Avian Sleep Homeostasis: Electrophysiological, Molecular and Evolutionary Approaches

The function of slow wave sleep (SWS) and rapid eye movement (REM) sleep in mammals is an unanswered question in neuroscience. Aside from mammals, only birds engage in these states. Because birds independently evolved SWS and REM sleep, the study of sleeping birds may help identify shared traits related to the function of these states. Throughout this dissertation, we apply such a bird’s perspective to the sleeping brain. We begin with a review on knowledge gained through the study of sleep in animals (Chapter 1). Next, we present results from the first electrophysiological study of sleep in the most basal group of living birds by studying ostriches (Chapter 2). Although ostriches engage in unequivocal SWS, their REM sleep electrophysiology is unique and resembles features of REM sleep present only in basal mammals. Thus, the evolution REM sleep may have followed a recurring sequence of steps in mammals and birds. The remaining chapters deal with the regulation of sleep (or sleep homeostasis). Sleep homeostasis refers to an increase in the intensity of sleep (typically quantified as slow wave activity, SWA) following an extended period of wakefulness. Although such a response has long been known to occur in mammals, it has been unclear whether birds are capable of similar changes in SWA following sleep loss. We provide the first experimental evidence for a mammalian-like increase in SWA following enforced wakefulness in birds (Chapter 3). In mammals, SWA increases locally in brain regions used more during prior wakefulness. To see if SWS is regulated locally in birds, we stimulated one part of the pigeon brain during enforced wakefulness and observed a local increase in SWA during subsequent sleep (Chapter 4). Brain regions not stimulated asymmetrically during wakefulness showed a symmetric increase in SWA. These patterns of a/symmetry may reflect changes in the strength of synapses, as they do in mammals, because they are mirrored by changes in the slope of slow waves during SWS – a purported marker of synaptic strength. Lastly, we investigate whether local increases in SWA in birds are mediated by similar molecular mechanisms to those of mammals (Chapter 5). Surprisingly, mRNA levels of such proteins did not respond to unilateral visual stimulation during enforced wakefulness in the manner predicted based on work derived from mammals, but further study is needed to resolve the meaning of this difference. Overall, this dissertation presents several novel findings on the evolution and regulation of avian sleep

Entorhinal cortex dysfunction in rodent models of dementia

As both the major input and output of the hippocampal formation, the entorhinal cortex (EC) occupies a pivotal position in the medial temporal lobe. The discovery of grid cells in the medial entorhinal cortex (mEC) has led to this region being widely implicated in spatial information processing. Importantly, the EC is also the first area affected by dementia pathology, with neurons appearing particularly susceptible to degeneration. Despite this, little is known about how pathology affects the functional output of mEC neurons, either in their ability to coordinate firing to produce network oscillations, or to represent information regarding the external environment. This thesis will use electrophysiological techniques to examine how dementia pathology contributes to the breakdown of mEC neuronal networks using the rTg4510 mouse model of tauopathy. The first 2 results chapters will show how the anatomical organisation along the dorso-ventral axis of the mEC has profound influence on the network activity that can be observed both in brain slices and awake-behaving mice. It will further show how deficits in network activity in rTg4510 mice occur differentially across this axis, with dorsal mEC appearing more vulnerable to changes in oscillatory function than ventral. The third results chapter will begin to explore the relationship between global network activity and the external environment, showing that rTg4510 mice display clear deficits in the relationship between oscillation properties and locomotor activity. Finally, the underlying basis for these changes will be examined, through the recording of single-unit activity in these mice. It will show a decreased tendency for mEC neurons to display firing rates modulated by running speed, as well as an almost complete breakdown of grid cell periodicity after periods of tau overexpression. Understanding how dementia pathology produces changes to neuronal function and ultimately cognition is key for understanding and treating the disease. This thesis will therefore provide novel insights into the dysfunction of the EC during dementia pathology

Long-Lasting Novelty-Induced Neuronal Reverberation during Slow-Wave Sleep in Multiple Forebrain Areas

The discovery of experience-dependent brain reactivation during both slow-wave (SW) and rapid eye-movement (REM) sleep led to the notion that the consolidation of recently acquired memory traces requires neural replay during sleep. To date, however, several observations continue to undermine this hypothesis. To address some of these objections, we investigated the effects of a transient novel experience on the long-term evolution of ongoing neuronal activity in the rat forebrain. We observed that spatiotemporal patterns of neuronal ensemble activity originally produced by the tactile exploration of novel objects recurred for up to 48 h in the cerebral cortex, hippocampus, putamen, and thalamus. This novelty-induced recurrence was characterized by low but significant correlations values. Nearly identical results were found for neuronal activity sampled when animals were moving between objects without touching them. In contrast, negligible recurrence was observed for neuronal patterns obtained when animals explored a familiar environment. While the reverberation of past patterns of neuronal activity was strongest during SW sleep, waking was correlated with a decrease of neuronal reverberation. REM sleep showed more variable results across animals. In contrast with data from hippocampal place cells, we found no evidence of time compression or expansion of neuronal reverberation in any of the sampled forebrain areas. Our results indicate that persistent experience-dependent neuronal reverberation is a general property of multiple forebrain structures. It does not consist of an exact replay of previous activity, but instead it defines a mild and consistent bias towards salient neural ensemble firing patterns. These results are compatible with a slow and progressive process of memory consolidation, reflecting novelty-related neuronal ensemble relationships that seem to be context- rather than stimulus-specific. Based on our current and previous results, we propose that the two major phases of sleep play distinct and complementary roles in memory consolidation: pretranscriptional recall during SW sleep and transcriptional storage during REM sleep

Visual cortical alpha rhythms : function and relation to other dynamic signatures in local networks

The alpha rhythm (8-12Hz) was the first EEG rhythm recorded by Hans Berger in 1929. Despite being the earliest rhythm discovered, alpha rhythms remain the most mysterious in terms of mechanism and function. In the visual system, post-stimulus alpha oscillations are observed upon closing of the eyes or removal of visual stimulus. Alpha rhythms have been implicated in functional inhibition and short term memory. This thesis presents a rat in vitro model of the cortical alpha rhythm. This was achieved by mimicking the neuromodulatory changes that occur upon the removal of visual stimulus. Beta oscillations were induced by excitation of the visual cortex slice using the glutamate agonist kainate [800nM] to mimic sensory stimulation. This excitatory drive was then reduced using the AMPA and KA receptor antagonist NBQX [5µM], followed by the blocking of neuronal Ih current with DK-AH269 [10µM] to produce alpha frequency oscillations.Alpha activity was seen throughout all cortical laminae, with alpha power predominating in layer IV of the V1. The rhythm was found to be criticallydependent upon NMDA receptor-mediated connections between neurons which required the need to be potentiated in the prior excitation phase leading to beta frequency oscillations. Alpha activity was also dependent upon gap junctional coupling and had neuromodulatory effects similar to the human profile of alpha.Alpha oscillations were generated by pyramidal neurons found in layer IV of the V1 which elicited burst discharges. The alpha rhythm was not dominated by synaptic inhibition despite the functional inhibition role it is thought to play. Instead, the alpha rhythm appeared to dynamically uncouple activity in the primary thalamorecipient neurons (layer IV regular spiking cells) from down-stream activity in both supragranular and infragranular layers. In this manner, the alpha rhythm appears to be ideally constructed to prevent ascending visual information from both passing on to higher order visual areas, and also being influenced by top-down signal from these areas