Nociceptin (1-13) amide: lab fragment of a natural pain-signal peptide

What this is

Nociceptin (1-13) amide is the first 13 residues of nociceptin — a 17-amino-acid brain neuropeptide that is the body's natural activator of a receptor called NOP (also known as ORL1). The full-length peptide, nociceptin/orphanin FQ, is stored on this platform as a separate card (/card/pep-10535). The 1–13 fragment is C-terminally amidated (the sequence ends with –NH₂ rather than a free carboxyl group), which protects it from carboxypeptidase degradation without changing its pharmacological activity. Research established that this truncated, amidated form is as active as the complete 17-residue peptide at the NOP receptor (Guerrini and colleagues, Journal of Medicinal Chemistry, 1997), making it the minimal potent fragment of the nociceptin family — and a standard scaffold for studying how structure drives NOP receptor activation.

History

The NOP receptor was cloned in 1994 as an "orphan" — a G-protein-coupled receptor with clear structural similarity to the classical opioid receptors (mu, delta, kappa) but no known endogenous ligand. In 1995, two independent groups deorphanized it simultaneously, in one of the earliest successful applications of reverse pharmacology. Meunier and colleagues (fractionating rat brain extract) called the 17-residue peptide nociceptin and published in Nature (Meunier and colleagues, Nature 1995); Reinscheid and colleagues (fractionating porcine brain extract) called the same peptide orphanin FQ and published in Science (Reinscheid and colleagues, Science 1995). Today it is universally abbreviated N/OFQ, and the receptor is officially named NOP by IUPHAR.

The 13-residue amidated fragment emerged quickly as a tool compound in the first wave of SAR studies that followed discovery. Guerrini and colleagues (University of Ferrara, 1997) used nociceptin (1-13) amide — NC(1–13)NH₂ — as the primary scaffold to map the structural requirements for NOP receptor activation, framing the analysis in terms of "message" sequences (residues needed for activation) and "address" sequences (residues needed for binding), terminology borrowed from classical opioid pharmacology but shown to apply differently here. The compound has been commercially synthesised ever since and remains in wide use as a reference agonist for NOP pharmacology.

What it does

Nociceptin (1-13) amide activates the NOP receptor in the same way as full-length nociceptin. The NOP receptor is a Gi/o-coupled receptor: activation inhibits adenylyl cyclase (lowering cAMP), opens inward-rectifying potassium channels (hyperpolarising neurons), and closes voltage-sensitive calcium channels — collectively suppressing neuronal excitability and reducing neurotransmitter release. Nociceptin (1-13) amide reproduces each of these downstream effects in isolated tissue preparations (mouse vas deferens, guinea-pig ileum) at potencies comparable to the parent peptide (Guerrini and colleagues, Journal of Medicinal Chemistry, 1997).

Despite acting at a receptor structurally adjacent to the opioid receptors, nociceptin and this fragment do not activate the classical mu, delta, or kappa opioid receptors at meaningful concentrations, and their effects are not reversed by the opioid antagonist naloxone.

The NOP receptor system has paradoxical effects on pain: administration into the brain (supraspinal) produces pro-nociceptive effects — increasing pain sensitivity by blocking stress-induced and opioid-mediated analgesia — while administration into the spinal cord produces antinociception. The same receptor also modulates anxiety, reward circuitry, food intake, memory consolidation, and cardiovascular function, as reviewed by Calo' and colleagues (British Journal of Pharmacology, 2000) and by Toll and colleagues (Pharmacological Reviews, 2016).

Evidence

• Human: No clinical trials are registered for nociceptin (1-13) amide itself. It is a research peptide and pharmacological tool, not a clinical drug candidate. The broader NOP system has entered clinical development through small-molecule drug programmes, most notably cebranopadol — a dual NOP/mu-opioid receptor agonist that completed Phase 3 trials (ALLEVIATE-1 and ALLEVIATE-2) for moderate-to-severe acute pain, demonstrating analgesic efficacy with reduced respiratory depression and lower abuse potential versus oxycodone (Tris Pharma, PAINWeek 2024 data). No registered trials on ClinicalTrials.gov for "nociceptin (1-13) amide" or "nociceptin 1-13."
• Animal: NC(1–13)NH₂ and its analogues have been tested extensively in mouse vas deferens, guinea-pig ileum, and rat vas deferens isolated tissue bioassays, consistently functioning as full NOP agonists (Guerrini and colleagues, 1997; Calo' and colleagues, 2000). In vivo rodent studies of the parent nociceptin peptide established the supraspinal hyperalgesia / spinal antinociception duality that also applies to this fragment.
• In vitro: Full agonism at recombinant human NOP expressed in CHO cells, demonstrated by inhibition of forskolin-stimulated cAMP accumulation (Calo' and colleagues, British Journal of Pharmacology, 2000). SAR studies using NC(1–13)NH₂ as the reference compound characterised the contribution of individual residues across positions 0, 1, 3, 4, and 10 to ORL1 binding affinity (Bobrova and colleagues, European Journal of Medicinal Chemistry, 2003).

Known effects

• NOP receptor agonism (full) — In vitro, preclinical; no clinical data for this compound
• Inhibition of adenylyl cyclase — Mechanistic; recombinant human NOP in CHO cells
• Pain modulation (paradoxical) — Supraspinal: pro-nociceptive; spinal: antinociceptive; preclinical animal models of full-length nociceptin
• Reward/addiction modulation — Suppression of dopaminergic reward signalling; preclinical; full-length nociceptin
• Antagonist scaffold — Structural template for [Nphe¹]nociceptin(1–13)NH₂, the first selective peptide NOP antagonist (Calo' and colleagues, 2000)

Mechanism

Nociceptin (1-13) amide has the sequence FGGFTGARKSARK-NH₂ — the first 13 residues of the 17-residue parent peptide, with the C-terminus amidated rather than present as a free acid. The stored raw sequence FGGFTGARKSARK does not show the C-terminal amide; the –NH₂ group is the biologically and chemically significant modification that distinguishes this compound from the simple truncation fragment.

Receptor binding and activation studies (Guerrini and colleagues, Journal of Medicinal Chemistry, 1997) defined a two-domain model for NOP interaction:

• Message domain (activation): The N-terminal tetrapeptide Phe¹-Gly²-Gly³-Phe⁴ (FGGF) is essential. Phe⁴ is the most critical residue for receptor activation; its substitution eliminates activity. Gly² is also indispensable — replacement with alanine causes a sharp drop in ORL1 affinity. Phe¹ is more tolerant of substitution (Tyr¹ analogs retain activity), but the overall N-terminal conformation and flexibility are required.
• Address domain (binding): The cationic residues — Arg⁸, Lys⁹, Arg¹², Lys¹³ — are required for receptor binding; alanine substitution at all four positions abolishes activity. These residues interact with the extracellular loops and binding pocket of the NOP receptor.

The NOP receptor couples to Gi/Go. Activated NOP inhibits adenylyl cyclase (reducing cAMP), opens Kir3/GIRK potassium channels (hyperpolarising the cell), and closes N-type and P/Q-type voltage-dependent calcium channels. MAPK cascades (ERK1/2, p38, JNK) are also recruited downstream of NOP activation. The combined effect is presynaptic inhibition of neurotransmitter release and postsynaptic neuronal silencing.

Nociceptin (1-13) amide does not bind classical opioid receptors (mu/OP1, delta, kappa/OP3) at pharmacologically relevant concentrations; selectivity for NOP/OP4 over the classical three receptors is high (Calo' and colleagues, 2000).

Related peptides

• Nociceptin / Orphanin FQ (/card/pep-10535) — the full 17-residue parent peptide from which this fragment is derived; the endogenous NOP receptor agonist discovered in 1995
• [Nphe¹]Nociceptin(1–13)NH₂ — the first selective NOP peptide antagonist; derived from this same scaffold by substituting phenylalanine at position 1 with α-aminophenylacetic acid (Nphe); described by Calo' and colleagues (2000) as a competitive NOP antagonist (pA₂ 6.0–6.65 in isolated tissues) with no activity at mu, delta, or kappa opioid receptors — not yet a separate platform card
• Leu-enkephalin (/card/pep-04456) — shares the YGGF N-terminal opioid motif with classical opioid peptides; illustrates the structural contrast between the enkephalin family (mu/delta agonists) and nociceptin (NOP-selective, naloxone-insensitive)