Chronic alcohol consumption shifts learning strategies and synaptic plasticity from hippocampus to striatum-dependent pathways

Abstract

Introduction: The hippocampus and striatum have dissociable roles in memory and are necessary for spatial and procedural/cued learning, respectively. Emotionally charged, stressful events promote the use of striatal- over hippocampus-dependent learning through the activation of the amygdala. An emerging hypothesis suggests that chronic consumption of addictive drugs similarly disrupt spatial/declarative memory while facilitating striatum-dependent associative learning. This cognitive imbalance could contribute to maintain addictive behaviors and increase the risk of relapse.

Methods: We first examined, in C57BL/6 J male mice, whether chronic alcohol consumption (CAC) and alcohol withdrawal (AW) might modulate the respective use of spatial vs. single cue-based learning strategies, using a competition protocol in the Barnes maze task. We then performed in vivo electrophysiological studies in freely moving mice to assess learning-induced synaptic plasticity in both the basolateral amygdala (BLA) to dorsal hippocampus (dCA1) and BLA to dorsolateral striatum (DLS) pathways.

Results: We found that both CAC and early AW promote the use of cue-dependent learning strategies, and potentiate plasticity in the BLA → DLS pathway while reducing the use of spatial memory and depressing BLA → dCA1 neurotransmission.

Discussion: These results support the view that CAC disrupt normal hippocampo-striatal interactions, and suggest that targeting this cognitive imbalance through spatial/declarative task training could be of great help to maintain protracted abstinence in alcoholic patients.

Keywords: addiction; alcohol; amygdala; dorsal striatum; hippocampus (CA1); learning strategies; memory systems; synaptic plasticity.

Figure 1

Experimental procedures. (A) Timeline describing the mice housing conditions (yellow), the drinking regimen (blue), and the temporal arrangement of experimental procedures including stereotaxic surgery (red), handling (gray), Barnes Maze task (green), and concurrent in vivo electrophysiological recordings (purple). Three mice groups were established based on their drinking regimen: AW (Alcohol Withdrawn, n = 22), CAC (Chronic Alcohol Consumption, n = 17), and Ctrl (Controls, n = 20). (B) Intracranial electrodes implanted by stereotaxic surgery for in vivo electrophysiology in freely moving mice. All mice received a stimulating electrode in the basolateral nucleus of the amygdala (BLA), and a recording electrode either in the dorsal CA1 of the hippocampus (dCA1) or in the dorsolateral striatum (DLS), to record evoked field potentials (EFPs) either in the BLA → dCA1 or BLA → DLS pathway. (C) Barnes Maze protocol used to assess spatial vs. non-spatial learning strategies. One of the 18 holes, called escape hole (red frame), led to a shelter under the board allowing to escape from the exposed area of the BM. After habituation (Hab, Trial 0, Day 4), mice were trained through 6 acquisition trials to enter the escape hole (Acq, Trials 1–6, Day 5). Remaining at the same location during trials 0–6, and signaled by a proximal cue (bottle), the escape hole could be found through Random, Serial, Spatial or Cued strategy. To dissociate the use of Spatial vs. Cued-strategy (S/C), a competition trial (Comp, Trial 7) was performed with two escape holes: the one at the same location that during Acq (S) and the one at the opposite where the proximal cue was relocated. (C) Three retention trials were performed on Day 6 (Ret, Trials 8–10). (D) Timeline of EFPs recordings (BL, E1, E15, E60; purple) and BM trials (t1–7; green) in a 4-mice cohort during Day 5 and 6. Mice were tested by cohort of maximum 4 individuals following the order of passage and inter-trial intervals depicted in top (Acq/Comp, Day 5) and bottom (Ret, Day 6) tables. A maximum of 4 cohorts (16 mice) were tested per day. Acq, acquisition; BL, Baseline recordings of EFPs in the BLA → dCA1 or DLS pathway; BM, Barnes Maze; E, Evoked field potentials in the BLA → dCA1 or DLS pathway from1 to 60′ after BM or HFS; Hab, habituation; Ret: retention; t, trial.

Figure 2

Search strategies in the Barnes maze task. (A) The Spatial/Cued strategy was defined as moving directly to the S/C hole (0 error) or to less than 3 holes before entering in the S/C hole (≤3 errors), during Habituation and Acquisition sessions. The Spatial strategy was defined as moving directly to the S hole (0 error) or to less than 3 holes before entering in the S hole (≤ 3 errors), during Competition and Retention sessions. Errors were made on holes either adjacent (direct), or non-adjacent (indirect) to the escape one. (B) The Cued strategy was defined as moving directly to the C hole (0 error) or to less than 3 holes before entering in the C hole (≤3 errors), during Competition and Retention sessions. Errors were made on holes either adjacent (direct), or non-adjacent (indirect) to the escape one. (C) The Serial strategy was allocated when the entry into the escape hole was preceded by visiting at least 4 adjacent holes in serial manner (clockwise or counter-clockwise direction). (D) The Random strategy was defined as hole searches separated by crossing through the center of the maze.

Figure 3

Effects of CAC and AW procedures on BLA → dCA1 and BLA → DLS neurotransmission. (A) Microphotographs of thionine-stained 50 μm-thick brain slices showing (black arrows) the localization of the recording electrode tip in the dCA1 (left) and the DLS (right); and of the stimulating electrode tip in the BLA (center). (B–D) I/O curves showing the variation of EFPs amplitudes as a function of various BLA stimulation intensities (0–0.7 mA; curves) and the BLA stimulation intensities used to evoked FPs representing 50%–70% of the maximal amplitude of the EFPs (C) in the dCA1 (B), or in the DLS (D). ^*p < 0.05: group effect. (E) Representative trace of BLA stimulation-EFPs in vivo recordings in dCA1 and DLS. (F–H) detailed locations of the recording electrode tip in the dCA1 (F) and the DLS (H); and of the stimulating electrode tip in the BLA (G).

Figure 4

Effects of CAC and AW on Spatial vs. non-Spatial learning strategies in the Barnes maze task. (A–C) Escape latencies (A), errors (B), and path lengths (C) over the course of the habituation trial (t0, Hab), the six acquisition trials (t1–6, Acq), the competition trial (t7, Comp), and the 3 retention trials (t8–10, Ret). Data are represented as mean ± SEM. ∑∑∑∑p < 0.0001: trial effect; ^*p < 0.05: group effect. (D–G) Relative use of each search strategy in AW, CAC and Ctrl group (from top to bottom) during: habituation (D), acquisition (E), competition (F), or retention (G). ^***p < 0.05: comparison with Ctrl group; ●●●p < 0.001: comparison with previous session. #p = 0.060: vs. Ctrl group, close to significance. (H,I) Escape latency (H) and errors (I) for all acquisition trials, pooled per search strategy. (J,K) Escape latency (J) and errors (K) for all retention trials, pooled per search strategy. ^*p < 0.05, ^***p < 0.001, ^****p < 0.0001: pairwise strategy comparison. (L) Score of preference for the Spatial strategy over the Cued strategy during retention trials 8 to 10. The score was calculated as followed: number of trials solved with a Spatial strategy, minus the number of trials solved with a Cued strategy.

Figure 5

Effects of CAC and AW procedures on the learning-induced BLA → dCA1 and BLA → DLS neurotransmission. (A,B) Changes in BLA → dCA1 (A) or BLA → DLS (B) amplitude (EFPs, in % change from baseline ± SEM) at different time points before and after the BM task. (C,D) Amplitude variation of signals (in % change from baseline ± SEM) 1, 15, 45, and 60 min after BLA HFS, recorded in the dCA1 (C) or DLS (D). ᴥp < 0.05, ᴥᴥp < 0.01, ᴥᴥᴥp<0.001: comparison with BL; ^*p < 0.05, ^**p < 0.01, ^***p < 0.001: comparison with Ctrl group.