RESUMEN
Ambient light alters the level of the transcriptional regulatory protein c-Fos in the suprachiasmatic nucleus, the site of an endogenous circadian clock in mammals, and in one other retino-recipient area, the intergeniculate leaflet of the lateral geniculate complex. Complementing previous work by ourselves and others on the photic and temporal regulation of c-Fos expression in the suprachiasmatic nucleus, the present studies investigated c-Fos regulation in the rat intergeniculate leaflet, revealing some important differences between the two brain regions. In the intergeniculate leaflet, the levels of c-fos mRNA (by in situ hybridization) and immunoreactive c-Fos protein (by immunohistochemistry) were elevated by light pulses administered either during the subjective day or subjective night. The regulation of immunoreactive Fos-B protein was similar to c-Fos, and 98% of Fos-B-expressing cells were also c-Fos-positive (by double-label immunofluorescence). By combining c-Fos immunofluorescence with stereotaxic injections of the retrograde tract tracer FluoroGold, we found photically-induced c-Fos in 15% of intergeniculate leaflet neurons projecting to the suprachiasmatic nucleus and in 34% of those projecting to the contralateral intergeniculate leaflet. Intergeniculate leaflet cells that express c-Fos after photic stimulation appear to represent a functionally-defined population that does not correspond to anatomically-defined categories based on connectivity or peptidergic phenotype.
Asunto(s)
Ritmo Circadiano/fisiología , Proteínas Proto-Oncogénicas c-fos/metabolismo , Núcleo Supraquiasmático/metabolismo , Percepción Visual/fisiología , Animales , Inmunohistoquímica , Hibridación in Situ , Masculino , Estimulación Luminosa , Ratas , Ratas Sprague-DawleyRESUMEN
Several authors have suggested that the transcriptional regulatory protein c-Fos might be part of the mechanism for photic entrainment of the circadian pacemaker in the suprachiasmatic nucleus (SCN) to environmental light:dark cycles. This hypothesis has been based on evidence gathered using single light pulses administered acutely to animals free-running in constant darkness. In order to begin to analyze SCN c-fos gene expression in animals during steady-state entrainment to photic cycles, we exposed male BALB/c mice to a skeleton photoperiod consisting of two 1-h light pulses separating a long (14 h) and a short (8 h) dark interval. The cycle was designed so that stable entrainment could be achieved in either one of two patterns (with rhythmic locomotor activity occurring during either the long or the short dark interval); SCN c-fos mRNA levels could then be measured during entrainment to light pulses at different phases of the circadian cycle, while controlling for the duration of preceding darkness. We found that c-fos was induced equally well by a light pulse that represented ZT 12 or ZT 3. The ZT 12 pulse functioned as an entraining pulse, because animals free-ran after it was removed from the lighting regimen, whereas removing the ZT 3 pulse caused little or no phase shift of activity onset. The data confirm that the expression of SCN c-Fos is not itself sufficient to reset rhythm phase, and they indicate that the role of this gene in the mechanism of photic entrainment is not yet fully understood.
Asunto(s)
Ritmo Circadiano/fisiología , Regulación de la Expresión Génica/fisiología , Genes fos/genética , Fotoperiodo , Proteínas Proto-Oncogénicas c-fos/biosíntesis , Núcleo Supraquiasmático/metabolismo , Animales , Autorradiografía , Encéfalo/anatomía & histología , Química Encefálica , Sondas de ADN , Hibridación in Situ , Masculino , Ratones , Ratones Endogámicos BALB C , Actividad Motora , Estimulación Luminosa , ARN Mensajero/biosíntesisRESUMEN
We measured immunoreactive Fos B protein levels in the ventrolateral subdivision of the rat suprachiasmatic nucleus (SCN) as a function of light and time of day. Immunohistochemistry revealed high levels of Fos B that were uniformly expressed throughout the 12 h:12 h light-dark cycle. Levels remained high in constant darkness and were modestly increased (about 2-fold) after a 2 h light pulse administered during the subjective night, but not after a light pulse during the subjective day. Fos B and c-Fos immunoreactivities could be colocalized within individual SCN cell nuclei using a double-label immunofluorescence method. Thus, despite their structural similarities, these two members of the fos gene family exhibit different patterns of expression in the rat SCN. These and previous data suggest that Fos/Jun DNA-binding complexes in the SCN are composed of constant, as well as variable, protein components; in at least some SCN cells, light-induced changes in the composition of these constituent proteins may lead to altered transcription of target genes.
Asunto(s)
Ritmo Circadiano/fisiología , Proteínas Proto-Oncogénicas c-fos/genética , Núcleo Supraquiasmático/fisiología , Animales , Encéfalo/fisiología , Expresión Génica/genética , Inmunoquímica , Masculino , Ratas , Ratas Sprague-Dawley , Factor de Transcripción AP-1RESUMEN
Vasoactive intestinal peptide (VIP) has been localized within the suprachiasmatic nucleus of the hypothalamus (SCN) and appears to play an important role in the entrainment of circadian rhythms with the light-dark (LD) cycle. The spontaneously hypertensive rat (SHR), an inbred strain used extensively in research on primary hypertension, has significantly more VIP mRNA in its brain than normotensive Wistar-Kyoto control (WKY) rats. Because VIP levels are abnormally high in SHR rats the present study examined whether the mechanisms controlling circadian rhythms are also altered in SHR rats. When entrained to a 24 h LD cycle, SHR rats began their wheel-running rhythm approximately 1.5 h earlier than WKY controls. SHR rats re-entrained to a phase delay in the LD cycle more slowly than did WKY rats, but tended to re-entrain to a phase advance more rapidly. The free-running period of SHR rats in both constant light and constant dark was significantly shorter than that of WKY rats. In SHR rats, phase delays produced by 1-h pulses of light were less than one-half the magnitude of the delays seen in WKY rats; however, the phase advances were nearly twice that of WKY rats. Using in situ hybridization, the SCN levels of mRNA encoding VIP were found to be significantly greater in SHR rats, but the mRNA levels of another peptide important for entrainment, gastrin releasing peptide, did not differ between SHR and WKY rats. These data indicate that the mechanisms controlling circadian rhythms in SHR rats differ significantly from those controlling rhythms in WKY rats and that VIP mRNA is significantly elevated within the SCN of SHR rats. The role of VIP in the entrainment of circadian rhythms is discussed.