RESEARCH ARTICLE Jonathan S. Carp Æ Ann M. Tennissen Xiang Yang Chen Æ Jonathan R. Wolpaw Diurnal H-reflex variation in mice Received: 15 April 2005 / Accepted: 17 June 2005 / Published online: 7 September 2005 Ó Springer-Verlag 2005 Abstract Mice exhibit diurnal variation in complex motor behaviors, but little is known about diurnal var- iation in simple spinally mediated functions. This study describes diurnal variation in the H-reflex (HR), a wholly spinal and largely monosynaptic reflex. Six mice were implanted with tibial nerve cuff electrodes and electrodes in the soleus and gastrocnemius muscles, for recording of ongoing and nerve-evoked electromyo- graphic activity (EMG). Stimulation and recording were under computer control 24 h/day. During a 10-day recording period, HR amplitude varied throughout the day, usually being larger in the dark than in the light. This diurnal HR variation could not be attributed solely to differences in the net ongoing level of descending and segmental excitation to the spinal cord or stimulus intensity. HRs were larger in the dark than in the light even after restricting the evoked responses to subsets of trials having similar ongoing EMG and M-responses. The diurnal variation in the HR was out of phase with that reported previously for rats, but was in phase with that observed in monkeys. These data, supported by those in other species, suggest that the supraspinal control of the excitability of the HR pathway varies throughout the day in a species-specific pattern. This variation should be taken into account in experimental and clinical studies of spinal reflexes recorded at differ- ent times of day. Keywords Electromyography Æ Implanted Æ Electrodes Æ Spinal cord Æ Monosynaptic Æ Reflex Æ Circadian rhythm Introduction Diurnal variation in motor behavior (defined as an ac- tion induced by muscle activation in response to a change in an animal’s internal or external environment) is a common feature among many species of animals (Turek 1985; Stupfel and Pavely 1990). They exhibit stereotypic variation in frequency and/or intensity of locomotor, feeding, grooming, and sexual activity in the course of the day. This variation is species-specific, and can even differ among strains of a given species. Diurnal variation also occurs during less complex motor behaviors. For example, the spinal stretch reflex and its electrical analog the H-reflex (HR), which are wholly spinal and mediated largely by monosynaptic pathways, are arguably the simplest motor behaviors of the mammalian nervous system. The size of these reflexes varies throughout the course of the day in monkeys and rats, although the phase of modulation differed between these two species (Wolpaw and Seegal 1982; Dowman and Wolpaw 1989; Chen and Wolpaw 1994). In rats at least, interruption of the corticospinal tract (CST) sub- stantially reduces this rhythm (Chen et al. 2002b). Operant conditioning of the HR has become a productive paradigm for the study of mammalian learn- ing and memory (Wolpaw 2001). HR conditioning is associated with plasticity at multiple sites within the brain and spinal cord, and it requires an intact CST. The use of mice in studies of learning and memory is growing, due in no small part to the current availability and continuing development of mutant and genetically modified animals (Vaillend et al. 2002; Morgan 2003). In order to take advantage of these experimental tools, our laboratory has recently developed methodology for performing long- term HR recordings in mice (Carp et al. in press). In this study, we present data that demonstrate the existence of diurnal variation in the HR in the mice that is not simply dependent on the expected within-day variations in the ongoing level of motoneuron pool activation. These re- sults provide important information on a potentially J. S. Carp (&) Æ A. M. Tennissen Æ X. Y. Chen Æ J. R. Wolpaw Laboratory of Nervous System Disorders, Wadsworth Center, New York State Department of Health and State University of New York, PO Box 509, Albany, NY 12201-0509, USA E-mail: carpj@wadsworth.org Tel.: + 518-486-4911 Fax: +518-486-4910 Exp Brain Res (2006) 168: 517–528 DOI 10.1007/s00221-005-0106-y