So far on The Naive Observer I’ve written mostly about large scale mapping of connections in the brain. While, as you have hopefully read, I think that neuroscience is going to be hugely advanced by untangling the brain on the cellular level, there are other important issues to consider. That neurons are intricately connected and pass information around their circuitry to make computations that give rise to the mind is daunting and amazing to think about, but we can’t forget that, at a more basic level, all that information is transmitted across tens of trillions of individual synaptic connections. Thus, exactly how synapses transmit information is clearly extremely important to understanding how the brain works, and as it turns out, synapses have worlds of complexity unto themselves. A study just published by Nouvian and friends in Germany deals with this finer point of neural computation and connectivity. Their paper looks at the inner hair cell ribbon synapse, a strange synapse necessary for hearing in the mamalian ear (in the paper, the mouse ear) that doesn’t seem to play at all by the rules. This study gives the perfect backdrop in which to frame normal synaptic transmission, and then look at a case where things are very different.
First of all, some basics.
A synapse is made of up three distinct parts:
1) The Presynaptic Terminal is where a neuron sends its signal from. These structures are located at the end of a long cellular appendage called an axon.
2) The Postsynaptic Terminal is where neurons receive signals sent from the presynaptic terminal. These structures are located at on appendages called dendrites, which are the information collecting arms of neurons.
3) The Synaptic Cleft is the very small space between the pre- and postsynaptic terminals(on the order of 10-50 nanometers wide). The signal is transmitted across this space by special chemicals called neurotransmitters that are released by the presynaptic terminal and detected by the postsynaptic terminal.
The vast majority of synapses in the brain, including the one described above, are chemical synapses, meaning that they transmit electrical activity between cells by releasing chemical messengers, or neurotransmitters, from a presynaptic terminal of one cell, to a post synaptic terminal of another. This involves momentarily transforming an electrical signal to a chemical signal and then back again. (There is also another type of synapse – the electrical synapse. They are less numerous, extremely important, but generally don’t get a whole lot of attention paid to them – an unfortunate circumstance that I am going to perpetuate for the time being.) When you get down and look really closely at a chemical synapse (which I will henceforth refer to as a synapse) you see that they are jam packed with machinery required for synaptic transmission. The most notable feature of the presynaptic terminal is a bunch of synaptic vesicles, which are essentially small spheres of bubbles of cellular membrane, filled with neurotransmitter. When a signal reaches a presynaptic terminal, it causes calcium channels in the terminal to open, and then, by a complex process involving a number of important proteins, the calcium that enters the cell through these channels causes a number of vesicles to fuse with the presynaptic membrane, releasing their neurotransmitter contents into the synaptic cleft. Since the cleft is so small, the neurotransmitters diffuse very rapidly to the postsynaptic terminal, where they bind to specialized receptor molecules embedded in the postsynaptic terminal that only recognize the specific type of neurotransmitter used by that synapse. These receptor molecules then trigger a new electical signal to be generated in the postsynaptic cell. Many of these postsynaptic signals, from many synapses, come together in a larger electrical signal and either cause the postsynaptic cell to fire its own signal or not.
There has been a lot of research done on how all of these molecules work, and as a result, the process of chemical synaptic transmission is extremely well characterized. While studying the hair cell inner synapse though, Nouvian and friends came across some peculiarities in the presynaptic machinery that leads to synaptic vesicle fusion and neurotransmitter release, so I am going to go into a bit more detail on this mechanism.
Generally, for a synaptic vesicle to fuse to the presynaptic membrane and release its neurotransmitter, the vesicle must be docked and primed for fusion at the active zone of the synapse. Docking and priming involves attachement of the vesicle to the inside of the membrane with 3 different proteins referred to collectively as SNAREs. When calcium enters the neuron, signalling the arrival of an electrical signal at the presynaptic terminal, another molecule called synaptotagmin detects that calcium and signals to the SNAREs to tighten, somewhat like a zipper, resulting in fusion of the vesicle with the membrane and neurotransmission. Incidentally, these SNAREs are the target of a group of rather notorious neurotoxins called botulinum toxins. These toxins are the most potent neurotoxins known, with lethal doses as low as 1ng of toxin per kg of whatever it is you are trying to kill. The wikipedia article estimates that this means, if distributed properly, 4 kg of this stuff would be enough to kill the entire human race – mind you 4kg of purified protein is a lot to amass. On an equally sinister note, BOtulinum TOXin is also the functional ingredient in Botox, whose mode of action is quite instructive as to the function of these toxins. Botox temporarily paralyzes the muscles in your face to prevent wrinkles, and meanwhile causes some swelling that gives the appearance of taught skin. To cause this paralysis, the toxins in Botox cleave the SNAREs that would otherwise be involved in vesicular fusion, synaptic transmission and hence muscular contraction and movement.
So generally, botulinum toxins shut down synaptic transmission. This makes these toxins very useful in neuroscience, since one of the best ways to study the function of a specific neuron or type of neuron is to shut it down. However, the synapse that Nouvian and friends study – the hair cell ribbon synapse in the ear – doesn’t seem to play by the rules. To begin with the, when the group tried using the botulinum toxins, they didn’t work; synaptic vesicle fusion, and thus presumably transmission, still occurred. Following this up from many different angles that you can read about in my In Depth critique, Nouvian and friends present strong evidence that hair cell ribbon synapses do not use the typical SNARE machinery for synaptic transmission. Now, this doesn’t necessarily mean that some strange new way of release has been discovered – there are other sets of vesicular fusion machinery found outside of the nervous system that are insensitive to the specific botulinum toxins that Nouvian and friends used.They did show that non-specific cleavage of protein with trypsin (an enzyme that used by our stomachs) did inhibit fusion, indicating that vesicle release in hair cells does require some type of protein. Most proteins are trypsin sensitive though, so this doesn’t tell us much, although it does rule out magical protein-independent vesicular fusion.