Neural activation in arousal and reward areas of the brain in day-active and night-active grass rats

Abstract

In the diurnal unstriped Nile grass rat (Arvicanthis niloticus) access to a running wheel can trigger a shift in active phase preference, with some individuals becoming night-active (NA), while others continue to be day-active (DA). To investigate the contributions of different neural systems to the support of this shift in locomotor activity, we investigated the association between chronotype and Fos expression during the day and night in three major nuclei in the basal forebrain (BF) cholinergic (ACh) arousal system - medial septum (MS), vertical and horizontal diagonal band of Broca (VDB and HDB respectively) -, and whether neural activation in these areas was related to neural activity in the orexinergic system. We also measured Fos expression in dopaminergic and non-dopaminergic cells of two components of the reward system that also participate in arousal - the ventral tegmental area (VTA) and supramammillary nucleus (SUM). NAs and DAs were compared to animals with no wheels. NAs had elevated Fos expression at night in ACh cells, but only in the HDB. In the non-cholinergic cells of the BF of NAs, enhanced nocturnal Fos expression was almost universally seen, but only associated with activation of the orexinergic system for the MS/VDB region. For some of the areas and cell types of the BF, the patterns of Fos expression of DAs appeared similar to those of NAs, but were never associated with activation of the orexinergic system. Also common to DAs and NAs was a general increase in Fos expression in non-dopaminergic cells of the SUM and anterior VTA. Thus, in this diurnal species, voluntary exercise and a shift to a nocturnal chronotype changes neural activity in arousal and reward areas of the brain known to regulate a broad range of neural functions and behaviors, which may be also affected in human shift workers.

Figure 1

Rostro-caudal illustrations depicting representative sampling areas used to quantify Fos expression in ACh and nACh cells of the MS, VDB, and HDB (A–E), as well as Fos expression in nTH and TH cells of the VTA and SUM (F–I). Boxes used for cellular counts had the following dimensions: (A–E) 600 µm × 300 µm and (F–I) 160 µm × 160 µm. Scale bar= 1 mm.

Figure 2

Patterns of Fos expression in ACh cells (A, B) and nACh cells (C, D) in the HDB. Panels A and C show significant ZT differences (asterisks) within groups, whereas panels B and D show significant group differences within ZT (p< 0.05). Note that group means with different letters are significantly different from each other, here as well as in Figure 3, Figure 4, and Figure 7.

Figure 3

Patterns of Fos expression in nACh cells in the MS. Panel A shows significant ZT differences (asterisks) within groups, whereas panel B shows significant group differences (letters) within ZT (p< 0.05).

Figure 4

Patterns of Fos expression in nACh cells in the VDB. Panel A shows comparisons within groups (no significant effects of ZT). Panel B shows significant group differences (letters) within ZT (p< 0.05).

Figure 5

Photomicrographs of ACh cells (ChAT- positive) expressing Fos in the HDB (A, B), MS (C, D), and VDB (E, F) of a control (A, C, E) and a NA grass rat (B, D, F) euthanized at ZT 16. Arrowheads indicate some double-labeled cells. Fos: blue nuclear staining. ChAT: brown cell body staining. Scale bar= 100 µm.

Figure 6

Photomicrographs of putative OXA fiber appositions (arrows) with ACh cells (arrowheads) in the HDB of a representative animal. Note that putative appositions are also observed in the vicinity of ACh cells. Scale bar= 30 µm.

Figure 7

Patterns of Fos expression in nTH cells of the aVTA (A, B) and SUM (C, D). Panels A and C show the distribution of Fos expression in nTH cells for each group at each sampled time. Neither the effect of ZT nor the interaction between ZT and group were statistically significant. Panels B and D show the overall distribution of Fos expression in nTH cells for each group. The effect of group was statistically significant (p< 0.05).

Figure 8

Photomicrographs of Fos and TH cells in the aVTA (A) and SUM (B). Double-labeled cells were scarce (arrow in B) or absent in both areas. In addition, note that TH cells were abundant in the aVTA. However, these cells were not positive for Fos, even though this protein was observed in the vicinity of those neurons (arrowheads). Fos: blue nuclear staining. TH: brown cell body staining. Scale bar= 100 µm.