Chronobiology International, Early Online: 1–14, (2015) ! Informa Healthcare USA, Inc. ISSN: 0742-0528 print / 1525-6073 online DOI: 10.3109/07420528.2015.1057640 ORIGINAL ARTICLE Rats with minimal hepatic encephalopathy show reduced cGMP-dependent protein kinase activity in hypothalamus correlating with circadian rhythms alterations Vicente Felipo 1 , Blanca Piedrafita 2 , Juan A. Barios 3,4 , Ana Agustı ´ 5 , Hanan Ahabrach 6 , Marı ´a Romero-Vives 3,4 , Luis C. Barrio 4 , Beatriz Rey 7 , Jose M. Gaztelu 3,4 , and Marta Llansola 1 1 Laboratory of Neurobiology, Centro de Investigacio ´n Prı ´ncipe Felipe, Eduardo Primo Yu ´fera, Valencia, Spain, 2 Medical Statistics Consulting C/Almazora, Valencia, Spain, 3 Unidad de Neurologı ´a Experimental, Instituto de Investigacio ´n «Ramo ´n y Cajal» (IRYCIS) Hospital Universitario Ramo ´n y Cajal, Madrid, Spain, 4 Centro de Tecnologı ´a Biome ´dica, Madrid, Spain, 5 Instituto de Investigacio ´n Sanitaria INCLIVA, Av/Blasco Iban ˜ez, Valencia, Spain, 6 Departament de Biologie, Faculte des Sciences, Universite ´ Abdelmalek Essaadi, Te ´touan, Morocco, and 7 Inter-University Research Institute for Bioengineering and Human Centered Technology (UPV), Camino de Vera s/n, Valencia, Spain Patients with liver cirrhosis show disturbances in sleep and in its circadian rhythms which are an early sign of minimal hepatic encephalopathy (MHE). The mechanisms of these disturbances are poorly understood. Rats with porta-caval shunt (PCS), a model of MHE, show sleep disturbances reproducing those of cirrhotic patients. The aims of this work were to characterize the alterations in circadian rhythms in PCS rats and analyze the underlying mechanisms. To reach these aims, we analyzed in control and PCS rats: (a) daily rhythms of spontaneous and rewarding activity and of temperature, (b) timing of the onset of activity following turning-off the light, (c) synchronization to light after a phase advance and (d) the molecular mechanisms contributing to these alterations in circadian rhythms. PCS rats show altered circadian rhythms of spontaneous and rewarding activities (wheel running). PCS rats show more rest bouts during the active phase, more errors in the onset of motor activity and need less time to re-synchronize after a phase advance than control rats. Circadian rhythm of body temperature is also slightly altered in PCS rats. The internal period length (tau) of circadian rhythm of motor activity is longer in PCS rats. We analyzed some mechanisms by which hypothalamus modulate circadian rhythms. PCS rats show increased content of cGMP in hypothalamus while the activity of cGMP-dependent protein kinase was reduced by 41% compared to control rats. Altered cGMP-PKG pathway in hypothalamus would contribute to altered circadian rhythms and synchronization to light. Abbreviations: HE: hepatic encephalopathy; MHE: minimal HE; PKG: cGMP-dependent protein kinase; PCS: porta-caval shunt; HPA: hypothalamus–pituitary–adrenal; CRH: corticotropic releasing hormone; NO: nitric oxide Keywords: Cyclic GMP, electroencephalogram, hypothalamus, light synchronization, porta-caval anastomosis, sleep–wake cycle INTRODUCTION Hepatic encephalopathy (HE) is a complex neuropsychi- atric syndrome present in patients with liver diseases, which may lead to a wide range of neurological and psychiatric disturbances. An early alteration in patients with liver cirrhosis is the impairment of sleep and of its circadian rhythm: patients cannot sleep well during the night and show sleepiness in the morning (Co ´rdoba et al., 1998). Sleep disturbances similar to those found in patients with liver cirrhosis have been reported in animal models of HE. Rats with porta-caval shunts (PCS), a model of minimal HE recommended by the International Society for Hepatic Encephalopathy (Butterworth et al., 2009) show sleep fragmentation and reduced total sleep time, indicating that it is an adequate animal model to study the mechanism involved in sleep alterations in liver cirrhosis (Llansola et al., 2012). The mechanisms by which liver failure leads to this altered circadian rhythm of sleep have not been clarified. Sleep-wake abnormalities do not seem to be related with hepatic encephalopathy (Montagnese et al., 2009), Correspondence: Marta Llansola, Laboratory of Neurobiology, Centro de Investigacio ´n Prı ´ncipe Felipe, Eduardo Primo Yu ´ fera, 3 46012 Valencia, Spain. E-mail: mllansola@cipf.es Color versions of one or more figures in the article can be found online at the publisher’s website. Submitted February 25, 2015, Returned for revision May 19, 2015, Accepted May 29, 2015 1