Long-term potentiation (LTP) is a form of synaptic plasticity that has been extensively studied as a putative mechanism underlying learning and memory. A late phase of LTP occurring 3-5 hours after stimulation and depending on transcription, protein synthesis and cyclic-AMP-dependent protein kinase (protein kinase A, or PKA) has been described, but it is not known whether transcription of presynaptic and/or postsynaptic genes is required to support late-phase LTP. Here we show that late-phase LTP can be obtained in rat hippocampal CA1 mini-slices in which the cell bodies of presynaptic Schaffer collateral/commissural fibres are removed. Thus, transcription of presynaptic genes is not necessary to support maintenance of late-phase LTP. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptor is the predominant mediator of the ionotropic response to synaptically released glutamate in the hippocampus and it has been implicated in LTP maintenance. We find that synthesis of AMPA receptor subunits is increased three hours after LTP induction: this effect on the synthesis of the AMPA receptor is blocked by inhibitors of PKA and of transcription. Our results support the idea of a postsynaptic mechanism maintaining late-phase LTP, in which AMPA receptor synthesis is increased as a result of PKA-dependent gene transcription.

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Long-term potentiation (LTP) is an increase in synaptic responsiveness thought to be involved in mammalian learning and memory. The localization (presynaptic and͞or postsynaptic) of changes underlying LTP has been difficult to resolve with current electrophysiological techniques. Using a biochemical approach, we have addressed this issue and attempted to identify specific molecular mechanisms that may underlie LTP. We utilized a novel multipleelectrode stimulator to produce LTP in a substantial portion of the synapses in a hippocampal CA1 minislice and tested the effects of such stimulation on the presynaptic protein synapsin I. LTP-inducing stimulation produced a long-lasting 6-fold increase in the phosphorylation of synapsin I at its Ca 2؉ ͞cal-modulin-dependent protein kinase II (CaM kinase II) sites without affecting synapsin I levels. This effect was fully blocked by either the N-methyl-D-aspartate receptor antagonist D(؊)-2-amino-5-phosphonopentanoic acid (APV) or the CaM kinase II inhibitor KN-62. Our results indicate that LTP expression is accompanied by persistent changes in presynaptic phosphorylation, and specifically that presynaptic CaM kinase II activity and synapsin I phosphorylation may be involved in LTP expression.Synaptic plasticity has been extensively studied in the mammalian hippocampal formation. Long-term potentiation (LTP) is one form of plasticity that has attracted considerable attention as a possible cellular substrate for learning and memory. LTP was first described by Bliss and Lomo (1), who demonstrated that a relatively brief burst of high-frequency stimulation (HFS) produces a long-lasting enhancement of synaptic responses in the mammalian hippocampus. Although presynaptic (2-4) and͞or postsynaptic (5-8) changes have been proposed to underlie LTP, the exact localization of these changes remains highly controversial. This has been due in part to the difficulty in resolving presynaptic and postsynaptic events electrophysiologically (9). In this study, we have utilized a biochemical approach to address this issue and attempted to identify molecular mechanisms that may underlie LTP.Numerous studies suggest that protein phosphorylation (specifically by protein kinase C and Ca 2ϩ ͞calmodulin-dependent protein kinase II [CaM kinase II)] plays a critical role in the induction and possibly the maintenance of LTP (10-12). However, identification of the specific phosphoproteins involved in LTP has been problematic. Synapsin I is a phosphoprotein that has a number of features which make it a likely candidate to mediate persistent presynaptic plasticity (13-15). It is found exclusively in neuronal presynaptic terminals, where it associates with synaptic vesicles (16-18). Considerable evidence suggests that synapsin I plays a key role in neurotransmitter release by regulating the availability of synaptic vesicles for exocytosis (19)(20)(21)(22)(23)(24) (however, see ref. 25). According to this model, synapsin I crosslinks synaptic vesicles to the cytoskeleton in a phosphorylation stat...