The expression of Fos within the suprachiasmatic nucleus of the diurnal rodent Arvicanthis niloticus

doi:10.1016/S0006-8993(97)01092-5

Brain Research

Volume 791, Issues 1–2, 27 April 1998, Pages 27-34

https://doi.org/10.1016/S0006-8993(97)01092-5 Get rights and content

Abstract

Rhythms in the expression of the nuclear phosphoprotein Fos, have been demonstrated in the suprachiasmatic nucleus (SCN) of nocturnal rodents. When rats are housed in a 12:12-h light/dark (LD) cycle the number of Fos-immunoreactive (-IR) cells within the SCN is higher during the day than at night 9, 23. In the two experiments reported here, Fos-IR was examined in the SCN of a diurnal murid rodent, Arvicanthis niloticus. First, thirty-six adult male A. niloticus housed in a 12:12-h LD cycle were perfused at six equally spaced time points beginning 1 h after lights on (n=6 per time point). Brains were sectioned and treated with immunohistochemical procedures for the identification of Fos. The number of Fos-IR cells in the SCN varied significantly as a function of time, and was highest 1 h after lights on and decreased thereafter. The distribution of Fos-IR within the SCN overlapped with that of arginine-vasopressin-IR (AVP-IR) and vasoactive intestinal peptide-IR (VIP-IR), but not with that of gastrin-releasing peptide-IR (GRP-IR). In the second study, double-labeling techniques revealed extensive Fos expression within SCN neurons containing AVP-IR, but not neurons containing GRP-IR. In conclusion, although the overall rhythm of Fos-IR in the SCN is similar in diurnal and nocturnal rodents, differences may exist with respect to the relative distribution of Fos-immunoreacte cells within different SCN cell populations.

Introduction

There is extensive evidence that the hypothalamic suprachiasmatic nucleus (SCN) is the anatomical substrate for the primary circadian pacemaker in mammals. Lesions of the SCN abolish a variety of circadian rhythms [39], and transplants of fetal SCN tissue into animals with SCN lesions can restore a number of these rhythms [26]. In addition, a variety of rhythms intrinsic to the SCN have been documented in several species. For example, both in vitro and in vivo rates of glucose metabolism are higher in the SCN during the day than at night 40, 41, as are rates of single and multiple unit activity 14, 19, 20, 44. Thus, the SCN generates its own rhythms and is responsible for many circadian rhythms found in mammals.

Within the SCN of rats and hamsters the ventrolateral and dorsomedial regions are anatomically and functionally distinct. The ventrolateral region receives projections from the geniculohypothalamic tract (GHT) and the retinohypothalamic tract (RHT), both of which transmit visual input to the SCN 5, 15, 30. At least some of the fibers of the GHT contain neuropeptide-Y (NPY; 5, 16). The ventrolateral SCN is populated by cells containing VIP 27, 48, whereas cells in the dorsomedial SCN contain AVP [47]. These different areas are also characterized by different patterns of rhythmicity with respect to the expression and release of these various peptides. In the ventrolateral SCN the concentration of NPY is high around the transitions between light and dark phases of a 24-h light/dark (LD) cycle [21], and the peptide VIP as well as its messenger RNA peak during the dark phase of a 24-h LD cycle 2, 33, 45, 49. In contrast, the in vivo expression of AVP messenger RNA is higher during the day than at night 49, 50as is the release of AVP from the dorsomedial SCN in vitro 8, 13.

Rhythms in the expression of the nuclear phosphoprotein Fos have also been documented within the SCN of nocturnal rodents kept in various lighting conditions 3, 7, 10, 18, 24, 38, 43. The proto-oncogene c-fos encodes for the nuclear phospho-protein Fos, which binds to the DNA as a heterodimer with Jun and is involved in the regulation of gene transcription. The SCN of rats housed in a 12:12-h LD cycle exhibits a 24-h rhythm of Fos-IR which peaks during the light phase 9, 23. When different sub-regions of the rat SCN are examined separately, it becomes apparent that Fos-IR in the ventrolateral SCN is much higher during the light phase than the dark phase, whereas Fos-IR in the dorsomedial SCN is somewhat higher during the dark compared to the light phase [9]. In mice kept on a 12:12-h LD cycle, Fos-IR in the SCN is high immediately after lights on, drops by the middle of the day, and remains low until the lights are turned on again [7]. In rats kept in constant light (LL), Fos-IR is higher in the ventrolateral SCN during the subjective night than during the subjective day, but it remains unclear whether there is a rhythm in Fos-IR in the dorsomedial SCN 9, 10. In the SCN of mice and rats housed in constant dark (DD), no rhythm of Fos-IR is seen 7, 9, whereas hamsters kept in DD exhibit a slight rhythm in Fos-IR in the rostral SCN, with a peak in the middle of the subjective day [6]. In nocturnal rodents housed in DD, light pulses during the subjective night induce a dramatic increase in Fos-IR, whereas light pulses during the subjective day do not 38, 43. Thus, Fos-IR in the SCN of nocturnal rodents is expressed rhythmically in animals kept in LD cycles and LL, and responds to light pulses during DD in a time-dependent manner.

Research on neural mechanisms underlying mammalian circadian rhythms has focused primarily on nocturnal rodents, and nothing is currently known about how these mechanisms differ in diurnal and nocturnal species. Theoretically the differences could be due to differences within some subpopulation of SCN neurons, to differences in responsiveness to SCN signals, or to some combination of these two factors. With respect to three variables, rhythms within the SCN are similar in diurnal and nocturnal species. Rates of glucose utilization and electrical activity are higher during the day than at night in the SCN of both nocturnal and diurnal mammals 13, 19, 25, 37, 42, and immunoreactive AVP within the SCN is elevated during the day compared to the night in both humans and nocturnal rodents [17]. In spite of these findings, it remains possible that some aspect of SCN function differs in nocturnal and diurnal species, and is responsible for their differing patterns of rhythmicity.

Here we present two additional tests of the hypothesis that some aspect of SCN function differs in nocturnal and diurnal species. The diurnal animal model we have used is Arvicanthis niloticus, a small murid rodent found in sub-Saharan Africa. This species is diurnal with respect to its patterns of wheel-running, general activity, body temperature and copulatory behavior 22, 28. In the first study, we documented the rhythm in Fos-IR within the SCN of members of this species sacrificed at different phases of a 12:12-h LD cycle. In the second study we examined patterns of double-labeling of Fos and two different peptides found within SCN neurons, AVP and GRP.

Section snippets

Materials and methods

Animals used in this study were adult A. niloticus bred in the laboratory from a wild group captured in 1993 in Kenya. All animals were housed in plexiglass cages (38×34×16 cm) with same-sex siblings until 24–72 h before sacrifice, at which time they were individually housed. Animals were provided rodent chow (Harlan 8640 Teklad 22/5 Rodent diet) and water ad libitum, and were kept on a 12:12-h LD schedule; a red light (<5 lux) was kept on constantly.

Experiment 1: single labeling

Fos-IR was clearly observable within the nuclei of cells in the SCN, and the number of Fos-IR cells in the SCN changed significantly as a function of time of day (Fig. 1 and Fig. 2; F=18.792, P<0.001). The number of Fos-IR cells was highest 1 h after lights-on (ZT 1) and decreased progressively over the next 23 h (Fig. 1). Cell numbers at ZT 1 were significantly higher than at any other time point (Fisher's t-test, P<0.002), and those at ZT 5 and ZT 9 were significantly higher than at ZT 13 (P

Discussion

In the first experiment the concentration of Fos-IR cells in the SCN of A. niloticus was higher during the light hours than during the dark hours, and the transition from ZT 21 to ZT 1 was marked by a dramatic increase in Fos-IR. The number of Fos-IR cells in the SCN was highest at ZT 1, lower at ZT 5 and ZT 9, and still lower during the dark phase of a 12:12 LD cycle (ZT 13, 17, 21). A slight decrease in the levels of Fos-IR occurred over the course of the dark period, but this was not

Acknowledgements

We are grateful to Antonio Nunez, Cheryl Sisk and Kay Holekamp for comments on an earlier draft of the manuscript. This work was supported by NIMH Grant RO1 MH534/NS534433-01, and by the Neuroscience Training Program at Michigan State University.

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Citation Excerpt :
In the SCN, we found that both species showed a clear increase in cFOS after the light pulse (Figs. 1 & 2). This pattern of activation has been demonstrated before in both grass rats and mice pulsed with light during the dark phase of a 24 hr LD cycle as in the current study (Grass rat: [66,72]; Mouse: [42,73]), as well as when they are kept in DD and exposed to a pulse of light during the subjective night (Grass rats: [74]; Mouse: [27,75]). The focus of research on induction of cFOS in the SCN by light has been on its role in phase shifting of the endogenous circadian oscillator, and there is good evidence that it does play such a role in nocturnal rodents [76,77].
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Photic responses of the circadian system are mediated through light-induced clock gene expression in the suprachiasmatic nucleus (SCN). In nocturnal rodents, depending on the timing of light exposure, Per1 and Per2 gene expression shows distinct compartmentalized patterns that correspond to the behavioral responses. Whether the gene- and region-specific induction patterns are unique to nocturnal animals, or are also present in diurnal species is unknown. We explored this question by examining the light-induced Per1 and Per2 gene expression in functionally distinct SCN subregions, using diurnal grass rats Arvicanthis niloticus. Light exposure during nighttime induced Per1 and Per2 expression in the SCN, showing unique spatiotemporal profiles depending on the phase of the light exposure. After a phase delaying light pulse (LP) in the early night, strong Per1 induction was observed in the retinorecipient core region of the SCN, while strong Per2 induction was observed throughout the entire SCN. After a phase advancing LP in the late night, Per1 was first induced in the core and then extended into the whole SCN, accompanied by a weak Per2 induction. This compartmentalized expression pattern is very similar to that observed in nocturnal rodents, suggesting that the same molecular and intercellular pathways underlying acute photic responses are present in both diurnal and nocturnal species. However, after an LP in early subjective day, which induces phase advances in diurnal grass rats, but not in nocturnal rodents, we did not observe any Per1 or Per2 induction in the SCN. This result suggests that in spite of remarkable similarities in the SCN of diurnal and nocturnal rodents, unique mechanisms are involved in mediating the phase shifts of diurnal animals during the subjective day.
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Previous studies have suggested that nocturnal and diurnal species of rodents differ in their circadian responses to light including phase shifts and early gene expression. Rhabdomys pumilio, the four-striped field mouse, is diurnal both in nature and in the laboratory. We studied in this species the response of the suprachiasmatic nucleus (SCN) to light stimuli at different time periods using light-induced expression of Fos as marker of neuronal activity. Fos induction in the SCN was investigated using immunohistochemistry and quantitative image analysis. The animals were exposed to a 15 min light pulse with monochromatic green light at different circadian times throughout a 24-h cycle. Animals maintained in constant darkness served as controls. R. pumilio exhibited an endogenous Fos rhythm in the SCN during constant darkness with highest expression during the subjective day at circadian time (CT) 2 and CT10. Photic stimulation resulted in significant Fos induction in the SCN at CT6, CT14, CT18 and CT22, compared to controls kept in constant darkness, with a peak of expression at CT22, i.e. during late subjective night, mainly due to expression in the ventral SCN. In tract tracing experiments based on the use of cholera toxin subunit B, we found that retinal fibres innervate mainly the contralateral ventral SCN. The intergeniculate leaflet received bilateral retinal innervation with overlap between ipsilateral and contralateral fibres. Altogether the data show that the rodent R. pumilio is a unique diurnal model for chronobiological studies.
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Research reportThe expression of Fos within the suprachiasmatic nucleus of the diurnal rodent Arvicanthis niloticus

Abstract

Introduction

Section snippets

Materials and methods

Experiment 1: single labeling

Discussion

Acknowledgements

Mol. Brain Res.

Brain Res.

Brain Res.

Exp. Neurol.

Neurosci. Lett.

Brain Res.

Brain Res. Bull.

Brain Res.

Brain Res.

Neurosci. Biobehav. Rev.

Brain Res.

Neurosci. Lett.

Neuron

Neuroscience

Physiol. Behav.

Trends Neurosci.

Brain Res.

Brain Res. Bull.

Brain Res.

Brain Res.

Neuroscience

Brain Res.

Mol. Brain Res.

Brain Res.

Peptides

Circadian modulation in photic induction of Fos-like immunoreactivity in the suprachiasmatic nucleus cells of diurnal chipmunk Eutamias asiaticus

J. Comp. Physiol. A

Research report
The expression of Fos within the suprachiasmatic nucleus of the diurnal rodent Arvicanthis niloticus