Impaired spatial memory performance in a rat model of neuropathic pain is associated with reduced hippocampus-prefrontal cortex connectivity

Abstract

Chronic pain patients commonly complain of working memory deficits, but the mechanisms and brain areas underlying this cognitive impairment remain elusive. The neuronal populations of the mPFC and dorsal CA1 (dCA1) are well known to form an interconnected neural circuit that is crucial for correct performance in spatial memory-dependent tasks. In this study, we investigated whether the functional connectivity between these two areas is affected by the onset of an animal model of peripheral neuropathic pain. To address this issue, we implanted two multichannel arrays of electrodes in the mPFC and dCA1 of rats and recorded the neuronal activity during a food-reinforced spatial working memory task in a reward-based alternate trajectory maze. Recordings were performed for 3 weeks, before and after the establishment of the spared nerve injury model of neuropathy. Our results show that the nerve lesion caused an impairment of working memory performance that is temporally associated with changes in the mPFC populational firing activity patterns when the animals navigated between decision points-when memory retention was most needed. Moreover, the activity of both recorded neuronal populations after the nerve injury increased their phase locking with respect to hippocampal theta rhythm. Finally, our data revealed that chronic pain reduces the overall amount of information flowing in the fronto-hippocampal circuit and induces the emergence of different oscillation patterns that are well correlated with the correct/incorrect performance of the animal on a trial-by-trial basis. The present results demonstrate that functional disturbances in the fronto-hippocampal connectivity are a relevant cause for pain-related working memory deficits.

Figure 1.

Apparatus and behavioral performance. A, Figure-eight maze, spatial alternation working memory task. Starting from the center of the maze (C), the animal had to alternately visit two reward sites (R) (feeder A and B) to obtain chocolate-flavored pellets. The animal was required to come back to the center from a given reward site before visiting the other reward site. The arrows indicate the direction of travel when going to the left and right goals. B, Training period performance for the spatial working memory task. Only rats that reach at least the threshold of 80% of correct alternations from feeder A to feeder B according to the task imposed rules were selected to be candidates to receive the surgery for electrode implantation. C, Movement map of a rat across days 2 and 10 of the training period. As shown, the rat frequently made more navigation errors in the early days of training (see, for example, the direct trajectories between reward locations across day 2). D, Level of sensibility to mechanical stimulation evaluated using von Frey filaments. A large decrease was observed in the threshold required to induce a paw response in the SNI group. E, Recording period performance for the spatial working memory task. F, A significant decrease in performance level and running velocity was observed in the SNI group after nerve lesioning. G, Total number of trials performed in each recording session. A large decrease of the number of performed trials was observed in the SNI groups after nerve lesioning. H, The SNI animals spent more time navigating in the delay zone of the behavioral test after lesioning. I, Also a significant increase of the average interval between correct alternations was observed in this experimental group. Values are presented as the mean ± SEM. SNI group, n = 5; sham group, n = 5. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by post hoc Bonferroni test. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 2.

Pain-related performance behavior and hippocampus function performance dependency. A, B, The chronic pain effects on the cognitive learning process performance was tested on a classic T-maze forced-choice task (A), and on the figure-eight spatial delayed alternation task (B). The nerve-lesioned animals (SNI: T-maze, n = 9; figure-eight maze, n = 6) showed a significantly lower performance when compared with control animals (sham: T-maze, n = 6; figure-eight maze, n = 6) across both tasks. A bilateral lesion of the dorsal CA1 hippocampal region was done with QA injection to evaluate whether the cognitive spatial performance depends on normal hippocampal function. C, D, A clear decrease of the performance level was observed in hippocampus-lesioned group (QA group) across both behavioral tasks (C, T-maze: QA, n = 6; aCSF control, n = 6; D, figure-eight: QA, n = 6; aCSF-control, n = 6). It should be noted that the performance decrease was larger in the figure-eight task. Values are presented as mean ± SEM. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by post hoc Bonferroni test. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3.

Stability of two cells simultaneously recorded in the same animal across experimental sessions and firing activity during task navigation. A, Illustration of the waveform shape of a hippocampal CA1 cell (yellow) and an mPFC cell (green). Only units with a >3:1 signal-to-noise ratio were considered. B, Off-line analysis of 3-D PC cluster stability from each channel shown across the whole recording sessions using the WaveTracker software (Plexon). C, In this view, 2-D PC clusters are projected as function of time (z-axis). Stability across time of the units isolated was used as an extra selection criteria. D, Correspondent oscillations of intracranial LFP channels. Raw recordings represent 10 s of ongoing LFP activity.

Figure 4.

Typical neuronal responses for correct and error alternations. A, Perievent time decision histograms illustrate the averaged firing rate of four mPFC and four CA1 representative neurons around the decision limit (bin resolution of 50 ms). Time = 0 on the x-axis corresponded to the time of decision of direction turn to yield the reward location. Across both recorded regions, some neurons changed their spiking activity and others remain unchanged in respect to correct and error alternations after the decision of direction (from left to right, mPFC: KS = 0.32, p = 0.0001; KS = 0.07, p = 0.8200; KS = 0.24, p = 0.0282; and KS = 0.10, p = 0.6725; from left to right CA1: KS = 0.29, p = 0.0003; KS = 0.15 p = 0.1349; KS = 0.25, p = 0.0026; and KS = 0.13, p = 0.4973, respectively). B, C, The majority of mPFC neurons elevated their average firing activity when the rat performed a correct alternation (B), and CA1 neurons decreased their firing activity across error alternations (C). In both cases, no differences were found between experimental groups. D, The percentage of neurons that elevated their average firing rate during the delay zone navigation with respect to the whole recording session decreased significantly after nerve lesioning in the case of mPFC neurons. Values are presented as mean ± SEM. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by Bonferroni post hoc test. *p < 0.05.

Figure 5.

Correlation between hippocampal theta oscillations and spiking activity. The relationship between the temporal structure of population neuronal spiking activity and the ongoing theta cycle of hippocampal LFPs (black dotted line), as calculated for the preoperation period and day 10 after sham (n = 5) or SNI surgery (n = 5). The black dotted line represents the phase histogram of the 4–9 Hz hippocampal local field potential (LFP_θ). A, B, Illustrations of the populational mPFC activity before (CT) and after sham surgery (A), and the activity before (CT) and after SNI surgery (B). The mPFC neurons increased their firing precision with respect to theta rhythms after nerve lesioning during the navigation across delay and choice zones (two-sample Kolmogorov–Smirnov test; delay zone: CT/SNI, KS = 0.24, p = 0.0459; sham/SNI, KS = 0.25, p = 0.0257; choice zone: CT/SNI, KS = 0.26, p = 0.0389; sham/SNI, KS = 0.28, p = 0.0291). C, D, Illustrations of the populational CA1 activity before (CT) and after sham surgery (C), and the activity before (CT) and after SNI surgery (D). The CA1 conserved their temporal structure of activity across reward and choice zones, but across delay zone increased their firing precision (CT/SNI: KS = 0.24, p = 0.0289; and sham/SNI: KS = 0.26, p = 0.0311). Values are mean ± SEM.

Figure 6.

Spectral analysis of mPFC-dCA1 LFPs channels. A, B, PSD of LFPs normalized by the percentage of total power within the frequency range analyzed (1–50 Hz) for mPFC (A) and CA1 (B) channels, comparing the CT and sham or SNI recording sessions after surgery. Qualitative illustration of PSD showed that spectral power patterns were partially conserved across the experimental groups.

Figure 7.

Spectral coherence. A, Spectral coherence between simultaneously mPFC and CA1 LFPs recorded channels showed similar global levels of coherence activity across navigation zones for experimental groups. B, The averaged coherence (in 1–50 Hz frequency range) across recording sessions indicated no significant differences between experimental groups; however, significant differences were found across recording sessions, particularly during the early recording sessions after lesioning. Values are presented as mean ± SEM. SNI group, n = 5; sham group, n = 5. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by post hoc Bonferroni test. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 8.

The oscillations of information flow between the two recorded regions were determined by partial directed coherence analysis. Data were presented individually across the three considered navigation zones of the behavioral task. A, B, The amount of information flow from dorsal hippocampal CA1 to mPFC after peripheral nerve lesioning decreased dramatically across choice zone navigation (A), and from mPFC to CA1 across the reward zone (B), indicating that less information was processed in the mPFC–dCA1 circuit after peripheral nerve lesioning.

Figure 9.

Partial directed coherence activity across recording days and frequency bands. A, B, The averaged PDC level in the 1–50 Hz frequency range across recording sessions indicates that less information was transmitted from CA1 to mPFC after peripheral nerve lesioning (A), and across frequency bands for reward (in all frequency bands except in delta) and choice (in all frequency bands) navigation zones (B). Note, however, that PDC activity was particularly conserved across the delay zone, and that only the theta frequency showed a decreased after lesioning. C, D, In the mPFC → CA1 direction, the averaged PDC level decreased for reward zone across recording days (C), and across frequency bands for reward (in all frequency bands) and choice (in theta and alpha bands) zones (D). No significant differences were found for delay zone across frequency bands. Frequency band analysis corresponds to the averaged PDC activity across postsurgery sham/SNI recording sessions. Frequency bands: delta (δ; 1–4 Hz), theta (θ; 4–9 Hz), alpha (α; 9–15 Hz), beta (β; 15–30 Hz), and slow-gamma (γ; 30–50 Hz). Values are presented as the mean ± SEM. SNI group, n = 5; sham group, n = 5. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by post hoc Bonferroni test. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 10.

Patterns of information flow activity for correct and error alternations. Different patterns of PDC activity were observed across correct and error alternations. A, Note, for example, a strong theta band activity during correct alternations for the CA1 → mPFC direction, which is altered after peripheral nerve lesioning. B, In terms of recording sessions, the averaged PDC activity (in the 1–50 Hz frequency range) indicates that significant differences were found between experimental groups. C, In terms of frequency bands, the PDC activity showed that less information was processed in both directions of the circuit across theta, alpha, and beta frequency bands for correct and error alternations after nerve lesioning. Frequency band analysis corresponds to the averaged PDC activity across postsurgery sham/SNI recording sessions. Frequency bands: delta (δ; 1–4 Hz), theta (θ; 4–9 Hz), alpha (α; 9–15 Hz), beta (β; 15–30 Hz), and slow-gamma (γ; 30–50 Hz). Values are presented as the mean ± SEM. SNI group, n = 5; sham group, n = 5. Comparisons between experimental groups are based on two-factor repeated-measures ANOVA, followed by Bonferroni post hoc test. *p < 0.05, **p < 0.01, ***p < 0.001.