Psychopharm ACH, Catecholamines, Glu and GABA and 5HT

Acetylcholine synthesis

synthesized in pre-synaptic terminal from two precursors: choline and acetyl coenzyme

Synthesis choline acetyltransferase (ChAT):

• transfers an acetyl group (-COCH3) from acetyl CoA to choline - producing ACh

• Rate Limiting Step in synthesis – determines rate of ACh synthesis

ACh synthesis part 2

Packaged into synaptic vesicles by vesticular ACh transporter (VAChT)

Protects from degradation by terminal enzyme

Can be blocked by drug VESAMICOL

Released by Ca++ dependent mechanisms (pre-synaptic potential)

A number of animal and bacterial toxins influence ACh release

ACh deactivation

Metabolism and reuptake

ACh Metabolism

ACh is metabolized by acetylcholine (AChE)

• in pre-synaptic terminal and on postsynaptic membranes

• Breaks ACh down into choline and acetic acid

Drugs that block AChE

• they prevent Acetylcholine from metabolizing

• Reversible and irreversible

Reversible AChE inhibitors

Physostigmine (crosses BBB, is CNS poison), neostigmine (treatment for muscular disorders with lone ACh tone)

Irreversible AChE inhibitor

Sarin (nerve gas for chemical warfare, death) (some used as pesticides)

ACh Reuptake

Choline transporters shuttle choline from synapse back to terminal for reuse in the synthesis of ACh - necessary for quick removal of the main ACh precursor from synapse

• choline transport back into terminals is blocked by drug hemicholinium-3 (HC-3)

Organization/Function of the ACh system

ACh neurons are clustered within striatum (interneurons) and the septum and pons (projecting neurons)

ACh Interneurons

Dopamine terminals from the nigrostriatal path inhibit ACh interneurons in the striatum - DA control of ACh here it’s important for normal motor control

DA neurons are lost in Parkinson’s disease, so DA inhibition of ACh becomes compromised, SO ACh blocking drugs are useful

Projecting ACh neurons

Innervate MANY brain sites

ACh receptors

2 main subtypes: muscarinic and nicotinic

Nicotinic (nAchR)

Found on muscles, ganglia

• ionotropic receptors - ion channel for Na+ and Ca++, stimulatory

• Both pre and post synaptically

5 subunits

• two a that are the binding sites, BOTH must be occupied to open the channel

• B,Y,d, subunits, varied across receptors

Can be open, closed, or desensitized

• high/continuous ACh exposure causes desensitizion

• Not all ACh receptors desensitize

  • Depolarization blockade

  • Cell cannot become active until agonist is removed and membrane is depolarized

    • drugs take advantage and strong drug agonists (succinylcholine - surgical muscle relaxant, curare) creates diaphragm paralysis

Muscarinic (mAChR)

5 different subtypes, distributed throughout CNS

Metabotropic receptors - act through several 2nd messenger systems

• some activate IP3/DAG while others inhibit cAMP (so can be stimulatory or inhibitory)

In CNS: mAChR func is good for cognition (cortex, hippocampus), motor behavior (striatum), and drug reward (basal forebrain)

In PNS: mAChR in secretory organs, so activation increase salivation, sweat, tearing up.

• reason why psychiatric meds that block mAChR produce dry mouth

• Antagonists are used to reduce secretions (like animal surgery) and agonists (poisons) used to produce exaggerated tearing, etc

Catecholamines

Dopamine and norepinephrine

• synthesized in presynaptic terminal from the amino acid tyrosine

Catecholamine synthesis

Starts with amino acid Tyrosine

TYR converted to DOPA by Tyrosine Hydroxylase (TH)

DOPA converted to dopamine by Aromatic Amino Acid Decarboxylase

DA converted to NE by Dopamine Beta-Hydroxylase

Catecholamine Packaging and Release

• Packaged into small synaptic vesicles by Vesicular Monoamine Transporter (VMAT)

  • Protects catecholamines from degradation by enzymes in the pre-synaptic terminal

  • VMAT function is inhibited by the drug Reserpine

• Released primarily by Ca++ dependent mechanisms (during pre-synaptic potential)

  • BUT some drugs can directly stimulate release in absence of cell firing (ex. Amphetamine/methamphetimines - directly stimulate DA/NE release

• Release inhibited by auto-receptors on pre-synaptic terminals

  • Receptors are in presynaptic terminal membrane

  • If no Ca++ then no Ca++ dependan release of DA/NE

Catecholine Inactivation - Metabolism

• COMT pathway - transfers a methyl group to catechol ring

• MAO pathway - replacement of amine

Catecholine Inactivation - Reuptake

Reuptake of DA and NE into the synaptic terminal by DA and NE transporter proteins

Catecholamine reuptake influenced by

Psychoactive drugs:

• cocaine (block NE, DA, 5HT uptake)

• Tricyclic antidepressants (block NE, 5HT uptake)

• Atypical antidepressants

  • Fluoxetine (Prozac) blocks SERT’s

  • Roboxetine blocks NERT’s (ne transporters)

DA Neural System in Rodent Brain

A9 neurons in Substantial Nigra

• DA neurons project to striatum

• Implicated in MOTOR function

A10 neurons in Ventral Tegmental Area

• DA neurons project to forebrain, cortex, hippocampus

• Implicated in REWARD

DA Neural System in Human Brain

A9 Neurons in Substantial Nigra

• DA neurons project to putamen (striatum area involved in motor control)

• when the neurons die here it causes tremors (Parkinson’s)

A10 Neurons in VTA

Ventral Tegmental Area (back of brain stem)

• Da neurons that project to basal forebrain, cortex and hippocampus (memory) - project into amygdala

  • Via the mesolimbic and mesocortical pathways

• DA neurons here play roles in REWARD, MEMORY and MOOD/EMOTIONAL REGULATION

• As Parkinson’s progresses, they have problems with memory, mood and go Adonic (ahydonic?) - loss of ability to feel pleasure

• That’s why DA reuptake helps mood

DA Receptors (DARs)

Use cAMP metabotropic pathway (like G proteins)

5 subtypes (D1-5), all metabotropic

D1, D5 - linked to stimulatory Gsa G proteins- stimulatory bc activates cAMP and excitatory - produce neuronal stimulation

DA activates behavior

DA activates behavior

• D1/D2: agonists increase locomotion, self grooming

  • Antagonists: induce catalepsy (lack of spontaneous movement)

• Receptors are in different parts of the brain that do different things - DA activates behavior, DA blocking drugs will slow behavior

D1, D5 DARs

Gs couples, activate adenylate Cyclades and cAMP production

D2, D3, D4 DARs

Gi coupled receptors inhibit adenylate cycles and cAMP production

NE organization and function

A6 neurons in the Locus Coeruleus (LC)

• NE cells project to forebrain, cerebellum, spinal cord

• Only 3000 neurons total!

• Used for attention and vigilance

NE Receptor subtypes

2 NE receptor subtypes: a and b’s

• Both metabotropic

• A1, a2, b1, b2 - adenoreceptors

  • All function as postsynaptic receptors

  • Can be stimulatory or inhibitory

A2 heteroreceptors

like D2, inhibit NE bc keeping Ca++ channels closed

NE is CNS adrenaline component

• startle response - auto receptors help tone down the NE release so you do not have high NE tone (unhealthy, increases heart rate and blood pressure, decreases blood flow to internal organs, fight or flight)

  • A1 receptors help bring neurons down to a base

Beta Blockers (b1 b2 NE)

• reduce NE tone on heart, so heart rate settles

  • Used to calm down

  • Less anxiety (propranolol)

  • Orchestra players use it

Serotonin (5HT)

• Synthesized in pre-synaptic terminal from amino acid tryptophan

• Used in depression, anxiety, obesity, violence, drug addiction, and so on and so forth

5HT Synthesis

2 step process

• L-tryptophan (TRP): adds a hydroxyl, takes away a carboxyl

• TRP is converted to 5-HTP by tryptophan hydroxylase (TRPH)

• 5HTP is converted to 5HT by AADC (same as for DA/NE)

• When u heat up a dairy product (warm milk), releases lots of TRP, starting product for serotonin so helps you sleep

5HT packaging and release

• Into vesicles using vesicular monoamine transporter (VMAT)

• Can be blocked by reserpine

 

• Released primarily by Ca++ dependent mechanisms

• Release inhibited by 5HT autoreceptors

What packaging is 5HT similar to

Catecholine

GLU glutamate

• An ionized amino acid used for protein synthesis

• Acts as a small molecule neurotransmitter

GLU synthesis

Most is synthesized in presynaptic terminal of GLU neurons from precursor GLUTAMINE

Synthesis by enzyme glutaminase (removes amine group)

GLU packaging

Into small clear synaptic vesicles by VGLUT 1-3

• provides means for releasing predetermined amount

• Protects from degradation

VGLUT’s

• ONLY in neurons that use GLU as neurotransmitter

• Neurons either pisses VGLUT 1 or 2 but NOT both

• VGLUT 3 is less abundant in brain

Glutamate Inactivation Reuptake

Glutamate transporters (EAAT1-3)

1 and 2 found on astrocytes, EAAT3 on neurons

1 and 2 take up GLU into neighboring astrocytes

3 RETURNS GLU to synaptic terminal

Glutamate Inactivation - Metabolism

Glutamate synthetase breaks down GLU that’s in astrocytes, adds back amine group to GLU, prod. GLUTAMINE

Glutamate transporters - transport glutamine OUT of astrocytes and INTO neurons

BOTH mech. Necessary for RAPID removal of GLU neurotransmitter from synapse

Function of GLU system

GLU is primary fast acting excitatory neurotransmitter in brain

• main transmitter type in large pyramidal neurons

• Also in cerebellum (motor) and hippocampus (learning/memory)

GLU receptors

2 subtypes of GLU receptors, metabotropic and ionotropic

• metabotropic (mGluR) - less used in CNS. 8 subtypes coupled to cAMP or PI 2nd messenger systems. Involved in motor, coordination, learning/memory

• Ionotropic (iGluR) - more used in CNS. 3 subtypes, each with subunits. AMPA (Na+ channel, most active, depolarization to to Na+ influx), Kainate (Na+ channel) (depol), NMDA (Na+/Ca++ channel (depol). ALL EXCITATORY

GLU AMPA/Kainate receptor agonists (NBQX)

Produce sedation, reduced locomotion, ataxia (impaired motor coord)

NMDA receptors

Different from AMPA and Kainate

• 2 additional binding sites

  • Mg++ binding site in receptor channel

  • Mg++ ion normally blocks channel at rest, is always there

    • BUT! When gLU binds nearby AMPA receptors and produces an EPSP, the Mg++ dissociates and the NMDA channel opens to allow Na+Ca++ to enter

  • PCP binding site (non-competitive agonist)

    • ONLY active in presence of agonist

    • Not active under normal conditions of functioning

NMDA in memory/learning

Long term potentiation (LTP) - lasting increase in synaptic activity in post synaptic neuron

• observe in Schaffer collaterals in hippocampus

• If single stimulus: slight EPSP prod by AMPA receptors, NMDA does not open, Mg++ stays in place

• If successive trains of EPSPs: tetanus, prolonged strong burst of GLU activity

  • AMPA and NMDA receptors open (Mg++ block dissociates) so Ca++ enters cell

    • Activates 2nd messengers that enhance synaptic sensitivity, SO a subsequent identical stimulus yields an enhanced EPSP

Molecular Mechinisms of LTP — Ca++ does work

Lots of EPSPs activate protein kinases (phosphorylation of enzymes)

• increase sensitivity to GLU

• Increase prod and insertion of additional AMPA receptors in membrane e

This all enhances synaptic strength long-term

Dangers of Glutamate

• high levels can be toxic to neurons, prod lesions, Excitotoxicity

  • Due to overstimulation of NMDA receptors

  • Prolonged GLU receptor stimulation by high levels of GLU: NECROSIS (increase salts in cells)

Rapid: cell death due to lysis

Delayed: associated with lower GLU levels and exposure time

• osmotic swelling is temporary and cells appear to return to normal BUT cell death occurs over hours (gradual)

• Dependent upon NMDA activation so can be blocked by NMDA agonists

Eg: stroke victims - excessive GLU release in ischemic brain sites

GABA

Inhibitory neurons use GABA as a small molecule neurotransmitter - primary inhibitory amino acid neurotransmitter

GABA synthesis

Synthesized in presynaptic terminals of GABA neurons from the amino acid precursor glutamate, Synthesized by glutamic acid decarboxylase (removes a carboxyl group)

GABA packaging

Into clear synaptic vesicles by vesicular GABA transporter (VGAT)

• means for releasing pre-determined amount

• Protects from degradation

• VGATs only found in neurons that use gaba as a transmitter, neurons posses VGAT 1

GABA is released by Ca++ dependent mechanisms

GABA reuptake

GAT 1-3, 1-2 located on astrocytes and neurons, 3 only on astrocytes

GAT1 returns GABA to synaptic terminal

Ex: gabitrl blocks GAT1, increases synaptic GABA, its an antiseizure medication

GABA metabolism

2 mech: neuronal and astrocytic

Astrocytic: GABA transaminase followed by glutamate synthetase

Glutamate transporters transport glutamine OUT of astrocytes and INTO neurons

GABA receptors

2: GABA A (ionotropic) and GABA B (metabotropic)

GABA B receptors

Metabotropic, coupled to Gi proteins that inhibit cAMP production (cAMP normally closes K+ and opens Ca++ channels)

GABA A receptors

Ionotropic, open Cl- ion channels, prod inhibition

• 85% of of all GABA receptors in CNS

• Counter neuronal depol Cl- shunt

• Has 5 subunits

GABA A receptor allosteric agonists

Benzos, barbituates

Sedative-hypnotic and anxiolytic effects

GABA A receptor agonist (direct acting agents)

Muscimol, potent sedative hypnotic effects

BZS inverse agonists

Bind and reduce GABA binding, PRODUCE fear and anxiety

Ex DMCM

5HT

Implicated in almost everything

5HT synthesis, release and activation

Synthesized in pre-synaptic terminal from amino acid tryptophan

• synthesis starts with Amino acid L-Tryptophan (TRP)

• Converted to 5-HTP by tryptophan hydroxylase

• 5-HTP is converted to 5HT by AADC (same and DA/NE)

5HT packaging and release

Packaged into small classical synaptic vesicles by VMAT

• means for releasing predetermined amount

• Protects from degradation by terminal enzyme

  • Can be blocked by drug reserpine

Released primarily by Ca++ dependent mech, inhibited by 5HT auto-receptors (slowing cell firing)

5HT inactivation metabolism

Metabolism: enzymatic breakdown of serotonin in synapse

• monoamine oxidase (MAO) pathway - oxidization replaces amine

Creates 5-HIAA

5HT inactivation reuptake

Reuptake of 5HT into synaptic terminal by serotonin reuptake transporters (SERTs)

• reuptake transporter proteins in terminal membrane

  • Diff from auto-receptors

Return NT to synaptic terminal for metabolism or repackaging into vesicles

Necessary for rapid removal of transmitters

Influenced by psychoactive drugs: cocaine, SSRIs