New research by neuroscientists from the University of Massachusetts Amherst has shown that dopamine is a crucial component of songbirds' ability to learn complex sounds.The Journal of Neuroscience published the discovery that dopamine drives plasticity within the auditory pallium of Zebra Finches. This new research will help to advance our understanding of how this neurotransmitter functions in an area of brain that encodes complex stimuli.Matheus Macedo–Lima, the lead author, says that people associate dopamine as reward and pleasure. He was a Ph.D student at UMass Amherst’s Neuroscience and Behavior graduate program. It is a well-known fact that dopamine plays a role in learning. However, there is very little information about dopamine's role in sensory processing. We wanted to find out if dopamine played a role in the brain's ability to learn new sounds and change with sound.Research on vocal learning in songbirds can provide insight into the process of speaking, says Remage-Healey (a behavioral neuroscientist and professor of psychological, brain, and brain sciences). It's not only the songbird who comes up with this strategy to use dopamine to create meaning and binding sounds. We humans are also interested in this parallel.Researchers conducted experiments in vitro as well as in vivo. They poked neurons under microscopes and in brains of birds that were listening to sounds and watching videos. The scientists were able to gather physiological, behavioral, and anatomical evidence that supports their hypothesis about dopamine's role.The researchers used antibodies to show that dopamine receptors can be found in many types neurons in the songbird's auditory brain. They can be either inhibitory or excitatory, and could also contain an enzyme that makes estrogens. Dr. Remage-Healey's work has shown that neurons in the auditory brains of songbirds produce estrogen when they are in social situations like listening to birdsong and seeing another bird. Dopamine and estrogens may be interconnected in sound learning, but we focused our research on dopamine as there was so much to learn about dopamine's effects on the songbird brain." Macedo-Lima is now a postdoctoral fellow at the University of Maryland.AdvertisementMacedo-Lima created a test similar to Pavlov's dog experiment. The birds were placed in a chamber, and presented with random sounds followed by silent videos of other birds. He says, "We wanted to concentrate on the association between the meaningless sound -- a tone - and the behaviorally relevant thing which is another bird in video."Researchers examined the bird's auditory brain regions following sound-video pairing. They used a gene marker that is known to be activated when a neuron undergoes change or plasticity. Macedo-Lima said, "We found this very intriguing increase in this gene expression at the left hemisphere in the ventral portion of the auditory area in dopamine receptor-expressing neuron, reflecting the learning process and paralleling human brainlateralization for speech learning."Researchers used whole-cell patch clamp techniques to measure and control the currents received by neurons in order to show dopamine's effect on basic signaling. In a dish, they found that dopamine activation decreased inhibition and increased excitation.Remage-Healey says that one modulator tunes the system so it ramps up the go and stops signals. It's a powerful yet simple control mechanism that can change how animals encode sound. It is a neurochemical lever which can alter how stimuli are registered in the brain and transmitted to this area.The team painlessly examined the brain cells of living birds. Macedo - Lima says that the dopamine delivered to live birds was exactly what we predicted from whole cell data. "We observed that the dopamine antagonist caused inhibition to decrease, and the dopamine agonist to increase firing of excitatory neurons."When the birds were given birdsong by other songbirds, the same effect was observed. The excitatory neurons responded better to dopamine activation than the inhibitory neurons. Macedo - Lima said, "We were happy that we replicated what we saw in the dish in a living animal listening to relevant sounds."These neurons were also unable to adapt for new songs that were presented to them by dopamine activation, strongly supporting the hypothesis of dopamine's involvement in sensory learning. "We don't currently know how dopamine affects sensorimotor learning in most animals," MacedoLima said. "But this research provides many clues about the mechanism that could work across vertebrates who need to learn complex sounds like humans," she added.
