Tonic Activation of Brain GnRH Immunoreactivity despite Reduction of Peripheral Reproductive Parameters in Opportunistically Breeding Zebra Finches

Abstract

Opportunistically breeding species offer the unique opportunity to understand mechanisms in reproductive physiology that allow for extreme flexibility in the regulation of reproduction. We studied a well-known opportunistic breeder, the zebra finch (Taeniopygia guttata) to test the hypothesis that the reproductive axis of opportunists is in a constant state of ‘near-readiness’. In wild zebra finches, reproduction is highly correlated with rainfall, and in the laboratory, water availability and humidity are the strongest cues to affect reproductive activation. We therefore subjected individuals to water restriction for eleven weeks followed by a two week period of ad libitum access to water. The control group had water freely available for the entire experiment. We measured the state of activation of the hypothalamo-pituitary gonad (HPG) axis at three levels: in the hypothalamus by measuring immunoreactive (ir) cGnRH-I and cGnRH-II; in the anterior pituitary gland by measuring plasma luteinizing hormone (LH); and in the gonads by measuring gonadal volume and function. We found that water restriction caused a reduction in circulating LH concentrations and that testis volume was more likely to decrease in water restricted than in control birds. Subsequent short-term return to ad libitum water availability caused LH to return to baseline in water restricted birds. These changes occurred without significant changes in ir-cGnRH-I, ir-cGnRH-II, or in testis function. These data suggest that in these opportunistic breeders, an inhibition of parts of the reproductive axis is not necessarily correlated with full inactivation of reproductive potential. GnRH-ir cells in the hypothalamus appear to remain active and able to respond to subsequent stimulation.

Photoperiod-Independent Hypothalamic Regulation of Luteinizing Hormone Secretion in a Free-Living Sonoran Desert Bird, the Rufous-Winged Sparrow <i>(Aimophila carpalis)</i>

“…This finding differs from the situation in many photoperiodic birds with highly seasonal breeding patterns in which cGnRH-I stores appear to decrease with increased release [Foster et al, 1987;Hahn and Ball, 1995;Parry et al, 1997;Cho et al, 1998;Deviche et al, 2000;Dawson et al, 2001;Pereyra et al, 2005]. However, stable cGnRH-I storage in the cell body is consistent with observations in Zebra Finch, Taeniopygia guttata [Perfito et al, 2006], Whitewinged Crossbills, Loxia leucoptera [MacDougall-Shackleton et al, 2001;Pereyra et al, 2005], Rufous-collared Sparrows, Zonotrichia capensis [Moore et al, 2006], Canaries, Serinus canaria [Bentley et al, 2003a], and Japanese Quail [Foster et al, 1988] which all exhibit little or no changes in GnRH-ir over the breeding season. Similar to Rufous-winged Sparrows, these species also exhibit flexible breeding schedules and/or terminate breeding due to relative photorefractoriness [Benkman, 1990[Benkman, , 1992Follett and Pearce-Kelly, 1990;Zann et al, 1995;Bentley et al, 2000;Leitner et al, 2003;Moore et al, 2005].…”

Identification, localization, and regulation of passerine GnRH-I messenger RNA

“…What is special about this case is that songbird research has been restricted to relatively gross analysis of GnRH regulation and has thus been severely hampered in terms of advance. For example, studies on GnRH-I immunoreactive peptide in zebra finches have been unable to detect changes in the peptide according to gonadal status because of the limitations of immunocytochemistry (Perfito et al 2006(Perfito et al , 2007. Our data on the correlation between gonadal status and GnRH-I mRNA indicate that measurement of GnRH-I mRNA allows us to determine small-scale differences in hypothalamic reproductive status of adult birds.…”

Molt–breeding overlap alters molt dynamics and behavior in zebra finches,Taeniopygia guttata castanotis

“…Testis volumes of males in this study fell into one of two distinctive size classes: 11. 3-13.6mm 3 [which we categorized as non-breeding individuals because these testis sizes are below those reported for being able to produce viable sperm (Sossinka, 1980;Vleck and Priedkalns, 1985;Perfito et al, 2006)] and 47. 1-57.8mm 3 (categorized as breeding males).…”