Samuel McKenzie - US grants

? DESCRIPTION (provided by applicant): Basic questions remain as to how information is conveyed by neural activity. In the hippocampus, the firing rate of single cells and the temporal ordering of those cells relative to one another correlate with experienced and planned spatial trajectories. These correlations suggest that the hippocampus supports memory function using both a rate code and a temporal code, but a causal link between temporal coding and memory is lacking. To establish if the temporal ordering of cells conveys information to the rest of the brain, it is necessary to perform simultaneous recordings from the hippocampus and an output region. Here, I propose three experiments in which I will record and optogenically stimulate the hippocampus while recording from one of its main post- synaptic cortical targets, the subiculum. Specific Aim 1 is to provide correlative evidence that, in the absence of stimulation, subicular cells are sensitive to hippocampal temporal coding. Statistical modeling of subicular activity wil test whether a significant amount of variance in firing rate can be explained by sequences of activity in area CA1 of the hippocampus. Specific Aim 2 will test whether subicular firing patterns are affected by disruptions to the hippocampal temporal code that is defined by small differences in spike timing. Spike-timing disruption will be achieved either directly by expressin light-activated opsins that excite pyramidal cells, or indirectly by expressing light-activated opsins that silence interneurons. If alterations in subicular activity occur when spike-timing in the hippocampus is disrupted, this would provide strong evidence that the temporal code can bias the activity of post-synaptic targets and therefore likely conveys information. Finally, Specific Aim 3 seeks to link disruption of fine spike-timing to memory. In this experiment, broader optogenetic stimulation of CA1 will be used and memory will be tested in a delayed alternation paradigm that depends upon the hippocampus. I predict that stimulations during the delay that disrupt the order in which hippocampal cells fire will also affect memory. The aim of these experiments is to show that information is conveyed from the hippocampus to downstream regions using a temporal code. Confirming a causal link between temporal coding and memory would greatly expand estimates of the hippocampal storage capacity and may offer insights into stimulation protocols that enhance memory.

Project Summary The aim of this project is to study the synaptic mechanisms that allow particular patterns of neural activity to become reinstated. In hippocampal circuits, the sequential pattern of neural activity observed during behavior is later replayed during oscillatory bursts of activity known as sharp-wave ripples (SPW-Rs). Computational models show that replay could arise due to plasticity of glutamatergic synapses onto excitatory neurons. However, such excitatory-excitatory connections are weak in hippocampal area CA1, where SPW-R replay is observed. Therefore, SPW-R replay in CA1 may be inherited from upstream region CA3, which has dense excitatory recurrents. Alternatively, it is possible that plasticity in inhibitory circuits supports changes in SPW-R dynamics. This grant will use SPW-R replay to study how neural patterns are learned and recalled. In the K99 Aims, I will examine whether neural activity is sufficient and synaptic plasticity necessary for subsequent neural reactivation during SPW-Rs. I will artificially induce patterns of activity in areas CA1 and CA3 and test whether those patterns are reactivated in the proceeding SPW-Rs and whether reactivation is restricted to recurrent-dense CA3. Next, I will test for the integrity of SPW-R replay while blocking synaptic consolidation in CA1 pyramidal cells. Replay disruptions would point to an unexpected role of CA1 plasticity in defining replay sequences. The R00 portion of the grant focuses on whether replay depends on synaptic plasticity in inhibitory circuits. First, I will establish whether the synaptic connectivity between CA1 pyramidal cells and interneurons changes with repetitive pairings in vivo. Next, I will block synaptic consolidation in CA1 GABAergic neurons to assess whether replay is also disrupted. Such a finding would demonstrate a novel role for plasticity in inhibitory circuits in defining network dynamics. Together, these experiments offer a direct test of the hypothesis that synaptic plasticity amongst a population of co-active neurons (excitatory and inhibitory) promotes subsequent reactivation of that population. To study how synaptic connectivity affects circuit dynamics, this grant combines, for the first time, cell-type specific control of synaptic consolidation and in vivo electrophysiology. The proposed training will set the foundation for a career that studies memory on the level of behavior, circuit dynamics, and synaptic function. The proposed experiments aim to inform clinical use of pharmacology and artificial neural stimulation to aid learning and recall in people with diseases that cause memory deficits.

The aim of this project is to study why rapid eye movement (REM) sleep is an anticonvulsant state and to test whether stimulation of REM-promoting brain regions prevents seizures. Experiments will focus on the pedunculopontine nucleus (PPT), a midbrain cholinergic region that densely innervates the thalamus. Electrophysiological and optical recordings will be done in the thalamus, neocortex, and hippocampus in healthy and epileptic rats. The central motivating hypothesis is that REM is a neuroprotective state due to the wide-spread cortical asynchrony observed in this state which arises, in part, due to cholinergic signaling in the thalamus. Specific Aim 1.1 will test how electrical stimulation of the PPT affects acetylcholine binding in the thalamus and the firing patterns of cortical and thalamic neurons in healthy rats. These experiments will establish a database of how the brain reacts to different kinds of stimulations so that if a therapeutic protocol is discovered, its neurophysiological mechanism of action can be better understood. Specific Aim 1.2 will test whether these same stimulation protocols change the seizure threshold in the kindling model of epilepsy. I hypothesize that those stimulations that induce strong thalamic acetylcholine binding will also be those that induce cortical asynchrony and most effectively suppress seizure spread in the evoked kindling model. Specific Aim 2 will then test whether electrical stimulation of the PPT is effective in suppressing seizures in a chronic epilepsy model induced by intra-hippocampal kainic acid injection. Seizures will be predicted online, and stimulation of the PPT will be given when seizures are likely. These experiments will further our understanding of the link between seizures and sleep and will guide future clinical studies to assess whether REM promoting brain regions should be targeted in patients suffering from epilepsy.