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Purinergic modulation of neurotransmission Brust, Tyson Brennan

Abstract

Signalling through adenosine- and adenine-nucleotide-gated receptors (purinoceptors) regulates a vast array of physiological processes in the central nervous system, including synaptic transmission. The research presented here describes a role for mitogen-activated protein kinases (MAPKs) in mediating molecular signalling cascades initiated by activation of purinoceptors in the hippocampus. In particular, it is proposed that MAPK activation is a requirement for purinoceptor-mediated presynaptic inhibition of neurotransmission. The three primary findings presented here are: (1) P2X₇ receptors are localized on mossy fiber terminals [i.e. mossy fibre terminals] where they function to inhibit mossy fiber-CA3 synaptic transmission in a pathway requiring p38 MAPK activation; (2) Adenosine A₁ receptors exist in a complex with p38 MAPK in which A₁ receptors activate p38 MAPK to decrease Schaffer-collateral-CA1 synaptic transmission; and (3) Sequential activation of C-Jun N-terminal kinase (JNK) following p38 MAPK activation is also required for A₁ receptor-mediated synaptic depression. Immunocytochemistry was used to demonstrate that P2X₇ receptors are abundant on presynaptic terminals of mossy fiber synapses in the rat hippocampus. Western blotting was used to show increases in the phosphorylation state of p38 MAPK and JNK following A₁ receptor stimulation in the CA1 region. Co-immunoprecipitation showed that A₁ receptors are physically associated with p38 MAPK and JNK in the hippocampus. Synaptic function was assessed using field excitatory postsynaptic potentials (fEPSPs) evoked in stratum lucidum in the CA3 region or in stratum radiatum in the CA1 region of rat hippocampal slices. Selective stimulation of P2X₇ receptors with the agonist Bz-ATP potently decreased mossy fiber-CA3 synaptic depression and this was blocked by the P2X₇ antagonist oxidized-ATP, but not by the P2X₁₋₃,₅,₆ antagonist, PPADs or the P2Y antagonist, RB2. Bz-ATP-induced synaptic depression was blocked by the p38 MAPK inhibitor SB203580. Stimulation of A₁ receptors with exogenous adenosine, endogenous adenosine released during hypoxia, or the agonist N⁶-cyclopentyladenosine (CPA) depressed evoked tEPSPs in the CA1 region. These inhibitory responses were blocked with the A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), the p38 MAPK inhibitors SB203580 and SB202190, and the JNK inhibitors SP600125 and JNK Inhibitor V. These results suggest that the inhibitory actions of purinoceptors requires the activation of p38 MAPK and JNK in the hippocampus.

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