Light-emitting diodes with
narrow spectral emissions or notch filters facilitate these investigations. Practicalities may force a reliance on incandescent and fluorescent lights but, because of their complex spectra, comparing light of different colors is more difficult. Illuminance measures suffice when wavelength per se is not a central focus. The photosensitivity of a physiological or behavioral response to light depends on what is being measured. This is important as the photosensitivity of one response cannot be generalized to other functions. As an example, measurements were made of the thresholds of entrainment of wheel-running DNA Damage inhibitor rhythms at three wavelengths, and these were compared with the thresholds of two other non-image-forming visual system functions, i.e. masking and the pupillary light reflex. Dim light that entrained mice failed to elicit either masking or pupillary light reflex; in general, circadian entrainment is more sensitive by 1–2 log units than other measures of the non-image-forming visual system. In an artificial photic environment, dim light can entrain circadian rhythms even when it fails to produce more easily measurable acute responses to light such as phase shifting and melatonin suppression (Butler & Silver, 2011). As mentioned previously, not only does the
circadian system influence feeding and metabolism, but food cues can also act to entrain circadian rhythms (Saper, 2006; Patton & Mistlberger, 2013). If food presentation is restricted to Niclosamide a short temporal window (typically a few hours), animals
exhibit increased Y-27632 in vitro activity in anticipation of feeding [food anticipatory activity (FAA)]. Because this synchronization of behavior with feeding persists in the absence of the SCN, a separate designation of the food entrainable oscillator was coined (Stephan et al., 1979). The identification of the neural locus of the food-entrainable oscillator has been challenging. The dorsomedial nucleus of the hypothalamus (DMH) probably plays a role in food entrainment (Gooley et al., 2006; Fuller et al., 2008), although mice and rats can entrain to food cues in the absence of a DMH (Landry et al., 2006, 2007; Acosta-Galvan et al., 2011). In mice, DMH lesions lead to reduced FAA, whereas lesions of both the SCN and DMH result in enhanced FAA (Acosta-Galvan et al., 2011). These findings suggest that the DMH participates in FAA, but is not the sole neural locus of the food-entrainable oscillator. It is likely that metabolic cues from the periphery, communicated to the central nervous system, participate in food entrainment. For example, ghrelin cells in the stomach that signal hunger express clock genes, ghrelin administration leads to increased activity in animals fed ad libitum, and ghrelin and clock gene rhythms in these stomach cells are synchronized to feeding (LeSauter et al., 2009). Consistent with these findings, FAA is greatly reduced in ghrelin receptor knockout mice (Blum et al., 2009; LeSauter et al.