Dendritic Calcium Spikes Are Tunable Triggers of Cannabinoid Release and Short-Term Synaptic Plasticity in Cerebellar Purkinje Neurons (original) (raw)
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Dendritic spikes mediate negative synaptic gain control in cerebellar Purkinje cells
Proceedings of the National Academy of Sciences, 2010
Dendritic spikes appear to be a ubiquitous feature of dendritic excitability. In cortical pyramidal neurons, dendritic spikes increase the efficacy of distal synapses, providing additional inward current to enhance axonal action potential (AP) output, thus increasing synaptic gain. In cerebellar Purkinje cells, dendritic spikes can trigger synaptic plasticity, but their influence on axonal output is not well understood. We have used simultaneous somatic and dendritic patch-clamp recordings to directly assess the impact of dendritic calcium spikes on axonal AP output of Purkinje cells. Dendritic spikes evoked by parallel fiber input triggered brief bursts of somatic APs, followed by pauses in spiking, which cancelled out the extra spikes in the burst. As a result, average output firing rates during trains of input remained independent of the input strength, thus flattening synaptic gain. We demonstrate that this "clamping" of AP output by the pause following dendritic spikes is due to activation of high conductance calcium-dependent potassium channels by dendritic spikes. Dendritic spikes in Purkinje cells, in contrast to pyramidal cells, thus have differential effects on temporally coded and rate coded information: increasing the impact of transient parallel fiber input, while depressing synaptic gain for sustained parallel fiber inputs. cerebellum | patch clamp | dendrite | synaptic integration A hallmark of active dendrites is their ability to produce regenerative events known as dendritic spikes (1-8). In pyramidal neurons, the inward currents associated with dendritic spikes provide a strong local depolarization that can boost distal synaptic inputs and enhance their effect on axonal action potential (AP) output (1-4), particularly during burst generation (5, 6). Furthermore, dendritic spikes can enhance the precision of axonal APs in hippocampal pyramidal neurons (7) as well as in neocortical pyramidal cells in vivo (8). Dendritic spikes thus have a boosting effect on the output of pyramidal cells, thus enhancing the gain of the synaptic input-output (I/O) function (9, 10). In contrast, the effect of dendritic spikes on AP output in Purkinje cells is not well understood. Purkinje cell dendritic spikes, originally discovered in alligator Purkinje cells (11, 12), can be triggered by strong parallel fiber (PF) activation (11, 13) or climbing fiber activation (14, 15) and are due solely to activation of dendritic voltage-gated calcium channels (13, 16-18), because Purkinje cells lack dendritic voltage-gated sodium channels and active backpropagation of APs (17,. Calcium influx driven by dendritic spikes has an important role in triggering synaptic plasticity (20-22) and dendritic release of neurotransmitters and neuromodulators (13, 23, 24). Dendritic spikes triggered by climbing fiber input have virtually no effect on the somatic complex spike waveform, probably due to the large synaptic and intrinsic conductances activated during the complex spike (14). However, the functional role of parallel fiber-driven dendritic spikes in regulating axonal output has not been addressed directly. This distinction is crucial, because the state of the Purkinje cell dendritic tree is very different during climbing fiber and parallel fiber excitation (25), and because climbing fiber input occurs only at ≈1 Hz in vivo (26, 27), whereas parallel fiber input occurs continuously at high rates. We have therefore directly probed the relationship between dendritic spikes and axonal AP output in Purkinje cells by using simultaneous dendritic and somatic whole-cell recordings. Our results show that a dendritic spike transiently increases synaptic efficacy by promoting short bursts of somatic APs but dampen AP output over longer timescales. The interplay between these two effects during sustained parallel fiber input results in a "clamping" of Purkinje cells output over long timescales and, thus, a flattening of synaptic gain, in striking contrast to pyramidal cells (9).
Coincidence detection in single dendritic spines mediated by calcium release
nature neuroscience, 2000
Here we show that pairing CF activation with bursts of PF activity triggers large (>10 µM) calcium signals in Purkinje cell dendrites. When PFs are densely activated, signals span whole dendritic branchlets and are mediated by voltage-dependent calcium entry. When PFs are sparsely activated, however, signals are restricted to single spines and blocked by metabotropic glutamate receptor antagonists. Single-spine signals and sparse-stimulation LTD are also blocked by thapsigargin, indicating that calcium must be released from stores. Single-spine signals and sparse-stimulation LTD are greatest when PF activation precedes the CF activation within 50-200 ms. This timing rule matches the properties of several forms of motor learning, providing a link between behavior and functional properties of cerebellar synaptic plasticity.
Neuron, 2001
Brief depolarization of cerebellar Purkinje cells was found to inhibit parallel fiber and climbing fiber EPSCs for tens of seconds. This depolarization-induced suppression of excitation (DSE) is accompanied by altered paired-pulse plasticity, suggesting a presynaptic locus. Fluorometric imaging revealed that postsynaptic depolarization also reduces presynaptic calcium influx. The inhibition of both presynaptic calcium influx and EPSCs is eliminated by buffering postsynaptic calcium with BAPTA. The cannabinoid CB1 receptor antagonist AM251 prevents DSE, and the agonist WIN 55,212-2 occludes DSE. These findings suggest that Purkinje cells release endogenous cannabinoids in response to elevated calcium, thereby inhibiting presynaptic calcium entry and suppressing transmitter release. DSE may provide a way for cells to use their firing rate to dynamically regulate synaptic inputs. Together with previous studies, these findings suggest a widespread role for endogenous cannabinoids in ret...
Neuron, 2012
Small-conductance Ca 2+-activated K + channels (SK channels) modulate excitability and curtail excitatory postsynaptic potentials (EPSPs) in neuronal dendrites. Here, we demonstrate long-lasting plasticity of intrinsic excitability (IE) in dendrites that results from changes in the gain of this regulatory mechanism. Using dendritic patch-clamp recordings from rat cerebellar Purkinje cells, we find that somatic depolarization or parallel fiber (PF) burst stimulation induce long-term amplification of synaptic responses to climbing fiber (CF) or PF stimulation and enhance the amplitude of passively propagated sodium spikes. Dendritic plasticity is mimicked and occluded by the SK channel blocker apamin and is absent in Purkinje cells from SK2 null mice. Triplepatch recordings from two dendritic sites and the soma and confocal calcium imaging studies show that local stimulation limits dendritic plasticity to the activated compartment of the dendrite. This plasticity mechanism allows Purkinje cells to adjust the SK2-mediated control of dendritic excitability in an activity-dependent manner.
Journal of Neuroscience, 2011
Cerebellar Purkinje cells have one of the most elaborate dendritic trees in the mammalian CNS, receiving excitatory synaptic input from a single climbing fiber (CF) and from ϳ200,000 parallel fibers. The dendritic Ca 2ϩ signals triggered by activation of these inputs are crucial for the induction of synaptic plasticity at both of these synaptic connections. We have investigated Ca 2ϩ signaling in Purkinje cell dendrites in vivo by combining targeted somatic or dendritic patch-clamp recording with simultaneous two-photon microscopy. Both spontaneous and sensory-evoked CF inputs triggered widespread Ca 2ϩ signals throughout the dendritic tree that were detectable even in individual spines of the most distal spiny branchlets receiving parallel fiber input. The amplitude of these Ca 2ϩ signals depended on dendritic location and could be modulated by membrane potential, reflecting modulation of dendritic spikes triggered by the CF input. Furthermore, the variability of CF-triggered Ca 2ϩ signals was regulated by GABAergic synaptic input. These results indicate that dendritic Ca 2ϩ signals triggered by sensory-evoked CF input can act as associative signals for synaptic plasticity in Purkinje cells in vivo and may differentially modulate plasticity at parallel fiber synapses depending on the location of synapses, firing state of the Purkinje cell, and ongoing GABAergic synaptic input. modulated by background synaptic inputs and by activation of neuromodulatory afferents via sensory stimulation .
Neuron, 2014
In cerebellar Purkinje cell dendrites, heterosynaptic calcium signaling induced by the proximal climbing fiber (CF) input controls plasticity at distal parallel fiber (PF) synapses. The substrate and regulation of this long-range dendritic calcium signaling are poorly understood. Using high-speed calcium imaging, we examine the role of active dendritic conductances. Under basal conditions, CF stimulation evokes T-type calcium signaling displaying sharp proximodistal decrement. Combined mGluR1 receptor activation and depolarization, two activity-dependent signals, unlock P/Q calcium spikes initiation and propagation, mediating efficient CF signaling at distal sites. These spikes are initiated in proximal smooth dendrites, independently from somatic sodium action potentials, and evoke high-frequency bursts of all-or-none fast-rising calcium transients in PF spines. Gradual calcium spike burst unlocking arises from increasing inactivation of mGluR1-modulated low-threshold A-type potass...
Cerebral Cortex, 2013
Endocannabinoids (eCBs) play a prominent role in regulating synaptic signaling throughout the brain. In layer 2/3 of the neocortex, eCB-mediated suppression of GABA release results in an enhanced excitability of pyramidal neurons (PNs). The eCB system is also involved in spike timing-dependent plasticity that is dependent on backpropagating action potentials (bAPs). Dendritic backpropagation plays an important role in many aspects of neuronal function, and can be modulated by intrinsic dendritic conductances as well as by synaptic inputs. The present studies explored a role for the eCB system in modulating backpropagation in PN dendrites. Using dendritic calcium imaging and somatic patch clamp recordings from mouse somatosensory cortical slices, we found that activation of type 1 cannabinoid receptors potentiated bAP-induced calcium transients in apical dendrites of layer 2/3 but not layer 5 PNs. This effect was mediated by suppression of GABAergic transmission, because it was prevented by a GABA A receptor antagonist and was correlated with cannabinoid suppression of inhibitory synaptic activity. Finally, we found that activity-dependent eCB release during depolarization-induced suppression of inhibition enhanced bAP-induced dendritic calcium transients. Taken together, these results point to a potentially important role for the eCB system in regulating dendritic backpropagation in layer 2/3 PNs.
Activation of CB1 cannabinoid receptors in the cerebellum acutely depresses excitatory synaptic transmission at parallel fibre–Purkinje cell synapses by decreasing the probability of glutamate release. This depression involves the activation of presynaptic 4-aminopyridine-sensitive K + channels by CB1 receptors, which in turn inhibits presynaptic Ca 2+ influx controlling glutamate release at these synapses. Using rat cerebellar frontal slices and fluoro-metric measures of presynaptic Ca 2+ influx evoked by stimulation of parallel fibres with the fluorescent dye fluo-4FF, we tested whether the CB1 receptor-mediated inhibition of this influx also involves a direct inhibition of presynaptic voltage-gated calcium channels. Since various physiological effects of CB1 receptors appear to be mediated through the activation of PTX-sensitive proteins, including inhibition of adenylate cyclases, activation of mitogen-activated protein kinases (MAPK) and activation of G protein-gated inwardly rectifying K + channels, we also studied the potential involvement of these intracellular signal transduction pathways in the cannabinoid-mediated depression of presynaptic Ca 2+ influx. The present study demonstrates that the molecular mechanisms underlying the CB1 inhibitory effect involve the activation of the PTX-sensitive G i /G o subclass of G proteins, independently of any direct effect on pre-synaptic Ca 2+ channels (N, P/Q and R (SNX-482-sensitive) types) or on adenylate cyclase or MAPK activity, but do require the activation of G protein-gated inwardly rectifying (Ba 2+-and tertiapin Q-sensitive) K + channels, in addition to 4-aminopyridine-sensitive K + channels.
Journal of Neurophysiology, 2010
Purkinje cell dendrites are excitable structures with intrinsic and synaptic conductances contributing to the generation and propagation of electrical activity. Voltage-gated potassium channel subunit Kv3.3 is expressed in the distal dendrites of Purkinje cells. However, the functional relevance of this dendritic distribution is not understood. Moreover, mutations in Kv3.3 cause movement disorders in mice and cerebellar atrophy and ataxia in humans, emphasizing the importance of understanding the role of these channels. In this study, we explore functional implications of this dendritic channel expression and compare Purkinje cell dendritic excitability in wild-type and Kv3.3 knockout mice. We demonstrate enhanced excitability of Purkinje cell dendrites in Kv3.3 knockout mice, despite normal resting membrane properties. Combined data from local application pharmacology, voltage clamp analysis of ionic currents, and assessment of dendritic Ca2+ spike threshold in Purkinje cells sugge...
Neuroscience, 2014
The spatial pattern of synapse activation may impact on synaptic plasticity. This applies to the synaptically-evoked endocannabinoid-mediated short-term depression at the parallel fiber (PF) to Purkinje cell synapse, the occurrence of which requires close proximity between the activated synapses. Here, we determine quantitatively this required proximity, helped by the geometrical organization of the cerebellar molecular layer. Transgenic mice expressing a calcium indicator selectively in granule cells enabled the imaging of action potential-evoked presynaptic calcium rise in isolated, single PFs. This measurement was used to derive the number of PFs activated within a beam of PFs stimulated in the molecular layer, from which the density of activated PFs (input density) was calculated. This density was on average 2.8 lm À2 in sagittal slices and twice more in transverse slices. The synaptically-evoked endocannabinoid-mediated suppression of excitation (SSE) evoked by ten stimuli at 200 Hz was determined from the monitoring of either postsynaptic responses or presynaptic calcium rise. The SSE was significantly larger when recorded in transverse slices, where the input density is larger. The exponential description of the SSE plotted as a function of the input density suggests that the SSE is half reduced when the input density decreases from 6 to 2 lm À2. We conclude that, although all PFs are truncated in an acute sagittal slice, half of them remain respondent to stimulation, and activated synapses need to be closer than 1.5 lm to synergize in endocannabinoid signaling.