Long-term potentiation induced by theta frequency stimulation is regulatedby a protein phosphatase-1-operated gate

Long-term potentiation (LTP) can be induced in the Schaffer collateral-->CA 1 synapse of hippocampus by stimulation in the theta frequency range (5-12 Hz), an effect that depends on activation of the cAMP pathway. We investiga ted the mechanisms of the cAMP contribution to this form of LTP in the rat hippocampal slice preparation. theta pulse stimulation (TPS; 150 stimuli at 10 Hz) by itself did not induce LTP, but the addition of either the beta - adrenergic agonist isoproterenol or the cAMP analog 8-bromo-cAMP (8-Br-cAMP ) enabled TPS-induced LTP. The isoproterenol effect was blocked by postsyna ptic inhibition of cAMP-dependent protein kinase. Several lines of evidence indicated that cAMP enabled LTP by blocking postsynaptic protein phosphata se-1 (PP1). Activators of the cAMP pathway reduced PP1 activity in the CA1 region and increased the active form of inhibitor-1, an endogenous inhibito r of PP1. Postsynaptic injection of activated inhibitor-1 mimicked the LTP- enabling effect of cAMP pathway stimulation. TPS evoked complex spiking whe n isoproterenol was present. However, complex spiking was not sufficient to enable TPS-induced LTP, which additionally required the inhibition of post synaptic PP1. PP1 inhibition seems to promote the activation of Ca2+/calmod ulin-dependent protein kinase (CaMKII), because (1) a CaMKII inhibitor bloc ked the induction of LTP by TPS paired with either isoproterenol or activat ed inhibitor-1 and (2) CaMKII in area CA1 was activated by the combination of TPS and 8-Br-cAMP but not by either stimulus alone. These results indica te that the cAMP pathway enables TPS-induced LTP by inhibiting PP1, thereby enhancing Ca2+- independent CaMKII activity.