Figure 9.

Schematic diagram of dendritic distribution of GluR1 immunogold particles in dopaminergic neurons of the PB and PN VTA after acute or chronic morphine administration. Each dendritic structure is divided by size (small, medium, and large; left to right, respectively) and dopaminergic dendrites of the PB VTA are represented on the left with dopaminergic dendrites of the PN VTA on the right side of the schematic. A, In saline controls, both the PB and PN VTA have similar, predominantly cytoplasmic (yellow circles) GluR1 gold particle distributions with sparse plasmalemmal (blue circles) GluR1 labeling. MVBs (green circles) are present in dendrites of both VTA regions. B, In the PB VTA (left), acute morphine administration results in a decrease in cytoplasmic GluR1 gold labeling in small and medium-sized dendrites with an increase in plasmalemmal (in small dendrites only) and synaptic (red) GluR1 labeling. There is also a decrease in the number of MVBs seen in these dendrites. In the PN VTA, there is a decrease in cytoplasmic GluR1 labeling in small and medium-sized dendrites with a slight increase in plasmalemmal GluR1 labeling in small dendrites; however, there is no change in synaptic GluR1 labeling or prevalence of MVBs. C, After chronic morphine administration, increased synaptic GluR1 labeling ensues in the PB VTA (left) with an increased amount of GluR1 at each synapse (red) and a concurrent decrease in the number of MVBs present within these dendrites. Mesolimbic dopaminergic dendrites of the PN VTA (right) demonstrate an increase in the number of synapses labeled with GluR1 (red) and a decrease in the number of MVBs after chronic morphine administration, which did not occur with acute morphine administration. Together, these findings suggest that early activation of the mesocortical dopaminergic pathway of the PB VTA may result in a decrease in mPFC glutamate release, leading to subsequent changes in GluR1 localization in the mesolimbic dopaminergic pathway of the PN VTA.