Dopaminergic neurons innervate multiple brain regions, including the mushroom body (MB), where they modulate learning. During olfactory classical conditioning, large subsets of dopaminergic neurons are activated, releasing dopamine across broad sets of postsynaptic neurons. These dopaminergic neurons respond strongly to the US and their activity can be sufficient as reinforcement when paired with odor delivery. However, the specific way in which dopamine alters the physiology of the MB neurons is not well understood.
Using in vivo calcium imaging, we studied the effect of manipulation of different subsets of dopaminergic neurons in the odor response of several mushroom body regions. Odor presentation was paired with activation of different populations of neurons with the application of an odor and odor-evoked calcium responses were compared before and after the treatment.
We previously reported that pairing the activation of dopaminergic neurons labeled by TH-Gal4 with odor exposure leads to calcium plasticity specifically in the gamma lobes of the MB (Boto et al., 2014), whereas activation of the neurons of the PAM cluster leads to a broader pattern of facilitation. In order to understand how dopamine affects the MB responses during differential conditioning, we paired an odor (CS+) with dopaminergic neuron activation, followed by a second odor (CS-) not associated with dopamine released. Our results show that pairing odor with dopaminergic stimulation generates positive change in the ratio of CS+/CS- responses: either an increase of the response to the CS+ and no change/decrease on the CS- response, or no change in the CS+ response, but a decrease in the CS- response. The specifics of this plasticity depend on the odors used for the CS+/CS-.
Using more specific drivers to study different dopaminergic cluster subdivisions, we saw that subdivisions of the TH-GAL4 driver produce distinct spatial effects on neuronal plasticity.