Changes in the responses of cerebellar nuclear neurons associated with the climbing fiber response of Purkinje cells.

Previous studies demonstrated that climbing fiber activity produces a short-term increase in the responsiveness of Purkinje cells to mossy fiber inputs. This led to the hypothesis that there are concomitant alterations in the discharge of cerebellar nuclear neurons. This series of experiments was initiated to test this hypothesis in simultaneously recorded Purkinje cell-nuclear cell pairs in related regions of the cerebellar cortex and nuclei. In decerebrate cats 50 pairs of Purkinje cells and nuclear neurons were identified and simultaneously recorded during spontaneous activity and during peripheral inputs. Auto-correlograms of nuclear cell activity and cross-correlograms of the simple spike and nuclear cell activity triggered on the occurrence of spontaneous complex spikes demonstrated little correlation between these events and the discharge of nuclear neurons. To examine the effect of evoked climbing fiber inputs on the Purkinje cell simple spike and nuclear cell responses, square wave mechanical stimuli which modulated the discharge of both cells of a pair were applied to the forepaw. A separation technique was used to construct one histogram illustrating the responses of the nuclear neuron and Purkinje cell in trials in which the peripheral stimulus evoked a climbing fiber input to the Purkinje cell and another histogram showing their responses in trials in which no climbing fiber input was activated. Using this separation technique it was shown that the amplitude of most Purkinje cell responses increased by 120-1200% in trials in which climbing fiber inputs were activated. The response amplitude of 68% of the nuclear cells was modified for these pairs. Most changes in nuclear cell responses were increases ranging from 120-220%. These changes were felt to reflect the action of many Purkinje cells converging on the isolated nuclear neuron. The modulation of the nuclear neuron was not due only to the effect of the related Purkinje cell, since the gain change of the Purkinje cell and nuclear cell of each pair was not correlated (r = 0.01). The discussion of these findings emphasizes that the increased responses of the nuclear cell are most likely produced by the intracortical action of the climbing fiber system on the responsiveness of Purkinje cells to mossy fiber inputs. Climbing fiber collateral input to nuclear neurons also may contribute to the changes in the nuclear cell responses observed in these experiments.