Synaptic Structure and Function Notes PDF

Synaptic Structure and Function Neurotransmitters: Synthesis, Release and Inactivation Neurotransmitter Receptor Superfamilies Tyrosine Kinase receptors Ionotropic receptors Metabotropic receptors Chapter 2/3 Synapse...

Synaptic Structure and Function Neurotransmitters: Synthesis, Release and Inactivation Neurotransmitter Receptor Superfamilies Tyrosine Kinase receptors Ionotropic receptors Metabotropic receptors Chapter 2/3 Synapse B A Pre-synaptic inhibition/facilitation birds eye view of dendritic spine astrocytic processes some kind of chemcial substance that influences how neurons function Neurotransmitters Traditional Criteria 1. The pre-synaptic terminal should contain a store of the suspected transmitter substance. 2. The effects of application of a suspected transmitter substance to a synapse should mimic the effects caused by stimulation of the pre-synaptic terminal at that synapse (which should release the substance from the terminals). 3. The substance should bind to receptors on the post-synaptic cell. 4. Application of an antagonist drug that blocks the receptors should inhibit both the action of the applied substance and the effect of stimulating the pre-synaptic neuron. 5. A mechanism must exist for the synthesis of the neurotransmitter. Therefore, the precursor and the appropriate enzymes should be present in the pre-synaptic terminal. 6. A mechanism must exist for inactivating the transmitter, such as a catabolic enzyme to degrade the transmitter (e.g., ACh), or an active re-uptake system in the pre-synaptic terminal (e.g., monoamines) or in adjacent glial cells (e.g., glutamate).  Important notes 1. Receptors often determine the effect of a neurotransmitter (e.g., different receptors/brain areas à different functions 2. Neuromodulators Released from neurons, glial cells or other secretory cells that alter neurotransmitter function (enhance, reduce, prolong), synthesis, release, receptor interactions, reuptake, and metabolism e.g., glucocorticoids (adrenal glands) influence synthesis of norepinephrine by controlling its synthesizing enzyme (tyrosine hydroxylase) à stress response 3. One axon can release several neurotransmitters, which can often occupy the same vesicles (coexistence/colocalization; e.g., ACh and galanin in basal forebrain). pretty much every neuron makes more than one neurotransmitter 4. Vertebrates and invertebrates share many of the same neurotransmitters - conservation of same code over millions of years of evolution  Neurotransmitters Synthesis Amino Acids Contain an amine group (-NH2) and a carboxyl group (-COOH) E derived from a single amino Monoamines Contain a single amine group - acid (contain single amine group Acetylcholine Quaternary amine (because of Nitrogen + 4 methyl groups) (water (synthesized from (proteins we eat - ionized at physiological pH soluble dietary precursors Small, water-soluble molecules that are ionized at physiological pH, thus reducing their tendency to diffuse through the blood-brain barrier dietary precursor transformed Synthesized from dietary precursors, and transformed into active compounds by a series of biochemical reactions that can take place either in the cell body or, most commonly, in the nerve terminals. That’s where the neurotransmitter is packed into vesicles for subsequent release. no recycling at terminal of large molecule recropeptides ↳ slower replenishment Neuropeptides 3 to 40 amino acids derived from protein precursors Clarger-molecule synthesized in the cell body. They are packed into · peptides vesicles by the Golgi apparatus and then transported via axonal pro-peptide transport to the nerve terminal for release. This process is slow! · = precursor (not end product) Neurotransmitters Release - Exocytosis out tell (out the all membran potentialas , a term · once sunthesized A Pre packaged in un synaptic vesicles. intra-cellulares culta. opens -exocytosis  EXOCYTOSIS Docking: SNARE proteins ↳ happens from proteins in presynaptic terminal + synaptic vessicles omio SWARESproteSept membran & SNARE proteins sunpriTrans  ut protein slics - Botulinum (BETOX) - SNARE proteins - ACH toxin blocks a the release of nicotinari Acetylcholine (which acts at nicotinic receptors at - botox NMJs) à paralysis JAMA. 2001;285:1059-1070  Release of Neurotransmitters: Rate- controlling factors It happens or It ~ doesn't (no smaller /bigger size Rate of cell firing: AP is all-or-none; strength of AP cannot determine amount of NT released – Frequency of firing: rapid firing à more NT release ↳ frequency alters how much NT released + pre-cursors , replenishment rates Transport of precursors and enzymes: different rates of replenishment of NTs (relatively fast) and NPs (more slowly).  Release of Neurotransmitters Rate-controlling factors Rate of cell firing; transport of precursors (NTs), synthesizing enzymes, and precursor proteins (NPs). 1. Heteroreceptors (diffNi) 2. Autoreceptors: (more common ↳ main function : 2. terminal somatodendritic 1. synapse or  Release of Neurotransmitters Rate-controlling factors CSAME NT) Autoreceptors: receptors for the NT they are releasing Provide negative feedback. Can help prevent ~exassire overstimulation (high levels of NT in link ana openpepare some to voltage dependentannels Obinding synapse). ca terminal t of effect different = NT a also reduce Somatodendri am mechanisms release different of NT release reduce rate bindings = reversible. Directly inhibit NT release - Inhibit rate of firing - Calcium (block (indirect calccum) k + /C) -  Neurotransmitter Inactivation 1. Enzymatic breakdown (ACh, neuropeptides, lipids ,and gases) - e.g., Acetylcholinesterase inactivates ACh 2. Reuptake (amino acids & amines): transporter proteins in cell membrane Isame all that releases NT - e.g., glycine and monamines that didn't produce/release WT Mdifr 3. Uptake: glial cells may ajaunt astrol I participate in inactivating certain NTs - e.g.,⑳ glutamate (astrocytes) exctotoro much can kills Many psychoactive drugs block reuptake mechanisms. (SSRI's) NT remains in cleft longer to prolong neurotransmission. ???  Heuptak ~ pumps -presynaptiferminal or - post-synaptic receptors  Receptor Superfamilies 3 main : Tyrosine Kinase receptors Not directly involved in neurotransmission Involved in neuronal growth during development & adulthood can influence structure/function of developing alls aka : ligand Ionotropic receptors/ligand-gated channels Operate at very short latency (few milliseconds) Involved in fast neuronal signaling Rapidly desensitize (loss of function after continued exposure) binas-> opens channel Immediatly (ex : GABA/ glutamate) ligand · lose their function /rapidly resensitize Coverstimulation Metabotropic receptors/G protein-coupled receptors Operate at longer latencies (100s of milliseconds) Response can outlast initial stimulation by minutes Involved in slow but sustained signaling Modulate action of fast neurotransmission allular require energy US ionotropic tals longr · outlast initial stimulation car response  development, growth, survival Tyrosine Kinase receptors - stimulationaol (phosphate that ↳ Kinase = Special enzym phosphorylizes a protein Activated by neurotrophic factors: greapfemme - Maintenance of synapses; neuronal ~ growth, survival, and development PNSTCNS ~ I Found in Nerve growth factor (NGF): this A G-binds turosine to Calcium trkA receptor.. l important growth FurmS for development t neuronal , survival - Brain-derived neurotrophic factor (BDNF) a and Neurotrophins-3 and -4 (NT-3 and memor second NT-4) - Stimulation - trkB receptor C activation Neurotrophin-3 (NT-3) trkC receptor Phosphorylation Protein kinases Addition of one or more phosphate groups (-PO42-). This alters the An enzyme that phosphorylates proteins structure and thus function of a protein  Ionotropic receptors Ligand-gated ion channels a typical thatgeth ↑ cminiproteins one was charmal Large proteins containing 4/5 subunits diff colours poreall These receptors tend to exhibit heterogeneity in their subunit composition, thereby leading to variations in function. They contain one or more neurotransmitter binding sites (orthosteric site), an intrinsic ion channel (pore) gated by the neurotransmitter, and binding sites for other molecules (allosteric sites), dimmer > - switch which modulate the function of the (turning light receptor. up/down) The result of activation of the receptor allosteastore ampipatina depends on the ions to which it is Chydrophobic + selective (NA+; Cl-; Ca+2; etc). hydrophilic FAST! Nicotinic ACh nicotinic receptor-colours = subunits of receptor receptor: Ionotropic -grey = phosphate reads of Na+ ion channel: membrane ??? excitatory 5 subunits portionlon Others: NMDA receptor: Ca2+ à second messenger GABAA: Cl- ??? Inhibitory (chloride) ↳ Inhibitory (hyper-polaries) Metabotropic receptors G protein-coupled receptors Made-up of a single protein with 7 - > must "folds (winds) (t) transmembrane domains, but NO pore DUAL MECHANISM OF ACTION can act on ion-channel g-protein - effector can activate an enzyme ·  Structure of G proteins G proteins are composed of 3 sub-units α (20 sub-types**) – divided into 4 subfamilies & contains guanyl nucleotide binding site β (5 sub-types**) lot's of. possible γ (8 sub-types**) variety G proteins are so named because their functioning is regulated in part by the binding of guanyl nucleotides, such as: GTP = guanosine triphosphate; when bound, G protein is active GDP = guanosine diphosphate; when bound, G protein is inactive is active or not determines if g-protein **Subtypes differ in effector recognition (i.e., which proteins they activate).  Mechanism of action Inactive state heterotrimer Hydrolysis of GTP by the α sub-unit dimer  Best characterized G proteins Gs and Gi = first identified Stimulate and inhibit Adenylyl cyclase, respectively. Two toxins traditionally used in identification and study of G proteins: Vibrio Cholerae bacterium Bortadella pertussim bacterium (whooping cough) O not linked to second messenger · prevelant in G-protein + on channels Cholera: blocks GTPase activity; persistently activates G protein causes diarrhea Pertussis: blocks ability of G protein to interact with receptors; inactivates G protein causes : Whooping cough. - Used to study structure and functions of G proteins. Direct interaction between G proteins and ion channels Effector protein (in this case, an ion channel rather than an enzyme) K+ channels (Go) and IPSP ??? huperpolarization somato dendritic this would. be G-protein autoreceptors G-protein regulated channel or second messenger regulated channel Metabotropic transmission effect of G-protein on effector enzyme: aka: G-coupled receptor G-protein stimulated by metabotropic receptor G-protein activates effector enzyome (important for stimulating second messengers) extracellular signalling agent --> neurotransmitter Biochemical cascade Second messenger = intracellular molecule regulated by extracellular signaling agent… Protein kinases phosphorylate substrate proteins: e.g., ion channels, enzyme, receptor, transporter, structural protein, transcription factor, etc. protein kinase--> phosphorylate other proteins can phosphorylate substrate proteins First messenger = ??? ligand of the receptor --> neurostransmitter  third messengers: enzymes or transcription factors phsophorylated by kinases ion channel regulated by G-protein membrane second messengers: cyclicAMP -> commonly implicated second messengers Ca -> very influential, mucking in all of secondary messengers important intracellulary regulation internal cellular calcium concentrations does not easily enter cell, when done its moved into stores Widespread changes to cellular structure/function possible. New proteins  IMP3 --> infleunces release of Ca from endoplasmic reticulum Calcium and calmodulin At resting conditions, the concentration of free cytoplasmic Ca2+ is maintained by Ca 2+ -Mg2+ ATPases (i.e., calcium pumps) that transport the ion either out Ca2+ - Mg2+ --> exchnages calcium for magnesium of the cell or into the endoplasmic reticulum for storage. However, cytosplasmic Ca2+ levels can be rapidly elevated by the activation of voltage-gated Ca2+ channels or ionotropic receptors like NMDA receptor. Ca2+ (second messenger) interacts with an intracellular Ca2+-binding protein, Calmodulin, to regulate a number of proteins. calcium calmodulin --> associates with calcium together as a second messenger Ca 2+/calmodulin-dependent protein kinase II (CaM-K II) Tyrosine & tryptophan hydroxylase, GABAA receptors, neurofilament proteins… Synthesis of other SMs; NTs Protein dephosphorylation: reversal of kinase effects Cytoskeletal structure: cell shape, etc CaM-K II and caM-K IIII very prominent in nervous system cyclic modifications of nucleotides --> ATP ATP converted to cAMP GTP converted cGMP Cyclic nucleotides include cAMP and cGMP Cyclic adenosine monophosphate (cAMP) Effector enzyme (AC or GC guanylyl cycase) Cyclic guanosine monophosphate (cGMP) ATP PKA or PKG GTP viagra --> has some tissue specificity so it works in this speciifc area PDE inhibitors: enhance effects of cAMP PDE inhibitors (Sildenafil) or cGMP by inhibiting their degradation. PDE inhibitors will prolong activity regulated by phosphodiesterases--> PDE that cyclic nucleotide cAMP has first messenger,can inhibit or stimulates Adenylylcyclase, upregulate the synthesis of cAMP cAMP & cGMP Mechanism of action cGMP: floats aorund in cytoplasm and waits for diff input Nitric oxide synthase (NOS) calcium stimulate nitrous oxide which stimulates guanylylcyclase Nitric oxide (NO) Intracellular messenger Many voltage-gated Photoreceptors, and modulation of channels, receptors, cAMP by stimulating catecholamine-synthesizing phosphodiesterase enzymes, transcription factors…  cAMP & cGMP Mechanism of action Nitric oxide synthase (NOS) Nitric oxide (NO) Intracellular messenger Many voltage-gated Photoreceptors, and modulation of channels, receptors, cAMP by stimulating catecholamine-synthesizing phosphodiesterase enzymes, transcription factors… focus on cAMP Gene regulation by neurotransmitters in the brain neurotransmitter or drug Neurotransmitters alter gene expression transmembrane protien (winds) by means of mechanisms involving second messengers and transcription factors. The second messenger (cAMP)-activated catalytic subunits of protein kinase (PKA) phosphorylates protein which can phosphorylate kinase transcription factors (cAMP response element binding protein – CREB), which bind to promoter regions (cAMP response element – CRE) on the DNA to promote transcription of a particular coding region of the DNA. This region codes for Immediate-early genes (IEG). First phase of gene activation… transcription factor have binding domain --> typically bind on the promotor region (adjacent to coding region which codes for the immediate early gene --> the protein we want to make first phase of gene activation --> gateway to genomic Inactivation of CREB impairs long-term memory (e.g., Bozon et al, 2003) response  Immediate early genes: “gateway to the genomic response” CREB (and other transcription factors) induce a group of IEGs that include: c-fos, c-jun, fos B, jun B, zif-268. The (protein) products of these IEGs are themselves transcription factors (Fos, Jun), which regulate the expression of other genes (often late response genes). mRNA is induced rapidly (within 15 min), but only transiently (remains elevated for only 30-60 min) = index of neuronal activation heterodimer Late response 21 Transcription factor immediate early gene genes  wanted to see what neurons are doing if they show rats familiar and novel objects -- > then do immunohistochemistry (using when they see novel vs familiar objects antibodies to label IEG “Imaging” of took rats brain, and stained it --> black dots are nucleotides displaying fos protein Brain Activity – expressing behavioural stimuli Fos = protein used rats as within subject control, rats trained with operant box with screen and lick product of the juice, rats had two stimulus in each eye, took our the hemispheres and had the rat as their own control (no confounds) cfos gene one eye sees novel, one eye sees familiar when you see something before, even your neurons dont more black dots in novel than familiar care (less neuronal activity) --> decrimental responsiveness neurons in perirhinal cortex respond less when you see object before --> simple signal of familiarity in the brain A signal for recognition memory in rat perirhinal cortex? Wan et al, 1999 area in MTL --> recognizes objects --> object memory take tissue out of living being, keep tissue alive in a petri dish c-fos tends to be nonspecific, what it binds with gives it specificity in-situ-hybridization immunohistochemistry knock out mice have impaired LTP and LTM very important for synaptic plastcity such as LTP (long term potentiation) zif268 Davis , Bozon, and Laroche (2003)