The secondary spikes of climbing fibre responses recorded from Purkinje cell somata in cat cerebellum

In Vivo Analysis of the Climbing Fiber-Purkinje Cell Circuit in SCA2-58Q Transgenic Mouse Model

Cerebellar Purkinje cells (PCs) and cerebellar pathways are primarily affected in many autosomal dominant cerebellar ataxias. PCs generate complex spikes (CS) in vivo when activated by climbing fiber (CF) which rise from the inferior olive. In this study, we investigated the functional state of the CF-PC circuitry in the transgenic mouse model of spinocerebellar ataxia type 2 (SCA2), a polyglutamine neurodegenerative genetic disease. In our experiments, we used an extracellular single-unit recording method to compare the PC activity pattern and the CS shape in age-matched wild-type mice and SCA2-58Q transgenic mice. We discovered no alterations in the CS properties of PCs in aging SCA2 mice. To examine the integrity of the olivocerebellar pathway, we applied harmaline, an alkaloid that acts directly on the inferior olive neurons. The pharmacological stimulation of olivocerebellar circuit by harmaline uncovered disturbances in SCA2-58Q PC activity pattern and in the complex spike shape when compared with age-matched wild-type cells. The abnormalities in the CF-PC circuitry were aggravated with age. We propose that alterations in CF-PC circuitry represent one of potential causes of ataxic symptoms in SCA2 and in other SCAs.

Heterogeneity of Purkinje cell simple spike-complex spike interactions: zebrin- and non-zebrin-related variations

KEY POINTS

Cerebellar Purkinje cells (PCs) generate two types of action potentials, simple and complex spikes. Although they are generated by distinct mechanisms, interactions between the two spike types exist. Zebrin staining produces alternating positive and negative stripes of PCs across most of the cerebellar cortex. Thus, here we compared simple spike-complex spike interactions both within and across zebrin populations. Simple spike activity undergoes a complex modulation preceding and following a complex spike. The amplitudes of the pre- and post-complex spike modulation phases were correlated across PCs. On average, the modulation was larger for PCs in zebrin positive regions. Correlations between aspects of the complex spike waveform and simple spike activity were found, some of which varied between zebrin positive and negative PCs. The implications of the results are discussed with regard to hypotheses that complex spikes are triggered by rises in simple spike activity for either motor learning or homeostatic functions.

ABSTRACT

Purkinje cells (PCs) generate two types of action potentials, called simple and complex spikes (SSs and CSs). We first investigated the CS-associated modulation of SS activity and its relationship to the zebrin status of the PC. The modulation pattern consisted of a pre-CS rise in SS activity, and then, following the CS, a pause, a rebound, and finally a late inhibition of SS activity for both zebrin positive (Z+) and negative (Z-) cells, though the amplitudes of the phases were larger in Z+ cells. Moreover, the amplitudes of the pre-CS rise with the late inhibitory phase of the modulation were correlated across PCs. In contrast, correlations between modulation phases across CSs of individual PCs were generally weak. Next, the relationship between CS spikelets and SS activity was investigated. The number of spikelets/CS correlated with the average SS firing rate only for Z+ cells. In contrast, correlations across CSs between spikelet numbers and the amplitudes of the SS modulation phases were generally weak. Division of spikelets into likely axonally propagated and non-propagated groups (based on their interspikelet interval) showed that the correlation of spikelet number with SS firing rate primarily reflected a relationship with non-propagated spikelets. In sum, the results show both zebrin-related and non-zebrin-related physiological heterogeneity in SS-CS interactions among PCs, which suggests that the cerebellar cortex is more functionally diverse than is assumed by standard theories of cerebellar function.

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.

Abstract

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.

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.

Duration of Purkinje cell complex spikes increases with their firing frequency

Climbing fiber (CF) triggered complex spikes (CS) are massive depolarization bursts in the cerebellar Purkinje cell (PC), showing several high frequency spikelet components (±600 Hz). Since its early observations, the CS is known to vary in shape. In this study we describe CS waveforms, extracellularly recorded in awake primates (Macaca mulatta) performing saccades. Every PC analyzed showed a range of CS shapes with profoundly different duration and number of spikelets. The initial part of the CS was rather constant but the later part differed greatly, with a pronounced jitter of the last spikelets causing a large variation in total CS duration. Waveforms did not effect the following pause duration in the simple spike (SS) train, nor were SS firing rates predictive of the waveform shapes or vice versa. The waveforms did not differ between experimental conditions nor was there a preferred sequential order of CS shapes throughout the recordings. Instead, part of their variability, the timing jitter of the CS’s last spikelets, strongly correlated with interval length to the preceding CS: shorter CS intervals resulted in later appearance of the last spikelets in the CS burst, and vice versa. A similar phenomenon was observed in rat PCs recorded in vitro upon repeated extracellular stimulation of CFs at different frequencies in slice experiments. All together these results strongly suggest that the variability in the timing of the last spikelet is due to CS frequency dependent changes in PC excitability.

Emergence of a 600-Hz buzz UP state Purkinje cell firing in alert mice

Purkinje cell (PC) firing represents the sole output from the cerebellar cortex onto the deep cerebellar and vestibular nuclei. Here, we explored the different modes of PC firing in alert mice by extracellular recording. We confirm the existence of a tonic and/or bursting and quiescent modes corresponding to UP and DOWN state, respectively. We demonstrate the existence of a novel 600-Hz buzz UP state of firing characterized by simple spikes (SS) of very small amplitude. Climbing fiber (CF) input is able to switch the 600-Hz buzz to the DOWN state, as for the classical UP-to-DOWN state transition. Conversely, the CF input can initiate a typical SS pattern terminating into 600-Hz buzz. The 600-Hz buzz was transiently suppressed by whisker pad stimulation demonstrating that it remained responsive to peripheral input. It must not be mistaken for a DOWN state or the sign of PC inhibition. Complex spike (CS) frequency was increased during the 600-Hz buzz, indicating that this PC output actively contributes to the cerebello-olivary loop by triggering a disinhibition of the inferior olive. During the 600-Hz buzz, the first depolarizing component of the CS was reduced and the second depolarizing component was suppressed. Consistent with our experimental observations, using a 559-compartment single-PC model - in which PC UP state (of about -43mV) was obtained by the combined action of large tonic AMPA conductances and counterbalancing GABAergic inhibition - removal of this inhibition produced the 600-Hz buzz; the simulated buzz frequency decreased following an artificial CS.

Encoding of oscillations by axonal bursts in inferior olive neurons

Inferior olive neurons regulate plasticity and timing in the cerebellar cortex via the climbing fiber pathway, but direct characterization of the output of this nucleus has remained elusive. We show that single somatic action potentials in olivary neurons are translated into a burst of axonal spikes. The number of spikes in the burst depends on the phase of subthreshold oscillations and, therefore, encodes the state of the olivary network. These bursts can be successfully transmitted to the cerebellar cortex in vivo, having a significant impact on Purkinje cells. They enhance dendritic spikes, modulate the complex spike pattern, and promote short-term and long-term plasticity at parallel fiber synapses in a manner dependent on the number of spikes in the burst. Our results challenge the view that the climbing fiber conveys an all-or-none signal to the cerebellar cortex and help to link learning and timing theories of olivocerebellar function.

Axonal propagation of simple and complex spikes in cerebellar Purkinje neurons

In cerebellar Purkinje neurons, the reliability of propagation of high-frequency simple spikes and spikelets of complex spikes is likely to regulate inhibition of Purkinje target neurons. To test the extent to which a one-to-one correspondence exists between somatic and axonal spikes, we made dual somatic and axonal recordings from Purkinje neurons in mouse cerebellar slices. Somatic action potentials were recorded with a whole-cell pipette, and the corresponding axonal signals were recorded extracellularly with a loose-patch pipette. Propagation of spontaneous and evoked simple spikes was highly reliable. At somatic firing rates of approximately 200 spikes/sec, <10% of spikes failed to propagate, with failures becoming more frequent only at maximal somatic firing rates (approximately 260 spikes/sec). Complex spikes were elicited by climbing fiber stimulation, and their somatic waveforms were modulated by tonic current injection, as well as by paired stimulation to depress the underlying EPSCs. Across conditions, the mean number of propagating action potentials remained just above two spikes per climbing fiber stimulation, but the instantaneous frequency of the propagating spikes changed, from approximately 375 Hz during somatic hyperpolarizations that silenced spontaneous firing to approximately 150 Hz during spontaneous activity. The probability of propagation of individual spikelets could be described quantitatively as a saturating function of spikelet amplitude, rate of rise, or preceding interspike interval. The results suggest that ion channels of Purkinje axons are adapted to produce extremely short refractory periods and that brief bursts of forward-propagating action potentials generated by complex spikes may contribute transiently to inhibition of postsynaptic neurons.

Effect of simple spike firing mode on complex spike firing rate and waveform in cerebellar Purkinje cells in non-anesthetized mice

Cerebellar Purkinje cells receive two different excitatory inputs from parallel and climbing fibers, causing simple and complex spikes, respectively. Purkinje cells present three modes of simple spike firing, namely tonic, silent and bursting. The influence of complex spike firing on simple spike firing has been extensively studied. However, it is unknown whether and how the simple spike firing mode may influence complex spike waveform and firing rate in vivo. We studied complex spike firing during tonic and silent mode periods in non-anesthetized mice. We found that complex spike firing rate is not influenced by simple spike firing modes, but that the complex spike waveform is altered following high frequency simple spike firing. This alteration is a specific decrement of the second depolarizing component of the complex spike. We demonstrate that the amplitude of the second depolarizing component is inversely proportional to the simple spike firing rate preceding the complex spike and that this amplitude is independent of previous complex spike firing. This waveform modulation is different from previously reported modulation in paired-pulse depression and refractoriness.

Ascending granule cell axon: an important component of cerebellar cortical circuitry

Physiologic evidence suggests that local activation of the cerebellar granule cell layer produces a much more restricted spatial activation of overlying Purkinje cells than would be expected from the parallel fiber system. These results have led to the suggestion that synapses associated with the ascending granule cell axon may provide a large, direct, excitatory input to Purkinje cells, whereas parallel fiber synapses may be more modulatory in nature. In the current experiments, serial electron microscopy was used to reconstruct synapses associated with these two segments of the granule cell axons in the cerebellar cortex of albino rats. The results indicate that there are significantly more presynaptic vesicles in ascending segment synapses than in parallel fiber synapses. Furthermore, a first-order linear regression analysis revealed positive correlations between all measures of pre- and postsynaptic morphology for parallel fibers, but not for ascending segment synapses. Perhaps most surprisingly, serial reconstructions of postsynaptic spines and their associated dendrites demonstrated that spines contacted by ascending segment synapses are located exclusively on the smallest diameter distal regions of the Purkinje cell dendrites, whereas parallel fiber synapses are found exclusively on intermediate- and large-diameter regions of the spiny branchlets. Based on two independent calculations, we estimate that 20% of the granule cell synapses onto a Purkinje cell are actually made by the ascending segment. By using computer simulations of a single Purkinje cell dendrite, we have also demonstrated that synchronous activation of these distal ascending segment inputs could produce a substantial somatic response. Taken together, these results suggest that the two different regions of granule cell axons may play very different physiologic roles in cerebellar cortex.

Modification of activity-dependent increases of cerebral blood flow by excitatory synaptic activity and spikes in rat cerebellar cortex

1. Mechanisms of activity-dependent increases in cerebral blood flow (CBF) were examined in rat cerebellar cortex using the laser Doppler flow technique and extracellular recordings of single unit activity and field potentials. 2. Stimulation of the monosynaptic climbing fibre system evoked long-lasting complex spikes in Purkinje cells, and extracellular field potentials with a characteristic profile that indicated contributions from both passive and active membrane mechanisms. The concomitant CBF increases were reproducible at fairly short intervals, and suggest that both synaptic activity and spikes may contribute to increased CBF. 3. Stimulation of the disynaptic parallel fibre system inhibited the spiking activity in Purkinje cells, while the postsynaptic activity increased as indicated by the simultaneously recorded field potential. Nevertheless, CBF always increased. The inhibition of spike firing activity was partly dependent on GABAergic transmission, but may also relate to the intrinsic membrane properties of Purkinje cells. 4. The CBF increases evoked by parallel or climbing fibre stimulation were highly correlated to the sum of neural activities, i.e. the negativity of field potentials multiplied by the stimulus frequency. This suggests a robust link between extracellular current flow and activity-dependent increases in CBF. 5. AMPA receptor blockade attenuated CBF increases and field potential amplitudes, while NMDA receptor antagonism did not. This is consistent with the idea that the CBF responses are of neuronal origin. 6. This study has shown that activity-dependent CBF increases evoked by stimulation of cerebellar parallel fibres are dependent on synaptic excitation, including excitation of inhibitory interneurones, whereas the net activity of Purkinje cells, the principal neurones of the cerebellar cortex, is unimportant for the vascular response. For the climbing fibre system, not only synaptic activity but also the generation of complex spikes from Purkinje cells contribute to the increases in CBF. The strong correlation between CBF and field potential amplitudes suggests that extracellular ion fluxes contribute to the coupling of brain activity to blood flow.

The secondary spikes of climbing fibre responses recorded from Purkinje cell axons in cat cerebellum

Responses evoked in Purkinje cells by climbing fibre activity were investigated by recording from Purkinje cell axons in the cerebellum of anaesthetized cats. Purkinje cell axons were identified by firing pattern and by latency of responses to stimulation of peripheral nerve and of the inferior olive. Axonal climbing fibre responses usually consisted of one to two spikes, suggesting that normally only the initial spike or, at most, this and one of the secondary spikes are propagated down the Purkinje cell axon. When two successive climbing fibre responses were evoked, the number of spikes in the second response was increased, usually up to three to five. This effect could be obtained at stimulation intervals of up to 100 ms. In a few cases it was possible for a climbing fibre response to be preceded by a parallel fibre volley evoked by stimulation of the cerebellar surface. This increased the number of spikes in the axonal climbing fibre response. The results suggest that the number of propagated spikes in the climbing fibre response can be modified by a preceding input to the Purkinje cell.