Estimating functional connectivity in an electrically coupled interneuron network

Abstract

Even though it has been known for some time that in many mammalian brain areas interneurons are electrically coupled, a quantitative description of the network electrical connectivity and its impact on cellular passive properties is still lacking. Approaches used so far to solve this problem are limited because they do not readily distinguish junctions among direct neighbors from indirect junctions involving intermediary, multiply connected cells. In the cerebellar cortex, anatomical and functional evidence indicates electrical coupling between molecular layer interneurons (basket and stellate cells). An analysis of the capacitive currents obtained under voltage clamp in molecular layer interneurons of juvenile rats or mice reveals an exponential component with a time constant of ∼20 ms, which represents capacitive loading of neighboring cells through gap junctions. These results, taken together with dual cell recording of electrical synapses, have led us to estimate the number of direct neighbors to be ∼4 for rat basket cells and ∼1 for rat stellate cells. The weighted number of neighbors (number of neighbors, both direct and indirect, weighted with the percentage of voltage deflection at steady state) was 1.69 in basket cells and 0.23 in stellate cells. The last numbers indicate the spread of potential changes in the network and serve to estimate the contribution of gap junctions to cellular input conductance. In conclusion the present work offers effective tools to analyze the connectivity of electrically connected interneuron networks, and it indicates that in juvenile rodents, electrical communication is stronger among basket cells than among stellate cells. To model the functional role of gap junctions (GJs) in the IN network and in cellular computation, it is necessary to determine the number of cells that are connected to a given cell, as well as the geometry of the network. Methods that have been developed to extract this information include dye coupling analysis (e.g., ref. 6), paired recordings coupled with anatomical descriptions (7), and frequency-dependent impedance measurements (8). The first two methods do not readily distinguish direct connections from indirect connections involving an intermediate IN (7,9,10), and all three methods are labor intensive and difficult to implement in a fully quantitative manner. In addition, they do not provide information on the spatial arrangement of the GJs. Therefore, the data that have been exploited for modeling GJ connectivity in IN networks are missing critical elements.In the cerebellum, Golgi cells and molecular layer interneurons (MLIs) have been shown to form anatomical and functional networks involving GJs that are specific to a given cell type (6,(11)(12)(13)(14). In both cases GJs may be involved in the generation of concerted oscillations under some pharmacological conditions (12, 15), and spikelets (spikes of coupled cells filtered through GJs) have been shown to encode sensory information in MLIs (16). MLIs are particularly interesting because th...

Optogenetic Mapping of Cerebellar Inhibitory Circuitry Reveals Spatially Biased Coordination of Interneurons via Electrical Synapses

“…However, it is known that strong electrical synapses occur between MLI (Mann-Metzer and Yarom, 1999; Alcami and Marty, 2013) and these could electrically couple MLI and thereby amplify the effects of a smaller number of direct, chemical inputs (Figure 3A, right). To distinguish between these two models, we examined the effects of disrupting electrical synapses: the model shown in Figure 3A (right) predicts that such disruption should reduce the number of converging interneurons and thereby reduce the input area, while the model of Figure 3A (left) predicts little or no effect.…”

“…Connexin 36 is thought to be the predominant component of gap junctions in CNS neurons (Connors and Long, 2004), is found in the molecular layer (Helbig et al, 2010), and is thought to mediate electrical coupling between MLI (Alcami and Marty, 2013). …”

“…We examined the ability of these compounds to inhibit electrical synaptic transmission between MLI by examining their effects on high-frequency “spikelets”, which result from electrical transmission of action potentials between neighboring MLI (Mann-Metzer and Yarom, 1999; Alcami and Marty, 2013). Wide-field photostimulation of MLI, in the presence of chemical synaptic blockers, produced both photocurrents due to activation of ChR2 expressed in the neuron as well as spikelets caused by light-evoked action potentials in other MLI (Figure S5A, arrow).…”

“…Anatomical studies indicate that a single PC is contacted by 3-7 basket cell axons, though only 1-2 of these form synapses within the pinceau structure (Palay and Chan-Palay, 1974; Somogyi and Hamori, 1976). In addition, MLI are often connected to each other through chemical and/or electrical synapses that could influence information flow between MLI and PCs (Mann-Metzer and Yarom, 1999; Mittmann et al, 2005; Alcami and Marty, 2013). Although traditional electrophysiological recordings can reveal the function of each of these connections, such measurements provide limited information about the spatial organization of these circuits.…”

Gap Junction Modulation of Low-Frequency Oscillations in the Cerebellar Granule Cell Layer

“…This suggests that gap junction blockers influenced local circuit interactions within the GCL. Gap junctions also connect molecular layer interneurons [53][54][55][56], and it is possible that the infusions also affected those synapses. However, given the paucity of direct connections between molecular layer interneurons and GCL elements [57,58], and the less prominent role of parallel fibers in the GCL rhythmicity [39], our findings are more consistent with the involvement of electrical synapses between Golgi cells.…”