Forebrain GABAergic cell-selective genetic manipulation in mice

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Since last year we have focused on the following four research questions. (1) How does oxidative stress exacerbate the behavioral phenotypes of Ppp1r2-Cre/fGluN1 KO mutants? We investigated whether oxidative stress is involved in the emergence of schizophrenia-like behaviors in this animal model. Consistent with this idea, post-weaning social isolation (PWSI) augmented oxidative stress levels particularly in cortical PV interneurons and precipitated anxiety-like behavior in 8 week old mutants, a point at which group-housed animals displayed no such behavior. Interestingly, in mPFC of adult mutants we found decreased expression (both mRNA and protein) of a master regulator of mitochondria energy metabolism and anti-oxidation, PGC1 (peroxisome proliferator activated receptor gamma coactivator 1), which is normally highly expressed in fast-spiking PV interneurons. PWSI exacerbated the down-regulation of PGC1 in the mutants. Therefore, elevation of oxidative stress level in mutants appears to be due to impaired anti-oxidant defenses system, which occurs as a downstream consequence of the depletion of NMDARs in early postnatal corticolimbic interneurons. Chronic treatment with the antioxidant/NOX inhibitor apocynin, beginning from postnatal 2 weeks, largely diminished the signs of oxidative stress and alleviated anxiety and the spatial working memory deficit. Our results suggest that oxidative stress plays a critical role in PV interneuron dysfunction presumably through PGC1 downregulation and in the pathogenesis of schizophrenia-like phenotypes (Jiang et al, Biol Psychiatry, 2013). We have also submitted a review manuscript entitled Convergence of genetic and environmental factors on parvalbumin-positive interneurons in schizophrenia to Frontier in Behav Neurosci. (2) Periodic auditory stimulation entrains the electro-encephalogram (EEG) to a specific phase and frequency, often referred to as the auditory steady-state response (ASSR). In both human and animal models, the ASSR has been used to assess the functional integrity of neural circuits that support synchronization. In order to investigate this phenomenon in a rodent model, we recorded LFPs directly from the primary auditory cortex of GABAergic interneuron-specific NMDAR hypofunction mice (Ppp1r2-Cre/fGluN1 KO mice), using the ASSR paradigm to assess both tone-evoked ASSRs and baseline LFP fluctuations. We then compared the relation of these two measures subjects. In mice with NMDAR hypofunction in cortical GABA neurons, we found that click train-evoked oscillatory LFP responses from auditory cortex are diminished. Specifically, these mutants show diminished power and phase locking at 40-Hz. Although the broadband baseline LFP power is also elevated, the signal-to-noise ratio reduction in the mutants appears to be largely due to external stimulis inability to evoke normal oscillatory power (Nakao et al., submitted). Because Ppp1r2-Cre-mediated genetic manipulation is confined to interneurons, it is unlikely that the defect is intrinsic to excitatory pyramidal neurons. Why, then, is the synchronized power of postsynaptic excitatory neurons diminished in Ppp1r2-cre/fGluN1 KO mutants? Postulating that the defect is one of synchrony between GABAergic inputs and excitatory neurons, we used double-patch recordings of sIPSCs from nearby pyramidal cells in tissue slices to test the theory. If synchronous GABAergic inputs are disturbed, IPSC events between nearby pyramidal neurons will be desynchronized. As expected, we found this to be true in mPFC slices from mutants at P28-P42. These results suggest that in our mutants, spike transmission, which must be precise to translate impulses into the correct postsynaptic responses, is disturbed. In order to identify the cellular mechanisms and impairments that best explain these mutant phenotypes, we assessed several mechanisms implicated in the desynchronization of IPSC events in postsynaptic pyramidal neurons. We applied brief pulses of current to presynaptic fast-spiking neurons to evoke unitary IPSCs in postsynaptic pyramidal neurons. This allowed us to see whether or not action potential impulses arriving at presynaptic terminals fail to evoke proper GABA release. In the mutants P28-P42, we found, GABA release is initially depressed, but subsequently IPSC amplitudes gradually increase. This pattern stands in sharp contrast to the unitary IPSC patterns observed in floxed controls, in which synapse transmission in response to the first impulse is robust and highly reliable. These results suggest that in these mutants, precise high-fidelity spike transmission at fast-spiking neuron/pyramidal neuron synapses is impaired (Zsiros et al, in preparation). (3) Is NMDAR hypofunction in interneurons really crucial for schizophrenia-related phenotypes? It is unclear whether NR1 ablation at other cell types, such as excitatory neurons, could also induce similar effects. Since last year we utilized the Cre/loxP system to generate transgenic conditional knockout mice (G35-3-Cre/fGluN1 KO), in which NR1 deletion is largely confined to cortical glutamatergic neurons. In contrast with interneuron-specific NR1 KO mice, we found that these mice do not exhibit comparable significant schizophrenia-like phenotypes in tests for working memory, social behavior, psychostimulant sensitivity, MK-801 response, and anhedonia or amotivation. Only prepulse inhibition (PPI) of acoustic startle was impaired in the mutants, suggesting attention deficits. This supports the refined hypothesis that NMDAR hypofunction must occur in cortical interneurons in order to model the hallmark symptoms of schizophrenia in rodents (Rompala et al, PLOS One, 2013). (4) What is the impact of GABA decrease or Gad67 reduction on physiology and behavior? To directly assess the impact of Gad67 reduction in psychiatric disorder phenotypes, using the same Ppp1r2-Cre line we have been characterizing a novel transgenic mouse line in which Gad67, was ablated following postnatal day 7 selectively in 50% of cortical and hippocampal interneurons. Gad67 expression was reduced by 50% with a concomitant increase in Gad65 and the GABA levels in the mutant cortex and hippocampus were also significantly reduced after adolescence. This manipulation recapitulated several specific features common to major depression including anhedonia, a lack of motivated behavior, and pronounced social withdrawal. However, there were no impairments in tests of behavioral despair or schizophrenia-related behaviors (prepulse inhibition and spatial working memory). Perhaps consistent with this specific behavioral phenotype, in the nucleus accumbens amphetamine-evoked dopamine release. In all, these results suggest a relationship between the anhedonia aspect of major depressive disorders and impairments in subcortical dopamine regulation through alterations in cortical interneuron networks (Kolata et al, submitted).