Differential susceptibility to synaptic plasticity reveals a functional specialization of ascending axon and parallel fiber synapses to cerebellar Purkinje cells

Abstract

Granule cell axons, via their parallel fibers, form synapses with Purkinje cells across large areas of the cerebellar cortex. Evidence for uniform transmission along parallel fibers to Purkinje cells is controversial, however, leading to speculation that the ascending axonal segment plays a dominant role in cerebellar processing. We have compared the relative susceptibilities of ascending axon and parallel fiber synaptic inputs to several forms of synaptic plasticity. We demonstrate that ascending axon synapses have a limited capability to undergo forms of long-term depression and potentiation compared with parallel fiber synapses. These results demonstrate that these two segments of the same axon play fundamentally different roles in cerebellar signaling, and, as such, the synapses formed between granule cells and Purkinje cells should not be treated as a homogenous population.

Figure 1.

Ascending axon synapses to Purkinje cells are resistant to long-term depression. a, The positioning of stimulating and recording electrodes in the granule cell and molecular layers for selective activation of ascending axon and parallel fiber synapses to Purkinje cells is shown. The effects of conjunctive activation of the CF input with parallel fiber (b) and ascending axon (c) pathway stimulation are shown. d and e illustrate the effects of 1 Hz raised intensity stimulation (1 Hz RIS) applied to the parallel fiber (n = 10) and ascending axon pathways (n = 8), respectively. The means and SEs are shown. Shown superimposed above are the means of six consecutive sweeps sampled before (darker traces) and 20 min after (lighter traces) the period of 1 Hz RIS for each pathway. Responses to parallel fiber activation are shown on the lefthand side and ascending axon responses are shown to the right. f shows the relative changes in CV⁻² values plotted against relative changes in mean EPSC amplitudes for each pathway with SEs before and 20 min after climbing fiber-independent LTD. Square symbols represent time-matched control data in which both pathways were activated alternately at 0.2 Hz throughout. g, The means and SEs of parallel fiber (gray) and ascending axon pathway EPSC amplitudes (black) sampled 30 min after control stimulation are shown, along with those after pairing each pathway with the climbing fiber input or after presentation of the climbing fiber-independent LTD protocol. The asterisks indicate significant differences between pathways (Wilcoxon matched pairs test; p < 0.05). Each data set represents a minimum of six separate experiments. In all panels, gray and black hues represent parallel fiber and ascending axon responses, respectively. Closed circles indicate when the conjunctive stimulation was applied to the parallel fiber pathway, and closed triangles represent the ascending axon pathway. bl, Baseline; aa, ascending axon; cf, climbing fiber; pf, parallel fiber.

Figure 2.

Ascending axon synapses to Purkinje cells are resistant to long-term potentiation. A comparison of the effect of applying 1 Hz stimulation for 5 min to parallel fiber (a) and ascending axon pathways to Purkinje cells. In each case, the alternate pathway was not stimulated during this period. Data are shown as in Figure 1, d and e. Shown superimposed above are the means of six consecutive sweeps sampled before (darker traces) and 20 min after (lighter traces) the period of 1 Hz stimulation for each pathway. Responses to parallel fiber activation are shown on the lefthand side, and ascending axon responses are shown to the right. Gray and black symbols represent responses to parallel fiber and ascending axon pathway activation, respectively. In c, the parallel fiber pathway was activated at baseline rates while 1 Hz stimulation was applied to the ascending axon. d illustrates the changes in CV⁻² that accompanied these stimulation protocols. bl, Baseline; aa, ascending axon; pf, parallel fiber.

Figure 3.

Ascending axon synapses to Purkinje cells are resistant to presynaptic potentiation. Bursts of stimuli were applied to parallel fiber (a) and ascending axon (b) pathway inputs to PCs at a rate of 16 Hz for 15 s. The alternate pathway in each case was not stimulated during this period. The data are presented as in Figure 2, a and b. The concomitant paired-pulse ratios are shown in c and d. e illustrates the changes in CV⁻² that resulted from these stimulation protocols. bl, Baseline; aa, ascending axon; pf, parallel fiber.

Figure 4.

The effects of PKA activation on ascending axon and parallel fiber synaptic inputs to Purkinje cells. The effects of a 10 min application of 10 μm forskolin on alternate activation of ascending axon and parallel fiber pathway responses are shown. The data are presented as in Figure 2, a and b. Each pathway was activated at 0.2 Hz throughout. b illustrates the resulting changes in CV⁻².

Figure 5.

LTD and LTP do not depend on activation of beams of parallel fibers. a illustrates the arrangement of stimulating and recording electrodes for dispersed activation of parallel fibers. Because granule cell ascending axons rise to different levels within the molecular layer, stimulation of the granule cell layer 150–300 μm lateral to the recorded PC should produce a more dispersed pattern of parallel fiber activation compared with molecular layer stimulation. The effects of conjunctive activation of the CF input with lateral GC stimulation (b), 1 Hz raised intensity stimulation (RIS) (c), 1 Hz stimulation (d), and 16 Hz stimulation of the lateral GC layer are shown (e) together with representative traces sampled before (black traces) and 20 min after (gray traces) each induction protocol. Pooled data represent the means and SEs of five to seven separate cells. f illustrates the resulting changes in CV⁻² for each protocol. The symbols match those used to display the results for each protocol in a–e. bl, Baseline; cf, climbing fiber; pf, parallel fiber; dpf, dispersed parallel fiber.