Cognitive & Neuroprotection

ACh is synthesised from choline + acetyl-CoA by choline acetyltransferase (ChAT) and degraded by acetylcholinesterase (AChE). Two receptor types: nicotinic (ligand-gated ion channels — at neuromuscular junction and autonomic ganglia) and muscarinic (GPCRs — M1-M5, in brain, heart, smooth muscle, glands). At the neuromuscular junction (NMJ): ACh release from motor neuron → nicotinic receptor activation on muscle endplate → sodium influx → endplate potential → muscle contraction. In myasthenia gravis, autoantibodies target NMJ components (AChR antibodies in 85%, MuSK antibodies in 5-8%), impairing neuromuscular transmission → fluctuating muscle weakness. Zilucoplan inhibits complement C5, preventing MAC-mediated damage to the postsynaptic membrane that worsens AChR loss. In the brain, cholinergic neurons in the basal forebrain project to the cortex and hippocampus — their degeneration in Alzheimer's disease causes cognitive decline, forming the rationale for cholinesterase inhibitor therapy.

PNS regeneration succeeds because: Schwann cells dedifferentiate after injury → clear myelin debris (with macrophages) → form bands of Büngner (aligned cellular tubes guiding regrowing axons) → produce neurotrophic factors (NGF, BDNF, GDNF) → axonal regrowth at approximately 1mm/day. CNS regeneration fails because: oligodendrocytes express inhibitory molecules (Nogo-A, MAG, OMgp binding NgR1/LINGO-1 receptor complex), reactive astrocytes form glial scar containing chondroitin sulphate proteoglycans (CSPGs, inhibiting axon growth), inflammation is poorly regulated (persistent rather than resolving), and adult CNS neurons have reduced intrinsic growth capacity (low expression of regeneration-associated genes like GAP-43). Strategies to promote CNS regeneration: blocking inhibitory molecules (anti-Nogo antibodies — in clinical trials for spinal cord injury), degrading glial scar (chondroitinase ABC), enhancing intrinsic growth (activating mTOR pathway), and providing neurotrophic support. Peptide approaches to axonal regeneration remain primarily preclinical.

BBB structure: brain endothelial cells connected by tight junctions (claudins, occludins, ZO proteins — creating paracellular resistance ~1800 Ω·cm²), surrounded by pericytes (providing structural support and regulating endothelial function) and astrocyte end-feet (inducing and maintaining BBB properties). Transport across BBB: transcellular lipophilic pathway (small lipophilic molecules passively diffuse — rule: MW <400 Da and <8 hydrogen bonds for significant BBB penetration), carrier-mediated transport (glucose via GLUT1, amino acids via LAT1), receptor-mediated transcytosis (transferrin receptor, LRP1 — exploited for drug delivery), and adsorptive transcytosis (cationic molecules). Most peptides are excluded because they are too large, too hydrophilic, and lack specific transporters. BBB drug delivery strategies for peptides: intranasal delivery (olfactory/trigeminal nerve pathways), receptor-targeting conjugates (transferrin receptor-binding peptides), cell-penetrating peptide conjugates, focused ultrasound BBB disruption, and nanoparticle encapsulation. Difelikefalin was deliberately designed with properties preventing BBB penetration (D-amino acid, high polarity) to restrict effects to the periphery.

BDNF is synthesised as a precursor (proBDNF, 32 kDa) that is cleaved to mature BDNF (14 kDa) by furin/proconvertases intracellularly or by plasmin/MMP-9 extracellularly. Crucially, proBDNF and mature BDNF have opposing effects: proBDNF binds p75NTR → promotes apoptosis and LTD (synaptic weakening); mature BDNF binds TrkB → promotes survival, LTP (synaptic strengthening), and neuroplasticity. The Val66Met polymorphism (rs6265) affects BDNF secretion — Met carriers have impaired activity-dependent BDNF secretion, associated with reduced hippocampal volume, impaired episodic memory, and increased susceptibility to mood disorders. BDNF levels in blood (serum/plasma): low BDNF is associated with depression, Alzheimer's disease, and obesity (BDNF also regulates appetite and energy homeostasis via hypothalamic TrkB receptors). Exercise robustly increases BDNF — one proposed mechanism for the cognitive benefits of physical activity. Trofinetide's mechanism (GPE tripeptide from IGF-1) does not directly involve BDNF but addresses overlapping neuroprotective pathways.

CSF is produced primarily by choroid plexus epithelium (approximately 500 mL/day, with total volume approximately 140 mL — complete turnover 3-4 times daily). Composition: clear, colourless, protein-poor (0.2-0.4 g/L vs 60-80 g/L in plasma), glucose approximately 60% of plasma, and low cell count (<5 cells/μL). CSF functions: mechanical protection (cushioning the brain), buoyancy (reducing effective brain weight from 1400g to approximately 50g), chemical stability (maintaining stable ionic environment for neural signalling), and waste clearance (glymphatic system — CSF flows through perivascular spaces, exchanging with interstitial fluid, clearing metabolic waste including amyloid-β during sleep). CSF analysis is clinically valuable: protein elevation (infection, inflammation), glucose decrease (bacterial meningitis), oligoclonal bands (MS), Aβ42 decrease + phospho-tau increase (Alzheimer's), and cytology (CNS malignancy). Intrathecal drug administration delivers directly into CSF, achieving high CNS concentrations with minimal systemic exposure.

Cognitive decline spectrum: subjective cognitive decline (SCD — self-perceived decline without objective impairment on testing, may represent earliest detectable stage), mild cognitive impairment (MCI — objective cognitive impairment beyond age expectations but preserved daily functioning; amnestic MCI converts to Alzheimer's at approximately 10-15% per year), and dementia (cognitive impairment affecting daily functioning — Alzheimer's disease accounts for 60-70% of cases). Biomarkers: amyloid PET and CSF Aβ42 (amyloid pathology), tau PET and CSF phospho-tau (tau pathology), FDG-PET (neuronal metabolism), MRI volumetry (hippocampal atrophy). Modifiable risk factors (Lancet Commission 2020): 12 risk factors account for approximately 40% of dementia risk — education, hearing loss, hypertension, obesity, smoking, depression, physical inactivity, diabetes, excessive alcohol, traumatic brain injury, air pollution, and social isolation. No peptide drug is currently approved for preventing cognitive decline, though research continues.

GDNF (134 amino acids, dimeric) is the most potent known survival factor for dopaminergic neurons — the neurons whose degeneration causes Parkinson's disease. GDNF signals through GFRα1 co-receptor → RET receptor tyrosine kinase → PI3K/Akt and MAPK survival pathways. Therapeutic development has been challenging: GDNF does not cross the BBB (requiring direct brain delivery), clinical trials of intraputamenal GDNF infusion in Parkinson's disease showed mixed results (possible efficacy confounded by delivery inconsistencies, placebo effects, and anti-GDNF antibody formation), and gene therapy approaches (AAV2-GDNF) are in clinical trials. GDNF also supports motor neurons (relevant to ALS research), enteric neurons (relevant to gut motility), and kidney development. The GDNF story illustrates both the therapeutic promise and the delivery challenges of neurotrophic factor-based therapies.

LTP phases: early LTP (E-LTP, lasting 1-3 hours — requires protein kinase activation but not new protein synthesis; mechanisms: CaMKII-mediated AMPA receptor phosphorylation increasing single-channel conductance, and exocytosis of AMPA receptor-containing vesicles) and late LTP (L-LTP, lasting >3 hours to lifetime — requires new gene transcription and protein synthesis; mechanisms: PKA/CREB pathway activating transcription of plasticity-related genes, BDNF-TrkB signalling promoting dendritic spine growth and new synapse formation). LTP cooperativity (multiple inputs must be co-active), associativity (weak input paired with strong input can be potentiated — the cellular basis of associative learning), and input specificity (only activated synapses are strengthened) are key properties that map onto behavioural learning phenomena. LTP can be artificially induced by high-frequency electrical stimulation (tetanus) or by glutamate uncaging at single synapses. Enhancing LTP is a theoretical mechanism for cognitive-enhancing peptides.

Myelin is a lipid-rich membrane wrapped around axons by oligodendrocytes (CNS) or Schwann cells (PNS). Myelin provides: saltatory conduction (action potentials jump between nodes of Ranvier — increasing conduction velocity from ~1 m/s unmyelinated to ~100 m/s myelinated), metabolic support (oligodendrocytes provide lactate to axons), and axonal protection. Demyelinating diseases: multiple sclerosis (autoimmune attack on CNS myelin/oligodendrocytes → inflammation, demyelination, axonal damage → neurological deficits), Guillain-Barré syndrome (autoimmune PNS demyelination). Glatiramer acetate (Copaxone) is a peptide-based disease-modifying therapy for relapsing MS. Its mechanism involves: promoting anti-inflammatory Th2 response, inducing regulatory T cells, and generating glatiramer-reactive T cells that cross-react with myelin antigens and produce neurotrophic factors (BDNF) at inflammatory CNS lesions. Glatiramer reduces relapse rate by approximately 30% and slows disability progression.

NGF (120 amino acids as the mature form) was discovered by Rita Levi-Montalcini and Stanley Cohen (Nobel Prize 1986). NGF binds TrkA (high affinity, promoting survival, differentiation, and axon growth of sensory and sympathetic neurons) and p75NTR (lower affinity, context-dependent effects). NGF is produced by target tissues innervated by NGF-dependent neurons — providing retrograde trophic support (neurons that successfully connect to target tissues survive; those that don't undergo apoptosis during development). In adults, NGF maintains sensory neuron function and is upregulated during inflammation and tissue injury (contributing to inflammatory pain/hyperalgesia — anti-NGF antibodies are in development for chronic pain). Trofinetide (GPE = Gly-Pro-Glu, the N-terminal tripeptide of IGF-1) acts through a different mechanism: it modulates neuroinflammation, glial function, and synaptic signalling through pathways that are not yet fully characterised, but early evidence suggests involvement of NF-κB and glutamate signalling modulation.

Common neurodegenerative diseases: Alzheimer's (amyloid-β plaques + tau tangles → hippocampal/cortical neuronal loss → memory/cognitive decline), Parkinson's (α-synuclein Lewy bodies → substantia nigra dopaminergic neuron loss → motor symptoms), ALS (motor neuron degeneration → progressive paralysis), Huntington's (huntingtin polyQ expansion → striatal neuron loss → movement/cognitive/psychiatric symptoms), and multiple sclerosis (autoimmune demyelination → axonal degeneration). Shared pathological mechanisms: protein misfolding and aggregation, mitochondrial dysfunction, oxidative stress, neuroinflammation (microglial activation), excitotoxicity (glutamate-mediated calcium overload), and impaired autophagy. Peptide-based neuroprotection strategies: targeting protein aggregation (peptide inhibitors of amyloid-β or α-synuclein aggregation), enhancing neurotrophic support (BDNF mimetics, NGF mimetics), modulating neuroinflammation (anti-inflammatory peptides), and protecting mitochondria (elamipretide-type approaches applied to neurodegeneration — currently preclinical for CNS indications).

Neuroinflammation involves activation of resident CNS immune cells: microglia (the brain's macrophages, existing in M1 pro-inflammatory and M2 anti-inflammatory phenotypes) and astrocytes (reactive astrocytes proliferate and form glial scars). Acute neuroinflammation is protective (clearing debris, fighting infection), but chronic neuroinflammation contributes to neurodegeneration through: sustained release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), reactive oxygen/nitrogen species production, complement activation, and blood-brain barrier disruption. Trofinetide (GPE) modulates neuroinflammation in Rett syndrome — proposed mechanisms include: reducing microglial activation, modulating astrocyte reactivity, and normalising glutamate neurotransmission. Glatiramer acetate modulates immune responses in multiple sclerosis by shifting T cell populations from pro-inflammatory Th1/Th17 to anti-inflammatory Th2/Treg phenotypes, indirectly reducing CNS inflammation.

NMJ structure: presynaptic motor nerve terminal (containing ACh-loaded synaptic vesicles, voltage-gated calcium channels) → synaptic cleft (~50nm, containing AChE) → postsynaptic muscle membrane (endplate, containing densely packed nicotinic AChRs, approximately 10,000/μm²). Transmission: action potential → Ca²⁺ influx through presynaptic VGCCs → SNARE-mediated vesicle fusion → ACh release (approximately 200 quanta per impulse) → ACh binds nicotinic AChR (two ACh molecules per receptor) → channel opens → Na⁺ influx → endplate potential (EPP) → if EPP exceeds threshold, muscle action potential → excitation-contraction coupling → muscle contraction. Safety factor: the EPP normally exceeds threshold by 2-3× (safety margin). In MG, autoantibody-mediated AChR loss reduces the safety factor → some endplates fail to reach threshold → muscle weakness. Zilucoplan prevents complement-mediated NMJ damage, preserving the remaining AChR population.

Neuroplasticity mechanisms: synaptic plasticity (LTP — strengthening of synaptic connections through repeated activation, mediated by NMDA receptor-dependent calcium influx → CaMKII activation → AMPA receptor insertion; LTD — weakening of connections through low-frequency stimulation → phosphatase activation → AMPA receptor removal), structural plasticity (dendritic spine growth/retraction — new spines form at sites of potentiated synapses, providing structural basis for memory), adult neurogenesis (new neurons generated in hippocampal dentate gyrus and subventricular zone — exercise, learning, and enriched environments promote neurogenesis; stress and ageing reduce it), and large-scale cortical reorganisation (remapping of cortical representations after injury or training). BDNF is a key molecular mediator of neuroplasticity — it promotes LTP, dendritic growth, neuronal survival, and neurogenesis. Exercise increases BDNF levels and enhances neuroplasticity, partly explaining the cognitive benefits of physical activity.

Neurotransmitter categories: amino acids (glutamate — primary excitatory; GABA — primary inhibitory; glycine — inhibitory in spinal cord), monoamines (dopamine — reward/motor; serotonin/5-HT — mood/sleep; norepinephrine — arousal/attention; histamine — wakefulness), acetylcholine (muscle activation, cognition), purines (ATP, adenosine), and neuropeptides (>100 identified — oxytocin, vasopressin, endorphins, substance P, NPY, CGRP, somatostatin, CCK, VIP). Neuropeptide neurotransmission differs from classical: neuropeptides are synthesised in the cell body (not the terminal), stored in large dense-core vesicles (not small synaptic vesicles), released from extrasynaptic sites (volume transmission, not point-to-point), act on GPCRs (slower, modulatory), and have no reuptake mechanism (terminated by enzymatic degradation). Understanding neurotransmitter systems provides context for how peptide drugs interact with neural circuits.

Neurotrophic factor families: (1) Neurotrophins (NGF, BDNF, NT-3, NT-4/5) — act through Trk receptor tyrosine kinases (TrkA for NGF, TrkB for BDNF/NT-4, TrkC for NT-3) and p75NTR (pan-neurotrophin receptor, mediating both survival and apoptotic signalling depending on context). (2) GDNF family ligands (GDNF, neurturin, artemin, persephin) — act through GFRα co-receptors and RET receptor tyrosine kinase. (3) Neurokines/neuropoietic cytokines (CNTF, LIF, IL-6) — act through gp130 signalling. Therapeutic delivery challenge: neurotrophic factors are proteins that do not cross the BBB, limiting systemic administration. Delivery strategies: intrathecal/intracerebroventricular injection, gene therapy (viral vectors encoding neurotrophic factors), cell-based therapy (implanting cells engineered to secrete neurotrophic factors), and small molecule/peptide mimetics that cross the BBB and activate neurotrophin receptors.

The original Giurgea criteria for nootropics: enhance learning and memory, protect the brain from physical/chemical injury, enhance tonic cortical/subcortical control mechanisms, lack usual pharmacological effects of psychotropic drugs (sedation, motor stimulation), and possess very few side effects with extremely low toxicity. Few substances meet all criteria. In the peptide space: selank (synthetic derivative of tuftsin/immunoglobulin fragment) is approved in Russia as an anxiolytic/nootropic; semax (synthetic ACTH analogue fragment) is approved in Russia for cognitive enhancement and stroke recovery — neither has FDA/EMA approval due to insufficient evidence by Western regulatory standards. The evidence gap between Russian-approved peptide nootropics (typically based on smaller, non-Western trials) and FDA/EMA standards (requiring large, multicentre, placebo-controlled Phase III trials) is significant and important for evidence-informed evaluation.

Synaptic plasticity forms: LTP (long-term potentiation — persistent strengthening, lasting hours to lifetime; the cellular mechanism of memory formation), LTD (long-term depression — persistent weakening, involved in memory flexibility and motor learning), spike-timing-dependent plasticity (STDP — strengthening or weakening depends on precise timing of pre- and post-synaptic firing), and homeostatic plasticity (synaptic scaling — global adjustment of all synaptic strengths to maintain stable network activity). LTP molecular mechanism (hippocampal CA1): glutamate release → AMPA receptor activation (depolarisation) + NMDA receptor activation (requires coincident depolarisation to relieve Mg²⁺ block) → Ca²⁺ influx through NMDA receptor → CaMKII activation → AMPA receptor phosphorylation (enhanced conductance) + AMPA receptor insertion (more receptors at synapse) → strengthened synaptic transmission. BDNF is required for late-phase LTP (protein synthesis-dependent, lasting >3 hours). Disrupted synaptic plasticity is implicated in cognitive decline and is a theoretical target for nootropic peptide research.