Low frequency hippocampal-cortical activity drives brain-wide resting-state functional MRI connectivity

The hippocampus, including the dorsal dentate gyrus (dDG), and cortex engage in bidirectional communication. We propose that low-frequency activity in hippocampal–cortical pathways contributes to brain-wide resting-state connectivity to integrate sensory information. Using optogenetic stimulation and brain-wide fMRI and resting-state fMRI (rsfMRI), we determined the large-scale effects of spatiotemporal-specific downstream propagation of hippocampal activity. Low-frequency (1 Hz), but not high-frequency (40 Hz), stimulation of dDG excitatory neurons evoked robust cortical and subcortical brain-wide fMRI responses. More importantly, it enhanced interhemispheric rsfMRI connectivity in various cortices and hippocampus. Subsequent local field potential recordings revealed an increase in slow oscillations in dorsal hippocampus and visual cortex, interhemispheric visual cortical connectivity, and hippocampal–cortical connectivity. Meanwhile, pharmacological inactivation of dDG neurons decreased interhemispheric rsfMRI connectivity. Functionally, visually evoked fMRI responses in visual regions also increased during and after low-frequency dDG stimulation. Together, our results indicate that low-frequency activity robustly propagates in the dorsal hippocampal–cortical pathway, drives interhemispheric cortical rsfMRI connectivity, and mediates visual processing

Frequency-selective control of cortical and subcortical networks by central thalamus

Central thalamus plays a critical role in forebrain arousal and organized behavior. However, network-level mechanisms that link its activity to brain state remain enigmatic. Here, we combined optogenetics, fMRI, electrophysiology, and video-EEG monitoring to characterize the central thalamus-driven global brain networks responsible for switching brain state. 40 and 100 Hz stimulations of central thalamus caused widespread activation of forebrain, including frontal cortex, sensorimotor cortex, and striatum, and transitioned the brain to a state of arousal in asleep rats. In contrast, 10 Hz stimulation evoked significantly less activation of forebrain, inhibition of sensory cortex, and behavioral arrest. To investigate possible mechanisms underlying the frequency-dependent cortical inhibition, we performed recordings in zona incerta, where 10, but not 40, Hz stimulation evoked spindle-like oscillations. Importantly, suppressing incertal activity during 10 Hz central thalamus stimulation reduced the evoked cortical inhibition. These findings identify key brain-wide dynamics underlying central thalamus arousal regulation

Combining Optogenetics and fMRI to Study Cerebral Networks in Animal Models

Functional magnetic resonance imaging is a well-established technique to examine brain activity and networks in animal models. In contrast to electrophysiology, optogenetics enables the control of specific cell types. A second advantage is that optogenetic stimulation can trigger excitatory as well as inhibitory effects. This study investigated optogenetic manipulation of neuronal activity and connectivity changes in mice and rats, using fMRI as an analytical method. The work addresses (1) glutamate release in mice and its use as a neurotransmitter and (2) oxytocin release in rats and its impact on neuronal networks. In fMRI, changes of metabolism and blood flow are described by the hemodynamic response function. The influence of the optogenetic stimulation on the hemodynamic response function was examined and characterized for both species. In the first part, glutamatergic neurons in the left hippocampus of mice were optogenetically stimulated via channelrhodopsin-2. Sham animals, without virus injection, were used as a control group to study unspecific effects induced by the laser stimulation. In transgenic (α-CamKII-Cre) mice, the direct hippocampal activation was investigated as well as its projections to other regions. Additionally, the impact of the dorsal-ventral position of the optical fiber on the hippocampal-prefrontal projections was investigated. We could demonstrate that the hemodynamic response measured in the hippocampus of mice reached its maximum earlier compared to the hemodynamic response function in humans. An explanation for this observation may be the smaller body size and the faster metabolism of mice. A highly significant increase of the BOLD signal was found for the optogenetic stimulation of glutamatergic neurons in the left hippocampus and its projecting areas, like the contralateral hippocampus and prefrontal regions. Furthermore, a negative correlation of prefrontal activation and the fiber depth was measured, which may be explained by the larger amount of stimulated neurons. In the second part, we optogenetically stimulated oxytocin-releasing neurons either in the amygdala or in the paraventricular nucleus of rats. We hypothesize that changes in the oxytocin associated networks of the basal ganglia and the olfactory system should result from the optogenetic stimulation. Long-term changes in connectivity were investigated. Therefore, changes in correlations between different brain regions were calculated using the resting-state measurements before and after the optogenetic stimulation. Moreover, seed regions of defined functional networks were determined and voxel-based changes in resting-state correlation to the seed region were investigated. Additionally, short-term connectivity changes were examined in a psychophysiological interaction analysis as well as the direct activation induced by the laser stimulation. We found that both channelrhodopsin-2 groups showed increased connectivity in the olfactory and basal ganglia networks compared to the control group. However, no short-term network changes were observed comparing the laser on and laser off condition. This might be explained by the hormonal characteristics of oxytocin, leading to a more global and prolonged response. To summarize, this thesis demonstrates the successful combination of optogenetics and fMRI as a tool for basic neuropsychiatric research. It proves the successful manipulation, not only of single neurons, but also of neuronal networks in vivo by optogenetic stimulation

Frequency-selective control of cortical and subcortical networks by central thalamus

Abstract Central thalamus plays a critical role in forebrain arousal and organized behavior. However, network-level mechanisms that link its activity to brain state remain enigmatic. Here, we combined optogenetics, fMRI, electrophysiology, and video-EEG monitoring to characterize the central thalamus-driven global brain networks responsible for switching brain state. 40 and 100 Hz stimulations of central thalamus caused widespread activation of forebrain, including frontal cortex, sensorimotor cortex, and striatum, and transitioned the brain to a state of arousal in asleep rats. In contrast, 10 Hz stimulation evoked significantly less activation of forebrain, inhibition of sensory cortex, and behavioral arrest. To investigate possible mechanisms underlying the frequencydependent cortical inhibition, we performed recordings in zona incerta, where 10, but not 40, Hz stimulation evoked spindle-like oscillations. Importantly, suppressing incertal activity during 10 Hz central thalamus stimulation reduced the evoked cortical inhibition. These findings identify key brain-wide dynamics underlying central thalamus arousal regulation

Optogenetic Stimulation of the Basolateral Amygdala Increased Theta-Modulated Gamma Oscillations in the Hippocampus

The amygdala can modulate declarative memory. For example, previous research in rats and humans showed that brief electrical stimulation to the basolateral complex of the amygdala (BLA) prioritized specific objects to be consolidated into long term memory in the absence of emotional stimuli and without awareness of stimulation. The capacity of the BLA to influence memory depends on its substantial projections to many other brain regions, including the hippocampus. Nevertheless, how activation of the BLA influences ongoing neuronal activity in other regions is poorly understood. The current study used optogenetic stimulation of putative glutamatergic neurons in the BLA of freely exploring rats to determine whether brief activation of the BLA could increase in the hippocampus gamma oscillations for which the amplitude was modulated by the phase of theta oscillations, an oscillatory state previously reported to correlate with good memory. BLA neurons were stimulated in 1-s bouts with pulse frequencies that included the theta range (8 Hz), the gamma range (50 Hz), or a combination of both ranges (eight 50-Hz bursts). Local field potentials were recorded in the BLA and in the pyramidal layer of CA1 in the intermediate hippocampus. A key question was whether BLA stimulation at either theta or gamma frequencies could combine with ongoing hippocampal oscillations to result in theta-modulated gamma or whether BLA stimulation that included both theta and gamma frequencies would be necessary to increase theta–gamma comodulation in the hippocampus. All stimulation conditions elicited robust responses in BLA and CA1, but theta-modulated gamma oscillations increased in CA1 only when BLA stimulation included both theta and gamma frequencies. Longer bouts (5-s) of BLA stimulation resulted in hippocampal activity that evolved away from the initial oscillatory states and toward those characterized more by prominent low-frequency oscillations. The current results indicated that one mechanism by which the amygdala might influence declarative memory is by eliciting neuronal oscillatory states in the hippocampus that benefit memory

Functional Connectivity of the Rodent Striatum

The striatum serves as the major input nucleus of the basal ganglia circuitry, important for its varied roles in cognition, motivation, and sensorimotor function. Despite decades of study, fundamental features of the striatum’s functional organization and broader role(s) within the basal ganglia circuitry remain contentious and/or poorly defined. Given the diverse and critical roles of striatal activity in normal brain function and a multitude of disease states (including neurodegenerative and psychiatric disorders), a better understanding of this nucleus’ functional organization is imperative. The use of electrophysiological tools, which predominate the field, allow for in-depth characterizations of discrete, pre-selected brain regions, but are not appropriate for delineating functional neural circuit interactions on large spatial scales in an unbiased manner. A complementary approach to these studies is the use of functional magnetic resonance imaging (fMRI), which provides global, unbiased measures of functional neural circuit and network connectivity. In the first two studies described herein (Chapters 2 and 3), we used fMRI to map the functional response patterns to electrical DBS of the rat nucleus accumbens (NAc; ventral striatum), as well as the dual striatal outputs: external globus pallidus (GPe), and substantia nigra pars reticulata. Notable findings included the presence of negative fMRI signals in striatum during stimulation of each nuclei, robust prefrontal cortical modulation by NAc- and GPe-DBS, and marked functional connectivity changes by high frequency DBS. We next used optogenetic tools to more selectively map the brain-wide responses to stimulation of GPe neurons in healthy and Parkinson’s disease model rats (Chapter 4), as well as dorsal striatal neurons and their motor cortical inputs (Chapter 5). Optogenetic stimulation of each nuclei elicited an intriguing dorsal striatal negative fMRI signal, observed during direct striatal stimulation as well as putative recruitment of both excitatory both inhibitory striatal inputs, and thus suggestive of neurovascular uncoupling. Additionally, results from our GPe experiments revealed that this signal may be compromised in certain neurological disease states (e.g., Parkinson’s disease). Collectively, the studies described in this dissertation have exploited fMRI tools to reveal novel features of striatal connectivity, which may shed light on striatal function in health and disease.Doctor of Philosoph

Activation of the pro-resolving receptor Fpr2 attenuates inflammatory microglial activation

Poster number: P-T099 Theme: Neurodegenerative disorders & ageing Activation of the pro-resolving receptor Fpr2 reverses inflammatory microglial activation Authors: Edward S Wickstead - Life Science & Technology University of Westminster/Queen Mary University of London Inflammation is a major contributor to many neurodegenerative disease (Heneka et al. 2015). Microglia, as the resident immune cells of the brain and spinal cord, provide the first line of immunological defence, but can become deleterious when chronically activated, triggering extensive neuronal damage (Cunningham, 2013). Dampening or even reversing this activation may provide neuronal protection against chronic inflammatory damage. The aim of this study was to determine whether lipopolysaccharide (LPS)-induced inflammation could be abrogated through activation of the receptor Fpr2, known to play an important role in peripheral inflammatory resolution. Immortalised murine microglia (BV2 cell line) were stimulated with LPS (50ng/ml) for 1 hour prior to the treatment with one of two Fpr2 ligands, either Cpd43 or Quin-C1 (both 100nM), and production of nitric oxide (NO), tumour necrosis factor alpha (TNFα) and interleukin-10 (IL-10) were monitored after 24h and 48h. Treatment with either Fpr2 ligand significantly suppressed LPS-induced production of NO or TNFα after both 24h and 48h exposure, moreover Fpr2 ligand treatment significantly enhanced production of IL-10 48h post-LPS treatment. As we have previously shown Fpr2 to be coupled to a number of intracellular signaling pathways (Cooray et al. 2013), we investigated potential signaling responses. Western blot analysis revealed no activation of ERK1/2, but identified a rapid and potent activation of p38 MAP kinase in BV2 microglia following stimulation with Fpr2 ligands. Together, these data indicate the possibility of exploiting immunomodulatory strategies for the treatment of neurological diseases, and highlight in particular the important potential of resolution mechanisms as novel therapeutic targets in neuroinflammation. References Cooray SN et al. (2013). Proc Natl Acad Sci U S A 110: 18232-7. Cunningham C (2013). Glia 61: 71-90. Heneka MT et al. (2015). Lancet Neurol 14: 388-40

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information – whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia