Morphine inhibits antidromic vasodilatation without affecting the excitability of C-polymodal nociceptors in the skin of the rat

Mechanisms of Transmission and Processing of Pain: A Narrative Review

Knowledge about the mechanisms of transmission and the processing of nociceptive information, both in healthy and pathological states, has greatly expanded in recent years. This rapid progress is due to a multidisciplinary approach involving the simultaneous use of different branches of study, such as systems neurobiology, behavioral analysis, genetics, and cell and molecular techniques. This narrative review aims to clarify the mechanisms of transmission and the processing of pain while also taking into account the characteristics and properties of nociceptors and how the immune system influences pain perception. Moreover, several important aspects of this crucial theme of human life will be discussed. Nociceptor neurons and the immune system play a key role in pain and inflammation. The interactions between the immune system and nociceptors occur within peripheral sites of injury and the central nervous system. The modulation of nociceptor activity or chemical mediators may provide promising novel approaches to the treatment of pain and chronic inflammatory disease. The sensory nervous system is fundamental in the modulation of the host's protective response, and understanding its interactions is pivotal in the process of revealing new strategies for the treatment of pain.

Distribution of functional opioid receptors in human dorsal root ganglion neurons

Preclinical evidence has highlighted the importance of the μ-opioid peptide (MOP) receptor on primary afferents for both the analgesic actions of MOP receptor agonists, as well as the development of tolerance, if not opioid-induced hyperalgesia. There is also growing interest in targeting other opioid peptide receptor subtypes (δ-opioid peptide [DOP], κ-opioid peptide [KOP], and nociceptin/orphanin-FQ opioid peptide [NOP]) on primary afferents, as alternatives to MOP receptors, which may not be associated with as many deleterious side effects. Nevertheless, results from several recent studies of human sensory neurons indicate that although there are many similarities between rodent and human sensory neurons, there may also be important differences. Thus, the purpose of this study was to assess the distribution of opioid receptor subtypes among human sensory neurons. A combination of pharmacology, patch-clamp electrophysiology, Ca imaging, and single-cell semiquantitative polymerase chain reaction was used. Our results suggest that functional MOP-like receptors are present in approximately 50% of human dorsal root ganglion neurons. δ-opioid peptide-like receptors were detected in a subpopulation largely overlapping that with MOP-like receptors. Furthermore, KOP-like and NOP-like receptors are detected in a large proportion (44% and 40%, respectively) of human dorsal root ganglion neurons with KOP receptors also overlapping with MOP receptors at a high rate (83%). Our data confirm that all 4 opioid receptor subtypes are present and functional in human sensory neurons, where the overlap of DOP, KOP, and NOP receptors with MOP receptors suggests that activation of these other opioid receptor subtypes may also have analgesic efficacy.

Local loperamide injection reduces mechanosensitivity of rat cutaneous, nociceptive C-fibers

Loperamide reverses signs of mechanical hypersensitivity in an animal model of neuropathic pain suggesting that peripheral opioid receptors may be suitable targets for the treatment of neuropathic pain. Since little is known about loperamide effects on the responsiveness of primary afferent nerve fibers, in vivo electrophysiological recordings from unmyelinated afferents innervating the glabrous skin of the hind paw were performed in rats with an L5 spinal nerve lesion or sham surgery. Mechanical threshold and responsiveness to suprathreshold stimulation were tested before and after loperamide (1.25, 2.5 and 5 µg in 10 µl) or vehicle injection into the cutaneous receptive field. Loperamide dose-dependently decreased mechanosensitivity in unmyelinated afferents of nerve-injured and sham animals, and this effect was not blocked by naloxone pretreatment. We then investigated loperamide effects on nerve conduction by recording compound action potentials in vitro during incubation of the sciatic nerve with increasing loperamide concentrations. Loperamide dose-dependently decreased compound action potentials of myelinated and unmyelinated fibers (ED50 = 8 and 4 µg/10 µl, respectively). This blockade was not prevented by pre-incubation with naloxone. These results suggest that loperamide reversal of behavioral signs of neuropathic pain may be mediated, at least in part, by mechanisms independent of opioid receptors, most probably by local anesthetic actions.

Nociceptor sensitization in pain pathogenesis

The incidence of chronic pain is estimated to be 20-25% worldwide. Few patients with chronic pain obtain complete relief from the drugs that are currently available, and more than half report inadequate relief. Underlying the challenge of developing better drugs to manage chronic pain is incomplete understanding of the heterogeneity of mechanisms that contribute to the transition from acute tissue insult to chronic pain and to pain conditions for which the underlying pathology is not apparent. An intact central nervous system (CNS) is required for the conscious perception of pain, and changes in the CNS are clearly evident in chronic pain states. However, the blockage of nociceptive input into the CNS can effectively relieve or markedly attenuate discomfort and pain, revealing the importance of ongoing peripheral input to the maintenance of chronic pain. Accordingly, we focus here on nociceptors: their excitability, their heterogeneity and their role in initiating and maintaining pain.

Opioid modulation of ferret vagal afferent mechanosensitivity

Despite universal use of opioids in the clinic to inhibit pain, there is relatively little known of their peripheral actions on sensory nerve endings, where in fact they may be better targeted with more widespread applications. Here we show differential effects of mu-, kappa-, and delta-opioids on mechanosensitive ferret esophageal vagal afferent endings investigated in vitro. The effects of selective agonists [d-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), 2-(3, 4-dichlorophenyl)-N-methyl-N-[(1S)-1phenyl-2-(1-pyrrolidinyl) ethyl] acetamide hydrochlorine (ICI 199441), and (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), respectively, on mechanosensory stimulus-response functions were quantified. DAMGO (10(-7) to 10(-5) M) reduced the responses of tension receptors to circumferential tension (1-5 g) by up to 50%, and the responses of mucosal receptors to mucosal stroking (10-1,000 mg von Frey hair) by >50%. DAMGO effects were reversed by naloxone (10(-5) M). Tension/mucosal (TM) receptor responses to tension and stroking were unaffected by DAMGO. ICI 199441 (10(-6) to 10(-5) M) potently inhibited all responses except TM receptor responses to tension, and SNC-80 (10(-5) to 10(-3) M) had no effect other than a minor inhibition of mucosal receptor responses to intense stimuli at 10(-3) M. We conclude that mu- and kappa-opioids have potent and selective peripheral effects on esophageal vagal afferents that may have applications in treatment of disorders of visceral sensation.

Endomorphin-1 causes synovial hypoaemia in rat knee joints via a capsaicin-sensitive neural pathway

In joints, synthetic mu-opioids reduce inflammatory changes such as protein extravasation and associated oedema formation. However, the effect of endogenous opioid peptides on other inflammatory processes such as altered tissue blood flow has not been investigated. The present study examined the peripheral effects of the endogenous mu-opioid ligand endomorphin-1 (EM-1) on rat knee joint blood flow using laser Doppler perfusion imaging. Topical application of EM-1 (10(-16)-10(-9) mol) to exposed rat knee joints resulted in a dose-dependent increase in synovial vascular resistance with a maximum rise of 56% occurring with the 10(-9) mol dose. Destruction of unmyelinated articular afferents by capsaicin treatment completely abolished the hypoaemic effects of EM-1. These findings suggest that EM-1 acts peripherally in knee joints to decrease synovial blood flow, and this hypoaemic response is dependent on the presence of capsaicin-sensitive nerves.

The ORL-1 (NOP1) receptor ligand nociceptin/orphanin FQ (N/OFQ) inhibits neurogenic dural vasodilatation in the rat

The effects of the ORL-1 (NOP(1)) receptor ligand nociceptin (N/OFQ) and the nociceptin antagonists [Nphe(1)]N/OFQ-(1-13)-NH(2) (Nphe) and nocistatin (NST) on neurogenic dural vasodilatation (NDV) in the rat dura mater evoked by electrical stimulation of a closed cranial window were studied. The middle meningeal artery was visualised using intravital microscopy, and the vessel diameter analysed using a video dimension analyser. N/OFQ (1, 10, 100 nmol kg(-1); i.v., n=10) significantly and dose-dependently suppressed NDV maximally by 65% (P<0.01). Neither Nphe (100 nmol kg(-1); n=5) nor NST (100 nmol kg(-1); n=4) alone had an effect on NDV (P>0.05). Baseline vessel diameter was not significantly affected by application of N/OFQ, NST or Nphe. Application of the selective N/OFQ antagonist Nphe (10, 100 nmol kg(-1) i.v., n=8) dose-dependently and significantly (P<0.01) reversed the inhibition of NDV induced by application of N/OFQ (10 nmol kg(-1)). NST (10, 100 nmol kg(-1); n=7) failed to reverse the effects elicited by N/OFQ. Application of N/OFQ elicited a dose-dependent transient decrease in arterial blood pressure (P<0.01). Nphe dose-dependently reversed the cardiovascular effects induced by application of N/OFQ (10 nmol kg(-1)) (P<0.01),while NST did not alter the blood pressure reaction elicited by N/OFQ. The results show that N/OFQ inhibits NDV, an effect which is antagonised by Nphe, but not by NST. ORL-1 (NOP(1)) receptors located on trigeminal sensory fibres may be involved in the regulation of dural vessel diameter and hence may play a role in migraine pathophysiology.

Frequency dependent changes in mechanosensitivity of rat knee joint afferents after antidromic saphenous nerve stimulation

The aim of the present study was to examine the effect of electrical saphenous nerve stimulation (14 V, 1-10 Hz) on the mechanosensitivity of rat knee joint afferents. The responses to passive joint rotations at defined torque were recorded from slowly conducting knee joint afferent nerve fibres (0.6-20.0 m/s). After repeated nerve stimulation with 1 Hz, the mechanosensitivity of about 79% of the units was significantly affected. The effects were most prominent at a torque close to the mechanical threshold. In about 46% of the examined nerve fibres a significant increase was obtained, whereas about 33% reduced their mechanosensitivity. The sensitisation was prevented by an application of 5 microM phentolamine, an alpha-adrenergic receptor blocker, together with a neuropeptide Y receptor blocker. An inhibition of N-type Ca(2+) channels by an application of 1 microM omega-conotoxin GVIA caused comparable changes of the mechanosensitivity during the electrical stimulation. Electrical nerve stimulation with higher frequencies resulted in a further reduction of the mean response to joint rotations. After stimulation with 10 Hz, there was a nearly complete loss of mechanosensitivity.In conclusion, antidromic electrical nerve stimulation leads to a frequency dependent transient decrease of the mechanosensitivity. A sensitisation was only obtained at 1 Hz, but this effect may be based on the influence of sympathetic nerve fibres.

Neuropeptide Y changes the excitability of fine afferent units in the rat knee joint

1. The aim of the present study was to examine the effects of the sympathetic co-transmitter Neuropeptide Y on primary afferent nerve fibres of the rat knee joint. The responses to passive joint rotations at defined torque were recorded from 41 slowly conducting afferent nerve fibres (0.9 - 18.8 m s(-1)) innervating the knee joint capsule. 2. About 70% of the joint afferents were significantly affected in their mechanosensitivity by topical application of Neuropeptide Y. Significant effects occurred at a concentration of 10 nM. 3. Decreased mechanosensitivity was observed in about 40% of nerve fibres, whereas 30% of the units increased the mechanosensitivity. In addition, in about 35% of the fibres resting activity was induced or increased. Neither the conduction velocity nor the mechanical threshold of the units correlated with the described effects of Neuropeptide Y. 4. NPY(13--36), a specific Y2-receptor agonist, only modulated the mechanosensitivity, with no effect on the resting activity. The effects on the mechanosensitivity were similar to Neuropeptide Y, i.e. increase and decrease of the response. 5. Studies with the Y1-agonist (Leu(31), Pro(34))-NPY showed that activation of the Y1-receptor predominantly resulted in an enhanced mechanosensitivity and an induction or increase of a resting activity. The opposite effect was observed by application of BIBP 3226 BS, a Y1-receptor antagonist. 6. In conclusion, these data indicate that Neuropeptide Y affects the excitability of sensory nerve fibre endings.

Effects of nociceptin and nocistatin on antidromic vasodilatation in hairless skin of the rat hindlimb in vivo

1. We tested whether nociceptin (NCE), the endogenous ligand of the opioid receptor-like 1 (ORL1) receptor, and nocistatin (NST), which reverses central NCE effects when applied intrathecally (i.t.), affect small-diameter afferent fibre-mediated vasodilatation in rat hairless skin. 2. Female Wistar rats were vagotomized. Ongoing sympathetic vasoconstrictor activity was abolished by bilateral section of the lumbar sympathetic trunk between ganglia L2 and L3. Sensory axons were selectively stimulated in the dorsal root L5 by 20 electrical impulses supramaximal for activating C-fibres at 1 Hz. Blood flow was measured on the plantar skin of the left hind paw in the L5 dermatome using laser Doppler flowmetry. 3. NCE injected intravenously (i.v.) as single boluses (1, 10 and 100 nmol kg(-1) 7 - 8 min before dorsal root stimulation (n=6) dose-dependently decreased blood pressure and local vascular resistance and suppressed antidromic vasodilatation maximally by 47% (P<0.01). When NCE was injected 2 min before stimulation (n=3), antidromic vasodilatation was reduced by 64% after NCE (1 nmol kg-1) and totally, or almost totally, abolished after the two higher doses. 4. NST (1 - 100 nmol kg(-1) i.v., n=6) was without significant effect on blood pressure and cutaneous vascular resistance. Applied 5 (n=6) or 2 min (n=3) before stimulation it also did not affect antidromic vasodilatation. NST (100 nmol kg(-1) i.v.) applied shortly before an equimolar dose of NCE did not antagonize NCE effects on vascular resistance, blood pressure and antidromic vasodilatation (n=4). 5. In conclusion, NCE inhibits antidromic vasodilatation, a component of neurogenic inflammation, in rat skin while NST is without effect. NST, at the small-diameter sensory ending, is not an effective antagonist of NCE.

Inhibition of calcium currents in rat colon sensory neurons by K- but not mu- or delta-opioids

Inhibition of calcium currents in rat colon sensory neurons by kappa- but not mu- or delta-opioids. J. Neurophysiol. 80: 3112-3119, 1998. We previously reported that kappa-, but not mu- or delta-opioid receptor agonists (ORAs) have selective, potentially useful peripheral analgesic effects in visceral pain. To evaluate one potential site and mechanism by which these effects are produced, we studied opioid effects on high-voltage activated (HVA) Ca2+ currents in identified (Di-I) pelvic nerve sensory neurons from the S1 dorsal root ganglion (DRG). Results were compared with opioid effects on cutaneous neurons from L5 or L6 DRG. Di-I-labeled DRG cells were voltage clamped (perforated whole cell patch clamp), and HVA Ca2+ currents were evoked by depolarizing 240-ms test pulses to +10 mV from a holding potential of -60 mV. Neither mu-ORAs (morphine, 10(-6 )M, n = 16; [D-Ala2, N-Me-Phe4, Gly-ol5] enkephalin, 10(-6 )M, n = 12) nor delta-ORAs ([D-Pen2, D-Pen5] enkephalin, 10(-7 )M, n = 16; SNC-80, 10(-7 )M, n = 7) affected HVA Ca2+ currents in colon sensory neurons. In contrast, the kappa-ORAs U50, 488 (10(-6 )M), bremazocine (10(-6)M), and nalBzoH (10(-6 )M) significantly attenuated HVA Ca2+ currents in colon sensory neurons; effects on cutaneous sensory neurons were variable. A nonreceptor selective concentration of naloxone (10(-5 )M) and nor-BNI (10(-6 )M), a selective kappa-opioid receptor antagonist, reversed the inhibitory effect of kappa-ORAs. In the presence of N-, P-, or Q-, but not L-type Ca2+ channel antagonists, the effect of U50,488 on HVA Ca2+ currents was significantly reduced. Pretreatment with pertussis toxin (PTX) prevented the inhibition by U50,488. These results suggest that kappa-opioid receptors are coupled to multiple HVA Ca2+ channels in colon sensory neurons by a PTX-sensitive G protein pathway. We conclude that inhibition of Ca2+ channel function likely contributes in part to the peripheral analgesic action of kappa-ORAs in visceral nociception.

DAMGO inhibits prostaglandin E2-induced potentiation of a TTX-resistant Na+ current in rat sensory neurons in vitro

We have tested the hypothesis that the mu-opioid agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO), inhibits prostaglandin E2 (PGE2)-induced modulation of a tetrodotoxin-resistant voltage-gated Na+ current (TTX-R INa) in putative nociceptors in vitro. Patch-clamp electrophysiological techniques were used on cultured dorsal root ganglion neurons from the adult rat. PGE2 (1 microM) induced a 103 +/- 22.8% increase in peak TTX-R INa. The PGE2-induced increase in TTX-R INa in the presence of 1 microM DAMGO (24.9 +/- 7.7%), was significantly less than that induced by PGE2 alone. In contrast, when DAMGO was applied after PGE2, PGE2-induced increase in TTX-R INa (85.3 +/- 19.6%) was not significantly different than the increase in the current induced by PGE2 alone. Preapplication of naloxone (10 microM) blocked DAMGO-induced inhibition of the PGE2-induced increase in TTX-R INa. DAMGO, alone, had no effect on peak TTX-R INa (1.4 +/- 1.5% of baseline). Our observation that DAMGO prevents PGE2-induced potentiation of TTX-R INa is consistent with the suggestion that modulation of TTX-R INa underlies the hyperalgesic agent-induced increase in the excitability of nociceptors associated with sensitization and hyperalgesia. Furthermore, our data suggest that inhibition of hyperalgesic agent induced modulation of TTX-R INa may be a novel mechanism underlying opioid-induced antinociception.

Afferent volley patterns and the spinal release of immunoreactive substance P in the dorsal horn of the anaesthetized spinal cat

Microprobes bearing immobilized antibodies to the C-terminus of substance P were used to measure release of this neuropeptide in the spinal cord of the anaesthetized spinal cat in response to peripheral nerve stimulation. Release of substance P was just detectable in laminae I, II with 150 stimuli (0.5 Hz, 5 min) and was near maximal with 300 stimuli. Using two periods of stimulation of 10 min separated by 15 min, greater levels of substance P were detected during the second period. Fifteen to 25 min after two periods of peripheral nerve stimulation levels of substance P detected by microprobes were still elevated above those present prior to stimulation. Stimulation with bursts of three impulses when delivering a fixed number of stimuli resulted in detection of increased levels of substance P at sites adjacent to the areas of maximal release. The results suggest that maximal release of substance P from the central terminals of primary afferent fibres occurs with relatively few impulses and at low frequencies in agreement with what is known of release from the peripheral terminals of these fibres.

Opioids suppress spontaneous activity of polymodal nociceptors in rat paw skin induced by ultraviolet irradiation

Changes in chemical sensitivity of peripheral nociceptors following injury or inflammation have been studied in in vitro preparation of the saphenous nerve-hind paw skin from adult rats. Heat hyperalgesia in the hind paw was induced by a prior ultraviolet irradiation and the skin from these animals was investigated five days later. Polymodal nociceptors were quiescent in normal skin but were spontaneously active in the majority of fibres after ultraviolet exposure. Capsaicin-induced activation of fine fibres was enhanced after ultraviolet pretreatment. Direct administration of morphine, DAGOL (mu-receptor agonist) and U-69593 (kappa-receptor agonist), but not DPDPE (delta-receptor agonist) to the receptive field produced a concentration-related and naloxone-reversible suppression of spontaneous firing in polymodal nociceptors of ultraviolet-treated skin. Morphine did not reduce the activity of fibres in normal skin when these were driven by KCl depolarization. These data show that polymodal nociceptors change their activity and sensitivity to exogenous chemicals following the induction of peripheral hyperalgesia by ultraviolet irradiation. Specifically, evidence is provided for the expression of opioid sensitivity and inhibition of polymodal nociceptor activity through mu- and kappa-opioid receptors. These observations may account for peripheral antinociceptive actions of opioids during specific states of peripheral hyperalgesia.