Calbindin and Fos within the suprachiasmatic nucleus and the adjacent hypothalamus of Arvicanthis niloticus and Rattus norvegicus

The suprachiasmatic nucleus is the site of the primary circadian pacemaker in mammals. The lower sub paraventricular zone that is dorsal to and receiv es input from the suprachiasmatic nucleus may also play a role in the regul ation of circadian rhythms. Calbindin has been described in the suprachiasm atic nucleus of some mammals, and may be important in the control of endoge nous rhythms. In the first study we characterized calbindin-expressing cell s in the suprachiasmatic nucleus and lower subparaventricular zone of noctu rnal and diurnal rodents. Specifically, Rattus norvegicus was compared to A rvicanthis niloticus, a primarily diurnal species within which some individ uals exhibit nocturnal patterns of wheel running. Calbindin-immunoreactive cells were present in the suprachiasmatic nucleus of Arvicanthis and were m ost concentrated within its central region but were relatively sparse in th e suprachiasmatic nucleus of Rattus. Calbindin-expressing cells were presen t in the lower sub-paraventricular zone of both species. In the second stud y we evaluated Fos expression within calbindin-immunoreactive cells in noct urnal Rattus and in Arvicanthis that were either diurnal or nocturnal with respect to wheel-running. All animals were kept on a 12:12 light/dark cycle and perfused at either 4 h after lights-on or 4 h after lights-off. In the suprachiasmatic nucleus in both species, Fos expression was elevated durin g the day relative to the night but less than 1% of calbindin cells contain ed Fos in Arvicanthis, compared with 13-17% in Rattus. In the lower sub-par aventricular zone of both species, 9-14% of calbindin cells expressed Fos, and this proportion did not change as a function of time. Among Arvicanthis , the number of calbindin expressings neurons in the lower sub-paraventricu lar zone was influenced by an interaction between the wheel running pattern s (nocturnal vs diurnal) and time of day. Thus, the number of calbindin-pos itive cells within the suprachiasmatic nucleus differed in Arvicanthis and Rattus, whereas the number of calbindin-positive cells within the lower sub -paraventricular zone differed in nocturnal and diurnal Arvicanthis. Our examination of R. norvegicus and A. niloticus suggests potentially impo rtant relationships between calbindin-containing neurons and whether animal s are nocturnal or diurnal, Specifically, rats had more Fos expression in c albindin containing cells in the suprachiasmatic nucleus than Arvicanthis. In contrast, Arvicanthis exhibiting diurnal and nocturnal patterns of wheel -running differed in the number of calbindin-containing cells in the lower sub-paraventricular zone, dorsal to the suprachiasmatic nucleus. (C) 2000 I BRO. published by Elsevier Science Ltd. All rights reserved.