The role of prolactin in blocking thyroxine-induced differentiation of tail tissue in larval and neotenic Ambystoma tigrinum

Reduced calcium and inhibition of protein kinase C mimic the enhancement of ornithine decarboxylase activity of prolactin in Ambystoma tigrinum tissues

We previously reported that prolactin (PRL) could increase the activity of ornithine decarboxylase (ODC) in liver slices taken from larval tiger salamanders (Ambystoma tigrinum). This action of the hormone was inhibited by oxytocin (OT), the calcium ionophore A23187, and diacyglycerol (DG) and was duplicated by 10 microM verapamil (VML), a calcium channel blocker. Here, we expand these results to show that 1) a higher dose of VML (50 microM) produces an additive effect with PRL; 2) addition of small amounts of calcium (0.1 mM) to the liver culture medium blocks PRL action; 3) neither nifedipine (NIF), a different type of calcium channel blocker, nor EDTA alter PRL action; and 4) gossypol, a reported inhibitor of protein kinase C, mimics PRL action. Additionally, we show that PRL increases ODC activity in tiger salamander tail skin in vitro, a tissue previously demonstrated to be a PRL target tissue in this species. The same set of treatments which we have shown to modify PRL effects on ODC in liver slices affects PRL action in the tail skin in a parallel manner. Thus, the mechanism whereby PRL enhances ODC activity appears to be the same in both these tissues. These results are discussed in conjunction with the findings from similar studies using mammalian tissues in an attempt to assess the current picture of the mechanism of PRL action and the possible role of inositol phospholipid turnover, calcium, and protein kinase C in the action of this hormone.

Observations on the interaction of prolactin and thyroxine in the tail of the bullfrog tadpole

Explants of tail fin from tadpoles of Rana catesbeiana maintained in tissue culture underwent resorption when exposed to a medium containing 150 nM thyroxine (T4). Increases in the specific activity of hexosaminidase (Hex) and of acid phosphatase (AP) were associated with this response. Ovine prolactin inhibited resorption of explants in a dose-responsive manner; however, prolactin had no influence on the specific activity of either Hex or AP cultured explants. This result contrasted with the effect of prolactin in vivo, where it inhibited both the resorption of the tail and the increase in specific activity of hydrolytic enzymes induced by T4. The actions of both hormones, when applied in vivo, were more pronounced in the tail fin than in the muscular core of the tail at the stage of regression tested, indicating that the failure of prolactin to influence enzyme induction in vitro did not result from an incapacity of the fin to respond to prolactin in this fashion. It is suggested that the inhibition of T4-induced enzymic activity by prolactin which is observed in the intact animal may come about in response to some related but indirect systemic action of prolactin that is lacking when explants are cultured in vitro. It is concluded that inhibition of the induction of hydrolytic enzymes is not an essential factor in the inhibitory effect of prolactin on the resorption of the tail fin.

Antagonistic effects of prolactin and oxytocin on tail fin regression and acid phosphatase activity in metamorphosing Ambystoma tigrinum

We have found that the inhibition of thyroxine-induced tail fin regression by prolactin in larval tiger salamanders is antagonized by oxytocin. Other workers have shown that prolactin blocks the rise in activity of several hydrolytic enzymes that occurs in regressing tissue during metamorphosis. Here, we examine the effects of prolactin and oxytocin--given alone and in combination--on tail fin regression and acid phosphatase specific activity in this tissue. Both long-term (12-day) and short-term (48-hr) treatment paradigms using prolactin and oxytocin are investigated. The results show that long-term prolactin treatment of metamorphosing larvae blocks fin regression and the rise in acid phosphatase specific activity seen in metamorphosing controls; short-term prolactin treatment of metamorphosing larvae inhibits fin regression within 48 hr, but does not block the rise in acid phosphatase activity seen in controls; oxytocin antagonizes the effects of prolactin on tail fin regression; and oxytocin treatment (long-term or short-term) of metamorphosing larvae causes an elevation of acid phosphatase activity above that seen in metamorphosing controls. With long-term treatment, this effect of oxytocin is slightly antagonized by prolactin; with short-term treatment, no antagonism is observed even though an effect of prolactin on fin height is still evident. We have interpreted these results as suggesting that the effect of prolactin on hydrolase activity is not a prerequisite for its inhibitory effect on fin regression to occur.

The effects of thyroidectomy on androgen and prolactin receptors in the dorsal skin and caudal fin of Triturus cristatus carnifex

The interactions between thyroid hormones and receptors for steroid hormones and prolactin in dorsal skin and caudal fin of Triturus cristatus carnifex were studied during the annual cycle. Thyroidectomy induces an increase of prolactin binding in the dorsal skin and caudal fin in the animals captured in March. In these thyrodectomized animals the androgen receptors became undetectable. Results indicate that in Triturus cristatus carnifex the thyroid induces an increase of androgen receptors and a decrease, that is removed by thyroidectomy, of prolactin receptors.

The effect of temperature on the action of thyroid hormone and prolactin in larvae of the salamander Hynobius retardatus

To elucidate the environmental factors and endocrine mechanisms which are responsible for inducing neoteny in the salamander Hynobius retardatus, the effect of temperature on the growth and metamorphosis of this amphibian, as well as the actions of thyroid hormone and prolactin at low temperature, were studied. (1) The metamorphosis of larvae cultivated at 10 degrees was significantly delayed compared with that of larvae cultivated at 22 degrees, but the metamorphosis was eventually completed. At 4 degrees, metamorphosis never occurred, even after 2 years. (2) Exogenously administered thyroid hormone accelerated metamorphosis at 10 degrees or 22 degrees, but was ineffective in larvae kept at 4 degrees, whether administered by injection or immersion. (3) If a higher concentration of thyroid hormone was given by a single injection to larvae cultured at 4 degrees, an appreciable acceleration of metamorphosis was observed after transferring the larvae to 22 degrees. (4) Unlike thyroid hormone, prolactin promoted growth at 4 degrees.