Integrated Physiology PH2130 Nervous System Lectures 2 & 3 PDF

Integrated Physiology PH2130 The Nervous System Lectures 2 &3 Stuart Cruickshank  Communication Neurons need to be able to conduct information in 2 ways: 1. From one end of a neuron to the other end. 2. Across the minute space separating one n...

Integrated Physiology PH2130 The Nervous System Lectures 2 &3 Stuart Cruickshank  Communication Neurons need to be able to conduct information in 2 ways: 1. From one end of a neuron to the other end. 2. Across the minute space separating one neuron from another. The 1st is accomplished electrically via APs. The 2nd is accomplished chemically via neurotransmitters.  We know signals get from one end of an axon to the other, but how exactly do APs send information? – Info can’t be encoded in AP size, since they’re “all or none.” In the diagram on the right, notice the effect that the size of the graded potential has on the frequency of AP’s and on the quantity of NT released. The weak stimulus resulted in a small amt of NT release compared to the strong stimulus.  Chemical Signals One neuron will transmit info to another neuron or to a muscle or gland cell by releasing chemicals called neurotransmitters. The site of this chemical interplay is known as the synapse. – An axon terminal (synaptic bulb) will abut another cell, a neuron, muscle fiber, or gland cell. – This is the site of transduction – the conversion of an electrical signal into a chemical signal.  Synaptic Transmission An AP reaches the axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open. Ca2+ rushes in, binds to regulatory proteins & initiates NT exocytosis. NTs diffuse across the synaptic cleft and then bind to receptors on the postsynaptic membrane and initiate some sort of response on the postsynaptic cell.  Effects of the Neurotransmitter Different neurons can contain different NTs. Different postsynaptic cells may contain different receptors. – Thus, the effects of an NT can vary. Some NTs cause cation channels to open, which results in a graded depolarization. Some NTs cause anion channels to open, which results in a graded hyperpolarization.  EPSPs & IPSPs Typically, a single synaptic interaction will not create a graded depolarization strong enough to migrate to the axon hillock and induce the firing of an AP. – A graded depolarization will bring the neuronal VM closer to threshold. Thus, it’s often referred to as an excitatory postsynaptic potential or EPSP. – Graded hyperpolarizations bring the neuronal VM farther away from threshold and thus are referred to as inhibitory postsynaptic potentials or IPSPs.  Summation One EPSP is usually not strong enough to cause an AP. However, EPSPs may be summed. Temporal summation – The same presynaptic neuron stimulates the postsynaptic neuron multiple times in a brief period. The depolarization resulting from the combination of all the EPSPs may be able to cause an AP. Spatial summation Multiple neurons all stimulate a postsynaptic neuron resulting in a combination of EPSPs which may yield an AP At all levels of the nervous system membrane excitability determined by neurotransmitters. Neuronal sensitivity determined by specific membrane receptors. Upon transmitter release: 1) Formation of transmitter/receptor complex. 2) Change in postsynaptic membrane permeability 3) Change in ionic flux across membrane 4) Results in change in membrane potential 5) Modifies activity of postsynaptic neurone 6) Effects may be EXCITATOR Y or INHIBITOR Y  Neurotransmitters e.g. Acetylcholine Noradrenaline Dopamine Glutamate 5-Hydroxytryptamine (Serotonin, 5-HT) Gamma Aminobutyric acid (GABA)  Integrated Physiology PH2130 The Nervous System Lectures 2 &3 Stuart Cruickshank  The Nervous System Central Peripheral (CNS) Efferent (Motor) Afferent (Sensory) Somatic (Voluntary) Autonomic (involuntary) Parasympathetic Sympathetic Enteric Nervous System  Acetylcholine Site Action Effect Neuromuscular Junction Excitatory Skeletal Muscle contraction Parasympathetic/ Excitatory Ganglionic sympathetic ganglia neurotransmission Parasympathetic Excitatory/ Smooth/cardiac muscle neuroeffector Junction Inhibitory & glands CNS Excitatory/ Learning, short-term Inhibitory memory  Acetylcholine Receptors Subdivided into 2: nicotinic (nAChR ) muscarinic (mAChR ) nAChR directly coupled to cation channels, mediate fast excitatory synaptic transmission. Differences occur between muscle and neuronal nAChR. Both occur presynaptically and postsynaptically & regulate transmitter release. mAChR are G-protein coupled receptors. Effects mediated by: Phospholipase C activation Adenylate cyclase inhibition K+ activation/Ca2+ inhibition mAChR effects postganglionic parasympathetic synapse Affecting heart, smooth muscle and gland and contribute to ganglionic excitation. 3 main subtypes: M1. Neuronal, slow excitation via  IP3 & DAG M2. Cardiac,  cAMP, Ca2+ & K+ conductance M3. Glandular, Smooth muscle contraction, vascular relaxation. Presynaptic neuron Postsynaptic neuron Acetylcholine Synaptic vesicles Na+ Action Potential Na + propagation Membrane depolarization Cha Ca2+ n Ca 2+ nel Nicotinic receptors  Cholinergic transmission 1) nAChR increase permeability to Na+,K+ and Ca2+ 2) Influx of Na+ causes depolarization 3) Neuromuscular junction known as endplate potential 4) In muscle fibre, localised epp spreads. If reach threshold action potential initiated and contraction 5) At synapse, fast excitatory postsynaptic potential (fast epsp) 6) Depolarizes axon. If epsp large causes action potential.  Acetylcholine Synthesis 1) Choline required (enters via carrier-mediated transport) 2) Acetylation via cytosolic enzyme choline acetyltransferase using acetyl-coenzyme A (CoA) 3) Rate limiting step = choline transport Vesicle accumulation ~ 100 mM. Vesicular transporter results in accumulation of H+. H+ actively pumped into vesicle. Uses “energy” of H+ gradient (low in cytosol, high in vesicle) in H+- Ach exchanger. Packaged Ach released through exocytosis.  Cholinergic Transmission Presynaptic nicotinic ACh receptor AcCoA Choline CAT CoA ACh Choline carrier Acetylcholinesterase AcCoA = acetyl coenzyme A CoA = Coenzyme A CAT = Choline acetyltransferase ACh = Acetylcholine  Acetylcholine Desensitisation. Caused by persistent exposure to nAChR agonist Conformational (shape) change of receptor. Agonist bound BUT not opening ion channel e.g. persistent exposure to ACh result in block of depolarising effect. BUT may also occur due to loss of electrical excitability Voltage-gated Na+ channels inactivated during sustained depolarisation  Neuron Astrocyte Glutamine transporter Gln Glutamate Gln Glutaminase transporter Glutamine Glu synthase Glu Glutamate  The Nervous System Central Peripheral (CNS) Efferent (Motor) Afferent (Sensory) Somatic (Voluntary) Autonomic (involuntary) Parasympathetic Sympathetic Enteric Nervous System Adrenergic receptors possess 2 categories α-adrenoceptor (noradrenaline > adrenaline > isoprenaline) β-adrenoceptor (isoprenaline > adrenaline > noradrenaline) Subtypes = α1 and α2 = β1, β2, β3 All are G-protein coupled receptors but differ in 2nd messenger pathways.  Adrenoceptor characteristics α1 α2 β1 β2 β3 PLC activation cAMP cAMP cAMP cAMP IP3 Ca2+ DAG K+ Ca2+  Secretion Presynaptic block  Heart Rate Bronchodilation  lipolysis  vasoconstriction  Insulin release  Force  vasodilation Use the table you constructed in PH1131!  Noradrenaline Site Action Effect Sympathetic neuro- Excitatory/ Increased heart rate effector junction inhibitory Vasoconstriction CNS Mainly inhibitory/some Blood pressure excitatory regulation  Noradrenaline Synthesis 1) Metabolic precursor is L-tyrosine (amino acid in body fluids taken up by adrenergic neurons) 2) Tyrosine hydroxylase (TH) is cytosolic enzyme found only in catecholamine containing cells 3) Catalyses conversion of tyrosine to dihydroxyphenylalanine (dopa) 4) Primary control of noradrenaline synthesis, i.e. TH is a rate-limiting step. ( TH can be inhibited by noradrenaline) 5) Dopa decarboxylase is a cytosolic enzyme found only in catecholimine- synthesising cells. 6) Catalyses the conversion of dopa to dopamine.  Noradrenaline synthesis cont. 7) Dopa decarboxylase is non-specific and catalyses decarboxylation of other amino acids such as L-histidine (histamine) and tryptophan (seratonin) 8) Dopamine-β-hydroxylase catalyses conversion of dopamine to noradrenaline. Small amount released with noradrenaline (provide index of sympathetic activity) 9) Phenylethanolamine N-methyltransferase (PNMT) catalyses methylation of noradrenaline to adrenaline PNMT located in adrenal medulla. ATP also released along with noradrenaline (responsible for the rapid excitatory synaptic potential and rapid contractile response in smooth muscle. (P2X receptors)  Noradrenaline Noradrenaline NA Transporter tyrosine R Tyrosine NA Hydroxylase DOPA Dopa Decarboxylase Dopamine Dopamine-β -hydroxylase Noradrenaline (NA) Regulation of noradrenaline release Release affected by presynaptic receptor activation: represents important control mechanism. Noradrenaline regulates its own release (and co- released ATP). Affect is inhibitory Known as auto-inhibitory feedback mechanism  Ca2+ NA Ca2+  cAMP ATP Adenylate cyclase  ATP exocytosis α2-adrenoceptor ATP NA So what about metabolism? Noradrenaline uptake and metabolism Action terminated upon reuptake primarily by noradrenergic nerve terminals Circulating noradrenaline and adrenaline degraded very slowly (contrast with Ach) 2 main metabolising enzymes found both intra and extracellularly Re-uptake important for efficient metabolism to take place 2 uptake mechanisms Differ in location, kinetic properties and inhibition. Called…………………. Uptake 1! Neuronal transporter. Has high affinity (i.e. able to transport very low concentrations). Is relatively selective for noradrenaline. Uptake rate is low. Uptake 2 Extra(non)-neuronal transporter. Has low affinity (i.e. able to transport only when concentration is high). Transports adrenaline and isoprenaline as well. Uptake rate is high. Amine transporters also co-transport Na+ and Cl- using electrochemical gradient as “energy source” Changes in gradient may affect uptake potentially reversing. Cocaine and amphetamines inhibit uptake 1 transporter Steroids e.g. corticosterone inhibits uptake 2  Noradrenaline DOMA =dihydroxymandelic acid Uptake 2 MAO NA DOMA VMA COMT COMT = catechol-O-methyl- transferase MAO = monoamine oxidase VMA = 3-methoxy-4- hydroxymandelic acid  Noradrenaline Metabolism MAO is abundant in noradrenergic nerves but also present in liver and intestinal epithelium. Converts NA to corresponding aldehyde. In sympathetic nervous system, MAO controls NA and dopamine content. Releasable store of either increases in MAO inhibited (MAO-inhibitors e.g. moclobemide) COMT is a ubiquitous enzyme (gut, liver kidney, brain) Acts either on NA itself or products of MAO (i.e. DOMA). Final metabolite is VMA. VMA excreted in urine. In some cancers (chromaffin tissue) VMA is increased and used as diagnostic test. What do you need to know? 1. Identify receptors/targets for Acetylcholine (Ach) and noradrenaline (NA). 2. Outline synthesis pathways for Ach and NA. 3. Outline transmitter degradation pathways.