Metabolic cues for puberty onset in free grazing Holstein heifers naturally infected with nematodes

Leptin is a new plausible candidate for the molecular link between nutritio nal status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and dela yed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the Leptin levels during d evelopment in heifers naturally parasitized versus those chronically treate d with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-I previously noted. Insulin levels were also me asured. Prolactin hormone was recorded as an indicator of immune system act ivation. We found a direct correlation between leptin and body weight durin g development and a prepubertal surge of the hormone 2 weeks before the fir st progesterone peak that indicates the onset of puberty. This suggests tha t leptin may act as a signal for this event. Insulin did not vary during gr owth and prepuberty. On the other hand, GH was not responsible for diminish ed IGF-I levels in parasitized animals as levels were similar in both group s. The GH levels were high at birth and then diminished rapidly and remaine d constant during development and puberty. The last hormone studied, prolac tin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection a nd are probably involved in the immune response of the host. (C) 2001 by El sevier Science Inc.