Clinical Neurochemistry “The Soup”

1. Clinical Neurochemistry “The Soup”

3. A good working knowledge of clinical neurochemistry is essential for understanding and treating neurological and psychiatric disorders. It is important to learn the basics now so you can update your clinical management as new information becomes available.

4. What You Should Know • Primary cell bodies, sites of action and metabolic pathways for dopamine, norepinepherine, serotonin, acetylcholine, GABA and glutamate • Know the main mechanism of action and termination of action of the most common neurotransmitters • Be aware of the most common receptor subtypes for each neurotransmitter • Be familiar with examples of the mechanism of action of commonly used drugs for each neurotransmitter and the diseases they treat

5. Advances in Neurochemistry • Slow neurotransmitters include the monoamines and work through G proteins and second messengers • Fast neurotransmitters include GABA and glutamate and bind directly on ion-gated channels

6. Clinical Neurochemistry • Monoamines • Dopamine • Norepinephrine • Serotonin • Others • Acetylcholine • GABA • Glutamate

7. Catecholamines • Dopamine • Norepinephrine • Epinephrine

8. Dopamine (Main Cell Bodies) • Long tracts • Substantia nigra  primarily to striatum • Ventral tegmental area  striatum plus the mesolimbic and mesocortical systems • Intermediate • Hypothalamic—pituitary (DA inhibits prolactin) • Short • Olfactory • Retina

9. Weigert stain of the midbrain SN=substantia nigra, VTA= ventral tegmental area, DR= dorsal raphe VTA SN Dopamine cell bodies and tracts DR

10. Phenylalanine Phenylalanine hydroxylase

11. Rate-Limiting Step and Termination of Action of Dopamine • Action of tyrosine hydroxylase is the rate-limiting step • The main termination of action for the monoamines is presynaptic reuptake • Monoamine oxidase (MAOB), catechol-O-methyltransferase (COMT)

13. Monoaminergic Receptors • Formed by 7 membrane spanning regions with an intracellular carboxy tail and an intracellular amino region • The structure of the receptors are highly conserved with small changes in amino acid sequence leading to changes in receptor affinity • Monoaminergic receptors exert their effect through G-proteins and other 2nd, 3rd and 4th messengers that often cause protein phosphorylation and regulation of an ion channel

16. Dopamine Receptors • D1 is the most common and thought to involve stimulation of adenylate cyclase and increased production of cyclic AMP • D1 receptors are found in the striatum but also abundantly in cortical and limbic regions • D2 receptors are located primarily in the striatum and inhibit adenylate cyclase • The D3, D4 and D5 receptors occur primarily in cortical and limbic regions

17. Drugs that affect the dopaminergic system Neuroleptics are classified as typical or typical based on their degree of blockade of the D2 receptor • Haloperidol is a potent D2 blocker and typical antipsychotic. It is an effective antipsychotic but can cause Parkinsonism, tardive dyskinesia (TD) and cognitive slowing. • Clozapine is an atypical antipsychotic with weak antagonism at D1 and D2 receptors and blocks 5HT2 serotonin receptors. It may exert its antipsychotic effect by blocking D4 receptors, thereby sparing the striatum. Clozapine does not normally cause extrapyramidal symptoms, TD, or increased prolactin.

19. Clinical Significance • Too much dopamine can cause euphoria, confusion and psychosis. Too little produces Parkinsonism • Dopamine does not cross the blood-brain barrier. Replace dopamine in Parkinson’s disease with L-dopa. • Cocaine blocks reuptake. Amantadine and amphetamine promote presynaptic release. • MAO-B inhibitors such as deprenyl are specific for blocking dopamine breakdown at the usual doses of 5 mg/bid.

21. Thalamus Cingulate Gyrus Amygdaloid Body Olfactory and Entorhinal Cortices Cerebellar Cortex Hippocampus To Spinal Cord Locus Ceruleus Lateral Tegmental NA Cell System NE cell bodies are in the locus ceruleus at the upper dorsal pons.

22. Noradrenergic Cell Bodies in the Dorsal Pons AS LC LC Weigert myelin stain of pons. LC=locus ceruleus, AS=aqueduct of Sylvius

23. Metabolism—Termination • Reuptake—main route of termination • COMT  Normetanephrine + MAO  VMA (3 methoxy 4 hydroxy-mandelic acid) • MAO  MHPG (3 methoxy-4 hydroxy-phenylglycol)

24. Alpha and beta receptors in a noradrenergic synapse

25. Noradrenergic Receptors • Phenoxybenzamine and phentolamine are A1 blockers and are used in the treatment of hypertension • Clonidine is an alpha2 presynaptic autoreceptor agonist and causes a decrease in sympathetic tone. It is useful in the treatment of hypertension and opiate withdrawal • Yohimbine is primarily an alpha2 presynaptic antagonist and causes an increase in sympathetic tone which may lead to increased arousal, panic anxiety and sexual potency. The beta receptors are thought to activate cyclic AMP

26. Clinical Significance • The amygdala is richly innervated by nonadrenergic neurons in the locus ceruleus. Norepinephrine plays an important role in panic disorder, maintenance of attention and transmission of pleasurable stimuli via the brainstem reticular activating system and medial forebrain bundle. NE enhances emotional memories and beta blockers can inhibit the formation of emotional memories • There is a dropout of noradrenergic neurons in the locus ceruleus in patients with Parkinson’s disease which may contribute to the high incidence of depression and anxiety in PD

27. Serotonin • Cell bodies • Main cell bodies are in the dorsal raphe nuclei surrounding the cerebral aqueduct in the midbrain. They project diffusely to the striatum, limbic system, cortex and cerebellum. Caudal raphe nuclei in the pons and medulla project to the spinal cord and probably play a role in the mediation of pain in the dorsal horn of the cord

28. Cingulum Thalamus Cingulate gyrus Striatum To hippocampus Neocortex Ventral striatum Amygdaloid body Cerebellar cortex Hypothalamus Olfactory and entorhinal cortices Caudal raphe nuclei Hippocampus To spinal cord Rostral raphe nuclei

29. Synthesis Availability of tryptophan is the rate limiting step in serotonin synthesis

30. Metabolism • Reuptake—primary method of inactivation • MAO 5-HIAA • Clinical significance • Serotonin has effects on: • Sleep induction • Mood • Pain/headache • Nausea • Anxiety • Extrapyramidal system • Pleasure • Vasomotor tone • Psychosis

31. Tryptophan 1 5-OH-tryptophan MAO inhibitorsdecrease degradation Reserpine depletesvesicular stores 5-HT MAO 4 5-HT 2 3 5-HT Fenfluramineincreases release Fluoxetine (Prozac) andtricyclics block reuptake 5 5-HT 6 LSD is an agonist 5-HIAA 7 Buspirone is an agonist Methysergide is an antagonist

32. Clinical Significance • Availability of tryptophan is the rate-limiting step, Activity of Tryptophan hydroxylase is also important • Reserpine depletes vesicular stores and may exacerbate depression • Fenfluramine promotes presynaptic release • MAOI pre- and postsynaptically slows metabolism • Tricyclic antidepressants such as amitriptyline, and fluoxetine inhibit reuptake

33. Serotonergic Receptors • A very active area of research. 5-HT1-7 receptors have been described; subtypes of each group have been identified • 5-HT1 works primarily  or  adenylate cyclase, Imitrex, used to treat acute migraine, is a 1D agonist • 5-HT receptors affect phosphatidylinositol systems methysergide, LSD • Ondansetron a 5-HT3 antagonist is a potent antiemetic

34. Serotonin Syndrome • MS—confusion, agitation, restlessness • Motor—myoclonus, rigidity, hyperreflexia • Autonomic-shivering, flushing, fever, diaphoresis • GI—nausea, diarrhea

35. Acetylcholine • Primary cell bodies • Found in the patchy forebrain nuclei of the nucleus basalis of Mynert and septal nuclei • Rich connections to the hippocampus and amygdala • Ach is the main neurotransmitter at the neuromuscular jct and in the autonomic nervous system • Termination of action by both: • Enzymatic cholinesterase- choline plus acetate • By reuptake of choline

36. NBM=nucleus basalis of Meynert NBM

38. Acetylcholine • Involved in: • Memory and attention • Induction of REM sleep • Regulation of behavior • Motor function • Autonomic nervous system

39. Clinical Significance • Choline acetyltransferase (CAT) is the enzyme involved in the synthesis of Ach, CAT decreases in AD • Botulinum toxin inhibits release of acetylcholine and is useful for the treatment of focal dystonia. Lambert-Eaton syndrome, a paraneoplastic disorder, leads to decreased release of Ach • Acetylcholinesterase inhibitors such as Aricept, Exelon and Reminyl are approved for the Rx of mild-mod AD. Reminyl also modulates presynaptic nicotinic receptors. Exelon also inhibits butyrylcholinesterase • Mestinon, a peripheral cholinesterase inhibitor, improves motor symptoms in myasthenia gravis

40. Cholinergic Receptors • Nicotinic at NMJ and ANS. Antibodies formed against nicotinic cholinergic receptors at the neuromuscular junction cause myasthenia gravis • M1-5 muscarinic receptors in the brain. Nicotinic receptors also in brain. M2 and 4 decrease cAMP and M1,3,5 work via PI • Atropine and scopolamine block muscarinic receptors. Atropine increases heart rate, slows GI motility and dilates the pupils. Scopolamine can cause memory disturbance. Urecholine, an autonomic agonist, promotes bladder emptying. Ditropan, an autonomic antagonist, promotes retention of urine

41. GABA • Distribution • The major inhibitory neurotransmitter in the brain • Ubiquitously distributed • High concentrations in the striatum, hypothalamus, spinal cord, colliculi and medial temporal lobe • Synthesis • Glutamate (amino acid precursor) •  Glutamic acid decarboxylase (GAD) • GABA

42. GABA Receptors • GABA A-chloride channel • GABA binding opens the chloride channel • Benzodiazepines enhance GABA affinity and activity • Bicuculline is a receptor antagonist and induces seizures • Barbiturates and alcohol help open the chloride channel at another site in the receptor • Picrotoxin inhibits the chloride channel and produces seizures • GABA is found to be decreased in the striatum in Huntington’s disease • GABA B-Baclofen

44. GABA Neuron

45. GABA Function • Benzodiazepines are used to treat anxiety, seizures, and muscle spasms • GABA transaminase inhibitor vigabatrin used in Europe for epilepsy • The anticonvulsant tiagabine (Gabatril) blocks reuptake of GABA • Topiramate (Topamax), divalproex (Depakote), gabapentin (Neurontin) and other AC’s modulate GABA

46. Glutamate • The most common excitatory neurotransmitter in the CNS. • Amino acid involved in excitotoxic injury, seizures, learning, memory, anxiety, depression, psychosis • Blockade of glutamate receptors may have a protective role for tissue at risk in acute stroke and for TBI. MK801 and PCP are NMDA antagonists and both cause psychotic symptoms • Riduzole and lamotrigine medication for ALS and epilepsy decrease glutamatergic transmission. Memanatine an NMDA antagonist is being tried for advanced AD

48. Glutamate Receptor Function • Glutamate, at NMDA receptors, leads to opening of an ion channel and influx of Ca and Na • The block of Mg is removed by activation of an AMPA receptor • Glycine must also bind to its receptor to allow Ca and Na influx • Some glutamate receptors are metabotropic and use 2nd messengers • Glutamate reuptake is tightly regulated

49. Feeling stuck? Check out the Neurotransmitter Table on p. 73

50. Questions 1. Why does cocaine chorea? a. It is a dopamine agonist b. It reduces GABA levels c. It enhances serotonin d. It increases endogenous dopamine 2. Which glutamate reactions are neurotoxic? a. Glutamate-ACh c. Glutamate-NMDA b. Glutamate-dopamine d. Glutamate-serotonin 3. Stimulation of which dopamine receptor(s) increases adenyl cyclase activity? a. D1 receptors c. Both D1 and D2 b. D2 receptors d. Neither