Adenosine is a modulator that has a pervasive and generally inhibitory effect on neuronal activity. Tonic activation of adenosine receptors by adenosine that is normally present in the extracellular space in brain tissue leads to inhibitory effects that appear to be mediated by both adenosine A1 and A2A receptors. Relief from this tonic inhibition by receptor antagonists such as caffeine accounts for the excitatory actions of these agents. Characterization of the effects of adenosine receptor agonists and antagonists has led to numerous hypotheses concerning the role of this nucleoside. Previous work has established a role for adenosine in a diverse array of neural phenomena, which include regulation of sleep and the level of arousal, neuroprotection, regulation of seizure susceptibility, locomotor effects, analgesia, mediation of the effects of ethanol, and chronic drug use.

Caffeine is believed to act by blocking adenosine A1 and A2A receptors (A1R, A2AR), indicating that some A1 receptors are tonically activated. We generated mice with a targeted disruption of the second coding exon of the A1R (A1R ؊/؊ ). These animals bred and gained weight normally and had a normal heart rate, blood pressure, and body temperature. In most behavioral tests they were similar to A1R ؉/؉ mice, but A1R ؊/؊ mice showed signs of increased anxiety. Electrophysiological recordings from hippocampal slices revealed that both adenosine-mediated inhibition and theophylline-mediated augmentation of excitatory glutamatergic neurotransmission were abolished in A1R ؊/؊ mice. In A1R ؉/؊ mice the potency of adenosine was halved, as was the number of A1R. In A 1R؊/؊ mice, the analgesic effect of intrathecal adenosine was lost, and thermal hyperalgesia was observed, but the analgesic effect of morphine was intact. The decrease in neuronal activity upon hypoxia was reduced both in hippocampal slices and in brainstem, and functional recovery after hypoxia was attenuated. Thus A1Rs do not play an essential role during development, and although they significantly influence synaptic activity, they play a nonessential role in normal physiology. However, under pathophysiological conditions, including noxious stimulation and oxygen deficiency, they are important. A denosine acts on four cloned and pharmacologically characterized receptors, A 1 , A 2A , A 2B , and A 3 (1). Adenosine is believed to play a particularly important role in hypoxia and ischemia, and there is evidence that adenosine serves to limit damage secondary to ATP loss (2, 3). However, adenosine may have important actions under more normal physiological circumstances as well. For instance, the effects of caffeine, at concentrations reached during habitual caffeine consumption, are believed to be a consequence of blockade of tonic activity at some A 1 and A 2A receptors (A 1 R and A 2A R) (4). Studies on mice lacking A 2A Rs show that adenosine tonically activates A 2A Rs and that this activation has functional effects, particularly on behavior, blood pressure, and blood platelets (5). A 1 Rs are more widely distributed than A 2A Rs (4, 6), but despite extensive pharmacological studies their physiological and pathophysiological roles remain unclear. Here we show that A 1 Rs mediate physiological as well as pathophysiological effects of endogenous adenosine. In particular, adenosine acts tonically to activate presynaptic and postsynaptic A 1 Rs to depress synaptic transmission and to reduce nociceptive signaling. At elevated levels seen during hypoxia, adenosine acting at A 1 Rs is responsible for the depression of neuronal activity, and in this situation elimination of A 1 Rs results in impaired functional recovery. Materials and MethodsGeneration of A1R Knockout Mice. A major part of the proteincoding sequence of the mouse A 1 R gene (7) corresponding to exon 6 of the human A 1 R gene described by Ren and Stiles (8) was cloned. The targeting construct was b...

A ketogenic diet (KD) is a high-fat, low-carbohydrate metabolic regimen; its effectiveness in the treatment of refractory epilepsy suggests that the mechanisms underlying its anticonvulsive effects differ from those targeted by conventional antiepileptic drugs. Recently, KD and analogous metabolic strategies have shown therapeutic promise in other neurologic disorders, such as reducing brain injury, pain, and inflammation. Here, we have shown that KD can reduce seizures in mice by increasing activation of adenosine A 1 receptors (A 1 Rs). When transgenic mice with spontaneous seizures caused by deficiency in adenosine metabolism or signaling were fed KD, seizures were nearly abolished if mice had intact A 1 Rs, were reduced if mice expressed reduced A 1 Rs, and were unaltered if mice lacked A 1 Rs. Seizures were restored by injecting either glucose (metabolic reversal) or an A 1 R antagonist (pharmacologic reversal). Western blot analysis demonstrated that the KD reduced adenosine kinase, the major adenosine-metabolizing enzyme. Importantly, hippocampal tissue resected from patients with medically intractable epilepsy demonstrated increased adenosine kinase. We therefore conclude that adenosine deficiency may be relevant to human epilepsy and that KD can reduce seizures by increasing A 1 R-mediated inhibition.

Adenosine and its receptors have been the topic of many recent reviews. These reviews provide a good summary of much of the relevant literature--including the older literature. We have, therefore, chosen to focus the present review on the insights gained from recent studies on genetically modified mice, particularly with respect to the function of adenosine receptors and their potential as therapeutic targets. The information gained from studies of drug effects is discussed in this context, and discrepancies between genetic and pharmacological results are highlighted.

In addition to affecting respiration and vascular tone, deviations from normal CO(2) alter pH, consciousness, and seizure propensity. Outside the brainstem, however, the mechanisms by which CO(2) levels modify neuronal function are unknown. In the hippocampal slice preparation, increasing CO(2), and thus decreasing pH, increased the extracellular concentration of the endogenous neuromodulator adenosine and inhibited excitatory synaptic transmission. These effects involve adenosine A(1) and ATP receptors and depend on decreased extracellular pH. In contrast, decreasing CO(2) levels reduced extracellular adenosine concentration and increased neuronal excitability via adenosine A(1) receptors, ATP receptors, and ecto-ATPase. Based on these studies, we propose that CO(2)-induced changes in neuronal function arise from a pH-dependent modulation of adenosine and ATP levels. These findings demonstrate a mechanism for the bidirectional effects of CO(2) on neuronal excitability in the forebrain.