Figure 5. LH GABAergic neurons oppose learning of cue-cue associations after reward learning.
Responding is represented as the time spent in the food port (%; ±SEM) during stimulus presentation when rats were learning the second-order contingencies (orange box; left panels early trials, right panels late trials), and during the probe test (gray box; left panels index group means, right panels indicate individual rat responses). Scatterplots represent rats’ individual responses, where equivalent rates of responding to the cues mean responses should congregate on the diagonal. Rats first learnt to associate A2 with food, and to differentiate that from B2, which did not predict food. Then, during second-order learning, rats were presented with A1→A2 and B1→B2 pairs. During this time, light (532nm, 16mW, green rectangle) was delivered into the brain during A1, resulting in inhibition of LH GABA neurons in our NpHR group (n=8 rats) but not our eYFP group (n=12 rats). Finally, we conducted a probe test, where we presented A1 and B1 alone and without reward. Across both learning and the probe test, rats in our NpHR group showed an enhancement of responding to A1 relative to B1 (stimulus: F1,18=10.576, p=0.004; stimulus x group: F1,18=4.657, p=0.045), which was maintained in the probe test without LH GABA neuronal inhibition (stimulus x session: F2,36=0.368, p=0.695). Follow up analyses showed a significant difference in responding to A1 and B1 in our NpHR group (F1,18=12.195, p=0.003) that was not present in the eYFP group (F1,18=0.748, p=0.05). This demonstrated that inhibition of LH GABA neuronal activity enhanced learning about cue-cue relationships after experience with reward learning. Data were analyzed with a repeated-measures ANOVA, with simple-main effects following a significant interaction, not necessitating control for multiple comparisons.