Functional MRI during hippocampal deep brain stimulation in the healthy rat brain

Deep Brain Stimulation (DBS) is a promising treatment for neurological and psychiatric disorders. The mechanism of action and the effects of electrical fields administered to the brain by means of an electrode remain to be elucidated. The effects of DBS have been investigated primarily by electrophysiological and neurochemical studies, which lack the ability to investigate DBS-related responses on a whole-brain scale. Visualization of whole-brain effects of DBS requires functional imaging techniques such as functional Magnetic Resonance Imaging (fMRI), which reflects changes in blood oxygen level dependent (BOLD) responses throughout the entire brain volume. In order to visualize BOLD responses induced by DBS, we have developed an MRI-compatible electrode and an acquisition protocol to perform DBS during BOLD fMRI. In this study, we investigate whether DBS during fMRI is valuable to study local and whole-brain effects of hippocampal DBS and to investigate the changes induced by different stimulation intensities. Seven rats were stereotactically implanted with a custom-made MRI-compatible DBS-electrode in the right hippocampus. High frequency Poisson distributed stimulation was applied using a block-design paradigm. Data were processed by means of Independent Component Analysis. Clusters were considered significant when p-values were <0.05 after correction for multiple comparisons. Our data indicate that real-time hippocampal DBS evokes a bilateral BOLD response in hippocampal and other mesolimbic structures, depending on the applied stimulation intensity. We conclude that simultaneous DBS and fMRI can be used to detect local and whole-brain responses to circuit activation with different stimulation intensities, making this technique potentially powerful for exploration of cerebral changes in response to DBS for both preclinical and clinical DBS

Electrical Stimulation of the Lateral Entorhinal Cortex Causes a Frequency-Specific BOLD Response Pattern in the Rat Brain

Although deep brain stimulation of the entorhinal cortex has recently shown promise in the treatment of early forms of cognitive decline, the underlying neurophysiological processes remain elusive. Therefore, the lateral entorhinal cortex (LEC) was stimulated with trains of continuous 5 Hz and 20 Hz pulses or with bursts of 100 Hz pulses to visualize activated neuronal networks, i.e., neuronal responses in the dentate gyrus and BOLD responses in the entire brain were simultaneously recorded. Electrical stimulation of the LEC caused a wide spread pattern of BOLD responses. Dependent on the stimulation frequency, BOLD responses were only triggered in the amygdala, infralimbic, prelimbic, and dorsal peduncular cortex (5 Hz), or in the nucleus accumbens, piriform cortex, dorsal medial prefrontal cortex, hippocampus (20 Hz), and contralateral entorhinal cortex (100 Hz). In general, LEC stimulation caused stronger BOLD responses in frontal cortex regions than in the hippocampus. Identical stimulation of the perforant pathway, a fiber bundle projecting from the entorhinal cortex to the dentate gyrus, hippocampus proper, and subiculum, mainly elicited significant BOLD responses in the hippocampus but rarely in frontal cortex regions. Consequently, BOLD responses in frontal cortex regions are mediated by direct projections from the LEC rather than via signal propagation through the hippocampus. Thus, the beneficial effects of deep brain stimulation of the entorhinal cortex on cognitive skills might depend more on an altered prefrontal cortex than hippocampal function

Storage, recall, and novelty detection of sequences by the hippocampus: Elaborating on the SOCRATIC model to account for normal and aberrant effects of dopamine

ABSTRACT: In order to understand how the molecular or cellular defects that underlie a disease of the nervous system lead to the observ-able symptoms, it is necessary to develop a large-scale neural model. Such a model must specify how specific molecular processes contribute to neuronal function, how neurons contribute to network function, and how networks interact to produce behavior. This is a challenging undertaking, but some limited progress has been made in understanding the memory functions of the hippocampus with this degree of detail. There is increas-ing evidence that the hippocampus has a special role in the learning of sequences and the linkage of specific memories to context. In the first part of this paper, we review a model (the SOCRATIC model) that describes how the dentate and CA3 hippocampal regions could store and recall memory sequences in context. A major line of evidence for sequence recall is the “phase precession ” of hippocampal place cells. In the second part of the paper, we review the evidence for theta-gamma phase coding

Use of functional neuroimaging and optogenetics to explore deep brain stimulation targets for the treatment of Parkinson's disease and epilepsy

Deep brain stimulation (DBS) is a neurosurgical therapy for Parkinson’s disease and epilepsy. In DBS, an electrode is stereotactically implanted in a specific region of the brain and electrical pulses are delivered using a subcutaneous pacemaker-like stimulator. DBS-therapy has proven to effectively suppress tremor or seizures in pharmaco-resistant Parkinson’s disease and epilepsy patients respectively. It is most commonly applied in the subthalamic nucleus for Parkinson’s disease, or in the anterior thalamic nucleus for epilepsy. Despite the rapidly growing use of DBS at these classic brain structures, there are still non-responders to the treatment. This creates a need to explore other brain structures as potential DBS-targets. However, research in patients is restricted mainly because of ethical reasons. Therefore, in order to search for potential new DBS targets, animal research is indispensable. Previous animal studies of DBS-relevant circuitry largely relied on electrophysiological recordings at predefined brain areas with assumed relevance to DBS therapy. Due to their inherent regional biases, such experimental techniques prevent the identification of less recognized brain structures that might be suitable DBS targets. Therefore, functional neuroimaging techniques, such as functional Magnetic Resonance Imaging and Positron Emission Tomography, were used in this thesis because they allow to visualize and to analyze the whole brain during DBS. Additionally, optogenetics, a new technique that uses light instead of electricity, was employed to manipulate brain cells with unprecedented selectivity

Déclenchement de la LTP hippocampique et de l'apprentissage par la dopamine : un signal d'apprentissage

L'hippocampe est la principale structure cérébrale impliquée dans la formation de la mémoire épisodique. Les mécanismes sous-jacents la mémoire hippocampique ont été étudié en détail chez les rongeurs, en particulier grâce à l'utilisation de tests de mémoire contextuelle. La potentialisation à long terme (PLT) est une augmentation de la transmission synaptique des afférences glutamatergiques ; elle sous-tend la formation des mémoires hippocampiques. Elle peut être déclenchée par une stimulation à haute fréquence (SHF). Ce mécanisme a permis de déchiffrer les mécanismes de la mémoire, montrant que la PLT, tout comme la mémoire, repose dans sa phase précoce sur des mécanismes de phosphorylation, ensuite, elle nécessite la formation de protéines de novo. Le lien entre la mémoire et la PLT est démontré par le fait que le blocage des différentes étapes de la PLT empêche la formation de la mémoire contextuelle et que celle-ci déclenche la PLT dans le CA1 de l'hippocampe. Étant donné que la PLT, tout comme la mémoire, est saturable, le système nerveux ne peut pas enregistrer tous les évènements vécus par l'animal. De plus, la SHF n'est pas compatible avec l'activité neuronale. Cela implique l'existence d'un signal d'apprentissage qui choisirait les entrées pertinentes à sauvegarder, et qui serait le déclencheur moléculaire de la PLT lors de l'apprentissage. La dopamine est un neuro-modulateur longtemps considéré comme indiquant la récompense. Cependant, la dopamine est libérée en réponse à tous les événements saillants, y compris aversifs. Les récepteurs dopaminergiques peuvent déclencher la phosphorylation et la formation de novo des protéines, et les récepteurs dopaminergiques D1/5 sont nécessaires pour la PLT tardive et la mémoire à long terme. De plus, la stimulation dopaminergique in vitro peut moduler la transmission synaptique du CA1. Dans ce travail, nous avons utilisé le comportement et l'électrophysiologie couplés aux manipulations optogénétiques des afférences dopaminergiques du mésencéphale et à l'inhibition pharmacologique des récepteurs dopaminergiques D1/5 pour étudier le rôle de la dopamine en tant que signal d'apprentissage déclenchant la PLT et l'apprentissage. En utilisant l'électrophysiologie, nous montrons que le couplage de stimulations optogénétiques des afférences dopaminergiques du mésencéphalique avec des entrées glutamatergiques du CA1 induit une PLT progressive de ces dernières, qui atteint un plateau 90 minutes après la dernière stimulation dopaminergique. Cette PLT dure au moins 5 heures, dépend des récepteurs D1/5 et occlue partiellement la PLT déclenchée par SHF. Ensuite, en utilisant le conditionnement de peur contextuel, nous montrons que l'infusion intra-hippocampique de de l'inhibiteur des récepteurs D1/5, SCH23390, bloque l'apprentissage du conditionnement de peur au contextuel mais pas à un indice auditif. Nous concluons que les récepteurs D1/5 hippocampiques sont nécessaires pour la mémoire de peur contextuelle. Enfin, nous avons utilisé une variante du conditionnement de peur au contexte appelée effet de facilitation par la préexposition contextuelle. Dans ce test, le conditionnement de peur a lieu le lendemain de l'apprentissage contextuel. Il permet ainsi d'étudier indépendamment chacune de ces deux étapes. Nous montrons que les récepteurs D1/5 sont nécessaires à l'apprentissage du contexte et à celui de la peur. Enfin, nous montrons que la stimulation optogénétique des axones dopaminergiques dans l'hippocampe favorise l'apprentissage contextuel et que leur inhibition empêche l'apprentissage contextuel. Ce travail nous permet de conclure que la voie dopaminergique du mésencéphale vers l'hippocampe a toutes les caractéristiques d'un signal d'apprentissage : elle déclenche la PLT sur les entrées sensorielles co-activées favorisant l'enregistrement d'informations contextuelles dans l'hippocampe indépendamment de toute information de valeur positive ou négative.The hippocampus is the main brain structure involved in episodic memory formation. The role of the hippocampus in learning, memory and their underlying mechanisms has been studied extensively in rodents, in particular by using contextual learning. Long-Term Potentiation (LTP) is an increase in synaptic transmission of glutamatergic afferents that lasts for hours, days or months and is thought to underlie hippocampal memory formation. It can be triggered in the hippocampus by an artificial High frequency Stimulation (HFS). This mechanism helped in deciphering memory mechanisms, showing that both memory and LTP rely firstly on phosphorylation and later on de novo protein synthesis. The link between memory and LTP was confirmed by showing that blocking LTP mechanisms hinders memory formation, and that contextual learning induces LTP in the CA1 of the hippocampus. Since LTP, just like memory, can be saturated, the nervous system cannot store every sensory input that the animal encounters. Moreover, HFS is not compatible with neuronal activity. Hence, there must be a teaching signal that would be the natural molecular trigger of LTP during learning, acting as a filter choosing the pertinent inputs to store. Dopamine is a neuromodulator that has historically been thought of as a value signal, for dopamine gets released during rewarding events. However, dopamine has later been shown to be released whenever a salient unrewarding, or even punishing, event occurs. Dopamine receptors can trigger both phosphorylation and de novo protein formation in most brain structures showing plasticity, and D1/5 Dopaminergic receptors are necessary for LTP maintenance and long-term memory. Moreover, dopaminergic stimulation in vitro can modulate synaptic transmission in CA1. Thus, we hypothesized that dopamine could act as a teaching signal. In this work, we use behavior and electrophysiology coupled with optogenetic manipulations of midbrain dopamine afferents and pharmacology inhibition of D1/5 dopaminergic receptors in order to study the role of dopamine as a teaching signal triggering LTP so that pertinent sensory inputs get stored. Using electrophysiology, we show that coupling optogenetic stimulations of midbrain dopamine with glutamatergic inputs in CA1 induces a progressive LTP that reaches its plateau 90 minutes after the pairing. This LTP endures at least 5 hours, is dependent on D1/5 receptors and partially occludes HFS-triggered LTP. Then, using contextual fear conditioning coupled with auditory cue conditioning we show that intraperitoneal injection of D1/5 receptor inhibitor, SHC23390, hinders both contextual and cue fear memories. Alternatively, intra-hippocampal infusion of SCH23390 blocks contextual memory but preserves cue fear memory intact. These results allowed us to conclude that hippocampal D1/5 receptors are necessary for contextual fear memories and in another brain structure for associative fear memories. Finally, we use a variation of contextual fear conditioning called contextual pre-exposure facilitation effect, which separates contextual learning from fear conditioning since the animal in this task learns each of them on two consecutive days. This allows studying dopamine as a teaching signal without the interference of any value inputs. We show that mice require between 2-8 minutes to encode contextual information. Furthermore, we show that D1/5 receptors are necessary for contextual and fear learning. Finally, we show that optogenetic stimulation of dopaminergic axons in the hippocampus promotes contextual learning and, conversely, their inhibition hinders contextual learning. This work allows us to conclude that the dopaminergic pathway from the midbrain to the hippocampus has all the characteristics of a teaching signal, namely, triggering LTP on co-activated sensory inputs promoting the storage of contextual information in the hippocampus without the need for any value information

D4 Dopamine Receptor-Mediated Modulation of Hippocampal Synaptic Efficacy and Network Activity in Behaving Mice

Dopamine (DA) is the predominant catecholamine neurotransmitter in the mammalian brain, where DA governs a variety of functions including locomotor activity, cognition, emotion, synaptic plasticity, learning and memory formation. In the hippocampus, activation of D1/5 receptors (D1/5Rs) is known to stabilize memory formation and late, protein-dependent long-term potentiation (LTP) in the stratum radiatum (RAD), whereas activation of D4 receptors (D4Rs) inhibits early LTP in the stratum oriens (OR) and depotentiates LTP in RAD. Interestingly, novelty exploration rescues memory impairment caused by blockade of D1/5Rs in the hippocampus. However, the function of D4Rs in synaptic plasticity, neural network activity and memory-associated behaviors remained to be elucidated in vivo. I implanted two recording electrodes targeting OR and RAD together with a bipolar stimulation electrode placed either in OR or RAD while monitoring depth profiles of stimulus-evoked local field potentials (eLFPs). At least one week later, eLFPs and spontaneous oscillatory activities (sLFPs) were recorded in the hippocampus of freely behaving mice to investigate the role of D4Rs under physiological condition. My results show that systemic, intraperitoneal treatment with the D4R agonist PD 168077 (PD) slightly decreased eLFPs both in OR and RAD and in parallel increased the paired-pulse ratio (PPR) between two eLFPs, indicating presynaptic mechanisms were involved in this modulation. PD treatment postponed the rapid eye movement (REM) sleep and, during REM onset, the theta peak frequency was shifted to lower band, the gamma band power was reduced and the strength of theta-gamma coupling was attenuated. Furthermore, D4R agonist treatment impaired late LTP (4 hours) both in OR and RAD, while early LTP (30 min) was reduced only in OR. When mice were transferred from their home cage to a fear box, band power of fast gamma increased in that novel environment, in particular after receiving an electric footshock on day 1 and during context exposure on day 2, and these increases persisted when the mice were returned to home cage immediately after. These changing patterns of oscillatory activities were not affected by PD treatment, and therefore the D4R-mediated layer-specific modulation of synaptic plasticity in the hippocampus is unlikely implicated in learning and memory during novelty exploration or fear conditioning

A topographical analysis of hippocampal field connectivity

The intrinsic connectivity of the hippocampus in the rat was studied using the retrograde tracers Fast Blue and rhodamine microspheres. In an initial double-labelling study the bilateral labelling within the CA3 field was observed following injection of the two dyes into homologous regions in the two CA1 fields. The labelling in the contralateral CA3 field was found to be identical both in distribution and in density to that observed in ipsilateral CAS, suggesting that the CAS-CA1 projection is bilaterally symmetrical. The observation that nearly 100% of backfilled cells were double-labelled indicates that homologous regions in the two CA1 fields receive information from the same cells in CAS, which indicates that the two CA1 fields act as one functional unit. Evidence for the existence of a sparse association/commissural system within CA1 was provided by the presence of a small number of labelled cells in the contralateral CA1 field. The topographic organization of the CAS-CA1 projection was studied following a series of injections of the two dyes into different locations across the extent of the CA1 field. In this experiment a new technique for the injections of tracers into the hippocampus was introduced, in which a microelectrode positioned adjacent to the tip of the injection pipette was used to record neuronal activity during the operation. This allowed the complex spike firing of hippocampal cells to be used as a marker for the dorsoventral location of the injection, thus greatly improving the accuracy of the injections. An analysis of the labelling in the extended hippocampus reveals that projections to CA1 arise from bands of cells organized diagonally across the CAS field. The CAS-CA1 projection was topographically organized in the septotemporal plane, with progressively more temporal parts of CA1 receiving inputs from bands located more temporally in CA3, Further analysis showed that cells positioned along a diagonal axis in CA1 were found to receive fibres from the same band of cells in CA3, and that this "CA1 axis" is similar to the axis of the diagonal bands in CAB. It therefore appears that the CA3-CA1 projection is organized in a diagonal fashion across the septotemporal length of the hippocampus both in terms of projecting and recipient cells. A number of injections was also placed into the CA3 field in order to study the CA3 association projection. In these cases the pattern of labelling in the CA3 field in the extended hippocampus was organized parallel to the septotemporal axis, and was topographically organized such that any given region in CA3 preferentially received fibres from cells located along the Scime transverse segment of CA3. These findings support the notion that the CA3 association pathway interconnects cells spread over a large fraction of the septotemporal extent of CAS. Preliminary data concerning the organization of the projection from the dentate gyrus to CA3 were collected following an injection of biocytin into the dentate gyrus. Additional data was taken from 3 animals in which rhodamine microspheres were found to be transported anterogradely along the mossy fibres in a previously unreported fashion. In all cases the projection was organized orthogonal to the septotemporal axis for most of the its length, with a shift in trajectory in proximal CA3 in the temporal direction. These results show that these three intrahippocampal pathways are angled with respect to each other, thus forming a lattice framework of connections similar to the "crossing fibre arrays" described by Tamamaki and Nojyo (1991a). Each pathway displays a specific directionality which, in contrast to the original lamellar hypothesis, allows the spread of information across all parts of the hippocampus. (Abstract shortened by ProQuest.