Behavioral Masking and cFos Responses to Light in Day- and Night-Active Grass Rats

Abstract

Light not only entrains the circadian system but also has acute effects on physiology and behavior, a phenomenon known as masking. Behavioral masking responses to bright light differ in diurnal and nocturnal species, such that light increases arousal in the former and decreases it in the latter. Comparisons made within a species that displays both diurnal and nocturnal patterns of behavior may provide insight into how masking differs between chronotypes and the association between mechanisms controlling masking and the circadian drive for activity. Nile grass rats (Arvicanthis niloticus) provide a useful model for studying such issues because when these animals are housed with running wheels, some run primarily during day, while others run at night. Here we compared behavioral masking responses to 2-h pulses of light and darkness given across a 12:12 light/dark cycle in day-active (DA) and night-active (NA) grass rats. Both wheel-running activity (WRA) and general activity (GA) were monitored. Light pulses at night tended to increase both WRA and GA overall in the DA grass rats, while in NA grass rats, light pulses significantly reduced WRA but had no effect on GA. Dark pulses during the day tended to decrease both WRA and GA in the DA grass rats, while in the NA grass rats, they tended to increase WRA in the early day but had no effect on GA overall. Next, we measured cFos expression within 2 brain areas potentially involved in masking, the intergeniculate leaflet (IGL) and the olivary pretectal area (OPT), of DA and NA grass rats either sacrificed on a control night or after a 1-h light pulse at ZT14. In DA grass rats, light at ZT14 induced cFos in the IGL and OPT, whereas in NA grass rats, cFos levels in both structures were high at ZT14 and were not altered by a 1-h light pulse. Overall, these results suggest that masking responses to light and darkness are dependent on the chronotype of the individual and that the responsiveness of the IGL and OPT to light may depend on or contribute to the behavioral response of these animals.

Neuronal substrates underlying stress resilience and susceptibility in rats

“…As recently shown in a mice study, light induces c-Fos activation in some brain areas such as LHb, but not in others such as DLG (dorsal lateral geniculate), where light rather cause a decrease of c-Fos activation [65]. It was reported that light does not induce changes in c-Fos in retinorecipient areas such as in the LHb, dorsal lateral geniculate (DGL), in the ventral subparalventricular zone (vSPZ), suprachiasmic nucleus (SCN) or in the VLGMC of night active grass rats [65, 66]. Our data are in accordance with other previous studies showing that CMS animals display circadian rhythm disturbances [67] which are common symptoms of MDD [68].…”

Intergeniculate leaflet lesions result in differential activation of brain regions following the presentation of photic stimuli in Nile grass rats

“…The OPT is retinorecipient [8], reciprocally connected to the IGL [4,9], and exhibits a Fos response to light [13,24]. The OPT mediates pupillary reflexes to light and is involved in the generation of eye movements [25], but may also be involved in sleep and circadian behavior [10,26,27].…”

“…For example, for grass rats that become night-active when given a running wheel, the OPT does not exhibit light-induced Fos at ZT14, whereas it does so in day-active wheel-runners [24]. Also, light induces Fos in the OPT of intact grass rats, but decreases it in intact mice [28]; this result stands in contrast to findings in laboratory rats [13], in which 2-h light pulses given at ZT19 induce increased Fos-ir in the OPT.…”

A Path to Sleep Is through the Eye

“…It should be noted that generalizations about the effects of light on sleep or arousal that are deduced from studies of nocturnal species may not apply to diurnal animals (for discussion, see Shuboni et al, 2012 ; Langel et al, 2014 ). In humans, for example, nocturnal light enhances alertness (for review, see Cajochen, 2007 ; Hubbard et al, 2013 ), contrary to the effects observed in small nocturnal laboratory rodents.…”