Uptake in Ground Squirrel Brain during Hibernation’

Autoradiographic patterns of [14C]2-deoxyglucose uptake are described throughout the brains of hibernating and euthermic ground squirrels. Autoradiographs of the brains of hibernating animals are generally homogeneous in comparison to euthermic animals; hence, the relative 2-deoxyglucose uptake (RBDGU) of gray to white matter for the majority of the 85 neural structures examined decreases during hibernation. Two categories of structures are identified as potentially important in hibernation: (I) structures that have the highest R2DGU during hibernation (cochlear nucleus, paratrigeminal nucleus, and superior colliculus) and (2) structures that undergo the least reduction in RBDGU in the transition from euthermia to hibernation (suprachiasmatic nucleus and lateral septal nucleus). The percentage of reduction in RZDGU that a structure undergoes in the transition from euthermia to hibernation is proportional to the RBDGU of that structure during euthermia. The suprachiasmatic, paratrigeminal, and cochlear nuclei undergo less of a reduction than would be predicted from this relationship and may be particularly important during hibernation. Sensory nuclei that receive primary afferent projections are among the structures with the highest R2DGU during hibernation. These metabolically active structures may be responsible for the sensitivity of the hibernator to environmental stimuli. The phenomenon of hibernation involves a complex suite of physiological adaptations. As body temperature falls from euthermic levels of 37°C to as low as 2°C in the ground squirrel, heart rate decreases from 250 to 300 beats/min to 7 to 10 beats/min and respiration slows from 100 to 150 breaths/min to 2 to 3 breaths/min (Twente and Twente, 1978). During hibernation, animals assume a stereotyped curled-up posture with the head tucked beneath the tail, indicating maintained activity in some motor control pathways. A hibernating animal retains muscle tonus and postural control, whereas the hypothermic animal at the same body temperature does not (Musacchia, 1976). Hibernating animals also remain responsive to auditory, vibratory, tactile, and thermal ’ This work was supported by National Institutes of Health Grant NS10367 to H. C. H., by Danforth and National Science Foundation Graduate Fellowships to T. S. K., and by a University of California, San Diego Academic Senate Grant to F. R. S. We would like to gratefully acknowledge the able assistance of Charles George, Nancy Thomas, and Lisa Moy in the preparation of animals for experiments and Scott Sakaguchi for aid in computer analysis of data. ’ To whom correspondence should be addressed at Department of Biological Sciences, Stanford University, Stanford, CA 94305. stimuli. Hibernators will deflect their ear pinnae to the source of auditory stimulation, orient to the stimulus, and in extreme cases, undergo a complete rewarming (arousal) to euthermia (Strumwasser, 1959). Hibernation also involves distinct changes in the nervous and endocrine systems. Action potentials are conducted in peripheral nerves of hibernating ground squirrels below 5”C, but conduction is blocked in the nonhibernating (euthermic) state at 5’ to 6°C (Kehl and Morrison, 1960). Involution of the endocrine system may be necessary for entrance into hibernation (Lyman and Chatfield, 1955) and the hypothalamohypophyseal neurosecretory system reaches its lowest activity during the hibernation season. Lower rates of synthesis, transport, and release of neurosecretory material from the axons of the paraventricular and supraoptic nuclei of the hypothalamus are observed in a deeply hibernating species than in a species which undergoes a shallower hibernation (Polenov and Yurisova, 1975). Hypothalamic thermosensitivity studies reveal that the decline in body temperature during entrance into hibernation is a regulated process (Heller et al., 1977). Hypothalamic regulation of body temperature remains functional throughout hibernation, enabling animals to