Repeated apomorphine administration alters dopamine D1 and D2 receptor densities in pigeon basal telencephalon

When pigeons are repeatedly administered a dose of apomorphine they show an increasing behavioral response, much as rodents do. In birds this expresses itself in an augmented pecking response. This sensitization is assumed to be largely due to a conditioning process. Here we present evidence that sensitization is accompanied by an alteration of the D1 to D2 dopamine receptor densities. An experimental group of pigeons was repeatedly injected with apomorphine, and a control group with saline. The basal forebrain tissue, known to be rich in dopamine receptors, was subjected to binding assays using tritiated specific D1 and D2 dopamine receptor antagonists. There was a trend towards an increase in D1 and a significant decrease in D2 receptor densities in apomorphine-treated birds compared to the saline-treated controls. We conclude that extended apomorphine treatment modifies the D1 dopamine receptor density in the opposite manner to the D2 dopamine receptor density.