Spike-timing-dependent plasticity in primate corticospinal connections induced during free behavior

Abstract

Motor learning and functional recovery from brain damage involve changes in the strength of synaptic connections between neurons. Relevant in vivo evidence on the underlying cellular mechanisms remains limited and indirect. We found that the strength of neural connections between motor cortex and spinal cord in monkeys can be modified with an autonomous recurrent neural interface that delivers electrical stimuli in the spinal cord triggered by action potentials of corticospinal cells during free behavior. The activity-dependent stimulation modified the strength of the terminal connections of single corticomotoneuronal cells, consistent with a bidirectional spike-timing-dependent plasticity rule previously derived from in vitro experiments. For some cells, the changes lasted for days after the end of conditioning, but most effects eventually reverted to preconditioning levels. These results provide direct evidence of corticospinal synaptic plasticity in vivo at the level of single neurons induced by normal firing patterns during free behavior.

Figure 1. Experimental design and protocol

A: Schematic showing action potentials of CM cell triggering intraspinal stimuli via neurochip (NC). B: Cortical recording (top) and SpTA of EMG (bottom) for CM spikes followed after delay of 25 ms by spinal stimulus. SpTA shows post-spike facilitation and post-stimulus response in same target muscle. C: SpTAs of EMG acquired before (Day 0) and after (Day 1) a 22h period of conditioning, showing analysis interval (pink square), baseline ± 2 SD of SpTA (horizontal gray lines), and MPI above baseline of feature (horizontal red lines and black numbers). Conditioning increased MPI by 66% (p=0.0003). Vertical bar calibrates 25% of baseline. Drawings represent monkey performing task on Days 0 and 1, and behaving freely during 21.8 h of conditioning, with mean spike frequency of 7.5 Hz.

Figure 2. Strengthening of corticospinal connections

A: SpTAs show PSpF in two flexor muscles and PSpS in an extensor. The PSpFs are superimposed on facilitatory synchrony effects in both flexor muscles. Numbers give MPI of the SpREs; vertical line corresponds to trigger event. First two columns show repeatability of SpREs for two successive days without intervening conditioning (Day -1 and Day 0). Open arrows designate periods without conditioning. Solid black arrows designate conditioning periods (50µA stimuli and 12 ms delay); duration and average stimulus frequencies given above arrows. Colors of SpTAs for subsequent days indicate significance of changes relative to Day 0: red: p< 0.01, blue p< 0.05, and black: no significant difference. Scales at right calibrate % of baseline. Numbers give MPI. FCU, flexor carpi ulnaris; PL, palmaris longus; ECR, extensor carpi radialis; n, number of triggering spikes for all SpTAs. B: Collision test for CM cell in A. Spontaneous spikes in cell triggered 2 spinal stimuli (arrows) with intensity sufficient to trigger antidromic spikes (seen after 2nd stimulus); antidromic response for 1st stimulus is absent due to collision. The current intensity (360µA) was higher than that used during conditioning. C: PSpEs for another CM cell, showing pure PSpF without synchrony. Conditioning times are 3.5 h on Day 0 and 3 h on day 1.

Figure 3. Summary of conditioning effect on output

A: MPI of spike-related effects in SpTAs compiled before and after conditioning (pre MPI and post MPI). Facilitation in a; Suppression in b. Data points are color coded for different spike-stimulus delays. Data points above the dashed line indicate SpREs that increased after conditioning. The red arrow identifies session in Fig. 2A. For sessions involving multiple days of conditioning, the post-conditioning values were obtained after the last conditioning period. B: Average MPI on pre-and post-conditioning day, and one and two days after the end of conditioning, for facilitation (a) and suppression (b). Error bars show standard deviation. Significance of comparisons is given above horizontal bars. C: Individual MPI values in successive days for cell-muscle pairs that were documented for two post-conditioning days. Open circles for subsequent days indicate significant changes (p<0.05) relative to Day 0. A & b: spike-related facilitation and suppression.

Figure 4. Changes in spike-related effects as function of delay between spikes and stimuli

The graphs plot average differences in MPI (ΔMPI) for pre-and post-condition days (including the unchanged cases). Numbers of separate observations are shown in parentheses. Error bars show standard deviation. Asterisks denote values significantly different from zero (* p<0.05, ** p<0.01, *** p<0.001). A: spike-related facilitation. B: spike-related suppression. For comparison, plots that include only cases in which the PSpEs changed significantly are shown in Supplementary Fig. S3.

Figure 5. Decreases in SpRE produced by intraspinal stimuli with zero delay after spike

The two cells were recorded in different monkeys (Sessions 3 & 4 in supplementary Table 1). Note the decrease in MPI values after conditioning.

Figure 6. Proposed mechanisms for conditioning effect

Spike-triggered stimulation with appropriate positive delay produces a strengthening of CM cell terminals, increasing direct excitatory post-spike effects. Changes in inhibitory effects could involve terminals of CM cell and/or inhibitory interneuron. The strengthened terminals are illustrated in red.