Response to selection for photoperiod responsiveness on the density and location of mature GnRH-releasing neurons

Abstract

-Natural variation in neuroendocrine traits is poorly understood, despite the importance of variation in brain function and evolution. Most rodents in the temperate zones inhibit reproduction and other nonessential functions in short winter photoperiods, but some have little or no reproductive response. We tested whether genetic variability in reproductive seasonality is related to individual differences in the neuronal function of the gonadotropin-releasing hormone network, as assessed by the number and location of mature gonadotropin-releasing hormonesecreting neurons under inhibitory and excitatory photoperiods. The experiments used lines of Peromyscus leucopus previously developed by selection from a wild population. One line contained individuals reproductively inhibited by short photoperiod, and the other line contained individuals nonresponsive to short photoperiod. Expression of mature gonadotropin-releasing hormone (GnRH) immunoreactivity in the brain was detected using SMI-41 antibody in the single-labeled avidin-biotin-peroxidase-complex method. Nonresponsive mice had 50% more immunoreactive GnRH neurons than reproductively inhibited mice in both short-and long-day photoperiods. The greatest differences were in the anterior hypothalamus and preoptic areas. In contrast, we detected no significant within-lines differences in the number or location of immunoreactive GnRH neurons between photoperiod treatments. Our data indicate that high levels of genetic variation in a single wild population for a specific neuronal trait are related to phenotypic variation in a life history trait, i.e., winter reproduction. Variation in GnRH neuronal activity may underlie some of the natural reproductive and life history variation observed in wild populations of P. leucopus. Similar genetic variation in neuronal traits may be present in humans and other species. genetic variation; artificial selection; seasonality; evolutionary physiology; brain variation; gonadotropin-releasing hormone WITHIN-POPULATION GENETIC VARIATION regulating the abundance, location, and connections of neurons must contribute to evolution of brain function. Similarly, natural genetic variation in neuronal traits is presumably responsible for some proportion of intraspecific functional variation in vertebrates. Because neural circuitry regulates reproductive physiology and behavior, neural variation is likely to explain some unknown proportion of life history variation within as well as among species. At present, almost nothing is known about natural levels of genetically based neuroendocrine physiological variation related to life history variation within species of mammals. The physiological link between genes and life history patterns is important because genes must act on life history traits through physiological mechanisms, and thus physiological variation may shape or constrain life history evolution (16). Understanding natural neuroendocrine variation related to life history traits might help us learn how rapidly brains adapt to current cha...

Phenotypic plasticity of reproductive traits in response to food availability and photoperiod in white-footed mice (Peromyscus leucopus)

“…We compared white-footed mice, P. leucopus, from two genetically distinct lines developed by artiWcial selection (Avigdor et al 2005;Heideman et al 1999aHeideman et al , 1999b in order to test for phenotypic plasticity in response to diVerent food treatments. One line was selected to strongly suppress gonadal development in short photoperiods [responsive (R) to short photoperiods].…”

Variation in levels of luteinizing hormone and reproductive photoresponsiveness in a population of white-footed mice (Peromyscus leucopus)

“…Natural populations of temperate-zone rodents contain substantial functional and genetic interindividual variation in this trait (5,8,10,17,19,33). Interindividual variation in the GnRH neuronal system has been correlated with variation in reproductive suppression (1,18,29), suggesting that variation in the GnRH neuronal system is related to interindividual variation in reproductive suppression in winter. Because GnRH neurons control LH secretion, interindividual variation in level of LH is a functional mechanism by which genetic variation in the HPG axis could directly affect reproductive phenotype.…”

Maternal Exposure to Dioxin Imprints Sexual Immaturity of the Pups through Fixing the Status of the Reduced Expression of Hypothalamic Gonadotropin-Releasing Hormone

“…Hence, the reason TCDD reduces the hypothalamic expression of GnRH does not seem to be explained by a change in epigenetic regulation. A number of GnRH neurons gather in the hypothalamic area, including the sexually dimorphic nucleus-preoptic area, a central region serving to produce sexual differentiation (Avigdor et al, 2005;Wray, 2010). Maternal exposure to a low dose of TCDD reduces the volume of the sexually dimorphic nucleuspreoptic area in male pups (Ikeda et al, 2005).…”