Research ReportUnilateral cortical application of tumor necrosis factor α induces asymmetry in Fos- and interleukin-1β-immunoreactive cells within the corticothalamic projection
Introduction
The issue of how the brain is organized to produce sleep is central to neurobiology. For instance, because sleep involves regulated unconsciousness, an understanding of this process may help provide a scientific explanation of consciousness. von Economo [66] presented evidence, based on brain lesions in humans, suggesting that sleep is an active process as opposed to the idea that was dominant at the time, that sleep results from the withdrawal of sensory input [28], [52]. In the intervening century, many investigations have supported the idea that sleep is regulated by specific brain circuits, e.g., the anterior hypothalamus [24], [25], [34], [49]. Perhaps the best evidence for this paradigm with regard to non-rapid eye movement sleep (NREMS) is that electrical, chemical, or thermal stimulation of the anterior hypothalamus induces NREMS [4], [21], [32], [55], [75]. However, this paradigm fails to address the issue of sleep homeostasis. Thus, the brain keeps track of its prior sleep/wake history over long periods of time; e.g., prolonged wakefulness results in increased sleep propensity [5]. The transfer of cerebrospinal fluid from a sleep-deprived animal (but not a control animal) into a fully rested recipient enhances sleep in the recipient. This finding suggests that the homeostatic processes for sleep involve sleep regulatory substances (SRSs) [7], [42]. Several SRSs thought to play a role in those experimental findings have now been identified including molecules such as tumor necrosis factor alpha (TNFα) and interleukin-1 beta (IL1β) [38].
The literature concerning sleep regulatory circuits also fails to adequately explain a number of findings suggesting that sleep can be targeted to brain areas disproportionately activated during prior wakefulness. For instance, unilateral stimulation of the somatosensory cortex in awake humans produces asymmetry in the electroencephalogram (EEG) slow wave (1/2–4 Hz) activity during subsequent NREMS [26]. Similar results were obtained from rats and mice [67], [68]. Furthermore, dolphins that only show EEG slow wave sleep on one side of the brain at a time [36] show sleep rebound only on the side that was sleep-deprived [40]. More recently, Huber et al. [22] showed that after a specific learning task, the subsequent EEG slow wave activity during sleep was enhanced in the area activated by the learning task. Others have also concluded that EEG slow wave activity of specific brain areas is, in part, dependent upon prior activity within that region [16], [33].
A more recent view of brain organization of sleep posits that sleep is initiated within neuronal assemblies as a function of prior cellular activity and this provides the mechanism for the localization and targeting of sleep as well as for sleep homeostasis [29]. In this view, for example, TNFα, adenosine, nerve growth factor (NGF), and other SRSs are produced and released in response to neuronal and glial activity [10], [38]. These substances in turn are posited to be made and act locally in autocrine, paracrine, and juxtacrine ways to alter the input–output relationships of neuronal assemblies (e.g., cortical columns, barrels, or a cortical–thalamic–cortical loop) and thereby induce a functionally different state in these assemblies. Indeed, expression of NGF within somatosensory pyramidal neurons is activity- and sleep-dependent [8]. Cortical TNFα levels also increase during prolonged wakefulness and during seizures; two conditions thought to be associated with enhanced cellular activity [18], [45], [48], [63], [64], [65]. Also, activity-dependent increases of TNFα appear to be important for the developing nervous system [41]. Other investigators have also supported the view that sleep may be important in synaptic homeostasis and specific types of learning [5], [6], [27], [56], [62], [69], [70].
Individual auditory cortical columns oscillate between two functional states as defined by the amplitude of auditory-induced evoked response potentials [9], [47]. One of the functional states, called the “S” state, usually occurs simultaneously with whole animal sleep. The “S” state is dependent upon its prior history in the sense that the probability of its occurrence is higher if, for the 15-min period prior to the measurement, it had been in the other functionally defined state. If such changes in the neuronal-group functional state are dependent upon activity-dependent SRSs, then one would predict that direct application of an SRS such as TNFα onto the surface of the cortex would drive the affected neuronal groups to simultaneously express the same functional state. Indeed, when TNFα is applied in this manner, EEG power is enhanced unilaterally [74]. We extend these observations; hereby reporting that the application of TNFα to the cortex enhances Fos expression within large nuclei present in the surrounding cortical area and the reticular nucleus of the thalamus (reticular thalamus), presumably from activation of the cortical projection to this area. We also report enhanced IL1β immunoreactivity (IR) within astrocytes in the cortex and reticular thalamus, suggesting that activation of the corticothalamic projection can control synthesis of IL1β in nearby astrocytes.
Section snippets
Agents
Rat recombinant TNFα was purchased from R&D, Inc. (Minneapolis, MN). TNFα was dissolved in pyrogen-free saline (PFS) and microinjected at 150 ng/2 μl.
Animals
Male Sprague–Dawley rats weighing 300–400 g were obtained from Taconic Farm, Inc. (Germantown, NY). The use of rats in these experiments was in accordance with Washington State University guidelines and was approved by the Animal Care and Use Committee. The rats were adapted to a 12:12-h light–dark cycle (lights on at 0200) at 23 ± 2 °C ambient
Cortical activation of Fos-IR by surface application of TNFα
A greater number of large nuclei expressing Fos-IR were observed in the primary somatosensory cortex underlying the injection site on the side receiving TNFα than on the contralateral side that received heat-inactivated TNFα (Figs. 1B and D). Only a few Fos-IR nuclei were evident on the side receiving the microinjection of heat-inactivated TNFα, suggesting that this preparation was not contaminated by endotoxins (Figs. 1A and C). The asymmetric increases in the number of Fos-IR nuclei were most
Discussion
A primary finding of this manuscript is that TNFα applied unilaterally to the primary somatosensory cortex enhances Fos-IR within large nuclei near the site of the injection, presumably within neurons. Further, Fos-IR is enhanced ipsilaterally in the part of the reticular thalamus that receives the corticothalamic projections, suggesting that a neuronal projection from the barrel field has been activated. These findings are consistent with prior work using anterograde tracers demonstrating
Acknowledgments
This research was supported by the NIH grants to J.M. Krueger, NS25378 and NS31453.
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Current address: Departamento de Fisiología, Facultad de Medicina, UNAM, A.P. 70-400, C.P. 04510, México DF, México.