Fredrik Bengtsson

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.

Cerebellar control of the inferior olive.

A subpopulation of neurones in the cerebellar nuclei projects to the inferior olive, the source of the climbing fibre input to the cerebellum. This nucleo-olivary projection follows the zonal and, probably also, the microzonal arrangement of the cerebellum so that closed loops are formed between the neurones in the olive, the cerebellar cortex and the nuclei. The nucleo-olivary pathway is GABAergic, but several investigators argue that its main effect is to regulate electrotonic coupling between cells in the inferior olive rather than inhibit the olive. However, there is now strong evidence that the nucleo-olivary fibres do inhibit the olive. Three functions have been suggested for this inhibition: (i) feedback control of background activity in Purkinje cells, (ii) feedback control of learning, and (iii) gating of olivary input in general. Evidence is consistent with (i) and (ii). Activity in the nucleo-olivary pathway suppresses both synaptic transmission and background activity in the olive. When learned blink responses develop, the blink related part of the olive is inhibited while blinks are produced. When the nucleo-olivary pathway is interrupted, there is a corresponding increase in complex spike discharge in Purkinje cells followed by a strong suppression of simple spike firing. Stimulation of the pathway has the opposite results. It is concluded that the nucleo-olivary fibres are inhibitory and that they form a number of independent feedback loops, each one specific for a microcomplex, that regulate cerebellar learning as well as spontaneous activity in the olivo-cerebellar circuit.

Sensory transmission in cerebellar granule cells relies on similarly coded mossy fiber inputs

The computational principles underlying the processing of sensory-evoked synaptic inputs are understood only rudimentarily. A critical missing factor is knowledge of the activation patterns of the synaptic inputs to the processing neurons. Here we use well-defined, reproducible skin stimulation to describe the specific signal transformations that occur in different parallel mossy fiber pathways and analyze their representation in the synaptic inputs to cerebellar granule cells. We find that mossy fiber input codes are preserved in the synaptic responses of granule cells, suggesting a coding-specific innervation. The computational consequences of this are that it becomes possible for granule cells to also transmit weak sensory inputs in a graded fashion and to preserve the specific activity patterns of the mossy fibers.

Intrinsic Plasticity Complements Long-Term Potentiation in Parallel Fiber Input Gain Control in Cerebellar Purkinje Cells

Synaptic gain control and information storage in neural networks are mediated by alterations in synaptic transmission, such as in long-term potentiation (LTP). Here, we show using both in vitro and in vivo recordings from the rat cerebellum that tetanization protocols for the induction of LTP at parallel fiber (PF)-to-Purkinje cell synapses can also evoke increases in intrinsic excitability. This form of intrinsic plasticity shares with LTP a requirement for the activation of protein phosphatases 1, 2A, and 2B for induction. Purkinje cell intrinsic plasticity resembles CA1 hippocampal pyramidal cell intrinsic plasticity in that it requires activity of protein kinase A (PKA) and casein kinase 2 (CK2) and is mediated by a downregulation of SK-type calcium-sensitive K conductances. In addition, Purkinje cell intrinsic plasticity similarly results in enhanced spine calcium signaling. However, there are fundamental differences: first, while in the hippocampus increases in excitability result in a higher probability for LTP induction, intrinsic plasticity in Purkinje cells lowers the probability for subsequent LTP induction. Second, intrinsic plasticity raises the spontaneous spike frequency of Purkinje cells. The latter effect does not impair tonic spike firing in the target neurons of inhibitory Purkinje cell projections in the deep cerebellar nuclei, but lowers the Purkinje cell signal-to-noise ratio, thus reducing the PF readout. These observations suggest that intrinsic plasticity accompanies LTP of active PF synapses, while it reduces at weaker, nonpotentiated synapses the probability for subsequent potentiation and lowers the impact on the Purkinje cell output.

Cerebellar molecular layer interneurons - computational properties and roles in learning

In recent years there has been an increased interest in the function of inhibitory interneurons. In the cerebellum this interest has been paired with successes in obtaining recordings from these neurons in vivo and genetic manipulations to probe their function during behavioral tasks such as motor learning. This review focuses on a synthesis of recent findings on the computational properties that these neurons confer to the cerebellar circuitry and on their recently discovered capacity for plasticity and learning in vivo. Since the circuitry of the cerebellar cortex is relatively well-defined, the specificity with which the potential roles of these interneurons can be described will help to guide new avenues of research on the functions of interneurons in general.

In vivo analysis of inhibitory synaptic inputs and rebounds in deep cerebellar nuclear neurons.

Neuronal function depends on the properties of the synaptic inputs the neuron receive and on its intrinsic responsive properties. However, the conditions for synaptic integration and activation of intrinsic responses may to a large extent depend on the level of background synaptic input. In this respect, the deep cerebellar nuclear (DCN) neurons are of particular interest: they feature a massive background synaptic input and an intrinsic, postinhibitory rebound depolarization with profound effects on the synaptic integration. Using in vivo whole cell patch clamp recordings from DCN cells in the cat, we find that the background of Purkinje cell input provides a tonic inhibitory synaptic noise in the DCN cell. Under these conditions, individual Purkinje cells appear to have a near negligible influence on the DCN cell and clear-cut rebounds are difficult to induce. Peripheral input that drives the simple spike output of the afferent PCs to the DCN cell generates a relatively strong DCN cell inhibition, but do not induce rebounds. In contrast, synchronized climbing fiber activation, which leads to a synchronized input from a large number of Purkinje cells, can induce profound rebound responses. In light of what is known about climbing fiber activation under behaviour, the present findings suggest that DCN cell rebound responses may be an unusual event. Our results also suggest that cortical modulation of DCN cell output require a substantial co-modulation of a large proportion of the PCs that innervate the cell, which is a possible rationale for the existence of the cerebellar microcomplex.

Feedback control of Purkinje cell activity by the cerebello-olivary pathway.

The pathway from the deep cerebellar nuclei to the inferior olive, the source of the climbing fibre input to the cerebellum, inhibits olivary transmission. As climbing fibre activity can depress the background firing of the Purkinje cells, it was suggested that nucleo‐olivary (N–O) inhibition is a negative feedback mechanism for regulating Purkinje cell excitability. This suggestion was investigated, in a set‐up with decerebrate ferrets, both by blocking and by stimulating cerebellar output while recording Purkinje cell activity. Blocking the N–O pathway was followed by an increased climbing fibre activity and a dramatic reduction in simple spike firing. Stimulation of the N–O fibres depressed climbing fibre responses and caused an increase in simple spike firing. These results are taken as support for the feedback hypothesis.

Segregation of Tactile Input Features in Neurons of the Cuneate Nucleus

Summary Our tactile perception of external objects depends on skin-object interactions. The mechanics of contact dictates the existence of fundamental spatiotemporal input features—contact initiation and cessation, slip, and rolling contact—that originate from the fact that solid objects do not interpenetrate. However, it is unknown whether these features are represented within the brain. We used a novel haptic interface to deliver such inputs to the glabrous skin of finger/digit pads and recorded from neurons of the cuneate nucleus (the brain’s first level of tactile processing) in the cat. Surprisingly, despite having similar receptive fields and response properties, each cuneate neuron responded to a unique combination of these inputs. Hence, distinct haptic input features are encoded already at subcortical processing stages. This organization maps skin-object interactions into rich representations provided to higher cortical levels and may call for a re-evaluation of our current understanding of the brain’s somatosensory systems.

Stimulation within the cuneate nucleus suppresses synaptic activation of climbing fibers.

Several lines of research have shown that the excitability of the inferior olive is suppressed during different phases of movement. A number of different structures like the cerebral cortex, the red nucleus, and the cerebellum have been suggested as candidate structures for mediating this gating. The inhibition of the responses of the inferior olivary neurons from the red nucleus has been studied extensively and anatomical studies have found specific areas within the cuneate nucleus to be target areas for projections from the magnocellular red nucleus. In addition, GABA-ergic cells projecting from the cuneate nucleus to the inferior olive have been found. We therefore tested if direct stimulation of the cuneate nucleus had inhibitory effects on a climbing fiber field response, evoked by electrical stimulation of the pyramidal tract, recorded on the surface of the cerebellum. When the pyramidal tract stimulation was preceded by weak electrical stimulation (5–20 μA) within the cuneate nucleus, the amplitude of the climbing fiber field potential was strongly suppressed (approx. 90% reduction). The time course of this suppression was similar to that found after red nucleus stimulation, with a peak suppression occurring at 70 ms after the cuneate stimulation. Application of CNQX (6-cyano-7-nitroquinoxaline-2,3-dione, disodium salt) on the cuneate nucleus blocked the suppression almost completely. We conclude that a relay through the cuneate nucleus is a possible pathway for movement-related suppression of climbing fiber excitability.

Spatio-temporal skin strain distributions evoke low variability spike responses in cuneate neurons.

A common method to explore the somatosensory function of the brain is to relate skin stimuli to neurophysiological recordings. However, interaction with the skin involves complex mechanical effects. Variability in mechanically induced spike responses is likely to be due in part to mechanical variability of the transformation of stimuli into spiking patterns in the primary sensors located in the skin. This source of variability greatly hampers detailed investigations of the response of the brain to different types of mechanical stimuli. A novel stimulation technique designed to minimize the uncertainty in the strain distributions induced in the skin was applied to evoke responses in single neurons in the cat. We show that exposure to specific spatio-temporal stimuli induced highly reproducible spike responses in the cells of the cuneate nucleus, which represents the first stage of integration of peripheral inputs to the brain. Using precisely controlled spatio-temporal stimuli, we also show that cuneate neurons, as a whole, were selectively sensitive to the spatial and to the temporal aspects of the stimuli. We conclude that the present skin stimulation technique based on localized differential tractions greatly reduces response variability that is exogenous to the information processing of the brain and hence paves the way for substantially more detailed investigations of the brains somatosensory system.