Potential mechanisms of tremor tolerance induced in rats by the repeated administration of total alkaloid extracts from the seeds of Peganum harmala Linn

Ethnopharmacological relevance: The seeds of Peganum harmala Linn have been widely used for the treatment of nervous, cardiovascular, gastrointestinal, respiratory, and endocrine diseases and many other human ailments. However, tremor toxicity occurs after overdose and is tolerated following multiple dosing. Thus far, little is known about the underlying mechanisms of tremors and tremor tolerance. Aim of the study: To investigate the potential mechanisms of tremors and tremor tolerance induced in rats by the repeated administration of total alkaloid extracts from the seeds of P. harmala (TAEP). Materials and methods: A tremor model was induced in male Wistar rats by administering TAEP at a dose of 150 mg/kg/day. To evaluate tremor action, behavioral assessment was conducted by using a custom-built tremor acquisition and analysis system. To investigate the relationships between tremors and neurotransmitter levels in the brain, various neurotransmitters were simultaneously quantified by an ultra-performance liquid chromatography combined with electrospray ionization-tandem mass spectrometry (UPLC-ESI-MS/MS) system, and the association between these two parameters was analyzed using Pearson correlation coefficients. To further elucidate the potential mechanisms of the alterations of neurotransmitter levels in cortical tissues, the protein expression levels of several important enzymes and transporters that are closely related to neurotransmitter levels were investigated. In addition, neuropathological analysis was conducted to assess the effect of TAEP on neurons in the brain. To further clarify the potential mechanisms of TAEP-induced neurodegeneration in the brain, c-fos was subjected to immunohistochemical analysis, and oxidative stress markers were examined. Results: Tremors initially occurred in rats after the oral administration of TAEP at a dose of 150 mg/kg/day. However, they were tolerated following repeated dosing. The levels of 5-hydroxytryptamine (5-HT) and glycine (Gly) in cortical tissues were most likely associated with the tremor response. Tremor tolerance also likely resulted from the degeneration of cerebellar Purkinje cells. Furthermore, the alteration of 5-HT levels was mainly attributed to the downregulated expression of monoamine oxidase A (MAO-A). The degeneration of Purkinje neurons might have resulted from the overexpression of c-fos and increased oxidative stress in the cerebellum after the multiple dosing of TAEP. Conclusion: The tremor response induced by TAEP at high doses is closely related to the concentrations of 5-HT and Gly in cortical tissues. Tremor tolerance may also be attributed to the degeneration of cerebellar Purkinje cells after the repeated dosing of TAEP. Further studies should be conducted to elucidate the interaction of the alkaloids on the neurotransmitter receptors, the expression of related neurotransmitter receptors, the specific signaling pathway involved in regulating MAO-A, and the mechanism of the loss and functional recovery of cerebellar Purkinje neurons.