Sympathectomy decreases formalin-induced nociceptive responses independent of changes in peripheral blood flow

Chronic inflammatory pain does not attenuate the development of tolerance to chronic morphine in adult male rats

The overall impact of chronic pain on the response to opioids is ambiguous in the literature, and comparisons between human and animal studies are complicated by vast differences between the manner and dosage of opioids given to humans treated for pain in comparison to rodents as well as a lack of healthy participant studies examining the impact of chronic opioids. The purpose of this study was to evaluate the impact of chronic pain on the development of tolerance to morphine and to assess how the concentration of drug affects this process. Twenty-four hours after the injection of CFA or normal saline in the left hind paw, the level of mechanical hypersensitivity was assessed and animals were randomly assigned to a morphine dose (1, 3 or 8 mg/kg or saline). Morphine was administered by subcutaneous injection twice a day for 5 days. On Day 6, animals were challenged with a single dose of 3 mg/kg morphine prior to formalin testing. Evidence of tolerance was mixed, and the results varied widely among the conditions. Analysis of mean paw withdrawal thresholds indicated that the analgesic efficacy of subcutaneous morphine diminished following repeated dosing. The presence of the chronic inflammatory pain condition during the morphine dosing period produced an increase in formalin pain behaviors compared to saline controls, such that animals given any dose of morphine during the 5-day dosing period showed higher responding to formalin following the 3 mg/kg dose than animals that had received saline injections. These results indicate that chronic pain does influence the development of opioid tolerance, but it does not prevent this phenomenon from occurring as suggested by some researchers.

Examining the role of the medial thalamus in modulating the affective dimension of pain

The purpose of this project was to explore the role of the medial thalamus (MT), including the medial dorsal thalamus (MD) and associated midline nuclei in pain processing. Experiment 1 explored the role of electrolytic lesions to the MT in the formalin test. It was hypothesized that animals with electrolytic lesions to the MT would have attenuated paw licking behavior during the second phase of the formalin tests as compared to sham lesion controls. This hypothesis was based on evidence of projections from the MD to the ACC, and previous research demonstrating attenuation of paw licking behavior in the second phase of the formalin test in animals with ACC lesions. Experiment 2 tested the effects of electrolytic MT lesions on mechanical paw withdrawal thresholds in the L5 nerve ligation model. It was hypothesized that lesions of the MT would not alter mechanical paw withdrawal thresholds. Experiment 3 tested the effects of electrolytic MT lesions on escape/avoidance behavior in the place escape avoidance paradigm. For experiment 1, animals with MT lesions were found to have slightly elevated paw licking behavior, but only across two time points. No differences in mechanical paw withdrawal thresholds and in escape/avoidance behavior were detected as compared to the sham lesion group. These results indicate a limited role for the medial thalamic nuclei in coding for pain intensity and the affective dimension of pain. Additional research is needed to explore the role of individual medial nuclei in pain processing.

Involvement of bradykinin, cytokines, sympathetic amines and prostaglandins in formalin-induced orofacial nociception in rats

1. This study characterises some of the mechanisms and mediators involved in the orofacial nociception triggered by injection of formalin into the upper lip of the rat, by assessing the influence of various treatments on behavioural nociceptive responses (duration of facial rubbing) elicited either by a low subthreshold (i.e. non-nociceptive; 0.63%) or a higher concentration of the algogen (2.5%). 2. The kininase II inhibitor captopril (5 mg kg(-1), s.c.) and prostaglandin(PG) E(2) (100 ng lip(-1)) potentiated both phases of the response to 0.63% formalin, whereas tumour necrosis factor (TNF alpha; 5 pg lip(-1)), interleukin(IL)-1 beta (0.5 pg lip(-1)), IL-6 (2 ng lip(-1)) and IL-8 (200 pg lip(-1)), or the indirectly acting sympathomimetic drug tyramine (200 microg lip(-1)), each augmented only the second phase of nociception. 3. Conversely, both phases of nociception induced by 2.5% formalin were inhibited by the bradykinin (BK) B(2) receptor antagonist HOE140 (5 microg lip(-1)) or the selective beta(1)-adrenoceptor antagonist atenolol (100 microg lip(-1)). However, the BK B(1) receptor antagonist des-Arg(9)-Leu(8)-BK (1 and 2 microg lip(-1)), antibody and/or antiserum against each of the cytokines, the adrenergic neurone blocker guanethidine (30 mg kg(-1) day(-1), s.c., for 3 days) and the cyclooxygenase(COX)-2 inhibitor celecoxib (50 and 200 microg lip(-1), s.c.; or 1 and 3 mg kg(-1), i.p.) reduced only the second phase of the response. The nonselective COX inhibitor indomethacin and the 5-lipoxygenase activating protein inhibitor MK886 did not change formalin-induced nociception. 4. Our results indicate that BK, TNF-alpha, IL-1 beta, IL-6, IL-8, sympathetic amines and PGs (but not leukotrienes) contribute significantly to formalin-induced orofacial nociception in the rat and the response seems to be more susceptible to inhibition by B(2) receptor antagonist and selective COX-2 inhibitor than by B(1) receptor antagonist or nonselective COX inhibitor.

Involvement of primary sensory afferents, postganglionic sympathetic nerves and mast cells in the formalin-evoked peripheral release of adenosine

Injection of formalin into the rat hind paw produces a dose-dependent local peripheral release of adenosine. Low doses of formalin (0.5-2.5%) evoke release during the first 10 min following injection, while a high dose of formalin (5%) evokes release lasting for 60 min. The current study was designed to determine the possible origin of release produced by two doses of formalin (1.5% and 5%). Microdialysis probes were implanted into the subcutaneous tissue under the glabrous skin of the hind paw of anaesthetized rats, and adenosine was determined by high performance liquid chromatography. Pretreatment with capsaicin, a neurotoxin selective for unmyelinated small diameter primary afferent nerves, markedly reduced the adenosine released by 1.5% formalin and the early phase of release by 5% formalin. Acute injection of 1% capsaicin to the hind paw of untreated rats also induced adenosine release. Pretreatment with 6-hydroxydopamine, a neurotoxin selective for sympathetic postganglionic nerve terminals, had no effect on release evoked by 1.5% formalin, but significantly reduced adenosine release during the late phase of release induced by 5% formalin. Pretreatment with compound 48/80, which degranulates mast cells, had no effect on adenosine release evoked by either concentration of formalin. We conclude that the origin of the adenosine released peripherally by formalin depends on the formalin concentration. At the lower concentration (1.5%), release is predominantly from unmyelinated sensory afferent nerve terminals, while at the higher concentration (5%), unmyelinated afferent nerve terminals are involved in the early phase, while sympathetic postganglionic nerve terminals are involved in the later phase. Mast cells do not contribute to release of adenosine evoked by either concentration of formalin.

Differential nociceptive responses in mice lacking the alpha(1B) subunit of N-type Ca(2+) channels

The role of N-type Ca(2+) channels in nociceptive transmission was examined in genetically engineered mice lacking the alpha(1B) subunit of N-type channels and in their heterozygote and wild-type littermates. In alpha(1B)-deficient mice, N-type channel activities in dorsal root ganglion neurons and spinal synaptoneurosomes were eliminated without compensation by other types of voltage-dependent Ca(2+) channels. The alpha(1B)-deficient mice showed a diminution in the phase 2 nociceptive responses more extensively than in the phase 1 nociceptive responses of the formalin test. The alpha(1B)-deficient mice exhibited significantly increased thermal nociceptive thresholds in the hot plate test, but failed to increase mechanical nociceptive thresholds in the tail pinch test. These results suggest a crucial role of N-type channels in nociceptive transmission, especially for persistent pain like phase 2 of the formalin test and for nociception induced by thermal stimuli.

Heat, but not mechanical hyperalgesia, following adrenergic injections in normal human skin

The development of adrenergic sensitivity in nociceptors has been suggested as a mechanism of neuropathic pain. We sought to determine if nociceptors in the skin of normal subjects exhibit adrenergic sensitivity. We investigated the effects of intradermal administration of norepinephrine, phenylephrine, and brimonidine on heat pain sensitivity. Norepinephrine and phenylephrine (in concentrations ranging from 0.1 to 10 microM by factors of 10), brimonidine (at 0.01-1 microM), and saline were injected (30 microl volume) in a random, double-blind manner to different sites on the volar surface of the forearm in ten subjects. Before and after the injections, heat testing was performed with a non-contact laser thermal stimulator. Heat pain threshold was measured by means of a 'Marstock' technique in which subjects pressed a reaction time key when they perceived that a slowly increasing heat stimulus (1 degrees C/s ramp from a 36 degrees C base) was painful. In addition, the subjects used magnitude estimation techniques to rate the intensity of pain to a suprathreshold heat stimulus (47 degrees C, 2 s). Mechanical testing was done using 200-microm diameter probes attached to calibrated weights that provided forces over the range of 16-512 mN. The intradermal injections of norepinephrine, phenylephrine and brimonidine produced little evoked pain. However, a dose-dependant decrease in heat pain threshold, but not mechanical pain threshold, was observed. At the highest drug dose injected, all three adrenergic compounds produced a significant decrease in heat pain threshold compared to the saline injection. A significant increase in response to the suprathreshold heat stimulus was also found. One possible explanation for this apparent heat hyperalgesia is that the decrease in perfusion due to the localized vasoconstriction may alter the heat response. However, in control studies we found that the non-adrenergic vasoconstrictors, angiotensin II and vasopressin did not produce heat hyperalgesia at doses that produced comparable decreases in blood flow. In addition, occlusion of blood flow with a blood pressure cuff did not lead to heat hyperalgesia. Thus, the heat hyperalgesia observed with the adrenergic agonists is not due to a decrease in perfusion associated with the injection. These results indicate that alpha(1)- and alpha(2)-adrenoceptor-mediated mechanisms may play a role in sensitization of nociceptors to heat stimuli in normal skin.