BMS 153: Neuroscience The chemical synapse + neurotransmitters- lectures 12-14 Dr Pen Rashbass (p.rashbass@shef.ac.uk) THESE NOTES ARE MEANT TO BE SUPPLEMENTAL TO YOUR LECTURES/OWN READING THEY DO NOT REPLACE THEM Nerves and neurons provide the means to communicate between regions of the brain. The synapse is the point of communication between neurons from (presynaptic) neurons to (postsynaptic) effector cells (e.g. muscle and glandular cells). Most synaptic communication is neurochemical and relies on neurotransmitters (NT). (However some rare synapses rely on electrical communication via Gap junctions) Synaptic control of neuronal activity provides the complex, elaborate, subtle and flexible mechanisms by which the brain is able to process information. When chemical synaptic transmission goes wrong, nervous system malfunction. Defective NT is the root cause of a large number of neurological and psychiatric disorders Synaptic neurotransmission is also the point at which we can modify neuronal function with drugs. Brain function has regional specificity Brain function has neurochemical (neurotransmitter) specificity. Mechanism of transmitter release- (peptides) Secretory granules are also Ca 2+ dependant and use exocytosis- but typically NOT at the active zone. Usually requires a high-frequency train of APs so that [Ca 2+ ] throughout the terminal can build up to level required. Release of peptides can take 50msec of more Mechanism of transmitter release-(amino acid/amines) AP arrives at the nerve terminal Ca 2+ influx into the terminal ( [Ca 2+ ] in local microdomain) vesicles triggered release contents by exocytosis occurs within a fraction of a millisecond of AP arriving Some neurons fire >1000x per second, releasing neurotransmitters each time. The rapid release is possible because some of the vesicles are docked to the plasma membrane and primed for fusion. Only a small proportion of vesicles fuse with plasma membrane in response to each AP For nerve terminal to respond rapidly and repeatedly, vesicles need to be replenished very quickly after discharged. Vesicles are retrieved from plasma membrane by endocytosis (see Alberts et al: Mol Biology of the Cell: 4 th ed pg 764-765)
THE MAJOR NEUROTRANSMITTERS AMINO ACIDS AMINES PEPTIDES Gamma- amino butyric acid (GABA) Acetylcholine (ACh) Cholecystokinin (CCK) Glutamate (Glu) Dopamine (DA) Dynorphin Glycine (Gly) Adrenalin (Epinephrine) Enkephalins (Enk) Histamine N-Acetylaspartylglutamate (NAAG) Noradrenalin (NA)/ Neuropeptide Y (Norepinephrine (NE)) Serotonin (5-HT) Somatostatin Substance P Thyrotropin-releasing hormone(trh) Vasoactive intestinal polypeptide(vip) Different neurons in the brain release different NTs. The amino acid and amine NTs are small organic molecules contain a nitrogen atom stored and released from synaptic vesicles The peptide NTs are: Large molecules Stored in secretory granules Secretory granules and synaptic vesicles are frequently observed in the same axon terminals. These different neurotransmitters are released under different conditions A neurotransmitter should be Localised in the presynaptic terminals Released in response to stimulation Able to interact with postsynaptic receptors Rapidly removed from the synapse implies cellular specialisation for mechanisms for synthesis and/or storage release post-synaptic recognition (receptors) transmitter release transmitter removal Agonist- mimics normal NT function Antagonist blocks effect of NTs and agonist Different neurotransmitters are synthesised in different ways Glutamate and glycine are among the 20amino acids (protein building blocks) they are abundant in all cells in the body GABA and the amines are only made by the neurons that release them. They require specific enzymes to synthesize them: these are situated locally at the axon terminal, Once synthesised the amino acid and amine NTs are concentrated into synaptic vesicles by transporter proteins. (Synaptic vesicles specialised class of tiny (~50nm diameter) vesicles- store amino acid and amine NTs.) Peptides are formed in the rer. Secretory granules from the peptide bud off from the Golgi apparatus and are carried to the axon terminal by axoplasmic transport
Transmitter-gated ion channels specialised for rapidly converting extracellular chemical signals into electrical signals concentrated in plasma membrane of postsynaptic cell open transiently in response to binding of NT brief permeability change in membrane change in membrane potential that are graded according to how much NT is released at synapse and how long it persists there An AP is triggered only if local membrane potential increases enough to open sufficient number of nearby voltage-gated cation channels in the same target cell membrane Transmitter-gated ion channels differ from one another Highly selective binding for the NT released from the presynaptic terminal Selective as the type of ions they let pass across the plasma membrane determines nature of postsynaptic response Excitatory NTs open cation channels influx of Na + depolarises postsynaptic membrane (excitatory post synaptic potential (EPSP)) toward the threshold potential for firing an AP Inhibitory NTs open either Cl - or K + channels surpresses firing (inhibitory post synaptic potential (IPSP)_= make it harder for excitatory influences to depolarise postsynaptic membrane. Inhibition is important: Strychnine binds to glycine receptors blocks action of glycine muscle spasms, convulsions death ACh, glutamate, 5-HT -usually excitatory GABA and glycine usually inhibitory Transmitter-gated ion channels are major targets for psychoactive drugs Curare- poison arrows /muscle relaxant in surgery block ACh receptors on skeletal muscle Barbiturates and tranquilizers e.g. diazepam (valium) bind to GABA receptors potentiate inhibitory action of GABA by allowing lower[gaba] to open Cl- channels Fluozetine (Prozac)- blocks uptake of 5-HT G-coupled receptors : NT binds to receptors activate small G-proteins activate effector proteins (either G-protein gated ion channels or 2 nd messengers). Synapses with transmitter-gated channels carry the bulk of specific information that is processed by the nervous system. But the effectiveness of these are modified by the many synapses with G-protein coupled receptors. The same NT can give different postsynaptic actions depending on what receptor it binds to e.g. ACh in heart coupled by a G -protein to a K + channel hyperpolarises the heart muscle fibre slows heartbeat ACh in skeletal muscle- binds to ACh-gated ion channel permeable to Na + rapid depolarises skeletal muscle contraction After neurotransmitter has been secreted it is rapidly removed- either destroyed by specific enzymes in synaptic cleft or taken up presynaptic nerve terminal or by surrounding glial cells. Reuptake is mediated by variety of Na + -dependent neurotransmitter carrier proteins allows neurotransmitter to be recycled allows cells to keep up with high rate of release. Rapid removal ensures spatial and temporal precision of synaptic signal. Decreases the chance of neurotransmitter influencing neighbouring cells Clears the synaptic cleft before next pulse is released
Dale s principle: A single neuron has only one NT e.g. cholinergic, glutamatergic, GABAergic etc. (Strictly speaking many peptide containing neurons violate this because the cells release an amino acid or amine AND a peptide, BUT it can be used to assign most neurons to distinct overlapping classes. Cholinergic neurons- Acetylcholine (ACh) First identified as substance released from vagus nerve to diminish heart rate ACh is transmitter in parasympathetic nerves, neuromuscular junction (therefore synthesised by all motorneurons of spinal cord and brain stem) and parts of brain e.g. 1)Neurons in the basal forebrain (nucleus basalis) innervate the cortex and hippocampus etc. These are involved in memory, coordination etc., and are lost in Alzheimer s disease. 2) Interneurons in the striatum are involved in motor function. A side effect of the treatment of parkinsonism with muscarinic ACh receptor antagonists (see below) is confusion. ACh formed from readily available substrates: Choline + Acetyl-CoA Ach. Synthesised by: Choline acetyltransferase (ChAT) ChAT is specific to cholinergic neurons and present in neuronal terminal in excess (i.e. enzyme is not saturated) Choline component of membrane lipids. Transport of choline into neuron = rate-limiting step (by altering amount can increase/decrease Ach synthesis AcetylCoA intermediary metabolism, deriving from glucose Following transmitter release, ACh is degraded by acetylcholinesterase (AChE): ACh choline + acetic acid Interfering with ACh pathway eg: 1.Prevent release of ACh eg-botulism-poisoning from Clostridium botulinum toxin or black-widow spider venom 2.AChE inhibitors: nerve gases, insecticides (organophosphates)+ treatments for Alzheimer s disease. Acute effects = decrease in heart rate + BP. Irreversible inhibition of AChE results in respiratory paralysis death 3.Block ACh receptors (skeletal muscle)-curare (see above) Catecholamines Neurotransmitters dopamine, noradrenalin and adrenalin are synthesised from the amino acid tyrosine Catecholaminergic neurons- found in regions of nervous system involved with regulation of movement, mood, attention and visceral function. Tyrosine has an active transport mechanism for uptake into the brain, shared with other large neutral amino acids. -Tyrosine L-Dopa (L-Dihydroxyphenylalanine) Dopamine (DA) Noradrenalin (NA) adrenalin Tyrosine hydroxylase (TH) Dopa decarboxylase (DDC) TH is the rate-determining step as it is normally saturated by substrate and is present only in neurons. DDC has high activity and is non-specific. Therefore the amount of dopamine is dependent on the amount of Dopa available. Removal of catecholamine neurotransmitters from synaptic cleft is by selective uptake back via Na + -dependent transporter into presynaptic terminal axon terminals. Amphetamine increases release of DA whilst cocaine blocks uptake therefore both prolong DA action In axon terminal- catecholamines may be reloaded into synaptic vesicle for reuse or enzymatically degraded by enzymes esp. Monoamine oxidase (MAO) on outer mitochondrial membrane MAO-A removes noradrenalin and 5-HT, MAO-B removes dopamine (MAO enzymes are present in the liver and gut, and provide protection from exogenous retroactive chemicals) MAO inhibitors have been used (a) in the treatment of: Depression and Parkinson s disease and (b) to increase levels of NT- BUT MAO-A inhibition can cause hypertensive crisis (the cheese effect ) due to the neuroactive effects of dietary amines such as tyramine, found in cheese, marmite etc
Parkinson s disease Tremor, rigidity, akinesia (slowing of movement, postural changes. No sensory loss and cognitive function is preserved until late stages. Primary pathology- progressive cell degeneration of pigmented dopaminergic cells in the substantia nigra which innervate the striatum (The striatum can be considered as a system that inhibits motor function). Dopamine, via its effects on D2 receptors, inhibits the cells of the striatum diminishes their inhibitory activity. As well as output neurons and dopaminergic terminals, the striatum contains cholinergic interneurons that have an excitatory effect (i.e. opposite to that of dopamine) on the striatum; If dopamine is lost, decreasing ACh activity will tend to restore the balance. So an antagonist at ACh receptors has similar effects to an agonist at dopamine receptors (Anticholinergic drugs blocking muscarinic receptors (a subtype of ACh receptors) can alleviate Parkinsonian symptoms Old treatments for Parkinson s used anticholinergic agents (belladonna alkaloids)) Pharmacological intervention in dopamine neurotransmission Synthesis: L-Dopa increases dopamine synthesis Storage: Reserpine destroys vesicular stores of dopamine induces Parkinsonism Release: Amphetamine increases dopamine release Reuptake: Cocaine blocks dopamine reuptake Metabolism MAO (B) inhibitor selegiline increases dopamine and used to treat Parkinson s disease. Receptors: Dopamine receptor agonists used to treat Parkinson s disease (Dopamine D2 antagonists induce parkinsonism) The pathway from the subs. nigra to the striatum is only one of several important dopaminergic systems in the brain innervating the: striatum (part of the basal ganglia - controlling motor function) cortical and limbic regions (emotion, memory, complex behaviours, addiction, psychosis) pituitary gland (hormonal secretion) The antipsychotic drugs are thought to act on dopamine receptors in cortical and/or limbic regions to improve some of the symptoms of schizophrenia. Their Parkinsonian side effects are due to antagonism of striatal dopamine receptors. There effects on dopamine receptors in the pituitary that control prolactin secretion can result in the overproduction of prolactin and gynaecomastia (enlarged breasts) and galactorrhoea (milk secretion) even in males! 5-Hydroxytryptamine (5-HT, serotonin) Synthesis: Similar to dopamine but from tryptophan via tryptophan hydroxylase. Availability of tryptophan is rate-determining. Controlled by binding to albumin. Removal of 5-HT by active uptake process; MAO-A 5-HT systems: Diffuse projection from brainstem (raphe nuclei in the reticular formation the reticular activating system ) to widespread areas of forebrain, particularly the cortex. The function of 5-HT systems include: Consciousness/arousal Circadian rhythms Mood Regulating aggression Drugs that increase synaptic 5-HT levels are used in the treatment of depression. These include the tricyclic antidepressants and the newer specific serotonin reuptake inhibitors (SSRIs) like fluoxetine (Prozac), that block reuptake removal of 5-HT into the presynaptic terminal. Tryptophan, as a precursor of 5-HT, has also been used as an antidepressant, as have MAO (A) inhibitors that prevent 5-HT breakdown
Amino acids, Glutamate, Glycine and GABA serve as NTs at most CNS synapses. Gamma-amino butyric acid (GABA) Most common inhibitory transmitter in brain. Very efficient and specific uptake processes remove GABA from synapse, not only into neurons but also into glia. GABA is found in striatum main output controlling motor function. GABA is also found throughout the brain, and esp. the cortex, in interneurons. Glutamate Most common excitatory transmitter in brain. Very efficient and specific uptake processes remove glutamate into neurons and glia. Found throughout the brain, notably as the transmitter in large pyramidal neurons (e.g. motor neurons) of the cortex that project to other regions of the brain or spinal cord. As GABA and glutamate are widely distributed throughout the brain they are therefore involved in all aspects of brain function. Imbalance between these two transmitter systems (increased excitatory glutamate, or decreased inhibitory GABA) occurs in epilepsy. This can be treated by e.g. GABA agonists. In the CNS, single neurons can receive inputs from 1000s of other neurons and can in turn synapse on may thousands of other selves The average motor neuron in the spinal cord has several thousand nerve terminals synapsing on its cell body + dendrites. The motor neuron must combine information from these sources and react either by firing APs along its axon or remaining quiet. Any single NT can have different effects depending on what receptor it can bind to. Divergence =The ability of one NT to activate more than one subtype of receptor and cause more than one type of postsynaptic response. Divergence may occur at any stage in the cascade of transmitter (e.g. which G-proteins and which effector systems are activated) Convergence. multiple transmitters activating their own receptor type can converge on the same effector system (again can occur at any stage in the signal cascade) Neurons integrate divergent and convergent signalling systems Recommended Reading: Anatomy & Physiology: The Unity of form of function 463-468- not extensive enough on pathways and neurochemistry Neuroscience Chapter 6 admittedly a bit deep but it s all there. Neuroscience at a Glance 24-25, 106, 122. Fundamentals of Psychopharmacology by Brain Leonard also has this material in Chapter 1 but the book isn t particularly user friendly for Level 1. Neuroscience: exploring the brain Chapter 5 and 6- good general overview with additional facts of special interest