tamate Overview How can one neurotransmitter have so many diverse functions? Darryle Schoepp, Ph.D. Senior Vice President and Franchise Head, Neuroscience
Control of Excitability via Amino Acid Neurotransmitters HO 2 C CO 2 H NH 2 tamate HO 2 C GABA NH 2 Pathology Hyper-excitability GABA (mrs, uptake), Deact. /, Desen. / Mg 2+ block Physiological Function / GABA (mrs, KA-R, AMPA-R, uptake ), Glycine/D-serine Hypo-excitability Pathology 2
Conditions Involving Pathological Excitatory Synaptic Transmission Anxiety Disorders / Stress Psychosis / Schizophrenia Depression / Mania / Bipolar Drug Withdrawal / Abuse Neurodegeneration Epilepsy Pain Cognitive Dysfunction Other.. 3
Control of glutamate: most highly regulated amino acid neurotransmitter Cloned tamate Receptors and Transporters Ionotropic Receptors NMDA Type m Receptors NR1*, NR2A, NR2B, NR2C, Group I (Gq/PI Coupled) NR2D, δ1, δ2 m1 AMPA Type m5 i1*, i2*, i3*, i4* Group II (Gi/cAMP Coupled) Kainate Type m2 i5*, i6*, i7, KA1, KA2 m3 *Form homomeric channels Group III (Gi/cAMP Coupled) m4 tamate Transporters m6 EAAT1 (GLAST) m7 EAAT2 (GLT1) m8 EAAT3 (EAAC1) EAAT4 EAAT5 Glycine / D-Serine / Cystine- Transporters GlyT1 / ASC-1 / xc-
Presynaptic vt m4/8 m4/8 m2 m2 EAAT - - - m3 m3? + m5 m5 Glial and astrocytes m5 m1/5 m1/5 NMDA AMPA m4/8 EAAT Swanson C.J., Johnson M.P., Linden A.M., Monn J.A., and Schoepp D.D. Nature Drug Discovery 4: 131 146, 2005. m7 m7 Kainate m3 camp Ca 2 + camp Ca 2 + Na + Cl - m3 GABAa GABAa GABAb GABA Postsynaptic GABA - - GABA m7 Modulation of Excitation GABAb GABA -
tamate Receptors in CNS Plasticity CNS plasticity is involved in: ability to learn and remember capacity to reorganize, recover from injury Basic mechanisms underlying plasticity Neurogenesis Activity dependent refinement of synapses Sprouting of synapses Pruning of synapses (eg. Apoptosis) NMDA / AMPA receptor activation Basic mechanism supporting brain plasticity Has applications to treat cognitive disorders, depression, neurodegenerative disorders Direct and indirect ways to modulate these receptors 6
Fear Potentiated Startle Model Light Sound Limbic Cortex Fear Conditioning Amygdala Fear Expression (+) AMPA-R EPSP Brainstem Spinal Cord m2/3 Agonists D-cycloserine Spinal Cord Shock NMDA-R Plasticity (Learning) Fear Potentiated Startle Reaction
The problem: physiology versus pathophysiology tamate is the major excitatory neurotransmitter in the mamamalian central nervous system, involved in most physiological and many pathological processes. Important mediator of brain excitability and plasticity, but... How can we selectively modulate glutamatergic synaptic transmission in the CNS? Can we selectively target pathological processes involving the glutamate system?
A HIGH PROPENSITY FOR GLUTAMATE-INDUCED NEURONAL INJURY EXISTS IN IN THE CNS mm glutamate throughout the CNS µm glutamate is toxic to neurons Energy dependent uptake from the synapse is critical to prevent excitotoxicity Toxic levels of glutamate lead to profound depolarization and further glutamate release Toxic levels of glutamate leads to neuronal degeneration and thus further glutamate release High levels of glutamate receptors are found throughout the brain and spinal cord A Role for Excitotoxicity Beyond Acute Brain Injury?
ECF ~10μM (K i 2.8 μm) m2 m m Kainate m5 (-) GABA GABA A m1/5 (+) Synaptic tamate is Highly Regulated (-) (-) (+) NMDA-R (K i ~5μM) (~150mM) (-) m7 (~1-10mM) AMPA Kainate Na +, Ca ++ Excitation Plasticity m8 (K i ~1μM) m4 (K i ~0.3 μm) m3 Glial Cell (K i ~860μM) Transporters m5 m2/3 m4,7,8
Pre-Synaptic Post-Synaptic m 2 tamate Neuron (-) (-) ir EPSP Physiological Pathological Stimulations GABA B m 2 Homeostasis GABA Neuron (-) (-) GABA A IPSP GABA B 11
Circuits matter - PCP increases limbic excitation via disinhibition NMDA antagonists (PCP, ketamine) induce psychosis in normal human volunteers and exacerbate psychosis in schizophrenic patients; correlation with limbic hyperexcitability. Human / ketamine Other / Excitatory Input 5-HT tamatergic / Excitatory Input GABA PCP Rat / PCP Increased Limbic Output Lahti et al. Neuropsychopharmacol. 21, S158-S169 (1999). 12 Weissman A.D., Dam M., and London E.D. Brain Res. 435: 29-40 (1987)
Effects of LY-379268 and PCP on Hippocampal Serotonin (Pete Hutson MRL) Male SD rats were administered LY-379268 (3mg/kg, i.p.) 30min before PCP (5mg/kg, i.p.) and hippocampal dialysates were analyzed. LY-379268 markedly inhibited PCP induced elevations in 5-HT. Area under curve for 5-HT was significantly decreased (p<0.01, T-test) in LY-379268 (3mg/kg, i.p.) treated animals compared to saline treated controls. 13
tamate pathways link to many other neurotransmitters Hypothetical Common Molecular Network for Drug Addiction Citation: Li CY, Mao X, Wei L (2008) Genes and (common) pathways underlying drug addiction. PLoS Comput Biol 4(1): e2. doi:10.1371/journal.pcbi.0040002 14
Drugs Which Act Via Neurotransmitter Pharmacology Neurotransmitter Class System / Function Drugs Approved Amino Acids GABA tamate Biogenic amines Dopamine Norepinephrine 5-HT Histamine Cholinergic / purinergic Acetylcholine Adenosine / ATP Neuropeptides Substance P CGRP Orexins Enkephalins Fast onset Excitability/plasticity Widespread Multiple targets Modulatory Widespread Multiple targets Modulatory Widespread Multiple targets Perturbed homeostasis Dynamic regulation Selective distribution T argeted function Sedative hypnotics Benzodiazapenes Anticonvulsants Memantine Ketamine Antipsychotics Tricyclics, SSRIs, SNRIs Stimulants Triptans Acetylcholine esterase inhibitors Nicotine Patch Chantix Substance P / EMEND Opiates Explosion of new information on neurotransmitters in the CNS function is creating new opportunities # of Top 60 Drugs 6 / 60 14 / 60 2 / 60 1 / 60 Source: Verispan, VONA, Drugs.com 15
tamatergic Therapeutic Approaches: Targets Galore m2 Receptor Potentiators m3 Receptor Antagonists m5 Receptor Potentiators ( ) m2 (+) m4/8 m2/3 Receptor Agonists m7 m8 m5 m1/5 Receptor Antagonists m2/3 m3 AMPA Kainate NMDA m5 16 m4/8 m1 ( ) Na +,Ca ++ (+) m4 Receptor Potentiators AMPA Receptor Potentiators AMPA/Kainate Receptor Antagonists Kainate Receptor Antagonists NMDA Receptor Modulators Excitability Plasticity
Many glutamate targets to pursue but few shown to be effective and safe. Are you sure there s a Drug in here?
Summary: How can one neurotransmitter have so many diverse functions? Physiology versus pathophysiology: How we can we dissociate these? Excitotoxicity as a concept Is it relevant beyond acute brain injury? How to avoid / manage as on-target pharmacology? Control of glutamate: the most highly regulated amino acid neurotransmitter Feedback mechanisms may alter long-term effects of drugs? Plasticity as well as excitability - Roles in brain pathophysiology create opportunities for new therapeutics? Presynaptic versus post-synaptic and glial regulation Tools to study all these aspects of the synapse are important to develop. Direct and indirect effects on other many other neurotransmitters May be important in pathophysiology and for therapeutic and on-target side effects of glutamate targeted drugs. Circuits matter. Not that simple to decrease or increase excitation via single targets Important to consider for pathophysiology and therapeutics. Many glutamate drug targets out there but few shown yet to be effective and safe What have we learned and what can be done to increase probability of success. 18