Time the Avenger: Unpredictable environmental disruptions induce changes in neural expression of glucocorticoid receptors in the vocal learning circuit of the male budgerigar.

Chronic stress can impair behaviors critical for survival, including cognitive traits such as vocal learning. Budgerigars (Melopsittacus undulatus), a parrot species capable of lifelong vocal plasticity, offer a valuable model for studying how stress affects vocal learning circuitry. Previous research has shown that chronic stress induced by unpredictable environmental disturbances reduces vocal plasticity in captive budgerigars, but the molecular mechanisms underlying this change remain unclear. To investigate physiological and molecular responses to these disturbances, we collected data across weekly timepoints in budgerigars subjected to a protocol of mild, unpredictable disturbances. We measured both baseline and stress-induced circulating corticosterone levels and used qPCR to assess expression of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) mRNA in three brain regions: the magnocellular nucleus of the medial striatum (MMSt), a vocal learning nucleus; the ventral striatum pallidum (VSP), a neighboring striatal region; and the hippocampus, which is involved in stress regulation. Contrary to expectations, there was no statistically significant difference in circulating corticosterone between control and experimental treatments nor among weeks. However, the presence of unpredictable disturbances was associated with significantly reduced GR expression in the MMSt, a region critical for maintaining the stereotypy of learned vocalizations. MR expression in all regions did not differ with treatment or week of sampling. These findings suggest that local downregulation of GR in vocal learning circuitry, rather than systemic hormonal changes, may mediate stress-induced alterations in vocal behavior. This study provides new insight into the neuroendocrine mechanisms by which exposure to unpredictable disturbances affects cognitive function in vocal learners.