Cholinergic projections to the suprachiasmatic nucleus and lower subparaventricular zone of diurnal and nocturnal rodents

In nocturnal species cholinergic agonists alter circadian rhythm phase when injected intraventricularly or directly into the suprachiasmatic nucleus (SCN), but the phase shifts obtained differ depending upon the site being injected. Cholinergic projections reach the SCN of nocturnal laboratory rats, however, nothing is known about these projections in diurnal rodents. The first objective of this study was to evaluate the hypothesis that cholinergic projections to the SCN are only present in nocturnal species. The second objective was to evaluate the hypothesis that the lower part of the subparaventricular zone (LSPV) is a candidate for being a site that mediates the phase shifts observed when cholinergic agonists are injected intraventricularly. These hypotheses were tested in the diurnal unstriped Nile grass rat (Arvicanthis niloticus) and the nocturnal laboratory rat. Additionally, we evaluated if the light-dark (LD) cycle had an effect on the expression of the vesicular acetylcholine transporter (VAChT) in the SCN, LSPV, and in two control areas. Animals were kept in a 12:12 LD cycle and perfused at six time points. VAChT immunoreactivity was observed in the SCN, LSPV, and in the control areas of both species. The SCN and LSPV showed a differential distribution and density of cholinergic projections between the two species, but similar temporal patterns of VAChT expression were found across species. These results provide evidence for a differential cholinergic stimulation of the SCN between grass rats and laboratory rats that may reflect a rewiring of neural projections brought about by the adoption of a diurnal activity profile.