We're not the only animals that begin shouting when we enter a noisy environment. Bats also adjust their calls to hear faint echoes above a noisy background. Although little was known about the neural pathways that control bats' vocal versatility, Michael Smotherman and colleagues from Texas A&M University, USA, explain that the basal ganglia – a suite of forebrain nuclei – are involved in vocal adjustments in other vertebrates through the action of a neurotransmitter, dopamine. Could the basal ganglia and dopamine have a function in controlling bats' echolocation calls? Knowing that a neurotoxin, 1-methyl-4-phenylpyridine (MPTP) can affect basal ganglia dopamine levels in other animals, Smotherman and his team decided to test the effect of the compound on free-tailed bats (Tadarida brasiliensis) to find out if dopamine and the basal ganglia are involved in controlling their voices (p. 3238).
Collecting bats from a local roost, the team recorded the animals' echolocation calls in both a silent and noisy environment and found that, in the noisy environment, the bats raised their voices and lengthened each chirrup to be heard above the racket. Then they gave the bats a small dose of MPTP and, after confirming that the toxin did not affect the animals' movements, the team tested their ability to adjust their calls in a loud environment. This time, the bats were unable to adjust their voices to compensate for the acoustic noise. So, the basal ganglia are involved in controlling the bat's voices in response to interference from background sound, but how?
Wondering how MPTP had affected the bats' dopamine levels, the team measured dopamine in the animals' brains and were surprised to see that, instead of falling, the level had risen 4-fold. Then the team tested which dopamine receptor is involved in mediating vocal control. Administering low doses of agonist compounds – which also bind to dopamine receptors – the team found that quinpirole, which binds type-2 dopamine receptors, produced the same changes in the bats' vocal response as the dopamine-enhancing MPTP.
So, basal ganglia are involved in bat vocal control, allowing the echolocating animals to adjust their calls in response to noisy backgrounds via type-2 dopamine receptor pathways. The team hope to find out more about the role of dopamine and the basal ganglia in bat and mammalian vocal control.