Diencephalic amnesia is thought to be the result of damage to a single thalamic structure that is responsible for the memory impairment. However, an alternative view is that different thalamic structures contribute to the memory impairment in subtly different ways. Paired-associate learning is one important measure of learning and memory that is highly sensitive to disruption in people with amnesia or dementia. The current study will investigate the influence of lesions to two thalamic subregions, the anterior thalamic nuclei (AT) and the lateral thalamic nuclei (LT) in an object-odour paired associate learning task. Each of these subregions has been suggested by the literature as critical for amnesia after thalamus injury. The current study does not involve a place/ space component. Both AT and LT lesions caused impairments in the object-odour paired associate task, but not in the simple discrimination tasks. The results of this study provide new evidence to suggest that the anterior thalamic region may be responsible for more than spatial memory processing. This result is inconsistent with those of Aggleton & Brown (1999) that consider the AT to be part of an 'extended hippocampal system'. The deficits observed from LT lesions in this study provide new insight into the lateral thalamic region's role in pattern processing.

Previous research has shown that nitric oxide signalling in the lateral nucleus of the amygdala is required for the consolidation of Pavlovian conditioned fear. Given the evidence that nitric oxide can act as a retrograde signalling molecule in in vitro models of memory consolidation the question arises whether this is also occurring within behavioural memory models? Using auditory fear conditioning this research shows that nitric oxide does indeed act as retrograde signalling molecule in the fear system. Its synthesis in the lateral nucleus of the amygdala regulates conditioning induced expression of the immediate early gene early growth response gene 1 (EGR-1) in cells of the auditory thalamus that project to the lateral nucleus of the amygdala. The regulation of EGR-1 expression by the lateral nucleus of the amygdala was proven to be dependent on amygdala-based cellular excitation, nitric oxide synthesis and NR2B-NMDA receptor activation but not ERK/MAPK activity. Using an EGR-1 antisense oligonucleotide to prevent training induced EGR-1 expressions in the auditory thalamus it was shown that this gene upregulation is necessary for the consolidation of conditioned fear. Finally, inhibition of EGR-1 upregulation in the auditory thalamus was proven to impair conditioning induced increases in the presynaptic proteins synaptophysin, and synapsin II and II back in the lateral nucleus of the amygdala. Overall, the results of this dissertation have shown that nitric oxide acts as a retrograde messenger in a mammalian memory system by modulating gene expression in presynaptic cells. This modulation of gene expression serves to increase levels of presynaptic proteins back at the origin of nitric oxide synthesis. This supports the long standing doctrine that nitric oxide acts as a retrograde signalling molecule to coordinate presynaptic changes associated with memory formation.

PhD / When modeling a system as complex as the brain, considerable simplifications are inevitable. The nature of these simplifications depends on the available experimental evidence, and the desired form of model predictions. A focus on the former often inspires models of networks of individual neurons, since properties of single cells are more easily measured than those of entire populations. However, if the goal is to describe the processes responsible for the electroencephalogram (EEG), such models can become unmanageable due to the large numbers of neurons involved. Mean-field models in which assemblies of neurons are represented by their average properties allow activity underlying the EEG to be captured in a tractable manner. The starting point of the results presented here is a recent physiologically-based mean-field model of the corticothalamic system, which includes populations of excitatory and inhibitory cortical neurons, and an excitatory population representing the thalamic relay nuclei, reciprocally connected with the cortex and the inhibitory thalamic reticular nucleus. The average firing rates of these populations depend nonlinearly on their membrane potentials, which are determined by afferent inputs after axonal propagation and dendritic and synaptic delays. It has been found that neuronal activity spreads in an approximately wavelike fashion across the cortex, which is modeled as a two-dimensional surface. On the basis of the literature, the EEG signal is assumed to be roughly proportional to the activity of cortical excitatory neurons, allowing physiological parameters to be extracted by inverse modeling of empirical EEG spectra. One objective of the present work is to characterize the statistical distributions of fitted model parameters in the healthy population. Variability of model parameters within and between individuals is assessed over time scales of minutes to more than a year, and compared with the variability of classical quantitative EEG (qEEG) parameters. These parameters are generally not normally distributed, and transformations toward the normal distribution are often used to facilitate statistical analysis. However, no single optimal transformation exists to render data distributions approximately normal. A uniformly applicable solution that not only yields data following the normal distribution as closely as possible, but also increases test-retest reliability, is described in Chapter 2. Specialized versions of this transformation have been known for some time in the statistical literature, but it has not previously found its way to the empirical sciences. Chapter 3 contains the study of intra-individual and inter-individual variability in model parameters, also providing a comparison of test-retest reliability with that of commonly used EEG spectral measures such as band powers and the frequency of the alpha peak. It is found that the combined model parameters provide a reliable characterization of an individual's EEG spectrum, where some parameters are more informative than others. Classical quantitative EEG measures are found to be somewhat more reproducible than model parameters. However, the latter have the advantage of providing direct connections with the underlying physiology. In addition, model parameters are complementary to classical measures in that they capture more information about spectral structure. Another conclusion from this work was that a few minutes of alert eyes-closed EEG already contain most of the individual variability likely to occur in this state on the scale of years. In Chapter 4, age trends in model parameters are investigated for a large sample of healthy subjects aged 6-86 years. Sex differences in parameter distributions and trends are considered in three age ranges, and related to the relevant literature. We also look at changes in inter-individual variance across age, and find that subjects are in many respects maximally different around adolescence. This study forms the basis for prospective comparisons with age trends in evoked response potentials (ERPs) and alpha peak morphology, besides providing a standard for the assessment of clinical data. It is the first study to report physiologically-based parameters for such a large sample of EEG data. The second main thrust of this work is toward incorporating the thalamocortical system and the basal ganglia in a unified framework. The basal ganglia are a group of gray matter structures reciprocally connected with the thalamus and cortex, both significantly influencing, and influenced by, their activity. Abnormalities in the basal ganglia are associated with various disorders, including schizophrenia, Huntington's disease, and Parkinson's disease. A model of the basal ganglia-thalamocortical system is presented in Chapter 5, and used to investigate changes in average firing rates often measured in parkinsonian patients and animal models of Parkinson's disease. Modeling results support the hypothesis that two pathways through the basal ganglia (the so-called direct and indirect pathways) are differentially affected by the dopamine depletion that is the hallmark of Parkinson's disease. However, alterations in other components of the system are also suggested by matching model predictions to experimental data. The dynamics of the model are explored in detail in Chapter 6. Electrophysiological aspects of Parkinson's disease include frequency reduction of the alpha peak, increased relative power at lower frequencies, and abnormal synchronized fluctuations in firing rates. It is shown that the same parameter variations that reproduce realistic changes in mean firing rates can also account for EEG frequency reduction by increasing the strength of the indirect pathway, which exerts an inhibitory effect on the cortex. Furthermore, even more strongly connected subcircuits in the indirect pathway can sustain limit cycle oscillations around 5 Hz, in accord with oscillations at this frequency often observed in tremulous patients. Additionally, oscillations around 20 Hz that are normally present in corticothalamic circuits can spread to the basal ganglia when both corticothalamic and indirect circuits have large gains. The model also accounts for changes in the responsiveness of the components of the basal ganglia-thalamocortical system, and increased synchronization upon dopamine depletion, which plausibly reflect the loss of specificity of neuronal signaling pathways in the parkinsonian basal ganglia. Thus, a parsimonious explanation is provided for many electrophysiological correlates of Parkinson's disease using a single set of parameter changes with respect to the healthy state. Overall, we conclude that mean-field models of brain electrophysiology possess a versatility that allows them to be usefully applied in a variety of scenarios. Such models allow information about underlying physiology to be extracted from the experimental EEG, complementing traditional measures that may be more statistically robust but do not provide a direct link with physiology. Furthermore, there is ample opportunity for future developments, extending the basic model to encompass different neuronal systems, connections, and mechanisms. The basal ganglia are an important addition, not only leading to unified explanations for many hitherto disparate phenomena, but also contributing to the validation of this form of modeling.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007. / Härtill 4 uppsatser.

Au cours de cette thèse, j’ai cherché à comprendre comment trois structures impliquées dans la modulation des états vigilance agissaient sur la dynamique du réseau cortical. Les noyaux du thalamus non-spécifique sont impliqués dans le transfert d’informations contextuelles. À l’inverse, les noyaux spécifiques convoient des informations sensorielles vers le néocortex. Ces entrées sensorielles activent fortement les interneurones fast-spiking du cortex qui limite la durée de l’activation du réseau. À l’inverse, mes travaux montrent que les entrées contextuelles entraînent l’activation des interneurones lents, générant une activation prolongée du réseau cortical. D’autre part, je me suis intéressée à la couche VIb du néocortex dont les neurons sont sensibles à deux modulateurs des états de vigilance : l’orexine et à l’acétylcholine. J’ai pu montrer que la couche VIb projette principalement dans les couches corticales infragranulaires où elle pourrait être servir d’amplificateur dépendant de l’orexine des entrées du thalamus non-spécifique. Finalement, j’ai cherché à comprendre si la transmission nicotinique endogène était médiée par une synapse ou par transmission volumique. Pour cela, j’ai comparé les courants nicotinques reçus par les neurones des couches I et VI. Mes travaux mettent en évidence l’existence d’une synapse mixte comprenant des récepteurs α4β2 et α7 dans la couche I et d’une synapse simple comprenant uniquement α4β2 en couche VI. Ces synapses sont activées par la stimulation phasique des neurones cholinergiques. Néanmoins mes résultats suggèrent aussi l’existence de récepteurs α4β2 extra-synaptiques activés par la libération tonique des fibres cholinergiques. / During my PhD, I investigated how three structures involved in the modulation of arousal states act on the dynamic of the cortical network. Nuclei of the non-specific thalamus convey information on environmental and behavioral context, whereas specific nuclei relay sensory information to the neocortex. These sensory inputs activate strongly the fast-spiking interneurons of the neocortex that limits response duration of the network. Conversely, I showed that contextual inputs target mainly the slow adapting interneurons allowing a long-lasting activation of the cortical network. I have also been interesting in the layer VIb of the neocortex whose neurons are sensitive to orexin and acetylcholine, two main modulators of the arousal states. I showed that layer VIb projects mainly onto infragranular cortical layers where its activation should act as an orexin-dependent amplifier of the non-specific thalamic inputs. Finally, I tried to decipher whether the endogenous nicotinic transmission is mediated by a synapse or by volume transmission. To do that, I compared nicotinic currents received by layer I interneurons and layer VI pyramidal cells. I showed that nicotinic transmission is likely to be mediated by a mixed synapse comprising α4β2 and α7 receptors in layer I and a simple α4β2 containing synapse in layer VI. These synapses are activated by a phasic stimulation of the cholinergic fibers. However, my results also suggest that a tonic activation of these fibers recruits extra-synaptic α4β2 receptors in both layer I and VI neurons.

La sérotonine a été impliquée dans le processus développemental de raffinement périnatal des connections synaptiques. D’autre rôle important est joue par des cellules immunitaires du cerveau, la microglie.Pendant ma thèse j’ai teste l’hypothèse qu’une interaction entre la microglie et la sérotonine est nécessaire pour l’établissement des circuits neuronaux correct dans le cerveau de la souris.Les résultats récents du laboratoire ont montré que la microglie exprime un des récepteurs à la sérotonine: le 5-HT2B. En utilisant comme model la ségrégation des axons retiennes dans le thalamus j’ai observé que la souris invalide pour ce récepteur présente des altérations anatomiques dans les régions des projections rétiniennes du thalamus.En parallèle, j’ai testé l’effet de la sérotonine sur la microglie. La libération locale de la sérotonine sur les coupes aigue du cerveau a un effet chemoattractant sur la microglie (2-photon microscopie).En plus, les analyses d'expression d’ARN de microglie dans une culture cellulaire primaire ont montres une augmentation de certains marqueurs d'activation dans la souris invalidée pour le récepteur 5-HT2B.Afin de tester quelle(s) cellule(s) sont responsable(s) pour les altérations observées dans le thalamus j’ai teste un nombre de souris avec une invalidation conditionnelle du récepteur 5-HT2B. Les résultats que j’ai obtenus pendant mon doctorat support l’hypothèse que la sérotonine interagit avec la microglie et que cette interaction pouvait être importante dans la maturation du cerveau. / Serotonin, besides its functions as a neurotransmitter, actively participates in postnatal establishment and refinement of brain wiring in mammals. Another important role is played by the brain resident macrophages, microglia, in developmentally-regulated neuronal death as well as in synaptic maturation or elimination.During my thesis, I tested the hypothesis of cross-regulations between microglia and serotonin during postnatal brain development in a mouse model. The laboratory data show a major expression of the serotonin 5-HT2B receptor by postnatal microglia, suggesting that serotonin could participate in temporal and spatial synchronization of microglial functions. Using an in vivo model of synaptic refinement during early brain development, the maturation of retinal projections to the thalamus, I observed that Htr2B-/- mice present anatomical alterations of the projecting area of retinal axons into the thalamus.Parallelly, I tested the effects of serotonin on microglial cells. A local delivery of serotonin attracted microglial processes on acute brain slices (two-photon microscopy).Moreover, after comparing mRNA expression level in microglial primary cultures, we have found that some activation markers are upregulated in microglia from Htr2B-/-.In the second part of my PhD, by using a number of conditional Htr2B-/- mice, I investigated which cell type(s) could be responsible for the altered segregation of retinal axons in the thalamus of the total Htr2B-/- mice.Overall, my results support the hypothesis that serotonin interacts with microglial cells and that these interactions could participate in brain maturation.

The purpose of this thesis was to determine the neuronal circuitry mediating U50,488-induced locomotion in preweanling rats. To this end, preweanling rats received bilateral electrolytic lesions of the ventromedial thalamus or superior colliculus and, two days later, the same rats received a challenge injection of U50,488. It was predicted that bilateral lesions of the ventromedial thalamus or superior colliculus would attenuate the U50,488-induced locomotor activity of 18-day-old rats.

Les neurones du cortex visuel primaire (V1) peuvent emprunter deux voies de communications afin d’atteindre les aires extrastriées : une voie cortico-corticale, et une voie cortico-thalamo-corticale à travers des noyaux thalamiques de haut niveau (HO) comme le pulvinar. Les fonctions respectives de ces deux voies restent toujours méconnues. Un pas vers une meilleure compréhension de celles-ci seraient d’investiguer la nature des signaux qu’elles transmettent. Dans ce contexte, deux grands types de projections cortico-thalamiques (CT) ont été identifiés dans le système visuel : les neurones de type I (modulator) et type II (driver) caractérisés respectivement par des axones minces dotés de petits boutons terminaux et par des axones plus épais et de plus grands boutons respectivement. Une proposition récente a aussi émis l'hypothèse que ces deux types pourraient également être distingués par leur expression de transporteur de glutamate vésiculaire. Cette hypothèse suggère que les projections de type II et de type I peuvent exprimer sélectivement VGLUT2 et VGLUT1, respectivement (Balaram, 2013; Rovo et al, 2012). Chez le chat, les projections de V1 vers le pulvinar se composent principalement de terminaux de type II, tandis que celles de l’aire PMLS présentent une combinaison de terminaux de type I et II suggérant ainsi que, la proportion de terminaux de type I augmente avec le niveau hiérarchique cortical des zones visuelles. Afin de tester cette hypothèse, nous avons cartographié la distribution des terminaux CT du cortex AEV (article 1) ainsi que de l’aire 21a (article 2). Nous avons aussi étudié l’expression de VGLUT 1 et 2 dans le système visuel du chat afin de tester si leurs expressions corrèlent avec les sites de projections de neurones de type I et II (article 3). Nos résultats indiquent que la grande majorité des terminaux marqués dans le pulvinar provenant de l’AEV et de l’aire 21a sont de type I (Article 1 et 2) alors que ceux de V1 sont majoritairement de type II. Une comparaison de la proportion des projections de type I à travers les aires V1, PMLS, 21a et AEV révèlent une corrélation positive de sorte que celle-ci augmente avec le degré hiérarchique des aires visuelles. Nos résultats indiquent que VGLUT 1 et 2 présentent une distribution complémentaire et que leur localisation dans des sites connus pour recevoir une projection de type ‘modulateur’ et ‘déclencheur’ proéminente suggère que leurs expressions peuvent montrer un biais pour celles-ci dans la voie géniculo-strié. Les résultats de cette thèse ont permis de mieux connaitre la nature des projections CT des aires visuelles extrastriées. Ces résultats sont d’autant plus importants qu’ils établissent un lien entre la nature de ces projections et le degré hiérarchique des aires visuelles, suggérant ainsi l’existence une organisation anatomofonctionnelle des voies CT passant par le pulvinar. Enfin, les résultats de cette thèse ont aussi permis une meilleure compréhension des vésicules VGLUT 1 et 2 dans le système visuel du chat et leurs affinités respectives pour les sites de projections de neurones de type I et II. / Visual signals from the primary visual cortex (V1), can take two main communication routes in order to reach higher visual areas: a corticocortical pathway and a cortico-thalamo-cortical (or transthalamic) pathway through high-order thalamic nuclei such as the pulvinar. While these pathways are receiving an increasing interest from the scientific community, their respective functions still remain largely unknown. An important step towards a better understanding of these pathways would be to investigate the nature of the signals they transmit. In this context, two main types of corticothalamic (CT) projections have been identified in the visual system: type I projections (modulators) and type II (drivers) characterized respectively by thin axons with small terminal and by thicker axons and larger terminals. A recent proposal has also hypothesized that these two types can also be distinguished by their expression of vesicular glutamate transporter (VGLUT) in their respective synaptic terminals such that type II (driver) and type I (modulator) projections can selectively express VGLUT 2 and VGLUT 1, respectively (Balaram, 2013; Rovo et al, 2012). In cats, projections from V1 to the LP-pulvinar are mainly composed of type II terminals, while those from the Posteromedial lateral suprasylvian (PMLS) cortex present a combination of type I and II terminals. This observation suggests that, in higher-order (HO) thalamic nuclei, the proportion of type I terminals increases with the hierarchical level of the visual areas. To test this hypothesis, we charted the distribution of CT terminals originating from the Anterior EctoSylvian visual cortex (AEV) (article 1) and from area 21a (article 2). We also studied the expression of VGLUT 1 and 2 in the cat's visual system in order to test whether their expressions correlate with the projection sites of type I and II axon terminals (article 3). Our results from article 1 and 2 indicate that the vast majority of terminals sampled in the pulvinar from the AEV and area 21a are of type I while projections from V1 projections to the pulvinar were mostly composed of type II terminals. A comparison of the proportion of type I projections across areas V1, PMLS, 21a and the AEV revealed a positive correlation such that its proportion increased with the hierarchical rank of visual areas. Our results also indicate that VGLUT 1 and 2 have a complementary distribution pattern which matches prominent projection of type I and II respectively in ascending visual projections but does not in extra-geniculate pathways involving the pulvinar (Article 3). Taken together, results from this thesis have allowed a better understanding of the nature of cortico-thalamic projections originating from extra-striate visual areas (21a and AEV). These results are all the more important in that they establish a link between the nature of these projections and the hierarchical degree of their cortical area of origin, thus suggesting that there is a functional organization of CT pathways passing through the pulvinar. Finally, results of this thesis also enabled a better understanding of the expression of VGLUT 1 and 2 in the visual system and their possible respective biases for type I and type II projections.

Trigeminal neuropathic pain (TNP) is a condition that occurs when one or more branches of the trigeminal nerve are insulted. Trigeminal neuropathic pain has been shown to be refractory to treatment. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels regulate neuronal excitability in both the peripheral and central nerve systems. Emerging evidence indicates that HCN channels are involved in the development and maintenance of chronic pain, however, the impact of thalamic HCN channel activity on TNP has yet to be elucidated. In this report, we used a chronic constriction of the distal infraorbital nerve (dIoN-CCI) to induce TNP in rats. By infusing HCN channel blockers into the ventral posteromedial (VPM) nucleus of the thalamus in dIoN-CCI rats, we demonstrated that inhibition of HCN channel activity ameliorated TNP. We found that the HCN blocker ZD7288 and the clinical drug ivabradine dose-dependently attenuated both evoked and none-evoked nociceptive behaviors in dIoN-CCI rats. Electrophysiological measurements showed the expression of HCN current (Ih) in the thalamocortical neurons in the VPM was sensitive to the HCN channel modulator cyclic adenosine monophosphate (cAMP), suggesting a contribution of the HCN2 subunit in thalamic HCN current. In the thalamus, surface expression of the HCN2 subunit was increased in dIoN-CCI rats. Taken together, we propose that an increase in HCN channel activity in the thalamus in the ascending nociceptive pathway contributed to trigeminal neuropathic pain.

The overarching purpose of this work is to investigate the effect of pupil-linked arousal systems and heart rate-linked arousal systems on behavior and the use of pupil dynamics as an index for adaptive behavior. These arousal systems are correlated with behavior states which have modulatory effects on perception and neural coding and are therefore integral in carrying out complex behaviors, such as decision-making. Additionally, cholinergic transmission is proposed to be critical for adaptive behavior via its modulation of thalamic neurons. While the first two chapters focus on the behavior output and the non-invasive ways to index arousal and behavior, the third aim attempts to investigate the neural circuits that underlie arousal’s effect on adaptive behavior by studying the pattern of cholinergic axons between brainstem nuclei and the thalamus, opening avenues for future investigation of their mechanistic impact on adaptive behavior. For characterization of the influence of arousal indexed by pupil dynamics and heartbeat dynamics, we simultaneously recorded electrocardiogram (ECG) and pupil size in head-fixed rats performing tactile discrimination tasks. We found both heartbeat dynamics and pupil size co-varied with behavioral outcomes, indicating behavior was dependent upon arousal indexed by both physiological signals. The potential difference between the effects of pupil-linked arousal and heart-rate linked arousal on behavior were estimated by constructing a Bayesian decoder predicting animals’ behavior from both signals prior to stimulus presentation. The decoder performed significantly better when using both physiological signals as inputs, suggesting both arousal systems, pupil-linked and heart rate-linked are not completely redundant. Additionally, the pupil size-based decoder failed to correctly predict animals’ behavior on a substantial portion of trials correctly predicted by the heart rate-based decoder, furthermore suggesting that both arousal systems exert different influences on animals’ behavior (Y.Liu, S. Narasimhan, B.J.Schriver, & Q.Wang, 2021). For characterization of how adaptive behavior in response to changing sensory environments depends on pupil-linked arousal, we recorded pupil size and behavioral output simultaneously during a similar tactile Go/No-Go discrimination task while systematically varying the statistics of the sensory environment. For each session, the probability of the presence of reward linked-stimuli (S+/Go) was randomly set at 80%, 50% or 20%. Animals adapted their behavioral responses and the task evoked pupil responses were bigger when the probability of S+ was lower. Impulsive licking (pre-stimulus response) decreased as the probability of S+ was lower. Animals became more liberal as the probability of S+ increased, in line with signal detection theory, indexed by a decrease in the decision criterion. We additionally found that reaction time decreased as the probability of S+ increased. A hierarchical drift diffusion model (HDDM) was used to model the decision-making process in these paradigms. We found the drift rate to monotonically vary with task difficulty. Animals performed sub-optimally to adaptively change their action in response to changes in the sensory environment and this adaptive adjustment in decision-making was indexed by their pupil dynamics. For studying the pattern of cholinergic axons between brainstem nuclei and the thalamus, we used two different AAV-retrograde constructs with two different reporters (mCherry and GFP) injected in the left and right hemisphere of the ventral-posterior-medial (VPM) thalamic nucleus of recombinant ChAT-cre mice. In agreement with previous studies done with conventional tracing methods, labelled projecting cells were traced to the LDT and PPN in the brainstem. Labelled cells were found in a clustered area of the LDT, suggesting a topographic distribution of the projections between the LDT and the VPM. A larger quantity of labelled cells was found in the PPN than the LDT. Additionally, bilateral injections with double reporters enabled us to find that a majority of cells project from the brainstem to the VPM project ipsilaterally while still displaying lateralization. This work provides methods and tools for future investigation of the functional impact of these projections between LDT, PPN and the VPM.