Reproductive Physiology: Songbird Study Removes Long-Standing Neuroendocrinology Research Roadblock

Abstract

Songbirds have long been favored as models to investigate the neuroendocrine mechanisms that regulate seasonal reproduction. Progress in this area has for many years been hindered by our inability to identify the songbird gene that codes for GnRH1, the hypothalamic decapeptide that stimulates the secretion of gonadotropins by the pituitary gland and, consequently, gonadal development (1–3). A new study removes this roadblock (4), thus creating new opportunities to address several long-standing questions. In this study, Stevenson et al. (4) cloned the GNRH1 cDNAs of two songbirds, the European starling and zebra finch, that are extensively used for research in reproductive endocrinology. The nucleotide sequence that is predicted to code for the GnRH1 decapeptidewas found topresentanumberofdifferencesbetween thezebra finch and starling and also between these species and nonsongbird avian species studied to date. Despite these differences, the amino acid structure of GnRH1 was found to be conserved across species examined. Immunoreactive GnRH1-containing neurons in songbirds are located primarily in the preoptic region (5–7). Confirming this observation, Stevenson et al. found GNRH1 mRNA expression, as determined by RT-PCR and in situ hybridization, to likewise be confined to this area of the brain. The activity of the reproductive system in many songbirds is regulated primarily by day length (1, 2, 8). In these birds, exposure to sufficiently long days stimulates reproductive development by increasing the production and secretion of GnRH1. Prolonged exposure to these long days, however, desensitizes the GnRH1 system to this stimulus. Birds in this condition are said to be photorefractory and undergo gonadal regression (8–10). The development of photorefractoriness is a key event in the annual reproductive cycle of seasonally breeding birds, but even though the reproductive consequences of photorefractoriness are well described, the process that initiates is speculative. Recently photorefractory birds generally retain a large pool of releasable GnRH1 whereas brain stores of GnRH1 in deeply photorefractory birds are low (6, 11–13). This observation led to hypothesize that photorefractoriness develops as a two-step process consisting in decreased secretion and then decreased production of GnRH1 (1). To test this hypothesis, Stevenson et al. (4) measured the gonad size and preoptic region expression of the GNRH1 gene in male starlings that they transferred from short days to chronically long days until birds became photorefractory. In this experiment, photoinduced changes in GNRH1 gene expression and gonad size followed parallel time courses as starlings became photorefractory. This finding is significant because it suggests that contrary to the current model, an early event associated with photorefractoriness in starlings consists in decreased expression of the GNRH1 gene and therefore presumably also of GnRH1 peptide production. The identification of the songbird GNRH1 gene will facilitate the use of these organisms to more directly address a number of unresolved issues in avian neuroendocrinology: Species such as the Japanese quail, Coturnix coturnix japonica, and desert sparrows of the genus Aimophila terminate a reproductive cycle not by becoming absolutely photorefractory, as do starlings but by developing relative photorefractoriness (14–16). By contrast with absolutely refractory birds, relatively photorefractory birds are thought to retain large hypothalamic storesofGnRH1thatcanbereleased inresponse tosufficiently long photoperiod or appropriate nonphotic or pharmacological stimulation (17).Does significantproductionofGnRH1continueas long asbirds remainrelativelyphotorefractory? Is thecontrolofGnRH1 production in relatively photorefractory species fundamentally different from that in absolutely photorefractory species? Studies comparing GNRH1 gene expression in absolutely and relatively photorefractory species should help answer these questions. As in other photoperiodic vertebrates, the activity of the reproductive system in songbirds is influenced by nonphotoperiodic factors such as water and food availability (18–20), tem-