Evidence that capsaicin hyperaemia of rat sciatic vasa nervorum is local, opiate-sensitive and involves mast cells

Activation of opioid μ-receptors, but not δ- or κ-receptors, switches pulmonary C-fiber-mediated rapid shallow breathing into an apnea in anesthetized rats

Rapid shallow breathing (RSB) is mainly mediated by bronchopulmonary C-fibers (PCFs). We asked whether this RSB could be modulated by opioids. In anesthetized rats right atrial bolus injection of phenylbiguanide (PBG) to evoke RSB was repeated after: (1) intravenously giving fentanyl (μ-receptor agonist), DPDPE (δ-receptor agonist), or U-50488H (κ-receptor agonist); (2) fentanyl (iv) following naloxone methiodide, a peripheral opioid receptor antagonist; (3) bilateral microinjection of fentanyl into the nodose ganglia; (4) fentanyl (iv) with pre-blocking histamine H(1) and H(2) receptors by diphenhydramine and ranitidine. Systemic fentanyl challenge, but not DPDPE or U-50488H, switched the PBG-induced RSB to a long lasting apnea. This switch was blocked by naloxone methiodide rather than diphenhydramine and ranitidine. After microinjecting fentanyl into the nodose ganglia, PBG also produced an apnea. Our results suggest that activating μ-receptors is capable of turning the PCF-mediated RSB into an apnea, at least partly, via facilitating PCFs' activity and this switching effect appears independent of the released histamine.

alpha(1)-Adrenoceptors augment thermal hyperalgesia in mildly burnt skin

The effect of the alpha(1)-adrenoceptor agonist phenylephrine on sensitivity to heat was investigated at three sites of mild burn injury in the cutaneous forearm of 19 healthy participants. Two of the sites were pre-treated with the alpha(1)-antagonist terazosin, to determine whether the effect of phenylephrine was mediated by alpha(1)-adrenoceptors. Terazosin was administered before the burn injury at one site, and after the burn injury at the other site. In another 15 participants, the nociceptive effect of the alpha(2)-adrenoceptor agonist clonidine was investigated with and without prior treatment with the alpha(2)-antagonist rauwolscine. Drugs were introduced into the skin by iontophoresis, and burns were induced by heating the skin to 48 degrees C for 2min. Heat pain thresholds to a temperature ramp (0.5 degrees C/s), and heat pain ratings to a thermal stimulus (45 degrees C, 7s), were determined before and after the administration of each drug. Thermal hyperalgesia provoked by phenylephrine was inhibited by terazosin administered after the burn injury, but not by terazosin administered before the burn injury. However, neither alpha(2)-adrenoceptor stimulation nor blockade affected sensitivity to heat in the mildly burnt skin. These findings suggest that stimulation of cutaneous alpha(1)-adrenoceptors increased the excitability of heat-sensitized nociceptive afferents. As terazosin was more effective when administered in burnt skin, an inflammatory response induced by the burn injury may have facilitated access of adrenergic agents to alpha(1)-adrenoceptors.

Mu opioid receptors and analgesia at the site of a peripheral nerve injury

Opioid ligands may exert antinociception through receptors expressed on peripheral afferent axons. Whether local opioid receptors might attenuate neuropathic pain is uncertain. In this work, we examined the function and expression of local mu opioid receptors (MORs) associated with the chronic constriction injury (CCI) model of sciatic neuropathic pain in rats. Low-dose morphine or its carrier were percutaneously superfused over the CCI site with the injector blinded to the identity of the injectate. Morphine, but not its carrier, and not equimolar systemic doses of morphine reversed thermal hyperalgesia in a dose-related, naloxone-sensitive fashion. Moreover, analgesia was conferred at both 48 hours and 14 days after CCI, times associated with very different stages of nerve repair. Equimolar local DAGO ([D-Ala2, N-Me-Phe4, Gly5-(ol)] enkephalin), a selective MOR ligand, provided similar analgesia. Local morphine also attenuated mechanical allodynia. MOR protein was expressed in axonal endbulbs of Cajal just proximal to the injury site, in aberrantly regenerating small axons in the epineurial sheath around the CCI site and in residual small axons distal to the CCI lesion. Sensory neurons ipsilateral to CCI had an increase in the proportion of neurons expressing MOR. We suggest that local MOR expressed in axons may be exploited to modulate some forms of neuropathic pain.

The orofacial capsaicin test in rats: effects of different capsaicin concentrations and morphine

The aim of this study was to develop a rat model of capsaicin-induced pain in the orofacial region. We examined the effects of subcutaneous injection of different doses of capsaicin (0.25, 0.4, 0.8, 1.5, 2.5, 25, 50, 100, 500 microg) on the face-grooming response. Injection of capsaicin into the vibrissa pad produced an immediate grooming of the injected area with ipsilateral fore- or hindpaw. A positive relationship between the amplitude of the grooming response and the capsaicin dose was observed until 1.5 microg, but with the highest concentrations (ranging from 25 to 500 microg) the amplitude of the response decreased. Morphine administered either systemically (in the neck, 0.5-4.0 mg/kg) or locally (0.25-1.0 mg/kg) reduced in a dose-dependent fashion the face grooming provoked by subcutaneous capsaicin (1.5 microg). The systemic and local morphine effects could be reversed by systemic (0.1 mg/kg) and local (0.05 mg/kg) administration of naloxone, respectively. The local administration of morphine (ED(50): 0.65 mg/kg) was more potent than systemic injection (ED50: 2.54 mg/kg) in reducing the grooming behavior. In conclusion, the orofacial capsaicin test appears to be a valid and reliable method for studying trigeminal pain mechanisms and testing analgesic drugs. The results of the present study also support the clinical use of peripheral opioid administration for the treatment of orofacial painful conditions.

The effect of sympathetic activity on thermal hyperalgesia in capsaicin-treated skin during body cooling and warming

An adrenergic mechanism is thought to contribute to pain in conditions that sometimes develop during chronic inflammation and after nerve or tissue injury. There is some doubt, however, about whether adrenergic activity influences nociception in acute inflammation. To investigate this issue, the noncompetitive alpha-(alpha) adrenergic antagonist phenoxybenzamine was introduced by iontophoresis into the skin of 16 healthy volunteers either before or after the topical application of capsaicin. When applied before capsaicin, phenoxybenzamine increased thermal hyperalgesia at normal ambient temperatures and during body warming. These findings suggest that phenoxybenzamine blocked an analgesic mechanism when applied before the onset of inflammation. However, this effect disappeared during body cooling. When applied after capsaicin, phenoxybenzamine inhibited thermal hyperalgesia at normal ambient temperatures, and during body warming and cooling. Thus, phenoxybenzamine blocked a hyperalgesic mechanism when applied after the onset of inflammation. It was concluded that the presence of inflammation influences the nociceptive effect of alpha-adrenergic blockage, possibly by increasing access to excitatory adrenergic receptors on nociceptive afferents. An excitatory adrenergic influence on nociception may overcome an inhibitory adrenergic influence during inflammation.

Evidence that nitric oxide- and opioid-containing interneurons innervate vessels in the dorsal horn of the spinal cord of rats

In the dorsal horn of the spinal cord, activation of small fibre nociceptive afferents leads to the release of nitric oxide and enkephalins by interneurons. In this work we encountered unexpected relationships among local spinal cord dorsal horn blood flow, specific forms of afferent input, nitric oxide and intrinsic opioids. Selective rises in rat lumbar dorsal cord blood flow using laser Doppler flowmetry and microelectrode hydrogen clearance polarography were generated by ipsilateral, 'nociceptive' low (3 Hz) frequency stimulation of sciatic afferents. Inhibitors of nitric oxide synthase (NOS) prevented rises in flow during stimulation without influencing baseline flow. Ipsilateral hindpaw intradermal injection of capsaicin, a nociceptive activator, also generated large rises in flow sensitive to NOS inhibition. During NOS blockade or morphine administration there were unexpected acute declines in the dorsal cord blood flow strictly confined to low frequency stimulation epochs. This acute vasoconstrictive effect was prevented by administration of an opioid receptor antagonist. Using immunohistochemistry, terminals apparently innervating dorsal spinal cord blood vessels were labelled with antibodies against neuronal NOS and met-enkephalin. We conclude that local nitric oxide and opioids, probably from interneurons, have competitive actions on dorsal horn microvessels once interneurons are activated during a nociceptive barrage. Collateral innervation of blood vessels may explain this property.

The microenvironment of injured and regenerating peripheral nerves

Local events in the milieu of injured peripheral nerve trunks may have an important influence on the likelihood of regenerative success or the development of neuropathic pain. Injury-related changes in the microcirculation of this milieu have provided some evidence that axonal endbulbs, structures that form at the proximal end of transected axons, dump peptides and other molecules into the injury milieu where they may exert local actions, including those on microvessels. During a later phase of nerve repair, macrophage influx and pancellular proliferative events appear to develop in a coordinated fashion. Nitric oxide is probably an important and prominent player in the injured nerve trunk, both at early and later stages of the repair process. A better understanding of the injured peripheral nerve microenvironment may allow therapeutic approaches that can enhance regeneration and diminish pain.

Pressure pain thresholds of upper limb peripheral nerve trunks in asymptomatic subjects

BACKGROUND AND PURPOSE

Palpation of peripheral nerve trunks has been advocated as a method of assessing the presence of hyperalgesic nerve tissue as a contributing factor to pain syndromes in musculoskeletal disorders of the upper quadrant. This study investigated, in the first instance, the pressure pain thresholds of the median, radial and ulnar nerve trunks of the upper limb in healthy, asymptomatic subjects.

METHOD

Forty-five male and 50 female healthy volunteer subjects participated in this study which involved measurement of pressure pain thresholds by use of pressure algometry bilaterally over the three peripheral nerve trunks in the upper limbs.

RESULTS

Pressure pain thresholds were shown to be lowest in the median nerve (p = 0.001) and lower in female subjects (p = 0.001). Laterality (p = 0.077) or the age of the subject (p = 0.254) did not significantly influence results.

CONCLUSIONS

The study demonstrated differences in pressure pain thresholds in the three nerve trunks of the upper limb. These findings should be taken into account when interpreting the findings of nerve palpation in musculoskeletal upper quadrant disorders.

Rat peripheral nerve components release calcitonin gene-related peptide and prostaglandin E2 in response to noxious stimuli: evidence that nervi nervorum are nociceptors

The presence of an intrinsic afferent innervation of nerves and their connective tissues (nervi nervorum) suggests that these neural elements participate in sensation and pathological processes affecting nerves. Primary afferent nociceptors contain and release neuropeptides including calcitonin gene-related peptide, implicated in inflammatory vasodilatation. We sought to evaluate the ability of different peripheral nerve components, in vitro, to release calcitonin gene-related peptide and prostaglandin E2 in response to electrical and noxious chemical stimuli, using sensitive enzyme immunoassays. We observed significant increases in both calcitonin gene-related peptide and prostaglandin E2 in response to a mixture of inflammatory mediators (bradykinin, histamine, and serotonin; 10(-5) M) applied to the intact nerves (+37% and +700%, respectively) and isolated sheaths (35% and 430%, respectively), but not when this mixture was applied to isolated axons. Proximal (antidromic) but not distal (orthodromic) electrical stimulation also evoked a comparable release of calcitonin gene-related peptide (+30%) from intact nerves. These results suggest that nervi nervorum nociceptors participate in neural inflammation. Capsaicin (10(-6) M) elicited a very large release of calcitonin gene-related peptide when applied to either the intact nerve (+400%), isolated sheaths (+500%), or isolated axons (1400%). The latter effect was substantially but not completely blocked by Ruthenium Red and capsazepine, and was completely blocked using a calcium-free bathing solution. The results support the presence of capsaicin receptors in peripheral nerves that can effect calcitonin gene-related peptide release from axons as well as from terminals.

Smooth muscle cells are the site of neurokinin-1 receptor localization in the arterial supply of the rat sciatic nerve

The occurrence and distribution of the preferred receptor for the neuropeptide, substance P (SP), the neurokinin-1 receptor (NK1R) was investigated in the vascular supply of the rat sciatic nerve. Messenger RNA for NK1R was demonstrated by RT-PCR in the epineurial layer where the majority of small arteries and arterioles feeding the endoneurial vasculature are located. Immunoreactivity to NK1 R-protein was localized on the smooth muscle cells of these arterial vessels by means of immunofluorescence using a polyclonal NK1R antiserum. This muscular localization of NK1R explains the previously reported [Zochodne, D.W. and Ho, L.T., J. Physiol., 444 (1991) 615-630] moderate vasoconstrictor rather than vasodilator effects of SP in this vascular bed.

The role of peripheral mu opioid receptors in the modulation of capsaicin-induced thermal nociception in rhesus monkeys

Capsaicin produces burning pain, followed by nociceptive responses, such as allodynia and hyperalgesia in humans and rodents. In the present study, when administered subcutaneously into the tail of rhesus monkeys, capsaicin (0.01-0.32 mg) dose-dependently produced thermal allodynia manifested as reduced tail-withdrawal latencies in 46 degrees C water, from a maximum value of 20 sec to approximately 2 sec. Coadministration of selective mu opioid agonists, fentanyl (0.003-0.1 mg) and (D-Ala2,N-Me-Phe4, Gly5-ol)-enkephalin (0.001-0.03 mg), dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by small doses of opioid antagonists, quadazocine (0.03 mg) and quaternary naltrexone (1 mg), applied locally in the tail. However, these doses of antagonists injected s.c. in the back did not antagonize local fentanyl. Comparing the relative potency of either agonist or antagonist after local and systemic administration confirmed that the site of action of locally applied mu opioid agonists is in the tail. These results provide evidence that activation of peripheral mu opioid receptors can diminish capsaicin-induced allodynia in primates. This experimental pain model could be a useful tool for evaluating peripherally acting antinociceptive agents without central side effects and enhance new approaches to the treatment of inflammatory pain.

Enhancement of thermal hyperalgesia by alpha-adrenoceptors in capsaicin-treated skin

This study aimed to investigate whether the endogenous release of noradrenaline would influence hyperalgesia to heat in skin sensitized by the topical application of 0.6% capsaicin. To release endogenous stores of noradrenaline, tyramine was introduced transcutaneously by iontophoresis into the volar aspect of the forearm of 19 healthy subjects. The heat pain threshold fell from 43.7 +/- 3.8 degrees C to 41.3 +/- 4.0 degrees C after the iontophoresis of tyramine in capsaicin-treated skin (P < 0.001), but did not change significantly after tyramine iontophoresis in untreated skin. The heat pain threshold decreased by 0.5 +/- 2.2 degrees C after the iontophoresis of saline, indicating that nonspecific factors did not fully account for the hyperalgesic effect of tyramine. Iontophoresis of the alpha-adrenergic antagonist, phenoxybenzamine, after the capsaicin treatment blocked the hyperalgesic effect of tyramine, suggesting that thermal hyperalgesia was mediated by alpha-adrenoceptors. However, iontophoresis of phenoxybenzamine before the capsaicin treatment was ineffective. These findings suggest that release of endogenous stores of noradrenaline increases sensitivity to heat in skin sensitized by capsaicin. In addition, neurogenic inflammation appears to increase access to the receptors that facilitate thermal hyperalgesia.

Neural connection between the ventral portion of the lumbar intervertebral disc and the groin skin

This study was designed to investigate neural mechanisms of referred pain in lumbar intervertebral disc lesions. Patients with a degenerative disc in lower lumbar segments occasionally complain of groin pain, which cannot be explained anatomically as having a radicular origin. In rats pretreated with intravenous application of Evans blue dye, the dye extravasation appeared in the groin skin after application of capsaicin to the ventral portion of the L5-6 intervertebral disc. This response occurred even in rats with a sectioned L-5 spinal nerve and sympathetic trunks, but did not occur in rats with a sectioned genitofemoral nerve. Capsaicin topically applied to the sciatic nerve did not cause dye extravasation in the hindpaw. Therefore, groin dye extravasation was not due to a direct effect of capsaicin but, rather, presumably was caused by an "antidromic axon reflex" of dichotomizing C fibers or to a segmental sympathetic reflex causing vascular permeability. The present results indicate that the ventral portion of the lumbar discs is neurally connected to the groin skin via the upper (L-2) lumbar spinal nerves in rats. Groin pain coincident with low-back pain may be explained as referred pain, indicating that a lesion is present in the ventral portion of the lumbar intervertebral disc space.