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Acceleration of Ambystoma Tigrinum Metamorphosis by Corticotropin-Releasing Hormone 1 1,2 GRAHAM C

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Acceleration of Ambystoma Tigrinum Metamorphosis by Corticotropin-Releasing Hormone 1 1,2 GRAHAM C JOURNAL OF EXPERIMENTAL ZOOLOGY 293:94–98 (2002) RAPID COMMUNICATION Acceleration of Ambystoma tigrinum Metamorphosis by Corticotropin-Releasing Hormone 1 1,2 GRAHAM C. BOORSE AND ROBERT J. DENVER * 1Department of Ecology and Evolutionary Biology,The University of Michigan, AnnArbor,Michigan48109-1048 2 Department of Molecular,Cellular and Developmental Biology,The University of Michigan, Ann Arbor,Michigan 48109-1048 ABSTRACT Previous work of others and ours has shown that corticotropin-releasing hormone (CRH) is a positive stimulus for thyroid and interrenal hormone secretion in amphibian larvae and that activation of CRH neurons may mediate environmental e¡ects on the timing of metamorphosis. These studies have investigated CRH actions in anurans (frogs and toads), whereas there is currently no information regarding the actions of CRH on metamorphosis of urodeles (salamanders and newts). We tested the hypothesis that CRH can accelerate metamorphosis of tiger salamander (Ambystoma tigrinum) larvae. We injected tiger salamander larvae with ovine CRH (oCRH; 1 mg/day; i.p.) and mon- itored e¡ects on metamorphosis by measuring the rate of gill resorption. oCRH-injected larvae com- pleted metamorphosis earlier than saline-injected larvae.There was no signi¢cant di¡erence between uninjected and saline-injected larvae. Mean time to reach 50% reduction in initial gill length was 6.9 days for oCRH-injected animals, 11.9 days for saline-injected animals, and 14.1 days for uninjected controls. At the conclusion of the experiment (day 15), all oCRH-injected animals had completed metamorphosis, whereas by day 15, only 50% of saline-injected animals and 33% of uninjected animals had metamorphosed. Our results show that exogenous oCRH can accelerate metamorphosis in urodele larvae as it does in anurans. These ¢ndings suggest that the neuroendocrine mechanisms controlling metamorphosis are evolutionarilyconserved across amphibiantaxa. J.Exp.Zool.293:94^98, 2002. r 2002 Wiley-Liss, Inc. Thyrotropin-releasing hormone (TRH), the neu- terrenal (adrenal) axes (Denver and Licht, ’89a; rohormone that regulates pituitary thyroid-stimu- Denver,’99). lating hormone (TSH) secretion in mammals, is Several lines of evidence support a physiological expressed in the tadpole hypothalamus (see Denver, role for CRH in regulating TSH secretion in anur- ’96), but lacks the ability to stimulateTSH secretion ans (frogs and toads). For example, CRH stimulated in the tadpole (Denver and Licht,’89a; see Kikuya- the secretion of thyrotropic bioactivity (Denver,’88; ma et al.,’93; Denver,’96) or in the neotenic axolotl Denver and Licht, ’89a; Jacobs and Kuhn, ’92) and (Darras and Kuhn,’83; Jacobs and Kuhn,’87). How- immunoreactive TSH b subunit (Denver, unpub- ever, TRH is active in stimulating thyroid activity lished results) by cultured tadpole and adult frog in adult frogs (Darras and Kuhn, ’82; Denver, ’88) pituitaries. Injections of CRH or CRH-like peptides and metamorphosed axolotls (induced by injections (i.e., sauvagine) elevated whole body thyroid hor- of thyroxine; Jacobs and Kuhn,’87), suggesting a de- mone (Gancedo et al.,’92; Denver,’93,’97a) and cor- velopmental switch in the neuroendocrine control ticosterone (Denver, ’97a) content and accelerated of the thyroid system in amphibia (see Denver,’96). metamorphosis of tadpoles of four frog species Current evidence supports a role for corticotropin- (Rana catesbeiana and Spea hammondii, Denver,’93; releasing hormone (CRH) in the positive regulation of pituitary TSH secretion in tadpoles (Denver,’96, Grant sponsor: NSF; Grant number: IBN-9974672. ’99). CRH is the primary neurohormone regulating *Correspondence to: Robert J. Denver, Department of Molecular, Cel- pituitary adrenocorticotropin (ACTH) secretion in lular and Developmental Biology, 3065C Natural Science Bldg.,The Uni- versity of Michigan, Ann Arbor, MI 48109-1048. E-mail: rdenver@ mammals (see Vale et al., ’97). In amphibians and umich.edu other nonmammalian vertebrates, CRH may play a Received 1 November 2001; Accepted 28 February 2002 Published online in Wiley InterScience (www.interscience.wiley. dual role by regulating both the thyroid and the in- com). DOI: 10.1002/jez.10115 r 2002 WILEY-LISS, INC. CRH ACCELERATES SALAMANDER METAMORPHOSIS 95 Rana perezi, Gancedo et al.,’92; and Bufo arenarum, cage. Digitized images were analyzed for body Miranda et al., 2000). These four species represent lengths using NIH image software. three anuran genera; however, more information from diverse amphibian taxa, especially urodeles, CRH injections is needed to evaluate how generalizable this action Based on the above experiment, CRH injections of CRH is in amphibians. were initiated in 13-week-old A. tigrinum larvae CRH has been shown to activate the thyroid and (mean body length=12.121cm, SE ¼ 0:157; range interrenal axes and to accelerate metamorphosis in 10.69^13.27 cm). Larvae were separated into three anuran larvae; however, there is currently no infor- treatment groups (n ¼ 6/treatment), which in- mation on the actions of CRH in urodeles (salaman- cluded uninjected controls, saline-injected controls ders and newts). In the present study, we tested the (daily i.p. injections of 50 ml phosphate buffered hypothesis that injection of CRH can accelerate saline [PBS]: 0.02 M sodium phosphate, 0.6% so- metamorphosis of larvae of the tiger salamander, dium chloride), and ovine CRH-injected experimen- Ambystoma tigrinum. We injected ovine CRH (i.p.) tals (daily i.p. injections of 1 mg oCRH dissolved in over a period of two weeks and monitored meta- 50 mlPBS). morphic progression by measuring gill length. Metamorphic progression was monitored by mea- suring gill length. Digital images were captured every two days starting at time zero and the length MATERIALS AND METHODS of the right middle gill rachis was measured. The Animals and animal husbandry time in days for each larva to achieve 50% gill re- sorption (GR50) was calculated. This method pro- A. tigrinum egg masses were collected from vides an accurate means to compare rates of ponds located near Pinckney,MI (Michigan scienti- metamorphosis in A. tigrinum (Norris et al., ’73; ¢c collecting permit # CO577). Egg masses were Norris and Platt,’74). placed in polystyrene cages (24 cm  45 cm  20 cm) containing ten liters of dechlorinated water. Upon Statistical analysis hatching, larvae were reared individually in poly- GR50 values were analyzed using Student’s un- styrene cages (20.5 cm  34 cm  11cm) in two liters paired t-test to make pairwise comparisons be- of water and fed brine shrimp, followed by tubifex tween uninjected and saline-injected or between worms, and then beef liver as they grew.Water was saline-injected and CRH-injected groups. Changes changed every two to three days and larvae were in gill length were analyzed over time (each time maintained on a constant 12L:12D light/dark cycle point was analyzed separately) by unpaired t-test 1 and a temperature range of 21^23 C. to compare uninjected or CRH-injected to saline- injected. Determination of body size at RESULTS AND DISCUSSION metamorphosis Growth rates of control and methimazole-treated Previous studies with A. tigrinum larvae in our larvae were similar; however, methimazole-treated laboratory (R. J. Denver, unpublished data) led us larvae never initiated metamorphosis (Fig. 1). Con- to hypothesize that a larva must reach a minimum trol larvae initiated metamorphosis 13 weeks after size or developmental threshold in order to become hatching and all had completed metamorphosis by competent to respond to environmental or neuroen- week 16 (Fig. 1). docrine stimuli. Thus, we ¢rst conducted a study Larvae receiving injections of oCRH metamor- aimed at determining the range of body size at phosed earlier than saline-injected larvae. Signi¢- which spontaneous metamorphosis is initiated in cant di¡erences in gill length were apparent two A. tigrinum larvae reared in the laboratory under days after beginning injections (Fig. 2; T ¼ constant environmental conditions. Two weeks À2:701; P ¼ 0:0223). No signi¢cant di¡erences were after hatching, larvae (n ¼ 10/treatment) were se- detected in gill length between saline-injected and parated into two groups and treated with or with- uninjected larvae (Fig. 2). GR50 values were signi¢- out the goitrogen methimazole dissolved in water cantly lower for oCRH injected animals compared to a concentration of 1mM (Denver et al.,’97). Digi- to saline-injected larvae (Table 1; T ¼ 2:629; tal images of individual larvae were captured P ¼ 0:0340). oCRH injection also resulted in a lar- weekly using an Olympus D-500L digital camera ger number of animals undergoing complete trans- mounted on a tripod at a ¢xed height above the formation than either uninjected or saline-injected 96 G.C. BOORSE AND R.J. DENVER TABLE 1. E¡ects of oCRH on time to 50% gill resorption (GR50) in A. tigrinum larvae1 Number of larvae completing metamorphosis Group GR50 (Days 7 SE) at end of experiment Untreated 14.1 70.7 2 Saline 11.9 7 1.4 3 oCRH2 6.9 7 2.0* 6 1 GR50, mean number of days to reach 50% gill resorption. 2 Larvae were given injections of oCRH (1 mg) every day for 15 days (n ¼ 6/ group). Asterisk designates signi¢cantly di¡erent from saline-treated larvae by unpaired t-test (Po0:05). Fig. 1. Analysis of growth and development of Ambystoma experiment metamorphosed over the subsequent tigrinum larvae for 18 weeks following hatching. Larvae were one to four weeks in the laboratory. reared in the absence (open circles) or presence (closed circles) of the goitrogen methimazole (1mM). Points on the graph repre- This is the ¢rst demonstration of a stimulatory sent means 7 SEM (n ¼ 10/treatment). Arrows indicate the fol- e¡ect of CRH on metamorphosis in a urodele. Pre- lowing progression of metamorphosis in control animals: A) vious studies of four anuran species showed that initial signs of metamorphosis; B) 50% of control animals com- CRH injections cause tadpoles to metamorphose pleting metamorphosis; C) 100% of control animals completing earlier than saline-injected controls (Gancedo metamorphosis.
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