Role of Electrotonic Coupling in the Olivocerebellar System

The level of electrotonic coupling in the inferior olive is higher than in any other brain region. Connexin36 is the main protein that forms the olivary gap junctions. Yet, the functional role of electrotonic coupling in the cerebellar motor control remains to be determined. In this thesis mice that lack coupling among their olivary neurons were subjected to classical eyeblink conditioning. Cx36 deficient mice showed impaired learning-dependent timing in that they were not able to fix the timing of their conditioned responses at the moment when the unconditioned stimulus is about to occur. The timing of spike activities generated in the olive of coupling- deficient mice was abnormal in that their latencies in response to the unconditioned stimulus were inconsistent and that their overall synchrony was reduced. Whole cell recordings of olivary neurons in vivo showed that these different spiking activities over time result in part from altered interaction! s with their subthreshold oscillations. These results, combined with analysis of olivary activities in a computer simulation of the cerebellar system, suggest that electrotonic coupling among olivary neurons is necessary for proper synchronous oscillations in the inferior olive, which in turn determine the pace of the olivary responses necessary for learning-dependent timing in cerebellar motor control

Modulation of murine olivary connexin 36 gap junctions by PKA and CaMKII

The inferior olive (IO) is a nucleus located in the brainstem and it is part of the olivo-cerebellar loop. This circuit plays a fundamental role in generation and acquisition of coherent motor patterns and it relies on synchronous activation of groups of Purkinje cells (PC) in the cerebellar cortex. IO neurons integrate their intrinsic oscillatory activity with excitatory inputs coming from the somatosensory system and inhibitory feedback coming from the cerebellar nuclei. Alongside these chemical synaptic inputs, IO neurons are coupled to one another via connexin 36 (Cx36) containing gap junctions (GJs) that create a functional syncytium between neurons. Communication between olivary neurons is regulated by these GJs and their correct functioning contributes to coherent oscillations in the IO and proper motor learning. Here, we explore the cellular pathways that can regulate the coupling between olivary neurons. We combined in vitroelectrophysiology and immunohistochemistry (IHC) on mouse acute brain slices to unravel the pathways that regulate olivary coupling. We found that enhancing the activity of the protein kinase A (PKA) pathway and blocking the Ca2+/calmodulin-dependent protein kinase II (CaMKII) pathway can both down-regulate the size of the coupled network. However, these two kinases follow different mechanisms of action. Our results suggest that activation of the PKA pathway reduces the opening probability of the Cx36 GJs, whereas inhibition of the CaMKII pathway reduces the number of Cx36 GJs. The low densities of Cx36 proteins and electrical synapses in βCaMKII knock-out mice point towards an essential role for this protein kinase in regulating the density of GJs in the IO. Thus, the level of olivary coupling is a dynamic process and regulated by a variety of enzymes modulating GJs expression, docking and activity

Deformation of network connectivity in the inferior olive of connexin 36-deficient mice is compensated by morphological and electrophysiological changes at the single neuron level

Compensatory mechanisms after genetic manipulations have been documented extensively for the nervous system. In many cases, these mechanisms involve genetic regulation at the transcription or expression level of existing isoforms. We report a novel mechanism by which single neurons compensate for changes in network connectivity by retuning their intrinsic electrical properties. We demonstrate this mechanism in the inferior olive, in which widespread electrical coupling is mediated by abundant gap junctions formed by connexin 36 (Cx36). It has been shown in various mammals that this electrical coupling supports the generation of subthreshold oscillations, but recent work revealed that rhythmic activity is sustained in knock-outs of Cx36. Thus, these results raise the question of whether the olivary oscillations in Cx36 knock-outs simply reflect the status of wild-type neurons without gap junctions or the outcome of compensatory mechanisms. Here, we demonstrate that the absence of Cx36 results in thicker dendrites with gap-junction-like structures with an abnormally wide interneuronal gap that prevents electrotonic coupling. The mutant olivary neurons show unusual voltage-dependent oscillations and an increased excitability that is attributable to a combined decrease in leak conductance and an increase in voltage-dependen