Biochemical study of prolactin binding sites in Xenopus laevis brain and choroid plexus

Localization of prolactin receptor in the newt brain

In the male newt Cynops pyrrhogaster, prolactin (PRL) acts directly on the central nervous system and induces courtship behavior. As a step to elucidate the localization of neurons on which PRL acts, we developed a polyclonal antibody against an oligopeptide having a sequence completely identical with a part of the sequence of PRL receptors (PRLRs) of two species of newts, C. pyrrhogaster and C. ensicauda, and performed an immunohistochemical study with this antibody. PRLR-immunoreactive cells were observed in the medial amygdala, anterior preoptic area, magnocellular preoptic nucleus, suprachiasmatic nucleus, nucleus of the periventricular organ, ventral hypothalamic nucleus, and choroid plexus. We also performed in situ hybridization with a (35)S-labeled newt PRLR antisense RNA probe and detected signals in the preoptic area and choroid plexus. Colocalization of both PRLR-like immunoreactivity and arginine vasotocin-like or mesotocin-like immunoreactivity was demonstrated in the magnocellular preoptic nucleus. This is the first report of PRLR localization in the amphibian brain.

Prolactin acts centrally to enhance newt courtship behavior

The effects of intracerebroventricular (ICV) and intraperitoneal (IP) injections of ovine prolactin (PRL), antiserum against newt PRL, and antibody against the newt PRL receptor on the expression of courtship behavior of male newts, Cynops pyrrhogaster, were studied to see whether PRL acts centrally or peripherally to induce the behavior. Injections of PRL by either route into gonadotropin-primed males enhanced the expression of the behavior dose-dependently. The minimum effective amount of ovine PRL administered intracerebroventricularly was 0.1 microg, whereas it was 100 microg when injected intraperitoneally. ICV injection of antiserum against newt PRL blocked the spontaneously occurring male courtship behavior when the anti-newt PRL serum was given either intracerebroventricularly or intraperitoneally. The minimum effective dose of the antiserum administered intracerebroventricularly was 0.05 microl, whereas it was 20 microl when injected intraperitoneally. Neither ICV nor IP injection of preimmune serum affected the expression of the behavior. Furthermore, ICV, but not IP, administration of 0.3 microg of anti-newt PRL receptor antibody, purified from antiserum against newt PRL receptor by use of an antigen-conjugated affinity column, blocked the spontaneously occurring courtship behavior in sexually developed males. Neither ICV nor IP injection of the same amount of normal rabbit IgG affected the expression of the behavior. The results strongly suggest that endogenous PRL enhances the behavior by acting centrally through the PRL receptors localized in the brain area.

Hormonal control of urodele reproductive behavior

Hormonal control of expression of courtship behavior and of development of structures related to the reproductive behavior in two species of Japanese newts, Cynops pyrrhogaster and Cynops ensicauda, was described. Prolactin (PRL) and androgen were essential factors for eliciting courtship behavior. In addition, arginine vasotocin markedly enhanced the expression of courtship behavior. PRL induced migration to water, in which courtship and oviposition take place, and converted the integument from the terrestrial type to the aquatic one. PRL also stimulated the growth of the tail fin, which was blocked by estrogen. Cellular and nuclear size and number of synapses on the somata of Mauthner cells, which are involved in tail movement, were also increased by PRL and androgen. Synthesis of sodefrin, a female-attracting pheromone, in the abdominal gland as well as that of mucopolysaccharides constituting the sac of sperm in the lateral gland was enhanced by PRL and androgen. Structural development of oviducts was elicited by estrogen or PRL to a certain extent, and full oviducal development by the combination of these two hormones, PRL being indispensable for the oviducal jelly secretion.

Cloning of a cDNA for Xenopus prolactin receptor and its metamorphic expression profile

A pituitary hormone, prolactin (PRL) shows various effects on cellular metabolism in amphibians, such as stimulation of larval tissue growth and inhibition of metamorphic changes. All these effects are mediated by its cell surface receptor. However, lack of information on PRL receptor (PRL-R) gene expression has made the physiological importance of the PRL/PRL-R system obscure in amphibian metamorphosis. Hence, a Xenopus PRL-R cDNA was cloned, its structure was characterized, and specific binding of PRL to Xenopus PRL-R expressed in COS-7 cells was confirmed. In adult tissues, high level expression was found in the lung, heart, brain, thymus and skin, and low level in the oviduct, kidney and spinal cord. The developmental expression pattern showed that PRL-R messenger ribonucleic acid (mRNA) was expressed in the brain and tail from premetamorphosis and the level increased toward late metamorphosis, suggesting that PRL may inhibit the metamorphic changes in those organs. The level of brain PRL-R mRNA reached a peak just at the start of the metamorphic climax stages and then decreased, whereas in the tail, mRNA expression peaked at late metamorphosis. In the kidney, mRNA expression increased and reached a maximum level at the end of metamorphosis. The results obtained were discussed in relation to metamorphosis.

Melatonin accelerates metamorphosis in Xenopus laevis

Delayed metamorphosis associated with large body size has been observed in Woodhousei fowleri tadpoles reared in continuous dark (DD). To evaluate the mechanism by which DD delayed metamorphosis, light-cycle exposure was controlled and thyroxine (T4), melatonin, or drugs that alter prolactin (Prl) concentrations were given to Xenopus laevis tadpoles. It was hypothesized that exogenous melatonin would delay metamorphosis and increase body size, and that elevation of Prl concentrations would have effects similar to melatonin exposure. Xenopus laevis tadpoles were randomized to three light conditions [light/dark (LD, 12 h/12 h), DD, and continuous light (LL)] and subgroups in each light condition were treated with T4, melatonin, bromocriptine (Bro), haloperidol (Hal), or no drug. Each subgroup included 12 tadpoles. Drugs were administered in the water either continuously or daily from 07.00 to 19.00 h (Intermittent). Measurements of total length, leg length, and stage of metamorphosis were obtained at regular intervals. DD resulted in delayed metamorphosis, while LL did not. T4 accelerated metamorphosis as expected, countering the delaying effects of DD. In contrast to the hypothesis, melatonin accelerated metamorphosis and impaired body size compared to controls. Intermittent Hal also accelerated metamorphosis, while Bro delayed it. In DD, both T4 and melatonin led to increased tadpole size in contrast to their counterparts in LD or LL. Delayed metamorphosis in DD is not caused by increased melatonin production. Melatonin and Hal (as given in this study) accelerate metamorphosis. Melatonin acceleration of metamorphosis may occur through alteration of the concentration of prolactin.

Modulation of prolactin receptors in the rat hypothalamus in response to changes in serum concentration of endogenous prolactin or to ovine prolactin administration

Specific binding of 125I-labeled rat prolactin (125I-rat PRL) to hypothalamic membranes was studied in Sprague-Dawley rats after ovine PRL administration and in relation to rat PRL serum variations induced by ectopic pituitary implants or by drugs which stimulate (domperidone) or inhibit (bromocriptine) PRL release. Repeated treatments with ovine PRL markedly increased specific binding values of 125I-rat PRL to hypothalamic membranes of female rats. Repeated treatments with domperidone also increased specific PRL binding in the hypothalamus. This effect was associated with an increase in PRL serum levels. Similar results were obtained in male rats after renal pituitary implants which resulted in a state of chronic hyperprolactinaemia. In contrast, a subchronic treatment with bromocriptine decreased specific PRL binding in the hypothalamus and concomitantly caused a sharp reduction in PRL serum levels. Scatchard analysis of data obtained from competition curves showed that the variations in the level of PRL binding to hypothalamic membranes were related to the number of PRL binding sites but not to the dissociation constant (Kd), which was unaffected by different treatments or by pituitary implantation. These results demonstrate a correlation between circulating concentrations of PRL and number of its receptors in the rat hypothalamus and give further support to the hypothesis that these binding sites may have a specific functional role in regulating the homeostasis of pituitary PRL secretion.

Prolactin and interrenal hormone balance in adult specimens of Xenopus laevis exposed to hyperosmotic stress for up to one week

Adult female specimens of Xenopus laevis were exposed to diluted artificial seawater for up to 1 week. Statistically significant increases were observed in serum levels of aldosterone and corticosterone, concomitant with a drop in levels of prolactin (PRL) receptors in the cell membranes of the epidermis and kidney, which can be taken as an indicator of the levels of circulating PRL. The changes in hormone levels were detected after 1 day, being followed by a gradual restoration of the initial mean levels of these hormones when the exposure to hyperosmotic stress was extended for up to 2 or 7 days. Comparison of the above results with those previously obtained in Xenopus after longer exposure (15 days) to brackish water substantiates the necessity of recording possible initial fluctuations, as well as any changes that occur over the course of longer exposure during which time adaptation to environmental changes are likely to take place. The reverse responses, which may be not independent of one another, in terms of levels of corticoids and PRL in the animals exposed to hyperosmotic stress, are discussed. A survey of the literature suggests that, in Xenopus, as in the case of other less strictly water-dwelling amphibians, PRL is one of the hormones required for life in fresh water.