Fluctuation of extracellular potassium and calcium in the cerebellar cortex related to climbing fiber activity

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

T-type channels buddy up

The electrical output of neurons relies critically on voltage- and calcium-gated ion channels. The traditional view of ion channels is that they operate independently of each other in the plasma membrane in a manner that could be predicted according to biophysical characteristics of the isolated current. However, there is increasing evidence that channels interact with each other not just functionally but also physically. This is exemplified in the case of Cav3 T-type calcium channels, where new work indicates the ability to form signaling complexes with different types of calcium-gated and even voltage-gated potassium channels. The formation of a Cav3-K complex provides the calcium source required to activate KCa1.1 or KCa3.1 channels and, furthermore, to bestow a calcium-dependent regulation of Kv4 channels via associated KChIP proteins. Here, we review these interactions and discuss their significance in the context of neuronal firing properties.

The Cav3-Kv4 complex acts as a calcium sensor to maintain inhibitory charge transfer during extracellular calcium fluctuations

Synaptic transmission and neuronal excitability depend on the concentration of extracellular calcium ([Ca](o)), yet repetitive synaptic input is known to decrease [Ca](o) in numerous brain regions. In the cerebellar molecular layer, synaptic input reduces [Ca](o) by up to 0.4 mm in the vicinity of stellate cell interneurons and Purkinje cell dendrites. The mechanisms used to maintain network excitability and Purkinje cell output in the face of this rapid change in calcium gradient have remained an enigma. Here we use single and dual patch recordings in an in vitro slice preparation of Sprague Dawley rats to investigate the effects of physiological decreases in [Ca](o) on the excitability of cerebellar stellate cells and their inhibitory regulation of Purkinje cells. We find that a Ca(v)3-K(v)4 ion channel complex expressed in stellate cells acts as a calcium sensor that responds to a decrease in [Ca]o by dynamically adjusting stellate cell output to maintain inhibitory charge transfer to Purkinje cells. The Ca(v)3-K(v)4 complex thus enables an adaptive regulation of inhibitory input to Purkinje cells during fluctuations in [Ca](o), providing a homeostatic control mechanism to regulate Purkinje cell excitability during repetitive afferent activity.

Changes in the volume of the intercellular space of the cerebral cortex in conditions of peripheral stimulation in rats

Changes in the volume of the intercellular space of the rat cerebral cortex in response to peripheral repetitive stimulation were studied. The volume of the intercellular space and its changes were assessed by a modification of the four-electrode impedance method. The results suggest that evoked electrical activity in the cerebral cortex was accompanied by 3-5% decreases in the volume of the intercellular space.

Noradrenergic influences on the cerebellar cortex: effects on vestibular reflexes under basic and adaptive conditions

Experiments performed either in decerebrate cats or in intact rabbits have shown that functional inactivation of the cerebellar anterior vermis or the flocculus decreased the basic gain of the vestibulospinal or the vestibulo-ocular reflex, respectively. These findings were attributed to the fact that a proportion of the vermal or floccular Purkinje cells, which are inhibitory in function, discharge out of phase with respect to the vestibulospinal or the vestibulo-ocular neurons during sinusoidal animal rotation, thus exerting a facilitatory influence on the gain of the vestibular reflexes. Intravermal injection of a beta-noradrenergic agonist slightly increased the gain of the vestibulospinal reflex, whereas the opposite result was obtained after injection of beta-antagonists. Similarly, intrafloccular injection of a beta-noradrenergic agonist slightly facilitated the gain of the vestibulo-ocular reflex in darkness (but not in light), whereas a small decrease of the reflex occurred after injection of a beta-antagonist. It was postulated that the noradrenergic system acts on Purkinje cells by enhancing their amplitude of modulation to a given labyrinth signal, thus increasing the basic gain of the vestibular reflexes. The Purkinje cells of the cerebellar anterior vermis and the flocculus also exert a prominent role on the adaptation of vestibulospinal and vestibulo-ocular reflexes, respectively. In particular, intravermal or intrafloccular injection of beta-noradrenergic antagonists decreased or suppressed the adaptive capacity of the vestibulospinal and vestibulo-ocular reflexes that always occurred during sustained out-of-phase neck-vestibular or visual-vestibular stimulation, whereas the opposite result was obtained after local injection of a beta-noradrenergic agonist. The noradrenergic innervation of the cerebellar cortex originates from the locus coeruleus complex, whose neurons respond to vestibular, neck, and visual signals. It was postulated that this structure acts through beta-adrenoceptors to increase the expression of immediate-early genes, such as c-fos and Jun-B, in the Purkinje cells during vestibular adaptation. Induction of immediate-early genes could then represent a mechanism by which impulses elicited by sustained neck-vestibular or visuovestibular stimulation are transduced into long-term biochemical changes that are required for cerebellar long-term plasticity.

Effects of intrastriatal injection of quinolinic acid on electrical activity and extracellular ion concentrations in rat striatum in vivo

Changes in neuronal activity and extracellular concentrations of ions were measured in rat striatum for 60-90 min after intrastriatal injection of quinolinic acid, an agonist of the N-methyl-D-aspartate receptor. The excitotoxin induced bursts of synchronous electrical activity which were accompanied by rises in [K+]e (to approximately 6 mM) and decreases in [Ca2+]e (by less than 0.1 mM); [H+]e usually increased (0.1-0.3 pH unit) after a short and small (< 0.1 pH unit) alkaline shift. The magnitude and frequency of these periodic changes decreased with time; after 90 min the amplitudes fell to 10-20% of the early values and the frequency to about one every 8 min as compared to one every 2-3 min immediately after quinolinate injection. By 90 min there was an increase in [K+]e from 3.3 mM to 4.2 mM and a decrease in [Ca2+]e from 1.34 mM to 1.30 mM. It is postulated that activation of the N-methyl-D-aspartate receptor causes disturbances in neuronal activity and ion gradients; restoration of the original ionic balances raises utilization of ATP and places an additional demand on energy-producing pathways. Increased influx of calcium into neurons may lead to an enhanced accumulation and subsequent overload of mitochondria with the cation. This, in turn, could result in dysfunction of the organelles and account for the decrease in respiration and [ATP]/[ADP] that have been observed previously in this model. The results of the present study lead to the conclusion that quinolinic acid produces early changes in activity of striatal neurons and movements of several cations which may contribute to subsequent abnormalities in energy metabolism and ultimately, cell death.

Interaction of the beta-carboline harmaline with a GABA-benzodiazepine mechanism: an electrophysiological investigation on rat hippocampal slices

An interaction of harmaline (HA), a beta-carboline, with benzodiazepine (Bzd) receptors, has been reported. HA perfusion induced a similar, although less potent, depressing effect as clonazepam (CLO) on the amplitude of the population spikes (PS) evoked by Schaffer collateral stimulation in the CA1 area of rat hippocampal slices. The suppressant effect of both CLO and HA on PS amplitude was reversed by simultaneous perfusion of the GABA antagonist picrotoxin. These results suggest that HA acts as a weak or partial agonist at Bzd receptors.

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates

While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 microC/phase. Charge density was held constant (approximately 75 microC cm-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation. EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 microC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 microC cm-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.

Neighboring cerebellar Purkinje cells communicate via retrograde inhibition of common presynaptic interneurons

Paired tight-seal whole-cell recordings were obtained from neighboring Purkinje cells in cerebellar slices. Under voltage clamp, spontaneous inhibitory postsynaptic currents resulted from the activity of GABAergic interneurons, stellate and basket cells. Up to 80% of inhibitory postsynaptic currents in paired recordings were in register. This correlation was not affected by antagonists of glutamate receptors, faded with distance, and was abolished by tetrodotoxin. Earlier work showed that voltage-gated Ca2+ entry into a Purkinje cell elicits a transient presynaptic inhibition of inhibitory postsynaptic currents. It is now shown that this inhibition is not restricted to the stimulated cell, but that it is transmitted to its neighbors. The results indicate that Purkinje cells exchange information by an unconventional mechanism involving retrograde control of inhibitory synapses.

Ryanodine and inositol trisphosphate receptors coexist in avian cerebellar Purkinje neurons

Two intracellular calcium-release channel proteins, the inositol trisphosphate (InsP3), and ryanodine receptors, have been identified in mammalian and avian cerebellar Purkinje neurons. In the present study, biochemical and immunological techniques were used to demonstrate that these proteins coexist in the same avian Purkinje neurons, where they have different intracellular distributions. Western analyses demonstrate that antibodies produced against the InsP3 and the ryanodine receptors do not cross-react. Based on their relative rates of sedimentation in continuous sucrose gradients and SDS-PAGE, the avian cerebellar InsP3 receptor has apparent native and subunit molecular weights of approximately 1,000 and 260 kD, while those of the ryanodine receptors are approximately 2,000 and 500 kD. Specific [3H]InsP3- and [3H]ryanodine-binding activities were localized in the sucrose gradient fractions enriched in the 260-kD and the approximately 500-kD polypeptides, respectively. Under equilibrium conditions, cerebellar microsomes bound [3H]InsP3 with a Kd of 16.8 nM and Bmax of 3.8 pmol/mg protein; whereas, [3H]ryanodine was bound with a Kd of 1.5 nM and a capacity of 0.08 pmol/mg protein. Immunolocalization techniques, applied at both the light and electron microscopic levels, revealed that the InsP3 and ryanodine receptors have overlapping, yet distinctive intracellular distributions in avian Purkinje neurons. Most notably the InsP3 receptor is localized in endomembranes of the dendritic tree, in both the shafts and spines. In contrast, the ryanodine receptor is observed in dendritic shafts, but not in the spines. Both receptors appear to be more abundant at main branch points of the dendritic arbor. In Purkinje neuron cell bodies, both the InsP3 and ryanodine receptors are present in smooth and rough ER, subsurface membrane cisternae and to a lesser extent in the nuclear envelope. In some cases the receptors coexist in the same membranes. Neither protein is observed at the plasma membrane, Golgi complex or mitochondrial membranes. Both the InsP3 and ryanodine receptors are associated with intracellular membrane systems in axonal processes, although they are less abundant there than in dendrites. These data demonstrate that InsP3 and ryanodine receptors exist as unique proteins in the same Purkinje neuron. These calcium-release channels appear to coexist in ER membranes in most regions of the Purkinje neurons, but importantly they are differentially distributed in dendritic processes, with the dendritic spines containing only InsP3 receptors.

Calcium is an intracellular mediator of the climbing fiber in induction of cerebellar long-term depression

In cerebellar Purkinje cells, conjunctive stimulation of parallel fibers and the climbing fiber causes long-term depression of parallel fiber-Purkinje cell transmission. It has been postulated that calcium is an intracellular mediator of the climbing fiber to induce this synaptic modification. To directly test the hypothesis, a calcium-chelating agent, EGTA, was intracellularly injected into Purkinje cells. In these injected cells, conjunctive stimulation failed to induce depression. Instead, it caused potentiation similar to that observed after repetitive stimulation of parallel fibers alone.

Long-term depression as a memory process in the cerebellum

When details of neuronal network structures of the cerebellum were uncovered in the 1960's, a hope emerged that functions of the cerebellum would eventually be explained in terms of operation of the cerebellar neuronal network. While various network models were proposed, involvement of synaptic plasticity in the cerebellar neuronal network as a memory process became a focus of discussion. The characteristic dual inputs to Purkinje cells, one from parallel fibers (axons of granule cells) and the other from climbing fibers, were suggested to represent such synaptic plasticity, and under this assumption, the cerebellar cortex was envisaged as a learning machine for pattern recognition. Despite these theoretical suggestions, earlier efforts to reveal the postulated synaptic plasticity in the cerebellar cortex were unsuccessful. It had then to wait for a decade before long-term depression (LTD) was finally found as its possible substrate. LTD is a long-lasting depression of parallel fiber-to-Purkinje cell transmission that occurs following conjunctive activation of parallel fibers and a climbing fiber both converging onto one and the same Purkinje cell. LTD has now been established by means of various testing methods, and recent efforts have been directed toward its molecular mechanisms. Efforts have also been devoted to demonstrate roles of LTD in motor learning through studies of adaptation of the vestibulo-ocular reflex, adaptive adjustment of hand movement, and more recently eyelid blink conditioned reflex. This article reviews recent efforts to characterize the LTD as a memory process, presumably the major, in the cerebellum.

28 K cholecalcin (CaBP) levels in abnormal cerebella: studies on mutant mice and harmaline- and 3-acetylpyridine-treated rats

The cerebellar Purkinje cells of both birds and mammals contain a specific calcium-binding protein, 28 K cholecalcin (CaBP). This is the same protein as the vitamin D-dependent kidney CaBP, but its Purkinje cell level is apparently vitamin D independent. The cerebellar CaBP contents of 3-acetylpyridine- and harmaline-treated rats and 5 mutant mouse strains (Purkinje cell degeneration, reeler, weaver, staggerer and nervous) were measured using a specific radioimmunoassay. The results indicate that the level of cerebellar CaBP is not dependent on the physiological state of the Purkinje cells but is an intrinsic measure of the size of the Purkinje cell population.

Variations in presynaptic grid size in the granular and molecular layer of the cerebellar cortex of the cat. I. A quantitative ultrastructural study on semithin E-PTA sections

The size distribution of synapses in the cerebellar cortex of the cat was defined on 0.5-micron semithin sections stained with ethanolic phosphotungstic acid (E-PTA). The surface area (SA) of synaptic grids was measured and the number of dense projections per grid (NDP) was counted. The results show large differences in mean values between molecular and granular layer. Within the molecular layer the differences in mean values at different levels below the pial surface were small; however, the frequency distributions differed significantly. In the granular layer a confined unimodal frequency distribution of SA and NDP was observed (mean NDP, 7.02 +/- 0.10), in the molecular layer a considerable variation in the size of the synaptic discs was observed (mean NDP, 20.40 +/- 0.43). Only a small percentage of the synaptic discs have less than 5 or more than 47 DPs. The sharply defined differences in synaptic size between the granular and molecular layer and the smaller differences within the granular and molecular layer are discussed in the context of the congruity hypothesis of Chan-Palay.

Phasic variations of extracellular potassium during fictive swimming in the lamprey spinal cord in vitro

The lamprey spinal cord in vitro can generate the motor pattern underlying locomotion, which can be recorded with suction electrodes in the ventral roots. To test if the extracellular level of potassium changed during rhythmic activity, potassium-sensitive microelectrodes were used to systematically (every 25 micrometers) explore the level of extracellular potassium [K+]e in different loci in the transverse plane of the spinal cord. During fictive locomotion the baseline level of [K+] increased with 0.08-0.4 mM in the grey matter. As a rule phasic variations of up to 0.2 mM, correlated to each ventral root burst, were superimposed on the tonic deviation of [K+]e. Changes in this range may cause a moderate depolarization of the spinal neurones and might also affect other neuronal functions including the rhythm-generating circuits.

Potassium accumulation around individual purkinje cells in cerebellar slices from the guinea-pig

K+-selective micropipettes were used to measure external K+ concentration [( K+]o) in the immediate vicinity of Purkinje cells in slices from guinea-pig cerebellum. The cells were either spontaneously active or were polarized via a separate intracellular micro-electrode. The level of [K+]o rose by 1-3 mM around the soma and dendrites of Purkinje cells during spike activity. The increases in [K+]o were usually greater during Ca2+-mediated spikes than during Na+-mediated spikes. This was even true at the soma where the Ca2+ spike only invaded electrotonically from the dendrites, in contrast to the Na+ spikes which were generated at the soma. No [K+]o changes were seen in the vicinity of Purkinje cells when the cells were hyperpolarized with current passage nor was any [K+]o change seen during subthreshold depolarizations. In glial cells, however, a hyperpolarizing current reduced [K+]o while a depolarizing current increased [K+]o in a symmetrical manner. When Ba2+ was substituted for Ca2+ in the bathing Ringer solution, prolonged plateau-potential spikes could be evoked from Purkinje cells. These spikes were accompanied by large [K+]o elevations but the plateau potentials outlasted the [K+]o elevations. These experiments suggest that large [K+]o increases can occur in the absence of Ca2+-mediated K+ conductances. Substitution of Mn2+ for Ca2+ in the Ringer solution removed some of the [K+]o increases at the Purkinje cell soma. Addition of tetrodotoxin to normal Ringer solution also reduced, but did not abolish the [K+]o increases at the soma. These experiments confirmed that both Na+ and Ca2+ spikes were involved in the [K+]o change. The diffusion characteristics of the slices were determined by an ionophoretic method using tetramethylammonium and ion-selective micropipettes. The extracellular volume fraction of the slice averaged 0.28 while the tortuosity averaged 1.84. These values were close to those found previously in the intact rat cerebellum. These data were used to make quantitative estimates of the expected [K+]o accumulation in the vicinity of a single cell (see Appendix). Such estimates showed reasonable agreement with the measured values. Our data show that quite large increases in [K+]o may occur around single Purkinje cells. Such increases have previously only been evident during the activation of cell populations in mammalian preparations. The present results are probably due to the superior recording conditions of the slice. Implications for intercellular communication are discussed.

The supernormal period of the cerebellar parallel fibers effects of [Ca2+]o and [K+]o

The nonmyelinated parallel fibers (Pfs) of the cerebellar cortex exhibit a pronounced supernormal period following a single conditioning volley. In the present investigation a comparison is made between the effects of changes in extracellular calcium ([Ca2+]o) and potassium ([K+]o) on the supernormal period of the Pfs. [Ca2+]o was monitored directly using ion-sensitive microelectrodes while rat cerebellar Pfs were continuously superfused with solutions containing varying concentrations of K+ (5-30 mM) or Ca2+ (0-6 mM). Pf recovery properties were studied by monitoring control (unconditioned) and test (conditioned by a previous impulse) response latencies. [Ca2+]o did not affect the activity-dependent relative increase in Pf excitability observed following conditioning stimulation (i.e. the supernormal period) although both control and test Pf volley latencies were related to [Ca2+]o. Relatively small increases in superfusate K+ concentration elicited a decrease in the control Pf volley latency but had no effect on the test latency. This resulted in the reduction or obliteration of the latency shift elicited by a conditioning stimulus. Simultaneously decreasing [Ca2+]o and increasing [K+]o decreased control Pf volley latency further than when each ion was altered separately. The test Pf volley latency was unchanged. Therefore, under these conditions, there was no Pf volley latency change following conditioning stimulation. These results are consistent with the hypothesis that activity-dependent changes in extracellular ionic concentrations may, in part, be responsible for the supernormal period in cerebellar parallel fibers.

Changes in extracellular potassium and calcium concentration and neural activity during prolonged electrical stimulation of the cat cerebral cortex at defined charge densities

In cats anesthetized with nitrous oxide and halothane, ion-selecting microelectrodes were used to monitor changes in the concentration of potassium [K+]0 and calcium [Ca2+]0 in the extracellular compartment of the cerebral cortex during as long as 4 h of continuous stimulation of the cortical surface. At stimulus charge densities shown to induce only minimal localized histologic changes [20 microC/cm2 . ph at 50 pulses per second (pps)], [K+]0 at a depth of about 750 micrometers underwent only a transient increase at the beginning of stimulation, followed by a rapid return to the prestimulus concentration. [Ca2+]0 was unaffected. At a higher charge density (100 microC/cm2 . ph at 20 pps) there was a rapid transient increase in [K+]0, followed by a more gradual return to a plateau about 1 mM above the prestimulus value. [Ca+]0 usually underwent an initial increase followed by a slow decrease to a plateau value above 0.5 mM. At a charge density of 100 microC/cm2 . ph and 50 pps (shown in histological studies to induce significant neural damage), [Ca2+]0 slowly decreased to near or below 0.5 mM in the middle layers of the cortex. After 30 to 40 min of stimulation, [K+]0 underwent episodic fluctuations about a plateau value 0.5 to 1 mM above the prestimulus concentration. Simultaneous recordings of the compound action potential in the ipsilateral pyramidal tract indicated that these fluctuations were due to local changes in the excitability of intracortical circuitry conditioned by the intense stimulation. The results have implications for the possible interrelation of the changes in extracellular ionic concentrations and the early stages of stimulation-induced neural damage.

Long-lasting depression of parallel fiber-Purkinje cell transmission induced by conjunctive stimulation of parallel fibers and climbing fibers in the cerebellar cortex

In rabbit cerebellar cortex, local stimulation of parallel fibers induced field potentials with two negative peaks, n1 representing conducting spikes of parallel fibers and n2 postsynaptic excitation in dendrites of Purkinje cells and other cortical cells. Conjunctive stimulation of parallel fibers and climbing fibers at 4 Hz for 30-120 sec caused a significant depression of n2 potential which lasted for at least 1 h. Such an effect could not be produced by stimulation of climbing fibers or parallel fibers alone. These observations support the plasticity assumption in the Marr-Albus model of the cerebellum.

Raphe - cerebellum interactions. I. Effects of cerebellar stimulation and harmaline administration on single unit activity of midbrain raphe neurons in the rat

The firing patterns of single raphe units at the posterior midbrain level were examined in chloralosed rats to assess the effects of cerebellar stimulation and/or harmaline administration. Raphe cells were grouped according to their spontaneous firing rate and other characteristics into two categories. From a total sample of 160 cells, 106 (66%) presenting a slow regular discharge pattern were classified as serotonergic (5-HT cells), whereas 35 (22%), having a faster firing rate, were considered non serotonergic (NS cells). Moreover, 19 (12%) raphe units were non categorized. Cerebellar juxtafastigial (JF) stimulation modified the discharge pattern of 56 (35%) raphe units. The remaining 65% were unaffected by the stimulation. Of the 41 5-HT cells affected by JF stimulation, 28 neurons (68%) showed a systematic increase of their firing rate, whereas of the 12 NS cells affected 8 neurons (66%) were inhibited. It thus appears that cerebellar stimulation has an opposite effect on raphe units according to the cell types. Harmaline administration suppressed the activity of 5-HT cells and increased the discharge rate of NS cells. Moreover, we noticed in the latter units a phase modulation of the firing pattern by pauses occurring with a fixed periodicity of 2.5 to 10 s. Considered in the context of previous studies, these results strongly suggest an inhibitory influence of the raphe system on the olivo-cerebellar circuitry.

The changes in Purkinje cell simple spike activity following spontaneous climbing fiber inputs

The purpose of these experiments was to systematically examine the characteristics of the excitability change occurring after the inactivation period evoked by the climbing fiber input to Purkinje cells. Ninety-eight Purkinje cells were isolated extracellularly in unanesthetized decerebrate cats. Simple spikes and complex spikes were discriminated separately. Post-stimulus time histograms were constructed from 100 consecutive trials triggered by the occurrence of spontaneous complex spikes. Seventeen Purkinje cells exhibited a reduction of simple spike discharge rate following the inactivation period. However, 14 cells showed no change in simple spike activity, and in 67 cells the discharge rate increased. These changes in excitability following a spontaneous complex spike were independent of the tonic simple spike activity of the Purkinje cell. Single traces of spike train data from Purkinje cells showed that the change in discharge rate was variable, some complex spikes being followed by an increase and others by a decrease in activity. The basis for these observations and the differences between these data and those from studies in which the climbing fiber input was evoked by electrical olivary stimulation are discussed.

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.

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

The effects of climbing fibre (CF) impulses on the discharges of cerebellar cortical cells has been studied in the cat. It was found that a brief burst of impulses in a CF can evoke a prolonged excitatory response in the Purkinje (P) cell it innervates. The response consisted of a burst of action potentials that lasted from 50 to 400 msec in different P cells. Bursts of CF impulses were not observed to accelerate the discharge of granule cells. It is therefore suggested that this powerful response results from a direct action of CF impulses on the P cell.

Prolonged depolarization elicited in Purkinje cell dendrites by climbing fibre impulses in the cat

1. Responses evoked in Purkinje cell dendrites by impulses in climbing fibres were studied by recording from the cat cerebellar cortex. Intra- and extracellular responses from dendrites of single Purkinje cells were recorded, as well as field responses from the intact cerebellar surface. The intracellular responses were presumably recorded from relatively proximal dendrites. The resting potential usually was 20-40 mV. The responses consisted of an initial spike-like component (amplitude 10-30 mV) followed by a plateau-like component (amplitude 2-12 mV) with a duration of about 100 ms. The duration of consecutive responses varied little. 3. The extracellular unitary responses were recorded from more distal dendrites. These responses were negative deflexions consisting of an initial component followed by a plateau-like component (amplitude 5-15 mV). The duration of consecutive responses varied widely from about 25 ms to more than 1 s. The negative deflexions are assumed to correspond to dendritic depolarizations. 4. The field responses recorded from the cerebellar surface consisted of a positivity followed by a negativity lasting several hundred milliseconds. The positivity signals the e.p.s.p.s generated by the climbing fibre synapses which do not extend to the most distal dendrites. The negativity presumably signals the plateau-like dendritic depolarizations which would involve also the most distal dendrites. 5. The nature and significance of the plateau-like depolarizations evoked by climbing fibre impulses in the purkinje cell dendrites are discussed.

Modulation of harmaline-induced rhythmic discharge of the inferior olive by juxtafastigial stimulation

We previously reported induction or suppression by juxtafastigial stimulation of the rhythmic complex spike discharge of Purkinje cells in harmaline treated rats. In this paper we show that this modulation of the cerebellar rhythmic activity implies the involvement of inferior olive neurons. These results are discussed in the general framework of the olivo-cerebello-bulbar circuitry. A modulatory control of the inferior olive neuron activity by the raphe system is suggested to explain part of these results.