Antiseizure Effects of Scoparone, Borneol and Their Impact on the Anticonvulsant Potency of Four Classic Antiseizure Medications in the Mouse MES Model-An Isobolographic Transformation

Abstract

Numerous botanical drugs containing coumarins and terpenes are used in ethnomedicine all over the world for their various therapeutic properties, especially those affecting the CNS system. The treatment of epilepsy is based on antiseizure medications (ASMs), although novel strategies using naturally occurring substances with confirmed antiseizure properties are being developed nowadays. The aim of this study was to determine the anticonvulsant profiles of scoparone (a simple coumarin) and borneol (a bicyclic monoterpenoid) when administered separately and in combination, as well as their impact on the antiseizure effects of four classic ASMs (carbamazepine, phenytoin, phenobarbital and valproate) in the mouse model of maximal electroshock-induced (MES) tonic-clonic seizures. MES-induced seizures were evoked in mice receiving the respective doses of the tested natural compounds and classic ASMs (when applied alone or in combinations). Interactions for two-drug and three-drug mixtures were assessed by means of isobolographic transformation of data. Polygonograms were used to illustrate the types of interactions occurring among drugs. The total brain content of ASMs was measured in mice receiving the respective drug treatments with fluorescent polarization immunoassay. Scoparone and borneol, when administered alone, exerted anticonvulsant properties in the mouse MES model. The two-drug mixtures of scoparone with valproate, borneol with phenobarbital and borneol with valproate produced synergistic interactions in the mouse MES model, while the remaining tested two-drug mixtures produced additivity. The three-drug mixtures of scoparone + borneol with valproate and phenobarbital produced synergistic interactions in the mouse MES model. Verification of total brain concentrations of valproate and phenobarbital revealed that borneol elevated the total brain concentrations of both ASMs, while scoparone did not affect the brain content of these ASMs in mice. The synergistic interaction of scoparone with valproate observed in the mouse MES model is pharmacodynamic in nature. Borneol elevated the brain concentrations of the tested ASMs, contributing to the pharmacokinetic nature of the observed synergistic interactions with valproate and phenobarbital in the mouse MES model.

Keywords: antiseizure medication; borneol; coumarin; isobolographic transformation; maximal electroshock; scoparone.

Figure 1

Chemical structure of scoparone and borneol.

Figure 2

Time course of the anticonvulsant effects of scoparone and borneol in the mouse MES model. (A,C) Time course and dose–response relationship curves for scoparone and borneol administered systemically (i.p.) in the mouse MES model. The dotted line indicates the approx. ED₅₀ values for scoparone and borneol, respectively. (B,D) Columns represent the ED₅₀ values for scoparone and borneol when administered i.p. at various pretreatment times.

Figure 3

Isobolographic transformation of data for interactions between classic antiseizure medications and scoparone in the mouse MES model. Interactions between scoparone and carbamazepine (a), phenytoin (b), phenobarbital (c) and valproate (d) are plotted on isobolograms. The constant dose of scoparone in mixture is parallel to the Y-axis. The oblique line connecting the ED₅₀ values of X- and Y-axes (as the line of additivity) reflects the theoretically calculated ED_50add values. Point A reflects the theoretically calculated ED_50add value for the two-drug mixture (scoparone + ASM), whereas the experimentally derived ED_50exp is illustrated graphically as point E.

Figure 4

Isobolographic transformation of data for interactions between classic antiseizure medications and borneol in the mouse MES model. Interactions between borneol and carbamazepine (a), phenytoin (b), phenobarbital (c) and valproate (d) are plotted on isobolograms. The constant dose of borneol in mixture is parallel to the Y-axis. The oblique line connecting the ED₅₀ values of X- and Y-axes (as the line of additivity) represents the theoretically calculated ED_50add values. Point A reflects the theoretically calculated ED_50add value for the two-drug mixture (borneol + ASM), whereas the experimentally derived ED_50exp is illustrated graphically as point E. ** p < 0.01 vs. the respective ED_50add value for valproate + borneol (point A).

Figure 5

Polygonogram illustrating interactions for two-drug (a) and three-drug (b) mixtures among scoparone, borneol and four classic antiseizure medications in the mouse MES model. Black lines illustrate the additive interactions, whereas the red lines indicate synergistic interactions.

Figure 6

Influence of scoparone and borneol on total brain concentrations of PB (a) and VPA (b) in experimental animals. Scatter plots with whiskers illustrate mean concentrations (in μg/g of wet brain tissue ± SD of 8 determinations) of PB (a) and VPA (b) in the brain of experimental animals. Statistical analysis of data was performed with one-way ANOVA followed by Dunnett’s multiple comparisons test. Brain tissue samples were quantified using fluorescence polarization immunoassay. All the drugs were administered i.p. at doses corresponding to their ED₅₀ values from the mouse MES model. PB—phenobarbital; VPA—valproate; * p < 0.05, ** p < 0.01 and *** p < 0.001 vs. the respective control (PB or VPA alone) animals.