Reproduction phase-related variations in the GnRH immunoreactive fibers in the pineal of the Indian major carp Cirrhinus mrigala (Ham.).

We studied GnRH immunoreactivity in the pineal gland of the Indian major carp Cirrhinus mrigala during different phases of reproductive cycle. In the resting phase (December-January), GnRH immunoreactive (-ir) fibers were organized as paired fascicles above the posterior commissure that ascend in the stalk and distribute widely in the pineal gland. The GnRH-ir fiber density significantly declined (P<0.001) during the preparatory phase (February-April) and the fibers disappeared thereafter. While no GnRH fibers were seen during the prespawning (May-June) and spawning (July-August), isolated GnRH-ir fibers reappeared in the postspawning phase. Since no GnRH cell bodies were detected in the pineal, these GnRH-ir fibers seem to be of central origin. The results reveal a distinct reciprocal relationship between the GnRH immunoreactivity in the pineal and the status of the ovarian maturity; the fibers appeared in the pineal only during the period of ovarian quiescence. While the functional significance of these cyclic changes in GnRH is yet to be determined, we suggest that the decapeptide may serve as a transmitter of central origin that modulates the activity of the pineal gland.