Downloaded By: [Universidad Granada] At: 18:42 9 May 2007 Day/night variations in membrane bound leucyl-2-naphthylamide hydrolysing activity in rat anterior hypothalamus, pituitary and retina GERMA ´ N DOMI ´ NGUEZ, MANUEL RAMI ´ REZ, & GARBIN ˜ E ARECHAGA Departamento Ciencias de la Salud-A ´ rea de Fisiologı ´a, Facultad de Ciencias Experimentales, Campus Las agunillas s/n, E-23071-Jae ´n, Spain Abstract It is well known that the suprachiasmatic nucleus of the anterior hypothalamus acts as pacemaker regulating circadian rhythms in mammals. The daily variations of neuropeptides as well as their receptors depend on this system (Moore 1983). Aminopeptidases play an important role in regulating the activity of brain peptides (Bauer 1982). In this study we investigated membrane bound leucyl-2- naphthylamide hydrolysing activity in the anterior hypothalamus, pituitary and retina of adult male rats at six time points of a 12:12h light:dark schedule (light from 7:00 to 19:00 h), in order to analyse its day/ night variation. The fluorometric assay evidenced significant differences between the three regions: in the anterior hypothalamus being higher during the dark period compared with the light period and in the pituitary higher during the light period compared with the dark period. In the retina the levels of this activity showed a higher heterogeneity during the day. Day – night differences in membrane bound leucyl-2-naphthylamide hydrolysing activity may reflect differences in its susceptible endogenous substrates. Keywords: Leucyl aminopeptidase, arylamidase activity, circadian rhythm, neuropeptides, photoneuroendocrine circuit Introduction In the course of biological evolution on our planet, living organisms have adapted to the daily rotation of the earth on its axis. One such adaptation consists of the acquisition of endogenous clocks that can be synchronised to the daily and seasonal changes in external time cues, light being one of the most important. While in other non-mammalian vertebrates circadian control is regulated mainly by the retina and pineal gland, in mammals the suprachiasmatic nucleus of the anterior hypothalamus (SCN) is considered to be the master circadian pacemaker. This nucleus is integrated in a photoneuroendocrine circuit that includes the retina and other selected regions of the neuroendocrine system. Firstly, nerve impulses are generated in the retinal ganglion cells through a novel opsin-like protein which has been recently proposed as a likely candidate for the photopigment in these cells (Brown & Robinson 2004). Subsequently, they reach the suprachiasmatic nuclei of the anterior Correspondence: Garbin ˜ e Arechaga, Departamento Ciencias de la Salud, Facultad de Ciencias Experimentales, Universidad de Jae ´n. Campus Las Lagunillas s/n 23071-Jae ´n, Spain. Tel: þ34 953212009. Fax: þ34 953212943. E-mail: arechaga@ujaen.es Biological Rhythm Research April 2007; 38(2): 127 – 132 ISSN 0929-1016 print/ISSN 1744-4179 online Ó 2007 Taylor & Francis DOI: 10.1080/09291010600903783