Blinking and Yawning in Epilepsy: The Role of Dopamine

Clinical data regarding the effects of dopaminergic drugs in idiopathic generalized epilepsies are scarce. The general observation that antipsychotic agents (dopaminergic antagonists) worsen seizures, has suggested that dopaminergic agonists would have antiepileptic effects. However, this has never been clearly demonstrated, besides in few limited studies (Mervaala, 1990 ; Quesney, 1980, 1981). More recently, Positron Emission Tomography (PET) investigations using dopaminergic markers (Fluoro-Dopa, SCH23390, DAT) have shown dopaminergic deficits in several epileptic syndromes: ring chromosome 20 syndrome (Biraben 2004), juvenile myoclonic epilepsy (Ciumas 2008), temporal lobe epilepsy (Bouilleret 2008), frontal lobe epilepsy (Fedi 2008). These data give rise to a renewal of interest for the involvement of the dopaminergic neurotransmission in epilepsies. Based on our experimental data from animal studies (see Deransart and Depaulis, 2002), the investigators propose an original study investigating the involvement of the dopaminergic system in idiopathic generalized epilepsies using behavioural as well as neurophysiological markers of the dopaminergic response, in conditions where seizing activities in patients are facilitated (EEG follow-up after sleep deprivation). This approach is based on the concept developed in our laboratory concerning the involvement of the basal ganglia, and more precisely the dopaminergic pathways, in the control of spike-wave discharges in idiopathic generalized epilepsies.

The primary objective is to assess dopaminergic reactivity using a behavioural marker (i.e. yawning) in patients with idiopathic generalized epilepsy compared to matched healthy controls after injection of either low dose of apomorphine or placebo. Other parameters will be recorded as secondary outcomes: behavioural (blinking), biochemical (prolactin, GH) and neurophysiological (Spike-Waves Discharge: SWD rating) markers. Safety parameters will be recorded to assess tolerance.

Experimental data: hyperdopaminergic response in a model of absence-epilepsy in the rat. Since the late 80's, our laboratory has demonstrated the existence of an endogenous neural mechanism that controls the occurrence of epileptic seizures in different animal models, a system based on the hypothesis that the basal ganglia modulate the synchronisation of epileptic rhythmic activities (Depaulis 1994). The studies performed in GAERS (Genetic Absence Epilepsy Rats from Strasbourg, a validated model of absence-epilepsy in rat) have shown that inhibition of the main output structure of basal ganglia (i.e., the substantia nigra pars reticulata) had antiepileptic effects (Depaulis 1994; Deransart 1996). Similarly, deep brain stimulation of the substantia nigra pars reticulata as well as of the subthalamic nucleus interrupts seizures (Vercueil 1998; Feddersen 2007). Systemic and intrastriatal dopamimetics injections in GAERS also suppress seizures, whereas antagonists worsen them (Warter et al, 1988; Deransart et al., 2000). Electrophysiological data showed respectively decreased and increased activity of DA neurons during and at the end of absence-seizures (Lücking et al. 2002). An increase in D3 receptor transcripts was also observed in adult GAERS as compared to inbred non epileptic control rats (NEC), within the ventral striatum (Deransart et al. 2001). These data suggest changes in the DA tone in GAERS with fully developed epileptic phenotype. According to the key role of DA D3 neurotransmission in the foetal cortical development (Levant B, 1995) and modulation of DA tone (Nissbrandt et al., 1995; Gilbert et al., 1995; Kreiss et al., 1995), the researchers investigated whether the putative impaired DA tone in GAERS correlates with functional changes in spontaneous and quinpirole-induced yawning behaviour (Kurashima et al. 1995; Collins et al.2005). The hypothesis of an increased dopaminergic tone in GAERS was thus addressed using pharmacology and microdialysis. In GAERS and NEC (i) spontaneous and quinpirole-induced yawning behaviour and (ii) changes in intra-accumbens dopamine contents induced by amphetamine and K+ and measured by microdialysis were investigated. Spontaneous yawning was significantly decreased in GAERS (0.3±0.2 yawn/hr, n=9) as compared to NEC (5.4±1.2, n=8) and Wistar Harlan rats (9.7±2.3, n=7). Quinpirole-induced yawning was significantly increased in GAERS (29.4±4.9) as compared to NEC (10.5±2.7) and Wistar-Harlan rats (22.6±3.5). Quinpirole also increased the number of absence-seizures in GAERS (+47.4±8.6%). When compared to NEC, basal levels of DA were 40% lower in GAERS whereas amphetamine and K+ produced a higher increase in extracellular dopamine in GAERS. The increased quinpirole-induced yawning in GAERS may account for an overexpression in D3 transcripts. The increased responsiveness of dopamine transmission observed in GAERS after pharmacological manipulations, as compared to NEC, suggests a " hyperdopaminergic " phenotype of GAERS. Altogether, these data support that GAERS have an impaired DA tone that may be associated with the development of mechanisms controlling absences seizures (Deransart et al, in preparation).

Altogether, these data suggest that a phasic involvement (" on request ") of the basal ganglia - notably under the influence of the dopaminergic neurotransmission - may underlie the rapid changes in extracellular activity recorded in output structures of the basal ganglia at the end of seizures in this model (Deransart 2003). Such a phasic functioning of the dopaminergic system in the control of seizures could also reconcile with the apparent discrepancy regarding the fact that seizures appear to escape chronic high-frequency stimulations of the substantia nigra (Feddersen 2007). These data strengthen the need for a re-appraisal of clinical approaches, especially based from a dynamic point of view regarding the involvement of the dopaminergic system in epilepsy.

Clinical data:

• Involvement of the dopaminergic system in idiopathic generalized epilepsies. Exposure to dopaminergic antagonists, like antipsychotic drugs, increases the risk for the patients to display epileptic seizures. The mechanism by which these treatments may lead to such an aggravation remains unknown and, up to date, there is no pharmacological strategy to decrease such a risk. Recent data suggested the involvement of the dopaminergic pathways in several epileptic syndromes: a significant homogeneous decrease in striatal dopamine uptake has been shown using PET in patients with epileptic seizures associated with ring chromosome 20 mosaicism (Biraben, 2004). In this epileptic syndrome, patients display prolonged seizures, reminiscent of an absence status epilepticus. A similar study, using a marker for the dopamine transporter, recently reported a decrease in dopamine re-uptake at the level of the substantia nigra in patients with idiopathic generalized epilepsies (Ciumas, 2008). These data differ from what is observed in neurodegenerative pathologies involving the dopaminergic system (e.g., Parkinson disease) where imaging studies of the dopaminergic transmission show a decrease in synaptic terminals linked to a progressive degenerative process. Data from epileptic patients rather suggest a modulation of the expression of some dopaminergic receptors as well as dynamic changes in the synaptic transmission. In line with our experimental data and the recently published neuroimaging studies, the investigators propose that the reactivity of the dopaminergic system displays special features specific to epileptic patients and may constitute a risk-level marker for epileptic seizures. The conceptual framework of the proposed translationnal study lies on the dynamic involvement of the dopaminergic system and the assessment of its reactivity in epileptic patients.
• Reactivity of the dopaminergic system in idiopathic generalized epilepsies. Low doses of apomorphine, a dopaminergic agonist, induce yawning and palpebral winking in healthy subjects (Blin, 1990). Similarly, higher therapeutic doses induce yawning immediately before therapeutic effect in patients with Parkinson's disease. These data suggest that higher affinity of apomorphine to presynaptic receptor could be involved in promoting yawning and blinking. The study of dopaminergic system response to low doses of apomorphine, below the side-effects threshold (nausea, vomiting, hypotension) represent an original, well-tolerated approach in both patients with epilepsy and healthy subjects. Similar studies have already been published in patients with migraine (Cerbo, 1997) and cocaine users (Colzato, 2008) in order to study dopaminergic reactivity in selected populations. The investigators propose to study dopaminergic reactivity in patients with idiopathic generalized epilepsy compared to matched healthy volunteers, during prolonged EEG after sleep-deprivation, using apomorphine doses of 1 µg/kg and 5 µg/kg. Indeed, apomorphine 0.5 to 2 µg/kg increased spontaneous blinking rate and yawning in healthy volunteers (Blin, 1990). However, a 10 µg/kg dose induced symptoms such as nausea, vomiting, sweating and dizziness in migraineurs (Cerbo, 1997). The study will focus on behaviour (spontaneous and apomorphine-induced yawning and blinking) and EEG (spontaneous and apomorphine induced SWD) as compared to placebo. Prolactin and GH dosing after apomorphine injection will provide a biochemical validation of dopaminergic stimulating effect. Indeed, prolactin secretion is inhibited by dopamine from the hypothalamus, while Growth hormone (GH) is stimulated. Thus, a decrease in prolactin and an increase in GH reflect central dopamine availability. Dosing prolactin and GH response to apomorphine subcutaneous injection represents a way to document the synaptic effects of the drug, as previously described (Aymard, 2003; Friess, 2001).

This project arises from concepts developed in our laboratory from experimental data obtained in an animal model of epilepsy and from recent clinical data from the literature. It aims at improving understanding of dopaminergic transmission dynamics in patients with epilepsy. It could therefore promote the emergence of new markers of susceptibility to epileptic seizures and constitute an opportunity to develop new pharmacological approaches based on dopaminergic neuromodulation.