Fig 1.
Schematics of setups used throughout this work.
(A) Schematic illustration of the interactions between axonal, dendritic and soma-specific quantities used in the model. (B) One neuron receives stimuli from one synapse (small colored circle). We stimulate this synapse by four different protocols (WTET, WLFS, STET, SLFS; for details see main text) to induce different dynamics of synaptic plasticity (viz. ELTP, ELTD, LLTP, LLTD). (C) For cross-tagging and heterosynaptic plasticity experiments, two synapses (S1 in red and S2 in blue) are connected to one neuron and stimulated independently. (D) Similar to setup (C) but with n stimulated synapses (S1, S2, …, Sn) and one non-stimulated synapse (Sn + 1). (E) For distance-dependent heterosynaptic plasticity protocols we considered that n synapses are stimulated (blue). This leads to the induction of a global postsynaptic calcium signal Cpost. Synapses (red) near the stimulated ones receive in addition a diffusive calcium signal Cdiff which depends on the distance d between stimulated (blue) and non-stimulated synapses (red).
Table 1.
Used parameters if not stated differently elsewhere.
Fig 2.
Consolidation of plasticity-induced changes depends on the initiation of protein synthesis.
(A) A weak high frequency stimulus (WTET; 0.2 sec) is not able to initiate protein synthesis (purple) and, therefore, only induces ELTP (blue) but not LLTP (green) leading to a fast decay of synaptic changes (red). (B) In contrast, a strong high frequency stimulus (STET) initiates protein synthesis (Δρ > θpro) and, thereby, causes long-lasting changes (LLTP). (C,D) Similar effects arise for the induction of LTD by low frequency stimulation; (C) WLFS leads to ELTD and (D) SLFS leads to LLTD. System Fig 1B is used.
Fig 3.
Consolidation of plasticity-induced changes depends on synaptic tagging and cross-tagging.
For the consolidation of synaptic changes two requirements have to be fulfilled [19]: postsynaptic protein synthesis (Fig 2) and synaptic tagging. (A,B) A brief (0.1 sec) and weak tetanus stimulation (WTET1) induces ELTP (S1; blue). (B) However, despite protein synthesis being initiated by another stimulus (STET at S2; red), ELTP at S1 cannot be consolidated. (C,D) If the same weak stimulus is provided for a longer duration (e.g., 0.2 sec; WTET2), the resulting early-phase changes are larger. (D) Now, the change reaches the tagging threshold and initiates a synaptic tag, therefore, enabling consolidation or cross-tagging of the synaptic change from ELTP to LLTP given protein synthesis initiated by S2. (E-H) As in experiments (Sajikumar et al., 2007), in the model, cross-tagging is independent of the sequence of stimuli. ELTD becomes LLTD supported by (E) a strong depression (SLFS) or (F) potentiation stimulus (STET). (G,H) The same holds for the consolidation of ELTP. Stimulation duration of WTET is 0.2 sec.
Fig 4.
Cross-tagging is not sensitive to the order of stimuli.
(A) ELTP (WTET; 0.2 sec) can be consolidated by protein synthesis initiated one hour after ELTP induction (S1; blue). Thereby, the strong stimulus at synapse S2 (red) can be an LTD- or LTP-induction (Fig 3H) signal. (B,C) The same effect arises for ELTD (WLFS) which can be consolidated supported by a (B) SLFS or (C) STET stimulus.
Fig 5.
The number of synapses with correlated inputs determines the type and consolidation of heterosynaptic plasticity.
(A-D) Varying the number of synapses n receiving Poisson spike trains of same frequency and different stimulation protocols enable many types of homo-(blue) and heterosynaptic (red) changes. For each n-value the synaptic weight change of Δρ = ρ/ρ0 (averaged over 10 trials [red and blue] and n synapses [blue]). (A) WTET (stimulation duration of 0.3 sec); (B) STET; (C) WLFS; (D) SLFS. (E-H) Applying the same stimulation protocol (here, STET) to a different number of synapses induces different heterosynaptic changes ΔW: (E) n = 4: ELTD; (F) n = 8: LLTD; (G) n = 11: ELTP; (H) n = 14: LLTP. (I-L) Similar to (E-H) with a SLFS stimulation: (I) n = 40: ELTD; (J) n = 50: LLTD; (K) n = 62: ELTP; (L) n = 63: LLTP. System Fig 1E is used.
Fig 6.
Correlation CC of inputs influences the induction of homo- and heterosynaptic plasticity.
Averages over 100 trials with standard deviations are shown. In general a higher correlation of input spike trains yields a stronger heterosynaptic effect. (A) STET; (B) WTET; (C) SLFS; (D) WLFS. Details see main text.
Fig 7.
Distance-dependent heterosynaptic plasticity.
n synapses receive independent Poisson spike trains with 100 Hz for 1 sec yielding a homosynaptic potentiation (not shown here). Due to the diffusion of messengers from the stimulated to unstimulated synapses, the unstimulated synapses (red) are heterosynaptically depressed (A-D) or potentiated and depressed (E-H) depending on their distance to the stimulated synapses. Synapses which are far away from a stimulated synapse (about 10 – 14 μm) are uneffected.