Investigaciones

Reprod. Nutr. Dev. 46 (2006) 379–390
c INRA, EDP Sciences, 2006
DOI: 10.1051/rnd:2006019

379

Review

Dynamic and integrative aspects of the regulation
of reproduction by metabolic status in male sheep
Dominique B*, Song Z, Graeme B. M
School of Animal Biology M085, Faculty of Natural and Agricultural Sciences, The University of
Western Australia, Crawley 6009, WesternAustralia

Abstract – Change in metabolic status, defined as a change in the availability of nutrients and energy to the tissues, is a powerful regulator of the reproductive function in small ruminants, especially
in genotypes that are not strongly responsive to photoperiod such as the Merino sheep. In this paper,
the dynamics of the response of the reproductive axis to changes in metabolicstatus are reviewed
in the light of recent studies. The nature and the roles of the various components of the pathways
linking metabolic status to reproduction are considered: nutrients and metabolites, the endocrine
system, and the nervous system. We discuss the role of leptin and insulin in detail because of the
central role of these two hormones in both the early gonadotrophin response toincrease in nutrition
and the long-term response of the testis to dietary stimulation. The possible roles of recently identified peptides, such as ghrelin and kisspeptin, are also considered as we develop a general hypothesis
that encompasses the different levels of integration necessary to explain the complex interactions
between reproductive function and metabolic status, and the possible existenceof a “metabolic
memory” in this interaction.
nutrition / GnRH / LH / leptin / insulin / testis

1. INTRODUCTION
In sheep and goats, reproductive activity is affected by a range of external factors, including socio-sexual cues, photoperiod and metabolic status (reviews: [1, 2]).
Metabolic status is defined as the amount
of nutrients and energy available to the animal’s tissues at a giventime, and it depends
on three factors: the mass of food consumed, the mass of body reserves and the
rate of expenditure of energy. Importantly,
changes in any of these three components
can influence the reproductive capacity in
both males and females [3, 4]. This review
aims at shedding light upon the signalling
systems involved in the regulation of repro* Corresponding author:dbla@animals.uwa.edu.au

ductive function by changes in metabolic
status, with a focus on male ruminants. We
begin by focussing on the metabolic target
in the reproductive axis and then we describe the concept of metabolic status and
its links to reproductive function. The response of the reproductive axis to an acute
increase in food intake is used to illustrate
the dynamic and integrative nature of therelationship between metabolic status and
reproductive activity. We then review the
role of three different classes of control
systems – nutrients, endocrine and nervous
systems – and illustrate the interactions
within and between these systems. Finally,
the regulation of reproductive function is
placed in perspective with the action of
other external factors that also controlreproduction.

Article published by EDP Sciences and available at http://www.edpsciences.org/rnd or http://dx.doi.org/10.1051/rnd:2006019

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D. Blache et al.

2. THE KEY METABOLIC TARGET
IN THE REPRODUCTIVE AXIS

4. EFFECTS OF VARIATION
IN METABOLIC STATUS
ON REPRODUCTIVE FUNCTION

The most fundamental driver of
reproductive function is the hypothalamicpituitary gonadal axis. Hypothalamicgonadotrophin-releasing hormone (GnRH)
controls the secretion of both luteinising
hormone (LH) and follicle-stimulating
hormone (FSH), the principal factors that
regulate the production of spermatozoa
and hormones by the testis. The frequency
of GnRH pulses, and consequently LH
pulses, is the code used by the nervous
system to control gonadal function. The
network of neurons that...