Purkinje Cell Simple Spike Firing Research Articles

In vivo cerebellar circuit function is disrupted in an mdx mouse model of Duchenne muscular dystrophy.

ABSTRACTDuchenne muscular dystrophy (DMD) is a debilitating and ultimately lethal disease involving progressive muscle degeneration and neurological dysfunction. DMD is caused by mutations in the dystrophin gene, which result in extremely low or total loss of dystrophin protein expression. In the brain, dystrophin is heavily localized to cerebellar Purkinje cells, which control motor and non-motor functions. In vitro experiments in mouse Purkinje cells revealed that loss of dystrophin leads to low firing rates and high spiking variability. However, it is still unclear how the loss of dystrophin affects cerebellar function in the intact brain. Here, we used in vivo electrophysiology to record Purkinje cells and cerebellar nuclear neurons in awake and anesthetized female mdx (also known as Dmd) mice. Purkinje cell simple spike firing rate is significantly lower in mdx mice compared to controls. Although simple spike firing regularity is not affected, complex spike regularity is increased in mdx mutants. Mean firing rate in cerebellar nuclear neurons is not altered in mdx mice, but their local firing pattern is irregular. Based on the relatively well-preserved cytoarchitecture in the mdx cerebellum, our data suggest that faulty signals across the circuit between Purkinje cells and cerebellar nuclei drive the abnormal firing activity. The in vivo requirements of dystrophin during cerebellar circuit communication could help explain the motor and cognitive anomalies seen in individuals with DMD.This article has an associated First Person interview with the first author of the paper.

Molecular layer interneurons shape the spike activity of cerebellar Purkinje cells

Purkinje cells receive synaptic input from several classes of interneurons. Here, we address the roles of inhibitory molecular layer interneurons in establishing Purkinje cell function in vivo. Using conditional genetics approaches in mice, we compare how the lack of stellate cell versus basket cell GABAergic neurotransmission sculpts the firing properties of Purkinje cells. We take advantage of an inducible Ascl1CreER allele to spatially and temporally target the deletion of the vesicular GABA transporter, Vgat, in developing neurons. Selective depletion of basket cell GABAergic neurotransmission increases the frequency of Purkinje cell simple spike firing and decreases the frequency of complex spike firing in adult behaving mice. In contrast, lack of stellate cell communication increases the regularity of Purkinje cell simple spike firing while increasing the frequency of complex spike firing. Our data uncover complementary roles for molecular layer interneurons in shaping the rate and pattern of Purkinje cell activity in vivo.

The dynamic relationship between cerebellar Purkinje cell simple spikes and the spikelet number of complex spikes.

Key points Purkinje cells are the sole output of the cerebellar cortex and fire two distinct types of action potential: simple spikes and complex spikes.Previous studies have mainly considered complex spikes as unitary events, even though the waveform is composed of varying numbers of spikelets.The extent to which differences in spikelet number affect simple spike activity (and vice versa) remains unclear.We found that complex spikes with greater numbers of spikelets are preceded by higher simple spike firing rates but, following the complex spike, simple spikes are reduced in a manner that is graded with spikelet number.This dynamic interaction has important implications for cerebellar information processing, and suggests that complex spike spikelet number may maintain Purkinje cells within their operational range. Purkinje cells are central to cerebellar function because they form the sole output of the cerebellar cortex. They exhibit two distinct types of action potential: simple spikes and complex spikes. It is widely accepted that interaction between these two types of impulse is central to cerebellar cortical information processing. Previous investigations of the interactions between simple spikes and complex spikes have mainly considered complex spikes as unitary events. However, complex spikes are composed of an initial large spike followed by a number of secondary components, termed spikelets. The number of spikelets within individual complex spikes is highly variable and the extent to which differences in complex spike spikelet number affects simple spike activity (and vice versa) remains poorly understood. In anaesthetized adult rats, we have found that Purkinje cells recorded from the posterior lobe vermis and hemisphere have high simple spike firing frequencies that precede complex spikes with greater numbers of spikelets. This finding was also evident in a small sample of Purkinje cells recorded from the posterior lobe hemisphere in awake cats. In addition, complex spikes with a greater number of spikelets were associated with a subsequent reduction in simple spike firing rate. We therefore suggest that one important function of spikelets is the modulation of Purkinje cell simple spike firing frequency, which has implications for controlling cerebellar cortical output and motor learning.

Effects of norepinephrine on spontaneous firing activity of cerebellar Purkinje cells in vivo in mice

Norepinephrine (NE), from the locus coeruleus (LC), has been supported to affect GABAergic system and parallel fiber (PF)-Purkinje cell (PC) synaptic transmission via adrenoceptor in cerebellum cortex. However, the effects of NE on the spontaneous spike activity of cerebellar PCs in living mouse have not yet been fully understood. We here examined the effects of NE on the spontaneous activity of PC in urethane-anesthetized mice by electrophysiological and pharmacological methods. Cerebellar surface application of NE (2.5–25μM) reduced the PC simple spike (SS) firing rate in a dose-dependent manner. The half-inhibitory concentration (IC50) was 5.97μM. In contrast, NE significantly increased the spontaneous firing rate of molecular layer interneuron (MLI). Application of GABAA receptor antagonist, gabazine (SR95531, 20μM) not only blocked the NE-induced inhibition of PC SS firing but also revealed NE-induced excitation of cerebellar PC. Blocking AMPA receptors activity enhanced NE-induced inhibition of PC spontaneous activity. Moreover, the effects of NE on PC spontaneous activity were abolished by simultaneously blocking GABAA and AMPA receptors activity. These results indicated that NE bidirectional modulated the spontaneous activity of PCs via enhancing both inhibitory inputs from MLIs and excitatory inputs of parallel fibers, but NE-induced enhance of inhibitory inputs overwhelmed the excitatory inputs under in vivo conditions.

The cerebellum for jocks and nerds alike.

Historically the cerebellum has been implicated in the control of movement. However, the cerebellum's role in non-motor functions, including cognitive and emotional processes, has also received increasing attention. Starting from the premise that the uniform architecture of the cerebellum underlies a common mode of information processing, this review examines recent electrophysiological findings on the motor signals encoded in the cerebellar cortex and then relates these signals to observations in the non-motor domain. Simple spike firing of individual Purkinje cells encodes performance errors, both predicting upcoming errors as well as providing feedback about those errors. Further, this dual temporal encoding of prediction and feedback involves a change in the sign of the simple spike modulation. Therefore, Purkinje cell simple spike firing both predicts and responds to feedback about a specific parameter, consistent with computing sensory prediction errors in which the predictions about the consequences of a motor command are compared with the feedback resulting from the motor command execution. These new findings are in contrast with the historical view that complex spikes encode errors. Evaluation of the kinematic coding in the simple spike discharge shows the same dual temporal encoding, suggesting this is a common mode of signal processing in the cerebellar cortex. Decoding analyses show the considerable accuracy of the predictions provided by Purkinje cells across a range of times. Further, individual Purkinje cells encode linearly and independently a multitude of signals, both kinematic and performance errors. Therefore, the cerebellar cortex's capacity to make associations across different sensory, motor and non-motor signals is large. The results from studying how Purkinje cells encode movement signals suggest that the cerebellar cortex circuitry can support associative learning, sequencing, working memory, and forward internal models in non-motor domains.

Direct Causality Between Single-Purkinje Cell Activities and Motor Learning Revealed by a Cerebellum–Machine Interface Utilizing VOR Adaptation Paradigm

A cerebellum-machine interface (CMI) was developed to test direct causality between single-unit cerebellar Purkinje cell activities and motor learning. The CMI converts Purkinje cell simple spike firing rates into a pulse width modulation signal that drives a single-joint robot arm. The CMI has no adaptive capability, thus any changes observed in the robot arm motion can be attributed directly to changes in the Purkinje cell's firing activities. We employed a vestibuloocular reflex (VOR) adaptation paradigm in goldfish as an example of motor learning where desired motion and control error signal of the robot arm were given to the fish as its head rotation and retinal slip, respectively. It is demonstrated that the control error of the robot arm decreased gradually, but not monotonically and in many cases only in one direction. This is the first direct evidence that a single Purkinje cell is capable of adaptive motor control. The results also suggest that a single Purkinje cell can be responsible for directional selective VOR motor learning previously reported in goldfish by Yoshikawa et al. (Conf Proc IEEE Eng Med Biol Soc 1:478-481, 2004) and monkeys by Hirata et al. (J Neurophysiol 85(5):2267-2288, 2002).

Evaluating the adaptive-filter model of the cerebellum.

The adaptive-filter model of the cerebellar microcircuit is in widespread use, combining as it does an explanation of key microcircuit features with well-specified computational power. Here we consider two methods for its evaluation. One is to test its predictions concerning relations between cerebellar inputs and outputs. Where the relevant experimental data are available, e.g. for the floccular role in image stabilization, the predictions appear to be upheld. However, for the majority of cerebellar microzones these data have yet to be obtained. The second method is to test model predictions about details of the microcircuit. We focus on features apparently incompatible with the model, in particular non-linear patterns in Purkinje cell simple-spike firing. Analysis of these patterns suggests the following three conclusions. (i) It is important to establish whether they can be observed during task-related behaviour. (ii) Highly non-linear models based on these patterns are unlikely to be universal, because they would be incompatible with the (approximately) linear nature of floccular function. (iii) The control tasks for which these models are computationally suited need to be identified. At present, therefore, the adaptive filter remains a candidate model of at least some cerebellar microzones, and its evaluation suggests promising lines for future enquiry.

Simple and Complex Spike Firing Patterns in Purkinje Cells During Classical Conditioning

Classical blink conditioning is known to depend critically on the cerebellum and the relevant circuitry is gradually being unravelled. Several lines of evidence support the theory that the conditioned stimulus is transmitted by mossy fibers to the cerebellar cortex whereas the unconditioned stimulus is transmitted by climbing fibers. This view has been dramatically confirmed by recent Purkinje cell recordings during training with a classical conditioning paradigm. We have tracked the activity of single Purkinje cells with microelectrodes for several hours in decerebrate ferrets during learning, extinction, and relearning. Paired peripheral forelimb and periocular stimulation, as well as paired direct stimulation of cerebellar afferent pathways (mossy and climbing fibers) causes acquisition of a pause response in Purkinje cell simple spike firing. This conditioned Purkinje cell response has temporal properties that match those of the behavioral response. Its latency varies with the interstimulus interval and it responds to manipulations of the conditioned stimulus in the same way that the blink does. Complex spike firing largely mirrors the simple spike behavior. We have previously suggested that cerebellar learning is subject to a negative feedback control via the inhibitory nucleo-olivary pathway. As the Purkinje cell learns to respond to the conditioned stimulus with a suppression of simple spikes, disinhibition of anterior interpositus neurons would be expected to cause inhibition of the inferior olive. Observations of complex spike firing in the Purkinje cells during conditioning and extinction confirm this prediction. Before training, complex spikes are unaffected or facilitated by the conditioned stimulus, but as the simple spike pause response develops, spontaneous and stimulus-evoked complex spikes are also strongly suppressed by the conditioned stimulus. After extinction of the simple spike pause response, the complex spikes reappear.

Single trial coupling of Purkinje cell activity to speed and error signals during circular manual tracking

The simple spike firing of cerebellar Purkinje cells encodes information on the kinematics of limb movements. However, these conclusions have been primarily based on averaging the discharge of Purkinje cells across trials and time and there is little information on whether Purkinje cell simple spike firing encodes specific motor errors during limb movements. Therefore, this study investigated single-trial correlations between the instantaneous simple spike firing of Purkinje cells with various kinematics and error signals. Purkinje cells (n = 126) were recorded in the intermediate and lateral zones centered on the primary fissure while three monkeys intercepted and tracked a target moving in a circle. Cross-correlation analysis was performed between the instantaneous simple spike firing rate and speed, the directional component of the velocity vector, and error signals during single movement trials. Significant correlations at physiologically relevant lags of +/-250 ms were observed with tracking speed for 37% of Purkinje cells, with the velocity component in 39%, with direction error in 6% and speed error in 25%. Simple spike firing of the majority of Purkinje cells with significant correlation showed a negative lag with respect to speed and a positive lag with respect to error signals. We hypothesize that the cerebellum is involved in movement planning and control by continuously monitoring movement errors and making intermittent corrections that are represented as fluctuations in the speed profile.

Acquisition, Extinction, and Reacquisition of a Cerebellar Cortical Memory Trace

Associative learning in the cerebellum underlies motor memories and probably also cognitive associations. Pavlovian eyeblink conditioning, a widely used experimental model of such learning, depends on the cerebellum, but the memory locus within the cerebellum as well as the underlying mechanisms have remained controversial. To date, crucial information on how cerebellar Purkinje cells change their activity during learning has been ambiguous and contradictory, and there is no information at all about how they behave during extinction and reacquisition. We have now tracked the activity of single Purkinje cells with microelectrodes for up to 16 h in decerebrate ferrets during learning, extinction, and relearning. We demonstrate that paired peripheral forelimb and periocular stimulation, as well as paired direct stimulation of cerebellar afferent pathways (mossy and climbing fibers) consistently causes a gradual acquisition of an inhibitory response in Purkinje cell simple spike firing. This conditioned cell response has several properties that matches known features of the behavioral conditioned response. The response latency varies with the interstimulus interval, and the response maximum is adaptively timed to precede the unconditioned stimulus. Across training trials, it matches behavioral extinction to unpaired stimulation and also the substantial savings that occur when paired stimulation is reinstated. These data suggest that many of the basic behavioral phenomena in eyeblink conditioning can be explained at the level of the single Purkinje cell.

Abnormal spontaneous and harmaline-stimulated Purkinje cell activity in the awake genetically dystonic rat.

The genetically dystonic rat is an autosomal recessive mutant with a movement disorder that closely resembles the generalized dystonias seen in humans. Abnormal activity of neurons within the cerebellar nuclei is critical to the dystonic rat motor syndrome. Increased glutamic acid decarboxylase activity, increased glucose utilization, and decreased muscimol binding within the cerebellar nuclei of the dystonic rat suggests that Purkinje cell firing rates are increased in these animals. However, under urethane anesthesia, Purkinje cell simple spike firing rates in dystonic rats were less than half the rates seen in normal littermates. In this study, both spontaneous and harmaline-stimulated single-unit Purkinje cell recordings were obtained from awake normal and dystonic rats. In striking contrast to previous results obtained under urethane anesthesia, there was no statistically significant difference in average Purkinje cell spontaneous simple spike frequency between dystonic and normal rats. Similar to previous studies obtained under urethane anesthesia, Purkinje cell spontaneous complex spike frequency was much lower in dystonic than in normal rats. Many Purkinje cells from dystonic rats, particularly those from the vermis or older animals, exhibited rhythmic bursting simple spike firing patterns. Cross-correlations showed that complex spikes produced less suppression of simple spikes in dystonic than in normal rats and harmaline-stimulated complex spike activity was, on average, faster and more rhythmic in normal than in dystonic rats. These findings indicate that olivocerebellar network abnormalities in the dystonic rat are not due to an inability of Purkinje cells to fire at normal rates and suggest that abnormal Purkinje cell bursting firing patterns in the dystonic rat are due to a defect in the pathway from the inferior olive to climbing fiber synapses on Purkinje cells.

Population code for tracking velocity based on cerebellar Purkinje cell simple spike firing in monkeys

Velocity is an important determinant of the simple spike discharge of cerebellar Purkinje cells. In a previous study, Purkinje cells in the intermediate and lateral cerebellum recorded during manual tracking were found to be tuned to a combination of direction and speed, (i.e. preferred velocity). In this study a population analysis of this simple spike discharge was used to determine whether the velocity of tracking could be predicted. For the majority (30/32) of direction-speed combinations, the population response accurately specified the target velocity. A temporal analysis showed how the population response gradually converged to the required velocity 200 ms prior to the onset of tracking. Therefore, the simple spike discharge of a Purkinje cell ensemble contains sufficient information to reconstruct target velocity, providing support for the hypothesis that the cerebellum controls or signals movement velocity.