Thermal dependence of serotonergic modulation of neural activity in the hamster

Neuronal Activity in the Hibernating Brain

Hibernation is a natural phenomenon in many species which helps them to survive under extreme ambient conditions, such as cold temperatures and reduced availability of food in the winter months. It is characterized by a dramatic and regulated drop of body temperature, which in some cases can be near 0°C. Additionally, neural control of hibernation is maintained over all phases of a hibernation bout, including entrance into, during and arousal from torpor, despite a marked decrease in overall neural activity in torpor. In the present review, we provide an overview on what we know about neuronal activity in the hibernating brain focusing on cold-induced adaptations. We discuss pioneer and more recent in vitro and in vivo electrophysiological data and molecular analyses of activity markers which strikingly contributed to our understanding of the brain's sensitivity to dramatic changes in temperature across the hibernation cycle. Neuronal activity is markedly reduced with decreasing body temperature, and many neurons may fire infrequently in torpor at low brain temperatures. Still, there is convincing evidence that specific regions maintain their ability to generate action potentials in deep torpor, at least in response to adequate stimuli. Those regions include the peripheral system and primary central regions. However, further experiments on neuronal activity are needed to more precisely determine temperature effects on neuronal activity in specific cell types and specific brain nuclei.

Serotonergic modulation of hippocampal pyramidal cells in euthermic, cold-acclimated, and hibernating hamsters

Serotonergic fibers project to the hippocampus, a brain area previously shown to have distinctive changes in electroencephalograph (EEG) activity during entrance into and arousal from hibernation. The EEG activity is generated by pyramidal cells in both hibernating and nonhibernating species. Using the brain slice preparation, we characterized serotonergic responses of these CA1 pyramidal cells in euthermic, cold-acclimated, and hibernating Syrian hamsters. Stimulation of Shaffer-collateral/commissural fibers evoked fast synaptic excitation of CA1 pyramidal cells, a response monitored by recording population spikes (the synchronous generation of action potentials). Neuromodulation by serotonin (5-HT) decreased population spike amplitude by 54% in cold-acclimated animals, 80% in hibernating hamsters, and 63% in euthermic animals. The depression was significantly greater in slices from hibernators than from cold-acclimated animals. In slices from euthermic animals, changes in extracellular K+ concentration between 2.5 and 5.0 mM did not significantly alter serotonergic responses. The 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin mimicked serotonergic inhibition in euthermic hamsters. Results show that 5-HT is a robust neuromodulator not only in euthermic animals but also in cold-acclimated and hibernating hamsters.

Drosophila ebony mutants have altered circadian activity rhythms but normal eclosion rhythms

Drosophila ebony mutants exhibit a syndrome of morphological and behavioral phenotypes that include an abnormally dark body color and defects in visual and courtship responses. We now show that mutants carrying any one of five ebony alleles display complex and variable locomotor activity rhythms. Although in the most extreme cases activity is essentially aperiodic, many individuals express short- and/or long-period activity components. Three different ebony mutants (e, e1, and e11) express free-running rhythmicity in a temperature-dependent manner; activity rhythms are robust at 28 degrees C, but weak or absent at 20 degrees C. Even while maintained in a light-dark (LD) cycle, ebony homozygotes characteristically display extremely disorganized patterns of activity; some individuals entrain with an apparently abnormal phase and/or express multiple rhythmic components. Interestingly, the visual system mutation norpA partially suppresses effects of the e1 allele, which suggests that aberrant visual system inputs might contribute to the rhythm deficits of ebony mutants. In contrast to their effects on the locomotor activity rhythm, ebony mutations have no apparent impact on the circadian rhythm of adult eclosion, and thus exert rhythm-specific effects on circadian periodicity.