Öne of the best-studied synapses in the brain continues to surprise neuroscientists. According to a May 18 study in Nature communication, Moss fiber synapses, so named because their ends look a bit like moss growing on the axons, have an unexpected way of regulating the flow of information in the hippocampus: the postsynaptic cells that receive neurotransmitter signals can make their own Release glutamate for transmission by the cell on the presynaptic side.
This so-called retrograde signal transmission was completely unexpected and depends on the influx of calcium into the postsynaptic cell, which means that researchers need to reconsider the results of previous experiments using in vitro conditions with different calcium availability.
The results are “a big deal,” for neuroscientists, says Chris McBain, a synaptic physiologist at the National Institutes of Health who was not involved in the study. “Retrograde glutamatergic signals are really rare in the central nervous system,” he notes, and when found in mossy fibers, “one of the most complex synapses adds another level of complexity”.
A mossy fiber synapse (green) overlaid with the positions and diagrams of the presynaptic and postsynaptic electrophysiological recordings
Nature protocols, in press / IST Austria
The researchers behind the new work, led by neurophysiologist Peter Jonas from the Institute of Science and Technology Austria, examined the plasticity of hippocampal neurons, the dynamic changes in the connections between cells that contribute to the functioning of neural circuits and ultimately underlie learning. Memory and other cognitive skills. János Szabadics, neurophysiologist at the Institute for Experimental Medicine in Budapest, puts it very simply: “Without synaptic plasticity, the brain would only be a cable bag,” he says.
Jonas’ team focused in particular on the physiology of post-tetanic potentiation (PTP), a phenomenon in which seconds to minutes after a high-frequency stimulus pulse, the amount of neurotransmitters released into the synapse is increased. PTP is one of the main mechanisms of plasticity in moss fiber synapses, which play a key role in the transmission of information in the hippocampus.
Moss fiber synapses have also been studied as a model for synapses in general because the bulbous tip or end of the cell’s axon that releases vesicles of neurotransmitters is several microns in diameter, making it easier to find and manipulate than most Neurons. In fact, McBain says, moss fiber synapses are so well studied that “you might think that all surprises have been found and that what’s left is somehow nuanced. . . . So for [this research team] I find it remarkable to show so clearly that a change in the postsynaptic calcium can actually trigger the presence or absence of this post-tetanic potentiation. “
It wouldn’t surprise me at all if there are different manifestations or flavors of this phenomenon that are everywhere.
—Chris McBain, National Institutes of Health
According to co-lead author Yuji Okamoto, the discovery was somewhat accidental. Both Okamoto and David Vandael, the other lead author of the study, are postdocs in Jonas’ laboratory and used a technique called “paired recording” developed by Jonas, which simultaneously takes electrophysiological measurements of the postsynaptic cell and the presynaptic terminal on either side of a synapse become.
Last year, Jonas’ team showed that PTP stems from an increase in the presynaptic terminal’s easily releasable vesicle pool – the amount of neurotransmitter that is packaged and ready for use when the neuron is activated. In their latest work, they wanted to determine the rules for creating this neurotransmitter cache.
One question they had was whether PTP induction is facilitated by activity in the postsynaptic cell, as it would be stimulated by its many other connections with different cells. This pattern of regulation, known as associative synapse plasticity, is often based on an increase in calcium in the postsynaptic cell.
To test this, the team dipped an electrode inside the postsynaptic cell and reduced the amount of calcium-binding compounds in the buffer solution, which made more calcium available to the cell. If anything, they expected that this increase in available calcium would increase PTP levels after the induction stimulus – 100 quick bursts to the presynaptic terminal and the observed postsynaptic cell. Instead, the potentiation was unexpectedly reduced or disappeared completely.
To clarify the role of calcium, the researchers conducted the opposite experiment, essentially removing calcium from the postsynaptic side by selecting the calcium-binding compounds – and PTP returned. That is, “the calcium influx into the postsynaptic cell somehow blocks the potentiation,” says Okamoto. In other words, when calcium is available, the postsynaptic cell can prevent PTP and therefore measure how much information in the form of neurotransmitters it is receiving from a particular synapse.
“We were of course surprised,” he says. “It’s really strange.”
BACK TALK: Communication across synapses is generally viewed as one way: neurotransmitters leave the presynaptic terminal (left cell in each panel) and bind to receptors on the postsynaptic cell (right cell of each panel). Under typical in vitro conditions, when neuroscientists examine so-called moss fiber synapses of the hippocampus, calcium availability is low (top row), and a high-frequency stimulation surge leads to excitation of the postsynaptic cell and an increase in the easily releasable pool of the neurotransmitter glutamate in the presynaptic terminal , a phenomenon known as post-tetanic potentiation. In a new study, researchers found that this post-tetanic potentiation can be blocked by reverse synaptic signaling. When calcium availability is high (bottom row), excitation of the postsynaptic cell leads to retrograde glutamatergic signaling – in which glutamate from the postsynaptic cell binds to receptors at the presynaptic end – which prevents this potentiation.
The surprises continued as Okamoto and his colleagues dug deeper. First, they tested whether the postsynaptic cell stopped PTP by signaling the presynaptic terminal with endocannabinoids, as retrograde endocannabinoid signaling occurs in other synapses and is the most established form of retrograde synaptic signaling. But bathing the presynaptic ends in a compound that blocks cannabinoid receptors had no effect.
Instead, the team tried to treat the presynaptic end with blockers for the glutamate receptors, which are known to be very common on moss fiber axons. That was enough to regain PTP, which was knocked out by increased calcium availability on the postsynaptic side. This suggests that under calcium abundant conditions, glutamate can migrate from the postsynaptic cell back to the presynaptic terminal to inhibit PTP.
This type of retrograde signal transmission wasn’t even thought of in mature moss fiber synapses, says Okamoto, especially not with glutamate.
Szabadics describes the methodology as “very clever” and adds that “there aren’t too many laboratories in the world” that can perform the kind of detailed electrophysiological experiments in the new work.
In a way, the results actually help answer long-standing questions in synapse biology, says McBain. The presence of glutamate receptors at presynaptic ends was established decades ago, but no one was sure where the glutamate that stimulates them came from. Many speculated that some of the glutamate released from the presynaptic terminal into the synapse diffuses back, especially when a large part of the neurotransmitter is released, he says. And while that could happen sometimes, “you now have a mechanism to contradict it,” he says, “and it kind of makes sense that you have this really nice barometer of postsynaptic activity” in the form of postsynaptic glutamate release. “It wouldn’t surprise me at all,” he adds, “if there are different manifestations or flavors of this phenomenon everywhere.”
Researchers can currently only guess why this type of feedback mechanism has developed, says Szabadics. One idea is that it could help prevent important memories from being overwritten by less useful data. but this or any other possible explanation is “very, very speculative” at this point, he warns.
The fact that PTP is regulated by the calcium available to the postsynaptic cell has far-reaching implications for the interpretation of previous neurophysiological research, notes McBain. “One of the uncomfortable truths of this article is that when you look at many other studies that have been done on this synapse, you find that the buffers used for electrophysiological recordings vary, which means calcium availability is also like that, he says . This could actually explain why labs often see different levels of PTP, he adds.
This lack of methodological standardization arose because “nobody thought that postsynaptic calcium would be confusing in determining the basic parameters of the synapse,” says McBain. “Everyone has to go back and rethink what the calcium dependence of most of the basic parameters we knew about the synapse is.”