Expression of Tgfα in the suprachiasmatic nuclei of nocturnal and diurnal rodents

Transforming growth factor alpha (TGFα) in the suprachiasmatic nuclei (SCN) has been proposed as an inhibitory signal involved in the control of daily locomotor activity. This assumption is based mainly on studies performed in nocturnal hamsters. To test whether the transcriptional regulation of Tgfα can be correlated with the timing of overt activity in other species, we compared Tgfα expression in the SCN of nocturnal Swiss mice and of diurnal Arvicanthis housed under a light/dark cycle (LD) or transferred to constant darkness (DD). In agreement with data on hamsters, Tgfα mRNA levels in the mouse SCN showed peak and trough levels around (subjective) dawn and dusk, respectively, roughly corresponding to the period of rest and activity in this species. In contrast, in Arvicanthis housed in DD, the circadian rhythm of SCN Tgfα was similar to that of the mice in spite of opposite phasing of locomotor activity. Furthermore, in Arvicanthis exposed to LD, Tgfα mRNA levels were constitutively high throughout the day. A tonic role of light in the regulation of Tgfα in Arvicanthis was confirmed by an increased expression of Tgfα in response to a 6-h exposure to light during daytime in animals otherwise kept in DD. In conclusion, this study shows that, contrary to what is observed in mice, Tgfα mRNA levels in the SCN of Arvicanthis do not match timing of locomotor activity and are modulated by light.