Cerebellar nucleus neurons were recorded in vitro, and dynamic clamping was used to simulate inhibitory synaptic input from Purkinje cells likely to occur in vivo. Inhibitory input patterns with varying synaptic amplitudes and synchronicity were applied to determine how spike rate and spike timing can be controlled by inhibition. The excitatory input conductance was held constant to isolate the effect of dynamic inhibitory inputs on spiking. We found that the timing of individual spikes was controlled precisely by short decreases in the inhibitory conductance that were the consequence of synchronization between many inputs. The spike rate of nucleus neurons was controlled in a linear way by the rate of inhibitory inputs. The spike rate, however, also depended strongly on the amount of synchronicity present in the inhibitory inputs. An irregular spike train similar to in vivo data resulted from applied synaptic conductances when the conductance was large enough to overcome intrinsic pacemaker currents. In this situation subthreshold fluctuations in membrane potential closely followed the time course of the combined reversal potential of excitation and inhibition. This indicates that the net synaptic driving force for realistic input levels in vivo may be small and that synaptic input may operate primarily by shunting. The accurate temporal control of output spiking by inhibitory input that can be achieved in this way in the deep cerebellar nuclei may be particularly important to allow fine temporal control of movement via inhibitory output from cerebellar cortex.Key words: cerebellum; Purkinje cell; synaptic; coding; inhibition; dynamic clamp; in vitro; whole cell; synchronizationThe final output of the cerebellum is generated by the neurons of the deep cerebellar nuclei (DCN). A major source of control over DCN neurons is derived from cerebellar cortex via GABA A -type inhibitory Purkinje cell input (Billard et al., 1993). Purkinje cell synapses provide 73% of the total synapses to DCN neurons, and almost all somatic synapses of DCN neurons are inhibitory (Palkovits et al., 1977;De Zeeuw and Berrebi, 1995). This arrangement leads to the question of how inhibitory input can control spiking in postsynaptic neurons accurately. The required accuracy of this control appears to be quite high, because the function of the cerebellum has been related to precise temporal aspects of movement control (Braitenberg, 1967;Ivry and Keele, 1989;Diener et al., 1993;Braitenberg et al., 1997;Wang et al., 1998). For example, a recent study by Timmann et al. (1999) showed that throwing a ball accurately required a temporal precision of the coordination between arm movement and finger opening of ϳ10 msec. Cerebellar patients were not able to throw accurately because the temporal precision was reduced. In the present study we investigated how the Purkinje cell projection onto the DCN might support such temporally precise control apparent from the behavioral level. Although the question of precise temporal coding of neural activity via ...

We performed whole-cell patch-clamp recordings in vitro to investigate the integration of excitatory and inhibitory inputs in neurons of the deep cerebellar nuclei (DCN) by applying synthetic synaptic input patterns with dynamic clamping. We explored an input regime in which excitation and inhibition had an ongoing baseline rate because both input pathways show ongoing activity in vivo. We found that spiking was time-locked to transients in the inputs, consisting of brief decreases in inhibitory or increases in excitatory conductance. Such input transients were caused by synchronization among multiple inputs. However, we found that temporal synchrony in the inhibitory input pathway had preferential access to the control of DCN spiking, because the large NMDA component of the excitatory inputs smoothed out temporal transients in this pathway. Thus, synaptic integration in the DCN appears to be tuned to allow the cerebellar cortical output from Purkinje cells preferential access to the control of DCN spiking. The effect of temporal modulations in the inhibition was further enhanced by the voltage dependence of the NMDA inputs. Thus, the presence of a baseline of mossy and climbing fiber inputs boosted depolarizing responses caused by reduced inhibition by the voltage-dependent increase in inward NMDA current. Overall, our results show that correlated activity or pauses in populations of Purkinje cells are well suited to the dynamic control of DCN spiking. In addition, strong transients in excitation can directly drive DCN responses that bypass cerebellar cortical processing.

The spatial distribution of low-voltage-activated (LVA) and high-voltage-activated (HVA) barium currents was investigated in neurons of the deep cerebellar nuclei (DCN) by combining barium imaging with voltage clamp. The current-induced fluorescence signal (DeltaF/F) of the HVA current was five times higher then the LVA-induced signal at the soma, but both signals were approximately equal in size in distant dendrites. This position-dependent shift of DeltaF/F indicates a non-uniform distribution of the underlying calcium channels. The higher weight of the LVA signal in the dendrites suggests that the LVA might be of particular relevance for the dendritic integration of synaptic inputs.