neurotaylor

The amazing astrocyte

Time was when neuroscience students learned about brain cells — meaning neurons. Other kinds of brain cells were recognized, but it was thought they played merely support roles. As a result, explanations of brain function were like explanations of society which only refer to men: incomplete and misleading. Not any more.

A new paper from PNAS (alas, not open access) pushes forward our understanding of one kind of ‘other’ brain cell: astrocytes. The name means ‘star cell’, but to date astrocytes have been outshone by neurons. However, evidence has been mounting that astrocytes are influential in brain processing, modifying the behaviour of neurons and generating their own electrical signals. The PNAS paper shows just how influential they can be.

The setup was mouse visual cortex, and the authors were interested in a set of inputs from the nucleus basalis of Meynert (NBM), a subcortical structure which lurks beneath the basal ganglia. The NBM projects widely to cortex, using acetylcholine (ACh), and these inputs, to quote the paper’s abstract, are ‘known to modulate cortical neuronal responses and instruct cortical plasticity’. What wasn’t so clear was how they change neuronal activity and make the neural circuitry more flexible. That was the question the authors set out to tackle.

In vivo, the reseachers paired visual stimulation (gratings of random orientations) with electrical stimulation of the NBM. They found that neurons responded more than to the gratings alone. Then they used two-photon imaging to show that astrocytes responded to NBM stimulation. When an ACh antagonist (blocker) was added, the astrocyte response didn’t happen, suggesting that the inputs driving the astrocytes were indeed using ACh. Like the neurons, the astrocytes responded to gratings, and responded still more to gratings + NBM excitation.

Using slices, and knock-out mice with impaired astrocytic responses, the researchers also showed that blocking the astrocyte activity affected the neurons, suggesting that the astrocyte activation (by ACh from the NBM projections to cortex) may cause the neuronal activation, not just happen alongside it. They think the astrocytes are influencing neurons via glutamate NMDA receptors — long associated with synaptic plasticity and learning.

Best of all, the increased response is specific to the circuits representing the particular visual stimulus with which the NBM stimulation has been paired. It’s not that the ACh input gives a kick to the whole of cortex; the focus is on the current visual input. So astrocytes may be contributing to the complex brain processes which allow us to concentrate attention on one small element of what we see — picking a signal out of all that cortical noise — and react to it quickly and efficiently. This is far more than just neuronal ‘support’.