Differential relationships between D1 and D2 dopamine receptor expression in the medial preoptic nucleus and sexually-motivated song in male European starlings (Sturnus vulgaris)

- Past data show that dopamine in the medial preoptic nucleus (POM) modulates vocal production in songbirds.
- Male starlings with intermediate levels of D1 expression in POM sang more than males with low or high levels of expression.
- Singing correlated positively with POM D1 expression in males with low and intermediate levels of D1 expression.
- Singing correlated negatively with POM D2 expression in males with low and intermediate levels of D1 expression.
- An optimal level of POM D1 expression may facilitate sexually-motivated song, and D2 receptors may have inhibitory effects.

Converging data in songbirds support a central role for the medial preoptic nucleus (POM) in motivational aspects of vocal production. Recent data suggest that dopamine in the POM plays a complex modulatory role in the production of sexually-motivated song and that an optimal level of dopamine D1 receptor stimulation is required to facilitate singing behavior. To further explore this possibility, we used quantitative real-time PCR to examine relationships between mRNA expression of D1 as well as D2 receptors in the POM (and also the lateral septum and Area X) and sexually-motivated singing behavior in male European starlings. Results showed that both males with the highest and lowest D1 expression in the POM sang significantly less than males with intermediate levels of expression. Furthermore, singing behavior rose linearly in association with increasing levels of D1 expression in POM but dropped abruptly, such that individuals with D1 expression values higher than the mean sang very little. Analysis of birds with low and intermediate levels of D1 expression in POM revealed strong positive correlations between D1 expression and song but negative relationships between D2 receptor expression and song. These findings support prior work suggesting an optimal level of POM D1 receptor stimulation best facilitates sexually-motivated singing behavior. Results also suggest that D2 receptors may work in opposition to D1 receptors in POM to modify vocal production.