The inhibitory neuromodulator adenosine is released in the brain in high concentrations under conditions of exaggerated neuronal activity such as ischemia and seizures, or electroconvulsive treatment. By inhibiting neural overactivity, adenosine counteracts seizure activity and promotes neuronal survival. Since stimulation of adenosineAdenosine, as a metabolite of ATP, nature's general energy source, has acquired early in evolution the general function of signaling and counteracting a dysbalance of energy supply and demand (Newby 1984). In the brain, adenosine acts as an inhibitory neuromodulator, which is released under conditions of neuronal activity and reduces this activity by inhibition of transmitter release and postsynaptic hyperpolarisation. Brain damage, e.g., by stroke, ischemia, and seizures leads to a pronounced increase in extracellular adenosine (for review see Rudolphi and Schubert 1996) which counteracts seizure activity (Dragunow 1988) and promotes neuronal survival (Deckert and Gleiter 1994;Picano and Abbracchio 1998;Dux et al. 1990).In addition to its role as an endogenous anticonvulsant and neuroprotective agent, adenosine is now recognized as an important regulator of sleep and wakefulness (for review see Porkka-Heiskanen 1999 (van Calker et al. 1978(van Calker et al. , 1979, but more recent findings have revealed coupling of adenosine receptors to other signaling systems including the phosphoinositol system, potassium, and calcium channels (for review see Fredholm et al. 1994;Ralevic and Burnstock 1998). Adenosine mediates neuroprotection by inhibiting presynaptically the release of several neurotransmitters including the excitatory neurotransmitter glutamate (Ribeiro 1995). Postsynaptically, adenosine increases conductances of various K ϩ -and Cl Ϫ -channels, which hyperpolarize the membrane potential and counteract a transmitter induced depolarisation with a subsequent reduction of Ca 2 ϩ -influx and stabilization of the Mg 2 ϩ blockage of NMDA receptors (Rudolphi et al. 1992a,b;Rudolphi and Schubert 1996). Most, if not all, of these actions of adenosine are mediated via the adenosine A 1 receptor, whereas the role of the other adenosine receptor subtype in neuroprotection is less clear (Schubert et al. 1997;Abbracchio and Cattabeni 1999;Heurteaux et al. 1995). Accordingly, upregulation of adenosine A 1 receptors by chronic treatment with A 1 -antagonists increases the neuroprotective effect of adenosine (Sutherland et al. 1991; Rudolphi et al. 1992a,b).Upregulation of adenosine A1-receptors is also observed after treatments that exert antidepressive effects in humans such as seizures and electroconvulsive treatment (ECT) (Newman et al. 1984;Gleiter et al. 1989;Angelatou et al..1993;Pagonopoulou et al. 1993), REM sleep deprivation (Yanik and Radulovacki 1987), and chronic treatment with carbamazepine (Daval et al. 1989;Biber et al. 1999;van Calker et al. 2000). While the upregulation by carbamazepine of adenosine A 1 receptors is readily comprehensible from carbamazepine's selective antag...