Stable expression of long-term synaptic plasticity is critical for the developmental refinement of neural circuits and for some forms of learning and memory. Although structural remodeling of dendritic spines is associated with the stable expression of long-term potentiation (LTP), the relationship between structural and physiological plasticity remains unclear. To define whether these two processes are related or distinct, we simultaneously monitored EPSPs and dendritic spines, using combined patch-clamp recording and two-photon time-lapse imaging in the same CA1 pyramidal neurons in acute hippocampal slices. We found that theta burst stimulation paired with postsynaptic spiking, which reliably induced LTP, also induced a rapid and persistent expansion of dendritic spines. Like LTP, this expansion was NMDA receptor dependent. Spine expansion occurred even when LTP was inhibited by postsynaptic inhibition of exocytosis or PKA (protein kinase A); however, under these conditions, the spine expansion was unstable and collapsed spontaneously. Furthermore, similar changes in LTP and spine expansion were observed when hippocampal neurons were treated with protein synthesis inhibitors. Like LTP, spine expansion was reversed by low-frequency stimulation (LFS) via a phosphatase-dependent mechanism, but only if the LFS was applied in a critical time window after induction. These results indicate that the initial expression of LTP and spine expansion is dissociable, but there is a high degree of mechanistic overlap between the stabilization of structural plasticity and LTP.

Trafficking of AMPA subtype glutamate receptors (AMPARs) from intracellular compartments to synapses is thought to be a major mechanism underlying the expression of long-term potentiation (LTP), a cellular substrate for learning and memory. However, it remains unclear whether the AMPAR trafficking that takes place during LTP is due to a targeted insertion of AMPARs directly into the synapse or delivery to extrasynaptic sites followed by translocation into the synapse. Here, we provide direct physiological evidence that LTP induced by a theta-burst pairing and tetanic stimulation protocols causes the rapid delivery of AMPARs to a perisynaptic site. Perisynaptic AMPARs do not normally detect synaptically released glutamate but can do so when the glial glutamate transporter EAAT1 is inhibited. AMPARs can be detected at this perisynaptic site before, but not after, the full expression of LTP. The appearance of perisynaptic AMPARs requires postsynaptic exocytosis, PKA signaling, and the C-terminal region of GluR1 subunit of AMPARs but not actin polymerization. Actin polymerization after LTP induction is required to retain AMPARs at the perisynaptic site after their appearance. Low-frequency stimulation given shortly after LTP induction leads to activity-dependent removal of perisynaptic AMPARs and suppresses the subsequent expression of LTP. These results demonstrate that AMPARs are rapidly trafficked to perisynaptic sites shortly after LTP induction and suggest that the delivery and maintenance of perisynaptic AMPARs may serve as a checkpoint in the expression of LTP.actin ͉ dendritic spine ͉ trafficking ͉ TBOA ͉ two-photon imaging A ctivity-induced modification of neuronal connections is essential for the development of the nervous system and may underlie some forms of learning and memory. One widely examined form of synaptic plasticity is long-term potentiation (LTP). LTP leads to the synaptic insertion of glutamate receptors of the AMPA subtype (AMPARs) (1-4). Recent studies also suggest that this synaptic incorporation of AMPARs may stabilize spine modifications associated with LTP (5, 6). Postsynaptic exocytosis (7,8), PKA activity (9), and AMPARs containing the GluR1 subunit (10) are implicated in the synaptic incorporation of AMPARs during LTP, but it remains unclear whether LTP leads to the direct insertion of AMPARs at the postsynaptic density (PSD) or to insertion at extrasynaptic regions followed by translocation into the PSD.Consistent with the latter idea, AMPAR targeting from intracellular compartments to the cell surface can occur via mechanisms that are distinct from AMPAR targeting from the cell surface to the synapse (11). By visualizing the movement of transfected AMPAR subunits or specific domains of AMPAR subunits in culture, trafficking of AMPARs can be observed in response to LTP induced by synaptic activity or chemical induction protocols. Using this approach, studies have found that AMPARs containing GluR1 can be inserted into the plasma membrane from intracellular stores (2). Lateral mobili...

Physiological and morphological alterations occur with long-term synaptic modifications, such as long-term potentiation (LTP) and long-term depression (LTD), but whether these two processes are independent or interactive is unclear. It is also unknown whether or how morphological modifications, like spine remodeling, may contribute to physiological modifications, such as trafficking of glutamate receptors which underlies, at least partially, the expression of LTP and LTD. In this study, we monitored spine size and synaptic responses simultaneously using combined two photon time-lapse imaging with patch-clamp recording in acute hippocampal slices. We show that spine shrinkage and LTD can occur independently of each other. We further show that changes in spine size are unrelated to trafficking of AMPA receptors (

How persistent synaptic and spine modification is achieved is essential to our understanding of developmental refinement of neural circuitry and formation of memory. Within a short period after their induction, both types of modifications can either be stabilized or reversed, but how this reversibility is controlled is largely unknown. We have shown previously that AMPA receptors (AMPARs) are delivered to perisynaptic regions after the induction of long-term potentiation (LTP) but are absent from perisynaptic regions after the full expression of LTP. Here, we report that perisynaptic AMPARs are GluR2-lacking and they translocate to synapses in a protein kinase C (PKC)-dependent manner. Once entering synapses, these AMPARs quickly switch to GluR2-containing in an activity-dependent manner. Absence of postinduction activity or blocking interactions between GluR2 and NSF, or GluR2 and GRIP/ PICK1 results in LTP mediated by GluR2-lacking AMPARs. However, these synaptic GluR2-lacking AMPARs are not sufficient to allow reversibility of LTP. On the other hand, postsynaptic inhibition of PKC activity holds AMPARs at perisynaptic regions. As long as perisynaptic AMPARs are present, both LTP and spine expansion remain labile: they can be reverted to the baseline state together with removal of perisynaptic AMPARs, or they can enter a stabilized state of persistent increase together with synaptic incorporation of perisynaptic AMPARs. Thus, perisynaptic GluR2-lacking AMPARs play a critical role in controlling the reversibility of both synaptic and spine modifications.dendritic spine | long-term potentiation | two-photon imaging P ersistent functional and structural changes in synaptic connections are generally believed to underlie long-lasting modifications in neuronal networks, such as developmental refinement of neural circuitry and memory formation in the adult (1-3). One widely studied form of such long-lasting changes is long-term potentiation (LTP), which is accompanied by long-lasting expansion of dendritic spines (2-4). Within a short time window after LTP induction, both types of modifications can be reversed (5, 6). What controls the labile period of these modifications is of great importance to our understanding of the consolidation of synaptic and spine modifications.Modification of existing synaptic AMPA receptors (AMPARs) and/or addition of new AMPARs to synapses appear to underlie the expression of LTP. Evidence supports a model that newly added AMPARs first appear at extrasynaptic/perisynaptic regions and are subsequently incorporated into synapses (7-11). These perisynaptic AMPARs can be removed by moderate synaptic activity (10) and this removal prevents the full expression of LTP and reverses spine expansion (6, 10). It has been suggested that perisynaptic AMPARs could play a critical role in regulating the persistence of LTP and spine expansion (10,12). This notion is consistent with the observation that stabilization of spine expansion requires synaptic incorporation of new AMPARs (6, 13). A few studies ...