Publication Date

April 2016

Advisor(s)

John Kirn

Major

Neuroscience & Behavior

Language

English (United States)

Abstract

The cortical pre-motor nucleus HVC plays a key role in regulating song learning, production, and maintenance in oscine birds. The HVC is located at the center of both major pathways involved in song learning and is believed to work as the implicit timekeeper of the song system. During the critical period, a time early in development during which a bird must be exposed to its species-specific song if it is to learn to sing, the HVC experiences high rates of neurogenesis that decrease considerably once the critical period has ended around 60 to 90 days post-hatch, although the nucleus continues to recruit new neurons throughout life. If a bird is isolated during the critical period so that it has no exposure to a male tutor, high rates of neuron addition to the HVC are prolonged and the bird can learn to copy another male’s song later into adulthood, although isolates generally produce a more variable song regardless of eventual tutor exposure. To date, investigations into the nature of neurogenesis to the HVC have focused on the addition of excitatory projection neurons that connect to RA, another nucleus involved in vocal production. However, it has been shown that inhibitory interneurons are also added to the HVC, at least during the critical period. Since parvalbumin-expressing interneurons are associated with plasticity in other sensory systems and have even been shown to induce ocular dominance plasticity in mice after the critical period has ended, we wondered what role parvalbumin-positive interneuron addition may play in regulating plasticity of the song system. To investigate this question, we quantified the rates of parvalbumin-expressing neuron addition to the zebra finch HVC during and directly after the critical period and analyzed these trends in relation to vocal learning. Neurons were birth-dated using thymidine analog injections at 20, 40, 60, or 120 days post-hatch in male zebra finches raised with male tutors. Birds were perfused at least 60 days after injection, and immunohistochemical techniques were used to quantify new parvalbumin-positive neurons added at these different time points. Before perfusion, birdsong was recorded, and computer-driven similarity scores between a bird’s song and its tutor’s were used to analyze the degree to which a bird had effectively copied its tutor. In addition, rates of parvalbumin-positive neurogenesis were investigated in a population of birds isolated from male tutors. In normally raised birds, rates of parvalbumin-positive neuron addition reflected what is known about neurogenesis to the HVC during the critical period, with a peak at 40 days post-hatch and a gradual decline thereafter. Rates of parvalbumin-positive neuron addition to the HVC at 60 days post-hatch positively correlated with tutor song copying. The addition of neurons expressing parvalbumin occurred at a higher rate in isolates than in birds reared with a tutor, and by 180 days post-hatch, isolated birds displayed significantly more parvalbumin-positive neurons throughout the HVC than birds reared with a tutor. These data suggest that parvalbumin-expressing neurons are at least one factor mediating plasticity in the zebra finch vocal learning system.

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