Secondary Thermal Hyperalgesia Research Articles

Heat hypersensitivity at a site proximal to a surgically induced osteochondral fracture in horses

ObjectiveTo evaluate thermal nociceptive thresholds (TNTs) before and after inducing a standardized radiocarpal bone osteochondral fracture (OCF) in horses. Study designProspective, controlled, randomized, masked study. AnimalsA group of 10 Thoroughbred fillies aged 2 years. MethodsSkin temperature and TNTs were measured on the skin over the triceps brachii muscle in both the thoracic limbs before (week 0) and weekly (weeks 1–8) after unilateral arthroscopic induction of a radiocarpal OCF (n = 4) or sham surgery (n = 6) followed by a standardized exercise programme. The contralateral, non-operated thoracic limb was used as a control within each horse. Percentage thermal excursion (%TE) defined as %TE = 100 ∗ (TNT – skin temperature)/(cut-off temperature – skin temperature) was calculated. Data were analysed with a mixed-effects model followed by Dunnett’s and Tukey’s tests for within and between-limbs comparisons, respectively; p < 0.05 was considered significant. ResultsSkin temperature in the control limb of OCF horses was significantly higher at week 7 than at week 0 (p = 0.0125). At week 1, TNTs and %TE values in operated limbs of OCF horses were significantly reduced compared with their baseline values at week 0 (p ≤ 0.0153) and their values in contralateral control limbs (p ≤ 0.0024) and operated limbs of sham-operated horses (p ≤ 0.0162). At week 2, TNTs and %TE values in operated limbs of OCF horses remained significantly reduced compared with values in operated limbs of sham-operated horses (p ≤ 0.0248). Conclusions and clinical relevanceCreation of an OCF in a radiocarpal bone induced transitory (<2 weeks) ipsilateral heat hypersensitivity proximal to the surgery site (skin over the triceps brachii muscle) in horses. Surgically induced OCF may cause somatosensory abnormalities consistent with secondary thermal hyperalgesia.

Evaluation of Secondary Thermal Hyperalgesia Resulting from Pulpal Inflammation in Patients with Symptomatic Irreversible Pulpitis

IntroductionInflammation can lead to hyperalgesia and allodynia by activation or sensitization of peripheral and central nervous system neurons. This study aimed to assess the occurrence of secondary thermal hyperalgesia in patients with symptomatic irreversible pulpitis (SIP). MethodsThe cold sensitivity test (visual analog scale) was performed for the tooth with SIP, its adjacent sound tooth, the same sound tooth in the opposite jaw, and the contralateral sound tooth in the opposite quadrant of the same jaw. Next, the tooth with SIP underwent root canal treatment, and 3 weeks later, after complete elimination of pain, the teeth underwent cold sensitivity testing again. ResultsA total of 64 patients, including 41 women and 23 men 18–65 years old, were evaluated in this study. The response to the cold sensitivity test significantly decreased in the tooth with SIP (P < .001), its adjacent sound tooth (P < .001), and the same sound tooth in the opposite jaw (P = .004) but not in the contralateral sound tooth in the opposite quadrant of the same jaw (P = .45) after endodontic treatment. No significant difference was noted between men and women in the groups (P > .05). ConclusionsHypersensitivity to cold test due to pulpal inflammation can also result in exaggerated response of the adjacent sound tooth and the same tooth in the opposite jaw to cold sensitivity test; these observations can be explained by the central and peripheral sensitization mechanisms.

Role of glutamate receptors in the dorsal reticular nucleus in formalin‐induced secondary allodynia

The role of glutamate receptors present in the medullary dorsal reticular nucleus (DRt) in the formalin test and formalin-induced secondary nociception was studied in rats. Secondary mechanical allodynia was assessed with von Frey filaments applied to the rat's hindpaw, and secondary thermal hyperalgesia was evaluated with the tail-immersion test. The selective glutamate receptor antagonists MK801 (N-methyl-D-aspartate receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (AMPA/KA receptor antagonist) and A841720 (metabotropic glutamate 1 receptor antagonist) were injected into the DRt before or 6 days after formalin injection in the rat. In the formalin test, the three antagonists significantly reduced the number of flinches in both phases of the test. DRt microinjection of MK801 or A841720, but not of CNQX, reduced both secondary nociceptive behaviors. Moreover, pre-treatment with the three antagonists injected into the DRt prevented the development of secondary mechanical allodynia and secondary thermal hyperalgesia. Similarly, in these rats, the number of c-Fos-like immunoreactive neurons were markedly reduced in both the superficial and deep lamina of the dorsal horn. Our findings support the role of DRt as a pain facilitator in acute and chronic pain states, and suggest a key role of glutamate receptors during the development and maintenance of formalin-induced secondary allodynia.

Estrogen-dependent, sex-specific modulation of mustard oil-induced secondary thermal hyperalgesia by orphanin FQ in the rat

Activation of opioid receptor-like 1 receptor (ORL 1) by intrathecal administration of orphanin FQ (OFQ), an endogenous ligand for the ORL 1 receptor, has been shown to produce antinociception. In addition, we have recently shown gonadal hormone-dependent, sex-specific modulation of acute spinal nociception such that estrogen attenuated OFQ-induced antinociception in the female whereas testosterone was required for the expression of antinociception in the male. However, sex-related differences in the role of OFQ under hyperalgesic conditions are unknown. Hence, we investigated whether OFQ produces sex-specific modulation of mustard oil-induced secondary thermal hyperalgesia in the rat. Mustard oil application to the hind limb significantly reduced the tail-flick latencies (TFL) in male, and ovariectomized (OVX), estradiol treated ovariectomized (OVX + E), proestrous (ProE) and diestrous (DiE) females. Intrathecal administration of OFQ not only attenuated mustard oil-induced decrease in TFLs, i.e. reversed hyperalgesia, but also led to a significant increase in TFLs above the baseline, i.e. produced antinociception in male, OVX, and diestrous rats. However, OFQ failed to alter TFLs in proestrous or OVX + E females, thus these two groups with elevated estrogen levels remained hyperalgesic following mustard oil treatment. These findings demonstrate that OFQ modulates mustard oil-induced secondary hyperalgesia in an estrogen-dependent, sex-specific manner.

(276) Transdermal cannabidiol has therapeutic potential for amelioration of experimental arthritis and secondary thermal hyperalgesia

(276) Transdermal cannabidiol has therapeutic potential for amelioration of experimental arthritis and secondary thermal hyperalgesia

NMDA receptor-mediated activation of medullary pro-nociceptive neurons is required for secondary thermal hyperalgesia

There is now direct evidence that a class of neurons in the rostral ventromedial medulla (RVM) exerts a net facilitatory influence on spinal nociception. The present experiments were designed to test whether activation of these neurons, referred to as “on-cells”, is required as part of a positive feedback loop leading to secondary hyperalgesia in acute inflammation produced by topical application of mustard oil. Activity of a characterized RVM neuron and paw withdrawals to heat (plantar surface) were recorded in barbiturate-anesthetized rats. Following three baseline trials, mustard oil was applied to the skin above the knee. Cell activity and paw withdrawal latencies were monitored for an additional 45 min. Application of mustard oil produced an increase in on-cell discharge that was associated with a substantial decrease in withdrawal latency of the ipsilateral paw. Blocking on-cell activation using local infusion of the NMDA-receptor antagonist AP5 into the RVM prevented hyperalgesia. Secondary thermal hyperalgesia following mustard oil was also associated with a significant decrease in the firing of “off-cells”, a cell population thought to exert a net inhibitory influence on nociception. Depression of off-cell firing was unaffected by AP5 microinjection. The firing of “neutral cells”, which have no documented role in nociceptive modulation, was unchanged following mustard oil and also unaffected by AP5 infusion in the RVM. Brainstem descending controls are receiving increasing attention in efforts to understand hyperalgesia and persistent pain states. The present experiments demonstrate that a novel, NMDA-mediated activation of on-cells is required for secondary thermal hyperalgesia in acute inflammation.

Role for Medullary Pain Facilitating Neurons in Secondary Thermal Hyperalgesia

The rostral ventromedial medulla (RVM) has recently received considerable attention in efforts to understand mechanisms of hyperalgesia and persistent pain states. Three classes of neurons can be identified in the RVM based on responses associated with nocifensive reflexes: on cells, off cells, and neutral cells. There is now direct evidence that on cells exert a net facilitating effect on spinal nociception and that off cells depress nociception. These experiments tested whether the secondary hyperalgesia produced by topical application of mustard oil involves an activation of on cells in RVM. Firing of a characterized RVM neuron and the latencies of withdrawal reflexes evoked by noxious heat were recorded in lightly anesthetized rats before and after application of mustard oil to the shaved skin of the leg above the knee. Mineral oil was applied as a control. Mustard oil produced a significant increase in ongoing and reflex-related discharge of on cells, as well as a decrease in the activity of off cells. neutral cell firing was uniformly unchanged after application of mustard oil. The alterations in on and off cell firing were associated with a significant decrease in the latency to withdraw the paw of the treated limb from the heat stimulus, and this hyperalgesia was blocked by microinjection of lidocaine within the RVM. Withdrawals evoked by heating the contralateral hindpaw, forepaw, and tail were unchanged after mustard oil application. These experiments support a pronociceptive role for on cells and suggest that these neurons contribute to secondary hyperalgesia in inflammation.

Isolation and culture of sensory neurons from the dorsal-root ganglia of embryonic or adult rats.

There is increasing use of isolated sensory neuronal preparations to examine the cellular mechanisms involved in pain signaling. Indeed, these in viro preparations have several advantages that make them beneficial for examining physiological and/or pathological processes affecting neuronal function. With isolated cells it can be determined whether various inflammatory mediators and algogenic agents have direct actions on sensory neurons. Additionally, the intracellular signaling pathways for agents that modulate the excitability and sensitization of sensory neurons can be examined. Finally, the concentrations of mediators and drugs that are used to alter cell function can be well controlled. The purpose of this chapter is to provide the reader with detailed methods for the harvest and growth of embryonic and adult rat sensory neurons (dorsal root ganglia neurons) in culture. Because numerous methods for growing sensory neurons exist, the rationale for certain aspects of the protocols described in the chapter are included, as are discussions of potential pitfalls.

Activation and up-regulation of spinal cord nitric oxide receptor, soluble guanylate cyclase, after formalin injection into the rat hind paw

Nitric oxide synthase is expressed abundantly in the spinal cord, and nitric oxide (NO) has been shown to play important roles in the central mechanism of inflammatory hyperalgesia. However, the expression and function of the NO receptor, soluble guanylate cyclase, is not fully understood in this processing at the spinal cord level. In the present study, we report that the soluble guanylate cyclase α 1 subunit but not the β 1 subunit was expressed in rat spinal cord, particularly in the dorsal horn. We showed that intrathecal administration of a selective inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, produced a significant anti-nociception demonstrated by the decrease in the number of flinches and shakes in the formalin-induced inflammatory pain model. This was accompanied by a marked reduction in formalin-induced c-fos expression in the spinal cord. During formalin-induced long-lasting inflammation, we found that the expression of the α 1 subunit of soluble guanylate cyclase was dramatically increased in the lumbar spinal cord on the second and fourth days after formalin injection into the dorsal side of a hind paw. Intraperitoneal pretreatment with an N-methyl- D-aspartate (NMDA) receptor antagonist, dizocilpine maleate (MK-801), and a neuronal NO synthase inhibitor, 7-nitroindazole, not only significantly blocked formalin-induced secondary thermal hyperalgesia but also suppressed formalin-produced increase in the α 1 subunit of soluble guanylate cyclase in the spinal cord. The present results indicate that peripheral inflammation not only initially activates but also later up-regulates soluble guanylate cyclase expression via the NMDA receptor–NO signaling pathway, suggesting that soluble guanylate cyclase might be involved in the central mechanism of formalin-induced inflammatory hyperalgesia in the spinal cord.

Group I metabotropic glutamate receptor antagonists block secondary thermal hyperalgesia in rats with knee joint inflammation.

Activation of ionotropic glutamate receptors has been shown previously to be essential for the development of secondary thermal hyperalgesia. The present study assessed involvement of group I metabotropic glutamate receptors (mGlu) in both the induction and maintenance phases of secondary thermal hyperalgesia initiated by knee joint inflammation in rats. The dose dependence of each drug in antagonism of thermal hypersensitivity was demonstrated in pre- and post-treatment paradigms. Knee joint inflammation was induced by injection of kaolin and carrageenan. Four hours later the paw withdrawal latencies were significantly shorter than baseline values. Rats were pretreated by spinal microdialysis infusion of group I mGlu receptor antagonists, LY393053 [(+/-)-2-amino-2-(3-cis and trans-carboxycyclobutyl-3-(9-thioxanthyl)propionic acid], LY367385 [(S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid], or AIDA [(R,S)-1-aminoindan-1,5-dicarboxylic acid/UPF 523] before knee joint injection. The paw withdrawal latencies measured 4 h after the injection were significantly longer in the presence of group I mGlu receptor antagonists than those of the artificial cerebrospinal fluid-treated arthritic control group. Post-treatment with the group I mGlu receptor antagonists LY367385 and AIDA allowed significant recovery of the paw withdrawal latencies after the onset of the knee joint inflammation. The knee joint inflammation itself was not affected by either treatment. The results of the present study indicate that secondary thermal hyperalgesia can be effectively attenuated during both the development and maintenance phases of acute knee joint inflammation by spinal application of specific group I mGlu receptor antagonists.

C-fos Antisense Oligodeoxynucleotide Decreases Subcutaneous Bee Venom Injection-Induced Nociceptive Behavior and Fos Expression in the Rat

Oligodeoxynucleotide complementary to c-fos mRNA was applied to characterize its effect on the spinal cord Fos expression and relevant nociceptive behaviors challenged by subcutaneous injection of bee venom to the rat hind paw. Nociceptive behavioral responses (spontaneous pain and hyperalgesia) following bee venom (0.2 mg/50 µl) injection were assessed in adult male Sprague-Dawley rats receiving intrathecal administra- tion of c-fos antisense oligodeoxynucleotide (ASO, 50 µg/ 10 µl), sense oligodeoxynucleotide (SO, 50 µg/10 µl) and saline (10 µl) 4 h prior to bee venom injection. The lumbar spinal cord expression of Fos protein 2 h after bee venom injection in the ASO-, SO- and saline-treated animals was observed by immunohistochemistry. The results showed that pretreatment of c-fos ASO markedly reduced the flinching response and primary thermal hyperalgesia, but without significant effects on mechanical hyperalgesia and secondary thermal hyperalgesia. At the same time, ASO treatment also significantly decreased the expression of Fos protein within the lumbar region of the spinal cord ipsilateral to the injection. The results provide further evidence that Fos protein contributes to the activation of the spinal dorsal horn neurons and the generation and/or maintenance of spontaneous pain and primary thermal hyperalgesia induced by subcutaneous injection of bee venom.

Differential roles of spinal neurokinin 1/2 receptors in development of persistent spontaneous nociception and hyperalgesia induced by subcutaneous bee venom injection in the conscious rat

To evaluate the roles of spinal neurokinin receptors in the development of persistent nociception and hyperalgesia to thermal and mechanical stimuli induced by subcutaneous (s.c.) bee venom injection, effects of intrathecal (i.t.) pre- or post-treatment with a non-selective antagonist of (NK1/2) receptors, [D-Arg1,D-Trp7,9,Leu11] substance P (spantide), and a selective NK3 receptor antagonist, (S)-(N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl) piperidin-3-yl)propyl)-4-phenylpiperidin-4-yl)-N-methyl acetamide (SR142801) were assessed in conscious rat. Injection of bee venom s.c. into the plantar surface of one hind paw resulted in a pathological pain phenomenon characterized by a 1–2h single phase of persistent spontaneous nociceptive behaviors (continuously flinching the injected paw) and a 72–96h profound primary thermal and mechanical hyperalgesia in the injection site and a secondary thermal hyperalgesia in the non-injected hindpaw. Pre-treatment with spantide i.t. at 0.05μg, 0.5μg and 5μg produced a dose-related suppression of the bee venom-induced flinching reflex during the whole time course and the inhibitory rate was 24±12.60% (35.38±4.12 flinches/5min, n=5), 48±6.75% (24.53±2.90 flinches/5min, n=5) and 60±7.69% (18.88±3.58 flinches/5min,n =5) respectively when compared with the saline control group (46.80±2.60 flinches/5min,n =5). Post-treatment of spantide i.t. at the highest dose (5μg) used in the present study 5min after bee venom injection also produced a 49% suppression of the flinching reflex in the control group [post-spantide vs saline: 19.42±3.15 (n=5) vs 38.42±3.25 flinches/5min (n=5)]. Moreover, i.t. pre-treatment with 5μg spantide partially prevented the primary and secondary thermal hyperalgesia from occurring, while it did not show any influence on the development of primary mechanical hyperalgesia. Neither the established thermal nor mechanical hyperalgesia identified in the above sites was affected by i.t. post-treatment with the same dose of spantide 3h after bee venom injection. Pre and post-treatment of SR142801 did not produce any significant effect on the bee venom-induced spontaneous pain and thermal and mechanical hyperalgesia. Our present result suggests that activation of spinal NK1/2 receptors is involved in both induction and maintenance of the persistent spontaneous nociception, while it is only involved in induction of the primary and secondary thermal, but not primary mechanical hyperalgesia induced by s.c. bee venom injection. The spinal NK3 receptor seems not likely to be involved in the bee venom-induced behavioral response characterized by spontaneous pain and thermal and mechanical hyperalgesia.

Changes in nitric oxide synthase isoforms in the spinal cord of rat following induction of chronic arthritis.

Nitric oxide (NO) possibly plays an important role in the events resulting in hyperalgesia. Nitric oxide synthase (NOS) is a key enzyme in the production of NO. In this study, the relationship between NOS and hyperalgesia in a rat chronic arthritis model was tested. Chronic arthritis was induced by injection of incomplete Freund's adjuvant into the knee joint cavity unilaterally. The paw withdrawal latency (PWL) to radiant heat was used to detect secondary thermal hyperalgesia induced by the arthritis. After 1 day the PWL of the arthritic hind-paw decreased and it reached its nadir at 3 days after induction of arthritis. The lumbar and cervical enlargement of the spinal cord were removed in different groups of animals 3, 7, 14, or 21 days after induction of arthritis, and frozen tissue sections were cut. Two series of sections were incubated with polyclonal antibodies to neuronal NOS (nNOS) or to inducible NOS (iNOS). nNOS was found to increase gradually in laminae I-III in the lumbar but not in the cervical enlargement. The change became most obvious 14 days after induction of arthritis as compared to the control animals. Ependymal cells around the central canal of the lumbar enlargement were more densely stained by anti-iNOS after arthritis. A corresponding change was also found in the cervical enlargement. Computer-assisted image analysis revealed that the mean density of the affected areas in the treated group increased significantly compared with the control animals. This study suggests that the expression of both nNOS and iNOS increase following induction of chronic arthritis, which in turn would presumably lead to an increase in the production of NO. This process could be involved in mediation of the secondary thermal hyperalgesia induced by chronic arthritis.