RHYTHMS NEUROREPORT 0959-4965 & Lippincott Williams & Wilkins Vol 11 No 14 28 September 2000 3261 Lithium lengthens the circadian period of individual suprachiasmatic nucleus neurons Michikazu Abe, Erik D. Herzog 1 and Gene D. Block CA NSF Center for Biological Timing, Department of Biology, Gilmer Hall, University of Virginia, Charlottesville, VA 22903, USA 1 Present address: Department of Biology, Washington University, St. Louis, MO 63130, USA CA Corresponding Author Received 4 July 2000; accepted 28 July 2000 Lithium treatment lengthens the period of circadian rhythms in most organisms. In the present study, we tested whether lithium acts directly on the mammalian suprachiasmatic nucleus (SCN) to lengthen rhythms of individual neurons. Lithium increased the circadian period of firing rate rhythms of cultured SCN neurons in a concentration-dependent manner. Lithium had no effect on the amplitude of these rhythms, but did affect the period of some cells more than others. The results indicate that lithium acts directly on the SCN to lengthen the free-running period of individual neurons. Neuro- Report 11:3261–3264 & 2000 Lippincott Williams & Wilkins. Key words: Depression; Diurnal; Hypothalamus; Multi-electrode; Pacemaker; Rhythm; SCN INTRODUCTION Some affective disorders may be related to disturbances of the circadian timing system. Depressed patients often experience diurnal variations in mood [1], and have altered circadian rhythms such as a early awakening, shortened latency to attain rapid eye movement sleep, advanced phase of hormonal rhythms (reviewed in [2]), and delayed phase of minimum core body temperature [3]. One hypoth- esis holds that abnormal phase relationships between circadian oscillators could produce manic-depression and other affective disorders. Although supporting evidence is limited, therapies that act on the circadian system appear to modulate mood disorders and vice versa [4,5]. For example, treatments that modify a patient’s sleep cycle or light–dark cycle can dramatically improve mood [6,7]. Perhaps the most compelling example is that lithium delays the advanced phase of sleep, melatonin and tem- perature rhythms in manic–depressive patients and im- proves their mood [2]. Lithium remains the drug of choice in cyclic bipolar affective disorder and several other periodic disorders [2,8]. One intriguing possibility is that lithium exerts its therapeutic effects directly through a circadian pacemaker in the brain. The suprachiasmatic nucleus (SCN) of the hypothalamus is the circadian pacemaker in mammals that is required for the control of daily rhythms in locomotor behavior, hormone production and body temperature [9]. In vitro, individual SCN neurons express circadian rhythms in spontaneous firing rate when cultured in dispersals [10– 12] or within the organized tissue [13]. Studies implicating the SCN in affective disorders and their treatment are virtually non-existent; however, Kleine–Levin syndrome patients, who have abnormal sleep–wake patterns, have been found to have damage in the area of the hypothala- mus that includes the SCN [14]. Lithium may act on the SCN since it lengthens the free-running circadian period of locomotor activity of hamsters which have received a SCN transplant to replace their lesioned SCN [15]. The objective of this study was to directly test whether lithium acts to increase the period of any or all SCN neurons. MATERIALS AND METHODS Neurons from the SCN were harvested from mouse pups (BALB/c 3 C57BL/6J hybrid; 3–5 days postnatal). Mice remained with their mothers on a 12:12 h light:dark sche- dule until the time of surgery. All procedures were approved by the University of Virginia’s Animal Care and Use Committee and conformed to NIH guidelines. Dispersed cultures of SCN cells were made and main- tained as described previously [10]. Cultures were grown on treated (5 ìg/cm 2 poly-D-lysine and 1.7 ìg/cm 2 laminin; Collaborative Research) multimicroelectrode plates (MMEPs) purchased from Guenter Gross (University of North Texas). After 1 week cultures were treated overnight with 100 mM Ara-C (Sigma), and after 3 weeks were transferred from the incubator to a recording station. Extracellular electrical activity was recorded from indivi- dual SCN neurons on MMEPs as described previously [10]. Signals from up to six electrodes were simultaneously monitored. Using off-line analysis similar to published methods [13], we assigned action potentials of similar amplitude and duration to individual cells and counted impulse frequency. The shape of the impulse and presence of a clear refractory period following the impulse were used as criteria for single-unit activity. We exchanged the full volume of recording medium with medium containing