Repetitive firing of cerebellar Purkinje cells in response to impulse in climbing fibre afferents

Simple and complex spike responses of mouse cerebellar Purkinje neurons to regular trains and omissions of somatosensory stimuli

Cerebellar Purkinje neurons help compute absolute subsecond timing, but how their firing is affected during repetitive sensory stimulation with consistent subsecond intervals remains unaddressed. Here, we investigated how simple and complex spikes of Purkinje cells change during regular application of air puffs (3.3 Hz for ∼4 min) to the whisker pad of awake, head-fixed female mice. Complex spike responses fell into two categories: those in which firing rates increased (at ∼50 ms) and then fell [complex spike elevated (CxSE) cells] and those in which firing rates decreased (at ∼70 ms) and then rose [complex spike reduced (CxSR) cells]. Both groups had indistinguishable rates of basal complex (∼1.7 Hz) and simple (∼75 Hz) spikes and initially responded to puffs with a well-timed sensory response, consisting of a short-latency (∼15 ms), transient (4 ms) suppression of simple spikes. CxSE more than CxSR cells, however, also showed a longer-latency increase in simple spike rate, previously shown to reflect motor command signals. With repeated puffs, basal simple spike rates dropped greatly in CxSR but not CxSE cells; complex spike rates remained constant, but their temporal precision rose in CxSR cells and fell in CxSE cells. Also over time, transient simple spike suppression gradually disappeared in CxSE cells, suggesting habituation, but remained stable in CxSR cells, suggesting reliable transmission of sensory stimuli. During stimulus omissions, both categories of cells showed complex spike suppression with different latencies. The data indicate two modes by which Purkinje cells transmit regular repetitive stimuli, distinguishable by their climbing fiber signals.NEW & NOTEWORTHY Responses of cerebellar Purkinje cells in awake mice form two categories defined by complex spiking during regular trains of brief, somatosensory stimuli. Cells in which complex spike probability first increases or decreases show simple spike suppressions that habituate or persist, respectively. Stimulus omissions alter complex spiking. The results provide evidence for differential suppression of olivary cells during sensory stimulation and omissions and illustrate that climbing fiber innervation defines Purkinje cell responses to repetitive stimuli.

Extinction of a classically conditioned response: red nucleus and interpositus

It is well established that the cerebellum and its associated circuitry are essential for classical conditioning of the eyeblink response and other discrete motor responses (e.g., limb flexion, head turn, etc.) learned with an aversive unconditioned stimulus. However, brain mechanisms underlying extinction of these responses are still relatively unclear. Behavioral studies have demonstrated extinction to be an active learning process distinct from acquisition. Accordingly, this current understanding of extinction has guided neural studies that have tried to identify possible brain structures that could support this new learning. However, whether extinction engages the same brain sites necessary for acquisition is not yet clear. This poses an overriding problem for understanding brain mechanisms necessary for extinction because such analysis cannot be done without first identifying brain sites and pathways involved in this phenomenon. Equally elusive is the validity of a behavioral theory of extinction that can account for the properties of extinction. In this study, we looked at the involvement of the interpositus and the red nucleus in extinction. Results show that, although inactivation of both nuclei blocks response expression, only inactivation of the interpositus has a detrimental effect on extinction. Moreover, this detrimental effect was completely removed when inactivation of the interpositus was paired with electrical stimulation of the red nucleus. These findings speak to the important role of cerebellar structures in the extinction of discrete motor responses and provide important insight as to the validity of a particular theory of extinction.

Bistability of cerebellar Purkinje cells modulated by sensory stimulation

A persistent change in neuronal activity after brief stimuli is a common feature of many neuronal microcircuits. This persistent activity can be sustained by ongoing reverberant network activity or by the intrinsic biophysical properties of individual cells. Here we demonstrate that rat and guinea pig cerebellar Purkinje cells in vivo show bistability of membrane potential and spike output on the time scale of seconds. The transition between membrane potential states can be bidirectionally triggered by the same brief current pulses. We also show that sensory activation of the climbing fiber input can switch Purkinje cells between the two states. The intrinsic nature of Purkinje cell bistability and its control by sensory input can be explained by a simple biophysical model. Purkinje cell bistability may have a key role in the short-term processing and storage of sensory information in the cerebellar cortex.

Climbing fibre modification of cerebellar Purkinje cell responses to parallel fibre inputs

Electrical stimulation has been used to activate, separately and independently, the climbing fibre inputs to Purkinje cells of the cat cerebellum. The effects of climbing fibre impulses on Purkinje cell discharges evoked by parallel fibre stimulation has been examined. It was found that impulses in the climbing fibre could block or reduce, in a graded manner, the Purkinje cell response to parallel fibre inputs. It has previously been shown that climbing a fibre inputs do not suppress the antidromic spike response of the Purkinje cell to cerebellar white matter stimulation. This suggests that the climbing fibre impulses modify the Purkinje cell response to parallel fibre inputs by reducing the excitatory action of parallel fibre impulses on the Purkinje cell.

Suppression of simple spike discharges of cerebellar Purkinje cells by impulses in climbing fibre afferents

Electrical stimulation has been used to activate climbing fibres (CFs) projecting to the cat cerebellar cortex, and the effects of low frequency (up to 10 Hz) trains of CF inputs on the discharges of cerebellar cortical neurons has been examined. Repetitive activation of the CF innervating a Purkinje (P) cell could reduce or completely suppress the ongoing simple spike discharges (SS) of the P cell. Suppression of SS was very readily produced by trains of short bursts of impulses in the CF. SS could remain suppressed for up to 4 min after such a train of CF impulses. Recording obtained from Golgi, basket and granule cells indicates that these cortical neurones did not mediate the suppression. It is proposed that the SS are suppressed as the result of a direct action of CF impulses on the P cell's dendritic tree which blocks the ongoing transmission of excitation from parallel fibres.