Probing the neural circuit basis of normal and disease-relevant cognitive function in mice

National Institute Of Neurological Disorders And Stroke

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Abstract

In the past fiscal year, we refined our capacity for conducting high-fidelity electrical recordings from multiple brain regions in freely behaving mice. We optimized electrode construction/implantation and pipelines for electrophysiological data analysis. We diversified our mouse colony to include unique dual- and triple-transgenic strains. We also established and refined our use of simultaneous optogenetic manipulation of neurons with various neuronal recording techniques (electrophysiology, fiber photometry, and in vivo fluorescent imaging) in behaving mice. We also created a pipeline for high-resolution confocal imaging and quantitative analysis of excitatory synapses in genetically defined neuronal populations. We applied these and other tools to advance understanding in three key domains: 1. Oscillatory synchrony and SWM in mouse models relevant to psychiatric disease. Dysregulated GSK3 signaling is implicated in schizophrenia and other psychiatric disorders. Our previous work showed that early postnatal treatment with a nonselective GSK3 (alpha and beta isoform) inhibitor can rescue deficits in SWM task acquisition and vHPC-mPFC synchrony in adult male Df(16)A+/ mice. Given reports of isoform-specific and sex-divergent GSK3 signaling, we characterized the effects of novel selective GSK3-alpha and beta inhibitors on SWM and vHPC-mPFC synchrony in male and female wild-type (WT) and Df(16)A+/ mice. GSK3-beta inhibition from P7-P28 rescued deficits in SWM task acquisition in adult male but not female Df(16)A+/ mice, and modulated task-related vHPC-mPFC theta coherence. Conversely, GSK3-alpha inhibition did not rescue deficits in task acquisition but did reverse deficits in task performance under increased SWM demand in male and female Df(16)A+/ mice. RNA sequencing of mPFC and vHPC tissue from WT and Df(16)A+/ mice has revealed sex-, genotype- and age-specific transcriptomic profiles that inform the molecular basis of our behavioral and neurophysiological findings. Heterozygous null mutations of SETD1A are definitively linked to increased risk for schizophrenia. Mice carrying similar mutations reportedly show deficits in SWM. To characterize the neurophysiological consequences of SETD1A haploinsufficiency, we examined synchrony between the mPFC, vHPC, dorsal hippocampus, and nucleus reuniens (Re) of the thalamus, prior to and during learning and performance of a SWM task. Contrary to a report of SWM deficits in Setd1a+/ mice, we observed that Setd1a+/ mice acquired and performed the SWM task comparably to WT controls. We included female mice in our study, and found they performed comparably to males. Our neural recordings revealed that Setd1a+/ mice show enhanced vHPC-mPFC broadband synchrony, enhanced vHPC-Re gamma synchrony, and reduced Re-mPFC broadband synchrony compared to WT mice. These findings implicate disruptions in prefrontal-thalamo-hippocampal synchrony in SETD1A-related genetic predisposition to schizophrenia. 2. vHPC-mPFC microcircuit interactions and plasticity. Inhibiting somatostatin-positive interneurons (SST-INs) in mPFC or vHPC-mPFC inputs during SWM encoding impaired vHPC-mPFC synchrony, mPFC neuron spatial encoding, and task performance. These impairments mimic those seen in Df(16)A+/ mice. This phenotypic convergence raised three questions: (1) How do vHPC inputs interact with mPFC inhibitory microcircuits in vivo? (2) Are these interactions disrupted in Df(16)A+/ mice? And (3) are these interactions plastic, offering a potential path to correcting circuit dysconnectivity? To address these questions, we used an all-optical approach to characterize in vivo dynamics and activity-induced plasticity of discrete mPFC interneuron population responses to vHPC input stimulation in wildtype and Df(16)A+/ mice. vHPC inputs were optogenetically stimulated and postsynaptic Ca2+ responses in mPFC SST-, VIP- and PV-INs were monitored with fiber photometry. SST-IN responses to vHPC terminal stimulation were weak at baseline in WT and Df(16)A+/ mice, but progressively increased over 50 days of minimal, periodic optogenetic stimulation. The potentiation of SST-IN Ca2+ responses was blunted in Df(16)A+/ relative to WT mice, and partially recovered with additional high-frequency optical vHPC input stimulation. In contrast, VIP- and PV-IN responses to vHPC input stimulation were initially strong but rapidly suppressed in WT and Df(16)A+/ mice that received additional high-frequency optical stimulation. Ongoing in vivo Ca2+ imaging is probing these divergent forms of plasticity at the level of neuronal ensembles. We are also exploring synaptic mechanisms of this plasticity and their influence on SWM performance. By reshaping the recruitment of mPFC INs by long-range inputs, these forms of plasticity stand to bias the routing of pathway-specific information through mPFC and may be leveraged to influence cognition-relevant circuit function and dysfunction. 3. Real-time modulation of cognition-relevant oscillatory synchrony. Dynamic changes in vHPC-mPFC synchrony correlate with various behaviors, but the causal contributions of such synchrony to behavior is unclear. Our prior work tested the causal contributions of theta-frequency synchrony to avoidance behavior using sinusoidal open-loop optogenetic stimulation of vHPC-mPFC projections during exploration of the elevated plus maze. We found a privileged role for vHPC-mPFC theta-frequency communication in generating avoidance behavior and showed direct evidence that synchronized oscillations play a role in facilitating neural transmission and behavior. Building on this work, we used open-loop sinusoidal optogenetic stimulation to characterize properties of vHPC-mPFC oscillatory dynamics more systematically. We recorded local field potentials (LFP) and/or single units from vHPC and mPFC during sinusoidal stimulation of ChR2 in vHPC-mPFC inputs. By systematically varying the frequency and amplitude of the optical stimulation, we revealed sensitivities of the vHPC-mPFC physiological response to our oscillatory manipulations. mPFC and vHPC oscillatory power were preferentially increased in response to open-loop sinusoidal stimulation at frequencies less than 15 Hz and above 10 Hz, respectively. vHPC-mPFC coherence was preferentially enhanced at frequencies above 10 Hz. These findings suggest a frequency-biased responsivity in vHPC-mPFC circuits whereby vHPC-mPFC input activity may preferentially promote synchrony at higher (e.g. gamma) frequencies over lower (e.g. theta) frequencies. A strong test of the causal contributions of long-range synchrony per se to cognitive function would be to promote (or disrupt) synchrony through real-time modulation of endogenous task-relevant oscillatory activity. To this end, we developed a novel closed-loop stimulation approach whereby we optogenetically activate vHPC projections to mPFC in a manner informed by ongoing vHPC theta-frequency oscillations. ChR2 or GFP was expressed in vHPC-mPFC terminals in WT mice, and blue laser light was delivered to the mPFC through optic fibers. Laser output was governed by the theta-filtered (7 Hz), half-wave rectified vHPC LFP, which could be delivered at various phase delays relative to ongoing vHPC theta oscillations. Closed-loop sinusoidal stimulation delivered near-synchronously with vHPC theta dynamics robustly enhanced vHPC-mPFC theta coherence in ChR2- but not GFP-expressing mice, with diminished effects at longer phase delays. Ongoing studies are examining the effects of our closed-loop approach on SWM performance. These findings demonstrate the promise of closed-loop optogenetics to probe the causal contributions of long-range oscillatory synchrony to cognition and behavior.