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

Propagated infra-slow intrinsic brain activity reorganizes across wake and slow wave sleep

Propagation of slow intrinsic brain activity has been widely observed in electrophysiogical studies of slow wave sleep (SWS). However, in human resting state fMRI (rs-fMRI), intrinsic activity has been understood predominantly in terms of zero-lag temporal synchrony (functional connectivity) within systems known as resting state networks (RSNs). Prior rs-fMRI studies have found that RSNs are generally preserved across wake and sleep. Here, we use a recently developed analysis technique to study propagation of infra-slow intrinsic blood oxygen level dependent (BOLD) signals in normal adults during wake and SWS. This analysis reveals marked changes in propagation patterns in SWS vs. wake. Broadly, ordered propagation is preserved within traditionally defined RSNs but lost between RSNs. Additionally, propagation between cerebral cortex and subcortical structures reverses directions, and intra-cortical propagation becomes reorganized, especially in visual and sensorimotor cortices. These findings show that propagated rs-fMRI activity informs theoretical accounts of the neural functions of sleep

GABAergic currents in RT and VB thalamic nuclei follow kinetic pattern of α3- and α1-subunit-containing GABAA receptors

Inhibitory postsynaptic currents (IPSCs) of the thalamic reticular (RT) nucleus are dramatically slower than in the neighboring ventrobasal (VB) neurons. It has been suggested that α3-subunit-containing receptors underlie slow IPSCs in RT neurons, while rapid synaptic currents in the VB nucleus are due to γ-aminobutyric acid A receptors (GABAARs), including the α1-subunit. In our recent study [Barberis et al. (2007) Eur. J. Neurosci., 25, 2726–2740] we have found that profound differences in kinetics of currents mediated by α3β2γ2 and α1β2γ3 receptors resulted from distinct binding and desensitization properties. However, a direct comparison between kinetics of neuronal GABAARs from RT and VB neurons and α3- and α1-subunit-containing receptors has not been made. For this purpose, current responses to ultrafast GABA applications were recorded from patches excised from neurons in VB and RT areas. Deactivation kinetics determined for RT and VB neurons closely resembled that in currents mediated by α3β2γ2 and α1β2γ2 receptors. In RT neurons, currents elicited by non-saturating [GABA] had a remarkably slow onset, a hallmark of α3-subunit-containing receptors. In VB and RT neurons, single-channel currents elicited by brief GABA pulses had similar characteristics to those of α1β2γ2 and α3β2γ2 receptors. However, in stationary conditions, similarity between single-channel currents in neurons and respective recombinant receptors was less apparent. We propose that the non-stationary kinetics of GABAergic currents in VB and RT nuclei mimic that of currents mediated by α1- and α3-subunit-containing receptors. The dissimilarity between stationary kinetics of neuronal and recombinant receptors probably reflects differences between GABAARs mediating phasic and tonic currents in these neurons

Starreveld scoring method in diagnosing childhood constipation

Four scoring methods exist to assess severity of fecal loading on plain abdominal radiographs in constipated patients (Barr-, Starreveld-, Blethyn- and Leech). So far, the Starreveld score was used only in adult patients. To determine accuracy and intra- and inter-observer agreement of the Starreveld scoring method in the diagnosis of functional constipation among pediatric patients. In addition, we compared the Starreveld with the Barr scoring method. Thirty-four constipated and 34 non-constipated children were included. Abdominal radiographs, obtained before treatment, were rated (Starreveld- and Barr) by 4 observers. A second observation after 4 weeks was done by 3 observers. Cut-off level for the Starreveld score, accuracy as measured by the area under the receiver operator characteristics curve, and inter- and intra-observer agreement were calculated. Cut-off value for the Starreveld score was 10. AUC for Starreveld score was 0.54 and for Barr score 0.38, indicating poor discriminating power. Inter-observer agreement was 0.49-0.52 4 (Starreveld) and 0.44 (Barr), which is considered moderate. Intra-observer agreement was 0.52-0.71 (Starreveld) and 0.62- 0.76 (Barr). The Starreveld scoring method to assess fecal loading on a plain abdominal radiograph is of limited value in the diagnosis of childhood constipatio

Activity of cortical and thalamic neurons during the slow (<1 Hz) rhythm in the mouse in vivo

During NREM sleep and under certain types of anaesthesia, the mammalian brain exhibits a distinctive slow (<1 Hz) rhythm. At the cellular level, this rhythm correlates with so-called UP and DOWN membrane potential states. In the neocortex, these UP and DOWN states correspond to periods of intense network activity and widespread neuronal silence, respectively, whereas in thalamocortical (TC) neurons, UP/DOWN states take on a more stereotypical oscillatory form, with UP states commencing with a low-threshold Ca2+ potential (LTCP). Whilst these properties are now well recognised for neurons in cats and rats, whether or not they are also shared by neurons in the mouse is not fully known. To address this issue, we obtained intracellular recordings from neocortical and TC neurons during the slow (<1 Hz) rhythm in anaesthetised mice. We show that UP/DOWN states in this species are broadly similar to those observed in cats and rats, with UP states in neocortical neurons being characterised by a combination of action potential output and intense synaptic activity, whereas UP states in TC neurons always commence with an LTCP. In some neocortical and TC neurons, we observed ‘spikelets’ during UP states, supporting the possible presence of electrical coupling. Lastly, we show that, upon tonic depolarisation, UP/DOWN states in TC neurons are replaced by rhythmic high-threshold bursting at ~5 Hz, as predicted by in vitro studies. Thus, UP/DOWN state generation appears to be an elemental and conserved process in mammals that underlies the slow (<1 Hz) rhythm in several species, including humans

Effectiveness of preoperative staging in rectal cancer: digital rectal examination, endoluminal ultrasound or magnetic resonance imaging?

In rectal cancer, preoperative staging should identify early tumours suitable for treatment by surgery alone and locally advanced tumours that require therapy to induce tumour regression from the potential resection margin. Currently, local staging can be performed by digital rectal examination (DRE), endoluminal ultrasound (EUS) or magnetic resonance imaging (MRI). Each staging method was compared for clinical benefit and cost-effectiveness. The accuracy of high-resolution MRI, DRE and EUS in identifying favourable, unfavourable and locally advanced rectal carcinomas in 98 patients undergoing total mesorectal excision was compared prospectively against the resection specimen pathological as the gold standard. Agreement between each staging modality with pathology assessment of tumour favourability was calculated with the chance-corrected agreement given as the kappa statistic, based on marginal homogenised data. Differences in effectiveness of the staging modalities were compared with differences in costs of the staging modalities to generate cost effectiveness ratios. Agreement between staging and histologic assessment of tumour favourability was 94% for MRI (kappa=0.81, s.e.=0.05; kappa(W)=0.83), compared with very poor agreements of 65% for DRE (kappa=0.08, s.e.=0.068, kappa(W)=0.16) and 69% for EUS (kappa=0.17, s.e.=0.065, kappa(W)=0.17). The resource benefits resulting from the use of MRI rather than DRE was 67164 UK pounds and 92244 UK pounds when MRI was used rather than EUS. Magnetic resonance imaging dominated both DRE and EUS on cost and clinical effectiveness by selecting appropriate patients for neoadjuvant therapy and justifies its use for local staging of rectal cancer patients