Photostimulation increases food intake, agouti-related protein (AGRP) and type II iodothyronine deiodinase (DIO2) gene expression in the medio-basal hypothalamus of Gambel's White-crowned Sparrow.

Abstract

Before migration, birds express hyperphagia leading to deposition of fuel in support of long-distance flight. Long days in spring stimulate a photoperiodic neuroendocrine cascade leading to heightened food intake. A major component of the response of the reproductive system to increased daylength in birds is the local conversion of thyroxine (T4) to triiodothyronine (T3) in the medial basal hypothalamus. However, mechanisms of photostimulation regulating hyperphagia in migratory birds have yet to be resolved. We report results from two studies of Gambel's White-crowned Sparrow (Zonotrichia leucophrys gambelii), a long-distance migrant. We used quantitative PCR to measure basal hypothalamic gene expression of several neuropeptides, glucocorticoid receptors, type II and type III iodothyronine deiodinase enzymes (DIO2 and DIO3), and α1 and α2 subunits of the cellular energy sensor AMP-activated protein kinase (AMPKα1 and AMPKα2). The first study involved birds on short days of 9L:15D exposed to 18 h food deprivation. In the second study, birds were photostimulated by one or two long days of 20L:4D. We observed no significant effects of food deprivation on hypothalamic gene expression. However, photostimulation significantly increased food intake on the first and second long days and was associated with significant increases in agouti-related protein (AGRP) and AMPKα2 mRNAs and in the ratio of DIO2/DIO3 expression. The pattern of increased DIO2 and decreased DIO3 gene expression is likely to have increased basal hypothalamic T3 content. This, in turn, may lead to altered local AMPK signaling to increase AGRP biosynthesis and thereby promote photostimulated hyperphagia.