Friedrich W. Johenning

Distinct Intracellular Calcium Transients in Neurites and Somata Integrate Neuronal Signals

Intracellular calcium signals have distinct temporal and spatial patterns in neurons in which signal initiation and repetitive spiking occurs predominantly in the neurite. We investigated the functional implications of the coexpression of different isoforms of ryanodine receptors (RyR) and inositol 1,4,5-trisphosphate receptors (InsP3Rs) using immunocytochemistry, Western blotting, and calcium imaging in neuronally differentiated PC12 cells. InsP3R type III, an isoform that has been shown to be upregulated in neuronal apoptosis, is exclusively expressed in the soma, serving as a gatekeeper for high-magnitude calcium surges. InsP3R type I is expressed throughout the cell and can be related to signal initiation and repetitive spiking in the neurite. RyR types 2 and 3 are distributed throughout the cell. In the soma, they serve as amplifying molecular switches, facilitating recruitment of the InsP3R type III-dependent pool. In the neurite, they decrease the probability of repetitive spiking. Use of a cell-permeant analog of InsP3 suggested that regional specificity in InsP3 production and surface-to-volume effects play minor roles in determining temporal and spatial calcium signaling patterns in neurons. Our findings suggest that additional modulatory processes acting on the intracellular channels are necessary to generate spatially specific calcium signaling.

Adenosine modulates transmission at the hippocampal mossy fibre synapse via direct inhibition of presynaptic calcium channels

The modulation of synaptic transmission by presynaptic ionotropic and metabotropic receptors is an important means to control and dynamically adjust synaptic strength. Even though synaptic transmission and plasticity at the hippocampal mossy fibre synapse are tightly controlled by presynaptic receptors, little is known about the downstream signalling mechanisms and targets of the different receptor systems. In the present study, we identified the cellular signalling cascade by which adenosine modulates mossy fibre synaptic transmission. By means of electrophysiological and optical recording techniques, we found that adenosine activates presynaptic A1 receptors and reduces Ca2+ influx into mossy fibre terminals. Ca2+ currents are directly modulated via a membrane‐delimited pathway and the reduction of presynaptic Ca2+ influx can explain the inhibition of synaptic transmission. Specifically, we found that adenosine modulates both P/Q‐ and N‐type presynaptic voltage‐dependent Ca2+ channels and thereby controls transmitter release at the mossy fibre synapse.

Analysis of Excitatory Microcircuitry in the Medial Entorhinal Cortex Reveals Cell-Type-Specific Differences

Medial entorhinal cortex (MEC) plays an important role in physiological processes underlying navigation, learning, and memory. Excitatory cells in the different MEC layers project in a region-specific manner to the hippocampus. However, the intrinsic microcircuitry of the main excitatory cells in the superficial MEC layers is largely unknown. Using scanning photostimulation, we investigated the functional microcircuitry of two such cell types, stellate and pyramidal cells. We found cell-type-specific intralaminar and ascending interlaminar feedback inputs. The ascending interlaminar inputs display distinct organizational principles depending on the cell-type and its position within the superficial lamina: the spatial spread of inputs for stellate cells is narrower than for pyramidal cells, while inputs to pyramidal cells in layer 3, but not in layer 2, exhibit an asymmetric offset to the medial side of the cells main axis. Differential laminar sources of excitatory inputs might contribute to the functional diversity of stellate and pyramidal cells.

An Approach for Reliably Investigating Hippocampal Sharp Wave-Ripples In Vitro

Background Among the various hippocampal network patterns, sharp wave-ripples (SPW-R) are currently the mechanistically least understood. Although accurate information on synaptic interactions between the participating neurons is essential for comprehensive understanding of the network function during complex activities like SPW-R, such knowledge is currently notably scarce. Methodology/Principal Findings We demonstrate an in vitro approach to SPW-R that offers a simple experimental tool allowing detailed analysis of mechanisms governing the sharp wave-state of the hippocampus. We combine interface storage of slices with modifications of a conventional submerged recording system and established in vitro SPW-R comparable to their in vivo counterpart. We show that slice storage in the interface chamber close to physiological temperature is the required condition to preserve network integrity that is necessary for the generation of SPW-R. Moreover, we demonstrate the utility of our method for studying synaptic and network properties of SPW-R, using electrophysiological and imaging methods that can only be applied in the submerged system. Conclusions/Significance The approach presented here demonstrates a reliable and experimentally simple strategy for studying hippocampal sharp wave-ripples. Given its utility and easy application we expect our model to foster the generation of new insight into the network physiology underlying SPW-R.

Dendritic Compartment and Neuronal Output Mode Determine Pathway-Specific Long-Term Potentiation in the Piriform Cortex

The apical dendrite of layer 2/3 pyramidal cells in the piriform cortex receives two spatially distinct inputs: one projecting onto the distal apical dendrite in sensory layer 1a, the other targeting the proximal apical dendrite in layer 1b. We observe an expression gradient of A-type K+ channels that weakens the backpropagating action potential-mediated depolarization in layer 1a compared with layer 1b. We find that the pairing of presynaptic and postsynaptic firing leads to significantly smaller Ca2+ signals in the distal dendritic spines in layer 1a compared with the proximal spines in layer 1b. The consequence is a selective failure to induce long-term potentiation (LTP) in layer 1a, which can be rescued by pharmacological enhancement of action potential backpropagation. In contrast, LTP induction by pairing presynaptic and postsynaptic firing is possible in layer 1b but requires bursting of the postsynaptic cell. This output mode strongly depends on the balance of excitation and inhibition in the piriform cortex. We show, on the single-spine level, how the plasticity of functionally distinct synapses is gated by the intrinsic electrical properties of piriform cortex layer 2 pyramidal cell dendrites and the cellular output mode.

Modulation of Elementary Calcium Release Mediates a Transition from Puffs to Waves in an IP3R Cluster Model

The oscillating concentration of intracellular calcium is one of the most important examples for collective dynamics in cell biology. Localized releases of calcium through clusters of inositol 1,4,5-trisphosphate receptor channels constitute elementary signals called calcium puffs. Coupling by diffusing calcium leads to global releases and waves, but the exact mechanism of inter-cluster coupling and triggering of waves is unknown. To elucidate the relation of puffs and waves, we here model a cluster of IP3R channels using a gating scheme with variable non-equilibrium IP3 binding. Hybrid stochastic and deterministic simulations show that puffs are not stereotyped events of constant duration but are sensitive to stimulation strength and residual calcium. For increasing IP3 concentration, the release events become modulated at a timescale of minutes, with repetitive wave-like releases interspersed with several puffs. This modulation is consistent with experimental observations we present, including refractoriness and increase of puff frequency during the inter-wave interval. Our results suggest that waves are established by a random but time-modulated appearance of sustained release events, which have a high potential to trigger and synchronize activity throughout the cell.

Role of RIM1α in short- and long-term synaptic plasticity at cerebellar parallel fibres

The presynaptic terminals of synaptic connections are composed of a complex network of interacting proteins that collectively ensure proper synaptic transmission and plasticity characteristics. The key components of this network are the members of the RIM protein family. Here we show that RIM1α can influence short-term plasticity at cerebellar parallel-fibre synapses. We demonstrate that the loss of a single RIM isoform, RIM1α, leads to reduced calcium influx in cerebellar granule cell terminals, decreased release probability and consequently an enhanced short-term facilitation. In contrast, we find that presynaptic long-term plasticity is fully intact in the absence of RIM1α, arguing against its necessary role in the expression of this important process. Our data argue for a universal role of RIM1α in setting release probability via interaction with voltage-dependent calcium channels at different connections instead of synapse-specific functions.

Complementary Sensory and Associative Microcircuitry in Primary Olfactory Cortex

The three-layered primary olfactory (piriform) cortex is the largest component of the olfactory cortex. Sensory and intracortical inputs converge on principal cells in the anterior piriform cortex (aPC). We characterize organization principles of the sensory and intracortical microcircuitry of layer II and III principal cells in acute slices of rat aPC using laser-scanning photostimulation and fast two-photon population Ca2+ imaging. Layer II and III principal cells are set up on a superficial-to-deep vertical axis. We found that the position on this axis correlates with input resistance and bursting behavior. These parameters scale with distinct patterns of incorporation into sensory and associative microcircuits, resulting in a converse gradient of sensory and intracortical inputs. In layer II, sensory circuits dominate superficial cells, whereas incorporation in intracortical circuits increases with depth. Layer III pyramidal cells receive more intracortical inputs than layer II pyramidal cells, but with an asymmetric dorsal offset. This microcircuit organization results in a diverse hybrid feedforward/recurrent network of neurons integrating varying ratios of intracortical and sensory input depending on a cells position on the superficial-to-deep vertical axis. Since burstiness of spiking correlates with both the cells location on this axis and its incorporation in intracortical microcircuitry, the neuronal output mode may encode a given cells involvement in sensory versus associative processing.

Signaling microdomains: InsP3 receptor localization takes on new meaning

A fundamental question in cell biology is how different receptor-mediated signaling cascades, despite utilizing many of the same intracellular components, can generate specific cellular responses. Delmas and colleagues (in this issue of Neuron) address this question in relation to the muscarinic acetylcholine receptor (M(1)AchR) and the B(2) bradykinin receptor (B(2)R). Using Trp channel isoforms as biosensors for PLC stimulation in response to agonist activation, they demonstrate a role for signaling microdomains in the induction of such selective responses.

Ryanodine Receptor Activation Induces Long-Term Plasticity of Spine Calcium Dynamics

A key feature of signalling in dendritic spines is the synapse-specific transduction of short electrical signals into biochemical responses. Ca2+ is a major upstream effector in this transduction cascade, serving both as a depolarising electrical charge carrier at the membrane and an intracellular second messenger. Upon action potential firing, the majority of spines are subject to global back-propagating action potential (bAP) Ca2+ transients. These transients translate neuronal suprathreshold activation into intracellular biochemical events. Using a combination of electrophysiology, two-photon Ca2+ imaging, and modelling, we demonstrate that bAPs are electrochemically coupled to Ca2+ release from intracellular stores via ryanodine receptors (RyRs). We describe a new function mediated by spine RyRs: the activity-dependent long-term enhancement of the bAP-Ca2+ transient. Spines regulate bAP Ca2+ influx independent of each other, as bAP-Ca2+ transient enhancement is compartmentalized and independent of the dendritic Ca2+ transient. Furthermore, this functional state change depends exclusively on bAPs travelling antidromically into dendrites and spines. Induction, but not expression, of bAP-Ca2+ transient enhancement is a spine-specific function of the RyR. We demonstrate that RyRs can form specific Ca2+ signalling nanodomains within single spines. Functionally, RyR mediated Ca2+ release in these nanodomains induces a new form of Ca2+ transient plasticity that constitutes a spine specific storage mechanism of neuronal suprathreshold activity patterns.