Neurochemistry Past Paper PDF

Neurochemistry Paper A Syllabic content 3.3 © SPMM Course We claim copyright for our own text material, productions and © SPMM Course adaptations. We claim no rights to Images/Figures with CC-BY-SA 1...

Neurochemistry Paper A Syllabic content 3.3 © SPMM Course We claim copyright for our own text material, productions and © SPMM Course adaptations. We claim no rights to Images/Figures with CC-BY-SA 1 license if they are used in this material. 1. Synaptic transmission  The presynaptic neuron synthesises, transports and stores the chemical messenger (neurotransmitter). Synthesis takes place in cell body / soma which contains the essential protein synthesis machinery. From here axonal transport occurs, and the neurotransmitter reaches the synaptic terminal. Before its eventual release, the neurotransmitter is stored within the synaptic vesicle. The release takes place through the process of membrane fusion and exocytosis.  Upon release, the neurotransmitter occupies receptors present on the surface of the postsynaptic neuronal membrane. Some of the neurotransmitter molecules will also act on autoreceptors that are present in the presynaptic neuronal membrane. Such autoreceptor activity is considered to be crucuial for feedback inhibition of the neurotransmitter synthesis and release.  Neurotransmitters exhibit specificity in receptor interaction. One neurotransmitter can have more than one receptor types, but within a given receptor site only a particular chemical conformation can be accommodated (lock and key).  Receptors have a finite number and thus get saturated if there is an over secretion of neurotransmitter.  Receptor binding is often competitive; relative synaptic concentrations of competing molecules decide the eventual degree of receptor activity. Most receptors are bound reversibly i.e. following dissociation of the neurotransmitter; the receptor falls back to its physiological status quo. Some molecules can act irreversibly producing structural alterations in the protein of receptor complexes.  After synaptic release and activity, cessation of neurotransmitter action takes place via 1. Reuptake back to presynaptic neuron via special transporters (e.g. monoamine transporters) 2. Enzymatic breakdown at the cleft (e.g. via COMT/MAO-A enzyme) 3. Removed by glia or plasma circulation (e.g. glutamate shuttle)  Feedback control of a neurotransmitter may exist at various points 1. Control of presynaptic synthesis 2. Regulation of release 3. Reuptake regulation 4. Autoreceptor mediated presynaptic inhibition 5. Independent postsynaptic inhibition via a different neuronal network Neurotransmitters Monoamines Amino acids Peptides Dopamine GABA Endorphins Norepinephrine Glycine Cholecystokinin Epinephrine Glutamate Neurotensin Serotonin Neuropeptide Y Acetylcholine Leptin Histamine Ghrelin © SPMM Course 2 2. Classification of receptors Receptors may be categorized into three categories: (1) Ligand-gated channels (ionotropic), in which binding of a chemical messenger alters the probability of opening of transmembrane pores or channels; (2) Those in which the receptor proteins are coupled to intracellular G proteins as transducing elements (metabotropic); (3) Those termed ligand-dependent regulators of nuclear transcription (nuclear receptors). Ionotropic or ion channel receptors result in fast response (GABAA benzodiazepine); G protein coupling (metabotropic) is comparatively a slower process (most antipsychotics, antidepressants). Ion channel receptors are made up of four or five protein subunits making up a pore like structure. The GABA-A receptor's structure is typical of most ligand-gated (ionotropic) receptors [‘doughnut with a hole in the centre’ or ‘rosette’ shaped]. Each protein subunit is a string of amino acids which passes in and out of the cell membrane four times. At the extracellular end of this string is a large N-terminal; this end-chain is thought to mediate GABA-channel interactions. In the middle of the string is a large intracellular loop of amino acids with four sites where phosphorylation occurs. Inhibitory neurotransmitter action leads to the entry of Cl- while excitatory action results in the entry of Ca2+ or other cations. Ionotropic receptors include GABAA, NMDA, the 5HT3 subtype of serotonin receptors. G-protein-coupled metabotropic receptors are proteins that span the cell membrane seven times (serpentine receptors). G protein-coupled receptors act via cyclase mediated second messenger activation (GTP, ATP, etc.). Gs-proteins are stimulatory; Gi-proteins inhibit the adenylate cyclase. A third variant of G-protein receptors acts via phospholipase C. Metabotropic receptors influence protein synthesis eventually thus producing longer lasting effects. Metabotropic receptors include DA receptors, most 5HT receptors except 5HT-3, NEN and neuropeptides including opioid receptors are G coupled. Nuclear receptors such as glucocorticoid receptors are part of a superfamily of receptors that have a cysteine-rich DNA-binding domain, a ligand-binding domain, and a variable amino terminal region. Upon appropriate ligand binding, a nuclear receptor becomes a transcription factor and binds in turn to DNA via zinc fingers. Other nuclear receptors include the receptors for progesterone, androgen, and 1,25- dihydroxycholecalciferol (Vitamin D). Many receptors of this family are orphan receptors, for which the ligands are still unidentified. The glucocorticoid receptor is located mainly in the cytoplasm but migrates to the nucleus as soon as it binds its ligand. In contrast, the estrogen and the triiodothyronine (T3) receptors are retained in the nucleus and bind hormones directly in the nucleus itself. © SPMM Course 3 3. Dopamine Source tyrosine l-dopa  dopamine Rate limiting step tyrosine hydroxylase Breakdown enzymes Monoamine oxidase (MAO) & Catechol-o-methyl transferase (COMT). MAO-A more selectively metabolizes norepinephrine and serotonin MAO-B more selectively metabolizes dopamine. Breakdown product Homovanillic acid Reuptake Dopamine transporter (cocaine inhibits this transported) Function Motivation, novelty seeking, reward circuitry (addictions), arousal and motor movement gating in basal ganglia Receptors 5 types; D1 to D5. All are G protein coupled D1-like  D1 & D 5; increase adenylate cyclase (stimulatory). D1 exclusively postsynaptic; resistant to antagonism. D5 more limbic in distribution; 10 times higher dopamine affinity D2-like  D2,3 & 4 ; decrease adenylate cyclase (inhibitory). D4 is found primarily in the frontal cortex and clozapine has a high affinity. D4-selective antagonists do not have antipsychotic efficacy. Disorders Levels low in Parkinson’s; high in psychosis especially at mesolimbic area; may be low in anhedonia and negative symptoms in mesocortical area. © SPMM Course 4 4. Noradrenaline Source tyrosine l-dopa dopamine  norepinephrine  epinephrine Rate limiting step tyrosine hydroxylase Synthetic enzymes dopamine-b-hydroxylase modulates norepinephrine production; phenylethanolamine-N-methyltransferase modulates conversion of NEN to epinephrine. Breakdown enzymes Monoamine oxidase (MAO – A especially) & Catechol-o-methyl transferase (COMT). Breakdown product 3-methoxy-4-hydroxyphenylglycol (MHPG) & VMA – vanillyl mandelic acid. MHPG is the major metabolite in CNS while VMA is major metabolite from peripheral nervous system/endocrine system. Reuptake noradrenaline reuptake channel (tricyclics, reboxetine inhibit this) Function arousal, anxiety, mood regulation, autonomic mediation Receptors 2 major types; α and β. α divided into a1 and a2 α1 receptors phospholipase C coupled; mostly postsynaptic α2 receptors Gi coupled ; mostly presynaptic autoreceptors β-receptors Gs coupled; predominate locus ceruleus – may regulate a β1-receptors – high affinity to norepinephrine and β2-receptors – high affinity to epinephrine. Disorders Levels low in depression and abnormal in panic/anxiety disorders. © SPMM Course 5 5. Serotonin Source tryptophan5 hydroxy l-tryptophan  serotonin Rate limiting step availability of tryptophan (hence it is possible to conduct tryptophan depletion studies and manipulate 5HT system) Synthetic enzymes tryptophan hydroxylase Breakdown enzymes MAO (preferentially MAO-A) Breakdown product 5-hydroxyindoleacetic acid (5-HIAA) Reuptake Serotonin reuptake channel (tricyclics, SSRIs inhibit this) Function mood, perception of pain, feeding, sleep-wake cycle, motor activity, sexual behaviour, and temperature regulation. Receptors 14 known subtypes of serotonin receptors (5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F, 5- HT2A, 5-HT2B, 5-HT2C, 5-HT3, 5-HT4, 5-HT5A, 5-HT5B, 5-HT6, and 5-HT7) All except 5-HT3 are G-protein-coupled receptors; 5HT3 predominant in gut; associated with motility. 5-HT1A receptors – Gi coupled postsynaptic; antidepressant response; sexual behaviour 5-HT1B receptors – Gi coupled presynaptic; 5-HT1D receptors – Gi coupled - both presynaptic and postsynaptic. 5-HT2 receptors - phospholipase C coupled; postsynaptic; antagonism leads to antipsychotic response (atypicals) and sedation; LSD causes 5-HT2 stimulation; down regulation noted after antidepressant treatment / ECT. 5-HT6 may be involved in antidepressant action 5-HT7 - regulation of circadian rhythm Disorders low serotonin levels  increased depression, aggression, suicide, and impulsivity; regulate dopamine system – role in psychosis © SPMM Course 6 Receptor Action 5HT1A Antidepressant (agonist), anxiolytics (partial agonist) 5HT1B Aggression 5HT1D Antimigraine (antagonist) 5HT2A Antipsychotic (antagonist); hallucinogens (agonist / partial agonist); implicated in working memory; also seen in platelets and smooth muscles 5HT2B Stimulation may produce cardiac valvular fibrosis (dexfenfluramine) 5HT2C Anxiogenic and anorexic effect (agonists) 5HT3 Antiemetic (antagonist) 5HT6 Possible antipsychotic/antidepressant action (antagonism) 5HT7 Regulation of circadian rhythm DOPA decarboxylase (DDC) “is an enzyme implicated in 2 metabolic pathways, synthesizing two important neurotransmitters, dopamine and serotonin (Christenson et al., 1972). Following the hydroxylation of tyrosine to form L-dihydroxyphenylalanine (L-DOPA), catalyzed by tyrosine hydroxylase, DDC decarboxylates L-DOPA to form dopamine. This neurotransmitter is found in different areas of the brain and is particularly abundant in basal ganglia. Dopamine is also produced by DDC in the sympathetic nervous system and is the precursor of the catecholaminergic hormones, noradrenaline and adrenaline in the adrenal medulla”. In the nervous system, tryptophan hydroxylase produces 5-OH tryptophan, which is decarboxylated by DDC, giving rise to serotonin. DDC is a homodimeric, pyridoxal phosphate-dependent enzyme. (Excerpt from www.omim.org) © SPMM Course 7 6. Acetylcholine Source choline and acetyl-coenzyme A Rate limiting step availability of choline Synthetic enzymes choline acetyltransferase Breakdown enzymes acetylcholinesterase – rapid metabolism Breakdown product Choline Reuptake no reuptake. Degraded choline is re up-taken and recycled. Function Modulate arousal, learning, memory, rapid eye movement sleep, pain perception, and thirst and parasympathetic mediation. Receptors Muscarinic receptors - G-protein-coupled. Five subtypes (M1, M2, M3, M4, and M5) Nicotinic receptors - ion channels; more in peripheral parasympathetic system; Less common than M receptors in CNS – mediates attention. Disorders reduced cholinergic function in Alzheimer's dementia; dopamine balance affected in Parkinson’s © SPMM Course 8 7. GABA Source Glutamic acid (glutamate) Rate limiting step glutamic acid decarboxylase (GAD) catalysis Synthetic enzymes glutamic acid decarboxylase (GAD) Breakdown enzymes GABA transaminase Breakdown product Broken down to glutamate, and then eventually to succinic acid Reuptake reuptake into both presynaptic nerve terminals and surrounding glial cells; uptake system is bidirectional and both temperature- and ion-dependent process; (inhibited by tiagabine) Function Mediates anxiety, seizure cessation, and actions of benzodiazepines, barbiturates, and alcohol. Receptors GABAA and GABAB GABAA – opens chloride channel; inhibitory – leads to hyperpolarization; made of five subunits and at least 14 subunit subtypes GABAB receptor is G-protein-coupled; baclofen is selective agonist Disorders Role in anxiety disorders and alcoholism; may have a role in many other disorders including epilepsy and Huntington’s. © SPMM Course 9 8. Glutamate Source 1. from 2-oxoglutarate and aspartate by aspartate aminotransferase, 2. from glutamine by glutaminase, or 3. from 2-oxoglutarate by ornithine aminotransferase Regulation accumulation of precursors such as glutamine or by end-product inhibition Synthetic enzymes glutaminase Breakdown enzymes Glutamate dehydrogenase, glutamine synthetase Breakdown product Broken down to glutamine or alpha-ketoglutarate Reuptake Largely glial uptake with conversion to glutamine Function Important metabolic role – intermediary in oxidation pathway (malate shuttle), immediate precursor of all GABA in CNS, intermediary in ammonia cycle; NMDA - memory acquisition, developmental plasticity, epilepsy, and ischemic brain injury. NMDA receptor mediates long-term potentiation Receptors metabotropic - 8 in total; 3 groups. Group I - mGluR1& mGluR5 – linked to phospholipase C Ionotropic: NMDA and non-NMDA NMDA - made up of subunits with distinct binding sites for glutamate, glycine, phencyclidine (PCP), magnesium, and zinc. Non NMDA – kainate binding or AMPA type. Disorders excitotoxic glutamate toxicity in stroke/schizophrenia/seizures suspected. NMDA antagonists can cause hallucinations – e.g. PCP, ketamine © SPMM Course 10 9. Glycine  Glycine is the primary inhibitory neurotransmitter in the spinal cord  It has the simplest structure of all aminoacids  It is synthesized primarily from serine by serine trans-hydroxymethylase and glycerate dehydrogenase, both of which are rate-limiting steps.  Glycine acts as a ‘mandatory adjunctive neurotransmitter’ for glutamate receptors; the excitatory glycine site on the NMDA receptor is called non-strychnine-sensitive glycine receptor.  Strychnine-sensitive glycine receptor is an inhibitory receptor seen in the spinal cord where glycine acts independently.  Facilitating glycine transmission can help reduce negative symptoms of schizophrenia. An experimental agent called bitopertin is a glycine reuptake inhibitor that has shown some early promise in reducing negative symptoms. 10. Endocannabinoids  Two endogenous cannabinoid substances - Anandamide (a weak ligand) and 2- arachnidonylglycerol (a strong ligand) are formed from arachidonic acid and ethanolamine.  The two types of cannabinoid receptors, central (CB1) and peripheral (CB2), both bind tetrahydrocannabinol (THC), the active ingredient of marijuana.  Anandamide lowers intraocular pressure, decreases activity level, and relieves pain. 11. Neurotrophins These are substances that act as polypeptide growth factors influencing proliferation and differentiation of neurons and glial cells. The best-characterised factors are Nerve growth factor (NGF); brain derived neurotrophic factor (BDNF), neurotrophin 3 and neurotrophin 4. According to neurotrophin hypothesis neurons compete during development for the limited resource of growth factors in the target region. Those neurons that are highly responsive, e.g. via high affinity binding sites, survive while others undergo programmed cell death. Incorrect targeting of axons may also lead to apoptosis (programmed cell death). BDNF may have a role in long-term potentiation (LTP) of memory. In animals, chronic stress leads to down regulation of BDNF. BDNF has been shown to have trophic effects on serotonergic and noradrenergic neurons. SSRIs and other antidepressants including ECT up regulate BDNF. The time course of this up regulation coincides with observed therapeutic actions of antidepressant interventions. A single nucleotide polymorphism in the BDNF gene on chromosome 11p13 results in an amino-acid substitution of valine (val) with methionine (met) at codon 66 (Val66Met) reducing BDNF activity. BDNF met/met mice demonstrate increased anxiety. Clinical studies in humans have demonstrated that subjects with the Val66Met allele have impaired hippocampal activation and performance. It is controversial if BDNF polymorphism increases the risk of clinical disorders or not. © SPMM Course 11 12. Some clinical implications Ach & LEWY BODY DEMENTIA β ADRENOCEPTOR Brain acetylcholine levels are reduced in DLB Chronic antidepressant treatment induces a similar to Alzheimer’s. Cortical choline acetyl reduction in β adrenoreceptor density around 2 transferase (ChAT) is reduced to a greater extent weeks after starting antidepressants; this correlates (85%) in patients with hallucinations in Lewy with therapeutic effects. body dementia than in those without hallucinations (50%). Unmedicated suicide victims show higher density This may partially explain the altered sleep-wake of β adrenoreceptors. patterns seen in DLB and also the response of β blockade can reduce peripheral features of hallucinations to acetylcholinesterase inhibitors anxiety driven by sympathetic overdrive. ABERRANT SALIENCE 5HT & DEPRESSION Kapur proposed that in the normal individual, the An increased density of 5HT2 binding sites has role of mesolimbic dopamine is to attach been shown in post mortem studies of depressed / significance or ‘salience’ to an external stimulus, suicidal patients. The increase in 5HT2A receptors or an internal thought. This converts a neutral is most prominent in dorsolateral prefrontal cortex piece of information into an attention grabbing one and in platelets of medication naïve patients. A (Kapur, 2003). reduction in 5HT1A receptors has also been noted in cortex In acute psychosis where hyperdopaminergic state is noted in mesolimbic system, insignificant events Long-term antidepressant treatment has been and perceptions receive inappropriate salience shown to reduce 5HT2 receptors and increase leading to delusional elaborations. 5HT1A function. But these changes may not be causative of antidepressant action as they predate Antipsychotics are claimed to "dampen the any clinical response to antidepressant therapy salience" of these abnormal experiences - do not erase the symptoms - but provide the platform for a Most directly acting 5HT1A agonists have poor process of psychological resolution. antidepressant activity. © SPMM Course 12 DISCLAIMER: This material is developed from various revision notes assembled while preparing for MRCPsych exams. The content is periodically updated with excerpts from various published sources including peer-reviewed journals, websites, patient information leaflets and books. These sources are cited and acknowledged wherever possible; due to the structure of this material, acknowledgements have not been possible for every passage/fact that is common knowledge in psychiatry. We do not check the accuracy of drug related information using external sources; no part of these notes should be used as prescribing information. Notes prepared using excerpts from:  http://omim.org/entry/107930  Kapur, S. Psychosis as a state of aberrant salience: a framework linking biology, phenomenology, and pharmacology in schizophrenia. Am J Psychiatry 2003; 160  Angelucci et al. BDNF in schizophrenia, depression and corresponding animal models. Molecular Psychiatry (2005) 10, 345–352  Artigas F. Serotonin receptors involved in antidepressant effects. Pharmacology & Therapeutics, 2013; 119-31 © SPMM Course 13