Effect of tremorigenic agents on the cerebellum: a review of biochemical and electrophysiological data

Effects of harmaline on anxiety-related behavior in mice

Harmaline (HA) is a beta-carboline commonly known to provoke motor alterations through activation of cells in the inferior olive. In addition, this pharmacological agent also induces cognitive disturbances such as motor and spatial learning impairments. In order to complete and extend these data, we examined the effects of this drug on state anxiety in mice, employing elevated plus maze test. We report here that lower doses of harmaline (5-10 mg/kg) have anxiogenic since higher doses (20 mg/kg) have anxiolytic-like properties. Overall pattern of our behavioral results provides evidence that harmaline also acts on emotional reactivity in mice by influencing their decision making when placed in an anxiogenic situation.

Citalopram, a selective serotonin reuptake inhibitor augments harmaline-induced tremor in rats

Citalopram, a serotonin reuptake inhibitor (SSRI) is one of the widely used antidepressants. Apart from its antidepressant activity citalopram is also used for anxiety, panic disorders, obsessive-compulsive disorder and behavioral disturbances of dementia. Tremor is the second most common neurological adverse effect in patients receiving treatment with SSRIs. Use of these agents in depressed patients with essential tremor has not been studied. The present study was undertaken to investigate the effect of chronic citalopram treatment on harmaline-induced tremors in rats. Female Sprague-Dawley rats weighing 70+/-2 g were given citalopram in doses of 0, 10, 20 and 40 mg/kg by gavage for 2 weeks. On the 15th day, the rats were given harmaline (10 mg/kg, i.p.) 30 min after the last dose of citalopram. The latency of onset, intensity and duration of tremor and EMG were recorded. Serotonin (5HT) and 5-hydroxy indole acetic acid (5HIAA) were measured in brain stem. Citalopram dose dependently exacerbated the duration, intensity and amplitude of EMG of harmaline-induced tremor. A significant decrease in 5HT turnover (5HIAA/5HT ratio) in the brain stem was observed suggesting a possible role of serotoninergic impairment in citalopram-induced augmentation of harmaline-induced tremor. Clinical implications of these observations warrant further investigation.

2-deoxy-D-glucose attenuates harmaline induced tremors in rats

Neuronal hyperactivity in essential tremor is accompanied by high energy demand in cerebellum, medulla and the thalamus. It has been suggested that brain regions that have increased metabolic demands are highly vulnerable to interruptions in glucose metabolism. In the present investigation attempt was made to study the effect of 2-deoxyglucose (2DG) a glycolytic pathway inhibitor on harmaline induced tremor in rats. Wistar rats of either sex weighing 100+/-3 g were given harmaline (10 mg/kg, i.p.) alone or along with 2DG (15 min before harmaline) in doses of 300, 600 and 900 mg/kg, respectively. The latency of onset, intensity and duration of tremor following harmaline administration were recorded. Neurobehavioral responses, electromyography (EMG) and levels of blood glucose and cerebellar serotonin (5HT) were determined after 40 min of harmaline administration. 2DG significantly and dose dependently attenuated severity of harmaline induced tremors and amplitude of EMG. Treatment of rats with 2DG alone reduced the locomotor activity, however, no significant change was observed in grip strength, landing foot splay, air righting reflex and response to tactile stimuli. Harmaline alone and along with 2DG had no effect on behavioral parameters except a decrease in landing foot splay. 2DG produced a dose-dependent hyperglycemia and attenuated harmaline induced increase in cerebellar 5HT levels. Our results clearly suggest the protective effect of 2DG in harmaline induced tremor. Further studies are warranted to assess the role of glucoprivation in the suppression of neuronal excitability in tremors.

Functional imaging of tremor

We review the insight that has been obtained from positron emission tomography (PET) and magnetic resonance imaging (MRI) activation studies on the functional anatomy underlying different forms of tremor. The effects of treatment, both pharmacologic and interventional approaches, on the dysfunctional cerebral systems involved in tremor, also are considered. Finally, we speculate on the neuropharmacologic basis of different types of tremor based on available in vivo PET evidence.

Brain neurotoxicity of Penitrem A: electrophysiological, behavioral and histopathological study

The neurotoxicity of Penitrem A (PA) in rats was assessed against neurophysiological, behavioral and histopathological parameters. Animals were acutely given intracerebroventricular (22-45 mg) or intraperitoneal injections (0.5-1.5 mg/kg) of PA. A typical trembling syndrome associated with PA was always noted. Depending on the dose administered, animals may convulse and eventually die (1-1.5 mg/kg). PA-induced transient alterations of the EEG involving an increase in the frequency and voltage of electrical activity recorded from the cerebral cortex. Hippocampal activity was not modified and some pathologic activities may be recorded at the thalamus. Generally these EEG alterations disappeared at d 3 after the injection and the animals progressively recovered. However in the most severe cases, neuromotor disturbances were maintained at d 7 (rotarod test). Coronal sections of the brain at the striatal, thalamic, hippocampal and pons levels mainly revealed that PA was able to induce dose related injuries in the cerebellum with massive degeneration of Purkinje cells and a significant vacuolization within the molecular layer. The neurotoxic mechanism remains unclear. Action of the mycotoxin on the cerebello-thalamo-cortical tract is discussed.

A positron emission tomography study of essential tremor: evidence for overactivity of cerebellar connections

The origin of essential tremor is unknown. Animal models have suggested that the inferior olivary nucleus may act as a tremor generator. We used positron emission tomography to study changes in regional cerebral blood flow associated with involuntary postural tremor and passive wrist oscillation in patients with essential tremor. Activation due to voluntary wrist oscillation and arm extension without tremor was studied in normal control subjects. The essential tremor group had bilaterally increased cerebellar blood flow at rest (without tremor) compared with the control group. Involuntary postural tremor was associated with further bilateral cerebellar activation, and also contralateral striatal, thalamic, and sensorimotor cortex activation. Voluntary wrist oscillation, maintained arm extension without tremor, and passive wrist oscillation were all associated with significant ipsilateral rather than bilateral cerebellar activation. We conclude that essential tremor is associated with increased bilateral cerebellar activity both at rest and during tremor.

Glucose metabolism in the brain of patients with essential tremor

Using positron emission tomography with [18F]fluoro-2-deoxyglucose, we determined the regional cerebral metabolic rate of glucose utilization at rest in 8 medication-free patients with essential tremor and in 10 normal subjects. Taking the metabolic values of regions of interest as ratios to the mean hemispheric metabolism, the patients showed significant glucose hypermetabolism of the medulla and thalami, but not of the cerebellar cortex. This study lends support to earlier suggestions that circuits involving the inferior olivary nuclei in the medulla and the thalmus are involved in the generation of essential tremor.

Electroencephalographic evidence for a dose-related biphasic effect of morphine on bicuculline-induced seizures in the rat

The effects of systemic pretreatment with morphine sulfate (MS) on electroencephalographic (EEG) manifestations of seizures due to bicuculline (BIC) were studied in freely moving rats. Electrodes were placed on frontal and occipital cortices, into the hippocampus and into a region of the prepiriform cortex, area tempestas (AT). BIC (0.1-0.25 mg/kg i.v.) elicited dose-dependent changes in electrocortical activity, consisting of 3 stages: stage 1, slight increase in periods of desynchronization; stage 2, trains of 4-6 Hz spike-and-wave complexes and of 2-4 Hz slow waves; stage 3, 'grand mal' seizures. Isolated spikes and hypersynchronous high voltage spikes occurred in the hippocampus during stage 2 and stage 3, respectively. In the AT, EEG abnormalities were observed during stage 3 only. During the recovery period, cortical synchronization occurred associated with the presence of occasional synchronous spikes in both deep areas. After pretreatment with MS (2, 12, and 25 mg/kg), stage 3 EEG seizures occurred with doses of BIC (0.12 or 0.18 mg/kg i.v.) that in the absence of MS produced only stage 1 or 2. In contrast, after pretreatment with a low dose of MS(0.5 mg/kg) the seizures induced by a maximally effective dose of BIC (0.25 mg/kg) were markedly attenuated. Thus, the effect of MS on BIC-induced seizure activity appears to be biphasic, potentiating seizures at doses at or above 2 mg/kg and inhibiting seizures at lower doses.

Foreign and endogenous serum protein extravasation during harmaline tremors or kainic acid seizures in the rat: a comparison

Cerebrovascular permeability to protein (CVP-p) was assessed in rats following the systemic injection of either kainic acid (KA) or harmaline. The extravasation of a foreign (horseradish peroxidase, HRP) or an endogenous (rat immunoglobulin G, IgG) tracer protein was determined using immunohistochemical methods. During KA-induced seizures, an extravasation of both HRP and presumed IgG occurred in similar forebrain loci; a lamina-specific extravasation occurred within the dorsal hippocampus. During harmaline-induced tremors protein extravasation also occurred, but was tracer dependent. HRP reaction product was observed within the inferior olive, the cortex of the cerebellar vermis and the neocortex. However, IgG-like immunoreactivity was only detected within the circumventricular organs of harmaline-treated rats. Because KA, but not harmaline, is neurotoxic, the results are consistent with an influence of endogenous serum protein extravasation on seizure-related hippocampal damage. Possible homeostatic properties of altered CVP-p are also considered.