Neuroestrogen synthesis modifies neural representations of learned song without altering vocal imitation in developing songbirds

Abstract

Birdsong learning, like human speech, depends on the early memorization of auditory models, yet how initial auditory experiences are formed and consolidated is unclear. In songbirds, a putative cortical locus is the caudomedial nidopallium (NCM), and one mechanism to facilitate auditory consolidation is 17β-estradiol (E2), which is associated with human speech-language development, and is abundant in both NCM and human temporal cortex. Circulating and NCM E2 levels are dynamic during learning, suggesting E2's involvement in encoding recent auditory experiences. Therefore, we tested this hypothesis in juvenile male songbirds using a comprehensive assessment of neuroanatomy, behavior, and neurophysiology. First, we found that brain aromatase expression, and thus the capacity to synthesize neuroestrogens, remains high in the auditory cortex throughout development. Further, while systemic estrogen synthesis blockade suppressed juvenile song production, neither systemic nor unilateral E2 synthesis inhibition in NCM disrupted eventual song imitation. Surprisingly, early life neuroestrogen synthesis blockade in NCM enhanced the neural representations of both the birds' own song and the tutor song in NCM and a downstream sensorimotor region, HVC, respectively. Taken together, these findings indicate that E2 plays a multifaceted role during development, and that, contrary to prediction, tutor song memorization is unimpaired by unilateral estrogen synthesis blockade in the auditory cortex. While many animals use sounds to communicate with one another (vocal communication), the ability to learn to vocally communicate is relatively rare 1. In vocal learning animals, such as humans and songbirds, vocal learning occurs across two main phases: an auditory memorization ('sensory') phase, followed by a sensorimotor phase ('babbling' , error correction/feedback) 2,3. While much is known about sensorimotor learning, the mechanisms that guide the formation and consolidation of auditory memories early in life are less clear. One brain region likely involved in storing auditory memories in songbirds is the caudomedial nidopallium (NCM) 4. NCM, comparable to mammalian secondary auditory cortex, is required for accurate song learning. Blocking ERK-signaling or mTOR bilaterally in NCM during tutoring leads to poor song imitation 5,6. Tutoring naïve juvenile songbirds leads to an increased proportion of tutor-song-selective neurons in NCM 7. Further, bilateral NCM lesions abolish innate preference for tutor song in adults 8 , but see 9,10. Thus, NCM contains a putative tutor 'engram'; however, the neuromodulatory mechanisms that shape this auditory memory formation and consolidation remain unknown. Consolidating recent experience in other contexts and systems require presynaptic signaling molecules ('neuromodulators'), such as brain-derived estrogens 11. The predominant estrogen 17β-estradiol (E2) is a candidate neuromodulator required for auditory memory consolidation, as is evident in a similar role in adult hippocampal-dependent cognit...

Aromatase and nonaromatase neurons in the zebra finch secondary auditory forebrain are indistinct in their song‐driven gene induction and intrinsic electrophysiological properties

“…Interestingly, the percentage of aromatase neurons detected by both the rabbit (Ikeda et al, 2017) and the sheep antibodies (present study) was slightly higher in the ventral NCM compared to the dorsal NCM (90.5% in vNCM, 86.5% in dNCM) which could partially account for this region-dependence. The dorsal NCM is also known to contain more cells as shown by DAPI staining (Vahaba et al, 2020) and confirmed in the present study that could explain the lower proportion of aromatase positive cells while the raw number between dNCM and vNCM might be similar.…”

Norepinephrine in the avian auditory cortex enhances developmental song learning

“…In this respect, it is possible that NE infusions into NCM allow for greater tutoring-dependent plasticity within sensorimotor circuitry. Supporting this notion is the finding that neuromodulation within NCM can affect sensory responses within sensorimotor circuitry (86,87). Revealing how NE manipulations in the NCM of juvenile birds affect auditory responses in NCM and HVC or how song learning is affected by NE infusions in NCM coupled with perturbations of HVC activity will be important to differentiate these possibilities.…”

Adult‐like neural representation of species‐specific songs in the auditory forebrain of zebra finch nestlings

“…Songbirds are altricial, and developmental processes in altricial species lag behind those in precocial species (Kubke & Carr, 2000). Still, sparse data in songbirds suggest at least that the auditory forebrain is largely developed before the sensitive period—cell density in NCM of 20 and 30 dph zebra finches is similar to, or even greater than, that in adults (Stripling et al., 2001; Vahaba et al., 2020)—and is mature before the song system. In song motor nuclei, neurogenesis, neural responses, and N‐methyl‐D‐aspartate receptor (NMDAR) expression change significantly during the sensitive period into adulthood (Burek et al, 1991; Nordeen & Nordeen, 2004; Whaling et al., 1997).…”

“…Extracellular single‐unit recording was performed in vivo in awake‐restrained birds as in previous studies (Miller‐Sims & Bottjer, 2014; Schneider & Woolley, 2013; Vahaba et al., 2020). Subjects were brought into an anechoic chamber and fixed to a stereotaxic apparatus (Herb Adams Engineering) on an air table at a 45° head angle.…”