Synbio Chemical Synapses PDF

**Synbio1 -- chemical synapses** - Electrical synapses = cells that are connected via a pore through which ions can go (gap junctions) Chemical synapses = main type of transmission. Each chemical synapse works in a certain way - Contains small vesicles filled with neurotransmitters. They...

**Synbio1 -- chemical synapses** - Electrical synapses = cells that are connected via a pore through which ions can go (gap junctions) Chemical synapses = main type of transmission. Each chemical synapse works in a certain way - Contains small vesicles filled with neurotransmitters. They are released into the synaptic cleft after calcium enters the cell (electrically activated cells) and depolarizes the cell. The calcium channels are voltage sensitive and open due to the depolarization of the membrane. - These transmitters reach the receptors on the post synapse. These are ligand gated channels. Once opened, influx of ions happen \--\> depolarizing this postsynaptic cell - The darker the tissue = the more proteins there are in the membranes - The presynaptic site contains the vesicles ( on top) - The postsynapse contains proteins part of the postsynaptic density which are important for the function of the receptors - Energy is needed for this process: comes from the mitochondria - Different types of synapses have different number of contact sites between the pre -and post synapse - Reason; to make the process of neurotransmission more reliable. Not everything that happens presynaptically will lead to postsynaptic activation. The more contact sites, the higher the chance that the signal will pass to the post synapse - The presynaptic site also has dense structures = a lot of proteins gather to make sure, that the vesicles are docked and released - Sound is evolutionary important therefore sound localization synapses have a lot of contact areas (make sure that the signal goes through the cleft) classical conditioning = - Some synapses are evolutionary conserved. These are very important synapse = hard-wired programs in the brain - These are all over the place. They do not need learning to get there. - This was found out by very old experiments: Nico Tinbergen showed an image to young goose. This image could be interpreted in two ways: either the mother goose moving to the left (long beak) of a predator (short beak) moving to the right. When this picture was moved to the left the baby goose thought it was their mother so they were not scared but when moving to the right they thought it was a predator. These goose are very young therefore this is hard-wired in their brain. - The non reliable synapses are much more plastic: they are being build when needed - These synapses cause association. Synaptic transmission at the neuromuscular junction - Neuromuscular junction = the presynapse depolarize and lead to the contraction of a muscle - There is a particular threshold that is needed to cause the formation of an action potential at the postsynaptic cell - One axon brings multiple synapses onto one muscle cell (a lot of release sites): this is therefore reliable - This is logical because our muscles are always moving - 85% of the signals cause a postsynaptic activation! - By stimulating with higher frequencies, more vesicles are released \--\> more activation of the postsynapse= so there is amplitude distribution - Nerve cells (presynaptic cells) also release transmitters when they should not (spontaneous events) - These are miniature excitatory potentials = lead to a sub threshold potential - There is release but no muscle contraction - So unwanted muscle contractions will most often not happen Unreliable synapses - Most synapses have a Release probability between 15 and 20 % - This means that a lot of signals that go through the brain that are intended to cause depolarization of the postsynapse will not lead to a signal = unreliable - Reliability can be improved! Relationship of synaptic vesicle exocytosis and quantal transmitter release - When not stimulated the vesicles stay in the presynapse. Only upon stimulation they fuse - The number of the vesicles that are being released are linearly correlated to the number of postsynaptic activations Short term plasticity - When giving stimulation to one synapse shortly after the other will cause increase in reliability of the activation of the second post synapse. This happens because the vesicles from the first action potential is already accumulated at the membrane - When giving a stimulation later to the same synapse then the reliability does not change - Giving many pulses together: the reliability increases a lot at the beginning but when kept stimulated then the reliability goes down. This is called fatigue because there is a particular number of vesicles that can be released from one synapse. Once these vesicles are used then there is less to no neurotransmitters that can be released - SO: the synapse and its reliability can change and this depends on how much the synapse is used - Mostly presynaptically organized Long term potentiation - The synapses are stimulated in a way that causes change to the synapse on long term (days-years) - Mostly postsynaptically organized **Synbio2 -- docking and priming of synaptic vesicles** Synaptic docking in order for vesicles to be released: - On the vesicle there is the synaptobrevin and synaptotagmin - On the host cell there is the SNAP25 and syntaxin - These proteins are found from yeast models Fusion of the vesicles on the synaptic cleft and reuse of the vesicles to be filled again: 1. Docking 2. Priming = ready to fuse 3. Fusion/exocytosis = triggered by calcium - The fusion proteins are used: synaptobrevin, syntaxin, SNAP25 and synaptotagmin 4. Endocytosis = to take the vesicle back up 5. Translocation 6. Endosome fusion 7. Budding 8. NT uptake Research on synaptobrevin and syntaxin = reverse genetics - Syntaxin and synaptobrevin function downstream of vesicle docking in drosophila - Current is driven by opening of ligand gated channels = AMPA and NMDA (glutamate neurotransmitters) \--\> depolarization \--\> voltage gated channels open \--\> sodium channels open \--\> causes action potential propagation - When doing the same experiment in a fly that is deficient for the gene synaptobrevin \--\> there is no action potential going through the postsynapse - This means that synaptobrevin is important in the formation of an action potential to the postsynapse - When giving glutamate on postsynapse, there is an action potential happening: the response to glutamate stays normal in the postsynapse when there is no synaptobrevin - Therefore = something is going wrong in the presynapse - Spontaneous events are also reduced in the presynapse when getting rid of the synaptobrevin (spontaneous fusion always normally happens. It does not cause action potential, but neurotransmitters **are** released) - There is a difference between the docking and priming stage of how close the vesicles are in to the membrane Forward genetics to see what molecules are involved in which steps - Acetylcholine is rapidly broken down by acetylcholinesterase: transmission happens very briefly - Aldicarb = compound breaks down acetylcholinesterase \--\> acetylcholine stays in the synaptic cleft \--\> muscles stay contracted (paralysis) \--\> death - Making mutant worms with Rab mutations and adding aldicarb caused no release of neurotransmitter \--\> survival - These worms can stay alive with mutation in Rab because the worms do not need nervous system for survival. Random uncoordinated muscle contractions happen instead of coordinated muscle movements: the random uncoordinated muscle contractions happen due to the unc genes. - Important unc genes - unc13 ortholog is munc13 - Unc18 =ortholog are munc18/nSec1 - This way molecules were found that play a role In synaptic transmission vesicle docking But in order to find how these molecules work together, reverse genetics is used again Reverse genetics to find out how unc13 and unc18 work: Munc13 and Munc18 in humans - These proteins play an essential role to ensure that neurotransmitter release is tightly regulated - Silencing munc18 caused silent mouse = so munc18 is a very important gene for synaptic neurotransmission - Measuring the increase in synaptic membrane length - Done by an electrode on the membrane: CA2+ is uncaged first and when light is shined, vesicles are docked \--\> the vesicles are docked on to the membrane, causing increase in membrane length Graph shows increase of membrane capacitance = the increase in membrane length when there are vesicles docked onto the membrane - Munc18 is important in fusing reactions: when the expression is blocked, there is no vesicle docking - Over expression of munc18 causes more vesicle docking - Cells with no munc18 can be given virus with munc18 in order to restore munc18 epxression - The munc18 gene is important in the fusion phase - Synaptobrevin important in priming - Munc13 = - Works like synaptobrevin = downstream of fusion and upstream of docking (priming) Munc18 \--\> munc13 \--\> synaptobrevin regulated secretion