Photostimulation Induces New Gene Transcription inGonadotropin-releasing Hormone Somata in a Seasonally BreedingSongbird Open Access

Abstract

Birds use a variety of environmental cues, such as day length, temperature, and social interactions, to time reproductive efforts. For most seasonal breeders, day length is the most important cue and takes precedence over all others. In both males and females housed on short days, exposure to a single long day induces a robust release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. The mechanisms underlying this response are only beginning to be understood. Previous research has shown that one long day causes striking upregulation of immediate early gene expression in regions of the mediobasal hypothalamus that contain GnRH axons and terminals. This upregulation is thought to represent the activation of tanycytes and astrocytes in the median eminence as well as neurons located in the infundibular nucleus, which may play a role in the retraction of glial processes that surround GnRH terminals. Although the photoperiodic response of the mediobasal hypothalamus has been well-studied, photo-induced activity in the GnRH neurons themselves has never been described. In this study, we used immunohistochemistry to assay the expression of the immediate early genes c-fos and egr-1 in the GnRH somata of male and female white-throated sparrows exposed to a single long day. We found that the protein products of both genes increased in GnRH neurons of the septo-preoptic area of the hypothalamus by 26 hours after dawn on the long day. These results suggest that photostimulation does in fact stimulate new gene transcription in the GnRH neurons on a relatively rapid time scale. Further research is required to determine whether the GnRH somata are themselves integrating photic cues, or whether they are simply responding to an increased demand for GnRH synthesis.

Table of Contents

Introduction 1

Methods 6

Animals 6

Photostimulation and Tissue Collection 6

LH Assay 7 Immunocytochemistry 7 Quantification of GnRH Neurons 8 Statistical Analysis 9 Analysis of the GnRH Promoter 9

Results 10

Plasma LH 10 Immediate Early Gene Induction in GnRH neurons 10 Egr-1 Induction in GnRH Neurons 11 FOS Induction in GnRH Neurons 11 Correlated FOS and Egr-1 Expression in GnRH Neurons 12 Number of GnRH Neurons 12 IEG Binding Sites in the GnRH Promoter 12

Discussion 13

References 20

Figures and Captions 25

Figure 1 25 Figure 2 26 Figure 3 27 Figure 4 28 Figure 5 29 Figure 6 30 Figure 7 31 Figure 8 32 Figure 9 33

About this Honors Thesis

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School	Emory College
Department	Neuroscience and Behavioral Biology
Degree	BS
Submission	Honors Thesis
Language	English
Research Field	Biology, Neuroscience
Keyword	white-throated sparrow GnRH FOS seasonality hypothalamus photostimulation reproduction Egr-1
Committee Chair / Thesis Advisor	Maney, Donna, Emory University
Committee Members	Frenzel, Kristen E, Emory University Starnes, Amanda, Emory University Patterson, Barbara (Bobbi), Emory University

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