Expression of μ- and δ-opioid receptors in song control regions of adult male zebra finches (Taenopygia guttata)

The opioid system is known to play a role in various aspects of learning and memory in diverse species of mammals and birds. Earlier studies have localized the endogenous opioids, met- and leu-enkephalin, in the song control regions of male zebra finches (Taenopygia guttata), a sexually dimorphic species of songbirds wherein only males sing. Recent research has shown that levels of enkephalin increase in some of the song control regions during singing and that blocking opioid receptors with the antagonist naloxone decreases the frequency of singing in songbirds. However, the distribution of receptors specific to the opioid system has not been studied in zebra finches. In the present study, we used reverse transcriptase PCR (RT-PCR) and quantitative real-time PCR (qRT-PCR) to demonstrate that the song control regions lateral magnocellular nucleus of the anterior nidopallium (LMAN), Area X, MSt (medial striatum), HVC and RA (robust nucleus of the arcopallium) expressed higher levels of μ-OR mRNA compared to δ-OR mRNA. In situ hybridization was used to demonstrate that neither LMAN nor Area X could be delineated from the surrounding brain regions [anterior nidopallium (ANP) and MSt, respectively], based on OR mRNA expression. However, HVC and RA neurons expressed marginally higher levels of μ-OR mRNA compared to the posterior nidopallium, which was confirmed by immunohistochemical localization. We also found that the dorsolateral subdivision of DLM (dorsolateral nucleus of the medial thalamus) demonstrated high levels of μ-OR immunoreactivity. Our results suggest that the ORs may be involved in modulating different aspects of vocalization and/or gating auditory input, as well as motor control.