Persistent protein kinase activity underlying long-term potentiation

Abstract

Long-term potentiation (LTP) of synaptic transmission in the hippocampus is a much-studied example of synaptic plasticity^1,2. Although the role of N-methyl-D-aspartate (NMDA) receptors in the induction of LTP is well established^3–5, the nature of the persistent signal underlying this synaptic enhancement is unclear. Involvement of protein phosphorylation in LTP has been widely proposed^6–15, with protein kinase C (PKC)^{6–8,10–12,14} and calcium-calmodulin kinase type II (CaMKII)^9,13 as leading candidates. Here we test whether the persistent signal in LTP is an enduring phosphoester bond, a long-lived kinase activator, or a constitutively active protein kinase by using H-7, which inhibits activated protein kinases¹⁶ and sphingosine, which competes with activators of PKC (ref. 17) and CaMKII (ref. 18). H-7 suppressed established LTP, indicating that the synaptic potentiation is sustained by persistent protein kinase activity rather than a stably phosphorylated substrate. In contrast, sphingosine did not inhibit established LTP, although it was effective when applied before tetanic stimulation. This suggests that persistent kinase activity is not maintained by a long-lived activator, but is effectively constitutive. Surprisingly, the H-7 block of LTP was reversible; evidently, the kinase directly underlying LTP remains activated even though its catalytic activity is interrupted indicating that such kinase activity does not sustain itself simply through continual autophosphorylation (see refs 9,13,15).