B. Gancedo

Corticotropin-releasing factor stimulates metamorphosis and increases thyroid hormone concentration in prometamorphic Rana perezi larvae

Attempts to identify a hypothalamic molecule that stimulates thyrotropin (TSH) secretion from amphibian pituitary have been unsuccessful to date. The effects of mammalian (ovine and human) corticotropin-releasing factor (CRF) on the thyroid function of prometamorphic (Taylor & Kollros stages XI-XVII) (Taylor and Kollros, 1946) Rana perezi larvae were studied. Chronic treatments with both ovine and human CRF (oCRF, hCRF) stimulated metamorphosis while delaying larval growth. Chronic hCRF (1 microgram) administration induced 3.2- and 5.3-fold increases in whole body concentration of thyroxine (T4) and triiodothyronine (T3), respectively. In contrast, the 0.5-microgram dose of hCRF stimulated a significant (3.4-fold) increase in whole body concentration of T4 but not of T3. Histological studies of the thyroid gland revealed a 22% increase in the number of follicles per section as a result of the chronic treatment with oCRF (1 microgram). Acute oCRF (2 micrograms) treatment induced a significant increase in T4 concentration at 4 hr (1.3-fold) and 8 hr (2.3-fold) postinjection. T3 concentration was not altered. These results support previous reports and lead us to conclude that a CRF-like peptide, and not TRH, is involved in the regulation of thyroid activity in anuran amphibians during metamorphosis.

Effect of α-Helical-CRF[9-41] on feeding in goldfish : Involvement of cortisol and catecholamines

The anoretic effect of corticotropin -releasing factor (CRF) was not dependent on adrenal activation in goldfish (Carassius auratus). Moreover, an interaction between CRF and the hypothalamic catecholaminergic system in the central regulation of food intake was observed. The intracerebroventricular (icv) administration of CRF increased cortisol levels and reduced food intake and hypothalamic norepinephrine and dopamine content at 2 hr postinjection, with these effects reversed by a-helical CRF[9_41] pretreatment. The anoretic effect of CRF was independent of the circulating cortisol increase, because it was only evoked after icv injections but not after intraperitoneal (ip) administration. Furthermore, the increase in plasma cortisol levels induced by ip administration of this steroid did not modify feeding.

Daily changes in thyroid activity in the frog Rana perezi: Variation with season☆☆☆

Plasma triiodothyronine (T3) and thyroxine (T4) levels, as well as thyroid free (f) and bound (b) thyroid hormones (TH) content, were determined by radioimmunoassay (RIA) in adult Rana perezi frogs during a 24 h cycle in winter, spring, summer, and autumn. Significant daily changes in plasma T3 levels were present in all the seasons except for winter, being the lowest values observed during the scotophase. In contrast, plasma T4 only showed significant changes in spring, following a similar pattern to the one described for T3. Thyroid fT3 content did present day/night significant changes only in spring showing high contents at early scotophase. Mean fT4 content was higher at the beginning of light phase than during the rest of daily photocycle in spring and autumn, but significant differences appeared only in autumn. Regarding the thyroid bound content of TH, bT3, and bT4 presented significant daily changes in spring and autumn. However, different profiles were observed in these two seasons. High bound contents were found at early photo- and scotophase with lower values at late dark phase in spring, whereas higher contents were detected at this time in autumn. The present results indicate the existence of seasonally changing daily fluctuations in thyroid activity in Rana perezi and it seems that an interaction between photoperiod and temperature plays a role in the regulation of these daily changes.

Characterization of Serotonin N-Acetyltransferase in the Lateral Eye of the Green Frog Rana perezi: Protective Action of EGTA

Abstract: The kinetics of seRotonin N‐acetyltransferase (NAT) from the lateral eye of Rana perezi have been characterized. NAT from ocular tissue reached maximal activity at a phosphate buffer concentration of 250 mM and a pH of 6.5. Reaction linearity was highly conserved within the homogenate fraction range tested (0.033‐0.33). The time course of ocular NAT reaction showed a high linearity at 25 and 35°C. Km and Vmax estimations for acetyl‐CoA at a 10 mM tryptamine concentration were 63.3 μM and 4.42 nmol/h per eye, respectively. Regardless of the acceptor amine (tryptamine or serotonin), the Km was not affected by the acetyl‐CoA concentration (50 or 250 μM), whereas the Vmax was significantly increased at a 250 μM acetyl‐CoA concentration. Ocular NAT showed a higher affinity for serotonin (Km= 20.7 μM) than for tryptamine (Km= 48‐60 μM); Vmax however, was similar for both substrates. Acetyl‐CoA does not protect ocular NAT; in contrast, the use of EGTA (4 mM) in the assay is essential to protect the enzyme because NAT in ocular crude homogenate shows rapid inactivation. This result suggests that intracellular calcium levels are involved in the NAT inactivation mechanisms in frog ocular tissue.

Ontogeny of Daily Changes in Extrathyroidal Thyroid Hormone Concentrations in Two Anuran Species (Rana Perezi and Xenopus Laevis)

The day/night changes in thyroxine (T4) and triiodothyronine (T3) whole-body concentrations in Rana perezi and Xenopus laevis throughout ontogeny have been studied. Daily cycles in thyroid hormones (TH) with higher nocturnal values appear during premetamorphosis in both species. Cyclicity disappears for T3, while it is reversed for T4, in prometamorphic larvae. In X. laevis higher nocturnal T3 and T4 values were observed at the end of climax.