Dopamine is a brain signalling chemical that is vital for our control of movement. It is depleted in people with Parkinson's disease, leading to mobility problems, tremors and speech impairments. But it also plays a pivotal role in many cognitive abilities at which humans excel, including learning, concentrating, pleasure-seeking and planning ahead.
Nenad Sestan and André Sousa of the Yale School of Medicine in New Haven, Connecticut and their colleagues measured the activity of individual genes in tissue samples from 16 brain regions, taken from six humans, five chimpanzees and five macaque monkeys.
They found elevated activities of two enzymes that make dopamine - tyrosine hydroxylase and DOPA decarboxylase - in two parts of the human brain, both vital for higher-level thought.
Dopamine dose
One was the striatum, which is involved in planning ahead, making decisions, perceiving rewards and feeling motivation. The other was the outer layer of our brain, the neocortex. This region is involved in storing and processing memories, experiencing conscious thought and processing language.
The researchers found that 1.5 per cent of the neurons in the human striatum were making dopamine, three times more than in the ape striatum. Likewise, they accounted for 0.2 per cent neurons in the neocortex, versus none at all in apes.
What's more, the extra dopamine in these regions was made almost exclusively by brain cells called interneurons. These form local connections, rather than linking distant parts of the brain. "It's possible these cells add more dopamine locally, to fine-tune local circuitry," says Sestan.
They confirmed this result in other primate species. "In the striatum, we see more of these cells than in any of the other species, and in the neocortex the difference was even more striking," says Sousa. "We couldn't find a single interneuron making dopamine in the corresponding brain areas of chimpanzees, bonobos or gorillas."
Brain evolution
"We're not yet sure of the extent to which our observations explain differences between the human, chimpanzee and other primate brains," says Sestan. "But we hypothesise that these cells could contribute to human-specific aspects of cognition or behaviour, and could be involved in the causes of Parkinson's disease or other disorders."
"Although we do not yet know how these neurons affect uniquely human behaviors, future studies can now focus on this specialised population of neurons as we seek to understand how we became human", says Alex Pollen at the University of California, San Francisco.
The findings also challenge two simplistic ideas about the evolution of our brains. One is that the crucial change was size - the human brain being treble the size of ape brains - and the other being that the only part of the brain important for intelligence is the neocortex.
"Most research is unjustifiably 'cortico-centric', and people routinely trot out that the neocortex is the interesting bit where change occurs, whereas this study shows it's a much more complex story," says Robert Barton of Durham University, UK. "And while brain size is clearly a highly relevant correlate, there's clearly a lot more going on 'under the bonnet'."
Journal reference: Science, DOI: 10.1126/science.aan3456
Jack Kruse...
"Managing dopamine is a huge issue with the latest generation, they have grown up with blue light and nnEMF everywhere. It’s in their schools, its on their phones where they get their porn, video games and the internet. It follows them home on their laptops, LED TV’s, and light’s illuminating their homes. Both destroy dopamine levels in the retina and brain. The collateral damage of a low dopamine level is a low ocular melatonin level in the eye. This generalizes to the frontal lobes as mentioned in Ubiquitination 24. This low level in the frontal lobes is capable of spreading like a prion disease to deeper levels in the brain. It spreads like a wild fire into the ventral segmental areas of the brain to cause many diseases. Dopamine is the neurohormone made from UV/IR light exposure that supports the development of the DC electric current in the retina. It is a neurohormone that is released by the hypothalamus under the direction of light exposure. Its action is as a hormone that is an inhibitor of prolactin release from the anterior lobe of the pituitary. Its level can dramatically affect enzymes at deeper levels in the brain to change function just from a spectral change of light at the retina. It does this by effecting several physical abilities in the local environment of cells and EZ water using “proton tunneling” to control our enzyme systems which modulate complex feedback controls on our hormone levels."