Harmaline-induced rhythmic activity of cerebellar and lower brain stem neurons

Regression of cerebellar syndrome with long-term administration of 5-HTP or the combination 5-HTP-benserazide

A quantitative evaluation of cerebellar ataxia, with an ataxia score (total, static, kinetic) and the measurement of objective values relating to the major symptoms, was used in 21 patients with hereditary ataxias treated for 12 months with high doses (16 mg/kg/day) of d-l-5-HTP, l-5-HTP or the combination d-l-5-HTP (16 mg/kg/day)--benserazide (6 mg/kg/day). The data obtained from regular examination were processed by computer. The ataxia showed a significant regression at the 12th month, mainly in the static forms and speed of speech. l-5-HTP appeared to be more effective than d-l-5-HTP. Regression of the cerebellar ataxia was also observed in non-degenerative conditions such as multiple sclerosis and surgical lesion of the anterior lobe vermis, showing that 5-HTP was active on the cerebellar syndrome in general. The regression of the cerebellar ataxia was very slow in inherited diseases and continued for 2 or 4 months after the treatment stopped. A serotoninergic cerebellar control of movement is discussed.

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.

Neural substrates of two different rhythmical vibrissal movements in the rat

Normal rats show two types of rhythmical vibrissal movement, one synchronized precisely with alpha waves (about 9 Hz) of the thalamocortical system, and the other often synchronized with theta waves (about 7 Hz) of the septohippocampal system. The alpha-synchronized vibrissal movements appear while the rat stands still with a slow respiratory pattern, and are of small amplitude (a fine tremor). The theta-synchronized vibrissal movements appear during exploratory sniffing behavior, and are of large amplitude. Thus, a group of facial motoneurons which constitute the final common pathway for vibrissal movement apparently receive input convergently from these two neural systems. In the present study, we observed the following: (1) the two types of movement rarely, if ever, appeared simultaneously and the same was true of the two brain wave patterns. Topographically, the predominant appearance of the alpha waves was in the frontal (sensorimotor) cortex, whereas that of the theta waves was in the occipital cortex and hippocampus. (2) Bilateral ablation of either the entire neocortex or just its anterior (but not posterior) half eliminated the vibrissal alpha-tremor movement while leaving vibrissal theta-sniffing movement normal. In anterior decorticate rats, the tremor movements started to recover by 1-4 months, and were abolished again by the removal of the remaining posterior cortex. (3) Lesions of the medial septum or the fornix eliminated hippocampal theta waves, but had no effect on vibrissal sniffing movement or alpha wave-vibrissal tremor. (4) Cerebellectomy and, to a lesser extent, pharmacological lesions of the inferior olive slowed, but did not block, alpha waves. In addition, vibrissal tremor movement became intermittent and less vigorous. The same manipulations, however, did not affect theta wave-vibrissal sniffing movement. (5) Harmaline (30 mg/kg, i.p.) did not induce alpha-tremor, which, in combination with the results with cerebellectomy and inferior olive lesions, indicates that alpha-tremor is generated by a neural mechanism that is different from that for harmaline-induced generalized tremor of 10 Hz. These findings confirmed that there exist two patterns of synchrony between vibrissal movement and rhythmical brain activity in the rat, i.e. alpha wave-vibrissal tremor movement and theta wave-vibrissal sniffing movement, and suggest that the two patterns reflect a rhythmical mode of functioning of two different neural systems, probably the thalamocortical and the septohippocampal system, respectively.

Central mediation of the conditioned taste aversion induced in rats by harmaline

The assumption that drugs used as unconditioned stimuli in conditioned taste aversion (CTA) studies act centrally was tested by comparing the effects of systemic and intracerebral injections of harmaline hydrochloride (H) in 340 rats. Intraperitoneal injection of 5-20 mg/kg but not of 2.5 mg/kg H administered 5 min after 15-min saccharin (0.1%) drinking decreased saccharin-water preference in a two-choice retention test, performed 48 h later, from 55% to 20%. Since CTA was not diminished when H (10 mg/kg) was injected into rats anesthetised immediately after saccharin drinking by pentobarbital (40 mg/kg), H (1.7-50 micrograms) was administered intracerebrally to anesthetised rats fixed in the stereotaxic apparatus. Injection of 3-6 micrograms H into the inferior olive elicited CTA comparable to that of systemic injection of 10 mg/kg H. Injections of 6 and 50 micrograms H into cerebellum and bulbar reticular formation elicited weaker CTA while neocortical, hypothalamic and mesencephalic applications were ineffective. CTA could also be elicited when 50 micrograms but not 6 micrograms H was injected into the inferior olive 1 or 2 h after saccharin drinking. This delay-dependent effect and failure of non-contingent H administration to change saccharin preference indicates that the H-induced CTA is not contaminated by a non-specific increase in neophobia. It is concluded that H probably elicits CTA by activation of caudal bulbar structures, including the nucleus of the solitary tract, area postrema and lateral reticular formation.

Harmaline induced tremor. III. A combined simple units, horseradish peroxidase, and 2-deoxyglucose study of the olivocerebellar system in the rat

Purkinje cells were recorded extracellularly and mapped in the cerebellar cortex of the rat under tremogenic doses of harmaline. Four different types of responses were encountered, of which two were considered as being responsible for the harmaline tremor. The latter had a regular firing pattern of complex spikes at 5 to 10 Hz and were mostly found in the vermis. Their number decreased in the more lateral region of the cerebellar cortex until they eventually disappeared. Horseradish peroxidase was injected into all the areas of the cerebellar cortex containing Purkinje cells with harmaline-induced activity. Labeled neurons were in all cases traced to the medial accessory olive. The metabolic activity of the inferior olive under harmaline was measured with 2-deoxyglucose. Increased labeling was only found in the medial accessory olive. Such an increase was demonstrated as being due to a direct effect of the drug on the inferior olivary neurons, indicating that the medial accessory olive is responsible for the harmaline tremor in the rat. Our results point out that, in the rat, there is an inverse relationship between serotoninergic innervation of a region in the inferior olivary nucleus and that with harmaline sensitivity, therefore a serotoninergic mechanism hypothesis for the harmaline tremor needs further investigation.

Effects of ethanol on rat cerebellar Purkinje cells

The effects of ethanol on cerebellar Purkinje cells were studied in awake rats immobilized and mechanically ventilated. Single unit activity of 58 cerebellar Purkinje cells was observed for 1 to 8 hr following a single intraperitoneal administration of ethanol doses ranging from 0.25 g/kg to 3.0 g/kg. The alteration in Purkinje cell activity was related to the dose and the blood level of ethanol. Low doses of ethanol tended to excite and high doses tended to inhibit the spontaneous activity of Purkinje cells. At the peak levels of blood ethanol, cerebellar Purkinje cells often exhibited rhythmic short bursting discharges and the periodic intense activation followed by suppression. In contrast to previous observations that actions of ethanol were short, the present data indicate that cerebellar Purkinje cell activity was continuously affected for a few to several hours following a single dose of ethanol. The data are compatible with behavioral observations that impaired coordination in acute ethanol intoxication also had a similar time course.

Harmaline effects on the sensory-motor reactivity: modifications of the acoustic startle pattern

The effects of harmaline, an indoleamine and a MAOI, were tested on the acoustic startle pattern. EMG measures of the startle reflex, the pinna reflex as well as the characters of the vertex evoked responses to brief intense tone bursts (60 msec, 110 dB, 8000 Hz) were simultaneously studied in 4 alert guinea-pigs. The basic experimental design was a 4 by 4 latin square, with the treatments being given at 2 day intervals. The four harmaline-HCl treatments were isotonic saline, 0.25, 5.0, and 10.0 mg/kg. Compared with saline baselines, all the doses resulted, throughout the 60 min session, in overall high significant depressions of the startle reflex, the pinna reflex and the initial wave of the acoustic evoked potential at the vertex. In contrast, harmaline had little or no influence on amplitude and latency of the late wave of the vertex response. The effects of harmaline on the general behavior of the guinea-pig are also reported. These results may support an involvement of serotonergic systems in the modulation of the sensory-motor reactivity at the brainstem level. Nevertheless, the probably more complex cortical processes involved in startle responsiveness do not appear univocally affected by the indoleamine drugs such as harmaline.

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. 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.

Rhythmic activity of spinal interneurons in harmaline-treated cats. A model for olivo-cerebellar influence at the spinal level

After harmaline administration, rhythmic discharges at 8-12 c/s were obtained by intracellular recording from interneurons in the lumbo-sacral cord of decerebrate paralyzed cats. Unitary extracellular recordings were also made from cerebellar Purkinje cells, showing a rhythmic complex spike activity. Simultaneous recordings of cerebellar and spinal interneurons reveal a precise time relation between rhythmic activities recorded at these two levels. Following each rhythmic cerebellar signal one can note an interruption of interneuron activity. It is also noted that sustained high frequency tonic firing of interneurons coincides with cessation of cerebellar rhythmic activity. These facts are related to the impingement of a disfacilitatory supraspinal influence on spinal interneurons. A model of the mechanisms of signal transfer along the olivo-cerebello-bulbo-spinal system is proposed, according to which tremorogenic supraspinal influences might act on motoneurons by disfacilitating excitatory or inhibitory interneurons.

The possible mechanisms of the high pressure-induced motor disturbances in the cat

High pressures elicit a high-frequency tremor (8-12 c/sec) in mammals, the mechanisms of which are still unknown. The present study shows that: (1) in spite of many similarities observed between the EMG characteristics of harmaline-induced tremor and pressure-induced tremor, cerebellar lesions which suppress the harmaline-induced tremor, do not modify the characteristics of the pressure-induced tremor; (2) at depth, the caudal part of the spinal cat (section at T9-T10 spinal level) displays irregular spontaneous EMG activities which can be clonic or rhythmic (4-8 c/sec), and a neuromuscular stretch hyperreflexivity. These data suggest that the origin of the pressure-induced tremor is spinal and neuromuscular rather than cerebellar.

Harmaline-induced tremor. I. Regional metabolic activity as revealed by [14C]2-deoxyglucose in cat

Changes of local cerebral glucose consumption under the effect of tremogenic doses of harmaline were studied. To find the brain structures activated by the drug, the autoradiographic method using [14C]2-deoxyglucose was applied to young cats. After administration of harmaline, the animals were paralized with flaxedil. Results were compared to a group of control animals not injected with the drug, but submitted to the same experimental protocol. Increases of neuronal activity were observed in several structures. A) Among the relays of the olivo-cerebello-fastigio (and vestibulo)-reticulo-spinal circuit that had been claimed to fire at the frequency of the tremor, labeling was found in: 1. selected portions of the inferior olive including the medial accessory olive and the caudolateral part of the dorsal accessory olive; 2. the molecular layers of the cerebellar cortex including vermian and paravermian zones. Labeling of the olivo-cerbellar system was therefore larger than the compartment controlling the fastigial nucleus and extended to that controlling the interpositus nucleus. B) Other structures not under the direct control of the olivo-cerebellar system displayed increased radioactivity under harmaline: lateral reticular nucleus, nucleus reticularis tegmenti pontis, red nucleus and basal ganglia. Part of the nucleus ambiguus, intensely labeled in the control animals, showed decreased radioactivity under harmaline. The experiments were repeated with the same protocol in another group of animals with unilateral sections of the inferior cerebellar peduncle in order to distinguish between a direct pharmacological influence and a nervous one. Marking of the basal ganglia was not affected by pedunculotomy, suggesting a direct "pharmacological" action of the drug in this cases. On the other hand, marking of the other labeled structures was asymmetric or suppressed by pedunculotomy, and therefore could result from a "nervous" effect secondary to activation of the inferior olive.

Harmaline-induced tremor. II. Unit activity correlation in the interposito-rubral and oculomotor systems of cat

Units were recorded extracellularly in the cat brainstem under the effect of tremogenic doses of harmaline. They were localized post mortem and the units discharging at the harmaline tremor frequency were mapped. Harmaline-sensitive neurons were found in the bulbo-pontine reticular formation, in particular, in the lateral reticular nucleus and the nucleus reticularis tegmenti pontis. The nucleus interpositus as well as the red nucleus also displayed numerous units discharging at the tremor frequency, indicating that the cerebello-interposito-rubro-spinal system controlling the flexor muscles participate in harmaline tremor. Participation of the oculomotor system in the harmaline-induced tremor was tested at the level of the vestibular neurons relaying the vestibulo-ocular reflex, the motoneurons, the eye muscles and the eye movements. No rhythmic discharge at the tremor frequency nor eye movements could be detected, indicating that harmalie tremor does not affect the oculomotor system.

Physiological correlates of ethanol intoxication tolerance, and dependence in rat cerebellar Purkinje cells

After single injections of ethanol (1-4 g/kg, i.p.), single unit extracellular recordings of rat cerebellar Purkinje cells show dramatically increased frequency of climbing fiber bursts and occasional increases in simple spike firing rate. These effects of ethanol are not observed in rats chronically treated with the substance for 11-14 days: firing patterns 0-3 h after the last ethanol administration do not differ significantly from controls. However, rats chronically treated and then withdrawn from ethanol slow significant, progressive decreases in climbing fiber activity and firing rates from 3 to 32 h after the last ethanol administration. These effects on cerebellar Purkinje cell firing appear to be physiological correlates of the phenomena of ethanol intoxication, tolerance, and dependence.

Activation of mossy and climbing fiber pathways to the cerebellar cortex by stimulation of the fornix in the rat

The fornix of the rat was electrically stimulated with bipolar concentric electrodes to determine the properties of single unit responses in Purkinje cells of the cerebellar cortex. Both climbing (CF) and mossy fiber (MF) pathways were activated by fornix stimulation. MF responses were indicated by single or double spike responses appearing at latencies of 5--10 ms. The MF spike responses, as quantified by histogram analysis, were further identified by appearance of graded responses with increasing stimulus strength and by following at frequencies up to and greater than 20/s. CF responses were identified by characteristic complex all-or-none burst responses with latencies usually between 10 and 20 ms and with following frequencies at no faster than 10/s. Experiments which involved movement of the stimulating electrode and production of lesions around it established that the activated fiber system was within the dorsal fornix and not in adjacent areas. The results indicate that hippocampal and other limbic areas can influence the cerebellar cortex by direct mossy and climbing fiber pathways, as has been demonstrated for other afferents. It is further suggested that motor patterns linked to hippocampal activity may be regulated by this system.

[Effects of juxtafastigial stimulation on the rhythmic activity of Purkinje cells in harmaline-treated rats]

In the anaesthetized rat, harmaline induces an olivary activation which results in rhythmic complex-spike (CS) discharges of Purkinje cells (4--8/sec) in the vermian cortex. The temporal organization of the rhythmic CS activity was studied. While some Purkinje cells present long periods of continuous rhythmic activity, the rhythmic CS discharge of other cells is modulated by periodical suppression of activity, with a total cycle length of about 10 sec. This organization can be modified by electrical stimulation of the juxtafastigial region (JF). During periods without spontaneous rhythmical CS firing, single JF shocks produce a late reflex response (200--300 msec) of the Purkinje cells, which appears as a repetitive sequence of CS (up to 30) at the harmaline-like induced frequency. The response obtained is the same whether the electrical stimulus is single or given in succession at regular intervals; however, in order to obtain such an effect, the frequency of the JF stimulation has to be inferior to the rhythmic CS frequency produced by the drug. At a higher stimulation rate (10 c/sec, 3 sec) the JF-induced response of Purkinje cells is abolished and we observe the suppression--4 to 8 sec following stimulation - of any rhythmic CS activity. This experimental modulation of rhythmic activity of the Purkinje cells (inducement or suppression of the CS firing), controlled indirectly by the olivary system, reproduces the spontaneous fluctuations of the thythmicity under harmaline. A neurophysiological model is presented and the functional significance of the results is discussed.

Olivo-cerebellar activity during harmaline-induced tremor.A 2-[14C]deoxyglucose study

The specific neuronal activity of the olivo-cerebellar system of the cat under the effect of harmaline has been studied using the 2-[14C]deoxyglucose method. The results obtained in animals treated with tremogenic doses of harmaline, but immobilized with flaxedil, have been compared with those of non-treated controls. An increased radioactivity in specific regions of the olivo-cerebellar system including parts of the medial and dorsal accessory olive and the corresponding receiving molecular layer have been found. No involvement of the granule cell layer nor of the nuclear cells has been observed. It is therefore suggested that the tremogenic rhythmicity of the cerebellar output is organized uniquely by an increased climbing fiber input.

Significance of central noradrenergic system on harmaline induced tremor

Since there is degeneration of substantia nigra concomitant with that of locus coeruleus (LC) in patients with Parkinson's disease, the study was performed to determine the role of central norepinephrine (NE) on harmaline induced tremor. The duration of harmaline (10 mg/kg IP) induced tremor was significantly reduced by intraventricular administration of L-thero-3,4-dihydroxyphenylserine (200 micrograms/rat) and 1-NE (50 micrograms/rat) was increased NE levels in the cerebral cortex, striatum, diencephalon, cerebellum and brain stem. Electrical stimulation of bilateral LC suppressed harmaline-induced 10-12/sec EMG activities in the neck muscle. Bilateral LC lesion upon electrocoagulation and 6-hydroxydopamine treatment resulted in a significant prolongation of the duration of harmaline induced tremor, reducing NE levels in the brain. These data suggest that central NE originating in the LC neurons has an inhibitory effect on the development of the tremor induced by harmaline.

Cholinergic mechanisms involved in head-shaking of infant rats

Central cholinergic mechanisms involved in D-amphetamine induced head-shaking (H-S) were explored in 9-day-old albino rats using anticholinergic, anticholinesterase and cholinomimetic drugs. Scopolamine (5 mg/kg, IP) blocks both spontaneous and D-amphetamine induced H-S. Physostigmine (0.10 mg/kg, IP), but not neostigmine, increases D-amphetamine induced H-S up to 400%. Pilocarpine (1-10 mg/kg, IP) per se induces H-S and strongly potentiates the amphetamine H-S effect. Cholinergic--catecholaminergic interactions in the CNS are discussed in relation to the expression of this motor item.

Spontaneous and amphetamine induced head-shaking in infant rats

A variable proportion of albino rats 6-11 days old exhibit spontaneous and infrequent rotatory head-shaking episodes. This motor pattern is slightly anticipated and significantly increased in occurrence and duration by the administration of D-amphetamine (5 mg/Kg), with a maximal effect of the drug on the 9th day. The rate of amphetamine induced rhythmic head oscillations increases with age from below 5 cps on the 5th day to about 9 cps on the 10th day. The results are discussed in relation to maturation of both the underlying catecholaminergic pathways, activated by D-amphetamine, and the stretch reflex systems of the head and neck muscles participating in the rhythmic activity. Emphasis is placed on the difference between head-shaking and stereotyped activity.

Drug-induced rhythmical activity in the inferior olivary complex of the rat

Experiments have been performed on pentobarbitone anesthetized or decerebrated rats. The nature of the synchronous rhythmical activity which occurs in the inferior olive following the electrophoretic or systemic administration of harmaline, harmine, dihydro-beta-erythroidine and various other compounds, is described. Harmine was shown to reduce the late phase of biphasic unitary action potentials and to evoke massed synchronous rhythmical activity on which the units were superimposed. The beta-carboline was more effective than ACh or DL-homocysteate (DLH) in increasing cell discharge rates. Synchronized rhythmical activity was recorded more than 500 mum from the site of ejection of the rhythm-inducing drugs. Developed rhythmical activity reduced the size of antidromic field potentials, but antidromic invasion could reset the rhythm of submaximal rhythmical activity. The effects of ACh and DLH, glycine, GABA, NA, DA and 5-HT were tested on established rhythmical activity. Of these, 5-HT was the only compound which almost invariably antagonized the rhythm. A number of tryptamine derivatives and reported 5-HT antagonists, as well as parachlorophenylalanine, have been tested, but the results were largely inconclusive. The hypothesis is advanced that the drug-induced rhythm results from the inhibition of a tonic inhibitory serotonergic input. This antagonism releases an innate tendency of olivary cells to discharge both rhythmically and synchronously.

Central mechanisms of tremor in some feline and primate models

For several years our interest has been in a postural Parkinson-like tremor at 4-6/sec. which can be produced in the monkey by lesions of the central nervous system. We have also studied the effects of harmaline, a drug which evokes or intensifies the Parkinson-like tremor in lesioned animals and which also induces a fine, generalized tremor at 7-12/sec. in normal animals. The results obtained so far indicate that these two types of tremor are generated by two independent central mechanisms which do not require the integrity of peripheral feedback loops. The experimental Parkinson-like tremor is generated by a thalamo-cortical mechanism while the olivo-cerebellar system is responsible for the faster "physiological" tremor. Similar tremor mechanisms may be involved in some movement disorders in man.