C-Jun N-terminal kinase regulates adenosine A1 receptor-mediated synaptic depression in the rat hippocampus Tyson B. Brust 1 , Francisco S. Cayabyab 1,2 , Brian A. MacVicar * Brain Research Centre, Department of Psychiatry, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC V6T 2B5, Canada Received 2 April 2006; received in revised form 31 August 2007; accepted 3 September 2007 Abstract Adenosine A1 receptors are ubiquitous mediators of presynaptic inhibition of neurotransmission in the central nervous system, yet the signalling pathway linking A1 receptor activation and decreased neurotransmitter release remains poorly resolved. We tested the contribution of c-Jun N-terminal kinase (JNK) to adenosine A1 receptor-mediated depression of field excitatory postsynaptic potentials (fEPSPs) in area CA1 of the rat hippocampus. We found that inhibition of JNK with SP600125 or JNK inhibitor V, but not an inactive analogue, attenuated the depression of fEPSPs induced by adenosine, hypoxia, and the A1 receptor agonist N 6 -cyclopentyladenosine (CPA). In contrast, the JNK in- hibitor SP600125 did not inhibit GABA B -mediated synaptic depression. In support of our electrophysiological findings, Western blot analysis showed that A1 receptor stimulation resulted in a transient increase in JNK phosphorylation in the membrane fraction of hippocampal lysates. The total amount of JNK in the membrane fraction was unchanged by CPA treatment. The increase in phosphorylated JNK induced by A1 receptor stimulation was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), indicating that A1 receptors specifically activate JNK in the hippocampus. Together with functional data indicating that JNK inhibition decreased CPA-induced paired pulse facilitation, these results suggest that JNK activation is necessary for adenosine A1 receptor-mediated synaptic depression occurring at a presynaptic locus The adenosine A1 receptor-JNK signalling pathway may represent a novel mechanism underlying inhibition of neu- rotransmitter release in the CNS. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: C-Jun N-terminal kinase; Adenosine; A1 receptor; Hypoxia; Hippocampus; Synaptic depression 1. Introduction Activation of adenosine A1 receptors rapidly causes potent depression of synaptic transmission in the central nervous sys- tem. The level of ambient adenosine in the extracellular space gates synaptic transmission in the hippocampus by inhibiting glutamate release. Adenosine concentrations increase dramat- ically in the extracellular space during trauma such as stroke and seizure, and acts as an endogenous neuroprotectant by preventing excessive excitation. Despite the importance of A1 receptor activation in mediating presynaptic inhibition, little is known about the signalling pathway leading from re- ceptor stimulation to decreased transmitter release. MAPKs are a large family of serine/threonine kinases widely expressed in brain that regulate diverse physiological processes such as cell survival, synaptic plasticity, and induc- ible gene expression (Nozaki et al., 2001; Pearson et al., 2001). There are three subfamilies of MAPKs: (1) extracellu- lar signal-regulated kinases; (2) p38 MAPK; and (3) the c-Jun N-terminal kinase (JNK). p38 MAPK and JNK are phosphor- ylated by MAPK kinases MKK3/6 and MKK4/7 respectively, in response to cellular stresses such as ischemia, osmotic shock, cytokines, growth factors, and ultraviolet light. Both p38 MAPK and JNK are activated by G protein-coupled receptors (Marinissen and Gutkind, 2001). * Corresponding author. Tel.: þ1 604 822 7797; fax: þ1 403 822 7299. E-mail address: bmacvica@interchange.ubc.ca (B.A. MacVicar). 1 These authors contributed equally to this work. 2 Present address: Department of Physiology, Neural Systems and Plasticity Research Group, University of Saskatchewan, Rm. A315 Health Sciences Building, 107 Wiggins Road, Saskatoon, SK S7N 5E5, Canada. 0028-3908/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropharm.2007.09.001 Available online at www.sciencedirect.com Neuropharmacology 53 (2007) 906e917 www.elsevier.com/locate/neuropharm