Habit Formation after Random Interval Training Is Associated with Increased Adenosine A(2A) Receptor and Dopamine D(2) Receptor Heterodimers in the Striatum

Striatal adenosine A(2A) receptors (A(2A)Rs) modulate striatal synaptic plasticity and instrumental learning, possibly by functional interaction with the dopamine D(2) receptors (D(2)Rs) and metabotropic glutamate receptors 5 (mGluR5) through receptor-receptor heterodimers, but in vivo evidence for these interactions is lacking. Using in situ proximity ligation assay (PLA), we studied the subregional distribution of the A(2A)R-D(2)R and A(2A)R-mGluR5 heterodimer complexes in the striatum and their adaptive changes over the random interval and random ratio training of instrumental learning. After confirming the specificity of the PLA detection of the A(2A)R-D(2)R heterodimers with the A(2A)R knockout and D(2)R knockout mice, we detected a heterogeneous distribution of the A(2A)R-D(2)R heterodimer complexes in the striatum, being more abundant in the dorsolateral than the dorsomedial striatum. Importantly, habit formation after the random interval training was associated with the increased formation of the A(2A)R-D(2)R heterodimer complexes, with prominant increase in the dorsomedial striatum. Conversely, goal-directed behavior after the random ratio schedule was not associated with the adaptive change in the A(2A)R-D(2)R heterodimer complexes. In contrast to the A(2A)R-D(2)R heterodimers, the A(2A)R-mGluR5 heterodimers showed neither subregional variation in the striatum nor adaptive changes over either the random ratio (RR) or random interval (RI) training of instrumental learning. These findings suggest that development of habit formation is associated with increased formation of the A(2A)R-D(2)R heterodimer protein complexes which may lead to reduced dependence on D(2)R signaling in the striatum.