Usage And Synthesis
Nociceptin/orphanin FQ was first identified as an
endogenous ligand for a GPCR that is homologous to opioid receptors. A 17-aa peptide that decreased forskolinstimulated cAMP production in vitro was isolated from
the rat brain in 1995 and named N, based on its ability
to increase reactivity to pain. The isolation of a peptide
with an identical sequence from the porcine brain was
simultaneously reported, and this peptide was named
OFQ. The term orphanin refers to the affinity of the peptide for an orphan opioid receptor, and F and Q reflect the
N- and C-terminal amino acid residues, respectively.
N/OFQ is generated from the precursor pronociceptin
by proteolytic cleavage. The amino acid sequence of
N/OFQ is common among the human, rat, and mouse.
The N-terminal pentapeptide sequence of N/OFQ, Phe-Gly-Gly-Phe-Thr, is related to those of Met-enkephalin
(Tyr-Gly-Gly-Phe-Met) and Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu)
The human prepronociceptin gene, PNOC, location
8q21, consists of four exons, and two cAMP responsive
elements are involved in the regulation of gene expression. Human preproN/OFQ (ppN/PFQ) is composed
of 167 aa residues, with a signal peptide consisting of
19 aa residues followed by a cysteine-containing the
N-terminal sequence and the region containing
N/OFQ.
A TATA-box motif upstream of the human PNOC
gene displays weak promoter activity. An increase of cellular cAMP levels by forskolin treatment upregulates
PNOC transcription. Estrogen also upregulates PNOC
transcription, whereas glucocorticoid downregulates
transcription.
N/OFQ is an agonist for N/OFQ receptors (NOP, also
known as nociceptin receptor, opiate receptor-like 1
[OPRL1], ORL receptor, ORL-1, N/OFQ receptor, etc.),
which are a subtype of opioid receptors belonging to
the GPCR family. Human OPRL1 is located on chromosome 20 (20q13.33).
UFP-102, Ro64-6198, and Ac-RYYRIK-NH2 are
agonists. UFP-101, SB 612111, J-113397, and JTC-801 are
antagonists.
The first behavioral effect of N/OFQ to be described
was a hyperalgesic response (lowering of nociceptive
thresholds). However, the effect of N/OFQ on nociceptive responses is complicated, as various studies report
hyperalgesia, blockade of hyperalgesia, analgesia, blockade of analgesia, no effect, or blockade of allodynia in
response to N/OFQ. N/OFQ is also associated with
locomotion, feeding, anxiety, spatial attention/learning,
endocrine effects in the hypothalamus, reward, and opiate tolerance and dependence. N/OFQ reduces dopamine levels in the nucleus
accumbens in rats, whereas it increases dopamine release
in the striatum. N/OFQ also inhibits the release of acetylcholine from the guinea pig trachea, substance P and the
calcitonin gene-related peptide from sensory nerve endings, and glutamate from cerebrocortical slices in rats.
N/OFQ stimulates prolactin release in both sexes, but
the effect is much greater in females. Conversely, growth
hormone release is stimulated only in male rats. The peptide inhibits electrically induced contractions of both the
guinea pig ileum and the mouse vas deferens. N/OFQ
causes hypotension and bradycardia in anesthetized rats,
whereas it increases the blood pressure and heart rate in
unanesthetized ewes.
The endogenous N/OFQ system has a physiological
role in mediating or regulating behavioral responses to
alcohol, and the activation of NOP suppresses ongoing
alcohol consumption or the reinstatement of responding
to alcohol. These findings encourage the development of
therapies targeted at the N/OFQ system for the treatment
of alcoholism in humans.
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