Synaptic integration and dendritic excitability in cerebellar Purkinje neurons

Dendrites receive most of the synaptic input to the neuron, but their contribution to synaptic integration is not well understood. In this thesis I investigate synaptic integration in Purkinje neurons in cerebellar slices, focusing on the contribution of dendritic excitability using simultaneous somatic and dendritic patch-clamp recordings. I first investigated the ionic conductances underlying intrinsic bistability in Purkinje cell action potential firing. The hyperpolarisation-activated mixed cation current Ih promotes the tonically firing state, with the firing rate being limited by the calcium-activated potassium current. Physiological modulation of Ih by serotonin is shown to enhance bistability triggered by inhibitory synaptic inputs. I next explored the interaction between excitatory postsynaptic potentials (EPSPs) and voltage-gated channels. Near action potential threshold, parallel fibre (PF) EPSPs are non-linearly amplified. Application of the sodium channel blocker TTX abolishes this subthreshold EPSP “boosting”. Strong depolarisation reveals an additional boosting mechanism which is mediated by calcium channel activation. Sodium channel-mediated boosting is perisomatic in origin, while calcium channel-mediated boosting is dendritic. The potassium channel blocker 4-aminopyridine abolishes the somato-dendritic polarity of calcium boosting and also hyperpolarises the voltage threshold of both sodium and calcium boosting. Blocking Ih increases the time constant of EPSP decay, thereby augmenting EPSP boosting. These results indicate that EPSPs are shaped by a complex interplay of several channel types depending on stimulus intensity and membrane potential. 3 I demonstrate that the very powerful synaptic input provided by the climbing fibre (CF) triggers an EPSP which depolarises the entire dendritic membrane to near zero mV. Pairing the CF with depolarisation or PF input triggered secondary calcium spikes which could be confined to dendritic branches and depended on the relative timing of CF and PF input. Inhibitory GABAergic synaptic input mimicked using dynamic clamp could completely suppress or delay CF-evoked dendritic calcium spikes in a temporally and spatially precise manner.