EPILEPSY

1. NEUROTRANSMITTERS M.Prasad Naidu MSc Medical Biochemistry, Ph.D.Research Scholar

2. Definition : - The chemical substance helpful for signal transmission in central nervous system &peripheral nervous system (via) the chemical synapses is neurotransmitters. • Synaptic transmission is the predominant means by which neurons communicate with each other.

4. The criteria for Chemical neurotransmitter • 1) found in presynaptic axon terminal. • 2) enzymes necessary for synthesis are present in presynaptic neuron . • 3) stimulation under physiological conditions results in release. • 4) mechanism exist for rapid termination of action. • 5) direct application to postsynaptic terminal mimics the activation of nerve stimulus.

5. 6) drugs that modify metabolism of the neurotransmitter should have predictable physiological effects invivo assuming that the drug is transported to the site where neurotransmitter acts. • Not all neuron to neuron transmission is by neurotransmitters , gap junctions provides direct neuron to neuron electrical conduction.

6. Neurotransmitter is stored in synaptic vesicle released in response to nerve impulse & controled by calcium influx. • Release of neurotransmitter is quantal event , that is a nerve impulse reaching presynaptic terminal result in release of transmitter from a fixed number of synaptic vesicle. • Neurotransmitter action is terminated by metabolic degradation , reuptake , or diffusion into other cell types.

7. Class :- 1 acetylcholine • Class : -2 The biogenic amines norepinephrine , epinephrine dopamine , serotonin . • Class : - 3 amino acids gamma amino butyric acid (GABA) , glycine , glutamate , aspartate. • Class : - 4 nitric acid (NO) carbonmonoxide ( co )

8. In addition to classical neurotransmitters many neuropetides are identified as definite or probable neurotransmitters, eg : - substance p , neurotensin , enkephalin , β – endorphin , histamine, vasoactive intestinal polypeptide, cholecystokinin , neuropeptide Y & somatostatin.

9. Neurotransmitters modulate the function of post synaptic cells by binding to specific receptors of 2 types • 1) ionotropic receptors ( direct ion channels that open after binding of neurotransmitters. ) 2) metabotropic receptors ( interact with G proteins stimulating production of second messengers & activating protein kinases , which modulate the cellular events. )

10. G proteins couple several receptors to intra cellular signaling system , linking neuronal excitability to energy metabolism & second messenger systems. • G protein binding receptors include adenosine , Ach ( muscarnic ), norepinephrine , dopamine , serotonin

11. Kinetics of ionotropic receptors are fast , (< 1 ms ) , because neurotransmitters directly alter the electrical property of the postsynaptic cell. • Kinetics of metabotropic receptors functions over longer time periods. This contributes to the potential for selective & finely modulated signaling by neurotransmitters

12. The membrane of neuronal cell maintains an asymmetry of inside outside voltage , & is electrically excitable. • Neuronal membranes are polarized to a potential of -90 mV by the activity of Na+_k+ ATPase transport system.

13. Factors that control the neuroexcitability 1)voltage gated ion channels 2) neurotransmitter activated ion channels. 3)neuromodulators 4)second messenger system. • The control of neuronal activity within normal limits is by the modulation of excitatory & inhibotory events simultaneously.

14. Ligand gated channels are responsible for communication between cells. • Voltage gated sodium channels are involved in propagation of action potential , rapid activation is at -60mV due to opening of fast transient channels. Voltage gated potassium channels contribute to repolarization ,this regulate repeated firing of action potential by prolonging after spike repolarization.

15. Voltage dependent calcium channels trigger neurotransmitter release , at rapid activation is around -70mV. • Autoantibodies to ca++ channels in motor nerve terminal leads to decreased release of Ach from nerve terminal , this is seen in eaton lambert myasthenic syndrome. • Voltage gated channels determine how inhibitory & excitatory influences are integrated .

16. Acetyl choline • Acetyl choline is the neurotransmitter used by all motor axons that arise from spinal cord, that is at neuromuscular junction. • Junction consist of a single nerve terminal separated from post synaptic region by synaptic cleft. • Motor end plate is the specialized portion of the muscle membrane involved in the junction.

18. Junctional folds are prominent they contain high density of Ach receptors. • Synthesis of Ach takes place in cytosol of nerve terminal . choline acetyl transferase acetyl coA+ choline Ach + coA • Ach is incorporated into membrane bound particle called synaptic vesicles. • Assembly of synaptic vesicle with cell membrane resembles assembly of transport vesicle involving SNAREs.

19. Release of Ach into synaptic cleft occurs by exocytosis , which involves fusion of vesicle with presynaptic membrane. • Nerve ending is depolarized by transmission of nerve impulse this opens the voltage gated Ca++ channels , permitting influx of Ca++ from synaptic cleft to nerve terminal , this Ca++ plays a role in exocytosis of Ach vesicle.

20. Approximately 200 vesicles are released into synaptic space. • Each vesicle contains 10000 molecules of Ach. • Ach binds Ach receptor , receptor undergoes conformational change opening the channel in the receptor that allows entry of Na+, k+ resulting in depolarization of muscle membrane.

21. Properties of Ach receptor of NMJ : nicotinic receptor (nicotine is an agonist for the receptor) a membrane glycoprotein containing 5 subunits. ( 2αβγδ subunits). only α subunit binds Ach with high affinity. 2 molecules of Ach binds receptor to open the ion channel which permits Na+ , K+ the receptor is thus transmitter gated ion channel. autoantibodies to receptors are implicated in causation of myasthenia gravis

23. Snake venom α bungarotoxin binds tightly to the α subunit & can used to label the receptor . Formation of autoantibodies to Ach receptors in NMJ damage to receptors by autoantibodies reduction in number of receptors Episodic weekness of muscles supplied by cranial nerves

24. When the channel closes Ach dissociates & it is hydrolyzed by acetyl choline esterase. acetyl choline esterase Ach + H2O Acetate +choline • Choline is recycled into nerve terminal by active transport , it can be used for synthesis of Ach.

25. The classical neurotransmitter of autonomic ganglia whether sympathetic or parasympathic is acetyl choline. • 2 classes of receptors are present in autonomic nervous system. • 1) nicotinic eceptors , • 2) muscarnic recptors. • Nicotinic receptors in autonomic ganglia are different from those on skeletal muscle.

26. Nicotinc & muscarnic receptors mediate excitatory postsynaptic potentials (EPSP) , but these potential have different time course. • Stimulation of presynaptic neuron elicits a fast EPSP followed by a slow EPSP. • Fast EPSP results from activation of nicotinic receptors which cause of ion channels to open.

27. Slow EPSP is mediated by activation of muscarnic receptors that inhibit the M current , a current that is produced by K+ conductance. • Besides acetyl choline sympathetic preganglion neurons may release enkephalin , substance p , LHRH , neurotensin or somatostatin.

29. Neurotransmitter in parasympathetic postganglionic neurons is acetyl choline. • Actions are mediated by 3 types of muscarnic receptors. • 1) M1 receptor (neural ) produces slow excitation of ganglia. • 2) M2 receptor (cardiac) activation slows the heart. • 3) M3 receptor (glandular) , causing secretion, contraction of visceral smooth muscle , vascular relaxation.

30. Muscarnic Ach receptors act by way of inosine triphosphate system & they may also inhibit adenyl cyclase & thus decreasing cAMP synthesis. • Muscarnic recptors also open or close ion channels particularly K+ or Ca++ this action occurs through G proteins. • Muscarnic receptors relax smooth muscle by an effect on endothelial cells which produces nitric oxide (NO) .

31. Nitric oxide ( NO ) relaxes smooth muscles by stimulating guanylate cyclase & there by increasing levels of cGMP which in turn activates cGMP dependent protein kinases. • The number of muscarnic receptors are regulated & exposure to muscarnic agonist decreases the number of receptors by internalization of rceptor.

32. The betz cells of motor cortex uses acetyl choline as their neurotransmitter. • Acetyl choline probably acts as an imporatant neurotransmitter in basal ganglia which is involved in control of movements. • Deficits in cholinergic path way in the brain implicated in some form of Alzheimer's disease.

33. GABA major fast inhibitory neurotransmitter in the fore brain. 30% synapses of C.N.S contain GABA. • Glutamic acid dehydrogenase synthesizes GABA from glutamate in nerve terminal . • 3 types of receptors GABA a GABA b GABA c

34. GABA a & GABA c are ionotropic receptors & are post synaptic linked to chloride channel. • GABA b receptors are metabotropic may be pre or post synaptic & are coupled to ca+ or k+ ion channels via GTP proteins. • Presynaptic GABA b receptors serve autoreceptors to inhibit release from nerve terminal.

35. Binding of GABA leads to an opening of chloride channels & resultant hyperpolarization. • Glycine is inhibitory neurotransmitter in brain stem & spinal cord. • Post synaptic receptor for glycine is ligand gated chloride channel that allows influx of Cl- to hyperpolarize the postsynaptic neuron

37. Glutamate & aspartate are excitatory neurotransmitters. • Glutamate is responsible for 75% of excitatory neurotransmission in brain. • Synthesis of glutamate & aspartate within central neuron & glial cells is from carbohydrates involved in TCA cycle.

38. Mitochondrial enzyme aspartate transaminase interconverts glutamate & aspartate. • Glia contains glutamine synthase which converts glutamate to glutamine. • Glutamine is subsequently transferred to neuron where it is deaminated to glutamate by glutaminase.

39. Glial inactivation & specific uptake systems for glutamate reduces interstitial glutamate levels to terminate neurotransmitter action & prevent excitotoxic damage. • Monosodium glutamate produces migrainous head ache. • Excessive glutamate can result in neurotoxicity , celldeath & neurodegeration seen alzheimer’s disease.

41. The receptors are subdivided into 5 classes. • 1 )NMDA (N – methyl –D –aspartate ) • 2 )AMPA (α amino 3 hydroxy 5 methyl 4 isoxazole propionic acid ) • 3 )The kainate recptor ( isolated from sea weed) • 4 )L –AP 4 ( synthetic agonist ) • 5 )Metabotropic receptors. • First four receptors are cation channels .

42. Metebotropic receptors are linked to intracellular production of diacylglycerol,& inositol triphosphate by phosphoinositide path way. • NMDA is receptor is complex contains 5 distinct sites for binding 1 ) site for transmitter binding glutamate 2 ) a regulatory site that binds glycine. 3 ) a voltage dependent Mg++ binding site 4 ) a site that binds phencyclidine 5 ) a site that binds Zn++.

43. NMDA receptor opens when glutamate binds & allows influx of Ca++ & Na++ into the cell. • Mg++ , zn++ , poly amines , & steroids can also modulate NMDA. • one of the most important controls on the ionic conductance through the NMDA receptor is voltage sensitive blocking by Mg++

44. Activation of AMPA receptor channels may depolarize the neuron sufficiently to remove the voltage dependent Mg++ block & activate NMDA channels. • AMPA & NMDA are co activated & are present on the same part of the neuron.

46. A separate site that modulates the gating of NMDA channel binds polyamines such as spermine & spermidine which are synthesized by neurons.different concentration dependent effects have observed. • Endogenous Zn reduces NMDA activated current. • Zinc is present in high concentrations in the hippocampus & released with some neurotransmitter in nervous system.

47. Hydrogen ions also modulate the ion conductance which is maximal at slightly alkaline pH , & reduces with increasing acidity. • During hypoxic ischemic injury , progressive acidification resulting from glycolytic metabolism , may turn off the NMDA receptor channel.

48. AMPA receptor is coupled to both Na++ , & K ++ channels. it’s activation opens the above channels, depolarizes the neuronal cell rapidly, it is responsible for the majority of rapid excitatory neurotransmission. • Kainate receptor is also coupled to Na++ , k++ channels. • Kainate receptor has slower rate of depolarizing capacity than AMPA receptor.

49. Excitatory aminoacids are also able to interact with metabotropic recptors that activate the second messenger system, these receptors are found both pre & post synaptically. • Activation result in presynaptic inhibition & post synaptic excitation.

50. The spectrum of neurological disorders mediated by excitotoxicity include epilepsy, stroke , neurodegerative disorders ( parkinson’s disease , amyotropic lateral sclerosis , AIDS dementia ) • Most strokes are caused by thromboembolic events causing diminished perfusion resulting in reduction of supply of oxygen & glucose.