Investigating a novel dopaminergic projection from Ventral Tegmental Area to Ventral Hippocampus in associative learning (21992)
Dopamine neurons in the ventral tegmental area (VTADA) have been traditionally viewed as assigning value to cues and context paired with rewards (e.g., food). However, recent work has revealed that VTADA neurons contribute in more complex ways to learning and behaviour than previously thought. This has catalysed efforts to understand how distinct dopamine circuits might contribute to different types of learning. Here, we investigated how an understudied dopamine circuit comprising the ventral hippocampus (vHP) contributes to associative learning. First, we established a novel projection from VTADA neurons to vHP by performing retrograde tracing. We then took advantage of fibre photometry of a genetically encoded dopamine sensor, GRABDA, to precisely record dopamine released in vHP as rats experienced a neutral cue (e.g., tone) paired with a reward. We compared dopamine signals across this reward learning task with that of fear conditioning, where rats experienced a new neutral cue (e.g., light) with mild footshock. In both cases, a dopamine signal developed to both the reward- and shock-paired cues across learning sessions. This signal showed a unique and interesting temporal dynamic, which does not reflect a quintessential dopamine prediction error (i.e., phasic increase of dopamine at cue onset). Specifically, we saw a delayed and slow increase in the dopamine response at cue onset, which reduced below baseline before cue offset. This mirrored the dopamine response during reward and shock receipt, which was reminiscent of a ‘stimulus-stimulus’ association. This reveals a unique role for dopamine in vHP in associative learning.