Differential roles of Ca²⁺/calmodulin-dependent protein kinase II and mitogen-activated protein kinase activation in hippocampal long-term potentiation

The roles of Ca^2-/calmodulin-dependent protein kinase II (CaM kinase II) and mitogen-activated protein kinase (MAPK) in longterm potentiation (LTP) were investigated in the CA1 area of hippocampal slices, using electrophysiological and biochemical approaches. A brief high-frequency stimulation, but not low-frequency stimulation, delivered to Schaffer collateral/commissural afferents produced a stable LTP and activated both CaM kinase II and 42 kDa MAPK. Different from the activity of CaM kinase II, the increase in MAPK activity was transient. At a concentration of 50 μM, but not of 30 μM, PD098059, a potent inhibitor of MAPK kinase, markedly inhibited the induction of LTP. Although the two concentrations had similar inhibitory effects on MAPK activity, only 50 μM PD098059 suppressed the activation of CaM kinase II. Application of calmidazolium, an antagonist of calmodulin, blocked both CaM kinase II activation and the LTP induction without affecting the increase in 42 kDa MAPK activity. Application of neurotrophin brain-derived neurotrophic factor (BDNF) promoted the induction of LTP, with concomitant activation of CaM kinase II. Under the same conditions, BDNF failed to activate MAPK in hippocampal slices. These results indicate that, although the LTP induction is accompanied by increases in two kinase activities, only CaM kinase II activation is required for this event.