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.