The seasonal reproduction of mammal means the reproduction experiences an annual period from quiescence to renaissance. Studies have shown that kisspeptin and RFRP play an important role in the reproductive seasonality. The non-breeding season is characterized by an increase in the negative feedback effect of estrogen on GnRH, and this effect is transmitted by kisspeptin neurons, which may be an important factor affecting the reproduction activities. The expression of RFRP depends on melatonin secretion, and shows an apparent inhibition on reproduction in non-breeding season. In addition, thyroid hormones influence termination of the breeding season. Dopaminergic neuron A14/A15 also contributes to the seasonal changes in estrogen negative feedback. These neural systems may synergistically modulate the seasonal changes of reproductive function with the photoperiod. This review makes a systematic expatiation on the relationship between seasonal reproduction and these neural systems.
The hypothalamo-pituitary-gonadal (HPG) axis integrates internal and external cues via a balance of stimulatory and inhibitory neurochemical systems to regulate reproductive function in mammals. However, RFRP-3 is a unique inhibitor of HPG axis at the hypothalamuic level in mammals to date. A large number of previous studies have confirmed that RFamide-related peptide (RFRP-3) suppresses gonadotropin-releasing hormone (GnRH) system and luteinizing hormone (LH) secretion, thereby affecting the reproduction. However, whether the inhibition of LH secretion by RFRP-3 occurs at the pituitary level or the hypothalamus level is not clear. It is interesting that RFRP-3 is also related to signal pathway of melatonin modulating mammal seasonal reproduction, but little is known about the effects of melatonin on the RFRP-3 neuron up to now. In addition, RFRP-3 also plays an important role in the regulation of energy balance and behavior. The regulatory mechanism of RFRP-3 in HPG axis and role of RFRP-3 in modulating mammalian energy balance, as well as behavior, are systematically elaborated and the remaining unsolved problems are also discussed in this paper.
We have reported (Ramaswami and Lakshman, 1958, 1959a, b)on the spawning reaction of the skipper-frog (Rana cyanophlyctis Schn.) to mammalian hormones. It was found that androgen, progesterone, 17-hydroxycorticosterone and deoxycorticosterone made the intact skipper-frog oviposit when these hormones were injected individually. The other mammalian hormones, viz., follicle-stimulating hormone (Armour), luteinizing hormone (Armour), growth hormone(Armour), thyroid-stimulating hormone (Armour), adrenocorticotrophic hormone(Dumex; Armour), pregnant mare serum (Antex:Dumex), chorionic gonadotrophin(Physex: Dumex;Antuitrin S: Parke Davis), thyroxine(Glaxo), sodium L-triiodothyronine (Glaxo), cortisone acetate(Glaxo), prednisone acetate (Glaxo), estradiol-dipropio-nate (Ciba), 19-nortestosterone (Organics Inc.) and insulin(Boots)brought about spawning or oviposition only when used along with a threshold dose of homo-plastic pituitary glands;the result with estrogen was very poor. Our recent experiments have also shown that lactogenic hormone (Panlitar:Armour) can also bring about ovulation in the skipper-frog only if the hormone is injected along with the threshold pituitary gland dose. Oxytocin (Parke Davis) could not bring about ripening of eggs even when combined with the threshold pituitary gland dose. Creaser and Gorbman (1939), Rugh (1948) and Burgers and Zwarenstein (1955) have described cases where chorionic gonadotrophin or even cortisone brought about ovulation in frogs or toads. It was our object to find out if the hormones that normally bring about ovula-tion in the skipper-frog would do so, if the test animals were devoid of their pituitary gland. It has been noted that a combination of mammalian follicle-stimulating hormone and luteinizing hormone brought forth ovulation in the intact and hypophysectomized Rana pipiens (Turner, 1955). In the newt, small doses of follicle-stimulating hormone or luteinizing hormone brought about ovulation fourteen days after hypophysectomy (Otsuka, 1956). MATERIALS AND METHODS Fresh gravid skipper-frogs (Rana cyanophlyctis Schn.) were hypophysectomized by the usual method and were maintained at room temperature. Some of the test animals which had their Received for publication November 26, 1959.
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