Neuroscientists at the University of Massachusetts Amherst have shown in new research that dopamine plays a key role in how songbirds learn complex new sounds.
Published in Journal of Neuroscience , the discovery that dopamine promotes plasticity in the auditory pallium of zebra finches provides a new basis for a better understanding of the functions of this neurotransmitter in an area of the brain that encodes complex stimuli.
“People associate dopamine with reward and joy,” says lead author Matheus Macedo-Lima, who does research in lead author Luke Remage-Healey’s lab as a Ph.D. Student in the graduate program Neuroscience and Behavior at UMass Amherst. “It is a very well-known concept that dopamine is involved in learning. But knowledge of dopamine in areas related to sensory processing in the brain is limited. We wanted to understand whether dopamine plays a role in how this brain region learns new sounds or changes with sounds. ”
Investigating vocal learning in songbirds provides insights into spoken language learning, added behavioral neuroscientist Remage-Healey, professor of psychology and brain sciences. “Not only the songbird comes up with this strategy of using dopamine to bind sounds and meanings. There is a parallel here that interests us – as humans. ”
The research team conducted a series of in vitro and in vivo experiments that examined neurons under the microscope and in the brains of live birds that watched videos and heard sounds. Ultimately, the scientists obtained anatomical, behavioral, and physiological evidence to support their hypothesis about the role of dopamine.
Using antibodies, the researchers showed that dopamine receptors are present in many types of neurons in songbirds’ hearing brain – they can be inhibitory or excitatory, and they can also contain an enzyme that produces estrogens. “DR. Remage-Healey research has shown that in the auditory brains of songbirds of both sexes, neurons produce estrogen in social situations, such as hearing a bird chirp or seeing another bird. We believe that dopamine and estrogens are involved in healthy learning work together, but that work focused on dopamine because we didn’t know so much about how dopamine affected songbirds’ brains, ”explains Macedo-Lima, now a Postdoc Associate at the University of Maryland.
Macedo-Lima developed a test similar to Pavlov’s well-known dog experiment, in which the birds sat alone in a chamber and a random sound was presented followed by a silent video from other birds. “We wanted to focus on the connection between a meaningless sound – a tone – and the behavioral thing that is another bird on video,” he says.
The researchers examined the birds’ auditory brain regions after this sound-video pairing, using a gene marker known to be expressed when a neuron undergoes a change or plasticity. “We found this very interesting increase in this gene expression in the left hemisphere, the ventral part of the auditory region, in dopamine receptor-expressing neurons, which reflects the learning process and is parallel to the lateralization of the human brain for language learning,” says Macedo-Limama.
To show the effect of dopamine on the basic signaling of neurons, the researchers used a whole-cell patch clamp technique that controlled and measured the currents received by the neurons. They found in a dish that dopamine activation decreased inhibition and increased arousal.
“This one modulator tunes the system so that the stop signals are shut down and the start signals are ramped up,” explains Remage-Healey. “It’s a simple but powerful control mechanism for how animals potentially code sounds. It is a neurochemical lever that can change how stimuli are registered and passed on in this part of the brain. ”
The team then painlessly examined the brain cells of live birds. “What happened when we administered dopamine was exactly as we had predicted from the whole cell data,” says Macedo-Lima. “We saw that inhibitory neurons fired less when we administered the dopamine agonists, while excitatory neurons fired more.”
The same effect occurred when the birds were played birdsong by other songbirds – the excitatory neurons reacted more and the inhibitory neurons less when dopamine activation occurred. “We were delighted to recreate what we saw in a bowl in a living animal that listens to the actual relevant noises,” says Macedo-Lima.
Dopamine activation also made these neurons unable to adapt to new songs presented to the animal, strongly confirming the hypothesis of dopamine’s role in sensory learning. “We don’t currently know how dopamine affects sensory learning in most animals,” says Macedo-Lima, “but this research gives a lot of clues as to how this mechanism might work in vertebrates that have to learn complex sounds, such as the one People.”