Distribution of D₁-like and D₂-like dopamine receptors in the brain of genetic epileptic WAG/Rij rats

Abstract

The densities of the dopamine (DA) D₁-like and D₂-like receptors were studied by autoradiography in brain regions of rats with (WAG/Rij strain) and without (ACI strain) genetic absence epilepsy. The core of the nucleus accumbens in WAG/Rij rats had a lower density of D₁-like receptors than in ACI rats, a reduction of both D₁-like and D₂-like DA receptors was also found for the dorsal striatum (dorsal caudate-putamen). On the other hand, the density of D₂-like receptors was higher in cortical (frontal and parietal) regions and lower in the CA3 region of the hippocampus of WAG/Rij, as compared to ACI rats. These results give new information about possible malfunction of the brain dopaminergic system in the WAG/Rij rat model of absence epilepsy. It seems that there are differences between WAG/Rij and other models of absence epilepsy, especially concerning the role of striatum.

Introduction

The WAG/Rij rat strain is used as a genetic animal model for absence epilepsy (Coenen and van Luijtelaar, 2003, van Luijtelaar et al., 2002). All adult WAG/Rij rats show absence seizures in the form of spontaneous occurring generalised spike-wave discharges (SWDs) in the EEG, accompanied by behavioural arrest, often with mild oral-facial twitching. SWDs in WAG/Rij rats are suppressed by anti-absence and aggravated by absence-provoking drugs (Peeters et al., 1988, van Luijtelaar et al., 2002). Given the validity of the model, neurochemical studies aimed at unravel basic mechanisms could be useful with respect to human absence epilepsy.

It is well known from clinical practice that neuroleptic drugs may cause a serious aggravation of absence seizures (Itil and Soldatos, 1980). Since the brain dopaminergic system is one of the main targets for neuroleptic drugs, an unknown dysfunction of the brain dopaminergic system was proposed to exist in absence epilepsy patients. Moreover, Buzsaki et al. (1990) proposed a common pacemaker mechanism for rhythmic EEG discharges of absence epilepsy and tremor in Parkinson patients. Recently, it was found that the dopamine (DA) transporter gene is specifically altered in human patients with idiopathic generalised absence epilepsy (Sander et al., 2000). Although the exact consequence of this finding is not known, it was suggested that it might lead to changes in the functioning of the DA-transporter protein. Thus, hypothetically, altered re-uptake of DA would contribute to enhanced network excitability (Sander et al., 2000) and, consequently, to a lower threshold for SWDs to occur.

Recently, some features of the brain dopaminergic system were proposed to account for the development of SWDs in WAG/Rij and apomorphine-susceptible (APO-SUS) rats. APO-SUS rats are also characterized by a high number of SWDs (De Bruin et al., 2000). They were originally bred because they showed an enhanced behavioural responsiveness to apomorphine and are considered as a model for schizophrenia (Cools et al., 1990). Increased activity in the mesolimbic dopaminergic system could account for the presence of absence seizures in WAG/Rij and APO-SUS rats (Cools and Peeters, 1992, De Bruin et al., 2000, De Bruin et al., 2001a). In good agreement with this, rats of the GAERS strain, which are also often used as a valid model for absence epilepsy, have an increased density of D₃ DA receptor mRNA in the core of nucleus accumbens (Deransart et al., 2001) and show a clear aggravation of absence epilepsy following injection of DA antagonists (Marescaux et al., 1992). Interestingly, APO-SUS rats show a predominance of the mesolimbic dopaminergic system over the nigrostriatal one, e.g. based on high amounts of novelty-induced ambulatory behavior, slow habituation and high susceptibility to an intermediate dose of amphetamine in the open-field. In contrast, apomorphine-unsusceptible (APO-UNSUS) rats have a low responsiveness to apomorphine (Cools et al., 1990, Cools and Peeters, 1992). This strain has opposite DA characteristics: a predominance of the nigrostriatal system over the mesolimbic one (opposite behavior features) and shows an extremely low incidence of SWDs (De Bruin et al., 2000). In the striatal projection area of the A9 substantia nigra neurons, the densities of D₂/D₃-binding sites, as well as D₁ receptor mRNA levels were significantly higher in APO-SUS than in APO-UNSUS rats. However, the D₂ receptor mRNA and the number of D₁-like receptor binding sites in the limbic regions, caudate putamen and nucleus accumbens were similar in APO-SUS and APO-UNSUS rats (Rots et al., 1996). Assuming that the elevated receptor numbers can be considered compensatory to insufficient receptor stimulation by its endogenous ligand, it was suggested that high apomorphine susceptibility could be related to malfunction of the nigrostriatal and tuberoinfundibular pathways (Rots et al., 1996).

In the present study, the regional distribution of D₁-like and D₂-like DA receptors was studied in rats of two inbred strains, WAG/Rij and ACI. These strains differ in respect to the amount of spontaneous SWDs (no or almost no SWDs in ACI rats (Inoue et al., 1990)), as well as in response to apomorphine (lower number of apomorphine-induced gnawing in ACI rats) (De Bruin et al., 2001a). The question in this study was whether these rat strains also differ in expression of D₁-like and D₂-like receptors. To test this, we used autoradiography to compare D₁-like and D₂-like DA receptor binding sites in brain areas of age matched rats of these two strains.