Abstract
Goal-directed behavior is dependent on neuronal activity in the prefrontal cortex (PFC) and extended frontostriatal circuitry. Stress and stress-related disorders are associated with impaired frontostriatal-dependent cognition. Our understanding of the neural mechanisms that underlie stress-related cognitive impairment is limited, with the majority of prior research focused on the PFC. To date, the actions of stress across cognition-related frontostriatal circuitry are unknown. To address this gap, the current studies examined the effects of acute noise-stress on the spiking activity of neurons and local field potential oscillatory activity within the dorsomedial PFC (dmPFC) and dorsomedial striatum (dmSTR) in rats engaged in a test of spatial working memory. Stress robustly suppressed responses of both dmPFC and dmSTR neurons strongly tuned to key task events (delay, reward). Additionally, stress strongly suppressed delay-related, but not reward-related, theta and alpha spectral power within, and synchrony between, the dmPFC and dmSTR. These observations provide the first demonstration that stress disrupts the neural coding and functional connectivity of key task events, particularly delay, within cognition-supporting dorsomedial frontostriatal circuitry. These results suggest that stress-related degradation of neural coding within both the PFC and striatum likely contributes to the cognition-impairing effects of stress.
Keywords: prefrontal cortex, striatum, synchrony, delay, working memory
The prefrontal cortex (PFC) plays a central role in regulating cognitive processes that guide flexible goal-directed behavior, including working memory (Goldman-Rakic 1996; Miller and Cohen 2001). PFC-dependent cognition involves topographically organized connections with the striatum (Heilbronner et al. 2016). For example, in rodents, the dorsal aspect of the medial PFC is closely associated with higher cognitive function and extends a robust innervation to the dorsomedial striatum (dmSTR; Voorn et al. 2004). Importantly, lesion and inactivation studies demonstrate that multiple PFC-dependent cognitive processes, including working memory, are similarly dependent on the dmSTR (Goldman and Rosvold 1972; Ragozzino 2007; Spencer et al. 2012; Spencer and Berridge 2019). Collectively, these findings identify a central role of frontostriatal circuitry in higher cognitive function.
It has long-been known that stress impairs cognitive processes dependent on frontostriatal circuitry and contributes to multiple behavioral disorders associated with frontostriatal cognitive dysfunction (Broadbent 1971; Hartley and Adams 1974; Arnsten 2009; Hilton and Whiteford 2010). However, our understanding of the neural mechanisms that underlie the cognition-impairing effects of stress is limited, impeding our ability to better treat stress-related cognitive dysfunction. In nonhuman primates and rodents engaged in delayed-response tasks of working memory, subpopulations of PFC neurons have been identified that respond to key task events, including the delay interval and reward (Fuster and Alexander 1971; Goldman-Rakic 1995; Devilbiss et al. 2017). Similar task-dependent neurons are found within the striatum, including rodent dmSTR (Schultz and Romo 1988; Levy et al. 1997; Akhlaghpour et al. 2016; Hupalo et al. 2019). In earlier studies we observed that stress selectively suppresses the activity of putative dorsomedial PFC (dmPFC) pyramidal neurons tuned to delay and reward (Devilbiss et al. 2017). To date, the effects of stress on working memory-related activity of dmSTR neurons are unknown.
In addition to the activity of individual neurons, local field potential (LFP) oscillations and oscillatory synchrony between regions have been posited to coordinate information processing (Buzsaki 2010; Roux and Uhlhaas 2014). In terms of PFC-dependent cognition in humans, frontal cortex theta activity (4–7 Hz) is positively correlated with working memory performance (Gevins et al. 1997; Mitchell et al. 2008; Hsieh and Ranganath 2014), whereas stress-related impairment in working memory is associated with decreases in theta activity (Gartner et al. 2014). Alpha (8–12 Hz) and beta (15–35 Hz) activities in the PFC have also been positively associated with higher cognitive function, including working memory and inhibitory control (Klimesch 1999; Freunberger et al. 2011; Roux and Uhlhaas 2014). The sensitivity of these latter frequencies to stress is currently unknown. In contrast to the positive relationship between lower frequency oscillations and PFC-dependent cognition, stress was observed to increase PFC gamma power (40–80 Hz; Minguillon et al. 2016). In rodents, striatal oscillatory activity is also linked to a diversity of cognitive/behavioral processes (Berke et al. 2004; DeCoteau et al. 2007; van der Meer and Redish 2009). However, this latter work has generally not involved the dmSTR or cognitive tasks dependent on dorsomedial frontostriatal circuitry. Currently, we know little about the effects of stress on task-related oscillatory activity and oscillatory synchrony within the broader dorsomedial frontostriatal system, representing a significant gap in our understanding of the neurobiology of both stress and higher cognitive function.
To address these issues, the current study examined the effects of working memory impairing acute noise-stress on single neuron spiking activity and spectral power within, and synchrony between, the dmPFC and dmSTR in rats engaged in a spatial working memory task.
Materials and methods
Subjects
Male Sprague–Dawley rats (n = 13, Charles River, Wilmington DE; 300–500 g) were housed individually with enrichment (Nylabone chews) and maintained on a 13/11-h light/dark cycle (lights on 0600 h) with ad lib water access. Food intake was limited to maintain motivation for food reward while avoiding weight loss (15–19 g chow per day). All procedures were in accordance with NIH and University of Wisconsin-Madison policies.
Working memory training
Animals were initially trained to perform a delayed non-match to position task as described previously (Devilbiss et al. 2012, 2017; Fig. 1). Briefly, animals were trained to enter the arm of a T-maze not chosen on the previous trial to receive food reward (one-half of a mini chocolate chip, 80 mg, Eilleen’s Candies, Green Bay, WI, or 45 mg sucrose pellet/trial, Bio-Serv, Flemington, NJ). Distal visual cues were minimized with black plastic sheeting and the maze was wiped with 70% ethanol to minimize olfactory cues. Masking white noise (60 dB) was played above the center of the maze (Devilbiss et al. 2017). Initially, a testing session consisted of 10 trials. Between trials, animals were placed in a start box at the base of the maze and prevented from exiting by a gate that was removed at the end of the delay period. On the first trial preceding the 10 trials, animals could choose either arm. Animals were considered trained when they reached an average accuracy of 80% for 2 consecutive sessions, each consisting of 10 trials with a 0-s delay between trials.
Once trained, animals were implanted with recording electrodes and allowed to recover 7–10 days with ad lib feeding (Fig. 1). Prior studies demonstrate the targeted region of the dmSTR receives direct projections from the dmPFC and is necessary for successful performance in this task (Spencer et al. 2012; Spencer and Berridge 2019). Following recovery from surgery, mild food restriction was reinstated and animals were trained to perform 2 40-trial sessions per day (excluding the first trial), each separated by 2 h. Trials were separated initially by a 5-s delay. During training, animals were connected to a dummy headstage. When animals reached an average accuracy level of 80–95% across 2 testing days (no more than 10% variation across days), they were randomly assigned to white noise-stress (95 dB) or no-stress control conditions for the subsequent testing day. On the testing day, animals were connected to the recording headstage and then tested in a baseline session and 2 h later a second testing session either under no-stress control or noise-stress conditions. Between testing sessions, the headstage remained connected, allowing within-neuron analyses across testing sessions (see below). No-stress control was identical to baseline testing conditions (i.e. 60 dB white noise).
Surgery
Under isoflurane (Halocarbon Laboratories, River Edge, NJ), animals were stereotaxically implanted with 50 μm stainless-steel chronic recording electrodes targeting both the dmPFC and dmSTR, as previously described (Fig. 1; Devilbiss et al. 2012, 2017). In the dmPFC, an 8-wire linear electrode array (NB Labs, Dennison, TX) oriented along the rostral-caudal axis was positioned in layer V (centered at A/P +3.0 mm, M/L ±0.7 to 0.9 mm, D/V −2.5 to 3.0 m). In the dmSTR, a 2 × 3 × 3 matrix electrode array (NB Labs, Dennison, TX) was implanted ipsilateral to the dmPFC electrodes (centered at A/P +0.45 mm, M/L ±3.5 mm with 11.5° angle, D/V −3.3 to 3.5 mm). Electrodes were secured with skull screws (MX-0080-16B-C, Small Parts, Inc., Logansport, IN) and dental acrylic (Plastics One, Roanoke, VA).
Testing and electrophysiological recording procedures
On testing days, animals were transported to the recording room and tethered to a 32-channel commutator and a multichannel electrophysiology acquisition processor (MAP, Plexon, Dallas, TX). Neural activity was amplified, discriminated, and time stamped (Devilbiss et al. 2017). Putative single units in the dmPFC and dmSTR exhibiting a ~3:1 signal-to-noise ratio were isolated (Fig. 1). The following measures were used to ensure that sorted waveforms arose from single neurons and remained stable throughout a testing session: (i) variability of peak waveform voltage, (ii) variability of waveform slope, (iii) scattergram distribution in the first 3 principal components, and (iv) refractory period (Fig. 1).
After spike-sorting, a 40-trial baseline recording session was conducted at ~11 AM, followed 2 h later by a second 40-trial session. During the second recording session, animals were tested either under conditions identical to baseline testing (no-stress control) or in the presence of 95 dB white noise (stress; Devilbiss et al. 2017). For animals exposed to noise-stress, the stressor was initiated immediately prior to and continued throughout the second session. This stressor is well documented to impair PFC-dependent cognition in humans, monkeys, and rodents (Arnsten et al. 1985; Becker et al. 1995; Arnsten and Goldman-Rakic 1998; Davis and Whalen 2001; Holmes and Wellman 2009; Szalma and Hancock 2011; Devilbiss et al. 2017) and, importantly, can be used concurrently with cognitive testing. Animals remained in the home cage and connected to the headstage between testing sessions. Analyses described above ensured stability of single-unit recordings throughout both testing sessions. Video recordings (80 frames/s) and IR beams were used to timestamp the animal’s location in the T-maze (Fig. 1; see Devilbiss et al. 2017).
Given performance in this task improves over multiple testing sessions, when animals exceeded the 80–95% performance criterion, delay duration was increased in 5–10-s increments. The maximal delay duration used in these studies was 40 s. Animals were tested with noise-stress no more than once a week. Data from 3 animals were excluded from analyses because they were not behaviorally impaired by more than 10% during a second exposure to the stressor. Three animals were also excluded from data analyses because of failure to run the task during stress. For these latter animals, failure to run was observed on the first stress testing session and on a subsequent session.
Neuron identification and event tuning
For the dmPFC, wide-spiking (WS), putative glutamatergic pyramidal vs. narrow-spiking (NS), putative inhibitory interneuron cell types were differentiated based on trough-to-peak average of the waveform (WS > 200 μs; NS, 100–200 μs; Mitchell et al. 2007). For the dmSTR, medium spiny neurons (MSNs) and fast-spiking (FS) interneurons were identified using the peak-to-valley duration (MSN > 250 μs; FS < 200 μs). Because of a limited number of NS dmPFC neurons and FS dmSTR interneurons identified, all subsequent analyses focused on WS PFC and MSN dmSTR neurons.
Distinct subpopulations of dmPFC and dmSTR neurons displayed excitatory responses to specific task events, including delay and reward (Figs. 2 and 3). Given high baseline task accuracy (control: 90.2 ± 1.5%; stress: 93.6 ± 1.0%; combined: 92.0 ± 0.9%), error responses of neurons could not be analyzed. Within both the dmPFC and dmSTR a relatively small number of neurons responded in a brief and time-locked manner to reward receipt. Instead, larger populations were observed that displayed a more sustained activation that began shortly prior to the physical receipt of reward, signaled by movement of the experimenter’s hand toward the animal. These are referred to as “reward proximity” neurons and may reflect positive response outcome or reward anticipation rather than the hedonic experience of the reward. Neurons “strongly tuned” to task events were identified by calculating the z-score of firing rate during the relevant baseline interval vs. activity in a 14-s interval surrounding the task interval. Delay-tuned neurons were identified by a z-score > 0.07 and a baseline firing rate > 0.5 Hz, whereas reward proximity neurons were identified by a z-score > 0.2 and a baseline firing rate > 0.5 Hz. The longer duration of the delay interval warranted a smaller z-score. Units were considered outliers if the percent change from baseline was >3 median absolute deviations. This was typically associated with low baseline firing rates and resulted in the exclusion of 1–2 neurons per group. This approach identified neurons with response properties similar to those described by others (Fuster and Alexander 1971; Batuev et al. 1990; Goldman-Rakic 1995; Horst and Laubach 2009). Units not meeting these criteria were considered “untuned” to task events.
Local field potential analyses: spectral density and coherence
LFPs were recorded from all electrodes in the dmPFC and dmSTR (MAP, Plexon). For each recording day, one wire/region was selected for analysis, based on recording quality (power of signal and minimal noise). Signals were referenced to ground, amplified, filtered, and digitized at 1 kHz, then down sampled to 200 Hz. Stationarity of the LFP was enforced across trials and time via a z-score transformation combined with differencing the timeseries across time, resulting in the spectrum scaling as a function of frequency (Oppenheim and Schafer 1989). LFP spectral density was analyzed using continuous wavelet transform (cwtft, MATLAB), with Morlet wavelets, and logarithmically increasing frequency from 4 to 100 Hz. The absolute power at each frequency was then calculated by squaring the absolute value of the wavelet coefficient (Cohen 2014). Average power within distinct frequency bands (theta 4–7 Hz; alpha 7–12 Hz; beta 15–35 Hz; gamma 40–80 Hz) was calculated for each trial over the interval of interest. To quantify synchrony, the cross-correlation as a function of frequency and time, wavelet transform coherence (wcoherence, MATLAB) with Morlet wavelets was calculated. Average synchrony per frequency band was calculated per trial.
Histology
Animals were deeply anesthetized and cathodal current (15 μA) was passed through each electrode for 10 s. Animals were then perfused with a 10% formalin solution. Brains were then removed, and immersed in 10% formalin for at least 24 h. Following fixation, brains were frozen and 40 μm coronal sections were collected through the dmPFC and dmSTR and stained with Neutral Red dye (Thermo Fisher Scientific, Waltham, MA).
Statistical analyses
Data were included only when histology verified accurate electrode placement and minimal tissue damage. Behavioral and electrophysiological measures were analyzed using a 2-way linear mixed-effects model, with session (baseline vs. treatment) and treatment (no-stress vs. stress) as fixed effects factors (JMP Pro Version 12.2.0, SAS Institute). When appropriate, pairwise comparisons of each group’s change from its own baseline were determined using Tukey HSD. For working memory performance, we analyzed the average working memory performance across each 40-trial testing session. For single unit activity, the mean spiking rate during task intervals was determined on a trial-by-trial basis using peri-event time histogram (PETH) analysis, with 8 independent analyses per combination of region (dmPFC or dmSTR), interval (delay or reward proximity), and neuronal tuning (tuned or untuned). For LFP recordings, the trial-by-trial LFP spectral density was analyzed as described above, with independent analyses done on region (dmPFC or dmSTR), interval (delay or reward proximity), and frequency band (theta, alpha, beta, and gamma). Trial-by-trial coherence was analyzed as described above with independent analyses done on task interval and frequency band.
Results
We previously characterized the effects of noise-stress on delay- and reward-related activity of individual dmPFC neurons (Devilbiss et al. 2017). A subset of 5 animals from this prior study was used to pilot simultaneous dmPFC–dmSTR recordings during stress. For the current study we revised methods used to identify and analyze task-related single unit activity, including: (i) determining neuronal tuning from baseline trials only, (ii) the use of reward-proximity interval rather than reward receipt, and (iii) statistical analyses that include the interaction of treatment and session, rather than treating each manipulation independently. Given these changes, and to minimize animal use, we included dmPFC single unit data from the 5 animals with dual dmPFC–dmSTR recordings from our prior study.
Effects of stress on working memory performance
Under baseline conditions animals displayed high task performance accuracy (92.0 ± 0.9% correct, range 82.5–100% correct). As shown in Fig. 1, noise-stress, initiated at the start of the second testing session (n = 16), significantly impaired working memory performance accuracy relative to both baseline (P < 0.0001) and no-stress control (treatment F(1,17.2) = 3.04, P = 0.099; session F(1,28) = 33.29, P < 0.0001; treatment × session F(1,28) = 23.91, P < 0.0001). In contrast, performance did not differ significantly from baseline under no-stress control testing conditions (n = 14; P = 0.930).
Effects of stress on task-related activity of dmPFC neurons
For dmPFC recordings, all electrodes were localized to layer V. Of the 343 recorded WS neurons identified, 57 (16.6%) were classified as strongly tuned to delay (Fig. 2). An additional 35 (10.5%) were identified as strongly tuned to reward proximity (Fig. 2). Only 2 neurons were strongly tuned to both events (3.5% of delay, 5.7% of reward proximity). Only 1 neuron displayed brief and time-locked responses to the physical receipt of food reward in the absence of reward-proximity activity and was excluded from analyses. The remaining neurons were classified as “untuned” to these task events.
Delay-related spiking activity of dmPFC neurons
As shown in Fig. 2, for delay-tuned neurons, noise-stress elicited a robust suppression of delay-related spiking activity of dmPFC neurons relative to baseline (P < 0.0001) and no-stress control (control, n = 21; stress, n = 36; treatment F(1,55) = 4.90, P = 0.031; session F(1,4501) = 77.22, P < 0.0001; treatment × session F(1,4501) = 69.51, P < 0.0001). In contrast, under no-stress control conditions spiking activity of delay-tuned WS dmPFC neurons did not differ from baseline conditions (P = 0.992). For neurons that were not tuned to the delay interval, stress significantly increased delay-related firing relative to baseline (P < 0.0001) and no-stress control (Fig. 2; control, n = 122; stress, n = 164; treatment F(1,284) = 29.49, P < 0.0001; session F(1,22592) = 43.53, P < 0.0001; treatment × session F(1,22592) = 4.06, P = 0.044). Delay-related activity of neurons not tuned to delay was moderately increased under no-stress control conditions relative to baseline (P = 0.013).
Reward-related spiking activity of dmPFC neurons
As shown in Fig. 2, stress also suppressed the spiking activity of dmPFC neurons tuned to reward proximity relative to baseline (P < 0.0001) and no-stress control (control, n = 16; stress, n = 19; treatment F(1,33) = 9.53, P = 0.004; session F(1,2319.6) = 20.05, P < 0.0001; treatment × session F(1,2319.6) = 6.31, P = 0.012). Spiking activity of reward proximity-tuned dmPFC neurons did not differ from baseline under no-stress conditions (P = 0.483). The activity of dmPFC neurons not tuned to reward proximity significantly increased during that interval under no-stress control conditions relative to baseline (Fig. 2; n = 127; p ≤ 0.0001). In contrast to that seen during the delay interval, stress significantly suppressed the activity of these neurons relative to no-stress control (n = 171; F(1,296.4) = 12.66, P = 0.0004; session F(1,19884) = 17.69, P < 0.0001; treatment × session F(1,19884) = 23.17, P < 0.0001), but not relative to baseline (P = 0.97).
Effects of stress on task-related activity of dmSTR MSNs
As with the dmPFC, and as shown in Fig. 3, a relatively large population of MSNs was observed to be strongly tuned to delay (n = 71, 20.5%), whereas a smaller population was strongly tuned to reward proximity (n = 25, 7.7%). Only a minimal number of neurons displayed a brief and time-locked response to food reward receipt (n = 3, 0.86%) and were excluded from the analyses.
Delay-related spiking activity of dmSTR neurons
As shown in Fig. 3, stress elicited a robust suppression of delay-tuned MSN dmSTR neurons relative to baseline (P < 0.0001) and no-stress control (control, n = 39; stress, n = 32; treatment F(1,69) = 0.15, P = 0.697; session F(1,5607) = 72.38, P < 0.0001; treatment × session F(1,5607) = 66.87, P < 0.0001). The magnitude of this suppression was comparable to that seen in the dmPFC. Under no-stress control conditions, spiking activity of delay-tuned dmSTR neurons did not differ from the baseline testing session (P = 0.995). For MSNs not tuned to the delay interval, stress had no significant effects on delay-related firing relative to baseline (P = 0.222) or no-stress control (Fig. 3; control, n = 109; stress, n = 167; treatment F(1,274) = 6.38, P = 0.012; session F(1,21802) = 2.82, P = 0.093; treatment × session F(1,21802) = 0.53, P = 0.467). Under no-stress control conditions, delay-related spiking activity of dmSTR neurons not tuned to the delay interval did not differ across the baseline and testing sessions (P = 0.928).
Reward-related spiking activity of dmSTR neurons
Stress also suppressed the firing rate of MSN dmSTR neurons tuned to reward proximity relative to baseline (P = 0.001) and no-stress control (Fig. 3; control, n = 11; stress, n = 14; treatment F(1,23) = 0.19, P = 0.666; session F(1,1580.3) = 6.45, P = 0.011; treatment × session F(1,1580.3) = 7.28, P = 0.007). The magnitude of this suppression was comparable to that seen in the PFC. Under no-stress control conditions, proximity reward-related spiking activity of dmSTR neurons did not differ from baseline (P = 0.999). As with the PFC, the activity of MSNs not tuned to reward proximity increased modestly, but significantly, under no-stress testing conditions relative to baseline (Fig. 3; n = 133; P = 0.005). However, unlike in the dmPFC, stress did not significantly change this effect relative to no-stress control (n = 166; treatment F(1,297.5) = 0.49, P = 0.484; session F(1,19793) = 28.15, P < 0.0001; treatment × session F(1,19793) = 0.24, P = 0.624).
Effects of stress on frontostriatal oscillatory power
The effects of stress on spectral power in the dmPFC and dmSTR during the delay and reward proximity intervals of the working memory task were also examined (control: n = 14; stress: n = 12). Under baseline conditions during the delay (Fig. 4) and reward proximity (Fig. 6) intervals, a dominant peak was observed centered at 8 Hz that spanned both theta (4–7 Hz) and alpha frequency bands (7–12 Hz). A second and broad peak, which included beta (15–35 Hz) and gamma (40–80 Hz) frequencies, was also seen throughout both intervals. This was most prominent in the dmPFC during delay and smallest in the dmSTR during reward proximity.
Stress and delay-related oscillatory activity
Stress robustly suppressed delay-related theta and alpha power in both the dmPFC and the dmSTR relative to baseline and no-stress control (Fig. 4; see Table 1 for statistical analysis results). No change in theta power was observed under no-stress control conditions in either the dmPFC or the dmSTR. However, there was a small, but significant, decrease in alpha power during the no-stress control session in both the dmPFC and the dmSTR relative to baseline. Stress elicited a weaker, but significant, suppression of delay-related beta power in the dmPFC and dmSTR relative to no-stress control, but not baseline (Fig. 4; see Table 1 for statistical results). However, in visual examination of the spectral density plots, stress appeared to differentially impact lower (15–25 Hz) vs. higher (25–35 Hz) beta frequencies during delay. Additional analyses demonstrated that stress significantly decreased delay-related low beta power, while significantly increasing higher beta power in both the dmPFC and dmSTR (Fig. 5). Stress also increased delay-related gamma power in the dmPFC and dmSTR relative to baseline and no-stress control (Fig. 4 and Table 1).
Table 1.
Interval | Frequency | Source | dmPFC | dmSTR |
---|---|---|---|---|
Delay | Theta | TRT | F 1,18.1 = 0.41, P = 0.528 | F 1,16.5 = 0.62, P = 0.441 |
SESS | F 1,2052 = 138.03, P < 0.0001 | F 1,2052 = 94.27, P < 0.0001 | ||
TRT*SESS | F 1,2052 = 148.31, P < 0.0001 | F 1,2052 = 121.72, P < 0.0001 | ||
Alpha | TRT | F 1,15.9 = 3.05, P = 0.100 | F 1,16.1 = 1.26, P = 0.278 | |
SESS | F 1,2052 = 275.25, P < 0.0001 | F 1,2052 = 592.1, P < 0.0001 | ||
TRT*SESS | F 1,2052 = 169.88, P < 0.0001 | F 1,2052 = 344.32, P < 0.0001 | ||
Beta | TRT | F 1,15.7 = 1.37, P = 0.259 | F 1,15.6 = 2.13, P = 0.164 | |
SESS | F 1,2052 = 0.61, P = 0.435 | F 1,2052 = 1.38, P = 0.241 | ||
TRT*SESS | F 1,2052 = 7.86, P = 0.005 | F 1,2052 = 16.18, P < 0.0001 | ||
Gamma | TRT | F 1,14.9 = 0.87, P = 0.366 | F 1,14.9 = 4.65, P = 0.048 | |
SESS | F 1,2052 = 8.13, P = 0.004 | F 1,2052 = 5.87, P = 0.016 | ||
TRT*SESS | F 1,2052 = 7.85, P = 0.005 | F 1,2052 = 4.18, P = 0.041 | ||
Reward | Theta | TRT | F 1,16.5 = 6.96, P = 0.018 | F 1,17.6 = 0.94, P = 0.346 |
SESS | F 1,1760.8 = 0.99, P = 0.321 | F 1,1760.8 = 0.57, P = 0.449 | ||
TRT*SESS | F 1,1761.9 = 0.70, P = 0.403 | F 1,1761.9 = 0.10, P = 0.757 | ||
Alpha | TRT | F 1,16.4 = 9.40, P = 0.007 | F 1,16.8 = 3.23, P = 0.090 | |
SESS | F 1,1753.8 = 4.58, P = 0.033 | F 1,1752.4 = 4.76, P = 0.029 | ||
TRT*SESS | F 1,1754.5 = 0.06, P = 0.799 | F 1,1752.9 = 0.53, P = 0.466 | ||
Beta | TRT | F 1,15.1 = 4.98, P = 0.041 | F 1,16.1 = 0.58, P = 0.458 | |
SESS | F 1,1750 = 0.45, P = 0.501 | F 1,1748.9 = 0.02, P = 0.889 | ||
TRT*SESS | F 1,1750.4 = 0.13, P = 0.719 | F 1,1749.1 = 2.17, P = 0.141 | ||
Gamma | TRT | F 1,15.2 = 0.81, P = 0.381 | F 1,15 = 0.16, P = 0.695 | |
SESS | F 1,1748.6 = 2.07, P = 0.150 | F 1,1748 = 0.68, P = 0.411 | ||
TRT*SESS | F 1,1748.8 = 6.52, P = 0.011 | F 1,1748.2 = 0.02, P = 0.879 |
Results of 2-way mixed model statistical analyses of local field potential recordings within the dorsomedial PFC (dmPFC) and dorsomedial striatum (dmSTR) during the delay and reward intervals across the 4 analyzed frequency bands: theta, alpha, beta, and gamma. TRT, treatment (stress vs. no-stress control); SESS, session (baseline vs. second testing session).
Thus, stress differentially affects lower vs. higher frequencies in both the dmPFC and dmSTR during the delay period. Stress-related changes in delay-related theta/alpha power were notably larger than stress-related increases in high-beta and gamma.
Stress and reward-related oscillatory activity
In contrast to that seen for delay, reward proximity-related oscillatory activity in the dmPFC was largely insensitive to stress (Fig. 6 and Table 1). The one exception was a small, yet significant, increase in reward-related gamma power. Under no-stress control conditions, there were no significant changes in reward-related spectral power within the dmPFC.
Within the dmSTR, neither no-stress control nor stress conditions affected reward-related oscillatory activity in any frequency band (Fig. 6 and Table 1).
Effects of stress on frontostriatal oscillatory synchrony
Delay-related synchrony
Under baseline conditions, there was a peak of dmPFC–dmSTR coherent theta and alpha activity during delay (Fig. 7). Stress significantly decreased delay-related theta and alpha synchrony relative to baseline and no-stress controls (Fig. 7 and Table 2). In contrast, stress elicited a small, yet significant, increase in dmPFC–dmSTR beta synchrony, which was largely driven by increased coherence in the higher beta frequencies (data not shown). Stress had no effect on delay-related gamma synchrony. In no-stress controls, delay-related dmPFC–dmSTR synchrony was not affected in any of the frequency bands relative to baseline.
Table 2.
Interval | Frequency | Source | Results |
---|---|---|---|
Delay | Theta | TRT | F 1,17.4 = 0.60, P = 0.447 |
SESS | F 1,2052 = 7.01, P = 0.008 | ||
TRT*SESS | F 1,2052 = 32.75, P < 0.0001 | ||
Alpha | TRT | F 1,16.5 = 6.32, P = 0.023 | |
SESS | F 1,2052 = 112.48, P < 0.0001 | ||
TRT*SESS | F 1,2052 = 52.87, P < 0.0001 | ||
Beta | TRT | F 1,15.7 = 1.20, P = 0.290 | |
SESS | F 1,2052 = 2.17, P = 0.141 | ||
TRT*SESS | F 1,2052 = 9.09, P = 0.003 | ||
Gamma | TRT | F 1,15.1 = 0.01, P = 0.907 | |
SESS | F 1,2052 = 5.24, P = 0.022 | ||
TRT*SESS | F 1,2052 = 1.99, P = 0.159 | ||
Reward | Theta | TRT | F 1,17.9 = 1.63, P = 0.218 |
SESS | F 1,1721 = 0.39, P = 0.531 | ||
TRT*SESS | F 1,1721 = 0.04, P = 0.844 | ||
Alpha | TRT | F 1,17.4 = 2.04, P = 0.171 | |
SESS | F 1,1720.8 = 12.91, P = 0.0003 | ||
TRT*SESS | F 1,1720.9 = 1.91, P = 0.167 | ||
Beta | TRT | F 1,18.4 = 0.80, P = 0.323 | |
SESS | F 1,1720.5 = 8.17, P = 0.004 | ||
TRT*SESS | F 1,1720.6 = 3.06, P = 0.080 | ||
Gamma | TRT | F 1,16.9 = 0.05, P = 0.818 | |
SESS | F 1,1720.4 = 4.66, P = 0.031 | ||
TRT*SESS | F 1,1720.5 = 0.002, P = 0.963 |
Results of 2-way mixed model statistical analyses of synchrony between the dorsomedial PFC (dmPFC) and dorsomedial striatum (dmSTR) during the delay and reward intervals across the 4 analyzed frequency bands: theta, alpha, beta, and gamma.
Reward-related synchrony
We also observed a peak of dmPFC–dmSTR theta and alpha synchrony during the reward-proximity interval during baseline (Fig. 7). However, unlike delay, stress had no significant effects on reward-related dmPFC–dmSTR synchrony in any frequency band (Fig. 7 and Table 2). During no-stress control conditions, theta and gamma synchrony were unchanged during reward proximity. However, there were reductions in alpha and beta synchrony in no-stress controls, neither of which were observed under stress conditions. Nonetheless, there were no significant differences in alpha or beta synchrony between stress and no-stress controls.
Discussion
Successful goal attainment requires the maintenance of information in the absence of sensory cues combined with the development and updating of action plans, often while confronting distractors and/or ambiguity. Frontostriatal circuits support a diversity of “executive” cognitive processes that support goal attainment. Stress impairs frontostriatal cognitive function, an action with strong relevance for health and safety (Broadbent 1971; Arnsten 2009). However, our understanding of the neurobiology underlying stress-related impairment in frontostriatal cognitive function is limited. To date, the majority of research in this area has focused on the PFC (Arnsten 2009; Devilbiss et al. 2017). Despite extensive evidence demonstrating a critical role of the dmSTR in “PFC-like” higher cognitive function, the impact of stress on this region has been overlooked. The current studies demonstrate that stress strongly disrupts working memory-related neural coding and functional connectivity within this cognition-supporting dorsomedial frontostriatal pathway. At the level of oscillatory activity and oscillatory synchrony, the effects of stress were largely limited to the delay interval. Collectively, these actions may contribute to stress-related impairment in higher cognitive function and stress-related psychopathology associated with impaired PFC-dependent cognition.
Effects of stress on task-related activity of individual neurons
At the single neuron level, stress robustly suppressed spiking activity of neurons strongly tuned to delay and reward in both the dmPFC and dmSTR. In both regions, there were larger populations of neurons tuned to delay than impending reward (reward proximity). Thus, delay-related neuronal activity within the dmPFC and dmSTR is a particularly prominent component of working memory performance. As observed previously (Devilbiss et al. 2017), stress resulted in an activation of the larger population of dmPFC neurons not tuned to delay, further degrading the population-level representation of information associated with delay. While dysregulated GABAergic transmission within the PFC has been observed in stress (McKlveen et al. 2016; Page and Coutellier 2019; Ghosal et al. 2020), the current observations indicate stress does not simply modulate overall inhibitory or excitatory tone within either the dmPFC or dmSTR. Instead, stress disrupts the encoding of key task-related information via complex actions across distinct subpopulations of neurons within these regions.
Stress increases catecholamine release in the PFC (Thierry et al. 1976; Dunn 1988; Abercrombie et al. 1989). The selective effect of stress on neurons strongly tuned to key task events is similar to that previously described for catecholamines in multiple regions, including the PFC (Berridge and Waterhouse 2003; Arnsten 2011). Of particular relevance, catecholamines exert an inverted-U shaped modulation of delay-related firing within the PFC, with higher rates of release engaging noradrenergic α1 and dopaminergic D1 receptors, resulting in a suppression of delay-related activity (Arnsten 2011). These observations suggest D1 and α1 receptors may participate in the degradative effects of stress on dmPFC neural coding.
The mechanisms responsible for stressor-induced suppression of task-related neuronal activity within the dmSTR are currently unclear. Given the dmPFC has direct, excitatory projections to the dmSTR, stressor-induced inhibition of task-related signaling of dmPFC neurons may well weaken task-related activity within the dmSTR. Stress also increases DA release within the dorsal striatum, although the magnitude of this is significantly less than seen in the PFC (Dunn 1988; Abercrombie et al. 1989). While stress-related elevations in DA could play a role in the neurophysiological actions of stress within the dmSTR, DA has been demonstrated to typically exert excitatory actions on striatal MSNs (Rebec et al. 1997; Surmeier et al. 2007). Beyond catecholamines, stressor-induced elevations in serotonin, glucocorticoids, or other neuromodulators could contribute to stress-related degradation of task-related spiking activity of dmSTR and dmPFC neurons.
Interestingly, while chronic stress was observed to decrease the activity of dmPFC neurons in animals tested in a cost–benefit conflict test, similar to that observed in the current study with acute stress, the opposite effect was observed for striosomal projection neurons in the dmSTR (Friedman et al. 2017). While the current study did not limit dmSTR recordings to striosomal projection neurons, these observations suggest the hypothesis that in contrast to acute stress, sustained exposure to stress may trigger neuroadaptations that results in disinhibition of the dmSTR.
Effects of stress on oscillatory activity within dorsomedial circuitry
At the level of LFPs, stress elicited event-, region-, and frequency-specific effects. The most robust of these was the suppression of delay-related theta (4–7 Hz) and alpha (7–12 Hz) spectral power within, and synchrony between, the dmPFC and dmSTR. Interestingly, this was not observed for reward proximity. The presence of delay-related theta and alpha activity in the dmPFC, as well as the sensitivity of this activity to stress, is consistent with prior observations in humans (Gevins et al. 1997; Klimesch 1999; Jensen and Tesche 2002; Bonnefond and Jensen 2012; Gartner et al. 2014; Manza et al. 2015; Popescu et al. 2016, 2019). Stress-related suppression of theta and alpha activity in the PFC could well contribute to an inability to maintain information over a delay period. While striatal oscillatory activity has been linked to a diversity of cognitive and behavioral processes (Berke et al. 2004; DeCoteau et al. 2007; van der Meer and Redish 2009), most of this research has not involved the dmSTR or working memory. The current studies provide the first demonstration that working memory performance is associated with a robust peak of theta and alpha activity in the dmSTR during both the delay and reward intervals. However, as with the PFC, stress potently suppressed delay-related, but not reward-related, theta and alpha power in the dmSTR as well as dmPFC–dmSTR theta/alpha synchrony. The differential sensitivity of delay- and reward-related theta/alpha activity suggests a prominent role of delay-related theta/alpha activity in the support of working memory as well as in the working memory impairing actions of stress. The suppression of delay-related theta/alpha synchrony in stress may reflect an impaired ability to maintain information flow between these regions necessary for successful task completion. This said, there was not a strong correlation between percent change in delay-related theta/alpha power or synchrony and percent change in task performance (data not shown). These latter observations could reflect the fact that oscillatory activity within this dorsomedial frontostriatal pathway is only one component of the neural architecture supporting higher cognitive function.
In contrast to the large suppressive actions of stress on theta and alpha activity, stress elicited relatively small increases in higher frequency oscillations within the dmPFC and dmSTR, particularly during the delay interval. This is consistent with previously observed increases in frontal gamma power in a variety of mental illnesses associated with impaired PFC cognitive function, including PTSD, depression, and schizophrenia (Basar-Eroglu et al. 2007; Strelets et al. 2007; Moon et al. 2018). The neural mechanisms that underlie the differential actions of stress across task events and oscillatory frequencies are currently unclear. However, similar to that concluded for single-unit activity, these task- and frequency-selective effects of stress indicate that stress is not simply altering inhibitory or excitatory tone within this frontostriatal circuit to influence oscillatory activity.
It should be noted that studies in humans typically define theta as oscillations in the 4–7 Hz range, and alpha as being comprised of 7–12 Hz activity. In contrast, some studies in rodents have more broadly defined theta as spanning 4–12 Hz activity (Buzsaki 2005). Nonetheless, in other studies in rodents, 4–7 Hz and 7–12 Hz have been, respectively, referred to as type II vs. type I theta (Kramis et al. 1975; Vanderwolf and Robinson 1981), low vs. high theta (Cervera-Ferri et al. 2011; Mouchati et al. 2020), or theta vs. alpha (Tort et al. 2010). Interestingly, there appear to be functional and neurochemical differences between these distinct frequency ranges in rodents. In particular, 8–12 Hz oscillations are enhanced with locomotion and resistant to cholinergic manipulations, whereas 4–7 Hz activity is more prominent in immobile animals and eliminated by the muscarinic antagonist, atropine (Kramis et al. 1975; Vanderwolf and Robinson 1981). Taken together, this suggests that even in rodents, the broader 4–12 Hz frequency range supports distinct processes that can be differentiated across the 4–7 Hz and 7–12 Hz frequency bands, regardless of whether these are referred to as subcategories of theta or as theta vs. alpha.
Summary
The current studies demonstrate that stress impairs neural coding within a cognition-supporting frontostriatal pathway. This was observed for single neuron activity as well as at the level of oscillatory power and synchrony. In terms of oscillatory activity, the most robust effects of stress were the suppression of delay-related theta (4–7 Hz) and alpha (7–12 Hz) activity within and between the dmPFC and dmSTR. These results indicate that stress elicits task-, neuron-, and frequency-specific degradation in frontostriatal neural coding of higher cognitive function.
Contributor Information
Craig W Berridge, Department of Psychology, University of Wisconsin, Madison, WI 53706, United States.
David M Devilbiss, Department of Cell Biology and Neuroscience, Rowan University.
Andrea J Martin, Department of Psychology, University of Wisconsin, Madison, WI 53706, United States.
Robert C Spencer, Department of Psychology, University of Wisconsin, Madison, WI 53706, United States.
Rick L Jenison, Department of Psychology, University of Wisconsin, Madison, WI 53706, United States.
Author contributions
Craig Berridge (Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing—original draft, Writing—review & editing), David M. Devilbiss (Conceptualization, Investigation, Methodology, Writing—review & editing), Andrea Martin (Data curation, Formal analysis, Investigation, Methodology, Project administration, Writing—review & editing), Robert Spencer (Methodology, Writing—review & editing), and Rick L. Jenison (Methodology, Writing—review & editing)
Funding
The National Science Foundation (IOS-0918555); the National Institutes of Health (MH081843, MH116526).
Conflict of interest statement: The authors declare no financial interests or potential conflicts.
Data availability
The datasets associated with this study are available from the corresponding author on reasonable request.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets associated with this study are available from the corresponding author on reasonable request.