Neurotransmitters, neuromodulators, neuropeptides

DA, NAD and AD synthesis

L-tyrosine (amino acid)
↓
L-DOPA (gains hydroxyl group)
↓
Dopamine (becomes decarboxylated) → stored into vesicles
↓
Noradrenaline (hydrolysed)
↓
Adrenaline (via PNMT)

5-HT synthesis

L-tryptophan (amino acid)
↓
5-hydroxyltryptophan (gains hydroxyl group)
↓
Serotonin (5-hydroxytryptamine, gets decarboxylated)

Monoamine storage

• Active transport into vesicles via VMATs
  → stored as bound complex, isn’t selective for types

• Prevents leakage into cytoplasm

• Driven by H⁺ electrochemical gradient generated by ATP H⁺ pump

Monoamine release

1. ‘Traditional’ release

   • Increasing excitability depolarizes, opens Ca²⁺ channels

   • Vesicles fuse to membrane and NTs released into synapse

2. Diffuse projection (5-HT and NAD)

   • Released from varicosities or buds along axons

   • Large amounts into extracellular space, diffuse along conc gradients

Monoamine reuptake

• Active transport into presynaptic cell for termination

• Via high affinity, Na⁺-dependent membrane transporter proteins

• Selective for type of monoamines i.e., SERT, NAT, DAT

Dopamine degradation

1. MAO_A and MAO_B

2. COMT

→ turns into homovanillic acid (HVA)

Noradrenaline degradation

1. MAO_A

2. COMT

→ turns into vanillylmandelic acid (VMA)

Monoamine functions and dysfunctions

Dopamine: hormone regulation, movement, reward, vomiting
→ mood disorders, schizophrenia, psychosis, drug abuse, movement disorders

Noradrenaline: arousal, attention, mood, autonomic function, analgesia
→ mood disorders, anxiety, autonomic dysfunction, pain

Serotonin: sleep, mood, feeding, sex, temp, BP, vomiting
→ mood disorders, anxiety, autonomic dysfunction, pain

Monoamines

1. Catecholamines (catecholamine ring)

   • Noradrenaline

   • Adrenaline

   • Dopamine

2. Indoleamines (indole ring)

   • Serotonin

   • Melatonin

SYNTHESIS: decarboxylated amino acids
CATALYST: cytosolic enzymes
ACTIVATION: GPCRs

Drugs targeting monoamine synthesis

• L-DOPA: increases synthesis, targets nigrostriatal pathway (can cross BBB, isn’t broken down in periphery)

• MDMA: inhibits 5-HT precursor to reduce synthesis

Drugs targeting monoamine storage and release

• Amphetamines: competes for VMAT, reverses vesicular uptake (dissipates H⁺ gradient), reverses transport (psychostimulant effects)

• Reserpine: disrupts H⁺ gradient to disrupt NAD storage, more in cytoplasm (mood and blood pressure)

Noradrenaline pathways

Locus coeruleus to:

• Broad cortical areas

• Amygdala

• Hypothalamus & thalamus

• Septum

• Nucleus solitarius and cerebellum

Serotonin pathways

Raphe nuclei to

• Broad cortical areas

• Amygdala & hippocampus

• Hypothalamus & thalamus

• Dorsal striatum

• Septum

• Cerebellum & spinal cord

Dopamine pathways

1. Mesolimbic: VTA to limbic regions (amygdala, hippocampus, NAC)

2. Mesocortical: VTA to prefrontal cortex

3. Nigrostriatal: Substantia nigra to basal ganglia (dorsal striatum)

4. Tuberoinfundibular: Hypothalamus to pituitary gland

Drugs targeting monoamine receptor action

• Bromocriptine (D₂): nigrostriatal, Parkinson’s disease

• Clozapine (D₂): mesolimbic/cortical, psychosis

• Clonidine (α₂): partial agonist for hypertension

• Mirtazapine (α₂): antagonist for depression

• Buspirone (5-HT_1A): agonist for anxiety

• Naratriptan (5-HT_1A,D,F): agonist for migraines

Drugs targeting monoamine transporters

• Cocaine: prevents DA reuptake, ‘salience’ (mesolimbic) pathway

• Amitriptyline: blocks NATs and SERTs, prevents reuptake

• Bupropion: blocks DATs and NATs, prevents reuptake

5-HT degradation

Only via MAO_B
→ turns into 5-hydroxyindole acetic acid (5-HIAA)

Drugs targeting monoamine degradation

• Moclobemide: inhibits MAO-A, increases cytoplasm conc and allows spontaneous leakage

• Selegeline: inhibits MAO-B, increases DA concentrations (Parkinson’s)

Neuropeptide

Category of large molecules that can act as NTs/NMs in the CNS and PNS

• For slower and longer durations than classical, small-molecule NTs

• Can also act as hormones in the blood stream at distant sites

• Long and linear amino acid chains joined by peptide bonds

Unique actions of neuropeptides as modulator/transmitter systems

• Different Ca²⁺ sensor location and sensitivity or release

• Released from synaptic cleft as well as dendrites, axons, soma

• Can act on auto-receptors, post-synaptic, or extra-synaptic

• Receptors have high affinity to respond to low agonist concentration

• No reuptake, broken down then resynthesised

• Stored in DCVs which are larger, slower, extrasynaptic, modulatory

Neuropeptide system vs glutamate neurotransmitter

SYNTHESIS

• NTs: Amino acid via enzyme

• NPs: Transcription & translation

STORAGE

• NTs: Small, clear vesicles, SCV

• NPs: Dense-core vesicles, DCV

RELEASE

• NTs: Only synaptic terminal

• NPs: Terminal, dendrites, axons, soma

ACTION

• NTs: One-on-one, fast

• NPs: GPCRs, high affinity, long distances

TERMINATION

• NTs: Reuptake via transporter

• NPs: Peptidase breakdown

Endogenous opioids function + methods

Mood, fear response, pain perception, GI function, stress responses, decision-making, attachment formation, drug addiction, reward
→ another layer of I/E to modify fast brain movement, different triggers

• Activating: agonists (lack for most NPs)

• Adjusting: peptidase inhibitors alters levels

• Blocking: antagonists, CRISPR, conditional knockouts

  • Agonists show system importance, but not opioid action
    e.g., codeine, morphine, heroin, fentanyl

GLP1R agonist

DEVELOPMENT

• Insulin vs glucagon → GLP-1 & glucagon come from same propeptide

• GLP-1 secreted from intestinal cells and brainstem (NTS and AP)

• Is a G_s GPCR (acts on adenyl cyclase), broken down ~2 mins

SOLUTION

• Range of chemical additions to reduce its breakdown = big peptide

• Targets NTS that responds blood hormones (appetite reduction)

• AP targeting also causes nausea (chemo trigger zone)

FUTURE

• Reduced cardiac events, stroke, possibly anti-inflammatory, lowers interest in addictive substances

• Development of other GIP & GLP1 agonists, glucagon receptors

  • Drugs to inhibit response and maintain weight loss after stopping

Endogenous opioid synthesis

From propeptides to neuropeptide family

1. Proopiomelanocortin (POMC) → β-endorphins

2. Proenkephalin → Enkephalins

3. Prodynorphin → Dynorphin A, B

Prepropeptides vs propeptides

↓ DNA transcribed into mRNA for ribosome template

Prepropeptide: largest precursor protein in synthesis
(signal peptide @ N-terminus + propeptide)

↓ signal peptide cleaved off ER

Propeptide: smaller precursor, contains 1+ copies of mature neuropeptides

↓ further cleavage via proteases in golgi apparatus

Biologically active neuropeptide

Endogenous opioid binding and release

• Sub-nanomolar (very high) receptor affinity (different from other NMs)

• Action in broad areas and on multiple receptors

  • δ, μ, κ receptors

  • All G_i/G_o receptors

• Receptors do the same thing, effects differ by system location

  • Circumscribed effects, only limited by location of release

  • Vs exogenous opioids acting everywhere

Endogenous opioid degradation and cellular effects

• Several peptidases involved

  • Enkephalins are very short = very susceptible to peptidases (chopped up rapidly)

• Actions increased by peptidase inhibition

Neuropeptides as drug targets

• Peptides are poor drugs as they’re broken down and can’t diffuse BBB

  • Develop smaller molecules with similar effects
    i.e., CRF, vasopressin, neurotensin, techykinin

• Clinically, antagonists can reduce alcohol abuse, gambling, constipation

• Can’t use opioid peptides as agonists