Prefrontal neurons encode context-based response execution and inhibition in reward seeking and extinction

Abstract

The prefrontal cortex (PFC) guides execution and inhibition of behavior based on contextual demands. In rodents, the dorsal/prelimbic (PL) medial PFC (mPFC) is frequently considered essential for execution of goal-directed behavior ("go") whereas ventral/infralimbic (IL) mPFC is thought to control behavioral suppression ("stop"). This dichotomy is commonly seen for fear-related behaviors, and for some behaviors related to cocaine seeking. Overall, however, data for reward-directed behaviors are ambiguous, and few recordings of PL/IL activity have been performed to demonstrate single-neuron correlates. We recorded neuronal activity in PL and IL during discriminative stimulus driven sucrose seeking followed by multiple days of extinction of the reward-predicting stimulus. Contrary to a generalized PL-go/IL-stop hypothesis, we found cue-evoked activity in PL and IL during reward seeking and extinction. Upon analyzing this activity based on resultant behavior (lever press or withhold), we found that neurons in both areas encoded contextually appropriate behavioral initiation (during reward seeking) and withholding (during extinction), where context was dictated by response-outcome contingencies. Our results demonstrate that PL and IL signal contextual information for regulation of behavior, irrespective of whether that involves initiation or suppression of behavioral responses, rather than topographically encoding go vs. stop behaviors. The use of context to optimize behavior likely plays an important role in maximizing utility-promoting exertion of activity when behaviors are rewarded and conservation of energy when not.

Keywords: contingency; electrophysiology; extracellular; instrumental; rat.

Fig. 1.

(A) Task design. Rats were presented with RS or NS. NS responses (gray) produced no outcome. RS responses ended the RS tone, retracted the lever, and illuminated the reward well. Post-RS well entry resulted in sucrose reward (black). In extinction sessions, all events were the same except that well entry produced no reward (dashed line). (B) Lever press and well entry behavior for rats providing PL and IL neural recordings as indicated. RS lever presses and well entries decreased significantly across extinction days. For these and all following figures, error bars are mean ± SEM. Statistics are provided in Task Performance. (C) Placement of recording wire tips for PL-implanted (red) and IL-implanted (blue) animals. All wires tips were histologically localized to the respective prefrontal region.

Fig. 2.

PL and IL neurons exhibited stimulus-evoked neural activity during DS-sucrose. (A) Single neuron examples of the prevalent response to RS presentation in each region. (B) Mean z-score activity for neurons that exhibited significant increases or decreases after RS or NS for PL (Left) or IL (Right) neurons. Solid/dashed lines represent activity of neurons with significantly increased/decreased activity, respectively. (C) Proportions of neurons that exhibited significant excitation (filled bars) or inhibition (empty bars) in response to RS or NS. (D) Mean absolute-value z-scores of all recorded neurons in response to RS or NS.

Fig. S1.

(A) Characterization of waveform clustering (Top) and the resultant waveforms (Bottom). (Right) Rare example of multiple neurons recorded on a single recording wire. (Left) more common (>50%) example of a single neuron recorded on a single recording wire. (B) Extracted mean waveform used for analysis of action potential width. Width measurements were taken as the time between spike valley and the following peak. (C) Distribution of spike widths and baseline firing rates for PL (Left) and IL (Right) neurons. Blue histograms show action potential widths, calculated as in B. Red circles indicate individual neurons with specific spike widths (x axis) and 5-min presession baseline firing rate (y axis). Note that the majority of the neurons are wide-spike and low-baseline firing, indicating that most neurons recorded are glutamatergic pyramidal neurons. A small number of neurons exhibited short-waveform action potentials, although baseline activity for these neurons were low in all cases but 2, arguing against their being GABAergic. Similarly, a small number of neurons exhibited relatively high baseline activity, but the combination of baseline activity and spike widths were not consistent with a GABAergic phenotype. We conclude that the vast majority of recorded neurons were glutamatergic, while noting that a small number may have been GABAergic interneurons.

Fig. S2.

z-scored activity for all recorded PL (Top) and IL (Bottom) neurons during DS-sucrose illustrate the diverse stimulus-evoked responses in both regions. Activity is aligned on presentation of the RS (Left) or NS (Right). These data demonstrate that PL activity was approximately equally split between excitation and inhibition, whereas IL activity was primarily inhibited (although note some excitatory responses). In addition, the data show similar, although slightly weaker, evoked responses between RS and NS presentation.

Fig. S3.

Activity of neurons in each trial of DS-sucrose recording sessions illustrate consistent evoked responses throughout the entire session. (A) An example of a RS-excited PL neuron. (B) An example of a RS-inhibited IL neuron. Trials are shown along the y axis and time in trial, aligned to RS presentation, is shown along the x axis. (C) Block-based responses for six PL (mauve; mean is red) and two IL (cyan; mean is blue) neurons that changed significantly across the recording session. Note the mild and inconsistent changes in most neurons across the session, even in this subset of significantly modulated neurons. These results indicate a minor influence of time and/or ingested sucrose on PL or IL activity during the DS-sucrose task.

Fig. 3.

Stimulus-evoked neural activity during extinction was similar to that during DS-sucrose. Color/line conventions are as in Fig. 2. (A) Proportions of neurons significantly excited or inhibited by RS or NS presentation in DS-sucrose (as in Fig. 2) compared with all four extinction days [numbers recorded, n = 89, n = 90, n = 88, n = 89 (prelimbic; Left) and n = 103, n = 101, n = 100, n = 103 (infralimbic; Right)]. (B) Mean z-score activity for significantly excited or inhibited PL and IL neurons on extinction day 2 (PL, Left; IL, Right). (C) Mean z-score activity for significantly excited or inhibited PL and IL neurons poststimulus for DS-sucrose and all extinction days. (D) Mean absolute-value poststimulus z-scores for all recorded PL and IL neurons in response to RS and NS presentation on DS-sucrose and all extinction days.

Fig. 4.

RS- and NS-evoked neural activity sorted by whether stimuli resulted in a lever press (lighter colors) or withhold (darker colors) during DS-sucrose. (A and B) Mean absolute-value z-scores of all recorded neurons in response to RS (A) and NS (B). Stimulus-evoked neural activity was strongest for RS-press (light red/blue) and NS-withhold (dark magenta/cyan) trials. (C and D) Neurons that were significantly excited (filled bars) or inhibited (empty bars) by RS (C) or NS (D) were analyzed with respect to how they responded to an executed press (RS/NS-press) or withheld press (RS/NS-withhold). Neurons significantly excited or inhibited by RS were more excited or inhibited when a press occurred after an RS (C). Neurons significantly excited or inhibited by NS were more excited or inhibited when no press occurred after an NS (D). Analysis described in Stimulus-Evoked PL/IL Activity Predicted Context-Appropriate Execution and Inhibition of Responding. ****P << 0.001, ***P < 0.001, and **P < 0.005.

Fig. 5.

Stimulus-evoked neural activity switched from signaling press in DS-sucrose to withhold in extinction. (A) RS-evoked neural activity of all recorded neurons sorted by whether RS presentation resulted in a lever press (lighter colors) or withhold (darker colors) during DS-sucrose and extinction. (B) NS-evoked neural activity of all recorded neurons sorted as in A and C. Neurons in PL and IL neurons that were significantly excited by RS presentation were more excited for pressing than for withholding during DS-sucrose, and were more excited for withheld presses than elicited presses during extinction. (D) Neurons that were significantly inhibited by RSs were more inhibited for pressing than withholding presses during DS-sucrose, and also more inhibited for withholding than for pressing during extinction. Transitions from neural representation of press to withhold were highly significant. Statistics are provided in Neurons in PL and IL Switched from Signaling Press During Sucrose-Seeking to Withhold During Extinction.

Fig. S4.

Neurons recorded in DS-sucrose and all four extinction sessions (PL, n = 65, IL, n = 72). Circles represent individual neurons (PL, red; IL, blue). For each neuron, we calculated an index of RS-evoked activity during press vs. withhold trials (SI Materials and Methods). Each figure represents a separate extinction session compared with DS-sucrose. Note that the majority of, although not all, neurons in PL and IL were strongly modulated for pressing during DS-sucrose and withholding during extinction. These points fall in the gray-shaded area. Data in scatter plots are based on absolute-value measurements of activity. Below each scatter plot is a measure of consistency of excitatory vs. inhibitory responses in each DS-sucrose pressing and extinction withholding. For neurons falling in the gray-shaded area, we measured whether neurons were excited or inhibited during DS-sucrose (when the animal pressed) and during extinction (when the animal withheld pressing). Note that, for both PL (red) and IL (blue), the majority of neurons were similarly modulated: excited or inhibited in both conditions (leftmost two bars). These data indicate that most individual neurons shifted their activity to represent pressing during DS-sucrose to withholding during extinction, and that these shifts in activity maintained the same valence of activity (excitation–excitation or inhibition–inhibition).

Fig. S5.

Pharmacological inactivation of PL (red) or IL (blue) using bac/mus (B/M) during DS-sucrose (A) or alternating days of extinction (B). ACSF was given on control days. Inactivation of IL, and not PL, decreased responding during DS-sucrose [PL, t(7) = 0.70; P > 0.05; IL, t(6) = 3.46; P < 0.05, paired t test]. During extinction, inactivation of PL and IL decreased lever presses, which rebounded on ACSF days [PL, χ²(4,28) = 9.53; P < 0.05; IL, χ²(4,20) = 14.67; P < 0.01, Friedman test]. Over the entire extinction session, inactivation of PL and IL neurons impaired extinction learning such that there were no significant differences between extinction day 1 and extinction day 6 (PL, signed-rank = 12.5; P > 0.05; IL, signed-rank = 7; P > 0.05, Wilcoxon test). These data indicate a broad influence of PL and IL manipulation on task performance not selectively limited to response execution or inhibition per se. Further discussion is provided in the main text.

Fig. S6.

PL and IL neurons exhibited changes in activity related to reward well entry and consumption. (A) Mean z-scores for all recorded neurons aligned on well entry following RS lever press during DS-sucrose. (B) Mean z-scores for the same neurons aligned on all other (nonrewarded) well entries. Neuronal activity in both areas is more strongly modulated for rewarded well entries than for nonspecific well entries. In addition, IL activity is strongly elevated and PL activity is mildly inhibited during sucrose consumption.