Alistair D. Corbett

75 years of opioid research: the exciting but vain quest for the Holy Grail

Over the 75‐year lifetime of the British Pharmacological Society there has been an enormous expansion in our understanding of how opioid drugs act on the nervous system, with much of this effort aimed at developing powerful analgesic drugs devoid of the side effects associated with morphine – the Holy Grail of opioid research. At the molecular and cellular level multiple opioid receptors have been cloned and characterised, their potential for oligomerisation determined, a large family of endogenous opioid agonists has been discovered, multiple second messengers identified and our understanding of the adaptive changes to prolonged exposure to opioid drugs (tolerance and physical dependence) enhanced. In addition, we now have greater understanding of the processes by which opioids produce the euphoria that gives rise to the intense craving for these drugs in opioid addicts. In this article, we review the historical pathway of opioid research that has led to our current state of knowledge.

Opioid activity of alkaloids extracted from Picralima nitida (fam. Apocynaceae)

Extracts of the seeds of Picralima nitida (fam. Apocynaceae) have been reported to have opioid analgesic activity. In this investigation, isolated tissue bioassays and radioligand binding assays have been used to determine the opioid activity of five alkaloids--akuammidine, akuammine, akuammicine, akuammigine and pseudoakuammigine--extracted from the seeds of P. nitida. Akuammidine showed a preference for mu-opioid binding sites with Ki values of 0.6, 2.4 and 8.6 microM at mu-, delta- and kappa-opioid binding sites, respectively. The agonist actions of akuammidine in the mouse-isolated vas deferens were antagonised by naloxone and the mu-opioid receptor selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) confirming an action at mu-opioid receptors. In contrast, akuammine also showed highest affinity for mu-opioid binding sites (Ki 0.5 microM) but was an antagonist at mu-opioid receptors with a pK(B) of 5.7 against the selective mu-opioid receptor agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO). Akuammicine has the highest affinity for kappa-opioid binding sites (Ki 0.2 microM) and was a full agonist at kappa-opioid receptors in the guinea pig ileum preparation but a partial kappa-opioid receptor agonist in the vasa deferentia of the mouse and the rabbit. Akuammigine and pseudoakuammigine showed little or no efficacy in the opioid bioassays. None of the alkaloids had significant activity for opioid receptor-like binding sites (ORL1-binding sites) with Ki values >> 10 microM. These data show that some alkaloids extracted from the medicinal plant P. nitida possess varying degrees of agonist and antagonist activity at opioid receptors but possess neither high affinity nor selectivity for mu-, delta- or kappa-opioid receptors or the ORL1-receptor.

The absence of apoeccrine glands in the human axilla has disease pathogenetic implications, including axillary hyperhidrosis

Background  The existence of a third type of sweat gland in human axillary skin, the apoeccrine gland, with a capacity to produce much higher sweat output than the eccrine gland, was proposed from examination of microdissected glands. However, previous studies of axillary skin glands did not examine the entire individual glandular structure via serial sections and the markers used to identify the different glands gave conflicting results and, hence, the existence of the apoeccrine gland remains controversial.

In vitro agonist effects of nociceptin and [Phe1ψ(CH2-NH)Gly2]nociceptin(1–13)NH2 in the mouse and rat colon and the mouse vas deferens

Nociceptin is an endogenous ligand of the opioid receptor-like (ORL1) receptor, a G-protein coupled receptor with sequence similarities to the opioid receptors. ORL1 receptors are present at both central and peripheral sites in several mammalian species but their functions are as yet poorly understood. The main aim of this investigation was to study the effects of nociceptin and the putative ORL1 receptor antagonist [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) in two peripheral tissues, the isolated proximal colon of the mouse and the distal colon of the rat. Nociceptin, [D-Ala(2), MePhe(4), Gly-ol(5)]enkephalin (DAMGO; mu-opioid receptor selective) and [D-Pen(2), D-Pen(5)]enkephalin (DPDPE; delta-opioid receptor selective) caused concentration-dependent contractions of mouse and rat isolated colon preparations (nociceptin EC(50)=1.20 and 0.28 nM in the mouse and rat, respectively). Des[Phe(1)]nociceptin (250 nM) had no contractile effect. Naloxone (300 nM) antagonised the effects of DAMGO and DPDPE but had no effect in either preparation on contractions seen in response to nociceptin. [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) also caused contractions in the colonic preparations (EC(50)=6.0 and 3.1 nM in the mouse and rat, respectively); there was no evidence of any antagonist activity. Tetrodotoxin (1 microM) abolished the contractile effects of nociceptin in the mouse colon but had no effect in the rat. In the vas deferens preparation isolated from DBA/2 mice, nociceptin caused concentration-dependent inhibitions of electrically-evoked contractions which were antagonised by [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) (apparent pK(B)=6. 31). However, [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) (0.3-10 microM) also possessed agonist activity in this preparation, as it inhibited the electrically-evoked contractions in a concentration-dependent manner. These observations do not support the proposal that [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) has agonist activity at central ORL1 receptors but is an antagonist in the periphery and that these differences in efficacy point to differences in the receptors. Rather, these data along with those of others suggest that [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin(1-13)NH(2) is a partial agonist and that differences in receptor reserve can account for the varied pharmacological actions of this pseudopeptide at central and peripheral sites.

The secretory clear cell of the eccrine sweat gland as the probable source of excess sweat production in hyperhidrosis

Abstract:  Primary hyperhidrosis is characterized by excessive sweating in palmar, plantar and axillary body regions. Gland hypertrophy and the existence of a third type of sweat gland, the apoeccrine gland, with high fluid transporting capabilities have been suggested as possible causes. This study investigated whether sweat glands were hypertrophied in axillary hyperhidrotic patients and if mechanisms associated with fluid transport were found in all types of axillary sweat glands. The occurrence of apoeccrine sweat glands was also investigated. Axillary skin biopsies from control and hyperhidrosis patients were examined using immunohistochemistry, image analysis and immunofluorescence microscopy. Results showed that glands were not hypertrophied and that only the clear cells in the eccrine glands expressed proteins associated with fluid transport. There was no evidence of the presence of apoeccrine glands in the tissues investigated. Preliminary findings suggest the eccrine gland secretory clear cell as the main source of fluid transport in hyperhidrosis.

Innervation and receptor profiles of the human apocrine (epitrichial) sweat gland: routes for intervention in bromhidrosis

Background  Human apocrine (epitrichial) sweat glands secrete in response to local or systemic administration of catecholamines and cholinergic agonists. As the process of secretion in human apocrine glands is not fully understood and no literature detailing the expression of adrenergic, cholinergic and purinergic receptors is available, there is a need to know the receptor types. Such data could provide new approaches for the treatment of axillary bromhidrosis.

Characterization of the ORL1 receptor on adrenergic nerves in the rat anococcygeus muscle

Nociceptin, the endogenous ORL1 receptor agonist inhibited the motor response to electrical‐field stimulation in the rat anococcygeus muscle. This effect was characterized using the peptide ligands acetyl‐Arg‐Tyr‐Tyr‐Arg‐Trp‐Lys‐NH2 (Ac‐RYYRWK‐NH2), acetyl‐Arg‐Tyr‐Tyr‐Arg‐Ile‐Lys‐NH2 (Ac‐RYYRIK‐NH2) and [Phe1ψ(CH2‐NH)Gly2]nociceptin(1‐13)NH2 ([F/G]NC(1‐13)NH2), and the non‐selective opioid antagonist naloxone benzoylhydrazone (NalBzOH). Nociceptin produced a concentration‐dependent inhibition of the adrenergic motor response to electrical‐field stimulation (EC50 19 nM, pEC50 7.7±0.1, n=8), but the response to exogenous noradrenaline (0.2–1 μM) was unaffected. The inhibitory nerve response was not affected by up to 1 μM nociceptin. After inhibition of nitric oxide synthase (Nω‐nitro‐L‐arginine 100 μM), and in the presence of peptidase inhibitors, nociceptin produced full inhibition of the pure adrenergic motor response (EC50 4 nM; pEC50 8.4±0.1, Emax 98.3±1.2%, n=12). Ac‐RYYRWK‐NH2 was a potent partial‐agonist (pEC50 9.0±0.1, Emax 66.4±5.2; n=11) but the efficacy of Ac‐RYYRIK‐NH2 (pEC50 8.0±0.2, Emax 36.7±3.5; n=12) was lower and the peptide could be tested as an antagonist (pA2 9.01). [F/G]NC(1‐13)NH2 and NalBzOH had little or no efficacy and were competitive antagonists with pKB values of 7.4 (95% c.l. 7.1, 7.7) and 6.9 (95% c.l. 6.7, 7.1) respectively. Both increased the response to field stimulation at high concentrations, suggesting the release of an endogenous agonist for the ORL1 receptor during stimulation. Rat nocistatin did not affect the response to electrical‐field stimulation, nor did it modify the inhibitory action of nociceptin. Our findings suggest there is a significant endowment of ORL1 receptors on sympathetic terminals of the rat anococcygeus, where nociceptin mediates a powerful inhibitory effect on adrenergic neuromuscular transmission.

Oral administration of Ginkgo biloba extract, EGb-761 inhibits thermal hyperalgesia in rodent models of inflammatory and post-surgical pain

Studies in vitro suggest that the standardised extract of Ginkgo biloba, EGb‐761 has anti‐inflammatory properties and modulatory effects on key pain‐related molecules. This study investigated the analgesic and anti‐inflammatory effects of EGb‐761 on carrageenan‐induced inflammatory and hindpaw incisional pain.

Nociceptin inhibits tonic nitric oxide release in the mouse isolated proximal colon

Nociceptin/orphanin FQ, the endogenous ligand of the opioid receptor-like (ORL1) receptor, caused contractions in the isolated colon of the mouse. Tetrodotoxin and the nitric oxide (NO) synthase inhibitor Nomega-nitro-L-arginine also produced contractions which were quantitatively similar to those seen in response to nociceptin. In the presence of either tetrodotoxin or Nomega-nitro-L-arginine, nociceptin was unable to cause a further contraction, whereas the muscarinic receptor agonist carbachol elicited a contractile response. Nociceptin had no contractile activity in colonic preparations contracted by Nomega-nitro-L-arginine then relaxed by addition of the NO donor sodium nitroprusside. These data suggest that nociceptin causes contractions of the mouse proximal colon by inhibiting the tonic, neuronal release of NO.

Activation of chloride secretion via proteinase-activated receptor 2 in a human eccrine sweat gland cell line – NCL-SG3

Abstract:  Proteinase‐activated receptor 2 (PAR‐2) has been shown to elicit secretion in a variety of secretory epithelial cells by the transepithelial movement of chloride ions across the apical membrane. However, it is not known whether these receptors are present and/or functional in the secretory epithelial cells of the human eccrine sweat gland. To investigate this possibility mRNA analysis, Ca2+ microspectrofluorimetry and the short circuit current (Isc) technique were used to quantify electrolyte transport in a cell line (NCL‐SG3) derived from human eccrine sweat gland secretory epithelia. The results provided molecular and functional evidence of the presence of PAR‐2 receptors in the NCL‐SG3 cells and show that these receptors can activate transepithelial Cl− secretion possibly via Ca2+‐activated Cl− channels.