Pharmacology Unit 4 lecture 4.pdf

GABAergic & Glutamatergic Neurotransmission & GABAergic and Glutamatergic Drugs GABA – Loca on in the brain GABA (always inhibitory) is widely and uniformly distributed throughout the brain (all over) This is in contrast to most other neurotransmi ers which have a localised, discrete distribu on (e....

GABAergic & Glutamatergic Neurotransmission & GABAergic and Glutamatergic Drugs GABA – Loca on in the brain GABA (always inhibitory) is widely and uniformly distributed throughout the brain (all over) This is in contrast to most other neurotransmi ers which have a localised, discrete distribu on (e.g. acetylcholine, noradrenaline, dopamine and serotonin) Synthesis (by glutamate), Storage, release, termina on, metabolism 1. Glutamate is taken into the neuron via carrier mediated transport 2. Glutamate is decarboxylated to GABA by glutamic acid decarboxylase 3. GABA is ac vely packaged into vesicles by an specific transporter 4. Release is via classical Ca2+-mediated exocytosis 5. Termina on is via uptake by a GABA transporter 6. Degrada on is via GABA transaminase Receptor Targets GABA exerts its effects via 2 main subtypes of receptor: - GABAA and GABAB receptors Main GABA receptor subtypes GABAA Mechanism of ac on Ligand-gated ion channel Subunit composi on Pentamer α, β, and γ subunits (3-6 subtypes of each subunit) GABAB Gi protein via AC and decreased cAMP Dimer Receptor Targets – The GABAA receptor (nb drug target) - Permeability - permeable to Cl- (cl- into cell, cell already nega ve – hyperpolarisa on – harder for cell to be ac vated by another NT - inhibits) - GABA binding site - binding of GABA & agonists/antagonists - Benzodiazepine binding site - benzodiazepines bind here and enhance the ac ons of GABA - Barbiturate binding site - barbiturates bind here and enhance the ac ons of GABA - Neurosteroid binding site - neurosteroids bind here and enhance the ac ons of GABA - Picrotoxin binding site - picrotoxin binds here and blocks Clchannel Physiological response - At the cellular level GABA is the main inhibitory transmi er in the brain Receptor Cellular loca on Response GABAA Postsynap c Fast postsynap c inhibi on GABAB Pre and postsynap c Mechanism of ac on Channel is selec vely permeable to ClIncreasing Cl- permeability hyperpolarises the cell, thereby reducing its excitability Pre and postsynap c inhibi on Inhibits voltage-gated Ca2+ channels (inhibits transmi er release) Opens K+ channels (reduces postsynap c excitability) Physiological response - At the behavioural level GABA is required for: - General CNS depression/inhibi on - Regulates/modulates the ac vity of other neurotransmi er systems Glutamate - Loca on in the brain Glutamate (opposite to GABA - excitatory) is widely and uniformly distributed throughout the brain This is in contrast to most other neurotransmi ers which have a localised, discrete distribu on (e.g. acetylcholine, noradrenaline, dopamine and serotonin) Synthesis (from glutamine), storage, release, termina on, metabolism 1. Glutamine is taken into the neuron via carrier mediated transport 2. Glutamine is converted to glutamate by glutaminase 3. Glutamate is ac vely packaged into vesicles by an specific transporter 4. Release is via classical Ca2+-mediated exocytosis 5. Termina on is via uptake by a glutamate transporter 6. Degrada on is via glutamine synthase Receptor Targets Glutamate exerts its effects via 4 main subtypes of receptor: - NMDA, AMPA, kainate and metabotropic receptors Main glutamate receptor subtypes NMDA AMPA Kainate Mechanism of Ligand-gated ion Ligand-gated ion Ligand-gated ion ac on channel channel channel Subunit Pentamer NR1 and Pentamer GluR1-4 Pentamer GluR5-7 & composi on NR2 subunits subunits KA1-2 subunits NMDA: N-methyl D-aspartate; AMPA: α-amino-3-hydroxy-5-hydroxy-5-methyl-4-isoxazoleproprionate Receptor Targets - The NMDA receptor The NMDA receptor: - Permeability - permeable to Na+, Ca2+ and K+ (lets in ca2+ - most of others let in sodium – increased cytoplasmic calcium has a lot of effects on ca2+ dependent ac ons in cell) Receptor Targets - The NMDA receptor Facilitatory sites on the NDMA ( ghtly controlled) receptor: - Glutamate site - for binding of glutamate and specific agonists/antagonists - Glycine site - for binding of glycine (required for channel opening) (2 receptors have to be bound to it before it opens) - Polyamine side - for binding of polyamines (facilitates channel opening) Metabotropic Gq-protein coupled PLC & increased IP3/DAG/Ca2+ Receptor Targets - The NMDA receptor Inhibitory sites on the NDMA receptor: - Mg2+ site - channel is normally blocked by Mg2+ when the cell is normally polarised but is overcome when the cell is depolarised - Zn2+ site - binding of Zn2+ inhibits receptor opening - Channel blocking drug site - certain drugs (e.g. PCP selec vely block the channel) Physiological response - At the cellular level Glutamate is the main excitatory transmi er in the brain Receptor NMDA Cellular loca on Postsynap c AMPA Postsynap c Kainate Pre and post-synap c Metabotropic Pre and post-synap c EPSP: Excitatory post-synap c poten al Response Slow EPSP Important in: Synap c plas city Excitotoxicity Fast EPSP Fast EPSP Synap c modula on Physiological response - At the behavioural level Glutamate is required for: - General CNS excita on/ac va on - Regulates/modulates the ac vity of other neurotransmi er systems Pathophysiological role - Head injury and stroke - A major drug target is the development of glutamatergic antagonists to reduce excitotoxic brain damage following head injury and stroke - Epilepsy - Some an -epilep c drugs work by antagonizing glutamate receptors, specifically the AMPA subtype (e.g. perampanel) Epilepsy - Epilepsy is a neurological disorder characterised by seizures - Seizures are caused by episodic high-frequency discharge of a group of neurons in the brain o This usually starts locally, but then spreads to other areas of the brain o The symptoms depend on the region of the brain affected - It affects ~0.5-1% of the popula on - In most cases there is no recognisable cause, but it may occur: o A er brain damage (trauma, infec on, tumours) o In certain inherited neurological disorders Epilepsy – types of epilepsy Generalised seizures – affect whole brain (in par al seizures if seizure spreads to re nacular forma on (area of brain involved in consciousness) – loose conscious) Epilepsy - Tonic-clonic seizures - EEG - The tonic phase consists of: o An ini al strong contrac on of the whole musculature o Rigid extensor spasm o Respira on may stop o Defeca on, micturi on and saliva on may occur - The clonic phase consists of: o A series of violent synchronous jerks o Lasts about 2-4 min o Pa ent recovers consciousness feeling ill and confused Epilepsy - Absence seizures – EEG - In absence seizures, the pa ent abruptly stops whatever he or she was doing and stares vacantly for a few seconds (unconcious) - The pa ent is unaware of his or her surroundings and recovers abruptly with li le a er-effects - Absence seizures frequently occur in children - EEG pa ern reflects neural oscilla ons between thalamus and cortex and is due to T type calcium channels (Drugs to treat absence seizures target T type calcium channels) Epilepsy - Neurochemical basis underlying seizures - The neurochemical basis for epilep c seizures is poorly understood - They may result from: o Enhanced excitatory amino acid (glutamate) transmission o Reduced inhibitory amino acid (GABA) transmission o Abnormal electrical proper es of the affected cells (ion channels – Na+ involved in Aps, Ca2+ involved in voltage-gated channels - exocytosis) - Note: Repeated epilep c discharge can cause neuronal death through excitotoxic mechanisms Lennox-Gastaut syndrome is a par cularly severe form of epilepsy that affects children. It is associated with progressive mental retarda on which probably occurs as a result of neurodegenera on. (There’s so much epilep c/neuronal ac vity that causes large release of glutamate. Glutamate acts on NMDA receptor to let in calcium. Too much calcium – ac va on random proteases and lipases (breakdown proteins and lipids) – damage structural proteins of cell and lipid bilayers of cell – neurons die) An -epilep c drugs - An epilep c drugs are fully effec ve in trea ng seizures in 50-80% of pa ents - They are also called an -convulsant drugs - Long-established an -epilep c drugs include: o *phenytoin, *carbamazepine, *valproate, *ethosuximide (poor safety profile – side effects. Most widely used. Valproate can’t be given t pregnant women with epilepsy – foetal abnormali es) o Barbituates (e.g. phenobarbital (anaestheric)) o Benzodiazepines (e.g. diazepam, clonazepam, lorazepam) - Newer an -epilep c drugs include: vigabatrin, gabapen n, lamotrigine, felbamate, agabine, topiramate, leve racetam, zonisamide, pregabalin, re gabine, perampanel, lacosamide, s ripentol Pa ents with epilepsy may need to take drugs con nuously for many years *These four are the most important an -epilep c drugs in use! o - An -epilep c drugs - Mechanisms of ac on - The various an -epilep c drugs control abnormal discharge through different mechanisms - At least four dis nct mechanisms have been iden fied: o Enhancement of GABA ac on (GABA – inhibitory – stops abnormal firing) o Inhibi on of voltage-gated sodium channel func on (Involved in APs – inhibit abnormal firing) o Inhibi on of voltage-gated calcium channel func on (Inhibit – stops release of NTs) o Antagonism of glutamate receptors (Excitatory – block – inhibits neurons) - The aim of drug treatment is to prevent abnormal discharge whilst leaving normal discharge intact An -epilep c drugs - Mechanism of ac on - Some an -epilep c drugs work by enhancing GABAergic transmission by: o Posi ve allosteric modula on (binds at different site – not agonists) of the GABAA receptor (e.g. barbituates and benzodiazepines) o Inhibi on of GABA transaminase (enzyme that breaks down GABA) (e.g. vigabatrin) o Inhibi on of GABA uptake (e.g. agabine) (binds to GABA transporter – GABA cant bind – stays in synapse) - Thus, all of these drugs enhance the ac on of the INHIBITORY neurotransmi er GABA An -epilep c drugs - Mechanism of ac on of benzodiazepines - Benzodiazepines act by enhancing GABAergic transmission at the GABAA receptor o Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmi er in the brain o The GABAA receptor is a ligand-gated ion channel receptor o It is permeable to Cl- and thus opening of the channel hyperpolarises the neuron and reduces its excitability - Benzodiazepines bind to the GABAA receptor at a different site to GABA and increase the affinity of GABA for the receptor (see next slide) An -epilep c drugs – Mechanism of ac on - Some an -epilep c drugs work by inhibi ng voltage-dependent sodium channel func on thereby reducing neuronal membrane excitability (e.g. *phenytoin, *carbamazepine, *valproate, lamotrigine) (*voltage-dependent sodium channel blocker) - This prevent propaga on of ac on poten als - Their blocking ac on shows the phenomenon of use dependence (prefer to bind to Na+ channel in inac ve state) (i.e. they preferen ally block the excita on of neurons that are firing repe vely) - Use-dependence occurs because these an -epilep c drugs preferen ally bind to the inac vated state of the Na+ channel An -epilep c drugs – Mechanism of ac on - Some an -epilep c drugs work by inhibi ng T-type voltage-gated calcium channel func on that underpins absence seizures (e.g. *ethosuximide and *valproate) - Other an -epilep c drugs work by binding to a subunit of P/Qtype voltage-gated calcium channels thereby preven ng it from trafficking to the membrane. This reduces calcium dependent exocytosis of synap c vesicles. (e.g. gabapen n and pregabalin) - Some an -epilep c drugs work by antagonizing glutamate receptors, specifically the AMPA subtype (e.g. perampanel) Anxiety - Normal - The normal fear response to threatening s muli comprises of: o Defensive behaviours o Autonomic reflexes o Arousal & alertness o Cor costeroid secre on o Nega ve emo ons - In anxiety states, these reac ons occur in an an cipatory manner independent of external events - Anxiety becomes pathological when these symptoms interfere with normal func on Anxiety Pathological Clinically recognised anxiety disorders include: - Generalised anxiety disorder (ongoing state of anxiety with no clear reason) - Social anxiety disorder (fear of being/interac ng with other people) - Panic disorder (a acks of overwhelming fear in associa on with marked soma c symptoms – swea ng, tachycardia, chest pains, trembling, choking etc.) (symptoms associated with ac va on of SNS) - Obsessive compulsive disorder (compulsive ritualis c behaviour driven by irra onal anxiety) - Phobias (strong irra onal fears of specific things or situa ons) - Post-trauma c stress disorder (anxiety triggered by insistent recall of past stressful experiences) Anxiety - Types of anxioly c drugs 1) Benzodiazepines (e.g. diazepam (Valium®); alprazolam (Xanax®)) And 1) Some drugs used for depression (e.g. SSRIs such as fluoxe ne (Prozac®)) 2) 5-HT1A receptor agonists (e.g. buspirone) 3) Beta-adrenoceptor antagonists (i.e. beta-blockers such as propranolol) (par cularly for panic disorders – triggers SNS – block effects) 4) Some drugs used for epilepsy (e.g. gabapen n, pregabalin etc.) 5) Some drugs used for schizophrenia (e.g. olanzapine, risperidone etc.) Benzodiazepines The main benzodiazepines are: - Ultrashort dura on o *Midazolam o *Zolpidem (Ambien®) (not strictly a benzo, but similar MoA) - Short dura on o Lorazepam o Temazepam - Medium dura on o Alprazolam o Nitrazepam - Long dura on o Diazepam (Valium®) o Chlordiazepoxide (Shorter dura on – sleeping pills. Longer dura on – anxiety/epilepsy) Benzodiazepines - Pharmacological effects – wanted or unwanted depending … - Reduc on of anxiety and aggression o Benzodiazepines are useful for acute anxiety states, behavioural emergencies, certain medical, surgical and dental procedures. - Seda on and induc on of sleep o Benzodiazepines are useful for transient/acute causes of sleep disturbance such as jet lag, emo onal upset. They are only recommended for short dura ons as tolerance/dependence can occur, as well as rebound insomnia. - Reduc on of muscle tone and coordina on o Increased muscle tone is a common feature of anxiety states and can cause addi onal symptoms (e.g. headache). Benzodiazepines can reduce this. They are also used to relax muscle spasms (e.g. associated with “slipped” disc). - An convulsant effects o Benzodiazepines are useful for epilepsy including life-threatening status epilep cus (unbroken series of seizures). - Anterograde amnesia o Benzodiazepines prevent forma on of memories of events experienced while under their influence. Flunitrazepam (Rohypnol®; the “date-rape” drug … vic ms have poor memory of events)