Priming of long-term potentiation in mouse hippocampus by corticotropin-releasing factor and acute stress: implications for hippocampus-dependent learning.

In the present experiments, we characterized the action of human/rat corticotropin-releasing factor (h/rCRF) and acute stress (1 hr of immobilization) on hippocampus-dependent learning and on synaptic plasticity in the mouse hippocampus. We first showed that h/rCRF application and acute stress facilitated (primed) long-term potentiation of population spikes (PS-LTP) in the mouse hippocampus and enhanced context-dependent fear conditioning. Both the priming of PS-LTP and the improvement of context-dependent fear conditioning were prevented by the CRF receptor antagonist [Glu(11,16)]astressin. PS-LTP priming and improved learning were also reduced by the protein kinase C inhibitor bisindolylmaleimide I. Acute stress induced the activation of Ca2+/calmodulin-dependent kinase II (CaMKII) 2 hr after the end of the stress session. The CaMKII inhibitor KN-62 antagonized the stress-mediated learning enhancement, however, with no effect on PS-LTP persistence. Thus, long-lasting increased neuronal excitability as reflected in PS-LTP priming appeared to be essential for the enhancement of learning in view of the observation that inhibition of PS-LTP priming was associated with impaired learning. Conversely, it was demonstrated that inhibition of CaMKII activity reduced contextual fear conditioning without affecting PS-LTP priming. This observation suggests that priming of PS-LTP and activation of CaMKII represent two essential mechanisms that may contribute independently to long-term memory.