A 'silent' population of brain cells in the olfactory network -
the part of the brain that processes smell information - may play a
key role in engaging local brain cell networks.
The finding comes from research led by scientists at the Medical
Research Council's National Institute for Medical Research
(NIMR; now part of the Francis Crick Institute).
The team discovered that a 'silent' subpopulation of nerve
cells, or neurons, in the early olfactory processing network, are
much more prominent than previously thought. They found that in the
awake state, these neurons show very little activity if odor
stimuli are not present, but they respond strongly to
stimulation.
The researchers used a method called whole-cell patch clamp
recordings to record electrical activity from a large sample
of 125 individual neurons in the brains of awake mice.
The technique is time consuming but avoids the problem of bias
towards recording active cells. In contrast, most previous studies
haven't looked at activity in the awake state, and silent and
weakly active neuron populations have remained undetected.
Dr Mihaly Kollo of NIMR explained: "Our findings suggest that
different neurons in this population communicate in different ways
with the local network.
"For instance some neurons, such as the silent but strongly
responding ones our team discovered, have better access to activate
interneurons - cells involved in local processing of smell
information."
Dr Andreas Schaefer of NIMR and University College London added:
"Our results demonstrate that, in contrast to previous conclusions,
overall neuronal activity in the awake state is not simply
increased compared to the anesthetised state. Instead, it is
characterised by an increase in variability and consequently both a
larger number of very active cells as well as a substantial
population of 'silent' neurons.
"Our large sample allowed us to make the striking finding that
sensory responses are strongly influenced by ongoing activity,
suggesting that a previously inaccessible population of 'silent'
cells can exert a powerful influence over the olfactory bulb in the
awake animal."
The work also has implications for other brain research. It
raises the possibility that commonly used large-scale methods for
measuring neuronal activity need to address the potential technical
difficulties and biases in recordings discovered here.
The paper, ' Silent' mitral cells dominate odor responses in the olfactory bulb
of awake mice, is published in Nature
Neuroscience.