The control of rate and timing of spikes in the deep cerebellar nuclei by inhibition.

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.

Spatial distribution of low- and high-voltage-activated calcium currents in neurons of the deep cerebellar nuclei.

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.

Spatial response properties of contralateral inhibited lobula plate tangential cells in the fly visual system.

This study describes the spatial response properties of a particular group of motion-sensitive and directionally selective neurons located in the lobula plate of the fly visual system. Their preferred motion direction is front-to-back (depolarization), and their null direction is back-to-front (hyperpolarization). They receive inhibitory input from the contralateral eye during pattern motion from back to front (regressive). In this study, we call these neurons regressive contralateral inhibited lobula plate tangential cells (rCI-LPTCs). Three physiologic groups of rCI-neurons (rCI-I, rCI-IIa, and rCI-IIb) can be distinguished on the basis of their ipsilateral pattern size dependence and their inhibitory contralateral input. rCI-I neurons depolarize during the motion of small ipsilateral patterns from front to back, but they become hyperpolarized by large ipsilateral patterns moving in the same direction. rCI-IIa and rCI-IIb neurons receive bidirectional inhibitory input from the contralateral eye. rCI-IIa neurons respond best to small ipsilateral pattern sizes, but unlike rCI-I neurons, their net response to large patterns is positive. rCI-IIb neurons respond best to large ipsilateral patterns. The anatomical and physiologic variability of the rCI-neurons suggests that more than three types of rCI-neurons exist. The suggested physiologic groups might be preliminary. We recorded one neuron that could mediate the bidirectional inhibitory input that rCI-IIa and rCI-IIb neurons receive from the contralateral eye. In the case of the rCI-IIa neurons at least one further contralateral inhibitory element has to be assumed. The tuning of rCI-IIa neurons to small ipsilateral pattern sizes is likely to be based on an on-center/off-surround organization of their synaptic input.

Synapse distribution on VCH, an inhibitory, motion-sensitive interneuron in the fly visual system.

In this study, the distribution of synapses in the ventral centrifugal horizontal (VCH)-a nonspiking, inhibitory, motion-sensitive interneuron in the third visual ganglion (lobula plate) of the blowfly Calliphora erythrocephala-was examined by electron microscopy and electrophysiology. The frequency histograms of excitatory and inhibitory postsynaptic potential amplitudes recorded from the VCH during contralateral stimulus presentation suggest the existence of three neurons, two excitatory and one inhibitory, mediating contralateral input. To localize input and output regions of the VCH, we investigated the ultrastructure of its two arborisation areas after intracellular iontophoretic injection of horseradish peroxidase. The VCH has input synapses in its arborisation in the protocerebrum and in its arborisation in the lobula plate. Output synapses were found exclusively in the lobula plate. Thus, the large dendritic arbor of the VCH in the lobula plate serves simultaneously as an input and an output region. There, we found input and output synapses in close vicinity (0.5-1.5 microm) to each other. Taking into account that the VCH receives retinotopicly arranged input from the ipsilateral eye in the lobula plate, the close location of input and output synapses in the VCH suggests that the spatial organization of its retinotopic synaptic input is more or less conserved in its inhibitory output pattern. The VCH has been proposed to inhibit the figure detection 1 (FD1), another neuron of the lobula plate, that responds preferentially to small moving objects. These results suggest that the FD1 may receive inhibitory inputs from the VCH in the lobula plate, where the dendritic arbors of both neurons overlap.