Neuroscience #4 PDF Lecture Notes

Lecture 012325 Ch 3, 4 Synaptic Transmission RQ 1, Ch 4 – chemical versus electrical – morphological studies – physiological studies Neurotransmitters – Ach, amino acids Cone snail paralyzing –...

Lecture 012325 Ch 3, 4 Synaptic Transmission RQ 1, Ch 4 – chemical versus electrical – morphological studies – physiological studies Neurotransmitters – Ach, amino acids Cone snail paralyzing – Amines and peptides its prey with neurotoxin – uptake and termination – pharmacology Chemical Synapses – common, diverse Electrical Synapses – rare, fastest 200 nm space pre / post synaptic versus 20 nm space pre / post synaptic need chemical neurotransmitter intermediary allows direct synaptic electrical flow post-synaptic receptors: ligand-gated channels (these are also called ionotropic receptors) pre- / post synaptic connexons NMJ – acetylcholine (Ach), excitatory CNS type I - glutamate, excitatory CNS type II – gamma amino butyric acid (GABA), or glycine, inhibitory Others - peptides, amines (dopamine, serotonin) GABA = Gamma Amino Butyric Acid  Synaptic morphology - neuromuscular junction (NMJ) Immunochemistry - identify pre- and post-synaptic membrane proteins NMJ = end plate region (a synapse) - spider toxin: pre-synaptic Ca channels pre-synaptic axon terminal of motor - cobra toxin: post-synaptic Ach receptors neuron (MN) and post-synaptic muscle fiber Rhodamine / Ab / spider toxin Fluorescein pre / Ab / cobra toxin post Don’t forget the synaptic futon ☺  Morphological evidence for exocytosis during neurotransmission Torpedo electric ray Transmission electron microscopy (TEM) stimulate nerve terminals TEM and SEM Ʊ omega figures vesicle / membrane fusion Scanning electron microscopy (SEM) during stimulation exocytosis or endocytosis? When Tylenol just doesn’t do it….. Freeze fracture technique - EM view within membranes Physiological view of exocytosis normal response at frog NMJ: pre-synaptic AP gives post-synaptic AP with NO pre-synaptic AP: see miniature end plate potentials (mEPPs) mEPPs are graded potentials – not voltage-gated vesicular hypothesis: mEPPs are response to 1 vesicle of NT mEPPs sum as quanta - end plate potential (EPP) EPPs are the NMJ equivalent of neuronal, excitatory post-synaptic potentials (EPSPs) Spontaneous mEPPs: no activity in MN EPPs evoked by stimulating MN 3 mEPP 2 mEPP 1 mEPP  Jellyfish aequorin protein - bioluminescent Ca++ signal measure pre-synaptic Ca++ during pre-synaptic depolarization post pre Squid giant synapse 1 2 1. weak depolarization no Ca++ increase no post-synaptic EPSP 2. strong depolarization observe Ca++ increase Increase in pre-synaptic Ca++ post-synaptic EPSP is required for release of transmitter Today: newer voltage-dependent calcium dyes (fura-2) often used in imaging of dendritic spines in studies of hippocampus, cortex, etc.  Na+ Ca++ K+ CaV++ channels regulate NT release arrival of Na+ / K+ action potential depolarization of pre-synaptic terminal voltage gated Ca++ channels open: with increased PCa: ECa is very positive - depolarizes EM ICa is prolonged - slow to inactivate ICa = inward current - why switch from (Na / K) AP? maybe Ca++ as ION is important Ca++ as ION promotes exocytosis long Ca++ spike needed to allow Ca++ to act by changing [Ca]INT, not just acting on EM Exocytosis and endocytosis physiological measure = capacitance of terminal membrane capacitance increases as membrane area increases - exocytosis ‘kiss and run’ exocytosis versus bulk exocytosis capacitance decreases as membrane area decreases - endocytosis SNARE complex - synaptic vesicle and membrane proteins Proteins that regulate release: Ca++ channel Ca++ binding proteins membrane fusion proteins  Disorders include: Toxins affecting Botulinum toxin Lambert-Eaton myasthenia (LEMS) Ca mechanism Tetanus toxin (autoimmune to CaV++ channel) (bind to SNARE proteins) * *  Summary of chemical Arrival of pre-synaptic AP - opens voltage gated Ca++ channels synaptic transmission Influx of Ca++ promotes exocytosis of transmitter Ligand-gated channels (i.e. nicotinic AchR) promote post-synaptic EPSP, graded potential EPSP triggers opening of nearby voltage-gated channels Post-synaptic action potential in neuron or (here) in muscle fiber Ach signal is terminated by enzymatic destruction (Acetylcholinesterase) Myasthenia - autoimmune attack on: 1. Ca channel - Lambert-Eaton; 2. nicotinic acetylcholine receptor - Myasthenia Gravis 1 2 Ligand-gated ionotropic receptors - fast chemical neurotransmission Excitatory post synaptic potential ionotropic receptor / channel EPSP Transmitter opens Na+ channel FAST response and termination Membrane depolarizes Graded potential gated by ligand ion channel part of receptor directly affect VM by ionic flux Inhibitory post synaptic potential IPSP Transmitter opens Cl- or K+ channel Membrane hyperpolarizes Graded potential  Neurotransmitters. 1. Acetylcholine Choline acetyltransferase (ChaT) directs Ach synthesis in terminal Acetylcholinesterase (AchE) directs breakdown of Ach in synaptic cleft some NTs synthesized in cell body uptake system for choline into pre-synaptic terminal AchE is a major target for pesticides, nerve gas But also a target in myasthenia gravis therapy Two Ach molecules gate AchR channel receptor / channel = IONOTROPIC receptor simultaneous K+ efflux / Na+ influx Glutamate - major CNS excitatory transmitter 2. Amino acid transmitters 1. AMPA / kainate receptor - opened by Glu - Na+ channel 2. NMDA receptor - opened by Glu AND +VM - Ca++ channel Glycine - minor inhibitory transmitter in CNS GABA - major inhibitory transmitter in CNS GABA synthesized from glutamate 2. Amino acid transmitters GABA - major inhibitory transmitter in CNS Glycine - minor inhibitory transmitter in CNS GABA receptor chloride channel barbitruate target IPSPs can “sculpt” ongoing neural activity or stop it. Cl- channels and inhibitory current ionotropic GABA, glycine oppose excitatory potentials oppose excitation by shunting excitatory current (shunting = create “leak” - decrease RM) 3. Neuropeptides: Short proteins synthesized in cell body, transport to terminal As transmitter for rapid actions As modulator for slow actions Amines or neuropeptides may be released along with classical transmitters enkephalin Substance P and enkephalin act in pain pathway: More on these “dueling neuropeptides” 012825  4. Biogenic Amines aromatic AA precursors NTs and modulators Catecholamines (L) dopamine (DA) (L-DOPA crosses BBB) epinephrine norepinephrine (NE) Indole-amines (top R) serotonin (5-HT) Imidazole-amines (btm R) histamine Here comes another story About life in grad school, sigh…..  Uptake and synthesis of NTs 1. diffusion terminates signal 2a. glial cells active in re-uptake (or) 2b. biogenic amine re-uptake by neurons * target of selective serotonin reuptake inhibitors (SSRIs)* 3. neurons interact with glial cells in transport 4a. synaptic terminal enzymes for re-synthesis 4b. synaptic terminal enzymes for breakdown GABA * target of monoamine oxidase inhibitors (MAOIs)* Why is this important clinically? Pharmacology. glutamate glycine Enzymatic breakdown of amines monoamine oxidase (MAO) Dopamine breakdown catechol-O-methyl transferase (COMT) targets of several drugs to treat: Parkinson’s Disease, depression, anxiety (MAO inhibitors increase amine levels) Serotonin Norepinephrine breakdown breakdown a methyl tyrosine Pharmacological intervention at aminergic synapses inhibit tyrosine hydroxylase (TH, catecholamine synthesis) Targets include: pheochromocytoma tumors TH, auto-receptors, MAO, post-synaptic receptors L-DOPA Crosses blood brain barrier, DA increase - treat PD MPTP – destroys DA neurons – elicit PD Reserpine depletes terminals of catecholamines (negative effect of hypertension treatment) Propanolol - blocks β adrenergic receptors Cocaine - Inhibit re-uptake of NE SSRIs, yohimbine Inhibit re-uptake of serotonin, NE Treatment of depression, anxiety MAO inhibitors Inhibit pre-synaptic destruction of amines Treatment of PD, depression, anxiety