Seasonal regulation of behaviour: what role do hormone receptors play?

Abstract

Many animals differentially express behaviours across the annual cycle as life stages are coordinated with seasonal environmental conditions. Understanding of the mechanistic basis of such seasonal changes in behaviour has traditionally focused on the role of changes in circulating hormone levels. However, it is increasingly apparent that other endocrine regulation mechanisms such as changes in local hormone synthesis and receptor abundance also play a role. Here I review what is known about seasonal changes in steroid hormone receptor abundance in relation to seasonal behaviour in vertebrates. I find that there is widespread, though not ubiquitous, seasonal variation in the expression of steroid hormone receptors in the brain, with such variation being best documented in association with courtship, mating and aggression. The most common pattern of seasonal variation is for there to be upregulation of sex steroid receptors with the expression of courtship and mating behaviours, when circulating hormone levels are also high. Less well-documented are cases in which seasonal increases in receptor expression could compensate for low circulating hormone levels or seasonal downregulation that could serve a protective function. I conclude by identifying important directions for future research.

Keywords: endocrine; hormone receptors; seasonality; steroid hormones; vertebrates.

Figure 1.

Potential relationships among seasonal changes in hormone receptor abundance, circulating hormones and behavioural responses. Four scenarios are shown in which the seasonal pattern of hormone secretion is the same, but the pattern of receptor abundance varies. For each scenario, the pattern of response (i.e. expression of a behaviour) generated by hormone-receptor binding is shown. (a) No seasonal changes in receptor abundance, variation in the behavioural response is driven entirely by changing hormone levels. (b) Enhancement: a seasonal increase in receptor abundance enhances the effect of the hormone. (c) Compensation: a seasonal increase in receptor abundance compensates for low hormone levels, sustaining expression of the behaviour when hormone level is low. (d) Protection: seasonal decline in receptor abundance reduces expression of behaviour despite high hormone levels. The scenarios shown are simplified for illustration; quantities of both hormones and receptors may further vary on timescales shorter than those shown here, and other regulatory elements such as hormone-binding proteins, hormone-metabolizing enzymes and receptor coactivators may also influence the behavioural response. (Online version in colour.)