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

Chemical suppression of harmaline-induced body tremor yields recovery of pairwise neuronal coherence in cerebellar nuclei neurons

Neuronal oscillations occur in health and disease; however, their characteristics can differ across conditions. During voluntary movement in freely moving rats, cerebellar nuclei (CN) neurons display intermittent but coherent oscillations in the theta frequency band (4-12 Hz). However, in the rat harmaline model of essential tremor, a disorder attributed to cerebellar malfunction, CN neurons display aberrant oscillations concomitantly with the emergence of body tremor. To identify the oscillation features that may underlie the emergence of body tremor, we analyzed neuronal activity recorded chronically from the rat CN under three conditions: in freely behaving animals, in harmaline-treated animals, and during chemical suppression of the harmaline-induced body tremor. Suppression of body tremor did not restore single neuron firing characteristics such as firing rate, the global and local coefficients of variation, the likelihood of a neuron to fire in bursts or their tendency to oscillate at a variety of dominant frequencies. Similarly, the fraction of simultaneously recorded neuronal pairs oscillating at a similar dominant frequency (<1 Hz deviation) and the mean frequency deviation within pairs remained similar to the harmaline condition. Moreover, the likelihood that pairs of CN neurons would co-oscillate was not only significantly lower than that measured in freely moving animals, but was significantly worse than chance. By contrast, the chemical suppression of body tremor fully restored pairwise neuronal coherence; that is, unlike in the harmaline condition, pairs of neurons that oscillated at the same time and frequency displayed high coherence, as in the controls. We suggest that oscillation coherence in CN neurons is essential for the execution of smooth movement and its loss likely underlies the emergence of body tremor.

Cerebellar nuclei neurons display aberrant oscillations during harmaline-induced tremor

Essential tremor, a common, debilitating motor disorder, is thought to be caused by cerebellar malfunction. It has been shown that rhythmic Purkinje cell firing is both necessary and sufficient to induce body tremor. During tremor, cerebellar nuclei (CN) cells also display oscillatory activity. This study examined whether rhythmic activity in the CN characterizes the occurrence of body tremor, or alternatively, whether aberrant bursting activity underlies body tremor. Cerebellar nuclei activity was chronically recorded and analyzed in freely moving and in harmaline treated rats. CN neurons displayed rhythmic activity in both conditions, but the number of oscillatory neurons and the relative oscillation time were significantly higher under harmaline. The dominant frequencies of the oscillations were broadly distributed under harmaline and the likelihood that two simultaneously recorded neurons would co-oscillate and their oscillation coherence were significantly lower. It is argued that these alterations rather than neuronal rhythmicity per se underlie harmaline-induced body tremor.

Stimulus-dependent activation of c-Fos in neurons and glia in the rat cerebellum

The intent of the present study was to use chemical or electrical stimulation of cerebellar afferents to determine how different stimulation paradigms affect the pattern of activation of different populations of neurons in the cerebellar cortex. Specifically, we analyzed immediate changes in neuronal activity, identified neurons affected by different stimulation paradigms, and determined the time course over which neuronal activity is altered. In the present study, we used either systemic (harmaline) or electrical stimulation of the inferior cerebellar peduncle (10 and 40 Hz) to alter the firing rate of climbing and mossy fiber afferents to the rat cerebellum and an antibody made against the proto-oncogene, c-fos, as a marker to identify activated neurons and glia. In control animals, only a few scattered granule cells express nuclear Fos-like immunoreactivity. Although no other cells show Fos-like immunoreactivity in their nuclei, Purkinje cells express Fos-like immunoreactivity within their somatic and dendritic cytoplasm in control animals. Within 15 min of chemical or electrical stimulation, numerous granule and glial cells express Fos-like immunoreactivity in their nuclei. Cells in the molecular layer express Fos-like immunoreactivity following harmaline stimulation in a time and lobule specific manner; they do not appear to be activated in the electrical stimulation paradigm. Following harmaline injections, there is an initial loss of Fos-like immunoreactivity in the cytoplasm of Purkinje cells; 90 min later, nuclear staining is observed in a few scattered Purkinje cells. Following electrical stimulation, the cytoplasmic staining in Purkinje cells is enhanced; it is never present in the nucleus. Data derived from this study reveal cell-specific temporal and spatial patterns of c-Fos activation that is unique to each paradigm. Further, it reveals the presence of an activity dependent protein in the cytoplasm of Purkinje cell somata and dendrites.

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.

Ultrastructure of synapses in the cerebellar cortex after long-term activation of climbing fibres

Adult male rats were treated for 24 or 48 h with harmaline which selectively activates neurones in the inferior olivary nucleus which give rise to climbing fibres projecting to the cerebellar vermis. Electrophysiological studies have shown that harmaline-induced climbing fibre activity completely blocks the responses of the Purkinje cell to parallel fibre input. Morphometric analysis of the ultrastructure of climbing and parallel fibre synapses revealed no significant differences in morphology between vermis (experimental) and hemisphere (control). These findings indicate that the decreased responsiveness of Purkinje cells to parallel fibre inputs induced by increased climbing fibre activity over 24 or 48 h is not accompanied by any observable structural changes in the cerebellar cortex.

Oscillatory behavior in inferior olive neurons: mechanism, modulation, cell aggregates

Inferior olive neurons, in brain slices maintained in vitro, display spontaneous, continuous oscillations of their membrane potential which are consonant with olivary rhythmic activity seen in vivo. This oscillatory behavior was studied with intracellular electrophysiological techniques. The 3-10 Hz rhythmicity of these cells from guinea pigs is tetrodotoxin resistant and dependent on a somatic calcium conductance. The oscillatory behavior can exhibit intrinsic frequency modulation and can be altered by synaptic processes. Synaptic alteration of the oscillatory behavior by afferent sources and extensive electrotonic coupling between cells in local aggregates (shown by Lucifer yellow dye-coupling) provide the substrate for a potent central pattern generator with a well established efferent pathway for control of motor functions.

Harmaline-induced changes in plasma membrane of Purkinje cells: a trans-synaptic effect mediated by climbing fibers

Fixed cerebellar cortex slices of adult male albino rats injected with saline, were exposed to the cholesterol probe filipin and freeze-fractured. Under these conditions, the plasma membrane of Purkinje cell dendrites was labeled with only a limited number of typical, 25-30 nm diameter filipin-sterol complexes. In contrast, in rats injected with harmaline, a drug that induces a rhythmic activation of climbing fibers, dendritic membranes showed an increased number of filipin-sterol complexes. This increased filipin labeling of Purkinje cell dendrites was not observed, however, when harmaline was injected into rats after destruction of climbing fibers. These results suggest that climbing fiber activation may induce a reorganization of the lipid matrix in the plasma membrane of Purkinje cells.

The distribution and synaptic organization of serotoninergic elements in the inferior olivary complex of the opossum

Immunohistochemistry and high-resolution autoradiography were used to analyze the distribution and synaptic organization of serotonin (5HT) - containing elements in the inferior olivary complex of the opossum. Immunoreactive beaded varicosities are present throughout the olivary complex. The densest 5HT immunostaining is present in subnucleus b of the caudal medial accessory nucleus. The rostral principal olive is sparsely populated with immunoreactive elements. Fine beaded fibers arborize throughout the neuropil of all the olivary nuclei except in subnucleus b of the caudal medial accessory nucleus where they also circumscribe neuronal cell bodies. In addition, a distinct population of large beaded fibers are occasionally encountered in the neuropil of the medial accessory nucleus. Ultrastructurally, labeled profiles that correspond in size to the smaller beads (less than 1 micron) contain tubulovesicular elements, large dense-cored vesicles, and clear vesicles. In contrast, larger profiles (greater than 2 microns) are characterized by numerous clear synaptic vesicles. Synaptic junctions were encountered in only 2% of the labeled elements. The majority of the labeled profiles were in juxtaposition to small-diameter dendrites (less than 2 microns) except in the caudal medial accessory nucleus, where they also were found in apposition to olivary cell bodies. Our results, when compared with other accounts, indicate that rather than major differences in the nuclear distribution of serotonin between species, there are differences in the density of serotoninergic elements in specific nuclei of the mammalian inferior olive. Based on the size of the labeled profiles and the distinct vesicle populations, our data suggest there are at least two populations of 5HT varicosities that are in juxtaposition to olivary neurons. Further, boutons containing 5HT primarily interact with the distal dendrites of olivary neurons except in the caudal medial accessory nucleus where cell bodies are in apposition to 5HT varicosities.

Rhythmic discharge of climbing fibre afferents in response to natural peripheral stimuli in the cat

The rhythmicity of inferior olivary neurones evoked by natural ipsilateral forepaw inputs was evaluated in the climbing fibre afferent discharge of Purkinje cells recorded in the cerebellar cortex of the decerebrate, unanaesthetized cat. Almost 50% of all Purkinje cells responding to the forepaw stimulus with an increase in complex spike activity exhibited periodic discharge, with the dominant periodicity being between 100 and 160 ms. In ten of twenty-five neighbouring, simultaneously recorded Purkinje cells the forepaw stimulus evoked similar periodicity in their complex spike discharge. For some cells two peaks of complex spike activity were evoked by a forepaw stimulus without an obvious third peak. By altering the stimulus duration the second peak of the response was shown to be temporally uncoupled to the 'off' phase of the displacement for many cells. The interdependence of the trials contributing to the periodic peaks in the peristimulus time histogram (p.s.t.h.) was examined by a 'separation technique'. This analysis indicated that the complex spikes contributing to a specific peak in the p.s.t.h. were generated with a high degree of independence (i.e. in different trials) from the complex spikes contributing to any other peak. It was hypothesized that the independence of the rhythmic complex spike peaks is due to the long relative refractoriness following a complex spike in a single cell. Therefore, the probability of a complex spike occurring at the next one or two cycles is decreased significantly. As a consequence, an inferior olivary neurone fires usually at only one of the various peaks in response to a single presentation of the forepaw stimulus. This hypothesis predicts that stimuli evoking a complex spike at the initial peak in a high percentage of trials should give rise to less periodicity. This prediction was tested by comparing the presence or absence of evoked oscillation with the probability of evoking a complex spike in the first peak of the p.s.t.h. Cells exhibiting a probability for complex spike discharge of over 50% in the first peak showed much less periodicity than cells with a complex spike occurring in less than 50% of the trials in the first peak. These results are discussed in the context of the inferior olive being viewed as a population of coupled elements with a tendency to oscillate. The natural forepaw stimulus is hypothesized as synchronizing the phases of spontaneously oscillating climbing fibre afferents, resulting in the observed periodicity in the complex spike p.s.t.h.

Cerebellar output regulation by the climbing and mossy fibers with and without the inferior olive

The activity of the olivocerebellar complex and the structures related in series with it have been studied using the complementary action of harmaline and 3-acetylpyridine to isolate the two principal inputs to the cerebellar Purkinje cells. The activities of the various nuclei as well as the entire brain have been simultaneously monitored using the [14C]2-deoxy-glucose method under the various combined effects of the pharmacological agents. (1) Tremogenic doses of harmaline increased the frequency of discharge in selected parts of the olivocerebellar system, increasing climbing fiber input and reducing Purkinje cell simple spike discharges in corresponding parts of the cerebellar cortex. The metabolic activity increased in the inferior olive and in the red nucleus. The results are interpreted as a net reduction of Purkinje cell inhibition on their target neurons, leading to a facilitatory cerebellar output. (2) Systemic injection of neurotoxic doses of 3-acetylpyridine selectively produced total degeneration of the neurons in the inferior olive, resulting in the suppression of complex spikes and a net increase in simple spike output from the Purkinje cells. The metabolic consequences were a reduction or absence in the inferior olive, decrease in the red nucleus, and increases in the Purkinje cell target neuron regions, including the intracerebellar and vestibular nuclei. The study of long survival times following the neurotoxic treatment revealed a transient metabolic marking of the inferior olive during the active glial processes accompanying the degeneration. In other parts the radioautographic changes caused by the destruction of the inferior olive persisted for about 1 month after the administration of the drug. (3) Tremogenic doses of harmaline were given to rats at different times following treatment with 3-acetylpyridine. It was demonstrated that: (a) intoxication of the inferior olive started within the second hour after 3-acetylpyridine administration, corresponding to the time at which the metabolic response to harmaline was also abolished; and (b) the increased metabolic activity produced by harmaline in the olivocerebellar complex was a consequence of an increased activity of the neurons of the inferior olive rather than a direct pharmacological effect of the drug. (4) Partial lesions of the inferior olive led to increased metabolic activity of those parts of the intracerebellar nuclei topographically related to the destroyed parts of the inferior olive. (5) In 3-acetylpyridine-treated animals, local ablation as well as local inactivation of the cerebellar cortex produced localized suppression of the intense labeling in the intracerebellar nuclei obtained in these animals. Since these regions receive synapses which are normally inhibitory, suppression of labeling clearly supports the hypothesis that regional marking may very well be produced by the activity of the presynaptic terminals themselves...

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

The effects of midbrain raphe stimulation and/or harmaline administration on cerebellar cell activities were examined in chloralosed rats. Cerebellar cortical cells were grouped into two categories. From a total sample of 68 cells, 48 were classified as Purkinje cells and the 20 others were unidentified. Midbrain raphe stimulation was found to inhibit for many sec the discharge of 40% of the Purkinje cells and 80% of the unidentified units. Other cells were unaffected, except 4 of them which were excited. Harmaline administration increased the CS firing rate of all Purkinje cells by inducing a rhythmic CS discharge at 7-12 Hz. Moreover, harmaline increased the discharge rate of unidentified units without inducing rhythmic activity. In both types of cerebellar cells a modulation of their firing pattern by periodical pauses at 0.1-0.4/s was noticed. These data are discussed in relation to the known influences of harmaline and cerebellar stimulation on raphe neurons. Taken in this context, results presented here confirm the existence of a modulatory influence of the raphe nuclei on the olivo-cerebellar circuitry. A general model of interactions is proposed.

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.