GABA Receptors and Cognition: Perspectives for Drug Development S. J. Enna, Ph.D. Summer School of Neuroscience Catania, Italy July 11, 2015

γ -Aminobutyric Acid (GABA) -Widely Distributed Throughout the CNS % of All CNS Neurons Are GABAergic -Major Inhibitory Neurotransmitter

Principles of GABA Pharmacology -Attractive Target Because of Importance in CNS Function -Generalized Activation Decreases CNS Activity Sedation Hypnosis Cognitive Impairment -Generalized Inhibition Increases CNS Activity Anxiety Seizures -Selective Manipulation of Receptor Subtypes Are Essential for Therapeutic Benefit

GABA Receptors GABA A - Pentameric Ligand-Gated Ion Channel GABA B - G Protein-Coupled (G o /G i )

GABA Recognition SiteMuscimol, THIPBicuculline Benzodiazepine SiteDiazepam, ZolpidemFlumazenil (Anexate) Ion Channel SitePhenobarbitalPicrotoxin Cellular Response: Hyperpolarization (  Cl - conductance) Pharmacology:Agonists - Anxiolytics, Hypnotics Antagonists – Convulsants Structural Properties:Pentameric Ligand-Gated Ion Channel Eighteen Subunits α 1-6 ; β 1-3 ; γ 1-3 ; δ 1 ; ε 1 ; θ 1 ; ρ 1-3 α 2 β 2 γ 2 (60%) GABA A Receptor

Cellular Localization of GABA A Receptors From Rudolph, U. and Knoflach, F., Nature Reviews Drug Discovery, 2011

Ion Channel β β γ α α Benzodiazepine Site GABA Site Benzodiazepine-Sensitive GABA A Receptor

DEFINITIONS – Orthosteric Compound – Attaches to the receptor recognition site – Allosteric Compound – Attaches to a receptor component distinct from the recognition site

Allosteric vs. Orthosteric Drugs - Allosteric Effects are Saturable in Physiological Range o Act only in the proximity of released transmitter - Greater Tissue Selectivity o Act only in tissues where transmitter is active - Greater Receptor Subtype Selectivity o Variable effects on receptor affinity and efficacy

GABA B Receptor Adapted from Urwyler, S., Pharmacological Reviews, 2011 Orthosteric Site Allosteric Site

GABA B Receptor Pharmacology Representative Allosteric Compounds: CGP7930 CS39783 o Reduce the sedative/hypnotic response thresholds to baclofen without displaying effects themselves o When administered alone they display anxiolytic, but not antidepressant, effects in laboratory animals without causing sedation, muscle relaxation or cognitive dysfunction

Inverse Agonist From

Neurotransmitter and Enzyme Modifications in Alzheimer’s Disease From Enna, S.J., et al, Brain Research, 1976

Effect of RO on Scopolamine-induced Impairment of the DMTP Task From Ballard, T.M., et al, Psychopharmacology, 2009

RO Effect in Cynomolgus Macaques From Ballard, T.M., et al, Psychopharmacology, 2009 Data are presented as mean ± SEM (n=12) ** p < 0.01 vs. vehicle (V)

Subtype Selective GABA A Receptor Compounds From Rudolph, U. and Knoflach, F., Nature Reviews Drug Discovery, 2011

Effect of SGS742 in Step-Down Passive Avoidance in Mice *** p < 0.001: generalized two-tailed Wilcoxon test From Froestl. W., et al, Biochemical Pharmacology, 2004

Summary and Conclusions -Cognition is Subject to GABAergic Regulation -GABA Receptor Subtype-Selective Agents Needed to Enhance Cognition -Allosteric Modulators and Partial Agonists and Antagonists Are Required to Minimize Side Effects and Enhance Efficacy