Surgical removal of the olfactory bulbs increases sensitivity of the reproductive system of female rats to the inhibitory effects of late afternoon melatonin injections

In rats, anosmia induced by olfactory bulbectomy sensitizes the neuroendocrine‐reproductive axis to the inhibitory effects of the pineal gland. Thus, combined blinding (which stimulates pineal antigonadotrophic activity) and anosmia cause a marked delay in the sexual maturation of female rats. The present study tested whether anosmia would also sensitize the neuroendocrine axis to the reproductive inhibitory effects of melatonin. In the present study, young female rats were rendered anosmic and maintained under light:dark cycles of 14:10; they were given melatonin injections daily (25 μg) at either 0900 hr (AM‐melatonin) or at 1800 hr (PM‐melatonin)—i.e., either 3 or 12 hr after lights on, respectively. The melatonin injections were begun when the rats were 23 days of age and were continued for 35 days. By comparison with AM‐melatonin, PM‐melatonin injections were considerably more effective in inhibiting bodily growth as well as the weights of the anterior pituitary, ovaries, and uterus of anosmic rats. Similarly, the changes in pituitary and plasma LH and PRL levels were more obvious in the PM‐treated anosmic rats than in those given melatonin in the morning. Thus, anosmia does increase the sensitivity of the neuroendocrine‐reproductive axis to melatonin, provided the indoleamine is administered late in the light phase of the light:dark cycle. Anosmia by itself had no influence on the daytime levels of radioimmunoassayable melatonin within the pineal gland, whereas in blinded control rats, melatonin levels were markedly augmented.