Clocks, nutrition, and reproduction [electronic resource] : complex regulation of gene expression in the brain
Keywords: Biological Sciences

In response to changing environments, the brains of vertebrates generate diverse behaviors change their morphology and physiology. Expression of genes in the brain is regulated by light, food, social stresses, and by internal signals including reproductive cycles and metabolism. To understand how these signals can have impact behaviors and the brain, I have studied regulation of reproduction, feeding, and light detection in a cichlid fish, Astatotilapia burtoni, and a circadian rhythm generation in the Siberian hamster, Phodopus sungorus. My experimentals involved measurement of mRNA by in situ hybridization and quantitative reverse transcription-PCR (qRT-PCR). To understand circadian rhythm entrainment, I studied when and where genes involved in circadian light detection are expressed in A. burtoni. mRNA encoding melanopsin, vertebrate ancient opsin, and pituitary adenylate cyclase-activating peptide were found in the inner retina of A. burtoni as well as in the brain. Their locations reveal a network homologous to circadian detection networks in other vertebrates. Melanopsin mRNA levels changed based on light input, as seen for some other opsin genes. Brief light stimuli can suppress circadian rhythms in several species. A two-pulse light treatment induces circadian arrhythmia in locomotor activity in P. sungorus. Loss of circadian rhythmicity in tissues and organisms could occur either through loss of intracellular rhythms or through loss of intercellular synchrony. Following the light treatment, P. sungorus had suppressed levels of mRNA encoding the core clock genes per1, per2, bmal1, and cry1 in the suprachiasmatic nucleus (SCN). Thus, circadian rhythm loss due to light stimuli involves inhibition of intracellular rhythms in the SCN. The brain controls reproductive behavior and physiology in response to relevant external and internal cues. Gonadotropin-releasing hormone (GnRH1) activates the reproductive system and GnRH1 peptide and mRNA levels are higher in dominant male A. burtoni than in subordinate males. Kisspeptin, a neuropeptide acting via the kisspeptin receptor (Kiss1r), increases GnRH1 release. In A. burtoni, I found kiss1r mRNA in GnRH1 and GnRH3 neurons and in many brain nuclei and kiss1r levels were higher in dominant males than in subordinate males. Kisspeptin signaling may regulate many brain functions, including responses to social status. Reproduction requires significant investment of metabolic energy, especially when parents provide care for offspring. Female A. burtoni carry eggs in their mouths for two weeks after spawning, during which time they do not eat. Food deprivation can affect reproductive and feeding behaviors and neuropeptides. To test the hypothesis that food deprivation and reproduction interact in regulating A. burtoni brain gene expression, I measured levels of several neuropeptide and receptor gene mRNAs following mouthbrooding or starvation. Mouthbrooding females had lower levels of gnrh1 mRNA than gravid females, but higher levels of cck and hcrt. In males, food deprivation decreased gnrh1 levels, but increased gnrh2, suggesting a novel feeding role for GnRH2 in fish. Mouthbrooding may inhibit reproduction and modulate feeding neural circuits. I also developed a technique for intracerebroventricular injections in A. burtoni that should facilitate manipulation of neuropeptide receptors including GnRH receptors. Matching behavior and physiology to accomodate changing lighting, social, feeding, and reproductive influences is important for maximizing organismal fitness and I have discovered how these factors regulate several genes that may contribute to adaptive responses in changing environments.