Circadian rhythm of AMPA receptor GluR2/3 subunit-immunoreactivity in the suprachiasmatic nuclei of Syrian hamster and effect of a light-dark cycle.

In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus are the site of the circadian clock that generates and coordinates many endogenous physiological and behavioral rhythms. SCN are normally entrained to light/dark (LD) cycle by direct retinal afferents using glutamate as neurotransmitter. N-Methyl-d-aspartate (NMDA) and non-NMDA receptors are involved in photic entrainment of SCN. In rodents, the presence of three of the four known 2-amino-3-(3-hydroxy-(-methylisoaxol-4-yl) propanoic acid (AMPA) receptor subunits has been demonstrated by in situ hybridization. This study analyzes the expression of GluR2/3 subunits in SCN of Syrian hamsters maintained under constant darkness (DD) or 12:12 LD cycle. In animals submitted to DD or LD, small immunoreactive neurons were located in the ventral and external latero-ventral parts of the rostral two-thirds of the SCN and along the symmetrical plane. The number of intensely labeled neurons with or without long process(es) were counted at six circadian times (CTs) in three groups of animals maintained in DD and six nycthemeral (zeitgeber time, ZT) times in one group of hamsters submitted to LD. In DD, we observed significantly more GluR2/3 subunit-immunoreactive (GluR2/3-ir) neurons during the subjective day than during the subjective night, with minima at CT 19-CT 23. The LD cycle significantly reduced the number of immunoreactive neurons, lessened the differences between LD phases and depressed immunoreactivity at light transition, i.e., at ZT 11 and ZT 23. This study demonstrates for the first time by immunohistochemistry the existence of a circadian dynamic of the expression of AMPA receptor subunits in SCN of rodents and the effect of the LD cycle on this dynamic.