br Discussion One binding site MB Fig

Discussion One binding site, MB327-1 (Fig. 4) is located in the extracellular domain between the γ and α subunits. MB327 adopts an extended conformation in the MB327-1 site and is oriented parallel to the channel. The second putative binding site, MB327-2 (Fig. 4) is situated deep inside the channel pore between the β and δ subunits. MB327 is docked in an extended conformation and intercalates between the β and δ subunits perpendicular to the channel with one pyridinium moiety oriented towards the inside channel pore and the other pyridinium moiety pointing away from the channel. The docking calculations are not able to discriminate which of the two putative Melphalan should be preferred, since both docking scores are to close (29.7 and 29.9, respectively). With the two docking scores being similar no conclusion can be drawn if only one or both binding sites are occupied by MB327. The two putative binding sites are surprisingly very asymmetrically lined with hydrophilic and lipophilic amino acids. In both sides one pyridinium ring with its tert-butyl-rest interacts mainly with hydrophilic residues while the other side of MB327 interacts mainly with lipophilic residues. In the MB327-1 site the hydrophilic side chains which are close to MB327 are aspartate, threonine and tyrosine. In the MB327-2 site the hydrophilic interactions are mainly with glutamate and tyrosine. In particular the presence of glutamate or aspartate in the putative binding pockets suggest a substitution of one of the pyridinium rings of MB327 with a hydrogen bond donor like a hydroxy- or alkylhydroxy- or amino-group whereas the other pyridinium ring of MB327 should be substituted with lipophilic rests. This asymmetric substitution of the two pyridinium rings of the bispyridinium compound might increase the affinity to these binding sites and possibly enhance the resensitizing effect on nAChR.
Conflicts of interest
Acknowledgement The study was funded by the German Ministry of Defense (E/U2AD/CF514/DF561).
Main Text Cholinergic neurons from the basal forebrain provide dense connections to cortex and are thought to support a range of behavioral functions from attention and learning to sensory processing and memory (Eggermann et al., 2014, Letzkus et al., 2011, Muñoz and Rudy, 2014). The release of acetylcholine can profoundly transform cortical processing by boosting firing rates, enhancing synaptic plasticity, and reducing network synchrony. These changes in turn can result in increased reliability of sensory responses, expansion of receptive fields, and reorganization of cortical maps. Although acetylcholine, like other neuromodulators, has been thought to act slowly via volumetric transmission and metabotropic receptors, recent data reveal a second, rapid, phasic mode of communication with point-to-point synapses and ionotropic transmission (Hangya et al., 2015, Muñoz and Rudy, 2014). Nevertheless, the relative extent and functional impact of these different types of communication remain unclear. At the cellular and synaptic levels, acetylcholine can produce a diversity of responses depending, in part, on the specific acetylcholine receptors present on the cell. Most cortical neurons express either metabotropic muscarinic or ionotropic nicotinic receptors in different complements according to neuronal subtype, cortical layer, and region (Hedrick and Waters, 2015). This heterogeneity is bound to affect the temporal dynamics of acetylcholine signals on cortical activity. In this issue of Neuron, Urban-Ciecko et al. (2018) use state-of-the-art techniques to examine how acetylcholine regulates the dynamics of a specific neuronal circuit ubiquitous in the superficial layers of cortex. The dendritic activity of cortical pyramidal neurons is controlled by a distinctive class of inhibitory interneurons called Martinotti cells that express somatostatin (SST), used as a marker in this study (Figure 1). In layer 5 of neocortex, pyramidal neurons can drive the activity of Martinotti cells, which in turn provide feedback inhibition to the dendrites of the same neurons (Isaacson and Scanziani, 2011). This disynaptic feedback inhibition forms a cortical circuit motif observed across cortical regions that is thought to gate dendritic excitation, calcium spikes, and plasticity.