Swiss neuroscientists Valerie Hinard and colleagues cultured mouse cortical neurons in dishes equipped with arrays of electrodes. This allowed them to record the electrical activity produced by the growing 'brain'. They also measured the expression of different genes in the neurons, and compared these to gene expression in real mouse brains.
They found that while cultures of neurons started out fired randomly, after about 10 days, the cultures entered a state of synchronized periodic firing, with the whole population of cells firing together in slow cycles of activity - with a frequency of 1 cycle every 5 to 15 seconds. This is extremely slow - by contrast the "slow waves" characteristic of animal sleep cycle about 30 times faster - but the authors say that such ultra-slow waves have been seen in sleeping animals too.
But the dishes could be 'woken up' by adding a mixture of neurotransmitters, which abolished the burst cycles. They reappeared about 24 hours later. Gene expression changes in the cells in the 'sleep' and 'wake' state were significantly correlated with changes seen in real mice deprived of sleep.
Finally - and this might end up being the most important bit - the authors compared the biochemistry of the 'sleep deprived' dishes to the 'well rested' ones. They found remarkably few major changes, but they did observe a significant increase in the levels of lysolipids.
Lysolipids are breakdown products of phospholipids, which make up the membranes of all living cells. When present in membranes, lysolipids can act as 'detergents', distorting their structure. That's bad. These results suggest that sleep might serve to prevent the build up of lysolipids. If that pans out, it would mean that the function of sleep is very primitive, a fundamental biological necessity for any connected network of neurons, even what amounts to a random medley thrown together on a plate.
This study used cultured mouse neurons, but it's possible to grow human brain cells in a dish too. The obvious next step will be to check if human neurons exhibit the same sleep/wake-like states - and whether the very slow synchronized firing is really like human sleep. If so, could this help understand insomnia? Narcolepsy? Maybe even jetlag?
It's also got implications for all other brain-in-a-dish research. Scientists may literally need to ensure that their dishes get enough sleep in future studies.
It's all very exciting. I don't study sleep in my own research, but I try to keep up with the literature as I find it very interesting. I've covered various aspects of sleep neuroscience previously. So while I'm no expert, this seems to me like truly groundbreaking stuff, and potentially a "game changer" for the whole of neuroscience.
Hinard V, Mikhail C, Pradervand S, Curie T, Houtkooper RH, Auwerx J, Franken P, and Tafti M (2012). Key electrophysiological, molecular, and metabolic signatures of sleep and wakefulness revealed in primary cortical cultures. The Journal of neuroscience : the official journal of the Society for Neuroscience, 32 (36), 12506-17 PMID: 22956841
9 comments:
"If that pans out, it would mean that the function of sleep is very primitive, a fundamental biological necessity for any connected network of neurons"
This would mean a function of sleep is such, since there are almost certainly many functions of sleep. My thought is that animals began to sleep as a means of conserving energy when little was required, and animals then evolved to accomplish things during sleep. Clearing lysolipid buildup may be one of these things, but I don't see why it's more primitive or fundamental than any of sleep's other functions.
This study doesn't make sense.
Interesting!
I guess it would be interesting to study the effects of "sleep deprivation" on short term memories—created by scientists in isolated rodent brain tissues (Robert A Hyde, Ben W Strowbridge. Mnemonic representations of transient stimuli and temporal sequences in the rodent hippocampus in vitro. Nature Neuroscience, 2012; DOI: 10.1038/nn.3208)... one possibility amongst many.
I did not read the paper. Would somebody that has read the paper please tell me how the response of the cultures of mouse neurons with and without added neurotransmitters can be said to be representative of wakefulness and sleep in whole animals? All I see so far is that X is added to a culture and Y changes. Are the changes in Y only seen during sleep in whole animals?
To somebody that has not read the paper this looks like a word game.
"It's also got implications for all other brain-in-a-dish research. Scientists may literally need to ensure that their dishes get enough sleep in future studies."
But the "sleep" occurred in the absence of the added neurotransmitters, right?
Do batteries sleep too? You can overcharge it, recharge it, let it rest, let it blow up but that doesn't mean it causes sleep. They should seriously screen medical researchers, make them do a course in psychology. Bulk of rigorous brain research looks to come from psychology.
DS: "Would somebody that has read the paper please tell me how the response of the cultures of mouse neurons with and without added neurotransmitters can be said to be representative of wakefulness and sleep in whole animals?"
Well the pattern of gene expression changes was significantly correlated between the activated cultured neurons and sleep deprived whole mice. However as you suggest this might not mean that both are 'sleeping' - it could be, say, that the cultured neurons are stressed by the transmitters, and mice are stressed by sleep dep. There's more to it though - the paper is pretty long.
Circadium rhythm in gene expression makes sense. You have resting states, polarized states, switching back and forth whatever. Sensationalizing to sleep - must've been lost in translation.
I have only read the electrophys section of the paper and skimmed the rest of the paper, but the activity they are recording does not look like slow wave osc or delta as recorded by Steriade to me. The period of the activity doesn't look right even if the bursts happen with a long interval between them. This is key because in vitro and in vivo research show the period of the slow osc and delta can be generated through resonance between tonic excitation and intrinsic conductance’s (often ih of the top of my head) making for a long period. Also in my experience slow wave osc and delta both are very rhythmic, not like the activity they recorded. As they've done no intracellular recordings it’s difficult to classify what they are recording as a model of slow wave sleep (or any kind of sleep). The activity to me looks more like the type of bursting activity you get during by applying a GabaA antagonist to slices. Apart from that I really like the approach of combining genetics and electrophys, I’m just not sure they’ve picked the correct activity to examine.
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