L-LTP

1. increase in spine area and the width

   Annotations:

   • and the width of the spine neck in the outer part of the molecular layer of the dentate gyrus following tetanic stimulation of the perforant path.
   1. -

      Annotations:

      • A problem with this work is that no recordings were made of evoked responses, so it is not possible to be sure that LTP was in fact induced.
2. increase in the length of the PSD
3. increase in cup-shaped spines
4. spine densitiy-

   Annotations:

   • LTP is not associated with an overall change in spine density, either in the dentate gyrus 6 hours after tetanus in vivo (Popov et al., 2004; see Trommald et al., 1996 for a counter claim) or in area CA1 in vitro (2 hours after tetanus (Sorra and Harris, 1998).
   1. EM- spines with perforated PSD

      Annotations:

      • increase in the proportion of spines that contained perforated PSDs when synapses were labeled 30 minutes after the induction of LTP, compared to the proportion in labeled spines in naïve tissue or in spines examined 5 minutes after LTP induction.
   2. early evidence- NOT changed

1. CA1,

   Annotations:

   • In area CA1, Lynch’s group also saw no change in spine density but detected a substantial increase (35%) in the number of synapses—presumed inhibitory—on dendritic shafts Lee et al., 1980
2. green fluorescent protein and used two-photon confocal microscopy

   Annotations:

   • transfected CA1 pyramidal cells in organotypic culture with green fluorescent protein and used two-photon confocal microscopy to image synaptic structures
   1. filopodia growth

      Annotations:

      • Within a few minutes of applying localized tetanic stimulation, they saw a rapid growth of filopodia-like structures emanating from the dendrite; filopodia were not seen in regions of the dendrite distant from the stimulating electrode and were not produced by low-frequency stimulation.
3. not correlated with enhanced response

   Annotations:

   • when new spines have been observed, it has not been demonstrated that they contribute to the enhanced response
4. Causally linked to LTP

   Annotations:

   • strongest evidence that morphological changes are causally linked to LTP because the increase in spine size occurred immediately, was blocked by the same drugs that block the induction of LTP, and was associated with an increase in the response to uncaged glutamate

Annotations:

• increase in the concentration of calcium that occurs in spines following tetanic stimulation. The method depends on precipitating calcium in smooth endoplasmic reticulum (SER) and rendering it electron-dense

1. When it is blocked by protein synthesis inhibitors/ transcriptional blockers
2. plateau after 1 to 2 hours

   Annotations:

   • This reveals slow-onset potentiation rising from baseline to reach a plateau after 1 to 2 hours. The slowly growing L-LTP induced by BDNF (Fig. 10–2D) or cAMP analogues has a similar time course.

1. protein synthesis inhibitors
   1. first evidence- Krug et al. (1984)
      1. protein synthesis inhibitors in-vivo

         Annotations:

         • infused the protein synthesis inhibitor anisomycin into the lateral ventricles of freely moving rats and found that the potentiated fEPSP in the dentate gyrus slowly declined to baseline over 3 to 5 hours. The baseline response was not affected in control experiments. This result appeared to establish an upper limit for the duration of posttranslational changes.
         1. potentiated fEPSP declined
   2. CA1- in-vitro
   3. Mossy fiber
   4. block L-LTP induced by cAMP/ BDNF
   5. block LTD induced by PP-LFS/ mGluR agonists
2. protein synthesis in presynaptic neurons
   1. LTP-associated increase in a number of presynaptic proteins

      Annotations:

      • an LTP-associated increase in a number of presynaptic proteins has been reported following the induction of LTP in perforant path-granule cell synapses (Kelly et al., 2000)

Annotations:

• The rapid effect of translational inhibitors suggests that the initial phase of L-LTP is dependent on protein synthesis from preexisting mRNA in the dendrites of potentiated synapses

1. +
   1. isolated dendrites can support L-LTP

      Annotations:

      • The demonstration that isolated dendrites of CA1 pyramidal cells can support BDNF-induced LTP and mGluR-dependent LTD provides direct evidence that dendritic protein synthesis is sufficient to support persistent synaptic plasticity (Kang and Schuman, 1996; Hüber et al., 2000); in both cases the effects are blocked by protein synthesis inhibitors
      1. blocked by protein synthesis inhibitors
   2. dendritic application of a protein synthesis inhibitor

      Annotations:

      • reduces L-LTP in intact slices
   3. mRNA, translation machinery and proteins invovled in induction present in detrides

      Annotations:

      • among them CaMKII, PKMζ arc/arg3.1, BNDF and its receptor trkB, and AMPA receptor subunits GluR1 and GluR2
   4. decay of tetanus induced LTP in isolated dendrites hastened by protein synthesis inhibitors

      Annotations:

      • Tetanus-induced LTP lasting a number of hours also occurs in isolated dendrites, and its decay is hastened by protein synthesis inhibitors (Frey et al., 1989). These experiments demonstrate that translation of existing dendritic mRNA can support a component of L-LTP that extends well beyond the duration of E-LTP.
2. -
   1. isolated dendrites don't support L-LTP

1. evidence for role of transcriptional factors
   1. inhibitors
      1. CA1- invitro

         Annotations:

         • block L-LTP
      2. DG in vivo

         Annotations:

         • block L-LTP
      3. Mossy fiber

         Annotations:

         • block L-LTP
   2. L-LTP induced by cAMP/ BDNF
      1. block by inhibitors
   3. upregultaion of immediate early genes

      Annotations:

      • upregulation of immediate early genes such as c-fos, zif268, and arc/arg3.1 reveal a correlation between transcription and LTP induction
   4. mutant mouse with targeted inactivation of the immediate early gene and transcription factor zif268

      Annotations:

      • The absence of L-LTP in the dentate gyrus confirms a causal role for gene transcription in the induction of L-LTP
   5. antisense mRNA to arc/arg3.1

      Annotations:

      • impairment of L-LTP
   6. transcription before translation

      Annotations:

      • Local protein synthesis alone cannot account for the observation that plasticity can be modulated in this associative manner by a geographically distant pathway; there has to be a transcriptional or translational signal from the soma that either supplies the required stabilizing proteins or transcripts or stimulates local protein synthesis to do so.
2. time-window
   1. 1-2 hours after L-LTP

      Annotations:

      • The effect of transcriptional inhibitors is delayed, beginning 1 to 2 hours after induction.
      1. transcriptional inhibitors- delayed affect
      2. signal- triggers mRNA transcription

         Annotations:

         • The delay is consistent with a model in which a signal from stimulated synapses is transported to the nucleus, where it triggers transcription of new mRNA species. The products of transcription, whether mRNA transcripts themselves or the proteins they encode, are delivered to dendrites to stabilize LTP at potentiated synapses.
3. 3. signalling between soma and dendrite
   1. long-range activation of nuclear transcription factors

      Annotations:

      • What is the signal, and how is it conveyed to the nucleus?
      1. CREB- target for kinases- role
         1. see signalling cascades
         2. genetically hypomorphic CREB mice

            Annotations:

            • supported the idea that CREB was important for LTP as well as for hippocampus- dependent memory (Bourtchuladze et al., 1994).
         3. CREB knockout mice
            1. failed to detect a robust effect on LTP

               Annotations:

               • could be explained by the upregulation of other members of the CREB family
         4. CREB knockout- all isoforms absent

            Annotations:

            • generated a mouse in which expression of all CREB isoforms was either absent or limited, and found no effect on L-LTP
            1. no affect on LTP
         5. targets of CREB
            1. c-fos- transcription of BDNF
            2. zif268
            3. arc/arg3.1
            4. 80

               Annotations:

               • coding for: neurotransmitters and peptides (n 35), growth factors and their receptors (n 7), structure-related proteins (n 4), proteins involved in cellular metabolism (n16), transcription factors (n 13), signal transduction proteins (n 8).
      2. Ca- activated signalling

         Annotations:

         • How is the signalling achieved? 
         1. 1. Ca-calmodulin- travels to some and activates kinases

            Annotations:

            • calcium is released from the endoplasmic reticulum as a traveling wave proceeding from dendrite to soma, where it binds to nuclear calmodulin to activate target kinases (Hardingham et al., 2001).
         2. 2. calcium/calmodulin translocates from synapse to nucleus

            Annotations:

            • Calcium is released fro the ER binds to calmodulin, and translocates to the nucleus like that
         3. 3. Ca2 enters the nucleus via voltagegated calcium channels in the somatic plasma membrane
            1. requires postsynaptic antidromic APs
         4. Kinase targets for ca-calmodulin
            1. CaMKIV- location- nuclear

               Annotations:

               • a member of the calcium/calmodulin-dependent kinase family whose location is predominantly nuclear (Lonze and Ginty, 2002).
               1. activation allows binding to CREB and initiate transcription
            2. Ras/MAP kinase
               1. activated by intra Ca- via tyrosine kinase receptors/ G-protein
                  1. stimulating gene expression in the nucleus
               2. MEK

                  Annotations:

                  • Synaptic activity leads to the phosphorylation of ERK1, 2 in both dendrites and soma. As with the calmodulin signal, the mechanism by which the activated ERK signal is translocated to the nucleus is not known.Although phospho-ERK does not phosphorylate CREB, inhibition of MEK, the kinase that phosphorylates ERK, prevents phosphorylation of CREB on ser133. There must therefore be intermediate kinase(s) that are substrates for phospho-ERK and allow it control over CREB phosphorylation; these intermediates are thought to be members of the RSK or MSK families of nuclear kinases
                  1. intermediate- RSK or MSK families of nuclear kinases??
                     1. ERK1/2 phospho
                        1. CREB phosphorylation
                  2. inhibition- prevents CREB phospho
            3. PKA pathways
      3. G-protein- D1/D5 dopamine receptors
         1. DA anta/ agonsits

            Annotations:

            • Late LTP in area CA1 is blocked by dopamine antagonists (Frey et al., 1990) and facilitated by D1/D5 agonists
         2. cAMP>> PKA>> CREB

            Annotations:

            • Dopamine receptor activation leads to upregulation of cAMP and presumptive translocation of activated PKA to the soma, where it can directly phosphorylate CREB on ser133 (Lonze and Ginty, 2002).
   2. Problems with synapse-soma signalling
      1. reaching a viable conc in nucleus

         Annotations:

         • how does the signal generated by one or a few potentiated synapses (in the limit, a single synapse) located at any position on the dendritic tree reach a concentration in the nucleus that allows activation of transcription factors?
         1. Action Potential Signaling
            1. Evidence
               1. Antidromic APs- convert E-LTP to L-LTP

                  Annotations:

                  • Antidromically driven action potentials can induce expression of immediate early genes, and antidromic stimulation of CA1 axons converts E-LTP to L-LTP, suggesting that action potentials can induce expression of soma-to-synapse plasticity factors (Dudek and Fields, 2002).
               2. HFS of NMDAR- drives a synapse to soma signal

                  Annotations:

                  • synaptic activation of NMDA receptors during highfrequency stimulation can drive somatic action potentials and so provide a rapid NMDAR-dependent signal from synapse to soma (Herron et al, 1986).
               3. synaptic NMDAR activation- somatic Ca entry

                  Annotations:

                  • synaptic activation of NMDA receptors can lead to somatic Ca2 entry via L-type voltage-gated Ca2 channels (Alford et al, 1993).
               4. strong induction protocols- NMDAR-independent L-LTP

                  Annotations:

                  • strong induction protocols that induce NMDAR-independent L-LTP in area CA1 also lead to a large somatic Ca2 signal that is blocked by L-type Ca2 channel inhibitors (Raymond and Redman, 2006).
                  1. large somatic Ca2 signal that is blocked by L-type Ca2 channel inhibitors
      2. How does the signal read there so quickly?

         Annotations:

         • transcription of immediate early genes such as c-fos and zif268 begins within a few minutes of LTP induction.
         1. Action Potential Signaling

            Annotations:

            • Because action potential(s) generally (though not always, see Section 10.3.6) occur during the induction of LTP, the nuclear signal could in principle be generated by Ca2 entering through somatic voltage-gated channels.
            1. Ca entry in the soma>> activate transcriptional mechanisms

               Annotations:

               • activated by action potentials
   3. how specific genes are targeted by different stimuli if the same signal cascades are engaged in each case.
      1. different combinations of kinases

         Annotations:

         • is likely that different combinations of kinases are activated by different stimuli and that expression of specific target genes is achieved by combinatorial selection from the cell’s portfolio of transcription factors
         1. e.g. immediate early gene c-fos

            Annotations:

            • transcription of the immediate early gene c-fos can be triggered by any one of several stimuli (growth factors, neuromodulators, neurotransmitters), all of which lead to phosphorylation of CREB on serine 133
            1. Neurotransmitters- increase Ca- CRF- CREB

               Annotations:

               • CRF- calcium response factor CREB- phosphorylation of CREB at ser141and ser143; phosphorylation at the latter two serine residues reduces the ability of ser133 phospho-CREB to bind to CBP.
               1. CRF and CREB phosphorylated at ser141 and ser143 but unassociated with CBP
                  1. transcription of BDNF
         2. immediate early gene zif268
            1. requires: CREB and Elk1 activation
   4. 4. Immediate early genes

      Annotations:

      • immediate early genes (IEGs) whose transcription is upregulated by activity without the need for prior protein synthesis.
      1. c-fos
         1. occurs within minutes of induction
      2. zif268,
         1. mRNA occurs within minutes of induction

            Annotations:

            • Enhanced mRNA expression for both c-fos and zif268 occurs within minutes of induction and peaks around 30 minutes after induction, returning to baseline levels within 2 to 4 hours
            1. Expression of proteins lags by 1 to 2 hours.
      3. arc/arg3.1
      4. Evidence for role in LTP
         1. upregulation of IEGs following LTP induction in DG

            Annotations:

            • in the sense that whenever a stimulus is sufficiently strong to induce L-LTP immediate early gene expression is triggered.
         2. block induction of LTP- block IEG expression
         3. inactivation of zif268- block L-LTP but not E-LTP
         4. -
            1. IEG expression- induced by non-LTP protocols

               Annotations:

               • enhanced expression of transcription factors such as zif268 and c-fos can be induced by procedures that do not induce LTP, such as nonsynaptic excitation of neurons by antidromic stimulation. The mechanical disturbance involved in cutting a hippocampal slice can also induce long-lasting upregulation of IEG expression in the dentate gyrus (French et al., 2001).
         5. DG vs CA1
            1. not readily switched on by LTPinducing stimulation in area CA1
            2. CA1 expressing arc/arg3.1 mRNA

               Annotations:

               • there is an increase in the number of pyramidal neurons in area CA1 expressing arc/arg3.1 mRNA following exposure to a novel environment (Guzowski et al., 1999).
            3. IEG- mainly upregulated in teh DG

Annotations:

• The existence of this time window implies that the synthesis of the proteins necessary to support L-LTP occurs at the time of or soon after the tetanus and that thereafter new protein synthesis is not required, even though expression of L-LTP can take some hours to reach its maximum and is then maintained at that level

1. inital L-LTP
   1. Drugs effective only during or immediately after LTP induction
   2. Drugs ineffective once LTP is established
   3. behavioral experiments
      1. protein synthesis inhibitors during learning- block long-term but not short-term memory

         Annotations:

         • protein synthesis inhibitors are given around the time of one-trial learning; short-term memory is unaffected by the inhibitors, and longterm memory is blocked(Squire and Barondes, 1972).
2. memory reconsolidation

Annotations:

• recognized by the mRNA or protein products that are transported into dendrites from the soma.

1. lifetime

   Annotations:

   • This tag must have a lifetime at least long enough to engage with the newly generated mRNA or proteins
2. 1. protein synthesis-independent.

   Annotations:

   • If this were the case, a tetanus to one pathway should still generate L-LTP even in the presence of a protein synthesis inhibitor if a second pathway was tetanized within a time window of 1 to 2 hours on either side of the tetanus to the first pathway. The tag would be set in the first pathway, notwithstanding the presence of the protein synthesis inhibitor, and would capture the plasticity-related proteins generated by the second pathway.
3. associativity

   Annotations:

   • Equally arresting was the demonstration that E-LTP, produced by a tetanus that by itself was too weak to produce L-LTP, could be converted to L-LTP by a preceding or subsequent strong tetanus given to a second pathway within a similar time window
   1. the weak stimuli- below the threshold to trigger protein synthesis, but to create a TAG
4. criteria
   1. (1) tags are generated by an activity- dependent, NMDAR-dependent, but protein synthesisindependent process
   2. (2) they have a lifetime of 1 to 2 hours but are susceptible to cancelation by LFS within the first few minutes of being set
   3. (3) at least two tags can be set, one induced by activity patterns that produce early or late LTP and LTD
   4. (4) tags are input-specific;
   5. (5) tags interact with proteins generated by de novo nuclear transcription (the process of “synaptic capture”) to trigger the conversion of early to late LTP or early to late LTD
5. LTP- protein kinases- PKA
6. LTD- protein phosphotases
7. Important events
   1. lead to long-term tags created and via synaptic capture

      Annotations:

      • via tagging and synaptic capture, with all other pathways carrying information about events of minor significance occurring an hour or two on either side of the significant event

Annotations:

• Transcriptional inhibitors given at the time of induction can take 2 hours or more to begin to affect LTP
• The relative delay in the action of transcriptional inhibitors presumably reflects the time taken for a signal to pass from synapse to nucleus for transcription first of IEGs and then of genes encoding effector proteins, followed by transport of mRNA and/or proteins into dendrites and their postulated capture by tagged spines

1. time for synapse-some, some-synapse transport