Glutamatergic Stimulation of Luteinising Hormone Secretion in Relatively Refractory Male Songbirds

Abstract

Seasonal breeding in two Sonoran desert passerines, the Cassin's (Aimophila cassinii) and Rufous-crowned (Aimophila ruficeps) Sparrows is thought to be terminated by the development of a decrease in responsiveness to photostimulation, a condition known as relative photorefractoriness. It was predicted that the development of relative refractoriness is a consequence of a decrease in gonadotrophin-releasing hormone (GnRH) synthesis and associated stores of releasable GnRH. This hypothesis was tested by determining the luteinising hormone (LH) responses to the excitatory amino acid glutamate agonist N-methyl-D,L-aspartate (NMA) in males of the two species subjected to photomanipulations aimed at generating five groups: Fully photosensitive with undeveloped testes on short days (8L : 16D); fully photosensitive with developed testes on 13L : 11D; relatively photorefractory with regressed testes on 13L : 11D, and groups with developed testes held on 15L : 9D or 16L : 8D. LH release was stimulated in the Cassin's Sparrow by NMA most in the 8L group; to a lesser, but similar extent in the two 13L groups; and not at all in the 15L and 16L groups. LH release was not stimulated by NMA in any of the photoperiodic regimes in the Rufous-crowned Sparrow. In both species, NMA induced Fos-like immunoreactivity in the anterior and basal hypothalamus, but not in GnRH cell bodies. It is concluded that the development of relative photorefractoriness in Cassin's Sparrows is a consequence of reduced GnRH synthesis, reflected in a reduction in releasable GnRH. The lack of LH response of the Rufous-crowned Sparrows to NMA administration may be a consequence of high responsiveness to handling stress.

Gonadotropin-releasing hormone plasticity: A comparative perspective

“…The administration of exogenous NMDA leads to an increase in peripheral LH concentrations in a large range of mammals and birds, and therefore, it has been claimed, provides a indicator of the amount of releasable GnRH1 [22, 54, 55, 117, 181]. In songbirds, NMDA treatment to photosensitive birds results in large increases in peripheral LH concentrations in the European starling [43] and Cassin's sparrow ( Aimophila cassinii ; [53]), but only in a moderate increase in photostimulated birds and a negligible change in photorefractory birds. However, white-crowned sparrows ( Zonotrichia leucophrys ) increase peripheral LH concentrations in response to NMDA regardless of their photoperiodic state [114, 116].…”

Up to the challenge? Hormonal and behavioral responses of free-ranging male Cassin's Sparrows, Peucaea cassinii, to conspecific song playback

“…In captive male Cassin's Sparrows, administration of the GnRH secretagogue NMA to photostimulated birds with high plasma LH had little effect on the plasma concentrations of this hormone (Deviche et al, 2008).…”

“…Plasma LH concentrations were determined using a micromodification of the radioimmunoassay described previously (Sharp et al, 1987). The assay has been validated for use in the Cassin's Sparrow (Deviche et al, 2008). Briefly, the reaction volume was 60 l, comprising 20 l of plasma sample or standard, 20 l of primary rabbit LH antibody, and 20 l of I 125 -labelled LH.…”

“…For example, in male Cassin's Sparrows, Peucaea carpalis, the increase in plasma LH in response to NMA injection is less when birds are photostimulated and have elevated plasma LH than when they are exposed to short days and have low plasma LH (Deviche et al, 2008). In the Green Anole, Anolis carolinensis, lightweight males have lower plasma T than heavyweight males, but GnRH administration does not increase plasma T in either form, suggesting that intact lightweight and heavyweight males are secreting this hormone at the maximum rate (Husak et al, 2009).…”

Gonadotrophin‐Releasing Hormone Neurones in a Photoperiodic Songbird Express Fos and Egr‐1 Protein After a Single Long Day

“…One long day fails to induce Fos protein expression in GnRH somata in mallards (18), Japanese quail (3) and white‐crowned sparrows (S. Meddle, unpublished data) but does induce massive Fos protein expression in the mediobasal hypothalamus (MBH) around the GnRH axons and terminals (3, 20). Similarly, NMDA injections induce Fos protein expression in the MBH but not in the GnRH somata in white‐crowned sparrows (19), white‐throated sparrows ( Zonotrichia albicollis ) (20), rufous‐crowned sparrows ( Aimophila ruficeps ) (21) and Cassin’s sparrows ( Aimophila cassinii ) (21). Clearly, GnRH secretion does not require the induction of Fos protein in the GnRH somata, and any such induction may therefore signify protein synthesis that is not necessarily related to release.…”