David J. Rowbotham

The burden of neuropathic pain: results from a cross-sectional survey

Background There are few published data on the treatment patterns and burden of neuropathic pain. We have investigated this in a large, observational, cross‐sectional survey.

Nociceptin induced inhibition of K+ evoked glutamate release from rat cerebrocortical slices

Nociceptin, an endogenous ligand for the orphan receptor ORL1, has recently been described. In this study we have shown that nociception inhibits 46 mM K+‐stimulated glutamate release from rat perfused cerebrocortical slices with an IC50 of 51 nM. At 100 nM the inhibition amounted to 68 ± 14% and was naloxone (10 μm)‐insensitive excluding an activation of μ, δ and κ opioid receptors. These data demonstrate the functional coupling of ORL1 in glutamatergic neurones and implicates a role for nociceptin in glutamatergic neurotransmission.

Ethnic differences in thermal pain responses : A comparison of South Asian and White British healthy males

&NA; The expression and report of pain is influenced by social environment and culture. Previous studies have suggested ethnically determined differences in report of pain threshold, intensity and affect. The influence of ethnic differences between White British and South Asians has remained unexplored. Twenty age‐matched, male volunteers in each group underwent evaluation. Cold and warm perception and cold and heat threshold were assessed using an ascending method of limits. Magnitude estimation of pain unpleasantness and pain intensity were investigated with thermal stimuli of 46, 47, 48 and 49 °C. Subjects also completed a pain anxiety questionnaire. Data was analysed using t‐test, Mann–Whitney and repeated measures analysis of variance as appropriate. There were no differences in cold and warm perception between the two groups. There was a statistically significant difference between the two groups for heat pain threshold (P=0.006) and heat pain intensity demonstrated a significant effect for ethnicity (F=13.84, P=0.001). Although no group differences emerged for cold pain threshold and heat unpleasantness, South Asians demonstrated lower cold pain threshold and reported more unpleasantness at all temperatures but this was not statistically significant. Our study shows that ethnicity plays an important role in heat pain threshold and pain report, South Asian males demonstrated lower pain thresholds and higher pain report when compared with matched White British males. There were no differences in pain anxiety between the two groups and no correlations were identified between pain and pain anxiety Haemodynamic measures and anthropometry did not explain group differences.

Simultaneous targeting of multiple opioid receptors: a strategy to improve side-effect profile

Opioid receptors are currently classified as mu (mu: mOP), delta (delta: dOP), kappa (kappa: kOP) with a fourth related non-classical opioid receptor for nociceptin/orphainin FQ, NOP. Morphine is the current gold standard analgesic acting at MOP receptors but produces a range of variably troublesome side-effects, in particular tolerance. There is now good laboratory evidence to suggest that blocking DOP while activating MOP produces analgesia (or antinociception) without the development of tolerance. Simultaneous targeting of MOP and DOP can be accomplished by: (i) co-administering two selective drugs, (ii) administering one non-selective drug, or (iii) designing a single drug that specifically targets both receptors; a bivalent ligand. Bivalent ligands generally contain two active centres or pharmacophores that are variably separated by a chemical spacer and there are several interesting examples in the literature. For example linking the MOP agonist oxymorphone to the DOP antagonist naltrindole produces a MOP/DOP bivalent ligand that should produce analgesia with reduced tolerance. The type of response/selectivity produced depends on the pharmacophore combination (e.g. oxymorphone and naltrindole as above) and the space between them. Production and evaluation of bivalent ligands is an emerging field in drug design and for anaesthesia, analgesics that are designed not to be highly selective morphine-like (MOP) ligands represents a new avenue for the production of useful drugs for chronic (and in particular cancer) pain.

Opioid receptor subtypes: fact or artifact?

There is a vast amount of pharmacological evidence favouring the existence of multiple subtypes of opioid receptors. In addition to the primary classification of µ (mu: MOP), δ (delta: DOP), κ (kappa: KOP) receptors, and the nociceptin/orphanin FQ peptide receptor (NOP), various groups have further classified the pharmacological µ into µ(1-3), the δ into δ(1-2)/δ(complexed/non-complexed), and the κ into κ(1-3). From an anaesthetic perspective, the suggestions that µ(1) produced analgesia and µ(2) produced respiratory depression are particularly important. However, subsequent to the formal identification of the primary opioid receptors (MOP/DOP/KOP/NOP) by cloning and the use of this information to produce knockout animals, evidence for these additional subtypes is lacking. Indeed, knockout of a single gene (and hence receptor) results in a loss of all function associated with that receptor. In the case of MOP knockout, analgesia and respiratory depression is lost. This suggests that further sub-classification of the primary types is unwise. So how can the wealth of pharmacological data be reconciled with new molecular information? In addition to some simple misclassification (κ(3) is probably NOP), there are several possibilities which include: (i) alternate splicing of a common gene product, (ii) receptor dimerization, (iii) interaction of a common gene product with other receptors/signalling molecules, or (iv) a combination of (i)-(iii). Assigning variations in ligand activity (pharmacological subtypes) to one or more of these molecular suggestions represents an interesting challenge for future opioid research.

Clinical characteristics and pain management among patients with painful peripheral neuropathic disorders in general practice settings

Alleviating chronic pain is a global healthcare priority. Understanding the medical profile and current treatment patterns in patients with painful neuropathic disorders (PNDs) is crucial to the development of effective pain management strategies. Thus, our objective was to describe the demographic and clinical characteristics of persons with PNDs and their use of pain medications. Using the general practice research database, we categorized PNDs in two ways: Pure PNDs (which include diabetic neuropathy, postherpetic neuralgia, etc.; N = 16,690) and Mixed PNDs (which include back/neck pain with neuropathic involvement; N = 14,309). On average, PND patients were 55 years old (Pure, 55.4 [SD = 16.9] years; Mixed, 54.3 [SD = 16.4] years). Over a third had other chronic pain‐related (Pure, 37.5%; Mixed, 37.1%) and nearly a quarter had non‐pain related (Pure, 28.1%; Mixed, 24.1%) comorbidities. Use of medications with clinically demonstrated efficacy in PNDs was higher among patients with Pure PNDs (tricyclic antidepressants [Pure, 16.6%; Mixed, 10.1%]; 2nd generation antidepressants [Pure, 11.0%; Mixed, 9.7%]; and antiepileptics [Pure, 12.2%; Mixed, 2.6%]), whereas use of NSAIDs (Pure, 43.1%; Mixed, 65.2%) and opioids (Pure, 8.5%; Mixed, 14.3%) was higher among patients with Mixed PNDs. Average daily doses of select neuropathic pain‐related medications among PND patients (Pure and Mixed) were lower than those recommended for neuropathic pain. Among both Pure and Mixed PND patients, use and doses of evidenced‐based neuropathic pain‐related medications was low, and lower than the use of NSAIDs (a medication class with no proven efficacy for PNDs) in each group, suggesting possible sub‐optimal neuropathic pain management among these patients.

Comparison of the effects of [Phe1Ψ(CH2‐NH)Gly2]Nociceptin (1–13)NH2 in rat brain, rat vas deferens and CHO cells expressing recombinant human nociceptin receptors

Nociceptin(NC) is the endogenous ligand for the opioid receptor like‐1 receptor (NC‐receptor). [Phe1ΨC(CH2‐NH)Gly2]Nociceptin(1–13)NH2 ([F/G]NC(1–13)NH2) has been reported to antagonize NC actions in peripheral guinea‐pig and mouse tissues. In this study, we investigated the effects of a range of NC C‐terminal truncated fragments and [F/G]NC(1–13)NH2 on NC receptor binding, glutamate release from rat cerebrocortical slices (rCX), inhibition of cyclic AMP accumulation in CHO cells expressing the NC receptor (CHONCR) and electrically evoked contractions of the rat vas deferens (rVD). In radioligand binding assays, a range of ligands inhibited [125I]‐Tyr14‐NC binding in membranes from rCX and CHONCR cells. As the peptide was truncated there was a general decline in pKi. [F/G]NC(1–13)NH2 was as potent as NC(1–13)NH2. The order of potency for NC fragments to inhibit cyclic AMP accumulation in whole CHONCR cells was NCNH2NC=NC(1–13)NH2>NC(1–12)NH2>>NC(1–11)NH2. [F/G]NC(1–13)NH2 was a full agonist with a pEC50 value of 8.65. NCNH2 and [F/G]NC(1–13)NH2 both inhibited K+ evoked glutamate release from rCX with pEC50 and maximum inhibition of 8.16, 48.5±4.9% and 7.39, 58.9±6.8% respectively. In rVD NC inhibited electrically evoked contractions with a pEC50 of 6.63. Although [F/G]NC(1–13)NH2, displayed a small (instrinsic activity α=0.19) but consistent residual agonist activity, it acted as a competitive antagonist (pA2 6.76) in the rVD. The differences between [F/G]NC(1–13)NH2 action on central and peripheral NC signalling could be explained if [F/G]NC(1–13)NH2 was a partial agonist with high strength of coupling in the CNS and low in the periphery. An alternative explanation could be the existence of central and peripheral receptor isoforms.

Effects of intravenous anesthetic agents on glutamate release: a role for GABAA receptor-mediated inhibition.

Background: Many anesthetic agents are known to enhance the &agr;1&bgr;2&ggr;2S&ggr;-aminobutyric acid type A (GABAA) chloride current; however, they also depress excitatory neurotransmission. The authors evaluated two hypotheses: intravenous anesthetic agents inhibit glutamate release and any observed inhibition may be secondary to GABAA receptor activation. Methods: Cerebrocortical slices were prepared from Wistar rats. After perfusion in oxygenated Krebs buffer for 60 min at 37°C, samples for glutamate assay were obtained at 2-min intervals. After 6 min, a 2-min pulse of 46 mM K+ was applied to the slices (S1); this was repeated after 30 min (S2). Bicuculline (1–100 &mgr;M) was applied when the S1 response returned to basal level, and 10 min later, thiopental (1–300 &mgr;M), propofol (10 &mgr;M), or ketamine (30 &mgr;M) were also applied until the end of S2. Perfusate glutamate concentrations were measured fluorometrically, and the area under the glutamate release curves was expressed as a ratio (S2/S1). Results: Potassium (46 mM) evoked a monophasic release of glutamate during S1 and S2, with a mean control S2/S1 ratio of 1.07 ± 0.33 (mean ± SD, n = 96). Ketamine and thiopental produced a concentration-dependent inhibition of K+-evoked glutamate release with half-maximum inhibition of release values of 18.2 and 10.9 &mgr;M, respectively. Release was also inhibited by propofol. Bicuculline produced a concentration dependent reversal of thiopental inhibition of glutamate release with a half-maximum reversal of the agonist effect of 10.3 &mgr;M. Bicuculline also reversed the effects of propofol but not those of ketamine. Conclusions: The authors’ data indicate that thiopental, propofol, and ketamine inhibit K+-evoked glutamate release from rat cerebrocortical slices. The inhibition produced by thiopental and propofol is mediated by activation of GABAA receptors, revealing a subtle interplay between GABA-releasing (GABAergic) and glutamatergic transmission in anesthetic action.A antagonist; mechanisms of anesthesia; neurotransmitter release; rat cerebrocortical slices.)

Partial agonist behaviour depends upon the level of nociceptin/ orphanin FQ receptor expression: studies using the ecdysone- inducible mammalian expression system

Partial agonism is primarily dependent upon receptor density and coupling efficiency. As these parameters are tissue/model dependent, intrinsic activity in different tissues can vary. We have utilised the ecdysone‐inducible expression system containing the human nociceptin/orphanin FQ (N/OFQ) peptide receptor (hNOP) expressed in Chinese hamster ovary cells (CHOINDhNOP) to examine the activity of a range of partial agonists in receptor binding, GTPγ35S binding and inhibition of adenylyl cyclase studies. Incubation of CHOINDhNOP cells with ponasterone A (PON) induced hNOP expression ([leucyl‐3H]N/OFQ binding) of 24, 68, 191 and 1101 fmol mg−1 protein at 1, 2, 5 and 10 μM PON, respectively. At 191 fmol mg−1, protein hNOP pharmacology was identical to that reported for other traditional expression systems. pEC50 values for GTPγ35S binding ranged from 7.23 to 7.72 (2–10 μM PON) for the partial agonist [Phe1ψ(CH2–NH)Gly2]N/OFQ(1–13)–NH2 ([F/G]N/OFQ(1–13)–NH2) and 8.12–8.60 (1–10 μM PON) for N/OFQ(1–13)–NH2 and Emax values (stimulation factor relative to basal) ranged from 1.51 to 3.21 (2–10 μM PON) for [F/G]N/OFQ(1–13)–NH2 and 1.28–6.95 (1–10 μM) for N/OFQ(1–13)–NH2. Intrinsic activity of [F/G]N/OFQ(1–13)–NH2 relative to N/OFQ(1–13)–NH2 was 0.3–0.5. [F/G]N/OFQ(1–13)–NH2 did not stimulate GTPγ35S binding at 1 μM PON, but competitively antagonised the effects of N/OFQ(1–13)–NH2 with a pKB=7.62. pEC50 values for cAMP inhibition ranged from 8.26 to 8.32 (2–10 μM PON) for [F/G]N/OFQ(1–13)–NH2 and 9.42–10.35 for N/OFQ(1–13)–NH2 and Emax values (% inhibition) ranged from 19.6 to 83.2 for [F/G]N/OFQ(1–13)–NH2 and 40.9–86.0 for N/OFQ(1–13)–NH2. The intrinsic activity of [F/G]N/OFQ(1–13)–NH2 relative to N/OFQ(1–13)–NH2 was 0.48–0.97. In the same cellular environment with receptor density as the only variable, we show that the profile of [F/G]N/OFQ(1–13)–NH2 can be manipulated to encompass full and partial agonism along with antagonism.

Identification of nociceptin in human cerebrospinal fluid: comparison of levels in pain and non-pain states

&NA; We have measured plasma and cerebrospinal fluid (CSF) concentrations of nociceptin, the endogenous agonist of the orphan opioid receptor‐like receptor (ORL‐ 1). We studied two groups of ten patients presenting for elective Caesarean section (Group E) or in established labour and requiring combined spinal epidural anaesthesia for pain relief (Group L). Nociceptin was identified in all CSF samples with mean±SD concentrations of 52.49±34.25 and 63.39±33.26 pg/ml in groups E and L, respectively. Nociceptin was identified in 16/20 plasma samples with mean±SD concentrations of 7.59±21.58 and 13 73±23.79 pg/ml in groups E and L, respectively. CSF concentrations were significantly higher than plasma concentrations and there were no differences between groups E and L. These data report the first measurements of CSF nociceptin in man and show no association with the acute pain of labour.