Daily changes in parameters of energy metabolism in brain of rainbow trout: Dependence on feeding Sergio Polakof, Rosa M. Ceinos, Begoña Fernández-Durán, Jesús M. Míguez, José L. Soengas ⁎ Laboratorio de Fisioloxía Animal, Departamento de Bioloxía Funcional e Ciencias da Saúde, Facultade de Bioloxía, Universidade de Vigo, E-36310 Vigo, Spain Received 25 May 2006; received in revised form 16 October 2006; accepted 16 October 2006 Available online 21 October 2006 Abstract We assessed the daily patterns of parameters involved in energy metabolism in plasma and brain of rainbow trout. Where daily rhythms were found, we analyzed the potential influence of feeding. Immature rainbow trout were randomly distributed in 3 groups: fish fed for 7 days, fish fasted for 7 days, and fish fasted for 7 days and refed for 4 days. On sampling day, fish of fed and refed groups were fed at 11.00 h, and all fish were sampled from each treatment group using the following time schedule: 14.00, 18.00, 21.00, 00.00, 04.00, 07.00, 10.00 and 14.00 h. The results obtained from metabolic parameters assessed in plasma and brain can be grouped into three different categories, such as (i) those displaying no 24 h changes in fed fish such as plasma lactate, protein or acetoacetate levels, as well as brain amino acid and protein levels, and lowKm (glucose) hexokinase, and aspartate aminotransferase activities, (ii) those displaying 24 h changes that were apparently dependent on feeding since they disappeared in fasted fish such as the case of plasma cortisol, glucose and triglyceride levels, as well as brain glycogen, glucose, and lactate levels, and pyruvate kinase and hexokinase IV activities, and (iii) those parameters displaying 24 h changes apparently not dependent on feeding such as plasma amino acids, brain acetoacetate levels as well as several enzyme activities measured in brain such as glucose 6-phosphate dehydrogenase, α-glycerophosphate dehydrogenase, glutamate dehydrogenase, and lactate dehydrogenase-oxidase. In general, 24 h changes dependent on feeding indicate an increased use of glucose in brain several hours post-feeding whereas those changes not dependent on feeding were characterized by reduced levels/activity at the night period suggesting a metabolic depression in brain during darkness. © 2006 Elsevier Inc. All rights reserved. Keywords: Rainbow trout; Energy metabolism; Brain; Daily changes; Food deprivation; Refeeding 1. Introduction Daily rhythms of activity in fish play an important role in the temporal organization and their adaptive value is to permit the adjustment of the life processes to the chronological arrange- ment of the external world (De Pedro et al., 1998). The light/ dark cycle is considered to be the predominant entrainer of the daily rhythms in fish although feeding time is also recognized as a major synchronizer (Boujard and Leatherland, 1992b). In mammals, plasma concentration of metabolic and hormonal products related to energy balance varies with a well-established dynamic, depending on whether the organism is well fed, fasted or starved (Escobar et al., 1998). Thus, fluctuations in plasma glucose concentration result from food intake, hepatic glucose production (from glycogen or gluconeogenesis) and/or changes in glucose tolerance. Fasting is the most frequently used approach to study whether a rhythm in glucose metabolism is independent of the (disturbing/masking) effect of feeding since those daily rhythms dependent on feeding should disappear in food deprived animals. In fish, the effects of fasting on daily changes of several Comparative Biochemistry and Physiology, Part A 146 (2007) 265 – 273 www.elsevier.com/locate/cbpa Abbreviations: Asp-AT, aspartate aminotransferase (EC. 2.6.1.1.); G3PDH, α-glycerophosphate dehydrogenase (EC. 1.1.1.8.); G6PDH, glucose 6-phos- phate dehydrogenase (EC. 1.1.1.49.); GDH, glutamate dehydrogenase (EC. 1.4.1.2.); HK-I, hexokinase I (EC. 2.7.1.1.); HK-IV, hexokinase IV (EC. 2.7.1.2.); LDH-O, lactate dehydrogenase-oxidase (EC. 1.1.1.27.); PK, pyruvate kinase (EC. 2.7.1.40.). ⁎ Corresponding author. Laboratorio de Fisioloxía Animal, Facultade de Bioloxía, Edificio de Ciencias Experimentais, Universidade de Vigo, E-36310 Vigo, Spain. Tel.: +34 986 812 564; fax: +34 986 812 556. E-mail address: jsoengas@uvigo.es (J.L. Soengas). 1095-6433/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2006.10.026