Light suppresses fos expression in the shell region of the suprachiasmaticnucleus at dusk and dawn: Implications for photic entrainment of circadianrhythms

The transcription factor Fos is implicated in neuronal signaling in the sup rachiasmatic nucleus, the mammalian circadian clock (Ikonomov and Stoynev, 1994; Klein et al., 1991; Kornhauser et al., 1996). Fos is expressed in two different regions within the suprachiasmatic nucleus. In the ventrolateral , retinorecipient, core region (Leak et al., 1999; Moga and Moore, 1997), F os is induced by light and expression is closely linked, both temporally an d functionally, to clock resetting and entrainment of circadian rhythms (Ar onin et al., 1990; Beaule and Amir, 1999; Hastings et al., 1995; Kornhauser et al., 1996; Kornhauser et al., 1990; Rea, 1989, 1998; Rusak et al., 1990 ; Wollnik et al., 1995). In the dorsomedial shell region (Leak et al., 1999 ), Fos expression is rhythmic (Guido et al., 1999a,b; Rusak et al., 1992; S umova and Illnerova, 1998; Sumova et al., 1998). Expression is high during the subjective day when photic sensitivity of the core is minimal, and low in the subjective night, when photic sensitivity of the core is maximal. Al though it has been shown that the pattern of Fos expression in the shell tr acks the photoperiod (Sumova et al., 2000), nothing is known about whether light influences the expression of Fos in the shell region or about the rol e of Fos expression in the shell in clock resetting and entrainment. In the present study we found that, in rats maintained in constant darkness, brie f exposure to light in the early subjective day or night induced Fos in the core, as expected, and acutely suppressed the levels of Fos immunoreactivi ty in the shell region. Similar changes in Fos expression in the core and s hell regions were seen after exposure to a brief entraining light. Light ex posure in the mid-subjective day or night differentially affected Fos expre ssion in the core, as previously shown, but had no effect on Fos expression in the shell region. The finding that Fos expression in the shell region o f the suprachiasmatic nucleus is suppressed by light at dawn and dusk sugge sts a critical role for the shell in photic entrainment of circadian rhythm s in nocturnal rodents. (C) 2001 IBRO. Published by Elsevier Science Ltd. A ll rights reserved.