Amino Acid Neurotransmitters. Paul Glue

Transcription

1 Amino Acid Neurotransmitters Paul Glue

2 Objectives Review: Relative abundance of AAs vs monoamines Pharmacology of glutamate, GABA Postulated role of glutamate, GABA dysfunction in neuropsych disorders

3 Relative abundance of neurotransmitters Glutamate ~60% of synapses GABA ~30% of synapses Monoamines, peptides, other AAs (e.g. glycine) <5%

4 GABA Inhibitory amino acid neurotransmitter; both pre- and postsynaptic receptors 2 receptors GABA-A ion channel GABA-B G-protein coupled receptor (heterodimer) Besides CNS, GABA also found in liver, GI tract, uterus, ovary, testis, lung, etc

5 BDZs bind to the GABA-A Receptor -Ligand-gated receptor complex -Made up of 5 helical columns surrounding a chloride channel -Separate binding sites for GABA, GABA agonists/ antagonists benzodiazepines barbiturates ethanol neurosteroids (pregnanolone) convulsants (picrotoxin, PTZ) video Resting state plus GABA plus GABA and BDZ outside GABA GABA BDZ Cell membrane inside Cl - Cl - Cl - Cl - Cl - Cl Cl - Cl - - Cl - Cl - Cl -

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7 Benzodiazepine pharmacology Partial Partial Inverse Inverse Agonists Agonists Antagonists Agonists Agonists Anxiolytic Neutral/ Anxiogenic Anticonvulsant no effect Convulsant Amnestic Promnestic Sedating Arousing Diazepam Abecarnil Flumazenil FG7142 Lorazepam Bretazenil DMCM Clonazepam (all BDZs and Z-drugs in clinical use)

8 Pharmacological theories of Anxiety (1) - GABA theories Observations: positive modulators of GABA-A receptor are anxiolytic (BDZs; barbiturates; ethanol) negative modulators are anxiogenic (FG7142; metrazol) in normals flumazenil (BDZ antagonist) is anxiogenic in panic disorder but not in healthy controls; BDZs are less sedating/impairing in anxious patients than in controls Normal Agonists Antagonists Inverse -anxiolytic -neutral/no effect Agonists -diazepam, etc -flumazenil -anxiogenic Panic Disorder Agonists are less sedating Antagonists are anxiogenic

9 Pharmacological theories of Anxiety (1) - GABA theories Observations (cont d): Altered GABA-A PET binding in panic disorder 15-BDZ naïve, drug free patients with panic disorder and 18 controls Statistical parametric map illustrating an area where benzodiazepine receptor binding (11C-flumazenil) was decreased in subjects with panic disorder vs control subjects (R dorsal anterolateral prefrontal cortex). Arch Gen Psych 2008:1166

10 GABA-A subtype-selective benzodiazepines - GABA-A receptor subtypes: most common type in the brain is a pentamer comprising 2 α's, 2 β's, and 1 γ (α2β2γ). Available BDZs are nonselective agonists. - Selective agonists for: > α1 subtype produce sedation and dependence > α2 and α3 are anxiolytic > α5 affect cognition and memory - MK-0343: α2/α3 partial agonist - reduced effects on alertness, memory and postural stability in healthy volunteers vs lorazepam - SL651498: full agonist at α2/3 subunits; partial agonist at α1 and α5 subunits > no reports of testing in anxious patients will they work??

11 Glutamate Pharmacology COOH Glutamate is one of the most common transmitters in the CNS Fast, excitatory transmitter; receptors on almost all neurons. Transmitter in ~60% of neurons, esp cortex, limbic structures. Glutamate binds to 4 classes of receptor three "ionotropic" receptor classes - ligand-gated ion channels which are characterized by the different ligands that bind to them: AMPA kainic acid N-methyl-D-aspartate or NMDA one G-protein coupled or "metabotropic" receptor class. H 2 N COOH

12 The Glutamate Synapse Interconversion of glutamate to glutamine Note significant Glu uptake (mainly astrocytes)

13 Glutamate Function COOH Under physiological conditions, activation of ionotropic receptors in neurons initiates transient depolarization and excitation. AMPA-Rs mediate the fast component of excitatory postsynaptic currents NMDA-Rs underlie a slower component. AMPA-Rs modulate Ca++ influx thru NMDA-Rs. Depolarization of the postsynaptic neuronal membrane via AMPA-Rs relieves the Mg++ block of the NMDA-R ion channel (this occurs in NMDA-R under resting conditions). This allows controlled Ca++ influx through the NMDA-R. This voltage-dependent modulation of the NMDA-R results in activity-driven synaptic modulation. Glutamate overactivity can lead to neuronal death due to Ca++ toxicity, other associated mechanisms. H 2 N COOH

14 NMDA-Receptors Structure - tetramers of two NR1 subunits and two NR2 subunits (some brain areas have NR3 subunits). Binding sites on the extracellular domain: NR1: coagonist glycine; NR2: glutamate. For efficient ion channel opening, the NMDA receptor requires both glutamate and the co-agonist glycine. Binding sites in the ion channel: Mg2+; PCP/ketamine site NMDA antagonists: Synthetic antagonists include: MK-801 (dizocilpine) Phencyclidine Ketamine Dextromethorphan Memantine, Amantadine Procyclidine Ketamine and NMDA modulators: Mg2+ blocks the NMDA channel in a voltage-dependent manner. - Na+, K+ and Ca2+ not only pass through the NMDA receptor channel but also modulate the activity of NMDA receptors. - Zn2+ blocks the NMDA current in a non-competitive and voltage-independent manner.

15 Metabotropic glutamate receptors Metabotropic receptors are coupled to their associated ion channel through a second messenger pathway. May be located pre-, post- or extra-synaptically Glutamate binding activates a G-protein and initiates an intracellular cascade There are 8 cloned mglurs (mglur1-mglur8) classified into three groups (I, II, and III) based on structural homology, agonist selectivity, and associated second messenger cascade Group I mglurs (mglur1 and mglur5) are coupled to the hydrolysis of fatty acids and the release of calcium from internal stores. Quisqualate and trans-acpd are Group I agonists. Group II (mglur2 and mglur3) and Group III (mglurs 4, 6, 7, and 8) receptors are considered inhibitory because they are coupled to the downregulation of cyclic nucleotide synthesis Appear to have neuroprotective effects in animal models

16 mglur1 mglur2 mglur3 mglur4 mglur5 mglur subtype mrna distribution in rat brain

17 Glutamate hypothesis of schizophrenia (1) Is DA antagonism alone enough for an effective antipsychotic agent? DA antagonism has limited effects on negative symptoms DA antagonists take several weeks to show clinical antipsychotic activity; other pharmacological effects ( PRL, EPSE) much more rapid. NMDA receptor antagonists (ketamine, PCP) are psychotogenic in normal individuals and schizophrenic patients; positive in animal models indicative of psychotogenic potential. Potency of antagonism correlates with ability to produce behavioral/ psychotogenic effects

18 Glutamate hypothesis of schizophrenia (2) Glutamate may have a significant role in the control of dopamine transmission in the striatum. Dopamine transmission occurs in two different temporal modes, phasic and tonic. Phasic DA release is transient and rapidly terminated, and selectively affects only receptors within or near the synapse. Phasic transmission is primarily dopamine dependent. Tonic release of dopamine results in a constant level of dopamine in the extracellular, extrasynaptic space and is regulated mainly by glutamate. Not all GluRs are realistic targets Ionotropic GluRs mediate most fast synaptic transmission in the CNS - too ubiquitous Excess Glu is neurotoxic; NMDA antagonism is psychotogenic Metabotropic glutamate receptors may be better targets These modulate synaptic neurotransmission mglur2 and 3 are primarily distributed in forebrain regions. Stimulation of these mediates presynaptic depression and decreases evoked release of glutamate. PCP and other NMDA antagonists increase glutamate efflux; this may increase DA activity (amongst others) Reduction of presynaptic glutamatergic activity by targeting group II mglurs may be a novel approach to treating schizophrenia

19 Clinical Trial: LY : (mglur 2/3 agonist in acute SCZ) (Nat Med 2007) Dopamine LY=OLZ, >pbo for: PANSS, CGI LY=pbo, >OLZ for: Weight, PRL

20 Glutamate Antagonists in Major Depression Rationale: NMDA-antagonists are effective in animal models of depression Elevated glutamate levels in occipital cortex of depressed patients Chronic antidepressants may work indirectly on NMDA systems (altered subunit transcription, binding density) Inhibitors of glutamate release (lamotrigine, riluzole) have antidepressant properties Clinical studies using single dose ketamine infusions Placebo-controlled, crossover, double-blind May work through effects on mtor (promotes synapse development) (Science 2010)

21 MADRS Responders (%) MADRS score Single Dose Ketamine Infusion Studies (1) Diazgranados; Arch Gen Psych Treatment refractory bipolar depression, unmedicated 0 Ketamine Placebo Randomized, double blind, 2 period crossover 80 Minutes Days Time after infusion Ketamine (0.5mg/kg) or placebo via 40 minute IV infusions Ketamine Placebo Assessments to 14 days Minutes Days Time after infusion

22 Single Dose Ketamine Infusion Studies (2) Zarate, Arch Gen Psych 2006 Treatment resistant MDD, unmedicated Single 0.5mg/kg IV infusion; placebo controlled, crossover design (%) Percent of responders (>50% HAMD) Percent in remission (HAMD <7) (%) 50 Ketamine Placebo mins hours (time) mins hours (time) Main side effects of ketamine: Perceptual disturbances and dizziness; confusion; elevated blood pressure; euphoria; increased libido Generally occurred in 1 st 20min of infusion.

23 mtor: mammalian target of rapamycin Serine/threonine protein kinase Regulates neuronal protein synthesis via enhancement of activity of particular components of protein synthesis machinery Key mechanism in learning and memory (LTP), neuronal development, etc

24 Glutamate and other disorders All effective mood stabilizers influence glutamate signalling (generally ) Li Glu transport; LTG Glu release, etc Excess Glu signalling in alcohol withdrawal, epilepsy,? anxiety