User:Kill Aids

I used to have a pretty intense relationship with amphetamine type stimulants and while that phase has passed, my fascination for pharmacology stuck.

Work in Progress. I will add some more findings and sources. Discord: bubatfresh

= Amphetamine Pharmacodynamics/Targets - Monoamine Releasing Agents and Activity Enhancers = Amphetamines are widely known for their euphoric and supposed cognition-enhancing effects yet, it’s surprisingly hard to find accurate and complete information on the targets and mechanisms by which they exert these. For an in silico monoamine releaser design project of mine, I had to dig through quite some literature and thought a little writeup might be useful.

WIP NAc & Mesocortical neurotransmission

Amphetamine-induced monoamine release must be differentiated into functional and non-functional increases in efflux. Non-functional increase is transporter-mediated and takes place independent of neuronal firing, while functional increase is an amplification of transmitter vesicle exocytosis in response to neuronal firing.

Mechanisms of functional increase in monoamine efflux

 * TAAR1 agonism biased toward Gαs G-protein activation leads to increased PKA pathway activation and counteracts the Gαi-mediated autoreceptor feedback of presynaptic D2sh, 5-HT1a and α2 Adrenergic receptors.
 * α2 Adrenergic autoreceptor antagonism directly inhibits negative feedback increasing PKA activity and monoamine biosynthesis

Intracellular TAAR1 is abundant in all types of monoaminergic neurons and the activation bias towards one or another is dependent upon the drugs corresponding monoamine transporter accessibility. α2 adrenoceptors are mainly localized presynaptically on noradrenergic neurons as well as dopaminergic, serotoninergic and glutamatergic neurons.

Protein Kinase A (PKA) then phosphorylates several membrane proteins on the Endoplasmic reticulum (ER) and cell membrane:


 * Phosphorylation of L- and N-type voltage-gated calcium channels increases their likelihood and duration of opening in response to action potentials
 * Phosphorylation of ryanodine receptors (RYR3) on the ER amplifies calcium-induced calcium release from the ER
 * Phosphorylation of the SERCA Calcium pump on the ER speeds up reuptake of Calcium into the ER

The increased cytosolic Calcium concentration in response to action potentials enhances calcium-mediated vesicle exocytosis.

PKA activation also induces tyrosine hydroxylase and aromatic L-amino acid decarboxylase thus increasing the biosynthesis and availability of serotonin, dopamine and noradrenaline.

Mechanisms of non-functional increase in monoamine efflux

 * Agonism of axonal α1B Adrenergic receptors on dopaminergic neurons activates the Protein Kinase C (PKC) pathway via Gαq G-protein subunits
 * Agonism of presynaptic 5-HT2b receptors on serotonergic neurons activates PKC via Gαq
 * Activated Gβγ G-protein subunits bind to monoamine transporters

As part of PKC pathway activation, IP3 receptors on the ER are activated and release additional Calcium to the Cytosol. The general increase in cytosolic Calcium activates Calmodulin and Calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII and PKC-dependent phosphorylation  of monoamine transporters in combination with Gβγ subunit binding leads to:


 * Reversal of transport direction
 * Transporters adopting an open conformation allowing for passive channeling of monoamines along their diffusion gradient
 * As well as Transporter internalization -> non-competitive reuptake inhibition

For efflux through membrane transporters, monoamines must be readily available in the cytosol and thus released from the synaptic vesicles. After entering the cell, amphetamines act on vesicular monoamine transporter 2 (VMAT2) by:


 * Competitive inhibition
 * Reversal of VMAT2 transport direction.

Vesicular monoamine transporters are antiporters operating on a pH gradient. Under normal conditions, they exchange vesicular protons for cytosolic transmitter molecules. Amphetamines likely offset the pH gradient by diffusing through the vesicle membrane in uncharged form and then protonating in the more acidic vesicular milieu using up protons. (weak base hypothesis)

Reserpine, a VMAT2 inhibitor, enhances amphetamine-induced stereotypies, probably by reducing functional dopamine efflux through reduced vesicular transmitter concentration. Interestingly TAAR1-KO mice show increased locomotor response to amphetamine, which indicates a lack of increase in functional relative to non-functional monoamine efflux.

Further modulatory effects [WIP]
TAAR1 coupling with K+ channels in plasma membrane -> hyperpolarization

++heterodimerization with D2sh effect on phasic signaling somewhat like MPH increasing D2sh autoreceptor feedback? 5-HT1a

Another Target that might play a role is the sigma 1 (σ1) receptor


 * Antagonism of σ1 receptors

σ1 is localized on the ER and antagonized by amphetamines with micromolar affinity. Agonized σ1 inhibits RYR3 and IP3 Receptors and antagonism by amphetamines could possibly counteract such an effect by endogenous agonists. σ1 agonism has been shown to modulate methamphetamines effect on dopaminergic neurotransmission and to prevent its excitotoxicity. This is likeley because RYR3 and IP3 inhibition prevents calcium depletion of the ER, which in turn would activate further calcium influx from the extracellular fluid, through STIM1-OARI1 activation, leading to overexcitation.
 * Antagonism of σ1 receptors