Circadian rhythm is the approximately 24-hour rhythmicity that regulates physiology and behavior in a variety of organisms. The mammalian circadian system is organized in a hierarchical manner. Molecular circadian oscillations driven by genetic feedback loops are found in individual cells, whereas circadian rhythms in different systems of the body are orchestrated by the master clock in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. SCN receives photic input from retina and synchronizes endogenous rhythms with the external light/dark cycles. SCN regulates circadian rhythms in the peripheral oscillators via neural and humoral signals, which account for daily fluctuations of the physiological processes in these organs. Disruption of circadian rhythms can cause health problems and circadian dysfunction has been linked to many human diseases.
Lily Yan, Laura Smale, and Antonio A. Nunez
Circadian rhythms are endogenous daily rhythms evident in behavior and physiology. In mammals, these rhythms are controlled by a hierarchical network of oscillators showing a coherent circadian coordination or coupling. The hypothalamic suprachiasmatic nucleus (SCN) sits on top of the hierarchy and coordinates the phase of oscillators in other brain regions and in peripheral organs, including endocrine glands. The phase of the SCN oscillator, in reference to the daily light-dark cycle, is identical across mammalian species regardless of whether they are most active during the day or night, that is, diurnal or nocturnal. However, the extra-SCN or peripheral oscillators are out of phase and are often reversed by 180° across diurnal and nocturnal mammals. In the endocrine system, with the notable exception of the pattern of pineal melatonin secretion, which features elevated levels at night regardless of the activity profile of the species, most endocrine rhythms show a 180° reversal when diurnal and nocturnal species are compared. There is also evidence of differences between nocturnal and diurnal species with respect to their rhythms in sensitivity or responsiveness to hormonal stimulation. One of the major unanswered questions in the field of comparative endocrinology relates to the mechanism responsible for the differential coupling in diurnal and nocturnal mammals of extra-SCN oscillators and overt circadian rhythms with the SCN oscillator and the light dark cycle. Viable hypotheses include species-specific switches from excitation to inhibition at key nodes between the SCN and its targets, the presence of extra-SCN signals that converge on SCN targets and reverse the outcome of SCN signals, and changes in oscillatory parameters between the oscillator of the SCN and those outside the SCN resulting in an anti-phase coupling among key oscillators.
Gerald H. Jacobs
Color is a central feature of human perceptual experience where it functions as a critical component in the detection, identification, evaluation, placement, and appreciation of objects in the visual world. Its role is significantly enhanced by the fact that humans evolved a dimension of color vision beyond that available to most other mammals. Many fellow primates followed a similar path and in recent years the basic mechanisms that support color vision—the opsin genes, photopigments, cone signals, and central processing—have been the subjects of hundreds of investigations. Because of the tight linkage between opsin gene structure and the spectral sensitivity of cone photopigments, it is possible to trace pathways along which color vision may have evolved in primates. In turn, such information allows the development of hypotheses about the nature of color vision and its utility in nonhuman primates. These hypotheses are being critically evaluated in field studies where primates solve visual problems in the presence of the full panoply of photic cues. The intent of this research is to determine which aspects of these cues are critically linked to color vision and how their presence facilitates, impedes, or fails to influence the solutions. These investigations are challenging undertakings and the emerging literature is replete with contradictory conclusions. But steady progress is being made and it appears that (a) some of the original ideas about there being a restricted number of tasks for which color vision might be optimally utilized by nonhuman primates (e. g., fruit harvest) were too simplistic and (b) depending on circumstances that can include both features of proximate visual stimuli (spectral cues, luminance cues, size cues, motion cues, overall light levels) and situational variables (social cues, developmental status, species-specific traits) the utilization of color vision by nonhuman primates is apt to be complex and varied.
Understanding of the brain mechanisms regulating reproductive behaviors in female laboratory animals has been aided greatly by our knowledge of estrogen receptors in the brain. Hypothalamic neurons that express the gene for estrogen receptor-alpha regulate activity in the neural circuit for the simplest female reproductive response, lordosis behavior. In turn, many of the neurotransmitter inputs to the critical hypothalamic neurons have been studied using electrophysiological and neurochemical techniques. The upshot of all of these studies is that lordosis behavior presents the best understood set of mechanisms for any mammalian behavior.