title: Differential responses of the somatotropic and thyroid axes to environmental temperature changes in the green iguana.
abstract: Growth hormone (GH), together with thyroid hormones (TH), regulates growth and development, and has critical effects on vertebrate metabolism. In ectotherms, these physiological processes are strongly influenced by environmental temperature. In reptiles, however, little is known about the direct influences of this factor on the somatotropic and thyroid axes. Therefore, the aim of this study was to describe the effects of both acute (48h) and chronic (2weeks) exposure to sub-optimal temperatures (25 and 18°C) upon somatotropic and thyroid axis function of the green iguana, in comparison to the control temperature (30-35°C). We found a significant increase in GH release (2.0-fold at 25°C and 1.9-fold at 18°C) and GH mRNA expression (up to 3.7-fold), mainly under chronic exposure conditions. The serum concentration of insulin-like growth factor-I (IGF-I) was significantly greater after chronic exposure (18.5±2.3 at 25°C; 15.92±3.4 at 18°C; vs. 9.3±1.21ng/ml at 35°C), while hepatic IGF-I mRNA expression increased up to 6.8-fold. Somatotropic axis may be regulated, under acute conditions, by thyrotropin-releasing hormone (TRH) that significantly increased its hypothalamic concentration (1.45 times) and mRNA expression (0.9-fold above control), respectively; and somatostatin (mRNA expression increased 1.0-1.2 times above control); and under chronic treatment, by pituitary adenylate cyclase-activating peptide (PACAP mRNA expression was increased from 0.4 to 0.6 times). Also, it was shown that, under control conditions, injection of TRH stimulated a significant increase in circulating GH. On the other hand, while there was a significant rise in the hypothalamic content of TRH and its mRNA expression, this hormone did not appear to influence the thyroid axis activity, which showed a severe diminution in all conditions of cold exposure, as indicated by the decreases in thyrotropin (TSH) mRNA expression (up to one-eight of the control), serum T4 (from 11.6±1.09 to 5.3±0.58ng/ml, after 2weeks at 18°C) and T3 (from 0.87±0.09 to 0.05±0.01ng/ml, under chronic conditions at 25°C), and Type-2 deiodinase (D2) activity (from 992.5±224 to 213.6±26.4fmolI(125)T4/mgh). The reduction in thyroid activity correlates with the down-regulation of metabolism as suggested by the decrease in the serum glucose and free fatty acid levels. These changes apparently were independent of a possible stress response, at least under acute exposure to both temperatures and in chronic treatment to 25°C, since serum corticosterone had no significant changes in these conditions, while at chronic 18°C exposure, a slight increase (0.38 times above control) was found. Thus, these data suggest that the reptilian somatotropic and thyroid axes have differential responses to cold exposure, and that GH and TRH may play important roles associated to adaptation mechanisms that support temperature acclimation in the green iguana.
authors.affiliation: Laboratorio de Bioquímica de Hormonas, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: José
authors.initials: J
authors.lastname: Ávila-Mendoza
authors.affiliation: Laboratorio de Bioquímica de Hormonas, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: Martha
authors.initials: M
authors.lastname: Carranza
authors.affiliation: Laboratorio de Fisiología Evolutiva, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: Patricia
authors.initials: P
authors.lastname: Villalobos
authors.affiliation: Laboratorio de Fisiología Evolutiva, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: Aurora
authors.initials: A
authors.lastname: Olvera
authors.affiliation: Laboratorio de Fisiología Evolutiva, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: Aurea
authors.initials: A
authors.lastname: Orozco
authors.affiliation: Laboratorio de Bioquímica de Hormonas, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico.
authors.forename: Maricela
authors.initials: M
authors.lastname: Luna
authors.affiliation: Laboratorio de Bioquímica de Hormonas, Departamento de Neurobiología Celular y Molecular, Instituto de Neurobiología, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qro. 76230, Mexico. Electronic address: aramburo@unam.mx.
authors.forename: Carlos
authors.initials: C
authors.lastname: Arámburo
mesh_terms: D000818:Animals; D001786:Blood Glucose / Q000032:analysis; D003345:Corticosterone / Q000097:blood; D013006:Growth Hormone / Q000235:genetics / Q000378:metabolism*; D007031:Hypothalamus / Q000187:drug effects / Q000378:metabolism; D007076:Iguanas / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000235:genetics; D007453:Iodide Peroxidase / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000097:blood; D013004:Somatostatin / Q000235:genetics; D013696:Temperature; D013961:Thyroid Gland / Q000187:drug effects / Q000378:metabolism*; D013963:Thyroid Hormones / Q000097:blood / Q000235:genetics / Q000378:metabolism; D013972:Thyrotropin / Q000235:genetics; D013973:Thyrotropin-Releasing Hormone / Q000008:administration & dosage / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D043562:Receptors, G-Protein-Coupled
chemical_list: D001786:Blood Glucose; D003345:Corticosterone; D013006:Growth Hormone; D007334:Insulin-Like Growth Factor I; D007453:Iodide Peroxidase; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D013004:Somatostatin; D013963:Thyroid Hormones; D013972:Thyrotropin; D013973:Thyrotropin-Releasing Hormone; C473103:FFAR2 protein, human; D043562:Receptors, G-Protein-Coupled
keywords: Environmental endocrinology; Environmental temperature; Green iguana; Growth hormone; Somatotropic axis; Thyroid axis