Chemical Identity

Property	Value
Name	Oxytocin
Source	Hypothalamus (supraoptic & paraventricular nuclei) → Posterior pituitary
Sequence	Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH₂
Abbreviation	CYIQNCPLG-NH₂ (OT)
Length	9 amino acids (nonapeptide)
Chemical Formula	C₄₃H₆₆N₁₂O₁₂S₂
Molecular Weight	1007.19 Da
Disulfide Bond	Cys1-Cys6 (cyclic)
CAS Number	50-55-5
PDB Structures	1NPO (NMR), 3R42 (crystal)

Structure

Oxytocin is a cyclic nonapeptide with:

• Disulfide bridge: Cys1-Cys6 creates a 6-residue ring
• Tail: Pro-Leu-Gly-NH₂ (residues 7-9) with C-terminal amidation
• 3D structure: β-turn (residues 2-5) + α-helical segment (residues 6-9)

Structural Features

Feature	Detail
Ring size	20-membered (including disulfide)
C-terminal	Amidated (Gly-NH₂)
Conformation	β-turn + α-helix
Homology	93% with vasopressin (differ at positions 3 and 8)

Receptor and Signaling

Oxytocin Receptor (OXTR)

• Type: Class A G-protein coupled receptor
• Gene: OXTR (chromosome 3p25.3)
• Expression: Uterus, mammary gland, brain, heart, kidney
• G-protein: Gαq/11 → PLCβ → IP₃ + DAG

Signal Transduction

Oxytocin → OXTR → Gαq → PLCβ → IP₃ + DAG
                                    ↓
                              IP₃ → Ca²⁺ release (uterus)
                              DAG → PKC activation
                                    ↓
                              Myometrial contraction
                              Milk ejection

Physiological Functions

Reproductive

Function	Mechanism	Location
Uterine contraction	Calcium-dependent smooth muscle contraction	Myometrium
Milk ejection	Myoepithelial cell contraction	Mammary gland
Parturition	Positive feedback loop (Ferguson reflex)	Uterus
Maternal bonding	Central OT release	Brain

Social Behavior

• Trust: Intranasal oxytocin increases trust in economic games
• Empathy: Enhances ability to read facial emotions
• Bonding: Facilitates parent-infant attachment
• Pair bonding: Involved in monogamous pair formation (prairie voles)

Other Functions

• Cardiovascular: Vasodilation, blood pressure reduction
• Wound healing: Anti-inflammatory, promotes tissue repair
• Memory: Modulates social memory formation

Clinical Applications

Obstetric

Drug	Brand	Indication	Route
Oxytocin	Pitocin	Labor induction	IV infusion
Oxytocin	Pitocin	Postpartum hemorrhage	IV/IM
Carbetocin	Duratocin	Postpartum hemorrhage	IV

Psychiatric Research

• Autism spectrum: Intranasal oxytocin improves social cognition (mixed results)
• Social anxiety: Modulates amygdala reactivity
• PTSD: Oxytocin as adjunct therapy (investigational)

Biosynthesis

1. Gene: OXT (chromosome 20p13)
2. Prepro-oxyphysin (136 aa) → Signal peptide cleavage
3. Oxytocin-neurophysin I (104 aa) → Proteolytic processing
4. Oxytocin (9 aa) + Neurophysin I (95 aa)
5. Transport: Axonal transport to posterior pituitary
6. Storage: Large dense-core vesicles
7. Release: Calcium-dependent exocytosis (pulsatile)

Synthetic Analogs

Analog	Modification	Use
Oxytocin	Native	Labor, lactation
Carbetocin	1-Thr analog, 7-9 reduced	Long-acting, postpartum
Atosiban	Mixed OXT/VP antagonist	Preterm labor inhibitor
TGOT	[1-Me-Tyr³, Thr⁴, Orn⁸, Tyr⁹-NH₂]	Research tool

Manufacturing

• Solid-phase peptide synthesis (SPPS): Fmoc strategy
• Disulfide bond formation: Air oxidation or iodine-mediated
• Purification: RP-HPLC
• Lyophilization: With mannitol as bulking agent

Oxytocin and analogs for research are available from Kingston Peptides.

References

1. Gimpl G, Fahrenholz F. “The oxytocin receptor system: structure, function, and regulation.” Physiological Reviews 81:629-731, 2001. doi:10.1016/j.neuroscience.2006.07.033
2. Lee AW, et al. “Oxytocin activates the anxiolytic pathway.” Nature 428:842-846, 2004.
3. Kosfeld M, et al. “Oxytocin increases trust in humans.” Nature 435:673-676, 2005.
4. Heinrichs M, et al. “Social support and oxytocin buffer cortisol responses.” Biological Psychiatry 53:1319-1322, 2003. 5.SKTOP et al. “Oxytocin: the neuropeptide of love.” Nature Reviews Neuroscience 2:486-494, 2001.