Bird neurons use three times less glucose than mammalian neurons

Summary: The brain neurons of birds are more energy efficient than those of mammals.

Source: Cell press

Birds have impressive cognitive abilities and demonstrate a high level of intelligence. Compared to mammals of approximately the same size, the brains of birds also contain significantly more neurons.

Now, a new study reported in Current biology September 8 helps explain how birds can afford to maintain more brain cells: their neurons get by on less fuel in the form of glucose.

“What surprised us the most was not, per se, that neurons consume less glucose – one might have expected this because of the differences in the size of their neurons,” says Kaya von Eugen of the University from the Ruhr to Bochum, Germany.

“But the magnitude of the difference is so great that the size difference cannot be the only contributing factor. This implies that there must be something more different in the birds’ brains that allows them to keep costs down. so low.

A landmark study in 2016 showed that the brain of birds contains significantly more neurons compared to a mammalian brain of similar size, the researchers explained. Given that brains are typically made of energetically expensive tissue, this raised a critical question: how are birds able to support so many neurons?

To answer this question, von Eugen and his colleagues set out to determine the neural energy balance of birds based on studies of pigeons. They used imaging methods that allowed them to estimate glucose metabolism in birds. They also used modeling approaches to calculate brain metabolic rate and glucose consumption.

Their studies revealed that the pigeon’s brain consumes a surprisingly low amount of glucose (27.29 ± 1.57 μmol of glucose per 100 g per minute) when the animal is awake. This results in a surprisingly low energy budget for the brain, especially when compared to mammals.

This means that the neurons in the brain of birds consume on average three times less glucose than those in the brain of mammals. In other words, their neurons are, for reasons that are not yet clear, less expensive.

Von Eugen says it’s possible the differences are related to the birds’ higher body temperature or the specific layout of their brains. The brain of birds is also smaller on average than the brain of mammals. But their brains retain impressive capabilities, perhaps in part thanks to their less expensive but more numerous neurons.

“Our finding explains how birds are able to support such high numbers of neurons without compromising processing power,” says von Eugen.

“During the long parallel evolution of birds and mammals, birds developed smaller brains with high numbers of neurons capable of advanced cognitive performance.

“And it appears that the combined effect of distinct elements for birds – small neuron size, high body temperature and specific bird brain layout – may have generated a possible advantage in neural information processing at an efficiency higher: cheap neurons with advanced technologies processing capacity.”

The researchers say they now want to better understand how bird neurons consume less glucose. Although they have ideas about how this might work, further study and testing is needed to uncover “the exact mechanistic explanation for how birds achieve such neural processing efficiency.”

Funding: This work was supported by the Deutsche Forschungsgemeinschaft.

About this neuroscience research news

Author: Kristopher Benke
Source: Cell press
Contact: Kristopher Benke – Cell Press
Image: Image is in public domain

Original research: Free access.
“Avian neurons consume three times less glucose than mammalian neurons” by Kaya von Eugen et al. Current biology

Summary

See also

Avian neurons consume three times less glucose than mammalian neurons

Strong points

• Brain tissue of awake pigeons consumes 27.29 ± 1.57 μmol of glucose per 100 g per minute
• This is equal to 1.86 × 10⁻⁹ ±0.2×10⁻⁹ μmol of glucose per neuron per minute
• The neuronal energy budget of pigeons is therefore about 3 times lower than that of mammals
• This may indicate more efficient neural processing in the avian clade

Summary

Brains are among the most energetically expensive tissues in the mammalian body.

This is mainly caused by expensive neurons with high glucose demands.

In mammals, the neural energy budget appears to be fixed, perhaps posing an evolutionary constraint on brain growth.

Compared to similarly sized mammals, birds have a higher number of neurons, and this advantage likely contributes to their cognitive prowess.

We set out to determine the neural energy budget of birds to elucidate how they can metabolically support such a high number of neurons. We estimated glucose metabolism using positron emission tomography (PET) and 2-[¹⁸F]fluoro-2-deoxyglucose ([¹⁸F]FDG) as a radiotracer in awake and anesthetized pigeons. Combined with kinetic modeling, it is the gold standard for quantifying cerebral metabolic rate of glucose consumption (CMR_glc).

We found that neural tissue in the pigeon consumes 27.29 ± 1.57 μmol of glucose per 100 g per min in the awake state, which translates to a surprisingly low neural energy budget of 1.86 × 10⁻⁹ ±0.2×10⁻⁹ μmol of glucose per neuron per minute. This is about 3 times lower than the rate in the average mammalian neuron.

The remarkably low neuronal energy budget explains how pigeons, and perhaps other avian species, can support such high numbers of neurons without associated metabolic costs or compromised neuronal signaling. The advantage of neural information processing with higher efficiency may have arisen during the distinct evolution of the avian brain.