Activity in the olivo-cerebello-bulbar system of the cat during ibogaline- and oxotremorine-induced tremor

Essential tremor (ET) is one of the most common and most disabling movement disorders among adults. The drug treatment of essential tremor remains unsatisfactory. Additional therapies are required for patients with inadequate response or intolerable side effects. Thus, we aimed to investigate the therapeutic effects of riluzole on harmaline-induced tremor and ataxia in rat, and determining whether riluzole exerts its effect through modulation of glutamate levels in cerebellum. The study included 5 groups of Wistar rats weighing 80–100 g, injected with harmaline (50 mg/kg i.p.) for inducing experimental tremors and ataxia. The rats in group 1 served as control (saline induced) and group 2 received harmaline alone, whereas the animals in groups 3, 4 and 5, were also given riluzole intraperitoneally at doses of 2, 4 and 8 mg/kg 10 min after harmaline administration, respectively. The intensity and duration of tremor were recorded. Rotarod test, distance traveled and number of crossings were used to evaluate motor performance. Results of this study demonstrated that riluzole dose dependently attenuated duration and intensity of harmaline-induced tremors. Also, riluzole significantly improves time to fall, distance traveled and number of crossings in combined riluzole and harmaline treated rats. Histological analysis indicated that harmaline could cause vermis Purkinje cell (PC) loss and riluzole prevented this toxic effect. Harmaline also could increase glutamate levels in vermis and treatment with riluzole restored glutamate levels. In conclusion, riluzole has relatively protective effects on harmaline-induced tremor, probably related to its inhibitory effect on glutamatergic neurotransmission.

Ibogaine is a tremorigenic hallucinogen that has been proposed for clinical use in treating addiction. We previously reported that ibogaine, administered systemically, produces degeneration of a subset of Purkinje cells in the cerebellum, primarily within the vermis. Ablation of the inferior olive affords protection against ibogaine-induced neurotoxicity leading to the interpretation that ibogaine itself is not directly toxic to Purkinje cells. We postulated that ibogaine produces sustained excitation of inferior olivary neurons that leads to excessive glutamate release at climbing fiber terminals, causing subsequent excitotoxic injury to Purkinje cells. The neuronal degeneration induced by ibogaine provides an animal model for studying excitotoxic injury in order to analyze the contribution of glutamate receptors to this injury and to evaluate neuroprotective strategies. Since non-N-methyl-d-aspartate (NMDA) receptors mediate Purkinje cell excitation by climbing fibers, we hypothesized that 1-4-aminophenyl-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-52466), which antagonizes non-NMDA receptors, may have a neuroprotective effect by blocking glutamatergic excitation at climbing fiber synapses. To test this hypothesis, rats were administered systemic ibogaine plus GYKI-52466 and the degree of neuronal injury was analyzed in cerebellar sections. The results indicate that the AMPA antagonist GYKI-52466 (10 mg/kg i.p.×3) does not protect against Purkinje cell injury at the doses used. Rather, co-administration of GYKI-52466 with ibogaine produces increased toxicity evidenced by more extensive Purkinje cell degeneration. Several hypotheses that may underlie this result are discussed. Although the reason for the increased toxicity found in this study is not fully explained, the present results show that a non-NMDA antagonist can produce increased excitotoxic injury under some conditions. Therefore, caution should be exercised before employing glutamate antagonists to reduce the risk of neuronal damage in human clinical disorders. Moreover, the contribution of different glutamate receptors to excitotoxic injury is complex and merits further analysis.

The indole alkaloids ibogaine and harmaline are β-carboline derivatives that cause both hallucinations and tremor. Reports that ibogaine may have potent anti-addictive properties have led to initiatives that it be tested for the treatment of opiate and cocaine addiction. In this study, ibogaine-treated rats were analysed for evidence of neurotoxic effects because human clinical trials of ibogaine have been proposed. We recently found that ibogaine induces a marked glial reaction in the cerebellum with activated astrocytes and microglia aligned in parasagittal stripes within the vermis.²⁸ Based on those findings, the present study was conducted to investigate whether ibogaine may cause neuronal injury or degeneration. The results demonstrate that, after treatment with ibogaine or harmaline, a subset of Purkinje cells in the vermis degenerates. We observed a loss of the neuronal proteins microtubule-associated protein 2 and calbindin co-extensive with loss of Nissl-stained Purkinje cell bodies. Argyrophilic staining of Purkinje cell bodies, dendrites and axons was obtained with the Gallyas reduced silver method for degenerating neurons. Degenerating neurons were confined to narrow parasagittal stripes within the vermis. We conclude that both ibogaine and harmaline have selective neurotoxic effects which lead to degeneration of Purkinje cells in the cerebellar vermis. The longitudinal stripes of neuronal damage may be related to the parasagittal organization of the olivocerebellar climbing fiber projection. Since these drugs produce sustained activation of inferior olivary neurons, we hypothesize that release of an excitatory amino add from climbing fiber synaptic terminals may lead to excitotoxic degeneration of Purkinje cells.

The genetically dystonic rat is insensitive to the tremorogenic effects of harmaline. This behavioral deficit has been linked to a defect in the olivocerebellar pathway, since few Purkinje cells of dystonic rats show a normal increase in rhythmic complex spike activity following harmaline. In normal rats, the Purkinje cell response to harmaline and tremor are initiated by a rhythmic increase in neuronal firing in the caudal inferior olive. The present single unit recording study was conducted, therefore, to determine if the inferior olive of the dystonic rat is activated by harmaline. Olivary unit responses to harmaline were also examined in normal rats made tolerant to harmaline tremor. These rats are behaviorally insensitive to harmaline and also fail to display rhythmic complex spike activity but do not have the motor deficits of the mutant rats. The spontaneous firing rate of neurons in the caudal and rostral inferior olive of the dystonic rat was significantly slower than that of phenotypically normal littermates. Despite this, all cells recorded in the caudal portion of the medial accessory olive of both dystonic and normal rats showed increased rhythmic activity following harmaline injection. Thus, the failure of the mutants to show harmaline tremor is not due to a failure of the drug to activate cells in the olive. Rather, the data suggest a defect in the subsequent transmission of this information. Unlike the control and dystonic rats, harmaline-tolerant rats failed to show sustained rhythmic activity in the inferior olive. These findings suggest that chronic treatment with harmaline may interfere with harmaline tremor at the level of the inferior olive.

Both normal and genetically dystonic (dt) rats show a high-frequency forepaw tremor in response to systemic administration of the serotonin (5-HT) agonist quipazine at 8 days of age. The response declines with age in normal, but not dystonic, rats. By 16 days of age and after the development of a generalized movement disorder, the dystonic rat exhibits enhanced sensitivity to the tremorogenic effects of the drug in comparison with normal rats. Tremor was blocked by pretreatment with ketanserin, suggesting that it is mediated by 5-HT₂ receptors. The dystonic rat has previously been shown to be insensitive to the tremorogenic effects of harmaline, a drug presumed to act indirectly through serotonergic neurons. This finding, coupled with the increased sensitivity to quipazine, suggests the presence of an abnormality in serotonergic systems in the mutants. Since there is evidence of abnormality in the olivo-cerebellar system in the dystonic rat, the alternative hypothesis that a nonserotonergic defect in the olivo-cerebellar system accounts for both the failure of behavioral response to harmaline and the persistent expression of a response to quipazine is also discussed.

Administration of the beta-carboline alkaloid, harmaline, causes the neurons of the inferior olive to fire synchronously and to act as a pacemaker for the generation of tremor. Rats treated daily with harmaline showed a progressive loss of drug-induced tremor. This tolerance was long-lasting and specific. No cross-tolerance was noted to the drug oxotremorine. Prevention or attenuation of tremor by pretreatment with diazepam or morphine preserved the tremorogenic capacity of harmaline when administered alone. These results suggest a relatively permanent change in the olivo-cerebello-bulbar pathway that underlies the generation of tremor induced by harmaline. Treatment with harmaline also increased cyclic 3′,5′-guanosine monophosphate (cGMP) in the cerebellum, presumably through activation of the climbing fiber pathway from the inferior olive to the cerebellar cortex. These increases were attenuated after repeated treatment. These results suggest that the site of tolerance to the tremogenic effects of harmaline lies within the olivo-cerebellar system.