30 days under long-day conditions. Castration increased hypothalamic ccka and cckb expression under short days, but decreased ccka after long-day exposure for 30 days. Photoperiod influenced whole-brain and hypothalamic ccks expression differently. Whole-brain ccks levels were generally higher under short days and lower under stimulatory long days, whereas these photoperiodic effects were largely absent in the hypothalamus. These findings suggest that hypothalamic cck genes may participate in feedback regulation on the BPG axis of the stickleback; however, FSH secretion and photoperiodic reproductive control likely involve additional factors.