Muscarinic antagonists impair multiple aspects of operant discrimination learning and performance

Abstract

Acetylcholine signaling can strengthen associations between environmental cues and reward availability. Diverse subtypes (M1-M5) of the muscarinic acetylcholine receptor (mAChR) family may have distinct roles in different learning and memory processes, such as encoding cue-reward associations and consolidating these associations in long-term memory. Using an operant discrimination learning task in which mice are trained to nose poke during a tone to receive a food reward, we found that acquisition of the task requires mAChR signaling in the central nervous system. In addition, post-session injections of a broad mAChR antagonist, scopolamine impaired consolidation of the cue-reward memory. Further, after successful learning of a cue-reward contingency across multiple training sessions, mice that received a single pre-session injection of scopolamine were unable to use the learned cue association to receive rewards. Taken together, these data demonstrate distinct roles for muscarinic signaling in acquisition, consolidation and recall of the operant discrimination learning task. Understanding mechanisms underlying natural reward-related responding may provide insight into other maladaptive forms of reward learning such as addiction.

Keywords: Acquisition; Consolidation memory; Muscarinic receptor (subtype); Muscarinic receptors; Operant learning; Reward learning.

Figure 1.. Pre-session injections of mAChR blockers impair the ability of mice to acquire appropriate operant discrimination responses.

(A) Timeline for behavioral training. (B) Mice that were administered saline (n = 9) or pirenzepine dihydrochloride (mAChR blocker that shows reduced CNS penetration; n = 10) prior to discrimination training obtain significantly more rewards compared to mice that were treated with scopolamine (mAChR blocker; n = 10) or benztropine mesylate (mAChR blocker; n = 10). (C) During tone and discrimination training, incorrect nose pokes were increased in mice that were administered scopolamine or benztropine mesylate compared to saline- or pirenzepine dihydrochloride-treated mice. (D) Scopolamine-treated mice made a higher number of inactive nose pokes compared to saline controls during both tone and discrimination training stages. (E) Saline- and pirenzepine-treated mice received a higher number of tone presentations compared to benztropine- and scopolamine-injected mice. (F) Compared to saline control mice or pirenzepine dihydrochloride-treated mice, central mAChR blockade increased hit rate (# of rewards obtained/# of tones delivered) during tone training but reduced hit rate during discrimination training. (G) Benztropine-injected mice have a higher number of receptacle entries compared to controls. Main effects and interactions are for discrimination training.

Figure 2.. mAChRs contribute to consolidation of operant discrimination learning.

(A) Timeline for behavioral training. (B) Mice injected with scopolamine (n = 12) immediately following each session show no impairment during tone training stage but earn significantly fewer rewards than saline-treated mice during discrimination training. when incorrect responses result in a timeout (n = 12). (C) Both groups decreased the number of incorrect nose pokes during discrimination training (note that error bars are very small and are therefore obscured by symbols). (D) Number of inactive nose pokes were higher in scopolamine treated mice during discrimination training. (E) Saline-treated mice received more tones per session during discrimination training compared to mice that were administered scopolamine, indicating that they made fewer errors and therefore had fewer timeout periods. (F) Post-session scopolamine administration significantly impaired the accuracy of nose poking when a tone was presented (hit rate: # of rewards obtained/# of tones delivered). (G) Scopolamine has no effect on the number of receptable entries. Main effects and interactions are for discrimination training.

Figure 3.. Pre-session scopolamine administration impairs performance of the operant discrimination task in trained mice.

(A) Timeline for behavioral training. Following 18 days of discrimination training (post-training) with post-session saline or scopolamine injections, mice were split into 4 groups (n = 6/group): 1) post-session saline during post-training/pre-session saline during performance evaluation; 2) post-session scopolamine during post-training /pre-session saline during performance evaluation; 3) post-session saline during post-training /pre-session scopolamine during performance evaluation; 4) post-session scopolamine during post-training/pre-session scopolamine during performance evaluation. (B) Pre-session injection of saline preserved the consolidation impairment observed in the group that received post-session scopolamine during discrimination training. Pre-session injections of scopolamine significantly disrupted performance in both groups. (C, D) Pre-session scopolamine administration increased the number of incorrect and inactive nose pokes in both groups. (E, F) Hit rate is impaired in mice that received scopolamine 30 min before the task. (G) Pre-session injections of scopolamine increased the number of reward receptacle entries. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.