Quail and other short-lived birds

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Abstract

Japanese quail are small galliforms that are migratory and generally live 2 to 3 years in the wild. Although there is evidence for other environmental cues, they primarily respond to long daylength for regulation of reproduction. In contrast to the Common Tern, a long-lived sea bird that shows little evidence of reproductive aging, Japanese quail follow a well-defined process of aging with evidence of declining function in reproductive, metabolic, and sensory systems. Our studies focus on neuroendocrine changes associated with reproductive aging in the Japanese quail, with emphasis on the male in order to study both endocrine and behavioral components of reproduction and the process of reproductive aging.

Introduction

The Japanese quail model system has been characterized during aging; many of the age-related changes found in mammalian neuroendocrine systems are mirrored in this species. Further, this species has two distinct advantages over mammalian models. First, hypothalamic systems exhibit neuroplasticity; even senescent males respond to testosterone replacement therapy with full recovery of reproductive behavior. This behavioral recovery is accompanied by restoration of specific hypothalamic neuropeptide systems, which regulate both sexual behavior and GnRH, thereby allowing identification of these neural systems. Further, males possess a cloacal gland, which similar to the prostate gland is androgen responsive. The second unique advantage to the avian model is a dynamic bone physiology, especially in females because the hollow bones serve as a depot for minerals used in egg production. As a result, aging females develop bone fragility; they have been well characterized as a model for hormone effects on osteoporosis and the role of vitamin D.

It appears that critical neuroregulatory systems, specifically catecholamine (norepinephrine [NE] and dopamine [DA]), aromatase enzyme (AROM) and opioid peptide (β-endorphin [END] and met-enkephalin [ENK]) systems must also remain young in order to prolong reproductive function. In the case of testosterone replacement therapy and restoration of male sexual behavior in the quail, it appears that these systems remain plastic and continue to respond to exogenous steroids, even in senescent animals. Therefore, rescue of these systems through steroid hormone replacement provides a tool for investigating systems affected by the process of aging and determining the processes involved in restoring some of their function.

Section snippets

Of quail and terns!

The Common Tern is a shore bird with a relatively long life span. This species exhibits clear and very interesting age-related changes in breeding strategies and behavior (Nisbet, 1989). As the pairs return from migration to their breeding grounds, the older pairs set about courtship and nesting immediately. These pairs, by virtue of their rapid nesting are able to select the prime territories for their nests. In addition, their immediate nesting allows the young to be hatched earlier and

Lifetime patterns in reproduction in the Japanese quail (Coturnix japonica)

The timing of the age-related decline is both species specific and varies with the individual (Ottinger et al., 1995, Nisbet, 1999). Japanese quail reach puberty about 6–8 weeks of age and attain full adult function within 2–3 weeks, with females maturing slightly later than males. Quail exhibit relatively high fertility and egg production and remain reproductive if maintained on a long photoperiod. On this regimen, females have a lifespan of 2.5 to 3 years, whereas males live 3–5 years (Ottinger,

Aging of hypothalamic systems: restoration by steroids?

Our approach has been to study hypothalamic systems in young reproductive quail and then compare these findings to senescent individuals. As mentioned earlier, the age-related decline in fertility in male quail coincides with decreasing reproductive behavior; both precede measurable hormone changes. Reproductive behavior therefore provides an index of reproductive status. Once separated according to behavioral activity, we found that sexually active males retained relatively higher circulating

Specific changes in the GnRH-I system during aging

In rats, age-related changes are relatively subtle in the female's GnRH system (Rubin et al., 1984, Leranth et al., 1986). However, there were morphological changes found in the synapses of GnRH neurons (Witkin, 1989). In birds, we have studied chicken GnRH-I, which is the biologically active form of the peptide. In quail, GnRH-I cell bodies are concentrated more rostral (preoptic-lateral septal; POA-SL region) and project to the median eminence (ME) (Sharp et al., 1990, Dunn et al., 1993).

Summary

In summary, gonadal steroids are likely to be a key element in the process of aging as they reflect hypothalamic and pituitary regulation and in turn modulate hypothalamic regulatory systems that impact behavioral and endocrine responses. In quail, testosterone is required for AROM and AVT immunoreactivity in the Preoptic area. In aging male quail, there is loss of AROM and AVT in the POA, which are restored with exogenous testosterone treatment. The POA-SL is also the site of GnRH-I cell

Acknowledgements

There have been a number of collaborators, graduate students, and post-doctoral fellows involved in this research. Some of them are cited in the text and others are referenced. This work would not have been possible without them. Support for this work was provided by the University of Maryland Agriculture Experiment Station, USDA-NRI grants # 88-37242 and 92-37203-7742 (MAO) and NATO CRG. 92-1267 (MAO and GCP).

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