Timing dependence of the induction of cerebellar LTD

Abstract

Long-term depression (LTD) of the granule cell to Purkinje cell synapse is thought to contribute to motor learning. According to the Marr/Albus/Ito model, sensory inputs drive granule cells to fire, thereby exciting Purkinje cells and influencing motor output. Inappropriate motor output causes neurons in the inferior olive to fire and activate Purkinje cells via the powerful climbing fiber (CF) synapse. CF activity is an error signal and the association of CF and granule cell parallel fiber (PF) activity results in LTD at coactivated PF synapses. Here we examine the timing dependence of LTD by using an induction protocol consisting of a single CF activation paired with a PF burst, with the relative timing of CF and PF activation systematically varied. LTD was most prominent when PF activation occurred before CF activation. A plot of LTD magnitude as a function of PF and CF timing was well approximated by a fit in which LTD peaked for PF activity approximately 80 ms before CF activation and the half width was approximately 300 ms. This indicates that the timing dependence of LTD is well suited to allow a CF to depress preceding PF inputs that generated inappropriate motor outputs. We also find that LTD induction and endocannabinoid release have a similar dependence on PF and CF timing. This suggests that the properties of endocannabinoid release may underlie the timing dependence of some forms of motor learning.

Figure 1. Timing dependence of the induction of LTD

Granule cell parallel fibers (PFs) and CFs were activated by extracellular electrodes and responses were measured using a whole cell electrode. PFs were stimulated 7 times at 100 Hz and the CF was stimulated once at time Δt, where positive Δt corresponds to PF activation preceding CF activation and Δt is measured from the middle of the stimulus train. This was repeated 30 times every 10 seconds. (A-C) A representative experiment is shown for Δt=150 ms with a response to conditioning stimulation (B) and the average EPSPs recorded before (C, black trace, -5 to -2 min) and after (C, grey trace, 20 to 23 min.). (D-F) Another representative experiment is shown for Δt=-150 ms. (G) Time course of EPSPs evoked by conditioning stimulation with indicated Δt’s. Traces are averages of 6 - 10 experiments and error bars are standard errors.

Figure 2. Climbing Fiber activation and CB1R activation is required for LTD under our experimental conditions

Experiments were conducted as in Figure 1 except that in (A-C) during the conditioning stimulation PFs were stimulated 7 times at 100 Hz without CF activation. Traces from a representative experiment are shown for the response to PF stimulation during the conditioning train (A, left), and the EPSPs before (A, right, black trace) and after the conditioning stimulation (A, right, grey trace). (B) The EPSP is plotted as a function of time for the experiment in (A), and in (C) the EPSP is plotted as a function of time for 6 experiments. (D-F) Experiments were also performed in which PFs were stimulated 7 times at 100 Hz and the CF was stimulated once at time Δt=50 ms in the presence of AM251. Traces from a representative experiment are shown (D), and the EPSP as a function of time are plotted in (E). (F) A summary of the normalized EPSP amplitude as a function of time is summarized for 5 experiments. Error bars in C and F are standard errors.

Figure 3. Summary of the timing dependence of the induction of LTD

(A) Normalized EPSP amplitude at 20 to 23 minutes post conditioning stimulation for individual experiments for the indicated Δt’s for PF (7 at 100 Hz) and single CF stimulation (open circles). Individual experiments are also shown for conditioning trains of PFs (7 times at 100 Hz) without CF activation (triangles) and PF and CF activation for Δt=50 ms in the presence of AM251 (squares). (B) Average normalized EPSP amplitude for the experiments in A. The fit to equation 1 is described in the text (solid line).

Figure 4. Comparison of the timing dependence of the associative effects of PF and CF stimulation on LTD, SSE and dendrtic calcium increases

Previous studies (Brenowitz and Regehr, 2005) have determined the timing dependence of pairing brief PF bursts (3 stimuli at 100 Hz) and CF bursts (5 stimuli at 100Hz) on peak synaptic suppression of excitatory synapses (SSE) (A), and on local calcium signals in Purkinje cell dendrites (B). Fits in A and B are to Equation 1, as described in the text. (C) The time courses of fits LTD (black), SSE, (red) and calcium increases (blue), are compared by normalizing the fits from Fig. 3B, Fig 4A and Fig. 4B respectively.