ELSEVIER Neuroscience Letters206 (1996) 97-100 NHllOSCHCI LHTEIIS Nerve growth factor phase shifts circadian activity rhythms in Syrian hamsters Keshavan G, Bina a, Benjamin Rusaka,b, * aDepartmentof Psychology, Dalhousie University, Halifax, Nova Scotia, B3H 4Jl, Canada bDepartment of Pharmacology, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada Received29 December1995;revisedversionreceived 2 February1996;accepted2 February1996 Abstract Nerve growth factor (NGF) receptors are found in high density in the rodent suprachiasmatic nucleus (SCN), a site which regulates mammalian circadian rh)~hms. We examined the effects of NGF (40 ng) or vehicle injections into the SCN, at circadian times (CT) 6, 14 or 22 on activity rhythms in hamsters maintained in constant darkness. NGF caused phase advances at CT6 (30.9 rain) and CT22 (36.9 min), and phase delays at CT14 (31.2 rain). Saline and cytochrome-c administration had no phase-shifting effects at CT6 and CT22, but at CT14 cytochrome-c produced large phase delays, implying that NGF-induced delays at this phase may be non-specific. Similarities between NGF-induced shifts and those elicited by the cholinergic agonist carbachol suggest a common mode of action. Keywords: Suprachiasmatic; Entrainment; Nerve growth factor; Phase response curve; Cholinergic; Acetylcholine The mammalian suprachiasmatic nucleus (SCN) is responsible for the generation of circadian rhythms and their synchronization (entrainment) to lighting cycles [9]. Acetylcholine (ACh) has been proposed to be involved in entrainment because iniections of carbachol, a cholinergic agonist, intraventricularly or into the SCN, phase shift activity rhythms in rodents, somewhat as light does [19]. These effects of carbachol may involve presynaptic modulation of the release of an excitatory amino acid [6]. Receptors for nerve growth factor (NGF; NGF-R) are synthesized in choline, rgic neurons and bind NGF re- leased by their postsynaptic target neurons, forming a NGF/NGF-R complex that exerts a biological effect criti- cal to the survival and maintenance of these neurons [24]. The SCN contains high levels of NGF-R immunoreactiv- ity [22], at least some of which is located on terminals of basal forebrain cholinergic neurons and retinal ganglion cells projecting to the SCN [3]. In addition, neurons in the SCN have been shown to contain both NGF [2] and its precursor [21], We therefore examined the effects of NGF applications into the SCN on circadian rhythms. A pre- liminary report of this study has been presented [2]. * Correspondingauthor. Tell.: +1 902 4942159; fax: +1 902 4942159; e-mail: rusak@ac.dal.ca. Fifty adult male hamsters (Mesocricetus auratus; LVG:lak, 80-100 g; Charles River, St. Constant, Qu6bec) were implanted under pentobarbital anesthesia (80 mg/kg) with stainless steel guide cannulas (22 gauge; Plastics One, Roanoke, VA) directed stereotaxically at the SCN (0.5 mm anterior to bregma, 0.3 mm lateral to midline, and 8.4 mm ventral to the skull surface, with the incisor bar 2 mm below the interaural line). After recovery from anesthesia, animals were housed individually in cages equipped with activity wheels and maintained under con- stant dim red light (DD). Wheel-running activity was monitored continuously on an event recorder (Esterline- Angus, Indianapolis) and activity counts were recorded every 10 min using an Apple computer. Event recorder charts were used to measure activity rhythm periods and phase shifts, while the actograms presented in the figures were plotted from computer files. After a stable free-running rhythm was established, hamsters were injected intracranially with 2/tl of one of the following: 0.9% saline, a saline solution of NGF (20 ng//~l saline; Sigma Chemical Co.), or a solution of cytochrome-c (CYT; 300 ng//tl; Sigma Chemical Co.), which is physicochemically similar to NGF, but lacks its biological activity [1]. Each injection was made over 2- 3 min using a Hamilton syringe connected via plastic 0304-3940/96/$12.00 © 1996ElsevierScienceIrelandLtd. All rights reserved PII: S0304-3940(96) 1243 2-9