The mechanisms that regulate the strength of synaptic transmission and intrinsic neuronal excitability are well characterized; however, the mechanisms that promote disease-causing neural network dysfunction are poorly defined. We generated mice with targeted neuron type-specific expression of a gain-of-function variant of the neurotransmitter receptor for glycine (GlyR) that is found in hippocampectomies from patients with temporal lobe epilepsy. In this mouse model, targeted expression of gain-of-function GlyR in terminals of glutamatergic cells or in parvalbumin-positive interneurons persistently altered neural network excitability. The increased network excitability associated with gain-of-function GlyR expression in glutamatergic neurons resulted in recurrent epileptiform discharge, which provoked cognitive dysfunction and memory deficits without affecting bidirectional synaptic plasticity. In contrast, decreased network excitability due to gain-of-function GlyR expression in parvalbumin-positive interneurons resulted in an anxiety phenotype, but did not affect cognitive performance or discriminative associative memory. Our animal model unveils neuron type-specific effects on cognition, formation of discriminative associative memory, and emotional behavior in vivo. Furthermore, our data identify a presynaptic disease-causing molecular mechanism that impairs homeostatic regulation of neural network excitability and triggers neuropsychiatric symptoms. IntroductionResearch has established a solid basis for our understanding of how different nerve cells interact, assemble into functional units, and influence behavior and mood (1-4). High-frequency oscillation of the neuronal membrane potential creates permissive time windows for induction of sensory context-dependent bidirectional plasticity of glutamatergic synaptic transmission (1, 5, 6), which is a synaptic correlate of discriminative associative memory (6-9). Thus, temporal precision of neuronal inputs relative to the actual membrane potential is an important determinant of information coding and memory formation (5, 10-12). GABAergic synaptic transmission is equally relevant for cognitive function, because GABAergic interneurons regulate neuronal excitability and provide a spatiotemporal control framework for the timing of synaptic glutamatergic transmission. Fast-spiking (parvalbumin-positive) interneurons, for example, regulate hippocampal neural network oscillation in cognitively relevant high-gamma frequency ranges (13,14). In conjunction with other interneuron types, they form a precision clockwork without which cortical operations are not possible (15,16). Thus, spatiotemporal coordination of glutamatergic and GABAergic synaptic transmission is essential for sensory processing and cognitive performance.

Cholinergic neuromodulation in the hippocampus controls the salience of background context memory acquired in the presence of elemental stimuli predicting an aversive reinforcement. With pharmacogenetic inhibition we here demonstrate that hilar perforant path-associated (HIPP) cells of the dentate gyrus mediate the devaluation of background context memory during Pavlovian fear conditioning. The salience adjustment is sensitive to reduction of hilar neuropeptide Y (NPY) expression via dominant negative CREB expression in HIPP cells and to acute blockage of NPY-Y1 receptors in the dentate gyrus during conditioning. We show that NPY transmission and HIPP cell activity contribute to inhibitory effects of acetylcholine in the dentate gyrus and that M1 muscarinic receptors mediate the cholinergic activation of HIPP cells as well as their control of background context salience. Our data provide evidence for a peptidergic local circuit in the dentate gyrus that mediates the cholinergic encoding of background context salience during fear memory acquisition.

Parvalbumin-positive (PV) basket cells provide perisomatic inhibition in the cortex and hippocampus and control generation of memory-related network activity patterns, such as sharp wave ripples (SPW-R). Deterioration of this class of fast-spiking interneurons has been observed in neuropsychiatric disorders and evidence from animal models suggests their involvement in the acquisition and extinction of fear memories. Here, we used mice with neuron type-targeted expression of the presynaptic gain-of-function glycine receptor RNA variant GlyR α3L185L to genetically enhance the network activity of PV interneurons. These mice showed reduced extinction of contextual fear memory but normal auditory cued fear memory. They furthermore displayed increase of SPW-R activity in area CA3 and CA1 and facilitated propagation of this particular network activity pattern, as determined in ventral hippocampal slice preparations. Individual freezing levels during extinction and SPW-R propagation were correlated across genotypes. The same was true for parvalbumin immunoreactivity in the ventral hippocampus, which was generally augmented in the GlyR mutant mice and correlated with individual freezing levels. Together, these results identify PV interneurons as critical cellular substrate of fear memory persistence and associated SPW-R activity in the hippocampus. Our findings may be relevant for the identification and characterization of physiological correlates for posttraumatic stress and anxiety disorders.

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