Rapid Effects of Hearing Song on Catecholaminergic Activity in the Songbird Auditory Pathway

Catecholaminergic (CA) neurons innervate sensory areas and affect the processing of sensory signals. For example, in birds, CA fibers innervate the auditory pathway at each level, including the midbrain, thalamus, and forebrain. We have shown previously that in female European starlings, CA activity in the auditory forebrain can be enhanced by exposure to attractive male song for one week. It is not known, however, whether hearing song can initiate that activity more rapidly. Here, we exposed estrogen-primed, female white-throated sparrows to conspecific male song and looked for evidence of rapid synthesis of catecholamines in auditory areas. In one hemisphere of the brain, we used immunohistochemistry to detect the phosphorylation of tyrosine hydroxylase (TH), a rate-limiting enzyme in the CA synthetic pathway. We found that immunoreactivity for TH phosphorylated at serine 40 increased dramatically in the auditory forebrain, but not the auditory thalamus and midbrain, after 15 min of song exposure. In the other hemisphere, we used high pressure liquid chromatography to measure catecholamines and their metabolites. We found that two dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, increased in the auditory forebrain but not the auditory midbrain after 30 min of exposure to conspecific song. Our results are consistent with the hypothesis that exposure to a behaviorally relevant auditory stimulus rapidly induces CA activity, which may play a role in auditory responses

Time course of the experiment.

<p>Female white-throated sparrows were collected from a free-living population and housed on short photoperiod for approximately 4 months. Each then received a subcutaneous silastic implant filled with estradiol (E2) to mimic breeding levels. Seven days later, each was isolated in a sound-attenuating chamber and exposed to either silence, 15 min song, or 30 min song. Brains were harvested immediately after the observation period and bisected into hemispheres for analysis by immunohistochemistry (IHC) or high pressure liquid chromatography (HPLC). The hemisphere used for each procedure (left or right) was balanced across playback conditions.</p

The effects of song exposure on phosphorylated tyrosine hydroxylase immunoreactivity (pTH-IR).

<p>A–F. Immunoreactive fibers in the auditory forebrain in a typical bird (median corrected gray value) from each group. G. pTH-IR increased in CMM and NCM after 15 min of song, and in NCM remained elevated above baseline after 30 min. H, I. pTH-IR did not increase in Ov or MLd. NCM, caudomedial nidopallium. CMM, caudomedial mesopallium. HA, apical hyperpallium. Ov, n. Ovoidalis (auditory thalamus). MLd, dorsal portion of the lateral mesencephalic nucleus (auditory midbrain). *significantly different from the silence (0 min song) condition, see text for p values.</p

Hearing song alters catecholamine content in auditory areas.

<p>DA, dopamine. DOPAC, dihydroxyphenylacetic acid. HVA, homovanillic acid. A. CMM, caudomedial mesopallium. B. NCM, caudomedial nidopallium. C. MLd, dorsal portion of the lateral mesencephalic nucleus (auditory midbrain). *significantly different from silence (0 min song), see text for p values. n = 4 for each time point. Total protein content was available for all samples only for NCM (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0039388#s4" target="_blank">Methods</a>). In order to plot the values for CMM and MLd, we normalized them using the average protein values for those regions. Note the different scales on the Y axes in each graph.</p

Parasagittal view of the auditory pathway in songbirds.

<p>Rostral is to the left. The auditory nerve enters the brainstem and projects to the cochlear nucleus (CN). From CN, projections extend to the auditory midbrain (MLd, the dorsal lateral mesencephalic nucleus). MLd projects to the auditory thalamus (Ov, nucleus ovoidalis), which projects to the thalamorecipient region of the auditory forebrain, Field L. From Field L, projections extend to the caudomedial nidopallium (NCM). NCM is reciprocally connected to the caudomedial mesopallium (CMM).</p