The axon initial segment (AIS) is a structure at the start of the axon with a high density of sodium and potassium channels that defines the site of action potential generation. It has recently been shown that this structure is plastic and can change its position along the axon, as well as its length, in a homeostatic manner. Chronic activitydeprivation paradigms in a chick auditory nucleus lead to a lengthening of the AIS and an increase in neuronal excitability. On the other hand, a long-term increase in activity in dissociated rat hippocampal neurons results in an outward movement of the AIS and a decrease in the cell's excitability. Here, we investigated whether the AIS is capable of undergoing structural plasticity in rat hippocampal organotypic slices, which retain the diversity of neuronal cell types present at postnatal ages, including chandelier cells. These interneurons exclusively target the AIS of pyramidal neurons and form rows of presynaptic boutons along them. Stimulating individual CA1 pyramidal neurons that express channelrhodopsin-2 for 48 h leads to an outward shift of the AIS. Intriguingly, both the pre-and postsynaptic components of the axo-axonic synapses did not change position after AIS relocation. We used computational modeling to explore the functional consequences of this partial mismatch and found that it allows the GABAergic synapses to strongly oppose action potential generation, and thus downregulate pyramidal cell excitability. We propose that this spatial arrangement is the optimal configuration for a homeostatic response to long-term stimulation.axon initial segment | chandelier cells | optogenetics | intrinsic plasticity | homeostatic plasticity N eurons receive a large number of synaptic inputs along the somato-dendritic compartment that integrate at the axon initial segment (AIS) to fire an action potential (AP) (1-3). As an important site for transforming graded synaptic inputs into all-or-none APs, it is also a potentially sensitive target for the modulation of neuronal excitability (4, 5). In fact, one interesting aspect of principal neurons in the hippocampus and cortex is the presence of a unique type of GABAergic axo-axonic synapse that forms onto the AIS and controls neuronal output (6, 7). These synapses are formed by a specific group of fast-spiking interneurons, the chandelier cells, that are generally found sparsely distributed in the brain and have therefore been difficult to study in the past (6, 8). However, the recently developed transgenic mouse lines that can label chandelier neurons more selectively have begun to shine light on their form and function (9). In the cortex, the axonal "cartridges" of synaptic boutons that form onto the AIS are generally found on the more distal AIS domain, where the AP is thought to initiate (9). Although there is little information on the role of these interneurons in network function, they have been implicated in a number of events, including driving negative feedback in the dentate gyrus (10), modulating the emergence of sha...

Neurons in the brain are highly plastic, allowing an organism to learn and adapt to its environment. However, this ongoing plasticity is also inherently unstable, potentially leading to aberrant levels of circuit activity. Homeostatic forms of plasticity are thought to provide a means of controlling neuronal activity by avoiding extremes and allowing network stability. Recent work has shown that many of these homeostatic modifications change the structure of subcellular neuronal compartments, ranging from changes to synaptic inputs at both excitatory and inhibitory compartments to modulation of neuronal output through changes at the axon initial segment (AIS) and presynaptic terminals. Here we review these different forms of structural plasticity in neurons and the effects they may have on network function.

Highlights d Axo-axonic synapses and Chandelier cell axons develop rapidly between P12 and P18 d Increasing network activity during this period reduces axoaxonic synapse number d The depolarizing nature of axo-axonic synapses suggests this plasticity is homeostatic d When axo-axonic synapses become hyperpolarizing, synapse plasticity is reversed

The regulation of hydrogen ion concentration (pH) is fundamental to cell viability, metabolism, and enzymatic function. Within the nervous system, the control of pH is also involved in diverse and dynamic processes including development, synaptic transmission, and the control of network excitability. As pH affects neuronal activity, and can also itself be altered by neuronal activity, the existence of tools to accurately measure hydrogen ion fluctuations is important for understanding the role pH plays under physiological and pathological conditions. Outside of their use as a marker of synaptic release, genetically encoded pH sensors have not been utilized to study hydrogen ion fluxes associated with network activity. By combining whole-cell patch clamp with simultaneous two-photon or confocal imaging, we quantified the amplitude and time course of neuronal, intracellular, acidic transients evoked by epileptiform activity in two separate in vitro models of temporal lobe epilepsy. In doing so, we demonstrate the suitability of three genetically encoded pH sensors: deGFP4, E2GFP, and Cl-sensor for investigating activity-dependent pH changes at the level of single neurons.