Facilitatory effects of opioids on the discharges of visceral nociceptors

Systemic morphine treatment induces changes in firing patterns and responses of nociceptive afferent fibers in mouse glabrous skin

Patients receiving opioids for pain may experience decreased effectiveness of the drug and even abnormal pain sensitivity-hyperalgesia and/or allodynia. We hypothesized that peripheral nociceptor hyperexcitability contributes to opioid-induced hyperalgesia and tested this using an in vitro mouse glabrous skin-nerve preparation. Mice were injected intraperitoneally with escalating doses of morphine (5, 8, 10, 15 mg/kg) or saline every 12 hours for 48 hours and killed approximately 12 hours after the last injection. Receptive fields of nociceptors were tested for mechanical, heat, and cold sensitivity. Activity was also measured during an initial 2-minute period and during 5-minute periods between stimuli. Aberrant activity was common in fibers from morphine-treated mice but rare in saline-treated mice. Resting background activity was elevated in C-fibers from morphine-treated mice. Both C- and Aδ-fibers had afterdischarge in response to mechanical, heat, and/or cold stimulation of the skin as well as spontaneous, unevoked activity. Compared to saline, morphine treatment increased the proportion of fibers displaying polymodal rather than mechanical-only responses. A significant increase in Aδ-mechanoreceptive fibers responding to cold accounted for most of this change. In agreement with this, morphine-treated mice showed increased sensitivity in the cold tail flick test. In morphine-treated mice, aberrant activity and hyperexcitability of nociceptors could contribute to increased pain sensitivity. Importantly, this activity is likely driving central sensitization, a phenomenon contributing to abnormal sensory processing and chronic pain. If similar changes occur in human patients, aberrant nociceptor activity is likely to be interpreted as pain and could contribute to opioid-induced hyperalgesia.

Functional role of peripheral opioid receptors in the regulation of cardiac spinal afferent nerve activity during myocardial ischemia

Thinly myelinated Aδ-fiber and unmyelinated C-fiber cardiac sympathetic (spinal) sensory nerve fibers are activated during myocardial ischemia to transmit the sensation of angina pectoris. Although recent observations showed that myocardial ischemia increases the concentrations of opioid peptides and that the stimulation of peripheral opioid receptors inhibits chemically induced visceral and somatic nociception, the role of opioids in cardiac spinal afferent signaling during myocardial ischemia has not been studied. The present study tested the hypothesis that peripheral opioid receptors modulate cardiac spinal afferent nerve activity during myocardial ischemia by suppressing the responses of cardiac afferent nerve to ischemic mediators like bradykinin and extracellular ATP. The nerve activity of single unit cardiac afferents was recorded from the left sympathetic chain (T₂-T₅) in anesthetized cats. Forty-three ischemically sensitive afferent nerves (conduction velocity: 0.32-3.90 m/s) with receptive fields in the left and right ventricles were identified. The responses of these afferent nerves to repeat ischemia or ischemic mediators were further studied in the following protocols. First, epicardial administration of naloxone (8 μmol), a nonselective opioid receptor antagonist, enhanced the responses of eight cardiac afferent nerves to recurrent myocardial ischemia by 62%, whereas epicardial application of vehicle (PBS) did not alter the responses of seven other cardiac afferent nerves to ischemia. Second, naloxone applied to the epicardial surface facilitated the responses of seven cardiac afferent nerves to epicardial ATP by 76%. Third, administration of naloxone enhanced the responses of seven other afferent nerves to bradykinin by 85%. In contrast, in the absence of naloxone, cardiac afferent nerves consistently responded to repeated application of ATP (n = 7) or bradykinin (n = 7). These data suggest that peripheral opioid peptides suppress the responses of cardiac sympathetic afferent nerves to myocardial ischemia and ischemic mediators like ATP and bradykinin.

High concentrations of morphine sensitize and activate mouse dorsal root ganglia via TRPV1 and TRPA1 receptors

Background

Morphine and its derivatives are key drugs in pain control. Despite its well-known analgesic properties morphine at high concentrations may be proalgesic. Particularly, short-lasting painful sensations have been reported upon dermal application of morphine. To study a possible involvement of TRP receptors in the pro-nociceptive effects of morphine (0.3 – 10 mM), two models of nociception were employed using C57BL/6 mice and genetically related TRPV1 and TRPA1 knockout animals, which were crossed and generated double knockouts. Hindpaw skin flaps were used to investigate the release of calcitonin gene-related peptide indicative of nociceptive activation.

Results

Morphine induced release of calcitonin gene-related peptide and sensitized the release evoked by heat or the TRPA1 agonist acrolein. Morphine activated HEK293t cells transfected with TRPV1 or TRPA1. Activation of C57BL/6 mouse dorsal root ganglion neurons in culture was investigated with calcium imaging. Morphine induced a dose-dependent rise in intracellular calcium in neurons from wild-type animals. In neurons from TRPV1 and TRPA1 knockout animals activation by morphine was markedly reduced, in the TRPV1/A1 double knockout animals this morphine effect was abrogated. Naloxone induced an increase in calcium levels similar to morphine. The responses to both morphine and naloxone were sensitized by bradykinin.

Conclusion

Nociceptor activation and sensitization by morphine is conveyed by TRPV1 and TRPA1.

Vagotomy prevents morphine-induced reduction in Fos-like immunoreactivity in trigeminal spinal nucleus produced after TMJ injury in a sex-dependent manner

Acute injury to the temporomandibular joint (TMJ) region activates neurons in multiple, but spatially discrete, areas of the trigeminal spinal nucleus as seen by an increase in Fos-like immunoreactive neurons (Fos-LI). Pretreatment with morphine greatly reduces Fos-LI produced in the dorsal paratrigeminal area (dPa5), ventrolateral pole of the subnucleus interpolaris/caudalis (Vi/Vc-vl) transition region, and laminae I-II at the subnucleus caudalis/upper cervical cord junction (Vc/C2) suggesting a role for these areas in processing pain signals from the TMJ region. To determine if vagal afferents contribute to neural activation after TMJ injury or reduction of activity after morphine, Fos-LI was quantified in the lower brainstem and upper cervical spinal cord of intact and vagotomized male and female rats under barbiturate anesthesia. Bilateral cervical vagotomy (VgX) did not affect Fos-LI produced by TMJ injury in males or females in the absence of morphine. By contrast, morphine-induced reduction in Fos-LI produced at the Vi/Vc-vl transition region was prevented by prior VgX in males and diestrus females, but not in proestrus females. Morphine inhibition of Fos-LI produced in laminae I-II at the Vc/C2 junction region was diminished in vagotomized males compared to intact animals, but not affected in females. In an autonomic control area, the caudal ventrolateral medulla (CVLM), VgX reversed the morphine-induced reduction in Fos-LI in males and females similarly compared to their respective intact controls. These results were consistent with the hypothesis that the Vi/Vc-vl transition region plays a unique role in deep craniofacial pain processing and may integrate autonomic and opioid-related modulatory signals in a manner dependent on sex hormone status.

Opioid-receptor-mediated excitation of rat mesenteric afferent fibres supplying the rat jejunum

The aim of the present study was to examine the sensitivity to opioid-receptor agonists of mesenteric afferents supplying the small intestine and to characterize the subpopulations of any responsive fibres. Mesenteric afferent discharge was recorded electrophysiologically in response to cumulative doses (1-400 microgram kg-1) of the mu-receptor agonist [D-ala,2 N- me-Phe4, Gly5-ol]-enkephalin (DAMGO), the delta-receptor agonist [D-ala,2 D-leu5]-enkephalin (DADLE) and the kappa-receptor agonist U-50488. DAMGO and DADLE, but not U-50488, markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05) that was unrelated to intestinal motor events. Subpopulations of afferent fibres responding to DAMGO were examined using waveform analysis to identify single units from within the whole mesenteric nerve bundles. One population was CCK-sensitive (15/15 fibres) and the other was a subpopulation of mechanosensitive afferents that responded to distension (17/28). 5-HT-sensitive afferents did not respond to DAMGO (0/11). We conclude that specific subpopulations of mesenteric afferents respond to mu- and possibly delta- but not kappa-receptor agonists. This sensitivity to opioids may contribute to the antinociceptive property of vagal afferents.

Differential effects of mu-, delta-, and kappa-opioid receptor agonists on mechanosensitive gastric vagal afferent fibers in the rat

Single-fiber recordings were made from the decentralized right cervical vagus nerve (hyponodosal) of the rat. A total of 56 afferent fibers that responded to gastric distension (GD) were studied: 6 fibers were stimulated by phasic balloon GD, 50 by fluid GD. All fibers gave increasing responses to increasing pressures of GD (5-60 mmHg). The effects of mu-opioid (morphine), delta-opioid (SNC80), and kappa-opioid (EMD61,753, U62,066) receptor agonists were tested on responses of afferent fibers to GD. Morphine, administered systemically over a broad dose range (10 microg to 31 mg/kg, cumulative), had no effect on either resting activity or responses of vagal afferent fibers to GD. Similarly, the delta-opioid receptor agonist SNC80 (0.05-3.2 mg/kg) did not affect resting activity or responses to GD. In contrast, cumulative intra-arterial doses of the kappa-opioid receptor agonist EMD61,753 or U62,066 dose dependently attenuated afferent fiber responses to GD. Doses producing inhibition to 50% of the control response to GD of EMD61,753 (8.0 mg/kg) and U62,066 (8.8 mg/kg) did not differ. The effect of U62,066 was moderately attenuated by a nonselective dose (4 mg/kg) of naloxone hydrochloride; the kappa-opioid receptor-selective antagonist nor-BNI (20 mg/kg) was ineffective. These results demonstrate that kappa-, but not mu- or delta-opioid receptor agonists modulate visceral sensation conveyed by vagal afferent fibers innervating the stomach. Given that kappa-opioid receptor agonists effects were only modestly antagonized by naloxone and not at all by nor-BNI, the results point to a novel site of action.

The effect of intradermal administration of lidocaine and morphine on the response to thermal stimulation

Opioids appear to exert a peripheral effect by gaining access to peripheral opioid receptors. It has been proposed that inflammatory processes and highly osmotic substances could alter the perineural barrier, thereby allowing easy access to opioid receptors. Although local anesthetics do not have osmotic activity, they are highly active on neural tissue and appear to work synergistically with opioids when administered for major conduction blockade. We therefore evaluated, in a double-blind fashion, the combination of lidocaine plus morphine in an attempt to provide a scientific basis for the use of a combination of morphine plus local anesthetics in the periphery. Seven thermal stimuli in 2 degrees C increments (range 40-52 degrees C) were delivered in a random sequence by a computer-controlled thermistor to one of three pretreated sites on 10 volunteers' forearms: reference site (no injection), lidocaine site (0.1-mL intradermal injection of lidocaine 0.5%), or lidocaine plus morphine site (0.1 mL of 0.5 mg of morphine plus lidocaine 1%). Pain responses to the thermal stimuli were rated by the volunteers using the method of magnitude estimation. Pain scores indicated that the combination of lidocaine plus morphine was not more effective than lidocaine alone in attenuating the heat-induced pain. Twenty and 120 min after injection, scores at the lidocaine plus morphine site were 37% and 20% greater than those at the lidocaine site. The addition of morphine to lidocaine did not result in an improvement in the analgesic efficacy and actually had an antianalgesic effect.

Sensory afferent processing in multi-responsive DRG neurons

The recent advance in molecular and neurobiological techniques disclosed the multi-responsive nature of DRG neurons. The survival, phenotype expression and electrical properties of these neurons are under the control of a variety of substances through their specific receptors. In pathological conditions, such as tissue inflammation or nerve injury, DRG neurons change their responsiveness through the dynamic reconstruction of their receptor system. This reconstruction is initiated by environmental stimuli. Thus the properties of polymodal nociceptors can be altered according to the environmental conditions. The whole story of this mechanism is not disclosed yet. In order to understand this mechanism, it is basically important to identify various receptor mRNAs in DRG neurons, precise localization of receptor proteins, site of synthesis and route of supply of ligands for these receptors.

Morphology of thalamocortical neurons projecting to the primary somatosensory cortex and their relationship to spinothalamic terminals in the squirrel monkey

This study examined the morphology of thalamocortical neurons projecting to the primary somatosensory cortex (SI; hand region of areas 3a, 3b, 1, and 2) and their relationship to the spinothalamic (STT) terminals in the squirrel monkey. Retrogradely labeled thalamocortical neurons were intracellularly filled with Lucifer yellow (LY), and the STT terminals were anterogradely labeled with biotinylated dextran. Both filled neurons and labeled terminals were differentially visualized in the same field by a dual immunocytochemical staining method. SI-projecting neurons appeared at the light level to be in contact with STT terminal boutons in the ventroposterior lateral (VPL), ventroposterior inferior (VPI), and centrolateral (CL) nuclei and the posterior complex (PO). The analyses of the neuronal morphology revealed that somatic and dendritic morphologies of SI-projecting neurons in these thalamic nuclei, as well as in the anterior pulvinlar (Pulo), centromedial (CM), and ventrolateral (VL) nuclei, were generally comparable with some exceptions: VL neurons had the largest soma sizes, the most primary dendrites, and the longest total dendritic length among all neurons studied; VPI neurons had the smallest soma sizes; VPL SI-projecting neurons were different from those in VPI in their soma sizes, shape factors, and orientations; in VPL the cells projecting to the superficial layers of SI were smaller than those projecting to the deeper layers, but in VPI the two groups of neurons were similar in soma sizes. In general, the SI-projecting neurons in VPL, VPI, and CL were similar in their dendritic morphologies and branching patterns, and varied from those in Pulo, PO, CM, and VL.

Influence of morphine on the activity of low-threshold visceral mechanoreceptors in cats with acute pericarditis

The purpose of this investigation was to test whether morphine (morphinum hydrochloricum) applied to the receptive field of the thoracic visceral afferent fibres modifies their activity. Experiments were performed on chloralose-anaesthetised cats, paralysed and artificially ventilated, in a state of pericarditis that was induced by intrapericardial injection of lambda-carrageenan and kaolin. Resulting acute inflammation was proven histopathologically and documented electrocardiographically. Single afferent fibres with receptive fields in thoracic viscera were dissected from thoracic sympathetic chain (19 fibres), as well as the vagus nerve (9 fibres). All tested fibres transmitted sensory information from the low-threshold mechanoreceptors. As a final result, it was found that morphine (0.001-1.0 mg/ml) when applied locally activates, depending on the dose, afferent fibres as follows: 12 sympathetic afferents (out of 12 tested), and 7 vagal afferents (out of 9 tested). In examining the specificity of morphine action, the preliminary local application of naloxone (1.0 mg/ml) just before morphine, blocked all excitatory responses. The excitatory response was present whether the receptive field was located in the inflammatory area, or outside it, in group III or IV 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.

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

The effect of morphine was examined under identical conditions on (a) antidromic vasodilatation, an 'efferent' function of C-nociceptors in the skin, and (b) afferent responses to heat and pressure of C-polymodal nociceptors. Morphine caused a large, naloxone-reversible, fall in peak antidromic vasodilatation (ADV). However it caused no significant change in heat or mechanical excitability of C-polymodal nociceptors in normal or mildly inflamed skin. The mechanisms by which morphine might affect efferent, but not afferent, functions of C-nociceptors are discussed.

Differential influence of naloxone on the responses of nociceptive neurons in the superficial versus the deeper dorsal horn of the medulla in the rat

Naloxone (200 micrograms/kg, i.v.) reduced the noxious thermal stimuli-evoked responses of 16/25 nociceptive neurons in the superficial laminae whereas it enhanced the responses of 6/10 nociceptive neurons in the deeper dorsal horn. However, a different picture emerged when selectivity of neuronal responsivity (nocireceptive or multireceptive) was considered. In the superficial dorsal horn, naloxone reduced the responses of the majority of (15/18) selectively nocireceptive neurons. The reduction in responses became apparent within 60 sec following naloxone administration and returned to control level within 48 min. In contrast, the responses of the majority of multireceptive neurons in the superficial (6/7), or the deeper (6/10) dorsal horn, were enhanced. The excitatory action in the superficial dorsal horn persisted for only 6-15 min, whereas it persisted for 40-70 min in the deeper dorsal horn. The firing of the majority of cold-receptive neurons (6/8) in the superficial dorsal horn was not altered. These effects were stereoselective since (+)-naloxone, the inactive isomer of naloxone, did not affect the responses of 14/16 nociceptive neurons. It is concluded that naloxone differentially, and selectively, affects the firing of nociceptive neurons in the superficial versus the deeper dorsal horn, and the firing of selectively nocireceptive versus multireceptive neurons. The relevance of these findings to the behavioral effects of naloxone, hyperalgesia and analgesia, is discussed.

Inhibitory effect of opiates on male rat sexual behavior may be mediated by opiate receptors outside the central nervous system

The importance of opiate receptors outside the central nervous system for the inhibitory actions of morphine on male rat sexual behavior was evaluated. Morphine (10 mg/kg) produced an almost complete inhibition of sexual behavior. This inhibition was antagonized by naloxone at a dose of 1 mg/kg but not at a dose of 0.25 mg/kg. The quaternary opioid antagonist methylnaloxone effectively blocked the inhibitory actions of morphine at a dose of 20 mg/kg but not at a dose of 5 mg/kg. Since the affinity of methylnaloxone for opiate receptors is about 5% of that of naloxone, it may be concluded that both antagonists were about equally effective in inhibiting the effects of morphine. Furthermore, the opiate-like drug loperamide was found to inhibit sexual behavior. This drug acts mainly outside the central nervous system. Its effect was blocked by both naloxone and methylnaloxone, suggesting that opiate receptors are involved. It was also shown that methylnaloxone is unable to block the reinforcing effects of morphine in the conditioned place preference procedure. Because the reinforcing effects of opiates seem to be localized to the central nervous system, it may be proposed that methylnaloxone does not antagonize morphine's central effects. Moreover, loperamide had no effect in the place preference procedure, suggesting that this drug does not act at central opioid receptors. Taken together, these data show that peripheral opioid receptors are responsible for at least some of the inhibitory actions of morphine on male sexual behavior. After treatment with morphine + methylnaloxone, ejaculatory mechanisms were facilitated, reflected in a reduced number of preejaculatory intromissions and a shortened ejaculation latency.(ABSTRACT TRUNCATED AT 250 WORDS)