ABSTRACT
Title
Role of the endocannabinoid system in a pharmacological model of absence epilepsy
Authors
R. Colangeli1, S. Cianci1,2, R.T. Ngomba3, J. Mairesse4, S. Gaetani1, V. Cuomo1
1Dept. of Physiology and Pharmacology “V. Erspamer”, Sapienza University of Rome, Italy; 2Dept. of Pharmacology and Human Physiology, University of Bari, Italy; 3Neuropharmacology Unit, Neuromed Institute Parco Technologico, Pozzilli, Isernia, Italy; 4NeuroPlasticity Team, UMR 8576 “Functional and Structural Glycobiology Unit”, CNRS/University Lille North of France
Abstract
Absence seizures are characterized by the presence of spike-wave discharges (SWDs) in electroencephalographic recordings and are associated with behavioural immobility (Blumenfeld, 2005). SWDs originate from a pathologic synchronization activity within a thalamic-cortical circuit involving ventrobasal thalamic nuclei, the somatosensory cortex, and the reticular thalamic nucleus (RTN) (reviewed by van Luijtelaar and Sitnikova, 2006). The RTN synchronizes burst firing between reciprocally interconnected glutamatergic thalamic relay neurons and neocortical principal neurons. The synchronization mediated by γ-aminobutyric acid (GABA)ergic neurons of the RTN drives the phasic oscillatory activity within the thalamocortical network that can generate normal rhythms as well as pathologic synchronous discharges such as SWDs (Huguenard and Mc Cormick, 2007).
In these regions of the brain, among others, both GABAergic and glutamatergic transmission are under the inhibitory control of the endocannabinoid system. This system consists of Gi-coupled cannabinoid receptors (CB1 and CB2), the two main endogenous ligands, anandamide and 2-arachidonoylglycerol, and the enzymes regulating their synthesis and metabolism (Piomelli, 2003). In the central nervous system CB1 are mainly localized pre-synaptically, where their activation leads to a transitory inhibition of neurotransmitter release.
In this study we evaluated the involvement of the endocannabinoid system in a validated pharmacological model of absence epilepsy, based on the systemic administration of pentylenetetrazole (PTZ) at a dose (20 mg/kg, i.p.) able to induce absence like seizures and loss of responsiveness in rats (Marescaux, 1984). In particular, we investigated whether PTZ treatment might affect CB1 expression in the thalamo-cortical circuits and whether the cannabinoid agonist WIN 55,212-2 (WIN, 5mg/kg, i.p.) might influence PTZ effects on neuronal activation and CB1 mRNA expression in these areas.
Neuronal activation was evaluated by analyzing the mRNA expression of the early gene c-fos. Both c-fos and CB1 mRNA levels were evaluated by in situ hybridization of rat brain coronal sections using [35S]-labelled riboprobes, followed by quantitative densitometric analysis of the autoradiographic films. The analyses were focused on discrete brain regions involved in the pathophysiology of SWDs, including the primary somato-sensory-forelimbic cortex (S1FL), the primary somato-sensory-peri-oral cortex (S1p.o.), the thalamic nuclei, and the RTN.
The statistical analyses, performed by one way ANOVA followed by Tukey’s post hoc test, revealed that PTZ treatment stimulated c-fos mRNA expression in the S1FL, unaffecting c-fos expression in the S1p.o. in the RTN and the thalamus. Furthermore, PTZ treatment caused a significant reduction of CB1 mRNA expression in RTN, leaving the cortex and the thalamic relay nuclei unaltered. When administered alone, WIN did not affect either CB1 or c-fos mRNA expression in any brain region considered. However, when administered after PTZ treatment, WIN increased c-fos mRNA expression in S1FL, S1p.o. and RTN, without influencing the effects of PTZ on CB1 mRNA expression.
These data indicate that the development of absence seizures in the PTZ-model is associated with plastic modifications of CB1 mRNA expression within the thalamo-cortical network, and raise the interesting possibility that CB1 receptors might represent a potential target for the development of novel anti-absence drugs.
References
In these regions of the brain, among others, both GABAergic and glutamatergic transmission are under the inhibitory control of the endocannabinoid system. This system consists of Gi-coupled cannabinoid receptors (CB1 and CB2), the two main endogenous ligands, anandamide and 2-arachidonoylglycerol, and the enzymes regulating their synthesis and metabolism (Piomelli, 2003). In the central nervous system CB1 are mainly localized pre-synaptically, where their activation leads to a transitory inhibition of neurotransmitter release.
In this study we evaluated the involvement of the endocannabinoid system in a validated pharmacological model of absence epilepsy, based on the systemic administration of pentylenetetrazole (PTZ) at a dose (20 mg/kg, i.p.) able to induce absence like seizures and loss of responsiveness in rats (Marescaux, 1984). In particular, we investigated whether PTZ treatment might affect CB1 expression in the thalamo-cortical circuits and whether the cannabinoid agonist WIN 55,212-2 (WIN, 5mg/kg, i.p.) might influence PTZ effects on neuronal activation and CB1 mRNA expression in these areas.
Neuronal activation was evaluated by analyzing the mRNA expression of the early gene c-fos. Both c-fos and CB1 mRNA levels were evaluated by in situ hybridization of rat brain coronal sections using [35S]-labelled riboprobes, followed by quantitative densitometric analysis of the autoradiographic films. The analyses were focused on discrete brain regions involved in the pathophysiology of SWDs, including the primary somato-sensory-forelimbic cortex (S1FL), the primary somato-sensory-peri-oral cortex (S1p.o.), the thalamic nuclei, and the RTN.
The statistical analyses, performed by one way ANOVA followed by Tukey’s post hoc test, revealed that PTZ treatment stimulated c-fos mRNA expression in the S1FL, unaffecting c-fos expression in the S1p.o. in the RTN and the thalamus. Furthermore, PTZ treatment caused a significant reduction of CB1 mRNA expression in RTN, leaving the cortex and the thalamic relay nuclei unaltered. When administered alone, WIN did not affect either CB1 or c-fos mRNA expression in any brain region considered. However, when administered after PTZ treatment, WIN increased c-fos mRNA expression in S1FL, S1p.o. and RTN, without influencing the effects of PTZ on CB1 mRNA expression.
These data indicate that the development of absence seizures in the PTZ-model is associated with plastic modifications of CB1 mRNA expression within the thalamo-cortical network, and raise the interesting possibility that CB1 receptors might represent a potential target for the development of novel anti-absence drugs.
References
Blumenfeld H. Brain Res. 2005;150:271-86.
Huguenard JR and McCormick DA. Trends Neurosci. 2007;30(7):350-6.
Marescaux C, et al. Epilepsia. 1984;25(3):326-31.
Piomelli D. Nat Rev Neurosci. 2003;4(11):873-84.
Van Luijtelaar, G Sitnikova E. Epilepsy Res. 2006 Oct;71(2-3):159-80.