Molecular signalling pathways in the cerebral cortex are required for retrieval of one-trial avoidance learning in rats

Abstract

Rats were implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus, the entorhinal cortex, anterior cingulate cortex, posterior parietal cortex, or the basolateral complex of the amygdala. The animals were trained in one-trial step-down inhibitory avoidance and tested 24 h later. Prior (10 min) to the retention test, through the cannulae, they received 0.5 microl infusions of a vehicle (2% dimethylsulfoxide in saline), or of the following drugs dissolved in the vehicle: the glutamate NMDA receptor blocker, aminophosphonopentanoic acid (AP5, 2.0 or 5.0 microg), the AMPA receptor blocker, 6,7-dinitroquinoxaline-2,3 (1H,4H)dione (DNQX, 0.4 or 1.0 microg), the metabotropic receptor antagonist, methylcarboxyphenylglycine (MCPG, 0.5 or 2.5 microg), the inhibitor of cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.1 or 0.5 microg), the PKA stimulant, Sp-cAMPs (0.5 microg), or the inhibitor of the mitogen-activated protein kinase (MAPK), PD098059 (10 or 50 microM). All these drugs, at the same doses, had been previously found to alter long-term memory formation of this task. Here, retrieval test performance was blocked by DNQX, MCPG, Rp-cAMPs and PD098059 and enhanced by Sp-cAMPs infused into CA1 or the entorhinal cortex. The drugs had similar effects when infused into the parietal or anterior cingulate cortex, except that in these two areas AP5 also blocked retrieval, and in the cingulate cortex DNQX had no effect. Infusions into the basolateral amygdala were ineffective except for DNQX, which hindered retrieval. None of the treatments that affected retrieval had any influence on performance in an open field or in a plus maze; therefore, their effect on retention testing can not be attributed to an influence on locomotion, exploration or anxiety. The results indicate that the four cortical regions studied participate actively in, and are necessary for, retrieval of the one-trial avoidance task. They require metabotropic and/or NMDA glutamate receptors and PKA and MAPK activity. In contrast, the basolateral amygdala appears to participate only through a maintenance of its regular excitatory transmission mediated by glutamate AMPA receptors.