Maturation, Behavioral Activation, and Connectivity of Adult-Born Medium Spiny Neurons in a Striatal Song Nucleus

Abstract

Neurogenesis continues in the adult songbird brain. Many telencephalic song control regions incorporate new neurons into their existing circuits in adulthood. One song nucleus that receives many new neurons is Area X. Because this striatal region is crucial for song learning and song maintenance the recruitment of new neurons into Area X could influence these processes. As an entry point into addressing this possibility, we investigated the maturation and connectivity within the song circuit and behavioral activation of newly generated Area X neurons. Using BrdU birth dating and virally mediated GFP expression we followed adult-generated neurons from their place of birth in the ventricle to their place of incorporation into Area X. We show that newborn neurons receive glutamatergic input from pallial/cortical song nuclei. Additionally, backfills revealed that the new neurons connect to pallidal-like projection neurons that innervate the thalamus. Using in situ hybridization, we found that new neurons express the mRNA for D1- and D2-type dopamine receptors. Employing DARPP-32 (dopamine and cAMP-regulated phosphoprotein of 32 kDa) and EGR-1 (early growth response protein 1) as markers for neural maturation and activation, we established that at 42 days after labeling approximately 80% of new neurons were mature medium spiny neurons (MSNs) and could be activated by singing behavior. Finally, we compared the MSN density in Area X of birds up to seven years of age and found a significant increase with age, indicating that new neurons are constantly added to the nucleus. In summary, we provide evidence that newborn MSNs in Area X constantly functionally integrate into the circuit and are thus likely to play a role in the maintenance and regulation of adult song.

Dynamic FoxP2 levels in male zebra finches are linked to morphology of adult-born Area X medium spiny neurons

“…6). Together with our previous data we conclude that reaching low FoxP2 levels is a sign that adult-born born MSN in Area X have reached maturity by 6 weeks after their generation in the VZ 34 . Since our previous work demonstrated that new MSNs participate in singing-associated neural activity in Area X 34 here we asked if this was linked to FoxP2 levels.…”

“…Together with our previous data we conclude that reaching low FoxP2 levels is a sign that adult-born born MSN in Area X have reached maturity by 6 weeks after their generation in the VZ 34 . Since our previous work demonstrated that new MSNs participate in singing-associated neural activity in Area X 34 here we asked if this was linked to FoxP2 levels. After singing FoxP2 mRNA and protein levels are lower in Area X tissue [25][26][27] than when birds were silent whereas the expression of the immediate early gene EGR-1 increases linearly the more birds sing undirected song 40,41 .…”

“…What might be the function of new MSNs in Area X and how is it affected by FoxP2 expression levels? Our previous study showed that once matured, new MSNs have similar characteristics as older, resident MSNs and are active during singing 34 . General MSNs function is feed forward inhibition within a cortico-striatal-thalamo-cortical loop during singing 59 .…”

“…FoxP2 high MSNs colocalize more frequently with a marker for new neurons than FoxP2 low MSNs 24 . Recently we showed that new MSNs mature and participate in singing activity -as measured by immediate early gene activation -within a timeframe of six weeks 34 . Whether FoxP2 influences not only the function but also the integration of new neurons into existing circuits is still an open question.…”

DARPP-32 Distinguishes a Subset of Adult-Born Neurons in Zebra Finch HVC

“…DARPP-32 is a reliable marker of dopaminoceptive neurons in both mammals (Hemmings, et al, 1987) and zebra finches Kosubeck-Langer, et al, 2017). To determine whether DARPP-32+ HVC NNs receive catecholaminergic input, we labeled tyrosine hydroxylase, the rate-limiting enzyme in the catecholamine synthetic pathway (Fernstrom & Fernstrom, 2007).…”