Loss of astrocytic A1 adenosine receptor is involved in a chemotherapeutic agent-induced rodent model of neuropathic pain.

INTRODUCTION/AIMS: Oxaliplatin is a commonly used platinum chemotherapy drug, whereas peripheral neurotoxicity is a widely observed adverse reaction lacking a satisfactory therapeutic strategy. Different adenosine receptors underlying the common neuropathic phenotype play different roles through varied pathophysiological mechanisms. In this study, we investigated the role of adenosine receptor A1 (A1R) in oxaliplatin-induced neuropathic pain and its potential use in an effective therapeutic strategy. METHODS: We established an oxaliplatin-induced neuropathic pain model simulating the mode of chemotherapy administration and observed the related neuropathic behavioral phenotype and implicated mechanisms. RESULTS: Five weekly injections of oxaliplatin for 2 weeks induced a severe and persistent neuropathic pain phenotype in mice. A1R expression in the spinal dorsal horn decreased during this process. Pharmacological intervention against A1R verified its importance in this process. Mechanistically, the loss of A1R expression was mainly attributed to its decreased expression in astrocytes. Consistent with the pharmacological results, the oxaliplatin-induced neuropathic pain phenotype was blocked by specific therapeutic interventions of A1R in astrocytes via lentiviral vectors, and the expression of glutamate metabolism-related proteins was upregulated. Neuropathic pain can be alleviated by pharmacological or astrocytic interventions via this pathway. DISCUSSION: These data reveal a specific adenosine receptor signaling pathway involved in oxaliplatin-induced peripheral neuropathic pain, which is related to the suppression of the astrocyte A1R signaling pathway. This may provide new opportunities for the treatment and management of neuropathic pain observed during oxaliplatin chemotherapy.

Ketamine decreases neuronally released glutamate via retrograde stimulation of presynaptic adenosine A1 receptors.

Ketamine produces a rapid antidepressant response in patients with major depressive disorder (MDD), but the underlying mechanisms appear multifaceted. One hypothesis, proposes that by antagonizing NMDA receptors on GABAergic interneurons, ketamine disinhibits afferens to glutamatergic principal neurons and increases extracellular glutamate levels. However, ketamine seems also to reduce rapid glutamate release at some synapses. Therefore, clinical studies in MDD patients have stressed the need to identify mechanisms whereby ketamine decreases presynaptic activity and glutamate release. In the present study, the effect of ketamine and its antidepressant metabolite, (2R,6R)-HNK, on neuronally derived glutamate release was examined in rodents. We used FAST methodology to measure depolarization-evoked extracellular glutamate levels in vivo in freely moving or anesthetized animals, synaptosomes to detect synaptic recycling ex vivo and primary cortical neurons to perform functional imaging and to examine intracellular signaling in vitro. In all these versatile approaches, ketamine and (2R,6R)-HNK reduced glutamate release in a manner which could be blocked by AMPA receptor antagonism. Antagonism of adenosine A1 receptors, which are almost exclusively expressed at nerve terminals, also counteracted ketamine's effect on glutamate release and presynaptic activity. Signal transduction studies in primary neuronal cultures demonstrated that ketamine reduced P-T286-CamKII and P-S9-Synapsin, which correlated with decreased synaptic vesicle recycling. Moreover, systemic administration of A1R antagonist counteracted the antidepressant-like actions of ketamine and (2R,6R)-HNK in the forced swim test. To conclude, by studying neuronally released glutamate, we identified a novel retrograde adenosinergic feedback mechanism that mediate inhibitory actions of ketamine on glutamate release that may contribute to its rapid antidepressant action.

Adenosine A1 antagonists and the cardiorenal syndrome.

Adenosine A1 antagonists are being developed for the treatment of renal dysfunction in patients with congestive heart failure. After early small studies prompted hope that these agents could increase urine output without worsening the glomerular filtration rate, larger studies published and presented in 2007 confirmed their beneficial impact on weight and renal function. However, in many studies the renal benefits disappear with higher doses, suggesting that specificity may be lost with higher doses of these drugs. Investigations in animals indicate that there may also be direct benefits on the myocardium and in the lung. Although studies have not shown adverse effects at optimal dosing, the widespread actions of adenosine mandate that safety be established. Ongoing studies should be able to demonstrate whether adenosine A1 antagonists can be used to improve renal function without adversely affecting patients with heart failure.

Partial adenosine A(1) receptor agonists inhibit sarin-induced epileptiform activity in the hippocampal slice.

Organophosphate poisoning can result in seizures and subsequent neuropathology. One possible therapeutic approach would be to employ adenosine A(1) receptor agonists, which have already been shown to have protective effects against organophosphate poisoning. Using an in vitro model of organophosphate-induced seizures, we have investigated the ability of several adenosine A(1) receptor agonists to inhibit epileptiform activity induced by the organophosphate sarin, in the CA1 stratum pyramidale of the guinea pig hippocampal slice. Application of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) or the partial adenosine A(1) receptor agonists 2-deoxy-N(6)-cyclopentyladenosine (2-deoxy-CPA) and 8-butylamino-N(6)-cyclopentyladenosine (8-butylamino-CPA) abolished epileptiform activity in a concentration-related manner. The rank order of potency was CPA (IC(50) 4-5 nM) >2-deoxy-CPA (IC(50) 113-119 nM)=8-butylamino-CPA (IC(50) 90-115 nM). These data suggest that partial adenosine A(1) receptor agonists, which have fewer cardiovascular effects, should be further evaluated in vivo as potential treatments for organophosphate poisoning.

Post-triptan era for the treatment of acute migraine.

There now is one realized and several attractive targets for the treatment of acute attacks of migraine that will follow and augment the use of serotonin 5-HT1B/1D receptor agonists, the triptans. Calcitonin gene-related peptide (CGRP) receptor blockade recently has been shown to be an effective acute antimigraine strategy; therefore, blockade of CGRP release by inhibition of trigeminal nerves would seem a logical approach. A number of targets are reviewed in this article including serotonin 5-HT1F and 5-HT1D receptors, adenosine A1 receptors, nociceptin, vanilloid TRPV1 receptors, and anandamide CB1 receptors. Development of one or more such compound offers the exciting prospect of new non-vasoconstrictor treatments for migraine and cluster headache.