Hibernation induces pentobarbital insensitivity in medulla but not cortex

Abstract

Hengen KB, Behan M, Carey HV, Jones MV, Johnson SM. Hibernation induces pentobarbital insensitivity in medulla but not cortex. Am J Physiol Regul Integr Comp Physiol 297: R1028-R1036, 2009. First published August 12, 2009 doi:10.1152/ajpregu.00239.2009.-The 13-lined ground squirrel (Ictidomys tridecemlineatus), a hibernating species, is a natural model of physiological adaption to an extreme environment. During torpor, body temperature drops to 0 -4°C, and the cortex is electrically silent, yet the brain stem continues to regulate cardiorespiratory function. The mechanisms underlying selective inhibition in the brain during torpor are not known. To test whether altered GABAergic function is involved in regional and seasonal differences in neuronal activity, cortical and medullary slices from summer-active (SA) and interbout aroused (IBA) squirrels were placed in a standard in vitro recording chamber. Silicon multichannel electrodes were placed in cortex, ventral respiratory column (VRC), and nucleus tractus solitarius (NTS) to record spontaneous neuronal activity. In slices from IBA squirrels, bath-applied pentobarbital sodium (300 M) nearly abolished cortical neuronal activity, but VRC and NTS neuronal activity was unaltered. In contrast, pentobarbital sodium (300 M) nearly abolished all spontaneous cortical, VRC, and NTS neuronal activity in slices from SA squirrels. Muscimol (20 M; GABAA receptor agonist) abolished all neuronal activity in cortical and medullary slices from both IBA and SA squirrels, thereby demonstrating the presence of functional GABAA receptors. Pretreatment of cortical slices from IBA squirrels with bicuculline (100 M; GABAA receptor antagonist) blocked pentobarbital-dependent inhibition of spontaneous neuronal activity. We hypothesize that GABAA receptors undergo a seasonal modification in subunit composition, such that cardiorespiratory neurons are uniquely unaffected by surges of an endogenous positive allosteric modulator.␥-aminobutyric acid receptors; ventral respiratory groups; nucleus tractus solitarius; respiratory control DURING WINTER HIBERNATING, mammals enter a state of torpor, defined by dramatically reduced metabolic and physical activity and low body temperature (T b ). The torpid state is interrupted periodically by interbout arousals to euthermia (T b ϭ 37°C) that generally last less than 24 h (3, 8). During torpor bouts, T b drops to just above ambient temperature and can approach 0°C, and respiration and heart rate drop as low as 1% of euthermic values (39,71). Neurons in the cortex, hippocampus, and thalamus are electrically silent in torpid hibernators (14, 67). Despite evidence for global forebrain depression, torpid hibernators maintain a robust, neuronally regulated cardiorespiratory output (37,25,41), suggesting that respiratoryand cardiovascular-related neurons in the brain stem remain active. The mechanisms that selectively depress the forebrain during hibernation are not well understood and may contribute to hibernators' unique ability to survive ischemia, hyp...

Changes in ventral respiratory column GABAaR ε- and δ-subunits during hibernation mediate resistance to depression by EtOH and pentobarbital

“…In a separate group of animals, tissues punches (diameter ϭ 0.75 mm, depth ϭ 0.75 mm; Stoelting, Wood Dale, IL) were taken from CTX, and two medullary regions previously reported to be seasonally insensitive to sodium pentobarbital, the VRC and the nucleus of the solitary tract (NTS; Ref. 22). Punches were taken from three slices (thickness ϭ 750 m) through the medulla inclusive of the BötC and extending through the cVRG (Fig.…”

“…Electrophysiological experiments were conducted during the summer active months (May to July) and during naturally occurring IBAs (November to February), which ruled out temperature-dependent effects. Detailed methods can be found in Hengen et al (22). Briefly, brains were removed and medullary and cortical slices (350 m thick) were cut with a vibrating microtome (Campden Instruments, Layfayette, IN).…”

“…In contrast, neuronal control of respiratory function is maintained during torpor (34). Throughout hibernation, between torpor bouts (3-21 days), squirrels spontaneously rewarm (T b ϭ 37°C) and resume activity for Ͻ24 h. Previous work in our laboratory (22) has shown that during these arousals (interbout arousal) neurons in the ventral respiratory column (VRC) are insensitive to high doses of barbiturates. In contrast, cortical neurons are highly sensitive to the inhibitory effects of pentobarbital throughout the year.…”

“…Identification of the VRC in squirrels is based on anatomical landmarks with reference to rats (37). Findings in squirrels suggest that medullary GABA A receptors are involved in preserving respiratory function during the extreme conditions that define mammalian hibernation (22). However, the specific molecular changes that take place in GABA A Rs in respiratory brain regions during hibernation are unknown.…”

“…There are three potential hypotheses to explain the previously reported pentobarbital insensitivity in VRC neurons during hibernation (22). First, there is insertion of ε-subunitcontaining GABA A Rs into VRC neurons during hibernation.…”