bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Acetylcholine disinhibits hippocampal circuits to enable rapid formation of overlapping memory ensembles Luke Y. Prince1,5, Krasimira Tsaneva-Atanasova2,3, Claudia Clopath4, and Jack R. Mellor1 1 Centre for Synaptic Plasticity, School of Physiology Pharmacology, and Neuroscience, University of Bristol, BS8 1TD, UK 2 Department of Mathematics and Living Systems Institute, University of Exeter, Exeter, EX4 4QF, UK 3 EPRSC Centre for Predictive Modelling in Healthcare, University of Exeter, Exeter, EX4 4QJ, UK 4 Bioengineering Department, Imperial College London, London, SW7 2AZ, UK 5 Current address: Department of Biological Sciences, University of Toronto Scarborough, Toronto, ON, M1C 1A4, Canada Correspondence: [email protected] Keywords: acetylcholine, hippocampus, computational modelling, electrophysiology, neuronal ensembles, dentate gyrus, CA3. 1 bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Abstract In the hippocampus, episodic memories are thought to be encoded by the formation of ensembles of synaptically coupled CA3 pyramidal cells driven by sparse but powerful mossy fiber inputs from dentate gyrus granule cells. Acetylcholine is proposed as the salient signal that determines which memories are encoded but its actions on mossy fiber transmission are largely unknown. Here, we show experimentally that cholinergic receptor activation suppresses feedforward inhibition and enhances excitatory–inhibitory ratio. In reconstructions of CA3 pyramidal cells, this disinhibition enables postsynaptic dendritic depolarization required for synaptic plasticity at CA3-CA3 recurrent synapses. We further show in a spiking neural network model of CA3 how a combination of disinhibited mossy fiber activity, enhanced cellular excitability and reduced recurrent synapse strength can drive rapid overlapping ensemble formation. Thus, we propose a coordinated set of mechanisms by which acetylcholine release enables the selective encoding of salient high- density episodic memories in the hippocampus. 2 bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. Introduction The hippocampus plays a central role in the formation of episodic memories by processing information from the entorhinal cortex sequentially through the dentate gyrus, CA3 and CA1 regions. Anatomical, functional and theoretical considerations propose separate computational properties for each of these regions in support of memory processing (Marr, 1971; McClelland and Goddard, 1996; Kesner and Rolls, 2015). In particular, the CA3 region is characterized by a recurrently connected set of excitatory pyramidal neurons, which are believed to encode auto-associative memories by selectively strengthening recurrent synapses between ensembles of neurons that provide a neural representation of the memory (Nakazawa et al., 2002; Rebola et al., 2017). Configuring a recurrent network in this way is proposed to endow the network with attractor dynamics, in which the network is driven towards a stable state of ensemble formation (Marr, 1971; Hopfield, 1982; Kesner and Rolls, 2015). This process is related to memory retrieval, in which external sources of input will alter the state of the network by activating subsets of neurons and through recurrent dynamics will be driven towards these stable states in which all neurons in the ensemble are reactivated – a process also referred to as pattern completion (Gold and Kesner, 2005; Yassa and Stark, 2011). Within this framework, memory encoding is believed to be the procedure of altering the network through synaptic plasticity to create or change the position of attractor states (Treves and Rolls, 1994; Tsodyks, 1999). However, not all memories are stored, indicating that there may be a gate to select which experiences should be encoded, but it is unclear how such a filter might operate. One potential filter mechanism is the release of neuromodulators such as acetylcholine that can rapidly reconfigure neuronal networks (Hasselmo, 2006; Prince et al., 2016). Acetylcholine is thought to promote encoding of new information by facilitating NMDA receptor function and induction of synaptic plasticity (Markram and Segal, 1992; Marino et al., 1998; Buchanan et al., 2010; Fernandez de Sevilla and Buno, 2010; Gu and Yakel, 2011; Dennis et al., 2016; Papouin et al., 2017) and selectively suppressing recurrent activity representing previously stored information in favour of feed-forward activity representing novel information (Hasselmo et al., 1995; Hummos et al., 2014). These properties are predicted to facilitate the encoding of new memories and allow greater overlap between representations (Hasselmo et al., 1995; Hasselmo, 2006). Commensurate with the concept of acetylcholine being important for encoding new information, acetylcholine release in the hippocampus and neocortex is associated with 3 bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. reward and arousal during periods of behavioural activity when it is beneficial to learn associated information in the environment to gain future rewards (Hangya et al., 2015; Teles-Grilo Ruivo et al., 2017). An alternative way to conceptualize the role of acetylcholine is as a signal for uncertainty, which may be resolved by familiarization through learning (Yu and Dayan, 2005). In either scenario, salience is indicated by increasing the release of acetylcholine in the hippocampus and other brain areas. The dentate gyrus receives excitatory glutamatergic input from layer II of the medial entorhinal cortex, and sparsifies this signal by suppressing the activity of most dentate gyrus granule cells through lateral inhibition while dramatically increasing the firing rate of a select few granule cells (O'Reilly and McClelland, 1994; Leutgeb et al., 2007). By this mechanism granule cells detect salient, novel information and accentuate minor contextual details related to familiar information (a process often referred to as pattern separation). Individual granule cells provide a strong, sparse, facilitating input to a small number of CA3 pyramidal cells that can be sufficiently powerful to engage 1:1 spike transfer after multiple spikes in a granule cell burst (Acsady et al., 1998; Henze et al., 2002; Sachidhanandam et al., 2009; Vyleta et al., 2016). This focal excitation by mossy fibers drives synchronisation between subsets of CA3 pyramidal cells potentially allowing recurrent CA3-CA3 synapses to engage Hebbian plasticity mechanisms to create ensembles of strongly coupled CA3 cells thereby initiating the storage of new information (O'Reilly and McClelland, 1994; Treves and Rolls, 1994; Kobayashi and Poo, 2004; Brandalise and Gerber, 2014; Guzman et al., 2016; Mishra et al., 2016). Mossy fibers also excite a broad and diverse set of inhibitory interneurons that provide a widespread ‘blanket’ of feed-forward inhibition over a large population of CA3 pyramidal cells (Acsady et al., 1998; Toth et al., 2000; Mori et al., 2007; Szabadics and Soltesz, 2009). This feed-forward inhibition prevents runaway excitation and ensures tight spike timing for spike transfer (Torborg et al., 2010; Restivo et al., 2015; Zucca et al., 2017), while also enhancing memory precision (Ruediger et al., 2011) but it is not known what impact it may have on synaptic plasticity within the CA3 recurrent network. Here we investigate the modulation by acetylcholine of ensemble creation and therefore memory encoding within the hippocampal CA3 network. Using slice electrophysiology and a hierarchy of experimentally constrained computational models of mossy fiber synaptic transmission and CA3 network activity, we demonstrate that acetylcholine suppresses feed-forward inhibition enabling plasticity at recurrent CA3-CA3 synapses and the 4 bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. formation of ensembles within the CA3 network. Furthermore, we show that acetylcholine increases the density of stable ensembles by enhancing permissible overlap between ensembles. 5 bioRxiv preprint doi: https://doi.org/10.1101/201699; this version posted February 19, 2018. The copyright holder for this preprint (which