Amino Acid Neurotransmitters

Paul Glue

Objectives

• Review:– Relative abundance of AAs vs monoamines– Pharmacology of glutamate, GABA– Postulated role of glutamate, GABA

dysfunction in neuropsych disorders

Relative abundance of neurotransmitters

• Glutamate ~60% of synapses

• GABA ~30% of synapses

• Monoamines, peptides, other AAs (e.g. glycine) <5%

Glutamate Pharmacology• 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.

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The Glutamate Synapse

Note – significant Glu uptake (mainly

astrocytes)

Interconversion of glutamate to glutamine

Glutamate Function• 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.

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NMDA-ReceptorsStructure - tetramers of two NR1 subunits and two NR2 subunits

(some brain areas have NR3 subunits).

Binding sites on the extracellular domain: NR1: co-agonist 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

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.

Ketamineand

- Zn2+ blocks the NMDA current in a non-competitive and voltage-independent manner. - The activity of NMDA receptors is also sensitive to the changes in H+ concentration, and is partially inhibited by the ambient concentration of H+ under physiological conditions.

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

mGluR1 mGluR2

mGluR3

mGluR5

mGluR4

mGluR subtype mRNA distribution

in rat brain

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

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

Dopamine

Clinical Trial:LY2140023:(mGluR 2/3agonist inacute SCZ) (Nat Med 2007)

LY=OLZ, >pbo for:PANSS, CGI

LY=pbo, >OLZ for:Weight, PRL

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)

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Single Dose Ketamine Single Dose Ketamine Infusion Studies (1)Infusion Studies (1)

•Diazgranados; Arch Gen Psych 2010

•Treatment refractory bipolar depression, unmedicated

•Randomized, double blind, 2 period crossover

•Ketamine (0.5mg/kg) or placebo via 40 minute IV

infusions

•Assessments to 14 days

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Percent of responders (>50% ↓HAMD) Percent in remission (HAMD <7)

•Main side effects of ketamine: Perceptual disturbances and dizziness; confusion; elevated blood pressure; euphoria; increased libido

•Generally occurred in 1st 20min of infusion.

• Zarate, Arch Gen Psych 2006• Treatment resistant MDD, unmedicated• Single 0.5mg/kg IV infusion; placebo controlled, crossover design

Single Dose Ketamine Infusion Studies (2)

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

GABA• Inhibitory amino acid neurotransmitter; both pre- and post-

synaptic 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

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)

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Resting state plus GABA plus GABA and BDZ

GABA

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BDZ

video

Benzodiazepine pharmacology

Partial Partial Inverse InverseAgonists Agonists Antagonists Agonists Agonists

Anxiolytic Neutral/ AnxiogenicAnticonvulsant no effect ConvulsantAmnestic Promnestic

Sedating Arousing

Diazepam Abecarnil Flumazenil FG7142LorazepamBretazenil DMCM

Clonazepam(all BDZs and

Z-drugs in clinical use)

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

Agonists Antagonists Inverse -anxiolytic -neutral/no effect Agonists

-diazepam, etc -flumazenil -anxiogenicNormal

Panic Disorder

Agonists are Antagonistsless sedating are anxiogenic

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