Cellular thalamic correlates of the slow (<1Hz) sleep rhythm

Sleep and wakefulness form an inherent, biological rhythm that defines our daily lives. Despite the fact that sleep is a constant interruption to the waking state, its purpose and the neural processes occurring during this behavioural state are not fully understood. However, it is now well established that sleep is not a period of the brain 'silence'. During the transition form light to deep sleep, the activity of the corticothalamic network becomes globally synchronised into consistent, characteristic rhythmic activities at <1Hz, quite in contrast to the so-called cortical 'desynchronisation' characterising states of brain alertness. The mechanism by which global synchronisation in the corticothalamic network arises is not fully defined and previous investigation has focused principally on the role of the cortex. However a clear understanding of the activities of thalamic, as well as cortical, neurones during sleep will aid our understanding into how, and why, global synchronisation occurs, and perhaps why sleep is so fundamental to life. In this thesis, I demonstrate a number of novel activities in two types of thalamic neurones recorded in vitro. Firstly, in thalamocortical neurones, the principal cell type and thalamic output neurones, I demonstrate the presence of an mGluRIa dependent slow (<1Hz) oscillation with identical properties to that seen in the intact brain during sleep. Thalamocortical neurones in relay nuclei subserving visual, somatosensory, auditory and motor systems displayed the slow (<1Hz) oscillation suggesting it could be the substrate for global thalamic synchronization at <1Hz. In addition, I provide a full characterisation of the cellular mechanism of this <1Hz oscillatory activity and demonstrate that during mGluRIa activation, the window component of the low-voltage activated Ca2+ current is unmasked, due to a reduction in the constitutive K+ leak current, inducing bistability-mediated activities that underlies the generation of the slow (<1Hz) oscillation. In neurones of the nucleus reticularis thalami, overlying the thalamus and providing an inhibitory drive to thalamocortical neurones, I also demonstrate a slow (<1Hz) oscillation, again with identical properties as seen in this cell type in the intact brain during sleep. I demonstrate that this slow (<1Hz) oscillation is dependent on mGluRIa activation and provide evidence suggesting that it is generated by a bistability-mediated mechanism as occurs in thalamocortical neurones. In light of these findings, I suggests that the thalamus, has a significant role in aiding, as well as maintaining, the global synchronisation of the corticothalamic network at <1Hz during the transition to, and during sleep. The ability of thalamic neurones to generate rhythmic activities at <1Hz due to cortical mGluRIa activation, that results simply in a reduction of the K+ leak current, will provide a strong excitatory drive to organise cortical activity at <1 Hz. A further novel observation was the presence of spikelets and burstlets (compounds of spikelets) in thalamocortical neurones. Investigations into the origin of these events indicated that they were electrophysiological manifestations of interneuronal electrotonic coupling. Furthermore, spikelets and burstlets had the ability to entrain the output of the neurones in which they were observed. Therefore, the presence of electrotonic coupling in thalamic neurones may have a hitherto unrealised role in the synchronization of thalamic activity during both sleep and awake states

Cellular thalamic correlates of the slow (< 1hz) sleep rhythm

Sleep and wakefulness form an inherent, biological rhythm that defines our daily lives. Despite the fact that sleep is a constant interruption to the waking state, its purpose and the neural processes occurring during this behavioural state are not fully understood. However, it is now well established that sleep is not a period of the brain 'silence'. During the transition form light to deep sleep, the activity of the corticothalamic network becomes globally synchronised into consistent, characteristic rhythmic activities at <1Hz, quite in contrast to the so-called cortical 'desynchronisation' characterising states of brain alertness. The mechanism by which global synchronisation in the corticothalamic network arises is not fully defined and previous investigation has focused principally on the role of the cortex. However a clear understanding of the activities of thalamic, as well as cortical, neurones during sleep will aid our understanding into how, and why, global synchronisation occurs, and perhaps why sleep is so fundamental to life. In this thesis, I demonstrate a number of novel activities in two types of thalamic neurones recorded in vitro. Firstly, in thalamocortical neurones, the principal cell type and thalamic output neurones, I demonstrate the presence of an mGluRIa dependent slow (<1Hz) oscillation with identical properties to that seen in the intact brain during sleep. Thalamocortical neurones in relay nuclei subserving visual, somatosensory, auditory and motor systems displayed the slow (<1Hz) oscillation suggesting it could be the substrate for global thalamic synchronization at <1Hz. In addition, I provide a full characterisation of the cellular mechanism of this <1Hz oscillatory activity and demonstrate that during mGluRIa activation, the window component of the low-voltage activated Ca2+ current is unmasked, due to a reduction in the constitutive K+ leak current, inducing bistability-mediated activities that underlies the generation of the slow (<1Hz) oscillation. In neurones of the nucleus reticularis thalami, overlying the thalamus and providing an inhibitory drive to thalamocortical neurones, I also demonstrate a slow (<1Hz) oscillation, again with identical properties as seen in this cell type in the intact brain during sleep. I demonstrate that this slow (<1Hz) oscillation is dependent on mGluRIa activation and provide evidence suggesting that it is generated by a bistability-mediated mechanism as occurs in thalamocortical neurones. In light of these findings, I suggests that the thalamus, has a significant role in aiding, as well as maintaining, the global synchronisation of the corticothalamic network at <1Hz during the transition to, and during sleep. The ability of thalamic neurones to generate rhythmic activities at <1Hz due to cortical mGluRIa activation, that results simply in a reduction of the K+ leak current, will provide a strong excitatory drive to organise cortical activity at <1 Hz. A further novel observation was the presence of spikelets and burstlets (compounds of spikelets) in thalamocortical neurones. Investigations into the origin of these events indicated that they were electrophysiological manifestations of interneuronal electrotonic coupling. Furthermore, spikelets and burstlets had the ability to entrain the output of the neurones in which they were observed. Therefore, the presence of electrotonic coupling in thalamic neurones may have a hitherto unrealised role in the synchronization of thalamic activity during both sleep and awake states.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Thalamic Gap Junctions Control Local Neuronal Synchrony and Influence Macroscopic Oscillation Amplitude during EEG Alpha Rhythms

Although EEG alpha (α; 8–13 Hz) rhythms are often considered to reflect an “idling” brain state, numerous studies indicate that they are also related to many aspects of perception. Recently, we outlined a potential cellular substrate by which such aspects of perception might be linked to basic α rhythm mechanisms. This scheme relies on a specialized subset of rhythmically bursting thalamocortical (TC) neurons (high-threshold bursting cells) in the lateral geniculate nucleus (LGN) which are interconnected by gap junctions (GJs). By engaging GABAergic interneurons, that in turn inhibit conventional relay-mode TC neurons, these cells can lead to an effective temporal framing of thalamic relay-mode output. Although the role of GJs is pivotal in this scheme, evidence for their involvement in thalamic α rhythms has thus far mainly derived from experiments in in vitro slice preparations. In addition, direct anatomical evidence of neuronal GJs in the LGN is currently lacking. To address the first of these issues we tested the effects of the GJ inhibitors, carbenoxolone (CBX), and 18β-glycyrrhetinic acid (18β-GA), given directly to the LGN via reverse microdialysis, on spontaneous LGN and EEG α rhythms in behaving cats. We also examined the effect of CBX on α rhythm-related LGN unit activity. Indicative of a role for thalamic GJs in these activities, 18β-GA and CBX reversibly suppressed both LGN and EEG α rhythms, with CBX also decreasing neuronal synchrony. To address the second point, we used electron microscopy to obtain definitive ultrastructural evidence for the presence of GJs between neurons in the cat LGN. As interneurons show no phenotypic evidence of GJ coupling (i.e., dye-coupling and spikelets) we conclude that these GJs must belong to TC neurons. The potential significance of these findings for relating macroscopic changes in α rhythms to basic cellular processes is discussed

Neuronal basis of the slow (<1 Hz) oscillation in neurons of the nucleus reticularis thalami in vitro

During deep sleep and anesthesia, the EEG of humans and animals exhibits a distinctive slow (<1 Hz) rhythm. In inhibitory neurons of the nucleus reticularis thalami (NRT), this rhythm is reflected as a slow (<1 Hz) oscillation of the membrane potential comprising stereotypical, recurring "up" and "down" states. Here we show that reducing the leak current through the activation of group I metabotropic glutamate receptors (mGluRs) with either trans-ACPD [(+/–)-1-aminocyclopentane-trans-1,3-dicarboxylic acid] (50–100 µM) or DHPG [(S)-3,5-dihydroxyphenylglycine] (100 µM) instates an intrinsic slow oscillation in NRT neurons in vitro that is qualitatively equivalent to that observed in vivo. A slow oscillation could also be evoked by synaptically activating mGluRs on NRT neurons via the tetanic stimulation of corticothalamic fibers. Through a combination of experiments and computational modeling we show that the up state of the slow oscillation is predominantly generated by the "window" component of the T-type Ca2+ current, with an additional supportive role for a Ca2+-activated nonselective cation current. The slow oscillation is also fundamentally reliant on an Ih current and is extensively shaped by both Ca2+- and Na+-activated K+ currents. In combination with previous work in thalamocortical neurons, this study suggests that the thalamus plays an important and active role in shaping the slow (<1 Hz) rhythm during deep sleep

Cellular Mechanisms of the Slow (<1 Hz) Oscillation in Thalamocortical Neurons In Vitro.

The slow (<1 Hz) rhythm is a defining feature of the electroencephalogram during sleep. Since cortical circuits can generate this rhythm in isolation, it is assumed that the accompanying slow oscillation in thalamocortical (TC) neurons is largely a passive reflection of neocortical activity. Here we show, however, that by activating the metabotropic glutamate receptor (mGluR), mGluR1a, cortical inputs can recruit intricate cellular mechanisms that enable the generation of an intrinsic slow oscillation in TC neurons in vitro with identical properties to those observed in vivo. These mechanisms rely on the “window” component of the T-type Ca2+ current and a Ca2+-activated, nonselective cation current. These results suggest an active role for the thalamus in shaping the slow (<1 Hz) sleep rhythm

The "window' T-type calcium current in brain dynamics of different behavioural states

All three forms of recombinant low voltage-activated T-type Ca2+ channels (Cav3.1, Cav3.2 and Cav3.3) exhibit a small, though clearly evident, window T-type Ca2+ current (ITwindow) which is also present in native channels from different neuronal types. In thalamocortical (TC) and nucleus reticularis thalami (NRT) neurones, and possibly in neocortical cells, an ITwindow-mediated bistability is the key cellular mechanism underlying the expression of the slow (< 1 Hz) sleep oscillation, one of the fundamental EEG rhythms of non-REM sleep. As the ITwindow-mediated bistability may also represent one of the cellular mechanisms underlying the expression of high frequency burst firing in awake conditions, ITwindow is of critical importance in neuronal population dynamics associated with different behavioural states

Nucleus- and species-specific properties of the slow (<1 hz) sleep oscillation in thalamocortical neurons

The slow (<1 Hz) rhythm is an electroencephalogram hallmark of resting sleep. In thalamocortical neurons this rhythm correlates with a slow (<1 Hz) oscillation comprising recurring UP and DOWN membrane potential states. Recently, we showed that metabotropic glutamate receptor activation brings about an intrinsic slow oscillation in thalamocortical neurons of the cat dorsal lateral geniculate nucleus in vitro which is identical to that observed in vivo. The aim of this study was to further assess the properties of this oscillation and compare them with those observed in thalamocortical neurons of three other thalamic nuclei in the cat (ventrobasal complex, medial geniculate body; ventral lateral nucleus) and two thalamic nuclei in rats and mice (lateral geniculate nucleus and ventrobasal complex). Slow oscillations were evident in all of these additional structures and shared several basic properties including, i) the stereotypical, rhythmic alternation between distinct UP and DOWN states with the UP state always commencing with a low-threshold Ca2+ potential, and ii) an inverse relationship between frequency and injected current so that slow oscillations always increase in frequency with hyperpolarization, often culminating in delta (?) activity at 1–4 Hz. However, beyond these common properties there were important differences in expression between different nuclei. Most notably, 44% of slow oscillations in the cat lateral geniculate nucleus possessed UP states that comprised sustained tonic firing and/or high-threshold bursting. In contrast, slow oscillations in cat ventrobasal complex, medial geniculate body and ventral lateral nucleus thalamocortical neurons exhibited such UP states in only 16%, 11% and 10% of cases, respectively, whereas slow oscillations in the lateral geniculate nucleus and ventrobasal complex of rats and mice did so in <12% of cases. Thus, the slow oscillation is a common feature of thalamocortical neurons that displays clear species- and nuclei-related differences. The potential functional significance of these results is discussed

Synchronized oscillations at alpha and theta frequencies in the lateral geniculate nucleus

In relaxed wakefulness, the EEG exhibits robust rhythms in the ? band (8–13 Hz), which decelerate to ? (2–7 Hz) frequencies during early sleep. In animal models, these rhythms occur coherently with synchronized activity in the thalamus. However, the mechanisms of this thalamic activity are unknown. Here we show that, in slices of the lateral geniculate nucleus maintained in vitro, activation of the metabotropic glutamate receptor (mGluR) mGluR1a induces synchronized oscillations at ? and ? frequencies that share similarities with thalamic ? and ? rhythms recorded in vivo. These in vitro oscillations are driven by an unusual form of burst firing that is present in a subset of thalamocortical neurons and are synchronized by gap junctions. We propose that mGluR1a-induced oscillations are a potential mechanism whereby the thalamus promotes EEG ? and ? rhythms in the intact brain