Distinct effects of ventral tegmental area NMDA and acetylcholine receptor blockade on conditioned reinforcement produced by food-associated cues

Abstract

Stimuli paired with rewards acquire reinforcing properties to promote reward-seeking behavior. Previous work supports the role of ventral tegmental area (VTA) nicotinic acetylcholine receptors (nAChRs) in mediating conditioned reinforcement elicited by drug-associated cues. However, it is not known whether these cholinergic mechanisms are specific to drug-associated cues or whether VTA cholinergic mechanisms also underlie the ability of cues paired with natural rewards to act as conditioned reinforcers. Burst firing of VTA dopamine (DA) neurons and the subsequent phasic DA release in the nucleus accumbens (NAc) plays an important role in cue-mediated behavior and in the ability of cues to acquire reinforcing properties. In the VTA, both AChRs and N-methyl-d-aspartate receptors (NMDARs) regulate DA burst firing and phasic DA release. Here, we tested the role of VTA nAChRs, muscarinic AChRs (mAChRs), and NMDARs in the conditioned reinforcement elicited by a food-associated, natural reward cue. Subjects received 10 consecutive days of Pavlovian conditioning training where lever extension served as a predictive cue for food availability. On day 11, rats received bilateral VTA infusion of saline, AP-5 (0.1 or 1μg), mecamylamine (MEC: 3 or 30μg) or scopolamine (SCOP: 3 or 66.7μg) immediately prior to the conditioned reinforcement test. During the test, nosepoking into the active (conditioned reinforced, CR) noseport produced a lever cue while nosepoking on the inactive (non-conditioned reinforced, NCR) noseport had no consequence. AP-5 robustly attenuated conditioned reinforcement and blocked discrimination between CR and NCR noseports at the 1-μg dose. MEC infusion decreased responding for both CR and NCR while 66.7-μg SCOP disrupted the subject's ability to discriminate between CR and NCR. Together, our data suggest that VTA NMDARs and mAChRs, but not nAChRs, play a role in the ability of natural reward-associated cues to act as conditioned reinforcers.

Keywords: NMDA receptor; conditioned reinforcement; incentive salience; muscarinic receptor; nicotinic receptor; ventral tegmental area.

Fig. 1

VTA cannula infusion sites, based on histological verification of cannula tip placements (adapted from (Paxinos and Watson, 2007). Black circles indicate correctly placed VTA cannula tips. Grey circles indicate misplaced cannula from rats that were excluded from analysis. B. Photomicrograph depicting localized spread of Chicago blue dye within the VTA. Abbreviations: VTA-ventral tegmental area; ml-medial lemniscus; SNR-substantia nigra reticulata.

Fig. 2

Conditioned reinforcement after training in non-cannulated animals receiving a cue predictive of food (paired group; n = 16) versus animals receiving a cue that is not explicitly paired with food (unpaired group; n = 7). **** p < 0.001; Bonferonni post-hoc test; CR = conditioned reinforced response. NCR = non-conditioned reinforced response.

Fig. 3

VTA infusion of AP-5 attenuates, but does not block, conditioned reinforcement. Comparison of nosepoke responses in subjects that received SAL (n = 19) or AP-5 (0.1 µg (n=12) or 1.0 µg (n=8)) VTA microinfusion immediately prior to the conditioned reinforcement test. The saline group is the same for Figs. 3, 4 and 5. Data shown as mean ±SEM. ** p < 0.01; Bonferonni post-hoc test; SAL = saline.

Fig. 4

VTA infusion of MEC does not alter conditioned reinforcement. Comparison of conditioned reinforcement immediately after SAL (n = 19) or MEC (3.0 µg (n = 11); or 30 µg (n = 13)) infusion into the VTA. The saline group is the same for Figs. 3, 4 and 5. Data shown as mean ±SEM. ** for p < 0.01; Bonferonni post-hoc test. MEC = mecamylamine.

Fig. 5

VTA infusion of high dose SCOP elevates NCR responding. Comparison of conditioned reinforcement in subjects receiving SAL (n = 19) or SCOP (3.0 µg (n = 11) or 66.7 µg (n = 12)) infusion into the VTA immediately prior to test. The saline group is the same for Figs. 3, 4 and 5. Data shown as mean ±SEM. ** for p < 0.01; Bonferonni post-hoc test. SCOP = scopolamine.

Fig. 6

Lever presses (A) and head entries (B) during the cue presentation, and head entries during the inter-trial interval (ITI; C) for sign trackers, goal trackers, and the intermediate group. Data shown as mean with standard error from the mean (SEM.). (D) Correlation between PCA score and CR nosepokes minus NCR nosepokes. Tukey’s post-hoc test comparing main effects of group (for A–C). ** p < 0.01; *** p < 0.001; **** p < 0.00001.