ELSEVIER 0031-9384(95)02019-R Physiology & Behavior, Vol. 59, No. 1, pp. 189-194, 1996 Copyright © 1995 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/96 $15.00 + .00 Auditory-Evoked Brainstem Responses in the Torpid Deermouse BHARTI KATBAMNA, .1 CHRYSSOULA THODI* AND JEROME B. SENTURIA~ *Department of Speech and Hearing, Cleveland State University, 1983 East 24th Street, Cleveland, OH 44115 USA ?Department of Speech Pathology and Audiology, Western Michican University, Kalamazoo, MI 49008-3825 USA, and ¢:Department of Biology, Cleveland State University, Cleveland, OH 44115 USA Received 1 June 1993 KATBAMNA, B., C. THODI AND J. B. SENTURIA. Auditory-evoked brainstem responses in the torpid deermouse. PHYSIOL BEHAV $9(1) 189-194, 1996.--This study examined auditory-evoked brainstem responses (ABR) in the deermouse (Peromyscus maniculatus) during torpor and arousal. The ABR of the euthermic deermouse consisted of five waves occurring in a time frame of 10 ms. During torpor, ABR wave I could be elicited at slow, but not fast stimulation rates indicating variability in neural activity along the auditory pathway. Arousal was heralded by the appearance of all the components of the ABR evoked in response to both slow and fast click rates signaling functional restoration of auditory neural activity during this phase. ABR Torpor Deermice INTRODUCTION HIBERNATION involves complex coordination of highly dy- namic activities of various levels of the brain, as well as the specific and nonspecific pathways to and from the brain. Such integrated activity promotes the sequential inhibition and facilita- tion required to enter into and arouse from the torpid state (16). Persistence of functional organization in certain pathways is indicated by many behavioral responses like the curled up pos- ture exhibited during torpor. Strumwasser (21) reported a behav- ioral response to auditory stimulation in the hibernating ground squirrel. The response was described as uncurling of the body and deflection of the pinnae towards the sound source for 20-30 min even after cessation of the sound stimulus; in some cases the animals aroused completely, rewarming to euthermia. Measure- ment of evoked electrical activity in response to olfactory, visual and auditory stimulation (6,13,14,16) provides further evidence of the functional integrity of sensory pathways during various stages of arousal from torpor. To define the magnitude of neural activity along various levels of the brain and brainstem in the hibernating ground squirrel Kilduff and his colleagues (12) measured transitions in [14C]2-deoxyglucose uptake in several neural structures. Mea- surements pertaining to the ascending auditory pathway showed sequential decline in metabolic activity along the pathway [i.e., minimal decline (3.8.9%) at the cochlear nucleus level] followed by the superior olivary nucleus and the nucleus of the lateral lemniscus (52.0% and 50.3% reductions, respectively), and fi- nally the inferior colliculus (greatest decrease--65.0%). Hamill and his associates (7), on the other hand, demonstrated intact auditory brainstem responses (ABR) at temperatures as low as 9.8°C during arousal in the same species. In small animals, ABR components I through V are known to emanate from the VIIIth cranial nerve, the cochlear nucleus, the superior olivary complex, the lateral lemniscus and the inferior colliculus, respectively (1,2,4,8,9,24). Thus, Hamill et al. (7) concluded that all nuclei within the brainstem auditory pathway were activated simultane- ously during the arousal phase. Katbamna et al. (11) measured ABR in the hibernating woodchuck. They were able to trace waves I and II, but not the rostral components, down to the lowest body temperatures supporting the conclusions of Kilduff et al. (12). The present study measured changes in ABR in torpid deermice; since the average body temperature during torpor in these animals is much higher (22-23°C) than the ground squirrel or the woodchuck, replication of either finding will shed light on the presence of auditory neural activity during hibernation. METHOD Subjects Seven, three-month-old deermice (Peromyscus maniculatus) obtained from the Peromyscus Stock Center at University of South Carolina, Columbia, served as experimental subjects. The mice were housed in pairs in a temperature-controlled environ- ment and received standard laboratory chow, and drinking water. i Requests for reprints should be addressed to Bharti Katbamna at her new address: Department of Speech Pathology and Audiology, Western Michican University, Kalamazoo, MI 49008-3825 USA. 189