Inversely Active Striatal Projection Neurons and Interneurons Selectively Delimit Useful Behavioral Sequences

Abstract

Understanding neural representations of behavioral routines is critical for understanding complex behavior in health and disease. We demonstrate here that accentuated activity of striatal projection neurons (SPNs) at the beginning and end of such behavioral repertoires is a supraordinate representation specifically marking previously rewarded behavioral sequences independent of the individual movements making up the behavior. We recorded spike activity in the striatum and primary motor cortex as individual rats learned specific rewarded lever-press sequences, each one unique to a given rat. Motor cortical neurons mainly responded in relation to specific movements regardless of their sequence of occurrence. By contrast, striatal SPN populations in each rat fired preferentially at the initiation and termination of its acquired sequence. Critically, the SPNs did not exhibit this bracketing signal when the same rats performed unreinforced sequences containing the same sub-movements that were present in their acquired sequence. Thus, the SPN activity was specifically related to a given repetitively reinforced movement sequence. This striatal beginning-and-end activity did not appear to be dependent on motor cortical inputs. However, strikingly, simultaneously recorded fast-spiking striatal interneurons (FSIs) showed equally selective but inverse firing patterns: they fired in between the initiation and termination of the acquired sequences. These findings suggest that the striatum contains networks of neurons representing acquired sequences of behavior at a level of abstraction higher than that of the individual movements making up the sequence. We propose that such SPN-FSI networks of the striatum could underlie the acquisition of chunked behavioral units.

Keywords: basal ganglia; chunking; corticostriatal; dorsolateral striatum; habit; sequence learning.

Figure 1. Rats Learn to Perform Specific Three-Step Lever-Press Sequences

(A) Task procedure with examples of a correct sequence (1–2-1, top) and an incorrect sequence (1-2-2, bottom), and top view of operant chamber configuration (right). (B) Percent correct in total session (solid) and in the best 3-min performance period (dashed) across training days (n = 13 rats). Only 4 unimplanted rats were trained past day 40. Shading indicates SEM. (C) Number of correct trials performed during best 3-min performance period across training days (n = 13 rats). (D) Head tracking data from 30 consecutive correct trials in one training session (top). Average session trajectory (dark blue to yellow) of rat performing the 1-2-1 sequence (bottom) with head locations for lever 1 presses (green), lever 2 presses (blue), and chocolate milk delivery (brown). (E) Side-to-side (top) and forward-and-back (bottom) session average head trajectories from 7 consecutive training days for a rat performing the 1-2-1 sequence (left) and another rat performing the 1-2-2 sequence (right). (F) Correlation coefficients of session average head trajectories between pairs of rats that learned different sequences (left, n = 42 pairs), session average trajectories within single rats (middle, n = 7 rats), and trial average trajectories within rats (right, n = 7 rats). Only trajectories from days 10+ of training with successful video tracking were used. Error bars indicate SEM. **p < 0.001 (Wilcoxon rank-sum test). (G) Ratio of trials performed in 30-min devaluation probe session and number of trials performed in the first 30 min of prior day’s training session as a function of number of days trained. Each data point corresponds to a single session.

Figure 2. Simple Lever-Press-Responsive Neurons Are More Prevalent in Forelimb Motor Cortex than in Dorsolateral Striatum

(A) Activity of a lever-press-related unit recorded in motor cortex during 1-1-2 correct (left) and 1-1-1 incorrect (right) sequences. Peri-event histograms for individual trial events were pasted together with window sizes determined by the session median time between each pair of successive events. Colored lines indicate events, and dashed lines indicate the time-window borders. This unit fired during lever 1 presses in both sequences. (B) Activity of a ‘non-motor-type’ unit in dorsolateral striatum during 1-2-2 correct (left) and 2-2-2 incorrect (right) sequences, showing selective spiking during the last press of the correct sequence, but not during repeated presses of the same lever. (C) Proportions of motor-type units in ten press-related time-points in 15 sessions with 10+ units recorded simultaneously in motor cortex and striatum (inset). Gray lines represent individual sessions. Error bars indicate SEM. *p < 0.05. (D) Distribution of overall z-scores for motor cortical pyramidal neurons (blue) and dorsolateral striatal SPNs (orange) in response to the events indicated in C. (E) Changes in the χ² statistic for motor cortical pyramidal neurons (MC Pyr; n = 670) and SPNs in dorsolateral striatum (DLS SPNs; n = 1708) when the beginning and end variables were added to the generalized linear model. ***p < 0.000001. (F) The difference in the proportions of SPNs and pyramidal neurons with given χ² statistic changes (x-axis) after adding the beginning and end variables to the model.

Figure 3. Striatal SPN Population Spiking Is Concentrated around the Initiation and Termination of the Learned Lever-Press Sequence but Not during Incorrect Lever-Press Sequences, Regardless of the Lever-Press Sequence Learned

(A) Activity of start (top) and end (bottom) SPNs in rats that learned 1-1-2 (left), 1-2-2 (middle) or 2-1-2 (right) lever-press sequences, plotted as in Figure 2A. (B) Activity of start SPN (top) that spiked during the first lever 1 press in the correct sequence, but not during the middle lever 1 press (left). This SPN did not respond during incorrect trials with repetitive lever 1 pressing (right). Activity of end SPN (bottom) that fired during the last lever 2 press in the correct sequence but not during the middle lever 2 press during the correct sequence (left). This SPN responded weakly during repetitive lever 2 pressing (right). (C) Mean peri-event SPN spiking during correct (first column), non-repeat incorrect (second column), lever 1 repeat (third column), and lever 2 repeat (fourth column) trials in 9 rats that learned different lever-press sequences indicated on the left. (D) Mean peri-event activity for all SPNs for the same trial types as in C (solid, n = 2501, 2501, 1143 and 1338, respectively) and for SPNs that did not surpass 5-Hz firing rate during correct trials (dashed, n = 1857, 1857, 842 and 1002, respectively). Shading indicates SEM. **p < 0.01. (E) Activity of sub-groups of SPNs responsive to first (blue), second (green) and third (red) lever press in the same trial types (n = 154, 125 and 157, respectively). (F) Proportion of highly responsive SPNs that fired maximally in the 0.5-s window around the first (blue), second (green) or third (red) lever press, or at other times in the correct trial (gray). See also Figures S1–3.

Figure 4. Beginning and End of Specific Sequences Are Emphasized Selectively in Rats That Were Trained on That Specific Sequence

(A) We calculated the difference between the mean population SPN activity in the one or two rats that were trained on a particular sequence and the population SPN activity in the rest of the 9 rats that were not trained to perform it, but did so occasionally as an incorrect trial. (B) Population spiking of SPNs recorded in dorsolateral striatum in all six matched correct (left) and incorrect (right) sequences. (C) SPN mean firing rates in correct sequences (solid) and the matched incorrect sequences (dashed). Shading indicates SEM. *p < 0.001 (Wilcoxon rank-sum test). (D) Difference in the SPN firing rates in correct sequences and matched incorrect sequences.

Figure 5. Inhibition of Motor Cortical Cell Bodies and Terminals Has Little Effect on Spiking in Dorsolateral Striatum

(A) Motor cortical injections result in selective halorhodopsin-expressing cortical terminals in dorsolateral striatum (left). Lesion marks (red) indicate tetrode tips within this termination zone (center and right). (B) Mean firing rate of 383 putative cortical pyramidal neurons (MC Pyr, left), 366 striatal SPNs (DLS SPN, second) and 106 striatal FSIs (DLS FSI, third) during pulses of laser light (yellow shading) delivered to cortical cell bodies in the freely moving rat, and proportions of significantly inhibited (blue) and activated (red) units (right). Insets in the second and third columns show, respectively, the subsets of 96 SPNs with firing rates of >1 Hz and 33 significantly activated FSIs. Shading indicates SEM. (C) Mean firing rate and proportions of modulated units for the same three cell groups (n = 383, 311 and 108, respectively) when the laser pulses were delivered to the corticostriatal terminals in the striatum, plotted as in B.

Figure 6. Motor Cortex Is Unlikely to Be Driver of Task-Boundary Activation in Dorsolateral Striatum

(A) Activity of an SPN during laser-off (gray) and laser-on (yellow) times across three training days (rows), showing task-time-selective modulation (arrow) by cortical terminal inhibition. Shading indicates SEM. (B) Activity of three SPNs recorded in one rat, with no differences in task-related spiking in laser-off (gray) and laser-on (yellow) times. (C) Proportions of all SPNs inhibited (blue) and activated (red) by cortical terminal inhibition in striatum during correct trial performance. (D) Beginning-and-end SPN firing during correct trials with (yellow) and without (gray) cortical terminal inhibition (n = 163 SPNs). (E) Session average activity of simultaneously recorded motor cortical pyramidal (blue) and striatal SPN (gray) units in correct trials (n = 51 sessions), non-repeat incorrect trials (n = 57), lever 1 repeat press trials (n = 34), and lever 2 repeat press trials (n = 20). See also Figure S4.

Figure 7. Narrow Spike-Width FSIs Fire in an Opposing Pattern to SPNs during Correct Sequence Performance

(A) Single units recorded in dorsolateral striatum putatively classified as narrow spike-width FSIs (red) and SPNs as used in previous analyses (blue) along features of spike waveform width, firing rate (FR), and proportion of interspike intervals (ISIs) greater than 1 s. (B) Normalized FSI activity recorded in nine rats performing different learned sequences indicated at left, plotted as in Figure 3C. (C) Mean FSI firing rates during incorrect trials (left, n = 311), correct trials (middle, n = 284), and trials in which the first two presses were correct but the last press was incorrect (right, n = 153). Inset shows activation of sub-groups of FSIs with different baseline firing rates in correct trials. Shading indicates SEM. (D and E) Normalized session average activity of simultaneously recorded SPNs (gray) and FSIs (red) in correct trials (D, n = 69 sessions) and correct start trials in which the last lever press was incorrect (E, n = 42 sessions). Shading indicates SEM. *p < 0.001. (F) Activity of SPNs with task-related spiking that was significantly correlated positively (green, n = 977) or negatively (blue, n = 1007) with that of simultaneously recorded FSIs. (G) Distribution of SPN-FSI pair correlation coefficients for start and end SPNs (purple) and middle SPNs (yellow). See also Figure S5.