Localization of androgen receptor mRNA-containing cells in avian respiratory-vocal nuclei: an in situ hybridization study.

Song development and song pattern formation in oscine songbirds are influenced by steroid hormones such as estrogens and androgens, and the control of vocal pattern generation is mediated via a network of interconnected vocal and respiratory nuclei. The main components of the respiratory part of the network are the expiratory and inspiratory premotor nuclei, known as retroambigualis (RAm) and the rostral ventral respiratory group (rVRG), respectively. These respiratory components play an integral role in song production either by providing the expiratory pulses of air required for each and every song syllable, or by controlling inspiration between syllables in the form of minibreaths, and between phrases for major replenishments of air. Here we analyze the distribution of androgen receptors (AR) and estrogen receptors (ER) in the midbrain and hindbrain of male and female zebra finches, and male canaries and green finches, using in situ hybridization with cRNA probes of the zebra finch AR and ER. ERmRNA was not expressed in any of the respiratory-vocal nuclei of the midbrain or hindbrain, but ARmRNA was expressed in the tracheosyringeal motor nucleus (nXIIts) and in RAm and rVRG. The size of the ARmRNA defined RAm and rVRG was similar in male and female zebra finches, but the size of ARmRNA defined nXIIts was slightly sexual dimorphic. Previously undescribed areas of ARmRNA expression outside the respiratory-vocal network in the brain stem were the nucleus semilunaris and layers 10-12 of the optic tectum. AR-mRNA expression in the respiratory-vocal nuclei of adult male songbirds, adult female zebra finches, and juvenile zebra finches suggests that the temporal pattern of learned and unlearned vocalizations is sensitive to androgen-dependent mechanisms mediated by RAm and rVRG.