E-LTP- pre or postsynaptic

1. Evidence against
   1. Glial currents

      Annotations:

      • See techniques for how it tests presynaptic neurotransmitter release
      1. CA1- Diamond et al., 1998; Luscher et al., 1998

         Annotations:

         • An increase in electrogenic current was observed during PTP but not during LTP in area CA1, providing evidence against increased release as an expression mechanism for LTP
      2. CA3- Kamamura et al., 2004

         Annotations:

         • An analogous result was obtained in area CA3. LTP at mossy fiber synapses, but not at associational-commissural synapses, was associated with a sustained increase in glial uptake current
   2. Paired-pulse ratio
      1. McNaughton, 1982

         Annotations:

         • LTP is not associated with a corresponding alteration in pairedpulse facilitation in the dentate gyrus in vivo.
   3. blockers
      1. open-channel blocker-AMPAR

         Annotations:

         • Philanthotoxin (PhTx) blocks GluR2-lacking, Ca2-permeable AMPA receptors in a use-dependent manner and so can be used to monitor Pr.
         1. Mainen et al., 1998

            Annotations:

            •  LTP was not associated with any change in the PhTx blocking rate in the GluR2 knockout mouse- 
            • no change in blocking rate= not a change in Pr= not presynaptic
      2. NMDAR non-competitive-ligand activated blocker
         1. Manabe and Nicoll, 1994

            Annotations:

            • using a ligand activated non-competitive anta of NMDARs (MK-801, see techniques)- It was found that LTP of AMPA receptor-mediated synaptic transmission was not associated with any change in the MK-801 blocking rate of NMDAR-mediated synaptic transmission
            • This was taken as evidence that LTP does not involve an increase in Pr.
      3. GluR2 knockout
2. COMMUNICATION- retrograde signalling

   Annotations:

   • the posysynaptic cell release retrograde messengers to modulate/ communicate with the presynaptic cell- and so this is presynaptic LTP
   1. Arachidonic Acid (AA)

      Annotations:

      • produced by the action of a calciumdependent enzyme, phospholipase A2, on membrane phospholipids
      1. inhibitor of AA production
         1. Williams and Bliss, 1988

            Annotations:

            • blocked chemically induced LTP in area CA1 and the dentate gyrus
      2. Role as retrograde in LTP
         1. Piomelli et al. (1987)
            1. Aplysia

               Annotations:

               • experiments in Aplysia that identified lipoxygenase metabolites of arachidonic acid as a novel class of second messenger
         2. Dumuis et al., 1988

            Annotations:

            • activation of NMDA receptors in cultured striatal neurons led to the release of AA into the culture medium
            1. NMDAR activation- led to AA release
      3. meets the criteria of retrograde

         Annotations:

         • Bliss et al. (1990), (1) LTP is accompanied by an increase in the concentration of AA in a postsynaptic membrane fraction and an increase in the extracellular concentration of AA; (2) inhibitors of phospholipase A2 block induction of LTP; (3) the application of arachidonic acid to active hippocampal synapses causes delayed but persistent potentiation of evoked responses
      4. Gaps in evidence

         Annotations:

         • For these reasons, AA has fallen out of fashion as a candidate messenger for LTP.
         1. Do extracellular scavengers of AA block induction?
         2. What is the presynaptic mode of action of AA that leads to an increase in transmitter release?
         3. evidence can be explained by other known properties of AA

            Annotations:

            • including inhibition of glutamate uptake into glia (Barbour et al., 1989) and its facilitatory action on NMDA currents (Miller et al., 1992).
   2. CBs

      Annotations:

      • retrograde mediator of heterosynaptic LTD at inhibitory synapses in area CA1

      Attachments:

      • Endocannabinoids in synaptic plasticity
   3. Nitric Oxide (NO)

      Annotations:

      • NO is a small membrane-permeable molecule with a short half-life The target of NO is soluble guanylyl cyclase which activated cGMP
      1. retrograde?

         Annotations:

         • In neurons, NO is derived from L-arginine in a reaction catalyzed by nitric oxide synthase, of which two isoforms, nNOS and eNOS, are expressed in dendrites of hippocampal neurons. Like AA, NO is released from cultured neurons exposed to NMDA
      2. NO inhibitors
         1. in-vitro
            1. Cummings et al., 1994

               Annotations:

               • don't block LTP
            2. Williams et al., 1989

               Annotations:

               • inhibition of LTP by NO synthesis inhibitors was seen at room temperatures but not at higher, more physiological temperatures
               1. temp diff
         2. in-vivo
            1. Bannerman et al., 1994

               Annotations:

               • don't block LTP
      3. application of NO
         1. produces LTP

            Annotations:

            • in combination with weak synaptic activation, produces LTP; two groups reported that it did
            1. O’Dell et al., 1991b; Bon and Garthwaite, 2003
         2. doesn't produce LTP
            1. Murphy et al., 1994

               Annotations:

               • based on the release of NO by flash photolysis from an inactive caged precursor, has been shown to reflect an interaction between NO and ultraviolet light, leading to artifactual inhibition of NMDA receptor currents
      4. Knockout of genes expressing NO isoforms

         Annotations:

         • hippocampal neurons
         1. Son et al., 1996

            Annotations:

            • fails to block LTP.However, LTP is compromised, though not abolished, in a double knockout of both genes
      5. Evidence linking postsynaptically generated NO to enhanced transmitter release
   4. Criteria for retrograde messengers

      Annotations:

      • The cellular machinery for synthesis of the messenger exists in the postsynaptic cell. • The synthesis of the messenger is upregulated by LTPinducing stimuli. • Postsynaptic injection of drugs that block synthesis inhibits LTP induction. • The messenger is released into the extracellular compartment following induction of LTPpre. • Perfusion with extracellular scavengers small enough to access the synaptic cleft inhibit LTPpre. • A target molecule in the presynaptic terminal, positively linked to transmitter release, is activated by the messenger. • Drugs that inhibit the link between messenger and exocytosis suppress LTPpre. • Exogenous application of the messenger induces LTPpre or lowers the threshold for its induction.
3. Evidence for
   1. increase in Pr
      1. decrease in PPF during LTP
         1. Schulz et al., 1994
      2. vesicle imaging
         1. Malgaroli et al., 1995

            Annotations:

            • Direct evidence for a presynaptic mechanism in cultured hippocampal neurons was obtained by measuring exocytoticendocytotic cycling with antibodies against the synaptic vesicle protein synaptotagmin
         2. Ma et al. (1999)

            Annotations:

            • showed that in hippocampal cultures treated with a cAMP analogue (a treatment that induces a late, protein synthesis-dependent form of LTP), there was a dramatic increase in the number of active boutons taking up the dye FM1-43
      3. decrease in failure rate
         1. Stevens and Wang (1994)

            Annotations:

            • LTP in area CA1 associated with decrease in failure rate without change in potency (the mean amplitude of responses excluding failures) Not silent synapses
      4. Ca indicators

         Annotations:

         • see techniques for explanantion
         1. Emptage et al., 2003

            Annotations:

            • PCa= probability of evoking a Ca2 transient in the visualised spine,
            • In the majority of spines examined, induction of LTP by high-frequency stimulation was accompanied by an increase in PCa at the imaged spine, whereas in experiments in which LTP was blocked by D-AP5, or in which the threshold for LTP was not reached, no change in PCa was seen.

1. Receptors
   1. Modifications
      1. P(o)

         Annotations:

         • probability of opening
      2. Channel conductance
         1. Benke et al., 1998
            1. single channel conductance

               Annotations:

               • nonstationary fluctuation analysis CA1 synapses of 2-week-old rats it was found that in most neurons increased sufficiently to account for LTP
               • In other neurons did not change, suggesting that any postsynaptic change was due to an increase in N, the number of receptors, or Po. The simplest explanation for the increase in is a modification of receptors already present at the synapse, but it could theoretically be due to the exchange of lower for higher receptors at the synapse. 
               • The estimate of derived from nonstationary fluctuation analysis is the weighted mean of all subconductance states
               • Because AMPA receptors can adopt multiple conductance states, it was suggested that LTP was due to an alteration in the relative time spent in these various states
               1. consistent evidence
                  1. CamKII

                     Annotations:

                     • Consistent with this idea, CaMKIImediated phosphorylation of ser831 on GluR1 not only occurs during LTP (Barria et al., 1997a,b) but leads to an increase in precisely this parameter in HEK293 cells expressing GluR1 (Derkach et al., 1999).
                     1. Poncer et al., 2002

                        Annotations:

                        • synaptic potentiation induced by postsynaptic application of a constitutive form of CaMKII is associated with an increase in
         2. mechanism

            Annotations:

            • Thus a simple scheme for the induction and expression of LTP is that Ca2 entering via NMDA receptors triggers CaMKII-dependent phosphorylation of GluR1 to increase the single-channel conductance of existing AMPA receptors. However, although this mechanism may occur, it cannot be the whole story, as CaMKII inhibitors applied alone fail to inhibit LTP at this stage of development
         3. CaMKII

            Annotations:

            • High-frequency stimulation leads to activation of CaMKII and phosphorylation of ser831, a state associated with increased single-channel conductance of the receptor (Derkach et al., 1999).
      3. mean open time
         1. Not changed

            Annotations:

            • Despite claims to the contrary (Kolta et al, 1998), it is usually accepted that the decay of AMPAR-mediated EPSCs is not changed by LTP.
      4. Benke et al., 1998

         Annotations:

         • following a pairing stimulus a decrease in mean amplitude of sucesses
   2. Insertion

      Annotations:

      • increase no. of receptors present
      • In the simplest scenario, the new synaptic receptors would have conductance properties identical to those of the existing ones. Alternatively, the new receptors could have a different subunit composition, thereby conferring different conductance properties (i.e., single-channel conductance, rectification, Ca2 permeability) to the potentiated synapses
      1. new receptors- modified
         1. Palmer et al., 2004

            Annotations:

            • early in development (around the first week of life) LTP was observed in some neurons that was associated with a decrease in channel conductance
         2. Lu et al., 2001; Pickard et al., 2001

            Annotations:

            • using antibody labelling tech- These studies identified the insertion of GluR1-containing AMPA receptors into synapses that previously lacked GluR1
      2. AMPAR inserion
         1. GFP-tagged subunits
            1. Shi et al., 1999

               Annotations:

               • LTP in organotypic slice cultures was found to be associated with delivery of GFP-GluR1 to dendritic spines
            2. Hayashi et al., 2000

               Annotations:

               • However, because imaging could not determine whether these constructs were inserted into the plasma membrane, mutated receptors were used that had a distinct electrophysiological signature
         2. antibody labelling
            1. Lu et al., 2001; Pickard et al., 2001

               Annotations:

               • insertion of native AMPA receptors during NMDAR-dependent LTP has been described
               1. additional GluR1

                  Annotations:

                  • These studies identified the insertion of GluR1-containing AMPA receptors into synapses that previously lacked GluR1
            2. silent synapses
               1. Pickard et al., 2001

                  Annotations:

                  • insertion of AMPA receptors into anatomically “silent” synapses (synapses where AMPA receptors could not be detected on the cell surface)
      3. new receptors- mostly identical
   3. GluR1 interactors
      1. LTP
      2. activation of CamKII needed?
         1. No- Hayashi et al., 2000

            Annotations:

            • Both spine delivery of GluR1 and LTP in organotypic slices requires activation of CaMKII, but deletion of ser831, the CaMKII phosphorylation site on the Cterminal tail of GluR1, did not prevent either of these effects
         2. Yes- Leonard et al., 1998

            Annotations:

            • point mutation on the PDZ domain at the extreme C-terminus of GluR1 (T887A) did prevent spine delivery and LTP suggesting that CaMKII may phosphorylate a protein that binds to this site. One such candidate is the membrane associated guanylate kinase (MAGUK) SAP97, which binds to GluR1 via its PDZ domain and has been implicated in trafficking to the synapse
            1. mutation in GluR1 PDZ domain
      3. PKA
         1. ser845

            Annotations:

            • ser845- glutamate receptor subunit 1(GluR1) Ser845 residue 
            • Mutagenesis is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation
            1. Esteban et al., 2003

               Annotations:

               • Mutagenesis of ser845 has shown that this PKA site is required though its phosphorylation is not sufficient for LTP, suggesting that this is one stage in a multi-step process.
            2. Oh et al (2006)

               Annotations:

               • PKA-dependent phosphorylation of ser845 is involved in the membrane targeting of AMPARs to extrasynaptic sites
      4. PKC

         Annotations:

         • Boehm et al, 2006- PKC is also implicated in the trafficking of GluR1 and LTP, via phosphorylation of ser818
   4. Tarps

      Annotations:

      • transmembrane AMPAR regulatory proteins
      1. AMPAR

         Annotations:

         • AMPA receptors also bind to stargazin, which in turn binds to PSD95 (Chen et al., 2000). Stargazin is one of a class of TARPs that are involved in the recruitment of AMPA receptors from extrasynaptic to synaptic sites during LTP
         1. regulating the number of AMPARs
         2. increase the affinity of AMPARs for glutamate
         3. increase conducatance

            Annotations:

            • They slow the rate of both deactivation and desensitization and enhance , by increasing the prevalence of high conductance substates (Tomita et al, 2005).
      2. gamma-8 TARP isoform knockouts

         Annotations:

         • loss of AMPAR subunits and a reduction in AMPAR-mediated EPSCs.
         1. reduced LTP
      3. mutaqtion to minic phospho

         Annotations:

         • mutation of these residues to aspartic acid mimic phosphorylation, drove AMPARs to synapses and blocked LTD (Tomita et al, 2005).
         1. block LTD
   5. GluR2 interactors
      1. LTD
      2. NSF

         Annotations:

         • N-ethylmaleimide-sensitive fusion protein (NSF), an ATPase involved in membrane fusion events, binds directly to the GluR2 subunit
         1. SNAP

            Annotations:

            • some evidence for a role of NSF and soluble NSF attachment proteins (SNAPs) in AMPA receptor insertion during LTP (Lledo et al., 1998).
      3. Block GluR2-NSF interaction
         1. decrease in AMPAR transmission

            Annotations:

            • rapid decrease in electrically evoked AMPAR-mediated synaptic transmission, suggesting that AMPA receptors can rapidly recycle at the synapse
            1. Nishimune et al., 1998
         2. internalisation of AMPARs

            Annotations:

            • suggesting that NSF regulates the membrane insertion or stabilisation of synaptic AMPARs
            1. Noel et al,, 1999, Luscher et al., 1999
      4. PDZ-containing proteins GRIP, ABP, PICK1

         Annotations:

         • GRIP (glutamate receptor interacting protein), ABP (AMPA receptor binding protein), and PICK1 (protein interacting with Ckinase 1). 
         1. GRIP

            Annotations:

            • These molecules presumably also localize other proteins involved in the LTP process, close to AMPA receptors. For example, GRIP binds to an associated protein, GRASP-1, which is a neuronal RasGEF and so may link AMPA receptors to Ras signaling (Ye et al., 2000).
            • ABP and GRIP contain multiple PDZ domains and probably serve to anchor AMPA receptors at both synaptic and intracellular locations
         2. ABP

            Annotations:

            • ABP and GRIP contain multiple PDZ domains and probably serve to anchor AMPA receptors at both synaptic and intracellular locations
         3. PICK1

            Annotations:

            • PICK1 can also bind PKC, suggesting that one of its functions is to target PKC to phosphorylate AMPA receptors.
            1. insertion of GluR2-containing AMPARs

               Annotations:

               • The transient insertion of calcium permeable AMPARs may provide a signal, or tag, to mark recently potentiated synapses. Given that PICK1 is able to regulate the GluR2-content of AMPARs, and is the only molecule known to do this, it is a prime candidate for mediating this effect.
               1. Plant et al., 2006

                  Annotations:

                  • LTP at CA1 synapses in two-week-old animals involves the rapid insertion of GluR2-lacking AMPARs followed by their replacement with GluR2-containing AMPARs, over a period of 20 minutes or so (Plant et al., 2006). 
            2. regulates GluR2
               1. Matsuda et al., 1999

                  Annotations:

                  • evidence that PICK1 may phosphorylate GluR2 on ser880 to inhibit the binding of AMPA receptors to ABP/GRIP
                  1. inhibit ABP/GRIP binding
               2. Daw et al., 2002

                  Annotations:

                  • PICK1-PKC phosphorylation of ser880 on GluR2 mobilizes AMPA receptors from intracellular sites during dedepression(i.e., reversal of LTD by an LTP induction protocol)
                  1. de-depression
               3. Terashima et al., 2004

                  Annotations:

                  • regulates the GluR2 content of AMPA receptors at synapses,, raising the possibility that it may play a role in LTP by altering the conductance properties of synaptic AMPA receptors
                  1. AMPAR conductance
2. Downstream changes
   1. voltage-gated or leak channels

      Annotations:

      • not much evidence for this
      1. Fan et al., 2005
3. evidence against presynaptic

   Annotations:

   • is evidence for postsynaptic

Annotations:

• Objective analysis of all of the evidence, however, points to a multitude of expression mechanisms that may vary according to the component of LTP under investigation (i.e., STP, E-LTP, L-LTP), the developmental stage of the animal, and even the experimental approach used to tackle the pro
• LTP is expressed by alterations in either L-glutamate release and/or AMPA receptor function, the extent of each component depending on several factors, of which the list above is unlikely to be exhaustive.