Letter to Neuroscience Letter to Neuroscience LIGHT SUPPRESSES FOS EXPRESSION IN THE SHELL REGION OF THE SUPRACHIASMATIC NUCLEUS AT DUSK AND DAWN: IMPLICATIONS FOR PHOTIC ENTRAINMENT OF CIRCADIAN RHYTHMS C. BEAULE è , A. ARVANITOGIANNIS and S. AMIR* Center for Studies in Behavioral Neurobiology, Department of Psychology, Concordia University, 1455 de Maisonneuve West, Montreal, QC, Canada H3G 1M8 Key words: circadian clock, Fos immunocytochemistry, phase shift, rat. The transcription factor Fos is implicated in neuronal sig- naling in the suprachiasmatic nucleus, the mammalian cir- cadian clock (Ikonomov and Stoynev, 1994; Klein et al., 1991; Kornhauser et al., 1996). Fos is expressed in two di¡erent regions within the suprachiasmatic nucleus. In the ventrolateral, retinorecipient, core region (Leak et al., 1999; Moga and Moore, 1997), Fos is induced by light and expression is closely linked, both temporally and func- tionally, to clock resetting and entrainment of circadian rhythms (Aronin 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; Sumova 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. Although it has been shown that the pattern of Fos expression in the shell tracks the photoperiod (Sumova et al., 2000), nothing is known about whether light in£u- ences the expression of Fos in the shell region or about the role of Fos expression in the shell in clock resetting and entrainment. In the present study we found that, in rats maintained in constant darkness, brief exposure to light in the early subjective day or night induced Fos in the core, as expected, and acutely suppressed the levels of Fos immu- noreactivity in the shell region. Similar changes in Fos expression in the core and shell regions were seen after exposure to a brief entraining light. Light exposure in the mid-subjective day or night di¡erentially a¡ected Fos expression in the core, as previously shown, but had no e¡ect on Fos expression in the shell region. The ¢nding that Fos expression in the shell region of the suprachias- matic nucleus is suppressed by light at dawn and dusk suggests a critical role for the shell in photic entrainment of circadian rhythms in nocturnal rodents. ß 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved. The e¡ect of light on the expression of Fos immunore- activity in the shell and core regions of the suprachias- matic nucleus (SCN) was assessed at four distinct phases of the 24-h cycle: dawn, dusk, which are the only phases implicated in photic entrainment, and the middle of the day or night. Groups of rats were housed under a 12h:12-h light:dark (LD) cycle for 15 days and were then placed in complete darkness for 36^48 h. Fos expression was assessed in 8 groups of rats (n = 6/group) killed at projected circadian times (CTs) corre- sponding to dawn (CT2; CT0 denotes to the time of onset of the light phase of the preceding LD cycle), day (CT8), dusk (CT14) and night (CT20) with or with- out exposure to 30 min of light (250 lux at cage level) at CT1, 7, 13 and 19. Representative images of Fos expression in the SCN as a function of CT in rats killed either in complete dark- ness or after exposure to a 30-min light pulse are shown in Fig. 1. In rats killed in complete darkness, Fos expres- sion in the shell region varied as a function of time with signi¢cantly more labeled nuclei seen at CT2, 8 and 14 than at CT20 (Fig. 2a). Exposure to 30 min of light suppressed Fos expression in the shell at CT2 and CT14, but light had no e¡ect on Fos expression in the shell at CT 8 and 20. Two-way analysis of variance (ANOVA) revealed a signi¢cant main e¡ect of CT (F 3;40 = 1.1, P 6 0.0001), a signi¢cant main e¡ect of light (F 1;40 = 34.8, P 6 0.0001), and a signi¢cant time^ light interaction (F 3;40 = 22.8, P 6 0.0001). In the core, Fos expression was consistently low in rats killed in com- plete darkness, with less than 25 Fos immunoreactive 249 *Corresponding author. Tel. : +1-514-848-2188 ; fax : +1-514-848- 2817. E-mail address : amir@csbn.concordia.ca (S. Amir). Abbreviations : ANOVA, analysis of variance ; CT, circadian time ; LD, light:dark; SCN, suprachiasmatic nucleus; TBS, Tris-bu¡- ered saline; ZT, zeitgeber time. NSC 5154 10-9-01 www.elsevier.com/locate/neuroscience Neuroscience Vol. 106, No. 2, pp. 249^254, 2001 ß 2001 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved PII:S0306-4522(01)00313-X 0306-4522 / 01 $20.00+0.00