Neuropeptide Y stimulates in vivo gonadotropin secretion in teleost fish

The effects of the neuropeptide Y (NPY), alone or in combination with a gonadotropin-releasing hormone analogue, D-Ala6-desGly10-Pro9-Net LHRH (LHRHa), have been studied on the in vivo secretion of the maturational gonadotropin (GtH2) in the rainbow trout Oncorhynchus mykiss and the common carp Cyprinus carpio. Depending on the species, two routes of administration were used: in trout, intraperitoneal injection (20 micrograms/kg body wt); in carp, direct infusion (3 micrograms/kg body wt) into the third ventricle via a temporary brain cannula. In both cases NPY alone induced a twofold increase in GtH2 secretion and peaked 2 to 4 hr administration regardless of the route of injection. The plasma gonadotropin levels returned to basal within 8 hr. The relative increases (peaked secretion/basal secretion) did not differ with the route of injection. When the animals were first treated with NPY and then LHRHa (20 micrograms/kg) 1 hr later, the magnitude of the response to LHRHa was greater in the animals pretreated with NPY, indicating either a potentiation of LHRHa action by NPY or additive effects of the two peptides. The return to basal levels also took longer in fish receiving NPY first. NPY may act directly at the pituitary level or activate central neuromediatory systems.