The endogenous activity patterns of Africa’s smallest terrestrial mammal, the pygmy mouse (Mus minutoides)

The endogenous rhythmicity of the locomotor activity and subsequent entrainment by light cycles of the pygmy mouse (Mus minutoides A. Smith, 1834) was investigated under laboratory-controlled conditions. Seasonal trapping in the field was used to assess the predominant activity phase in their natural habitat, and determine whether seasonal variation in activity occurs in the field. Mus minutoides were subjected to a series of light cycles starting with a 12 h light (L) : 12 h dark (D) cycle (2 weeks) to determine whether they entrain their activity patterns to light cues, after which they were maintained in constant darkness (3 weeks) and the endogenous rhythm allowed to free run; the tau for each animal’s endogenous activity rhythm was then calculated. This was followed by another 12 h L : 12 h D cycle (2 weeks) before the cycle was inverted to 12 h D : 12 h L (2 weeks) to assess the rate of re-entrainment. The animals were then exposed to long (16 h L : 8 h D) and short (8 h L : 16 h D) photoperiods for a 6-week period under each lighting regime. Changes in foraging behaviour and body mass were recorded throughout the study. Mus minutoides is strictly nocturnal in both the laboratory and the field, it caches food in its nest, and it cannot be trapped during the winter months in this environment.

Endocrine regulation of circadian physiology

Endogenous circadian clocks regulate 24-h rhythms of behavior and physiology to align with external time. The endocrine system serves as a major clock output to regulate various biological processes. Recent findings suggest that some of the rhythmic hormones can also provide feedback to the circadian system at various levels, thus contributing to maintaining the robustness of endogenous rhythmicity. This delicate balance of clock–hormone interaction is vulnerable to modern lifestyle factors such as shiftwork or high-calorie diets, altering physiological set points. In this review, we summarize the current knowledge on the communication between the circadian timing and endocrine systems, with a focus on adrenal glucocorticoids and metabolic peptide hormones. We explore the potential role of hormones as systemic feedback signals to adjust clock function and their relevance for the maintenance of physiological and metabolic circadian homeostasis.

Mammalian clock output mechanisms

In mammals many behaviours (e.g. sleep–wake, feeding) as well as physiological (e.g. body temperature, blood pressure) and endocrine (e.g. plasma corticosterone concentration) events display a 24 h rhythmicity. These 24 h rhythms are induced by a timing system that is composed of central and peripheral clocks. The highly co-ordinated output of the hypothalamic biological clock not only controls the daily rhythm in sleep–wake (or feeding–fasting) behaviour, but also exerts a direct control over many aspects of hormone release and energy metabolism. First, we present the anatomical connections used by the mammalian biological clock to enforce its endogenous rhythmicity on the rest of the body, especially the neuro-endocrine and energy homoeostatic systems. Subsequently, we review a number of physiological experiments investigating the functional significance of this neuro-anatomical substrate. Together, this overview of experimental data reveals a highly specialized organization of connections between the hypothalamic pacemaker and neuro-endocrine system as well as the pre-sympathetic and pre-parasympathetic branches of the autonomic nervous system.