Dwayne Godwin - US grants

DESCRIPTION (provided by applicant): The dorsal lateral geniculate nucleus (LGN) is a vital link in the chain of perception. Retinal ganglion cells encode the visual world and transmit it to the LGN. Most agree that gating of this information occurs at the retinogeniculate synapse, but a complete mechanism is elusive. Our recent findings demonstrate several new and exciting sources of dynamic control of visual information at the LGN. Two of these are known modulators of relay neurons: the cholinergic parabrachial brainstem (PBR) and the massive corticogeniculate (CG) feedback. The PBR releases nitric oxide (NO), and our data indicate an amazing difference in the way NO affects retinal and cortical inputs. NO has a powerful and selective inhibitory influence on retinogeniculate transmission in the LGN through interaction with the NMDA receptor; however, transmission through the CG pathway is enhanced by two separate mechanisms. The other great remaining mystery of thalamic function is the purpose of the cortical feedback to LGN from layer 6, which we now see as intimately linked with the PBR and NO. We propose a new series of experiments to reveal the contributions of the PBR, and cortical feedback influences, with a global hypothesis that vision requires cooperative activity patterns of both of these pathways. Aim 1: How does NO affect LGN relay cell membrane properties? NO suppresses NMDA receptor function in the retinogeniculate pathway. We hypothesize that NO also targets the low threshold Ca2+ current (I(T)) and a key K+ currents (I) (As)). We will probe I(T) and I(As) with intracellular patch recordings from slices of the LGN, while delivering NO donors and scavengers, and by stimulating intrinsic NO production through the enzyme bNOS. Aim 2: How does NO affect fast synaptic inputs to LGN relay cells? We will stimulate the retinal and CG pathways (to evoke GABAergic IPSP/JPSCs) and the CG pathway (to evoke glutamatergic EPSP/EPSCs) in slices while manipulating NO levels. We hypothesize a stark difference in how NO affects retinal and cortical EPSP/EPSCs; cortical EPSP/EPSCs are enhanced, indicating that the PBR, through NO, may shift the balance away from retinal feedforward processing and toward cortical feedback processing. Aim 3: How does the corticogeniculate projection control the thalamocortical dialog? Relay cells respond to retinal inputs in one of two modes, burst or tonic. We hypothesize that layer 6 promotes a visuotopic gradient of burst and tonic responses, and synchronous firing, in the LGN. We will record from thalamic ensembles during visual processing, while activating and inactivating layer 6 of visual cortex, and while activating brainstem pathways. The impact of retinal inputs to LGN is well known, but the field is struggling with the nature and scope of extraretinal synaptic influences. The answers to these questions will completely transform our view of thalamic function from that of a slave of the periphery to a partner with cortex in binding together the threads of visual perception.

DESCRIPTION (provided by applicant): Drinking alcohol makes you sleepy. For some insomniacs, this effect is the pathway to bedtime alcohol consumption and eventual abuse. Sleep disturbances are common in alcoholic patients, with a number of serious health consequences. The most prominent and best understood of brain rhythms are the spindle waves associated with Stage II sleep, and this specific form of sleep is enhanced in response to acute alcohol administration. Perhaps the most promising brain region in which to explore alcohol influences on sleep - the thalamus- has been so far ignored. The thalamus is a primary generator of sleep/wake cycles and the brain rhythms that are the hallmark of sleep staging. Slices of the ferret thalamus possess all of the necessary circuitry for the generation of spindle waves. The mechanisms underlying spindle wave generation are known to depend on specific synaptic activation patterns of GABAergic circuitry within the thalamus', with both ascending and descending control from the brainstem and cortex, respectively. GABAergic and glutamatergic systems (particularly NMDA) are known targets of ethanol, and synaptic transmission is therefore our primary target in this proposal. Ethanol has been shown to potentiate evoked GABAa IPSCs in a number of brain regions, via several known mechanisms, including enhancement of the underlying GABAa receptor-mediated channel conductance. NMDA influences are known to entrain thalamic rhythms. The following specific aims will determine the influence of ethanol on the spindle wave circuitry of the thalamus, and will examine GABAa, and NMDA mediated synaptic transmission as touchstones of these effects: Aim 1: We will examine the influence of ethanol on GABAa receptor-mediated IPSPs and IPSCs within the thalamus using intracellular recording techniques. We hypothesize that ethanol will potentiate the amplitude of GABAa IPSPs and IPSCs by postsynaptic mechanisms that favor the generation of spindle waves, as predicted by our preliminary modeling data. Aim 2: We will examine the influence of ethanol on NMDA receptor- mediated EPSPs and EPSCs within the thalamus. Stimulation of the corticothalamic pathway specifically activates glutamate receptors and can synaptically synchronize spindle waves. We hypothesize that ethanol will attenuate NMDA receptor-mediated potentials, disrupting cortical control of spindle waves, consistent with our preliminary data. Aim 3: We will examine the effect of ethanol on a low threshold calcium current that is vital to spindle oscillations. Our preliminary data show an enhancement of this current during ethanol exposure, which could underlie increases in sleep spindles by ethanol. This research is an opportunity to work out the mechanisms underlying reported acute perturbations of normal sleep by ethanol in a new, yet well-characterized and accessible model system. These acute changes may set the stage for disruption of sleep due to chronic abuse, disruptions that last well beyond withdrawal. Because the targets of ethanol to be examined here are vital links to ethanol's influence in other systems, our results will extend to basic mechanisms of ethanol effects in the CNS as a whole.

Sleep perturbations by ethanol play a key role in the progression of alcoholism, and are predictive of relapse. For some insomniacs, the sedative effects of ethanol are the pathway to bedtime alcohol consumption and eventual abuse. Continued abuse of ethanol leads to long-term changes in sleep circuitry that last well beyond the cessation of ethanol administration. The overall goal of this project is to determine the key molecular events that underlie cellular adaptation of sleep circuitry to alcohol, with the eventual goal of identifying novel drug targets for treatment of ethanol's disruption of normal sleep. The thalamus is a primary generator of sleep/wake cycles and the brain rhythms that occur during sleep. The best understood of these rhythms is the thalamic spindle oscillation associated with Stage II sleep, which is enhanced in response to acute alcohol administration. In alcoholics, spindle waves are diminished and are replaced with less-restful random eye movement (REM) sleep. Understanding the fate of spindle wave sleep is thus a vital question that directly relates to the reinforcement effects of ethanol, since some alcoholics return to drinking in an effort to improve the quality of their sleep. No laboratory has addressed the mechanism of these changes in a primate model, or in animals with well-characterized drinking schedules. The engine that allows spindle waves to flow through the brain is a low threshold calcium current, mediated by T-type calcium channels that come in three known varieties. We have recently shown that the T-channel transcript found in thalamic relay cells is exquisitely sensitive to ethanol, showing acute enhancement at the low end of physiologically meaningful ethanol concentrations (10-17mM). Amazingly, this current appears to be inhibited by concentrations of ethanol much above this range. In Aim 1 of this project we will determine whether the T channel is functionally impaired in the dorsal lateral geniculate nucleus of chronic drinking animals by performing whole cell patch recordings in two preparations, monkeys and rats, in a condition of excessive drinking and in a condition near the peak of withdrawal. In Aim 2, we will determine with patch recordings whether the T channel function is reduced in the thalamic reticular nucleus of chronic drinking animals. The experiments of Aim 3 will provide a complementary examination of the molecular expression patterns of T-type channels in monkeys and rats using quantitative RT-PCR techniques.

DESCRIPTION (provided by applicant): One of the most compelling and unresolved issues of visual processing is the role of the massive cortical feedback that occurs in a number of sites in the visual system. Activation of this feedback is thought to underlie processes such as attention. It may also serve as a partial solution to the binding problem, which has been proposed to rely on correlated firing in ensembles of neurons. The earliest cortical feedback does not occur in cortex at all, but in the thalamus. Similar to feedback systems in the cerebral cortex, corticothalamic feedback is characterized by extensive reentry of processed data from Layer 6 of cortex to lower processing levels in sensory thalamic nuclei such as the lateral geniculate nucleus (LGN). We propose to characterize the rules of communication between corticothalamic neurons and their target thalamic relays (e.g., what proportion of cells engage, and what patterns of activity promote this communication?);how might the resulting activity patterns within the thalamus be expressed topographically (e.g., does feedback promote certain types of activity in certain topographic regions and not others?);and how are the differences in these patterns critical in allowing certain kinds of information access to cortex (e.g., thalamic neurons have two firing regimes: burst and tonic, will these have differing impact at thalamocortical (TC) synapses?). Is information selectively affected in well-known physiological types of neurons? (e.g., ON/OFF, X/Y-like, and/or eye specific information?) To answer these questions, multielectrode arrays will be used to record thalamic ensembles and their ongoing dialog with Layer 6 of cortex. We will test the following hypotheses: (H1) that Layer 6 feedback to the LGN will produce correlations among ensembles of LGN cells that depend upon visuotopic register with spiking Layer 6 neurons;(H2) that LGN cells will preferentially activate Layer 6 neurons under certain firing conditions and activity patterns;and (H3) that brainstem activation will increase correlated firing across LGN neurons in response to Layer 6 feedback. The purpose of corticothalamic interactions is an enduring mystery, but one whose resolution will yield fundamental details of the defining role of feedback in neural systems. In addition to the details of normal corticothalamic processing, our proposed studies will provide a foundation for understanding abnormal thalamic network states in CNS disorders such as epilepsy, chronic pain and Parkinson's disease. Relevance: Information from peripheral sensory systems is processed at ascending stages of the thalamus and cortex. The cortex sends a massive projection back to lower processing levels of the brain, but the function of this feedback is poorly understood. This project proposes the detailed examination of how the cortex may instruct the thalamus to allow certain types of information to proceed to higher processing stages based on specific timing of stimulation or arrangement of objects in space. By recording from large numbers of brain cells in both regions simultaneously, we will discover rules that the regions use to communicate. These are perturbed in a number of brain disorders, thus the information we will learn about the basic rules of communication is vital for the understanding of these abnormal brain conditions.

DESCRIPTION (provided by applicant): There are fundamental gaps in understanding how alcoholism leads to alcohol withdrawal syndrome, with its range of serious clinical symptoms, including profound seizures. Withdrawal seizures become progressively worse with each withdrawal, complicate treatment, and can be fatal. It is therefore vital to idenify safe and effective treatment alternatives. Calcium channels may offer such an alternative for treatment. The long-range goal of this work is to better understand the cellular and molecular mechanisms underlying alcohol withdrawal. The immediate goals of this proposal are: 1) to define the cellular and molecular alterations in T-type Ca2+ channels that occur in response to multiple WDs~ 2) to link these changes to the development of seizure and other features of alcohol WD syndrome~ 3) to examine whether treatments targeting these channels are effective~ 4) to understand how the acute sensitivity of the T-channel isoform implicated in our studies relates to alcohol WD syndrome and seizures involving this same channel~ and 5) to determine how relevant brain circuitry is recruited into WD seizure. Guided by strong preliminary data and published findings, and enabled by an innovative method of studying WD seizures, the following specific aims will be pursued: Aim 1 will determine how acute inhibition of T-type channels by ethanol leads to WD hyperexcitability. Using the finding that PKC mediates the acute inhibitory effect of ethanol, whole cell recordings will be used to test the hypothesis that PKC inhibitors will reveal a change in ethanol-mediated inhibition of T-type currents during WD. Aim 2 wil determine the contribution of T-type channels to seizures resulting from multiple ethano withdrawals. Quantitative EEG recordings will be used to test whether T-type channel blockers will inhibit WD seizure and will protect against the progression of seizures due to multiple, intermittent WDs. Aim 3 will determine the site of origin and propagation patterns of WD seizure. The progression of WD seizure will be characterized using in vivo, multisite recordings coupled with novel optogenetic activation of this network using channelrhodopsin 2. The studies are highly significant because of the vertically integrated investigation of the role of T-type calcium channels in alcohol withdrawal seizure, which is an enduring problem in the treatment of abstaining alcoholics. The principles of channel modulation and network involvement promise to extend to other neural systems that include these channels. The studies are innovative because they deliver a new optogenetic approach to studying WD seizures, which will be used to understand the progression of WD seizure in unprecedented detail, and a potential new therapy based on a current antiepileptic drug.

Project Summary Gene therapy is an emerging treatment strategy for epilepsy that promises to dampen activity in specific seizure related circuitry in order to prevent or lessen the intensity of seizures. Finding the appropriate target for delivery represents a significant challenge for preclinical seizure models. In order to optimize delivery parameters, multiple strategies, locations, and doses must be compared. We have developed a novel screening tool that uses optogenetic intensity-response curves to precisely determine thresholds for population discharge (aka. interictal spikes), the oPDT. Once this threshold is known, suprathreshold stimulus trains of varying length can be used to determine an after discharge threshold, a measure of seizure susceptibility. These two metrics can be collected in the same animals, compared, and tracked. Thresholds vary predictably with behavioral state (sleep/wake), but are stable over time allowing for multiple within subject experiments. A chronic multi-site array in hippocampus and connected structures allows for detection of network wide stimulus responses and also continuous monitoring of normal activity. We propose to test and optimize two promising gene therapy strategies using our optogenetic thresholding technique, in non-epileptic animals, in order to assess therapeutic potential. Kv1.1 overexpression in neurons reduces excitability by raising the functional threshold for activation and decreasing burst production. Kir4.1 overexpression in astrocytes improves their ability to absorb extracellular K+, preventing K+ build up and the resulting ictogenesis. In Aim 1, we will locate effective target areas and optimize the dose of Kv1.1 in order to balance efficacy with impairment of normal function. An AAV vector developed by our collaborator Edward Perez-Reyes will be used to deliver Kv1.1. Baseline activity, the oPDT, and the oADT will be tracked over time with multiple measurements taken before expression occurs (<2 weeks), while expression builds (2-6 weeks), and when expression levels stabilize (>6 weeks). Changes in these metrics over time will reveal important information about the circuit level effects of Kv1.1 overexpression and its viability as a treatment for epilepsy. In Aim 2, we will determine if Kir4.1 overexpression in astrocytes is sufficient to reduce seizure suseptability. An AAV vector specific for astrocytes (using the GFAP promoter), will be used to overexpress Kir4.1. Success in these experiments will help to identify potential targets for therapeutic intervention, assess the therapeutic window, and provide critical clues about the nature of population discharge and seizure generation.