Biphasic modulation of spinal nociceptive transmission from the medullary raphe nuclei in the rat

The modulatory effects of electrical and chemical (glutamate) stimulation in the rostral ventromedial medulla (RVM) on spinal nociceptive transmission and a spinal nociceptive reflex were studied in rats. Electrical stimulation at a total 86 sites in the RVM in the medial raphe nuclei (n = 54) and adjacent gigantocellular areas (n = 32) produced biphasic (facilitatory and inhibitory, n = 43) or only inhibitory (n = 43) modulation of the tail-flick (TF) reflex. At these 43 biphasic sites in the RVM, facilitation of the TF reflex was produced at low intensities of stimulation (5-25 microA) and inhibition was produced at greater intensities of stimulation (50-200 microA). At 43 sites in the RVM, electrical stimulation only produced intensity-dependent inhibition of the TF reflex. Activation of cell bodies in the RVM by glutamate microinjection reproduced the biphasic modulatory effects of electrical stimulation. At biphasic sites previously characterized by electrical stimulation, glutamate at a low concentration (5 nmol) produced facilitation of the TF reflex; a greater concentration (50 nmol) only inhibited the TF reflex. In electrophysiological experiments, electrical stimulation at 62 sites in the RVM produced biphasic (n = 26), only inhibitory (n = 26), or only facilitatory (n = 10) modulation of responses of lumbar spinal dorsal horn neurons to noxious cutaneous thermal (50 degrees C) or mechanical (75.9 g) stimulation. Facilitatory effects were produced at lesser intensities of stimulation and inhibitory effects were produced at greater intensities of stimulation. The apparent latencies to stimulation-produced facilitation and inhibition, determined with the use of a cumulative sum method and bin-by-bin analysis of spinal neuron responses to noxious thermal stimulation of the skin, were 231 and 90 ms, respectively. The spinal pathways conveying descending facilitatory and inhibitory influences were found to be different. Descending facilitatory influences on the TF reflex were conveyed in ventral/ventrolateral funiculi, whereas inhibitory influences were conveyed in dorsolateral funiculi. The results indicate that descending inhibitory and facilitatory influences can be simultaneously engaged throughout the RVM, including nucleus raphe magnus, and that such influences are conveyed in different spinal funiculi.     

Solution and gel rheology of a new polysaccharide excreted by the bacterium Alteromonas sp. strain 1644

The present series of experiments were designed to examine a potential role for central descending pain facilitatory systems in mediating secondary hyperalgesia produced by topical application of mustard oil and measuring the nociceptive tail-flick reflex in awake rats. Topical application of mustard oil (100%) to the lateral surface of the hind leg produced a facilitation of the tail-flick reflex that was significantly reduced in spinal transected animals. Mustard oil hyperalgesia was also inhibited in animals that had received electrolytic lesions in the rostral ventromedial medulla (RVM). Intrathecal (i.t.) administration of the non-selective cholecystokinin (CCK) receptor antagonist proglumide (10 micrograms) prior to mustard oil application completely blocked both the lesser and greater hyperalgesic responses observed in spinal transected and normal animals, respectively, and produced an inhibition of the tail-flick reflex in normal animals. Administration of the selective CCKB receptor antagonist L-365260 i.t. dose-dependently inhibited mustard oil hyperalgesia (ID50 = 364 ng) at doses approximately 5-fold less than the CCKA receptor antagonist devazepide (ID50 = 1760 ng). Similar to spinal proglumide, microinjection of the neurotensin antagonist SR48692 (3.5 micrograms) into the RVM blocked mustard oil hyperalgesia and inhibited the tail-flick reflex. These data suggest that secondary hyperalgesia produced by mustard oil is mediated largely by a central, centrifugal descending pain facilitatory system which involves neurotensin in the RVM and spinal CCK (via CCKB receptors). The inhibition of the tail-flick reflex produced by mustard oil following spinal or supraspinal administration of receptor antagonists suggests concurrent activation of central descending facilitatory and inhibitory systems.     

Tolerance to the antinociceptive effect of morphine in spinally transected rats.

Examined the effect of a spinal transection (ST) on morphine (MOR)-induced tolerance in rats with the tail withdrawal reflex (tail flick; TF), elicited by noxious thermal stimulation. Intact Ss became tolerant to sc MOR injections if they were tested on the TF after each injection. MOR administration alone did not produce tolerance; TF tests alone did, although not always to a significant extent. However, when MOR only, TF tests only, or both were administered prior to ST (acute spinal Ss), all groups were tolerant when tested 1 day after spinalization. When the same treatments were administered to Ss 3 wks after ST (chronic spinal Ss), neither MOR nor TF tests alone produced tolerance. Chronic spinal Ss became tolerant only if they were tested after each injection. Results suggest that tolerance develops at the spinal cord as a result of either chronic opiate exposure or performance of the nociceptive response, but in intact Ss, tolerance is inhibited or suppressed by a supraspinal action of MOR. Results also suggest that such tolerance is mediated by descending input or that ST produced intrinsic changes in the spinal cord that preclude the development of tolerance induced only by opiate or behavioral stimulation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Midday exposure to bright light changes the circadian organization of plasma melatonin rhythm in humans

With the aim to develop a simple behavioural method for the study of hyperalgesic processes and for the evaluation of anti-hyperalgesic properties of analgesic drugs, the effect of the tail injection of formalin (10% formaldehyde intradermally) on hindpaw nociceptive thresholds to thermal stimulation was evaluated in the rat. The formalin injection in the tail induced a significant reduction of plantar test latencies. The pretreatment with the competitive N-methyl-D-aspartate (NMDA) receptor antagonist CGP 37849 (2.0 mg/kg) completely blocked the hyperalgesic action of the formalin. The analgesic drug paracetamol (25 mg/kg) was able to prevent hyperalgesia. However, this drug was unable to block hyperalgesia when already established. Our results suggest that this method could be used for the evaluation of analgesic drugs in an experimental setting representative of clinical pain.     

Spinal cholinergic and monoamine receptors mediate the antinociceptive effect of morphine microinjected in the periaqueductal gray on the rat tail, but not the feet

The antinociceptive effects of morphine (5 micrograms) microinjected into the ventrolateral periaqueductal gray were determined using both the tail flick and the foot withdrawal responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of appropriate antagonists was used to determine whether the antinociceptive effects of morphine were mediated by alpha 2-noradrenergic, serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray was reversed by intrathecal injection of the cholinergic muscarinic receptor antagonist atropine, but was not affected by the alpha 2-adrenoceptor antagonist yohimbine, the serotonergic receptor antagonist methysergide, or the opioid receptor antagonist naloxone. In contrast, the increase in the tail flick response latency produced by morphine was reduced by either yohimbine, methysergide or atropine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibits the nociceptive responses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic and alpha 2-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptors appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.     

NMDA receptor-dependent long term hyperalgesia after tail amputation in mice

Amputation of the mouse tail tip (2.5 cm) caused long term thermal and mechanical hyperalgesia in the remaining part of the tail. Hyperalgesia of the hindpaw to noxious heat (55 degrees C) and cold (0 degrees C) stimuli were also observed. Hyperalgesia at both the tail and hindpaw had a rapid onset (< or = 30 min) and long lasting (> or = 7 days) effect. Skin temperature of the remaining tail or hindpaw was not significantly affected by the amputation. Heat injury of the tail in normal mice induced short but not long term hyperalgesia (< or = 48 h). Intrathecal pretreatment with NMDA receptor antagonists significantly attenuated long term hyperalgesia caused by tail amputation. These results strongly suggest that spinal NMDA receptors are critical for the induction of hyperalgesia by tail amputation, and the current mouse model may prove useful for investigating mechanisms of persistent pain after amputation.     

Nociceptive c-fos expression in supraspinal areas in avoidance of descending suppression at the spinal relay station

The number and distribution of Fos-like-immunoreactive neurons in different supraspinal brain areas induced by formalin injection into one hindpaw was estimated in rats with transected dorsal half of the spinal cord at the thoracic level in an attempt to avoid most of the descending modulatory actions. The results showed that: (i) after spinal lesion, the peripheral noxious inputs, going up mainly through the ventral spinal cord, elicited a more widespread and densely located Fos-like-immunoreactive neurons in subcortical areas, many of them showed no Fos expression when noxious stimulation was given in rats with intact spinal cord; (ii) at the same time, a small number of subcortical areas, such as the lateral ventroposterior thalamic nucleus and dorsal raphe nucleus, exhibited no significant increase of nociceptive Fos-like immunoreactive neurons after spinal lesion as compared to that with intact spinal cord; and (iii) there appeared a prominent expansion of cortical areas with densely located Fos-like-immunoreactive neurons in spinal-lesioned rats as compared with the limited labelled areas in the control group with intact spinal cord. These results indicate that: (i) in avoiding the spinally descending modulatory mechanisms, more widespread supraspinal and cortical neurons will be recruited and activated in response to the noxious stimulation; and (ii) the descending systems exert differential actions on the spinal targets which project nociceptive signals to different supraspinal regions. The implication of these facts is discussed.     

Hyperalgesia induced by intrathecal administration of nitroglycerin involves NMDA receptor activation in the spinal cord]

Spinal NMDA receptors are involved in hyperalgesia and chronic pain. The activation of spinal NMDA receptor results in the production of nitric oxide in the second order neurons in the spinal cord dorsal horn. We investigated the effects of intrathecally administered nitroglycerin (NTG) which releases nitric oxide in the cell. Formalin test which reflects phasic and tonic nociception was used as a nociceptive measure in rats with chronically implanted intrathecal catheters. Intrathecal injection of NTG resulted in the increase of flinching behavior induced by formalin injection to one paw in phase 1 (phasic) and phase 2 (tonic) responses in a dose-dependent manner. Intrathecally administered NMDA antagonist, MK-801 (MK) dose-dependently inhibited the effect of NTG but the effect was significant only in the phase 2 of the formalin test. MK given after formalin injection had significantly less effect on the phase 2 response. L-NAME (NOS inhibitor), MB (guanylate cyclase inhibitor) and HB (nitric oxide scavenger) significantly antagonized the hyperalgesic effect of NTG in the phase 2 of the formalin test. These results show that nitric oxide plays an important role in producing hyperalgesia in the spinal cord acting postsynaptically as well as pre-synaptically.     

No evidence for the H133Y mutation in SONIC HEDGEHOG in a collection of common tumour types

Subcutaneous formalin injection into the hindpaw produces two phases of nociceptive response: phase 1 and phase 2. Activation of N-methyl-D-aspartate (NMDA) receptors in the spinal cord during phase 1 is important for phase 2. We report here that phase 2 but not phase 1 requires new RNA and protein synthesis in the spinal cord.     

Behavioral analysis of diffuse noxious inhibitory controls (DNIC): antinociception and escape reactions

'Diffuse noxious inhibitory controls' or DNIC is the inhibition of multireceptive neurons in the dorsal horn of the spinal cord that results when a noxious stimulus is applied to a region of the body remote from the neuron's excitatory receptive field. Although this phenomenon is well-documented, the behavioral consequences of DNIC are not clear. The present study was undertaken to determine how nocifensor withdrawal reflexes evoked by a noxious stimulus are altered by application of a second noxious stimulus to a distant part of the body. The tail flick or hindpaw withdrawal reflex of lightly anesthetized (0.6-1.0% halothane) rats was measured before, during and after another appendage was placed in water ranging in temperature from 45 to 54 degrees C. When the forepaw or hindpaw was placed in water exceeding 49 degrees C the tail flick reflex to acute noxious radiant heat was inhibited. In contrast, noxious conditioning stimuli, regardless of temperature or location, had no effect on the latency for hindpaw withdrawal evoked by an acute noxious stimulus, but did produce a change in reflex topography from flexion to extension. These results, along with previous research on DNIC, suggest that intense noxious stimuli: (1) inhibit the tail flick reflex via inhibition of multireceptive neurons in the dorsal horn; (2) disinhibit hindpaw extensor motoneurons by inhibiting the activity of multireceptive neurons involved in hindlimb flexion; and (3) reduce pain sensation by inhibiting multireceptive neurons projecting to the brain (see Model in Discussion).     

Radiation enhancement by biochemical modulation and 5-fluorouracil

BACKGROUND: Nociceptin is a 17-amino acid peptide and acts as a potent endogenous agonist of the opioid receptor-like1 receptor. Nociceptin is reported to depress glutamatergic transmission and to block the spinal facilitation that is thought to be mediated by the N-methyl-D-aspartate (NMDA) receptor. In the present study, the authors investigated the effect of intrathecally administered nociceptin and NMDA antagonists on the level of thermal hyperalgesia after partial sciatic nerve injury in the rat. METHODS: Partial sciatic nerve injury was created by tight ligation of one third to one half of the right sciatic nerve. The level of thermal hyperalgesia was evaluated by the difference score, which was calculated by subtracting the paw withdrawal latency against thermal nociceptive stimulation in the uninjured paw from that in the injured paw. Drugs were administered intrathecally 7 or 11 days after the nerve injury, and the level of thermal hyperalgesia was measured 5, 15, 30, 60, and 90 min after the drug injection. RESULTS: Intrathecal injection of nociceptin, but not of NMDA antagonists, attenuated the level of thermal hyperalgesia in a dose-dependent manner at a dose of 0.17-17 nM (post-drug difference score: saline-treated rats, -4.9 +/- 2.2 s; 17 nM nociceptin-treated rats, -1.3 +/- 0.9 s). CONCLUSIONS: Intrathecal injection of nociceptin attenuated the level of thermal hyperalgesia induced by partial sciatic nerve injury, and NMDA receptor-dependent spinal facilitation does not play an important role in maintaining thermal hyperalgesia in rats with partial sciatic nerve injury.     

Neonatal capsaicin treatment abolishes formalin-induced spinal PGE2 release

We investigated the effect of neonatal capsaicin treatment on formalin-evoked pain behavior and spinal levels of nociceptive neuromodulators using in vivo intrathecal microdialysis in conscious adult rats and age-matched controls. Capsaicin-treated rats displayed thermal hypoalgesia and a significant decrease in tissue content of calcitonin gene-related peptide. Paw swelling, flinching and release of spinal prostaglandin E2 induced by injection of formalin into the hindpaw were also reduced in capsaicin-treated rats compared with controls, whereas glutamate, aspartate and taurine release was unaffected. These data suggest that formalin-induced inflammation, pain behavior and spinal prostaglandin E2 release are mediated by mechanisms sensitive to neonatal capsaicin while the formalin-evoked release of amino acids in the spinal cord is not.     

Mechanisms of rapid growth in the neonatal pig heart

Physiological studies were conducted to examine the effects of noxious stimulation of one hindpaw or one forepaw on the latency of the withdrawal reflex in the tail-flick test in lightly anesthetized spinally intact or transected rats. Male Sprague-Dawley rats were anesthetized with an intraperitoneal injection of a mixture of Na-pentobarbital (20 mg/kg) and chloral hydrate (120 mg/kg). After baseline readings were taken in the tail-flick test, the effects of various noxious stimuli applied to remote body regions were assessed. The noxious stimuli included unilateral or bilateral hindpaw or unilateral forepaw thermal (immersion in water at 55 degrees C for 90 s), unilateral or bilateral chemical (subcutaneous hindpaw injection of 50 microliters of 5% formalin) and unilateral or bilateral mechanical (pinch with clamp exerting a force of 14.75 or 27 N) stimulation. Bilateral chemical and thermal, and unilateral thermal stimulation induced an antinociceptive response, consisting of an increase in tail-flick latency, peaking at 30 s after stimulation. Recovery to baseline levels occurred over the next 3-6 min. The antinociceptive effect of noxious thermal stimulation was attenuated or absent in chronically spinalized animals (T6/7) following hindpaw or forepaw immersion, respectively. Noxious mechanical stimulation had no effect on tail-flick latency. The data provide evidence that a noxious thermal or chemical stimulus produces a heterosegmental antinociceptive effect which is mediated in part via a supraspinal mechanism and in part via a local spinal mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Preparation and in vitro release studies of ibuprofen-loaded films and microspheres made from graft copolymers of poly(L-lactic acid) on acrylic backbones

Several studies have shown that the anterior pretectal nucleus (APtN) is involved in descending inhibitory pathways that control noxious inputs to the spinal cord and that it may participate in the normal physiological response to noxious stimulation. Among other brain regions known to send inputs to the APtN, the dorsal column nuclei (DCN), pedunculopontine tegmental nucleus (PPTg), deep mesencephalon (DpMe), and dorsal raphe nucleus (DRN) are structures also known to be involved in antinociception. In the present study, the effects of stimulating these structures on the latency of the tail withdrawal reflex from noxious heating of the skin (tail flick test) were examined in rats in which saline or hyperbaric lidocaine (5%) was previously microinjected into the APtN. Brief stimulation of the PPTg, DpMe or DRN, but not the DCN, strongly depressed the tail flick reflex. The antinociceptive effect of stimulating the DRN, but not the PPTg or DpMe was significantly reduced, but not abolished, by the prior administration of the local anaesthetic into the APtN. The antinociception induced by stimulation of the PPTg or DpMe, therefore, is unlikely to depend on connections between these structures and the APtN. Similar inhibition of the effect of stimulating the DRN was obtained from rats previously microinjected with naloxone (2.7 nmol) or methysergide (2 nmol) into the APtN. Strongly labelled cells were identified in the DRN following microinjection of the fluorescent tracer Fast Blue into the APtN. These results indicate that the APtN may participate as a relay station through which the DRN partly modulates spinal nociceptive messages. In addition, they also indicate that endogenous opioid and serotonin can participate as neuromodulators of the DRN-APtN connection.     

Peripheral noxious stimulation releases spinal PGE2 during the first phase in the formalin assay of the rat

Injection of formalin (5%; 50 microl) into the dorsal surface of the hind paw of rats evoked a characteristic biphasic flinching behaviour of the injured paw accompanied by a significant increase in the interstitial prostaglandin E2 (PGE2) concentration of the dorsal lumbar spinal cord. Interestingly, the increase in PGE2 concentration was only observed during the first phase of the formalin behavioural response (during the 0-10 and 10-20 min microdialysis-sample). Saline paw injection did not have a significant effect on behaviour or on PGE2 concentration. These data suggest that spinal release of PGE2 is involved in nociceptive processing in the formalin-induced hyperalgesia model of the rat during the first but not second phase.     

Comparison of morphine-induced effects on thermal nociception, mechanoreception, and hind limb flexion in chronic spinal rats.

The effect of systemic morphine on 3 behaviors in the same group of chronic spinal rats was examined: the tail-flick (TF) reflex to a noxious thermal stimulus, limb withdrawal (LW) to mechanoreceptor (von Frey hair) stimulation, and hind limb flexion (flexor reflex [FR]) elicited by innocuous electrical stimulation of the toes. Compared with intact rats, the potency of morphine on both the TF and the hind paw (but not the forepaw) LW response was significantly reduced. Morphine's effect on the FR depended on the dose. The lowest dose (1.0 mg/kg) produced no change, 4.0 mg/kg decreased response magnitude by approximately 50% (indicating an antispastic effect), and 8.0 mg/kg increased flexor magnitude by 100%. The concurrent TF and FR assays revealed a dissociation of morphine's effects in that the highest dose (8.0 mg/kg) significantly inhibited the nociceptive TF response but facilitated the FR in the same chronic spinal rats. This outcome may be relevant to the phenomenon of "opioid-related myoclonus" recently described in cancer patients, which "was highly associated with nerve dysfunction due to spinal cord lesions" (S. Mercadante, 1998, p. 6). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of spinal cyclooxygenase (COX)-2 on thermal hyperalgesia evoked by carageenan injection in the rat

Prostaglandins, which are known to play an important role in the nociceptive transmission in the spinal cord, are produced by cyclooxygenase (COX). Two forms of COX have been identified, COX-1 (constitutive form) and COX-2 (a form highly inducible in response to inflammatory stimuli). COX-2 mRNA was reported to be expressed in the brain in normal rats in the absence of inflammation. We investigated the role of spinal COX-2 in the maintenance of thermal hyperalgesia induced by paw carageenan injection in the rat using NS-398, a selective COX-2 inhibitor. Intrathecally administered NS-398 attenuated the level of thermal hyperalgesia in a dose-dependent manner. This suggested that spinal COX-2 plays an important role in the maintenance of thermal hyperalgesia induced by paw carageenan injection.     

Intrathecal CGRP(8-37) results in a bilateral increase in hindpaw withdrawal latency in rats with a unilateral thermal injury

The present study was performed to explore the effects of intrathecal administration of calcitonin gene-related peptide8-37 (CGRP(8-37)) on the hindpaw withdrawal latency (HWL) to pressure in rats with one thermally injured hindpaw. Furthermore, the interaction of CGRP(8-37)and naloxone was studied. Thermal injury was performed by dipping the left paw into 60 degrees C for 20 s. This induced a significant increase in the volume of the left hindpaw (P<0.001) and significant bilateral decreases of the latency of hindpaw withdrawal response to mechanical stimulation (Left: P<0.001; right: P<0.05). Intrathecal administration of 10, 20 and 40 nmol of CGRP(8-37), but not of 1 or 5 nmol, induced a significant bilateral increase in HWLs (P<0.001). The effect of CGRP(8-37) was partly reversed by intrathecal injection of naloxone at a dose of 32 and 64 microg respectively. Using radioimmunoassay, we found a significant bilateral increase in the concentration of CGRP-like immunoreactivity in the perfusate of both hindpaws 24 h after unilateral thermal injury (left: P< 0.001; right: P< 0.05). There was also an increase in the amount of CGRP-like immunoreactivity in the cerebrospinal fluid (P< 0.001), but not in plasma. The results indicate that CGRP plays a role in the transmission of nociceptive information in the spinal cord of thermally injured rats. Furthermore, our findings suggest that opioids can modulate CGRP-related effects in the spinal cord.     

Preproenkephalin mRNA is regulated by an interaction between steroid hormones and nociceptive stimulation

The expression of preproenkephalin (PPE) mRNA has previously been shown to be regulated by steroid hormones in the ventromedial nucleus of the hypothalamus (VMH) and to be regulated by noxious stimuli in the dorsal horn of the spinal cord (DH). The present in situ hybridization study in ovariectomized rats showed that PPE mRNA expression in both the VMH and the lumbar DH, responds to the interaction between a noxious peripheral stimulus and ovarian steroid hormones. In the VMH, either estradiol or estradiol + progesterone increased the mean PPE mRNA content per cell by 100% compared with vehicle-treated rats. Unilateral hindpaw injection of 5% formalin, as compared to saline, significantly increased mean PPE mRNA content per VMH cell in rats treated with vehicle or estradiol but not those treated with estradiol + progesterone. Regression analysis for mean PPE mRNA content per VMH cell as a function of intensity of hindpaw inflammation showed a significant positive correlation coefficient after vehicle and estradiol treatment (P < 0.02) but a strong trend towards a negative correlation coefficient after estradiol + progesterone treatment (P < 0.06). ANOVA for homogeneity of regression coefficients showed a significant difference across hormone groups (P < 0.01). In the lumbar DH, mean PPE mRNA content per cell was greater in rats injected with formalin than with saline and was greatest in rats given steroids + formalin. Mean PPE mRNA content per DH cell was greater ipsilateral than contralateral to the formalin injection in estradiol-treated rats, but no laterality difference was seen in the other hormone groups. No significant differences in mean PPE mRNA levels per DH cell were found among the rats treated with saline + hormone, saline + vehicle, formalin + vehicle, or uninjected rats. For all hormone groups combined, mean PPE mRNA per DH cell showed a significant positive regression on intensity of hindpaw inflammation (P < 0.05). Taken together these data are consistent with reports of increased pain threshold during pregnancy, descending control of antinociception from the basomedial hypothalamus and positive correlations between VMH levels of PPE mRNA and lordosis, a behavior evoked by somatosensory stimulation below nociceptive threshold.