The effects of A- and C-fiber stimulation on patterns of neuropeptide immunostaining in the rat superficial dorsal horn

Contribution of Histamine to Nociceptive Behaviors Induced by Intrathecally Administered Cholecystokinin-8

The involvement of spinal release of histamine in the nociceptive behaviors induced by cholecystokinin-8 (CCK-8) was investigated in mice. Intrathecal (i.t.) injection of CCK-8 elicited the nociceptive behaviors consisting of biting and licking. The nociceptive behaviors induced by i.t. treatment with CCK-8 showed two bell-shaped patterns. The histamine H₃ receptor antagonist significantly promoted the nociceptive behaviors induced by CCK-8 at doses of 1–100 fmol and 100 pmol. The nociceptive behaviors elicited by CCK-8 was inhibited by i.t. administration of the CCK-B receptor antagonist in a dose-dependent manner, but not by the CCK-A receptor antagonist. The nociceptive behaviors induced by CCK-8 were markedly suppressed by i.t. pretreatment with antiserum against histamine and were abolished in histidine decarboxylase-deleted gene mice. In histamine H₁ receptor-deleted gene mice, the nociceptive behaviors induced at both 10 amol and 10 pmol of CCK-8 were not affected. The tachykinin neurokinin-1 (NK₁) receptor antagonists inhibited CCK-8 (10 pmol)-induced nociceptive behaviors in a dose-dependent manner. CCK-8 (10 amol)-induced nociceptive behaviors was not antagonized by co-administration with the tachykinin NK₁ receptor antagonists. The nociceptive behaviors elicited by CCK-8 were inhibited by i.t. administration of the antagonist for the N-methyl-D-aspartate (NMDA) receptor in a dose-dependent manner. Our results suggest that the nociceptive behaviors induced by i.t. administration of CCK-8 (10 pmol) are mediated through the spinal release of histamine and are elicited via activation of the tachykinin NK₁ and NMDA receptors, whereas the nociceptive behaviors induced by i.t. administration of CCK-8 (10 amol) are mediated through the spinal release of histamine and elicited via NMDA receptor activation.

Different phosphoinositide 3-kinase isoforms mediate carrageenan nociception and inflammation

PI3K-α, -δ, and -γ all participate in inflammation induction. Antagonism of only PI3K-γ blocks nociception, which is indicative of a role for this isoform within the afferent.

Phosphoinositide 3-kinases (PI3Ks) participate in signal transduction cascades that can directly activate and sensitize nociceptors and enhance pain transmission. They also play essential roles in chemotaxis and immune cell infiltration leading to inflammation. We wished to determine which PI3K isoforms were involved in each of these processes. Lightly anesthetized rats (isoflurane) were injected subcutaneously with carrageenan in their hind paws. This was preceded by a local injection of 1% DMSO vehicle or an isoform-specific antagonist to PI3K-α (compound 15-e), -β (TGX221), -δ (Cal-101), or -γ (AS252424). We measured changes in the mechanical pain threshold and spinal c-Fos expression (4 hours after injection) as indices of nociception. Paw volume, plasma extravasation (Evans blue, 0.3 hours after injection), and neutrophil (myeloperoxidase; 1 hour after injection) and macrophage (CD11b+; 4 hour after injection) infiltration into paw tissue were the measured inflammation endpoints. Only PI3K-γ antagonist before treatment reduced the carrageenan-induced pain behavior and spinal expression of c-Fos (P ≤ 0.01). In contrast, pretreatment with PI3K-α, -δ, and-γ antagonists reduced early indices of inflammation. Plasma extravasation PI3K-α (P ≤ 0.05), -δ (P ≤ 0.05), and -γ (P ≤ 0.01), early (0-2 hour) edema -α (P ≤ 0.05), -δ (P ≤ 0.001), and -γ (P ≤ 0.05), and neutrophil infiltration (all P ≤ 0.001) were all reduced compared to vehicle pretreatment. Later (2-4 hour), edema and macrophage infiltration (P ≤ 0.05) were reduced by only the PI3K-δ and -γ isoform antagonists, with the PI3K-δ antagonist having a greater effect on edema. PI3K-β antagonism was ineffective in all paradigms. These data indicate that pain and clinical inflammation are pharmacologically separable and may help to explain clinical conditions in which inflammation naturally wanes or goes into remission, but pain continues unabated.

Management of acute postoperative pain after oral surgery

This article provides a brief review of the acute pain mechanism as it relates to the effects of a surgical insult. A brief understanding of the physiologic modulation of acute pain establishes a rational framework for the concept of preemptive and postoperative analgesia. A brief review of commonly used analgesic agents is presented. Research in pain management and new drug development is ongoing as new concepts in neurophysiology and pharmacology are being elucidated.

Enhanced vascular permeability in rat skin induced by sensory nerve stimulation: evaluation of the time course and appropriate stimulation parameters

Activation of nociceptors causes them to secrete neuropeptides. The binding of these peptides to receptors on blood vessels causes vasodilation and increased vascular permeability that allows loss of proteins and fluid (plasma extravasation, PE); this contributes to inflammation. This study defines the relationship between electrical activation of nociceptors and PE and evaluates the time course of this response in the skin of rats. We measured the time course and extent of PE by digital imaging of changes in skin reflectance caused by leakage of Evans Blue (EB) dye infused in the circulatory system before stimulation. Stimulation of the exclusively sensory saphenous nerve caused the skin to become dark blue within 2 min due to accumulation of EB. While PE is usually measured after 5-15 min of electrical stimulation, we found that stimulation for only 1 min at 4 Hz produced maximum PE. This response was dependent on the number of electrical stimuli at least for 4 Hz and 8 Hz stimulation rates. Since accumulation of EB in the skin is only slowly reversible, to determine the duration of enhanced vascular permeability we administered EB at various times after electrical stimulation of the saphenous nerve. PE was only observed when EB was infused within 5 min of electrical stimulation but could still be observed 50 min after capsaicin (1%, 25 microl) injection into the hind paw. These findings indicate that enhanced vascular permeability evoked by electrical stimulation persists only briefly after release of neuropeptides from nociceptors in the skin. Therefore, treatment of inflammation by blockade of neuropeptide release and receptors may be more effective than treatments aimed at epithelial gaps. We propose, in models of stimulation-induced inflammation, the use of a short stimulus train.

Effects of the neuropeptides substance P, calcitonin gene-related peptide, vasoactive intestinal polypeptide and galanin on the production of nerve growth factor and inflammatory cytokines in cultured human keratinocytes

Neuropeptides released from the cutaneous sensory nerve endings have neurotransmitter and immunoregulatory roles; they exert mitogenic actions and can influence the functions of different cell types in the skin. The aims of this study were a systematic investigation of the effects of the neuropeptides substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP) and galanin (GAL) on the inflammatory cytokine production (IL-1alpha, IL-8 and TNF-alpha) of the keratinocytes, and a study of their role in the production and secretion of nerve growth factor (NGF) and its precursor molecule (proNGF). Cultures of normal human keratinocytes were treated with 10(-8)M SP, CGRP, VIP or GAL for 30 min. After different time intervals, cells were harvested for total RNA isolation; in addition, cell lysates and supernatants were collected. The effects of the neuropeptides on the mRNA expressions of the different cytokines and NGF were investigated by Q-RT-PCR and the protein levels were studied by means of ELISA assays and Western blotting. Each of the four neuropeptides induced increases in the expressions of IL-1alpha, IL-8 and TNF-alpha mRNA. Increases appeared in the amount of the IL-1alpha protein in the supernatants of neuropeptide-treated cells, and the IL-8 secretion was mildly elevated, while secretion of TNF-alpha remained undetectable. The four neuropeptides increased the NGF mRNA expression to different extents. In the cell lysates of the keratinocytes, only proNGF could be detected, its concentration in the neuropeptide-treated cells being approximately twice that in the time-matched controls. Both control cultures and neuropeptide-treated cultures were found to secrete proNGF and mature NGF, but neuropeptide-treated cell cultures produced markedly higher (3-7-fold) amounts of NGF-like immunoreactive materials. The results demonstrated that neuropeptides released from cutaneous nerves after an injurious stimulus are able to induce an upregulation of IL-1alpha and IL-8 production; they are additionally able to influence the expressions of proNGF/NGF and their secretion from the keratinocytes. These findings may contribute toward an understanding of the neural influence on skin health and disease.

Effect of Moxibustion on Dopaminergic and Serotonergic Systems of Rat Nucleus Accumbens

Acupuncture and moxibustion are traditional medical treatments that have come to play important roles in complementary and alternative medicines. Moxibustion also has a long history as a folk remedy in Japan, particularly due to the technical simplicity and selective efficacy on certain types of disease and distress. This study examined the effects of moxibustion focusing on the brain reward system, particularly in the nucleus accumbens. The effects of moxibustion stimulation at various sites and frequencies on monoamine levels of adult male Sprague-Dawley rats were examined using high-preformance liquid chromatography of dissected nucleus accumbens tissues. The rats weighing 290-310 g were divided into 3 groups according to the moxibustion point used: hindlimb, lumbar or parietal points. Each group was further divided into 3 subgroups, with stimulation for 10 consecutive days, for 1 day, or sham treatment (control). On each day of stimulation, 5 moxibustion cones with a peak temperature of 200 degrees C were applied consecutively. Stimulation of any point on 1 day only did not change dopamine or serotonin levels, but lumbar stimulation significantly increased the metabolic turnover of dopamine. Conversely, stimulation for 10 consecutive days resulted in significantly decreased serotonin levels for hindlimb and parietal stimulations, and significantly increased 5-hydroxyindolacetic acid/serotonin ratio for hindlimb stimulation. These results suggest that the metabolic turnover of serotonin release may be accentuated by moxibustion in a reward-related brain area. Moxibustion over consecutive days, especially that to peripheral regions, appears most efficient to influence on monoamine levels in the nucleus accumbens.

The coding of noxious mechanical and thermal stimuli of deep dorsal horn neurones is attenuated in NK1 knockout mice

Previous behavioural evidence has shown that NK1 receptor gene knockout (NK1 -/-) mice display altered nociceptive responses following tissue or peripheral nerve injury. A single electrophysiological study reported an attenuation of wind-up and responses to mustard oil application. Although the behavioural results implicate SP and its receptor (NK1-R) in the transmission of noxious high intensity pain, little is known regarding the spinal neuronal substrates and the modalities involved. We have addressed this using in vivo electrophysiology and recordings of deep dorsal horn neurones in urethane-anaesthetised C57B6 x 129/sv mice to reveal a marked deficit in mechanical and thermal coding, selectively encompassing the suprathreshold range of noxious stimuli. The frequency-dependent increase in neuronal activity following repetitive C-fibre stimulation (wind-up) was also abolished in spinal neurones of NK1-R knockout mice. Quantification of the receptive field size of spinal neurones, mapped with low- and high-intensity mechanical punctate stimuli, revealed no differences between NK1 -/- and wildtype mice. We conclude that NK1-Rs are important in the high intensity noxious signalling of acute peripheral (mechanical/thermal) stimuli and this may result from the lack of wind-up and/or the disruption of spinal-bulbo-spinal loops.

Substance P

Substance P is considered to be an important neuropeptide in nociceptive processes. Although substance P was described more than 60 years ago, there is still controversy about its exact role in nociception. This article reviews the current knowledge about the function of substance P in pain. Special emphasis is put on how to use this knowledge in the development of new ways to treat pain.

Acute pain mechanisms

The systems activated by tissue-injuring stimuli are complex. The nociceptive primary afferents have little spontaneous activity under normal conditions; however, after tissue injury, they display longlasting, ongoing activity. This results, in part, because the injury elicits the release of active factors that sensitize or excite the peripheral nerve terminal. A threshold that is lowered to the extent that body temperature and the pressure of edema are adequate stimuli results in spontaneous pain. This phenomenon is mediated by a variety of blood-borne active factors released during plasma extravasation, by agents released from local inflammatory cells, and by neurotransmitters released from the peripheral terminals of the primary afferent fibers themselves. Well-defined projections into the dorsal horn convey the "pain message" to at least two well-defined populations of neurons: those that are nociceptive specific and those that display an intensity-linked discharge over a range of stimuli from innocuous to noxious. Convergence from various fiber types, modalities, and end organs permits the encoding of afferent traffic with respect to intensity and location. The convergence of axons from somatic and visceral structures reflects the mechanism for the so-called "referred pain state." Most importantly, these dorsal horn systems have a dynamic component in addition to the hard-wiring; their output can be regulated both up and down. The up-regulation provides the basis for much of the facilitated processing that is believed to account for a significant percentage of the postinjury pain state. The facilitated state has a unique pharmacology, with the underlying mechanisms reflecting a cascade of actions that starts with the NMDA receptor and proceeds through the spinal release of intermediaries, such as prostaglandins and nitric oxide. Conversely, the ability to down-regulate the dorsal horn stimulus response function accounts for the powerful control exerted by a wide variety of diverse factors, including the spinal delivery of opioid and nonopioid analgesics and the "endogenous analgesia system." These linkages reflect the complexity of the encoding mechanisms that transduce the tissue injury into the behavioral sequela known as pain. This article also emphasizes that, although considerable progress has been made in the past decade, the current pace of research promises greater insights.

Primary afferent fibers that contribute to increased substance P receptor internalization in the spinal cord after injury

Upon noxious stimulation, substance P (SP) is released from primary afferent fibers into the spinal cord where it interacts with the SP receptor (SPR). The SPR is located throughout the dorsal horn and undergoes endocytosis after agonist binding, which provides a spatial image of SPR-containing neurons that undergo agonist interaction. Under normal conditions, SPR internalization occurs only in SPR+ cell bodies and dendrites in the superficial dorsal horn after noxious stimulation. After nerve transection and inflammation, SPR immunoreactivity increases, and both noxious as well as nonnoxious stimulation produces SPR internalization in the superficial and deep dorsal horn. We investigated the primary afferent fibers that contribute to enhanced SPR internalization in the spinal cord after nerve transection and inflammation. Internalization evoked by electrical stimulation of the sciatic nerve was examined in untreated animals, at 14 days after sciatic nerve transection or sham surgery and at 3 days after hindpaw inflammation. Electrical stimulation was delivered at intensities to excite Abeta fibers only, Abeta and Adelta fibers or A and C fibers as determined by the compound action potential recorded from the tibial nerve. Electrical stimuli were delivered at a constant rate of 10 Hz for a duration of 5 min. Transection of the sciatic nerve and inflammation produced a 33.7 and 32.5% increase in SPR and immunoreactivity in lamina I, respectively. Under normal conditions, stimulation of Adelta or C fibers evoked internalization that was confined to the superficial dorsal horn. After transection or inflammation, there was a 20-24% increase in the proportion of SPR+ lamina I neurons that exhibited internalization evoked by stimulation of Adelta fibers. The proportion of lamina I SPR+ neurons that exhibited internalization after stimulation of C-fibers was not altered by transection or inflammation because this was nearly maximal under normal conditions. Moreover, electrical stimulation sufficient to excite C fibers evoked SPR internalization in 22% of SPR+ lamina III neurons after nerve transection and in 32-36% of SPR+ neurons in lamina III and IV after inflammation. Stimulation of Abeta fibers alone never evoked internalization in the superficial or deep dorsal horn. These results indicate that activation of small-caliber afferent fibers contributes to the enhanced SPR internalization in the spinal cord after nerve transection and inflammation and suggest that recruitment of neurons that possess the SPR contributes to hyperalgesia.

The N-methyl-D-aspartate (NMDA) receptor is postsynaptic to substance P-containing axon terminals in the rat superficial dorsal horn

The N-methyl-D-aspartate (NMDA) receptor is thought to mediate the postsynaptic effects of excitatory amino acids released from primary afferent terminals in the superficial layers of the dorsal horn of the spinal cord where synergistic associations with substance P (SP) have been implicated in the production of hyperalgesia. We examined the electron microscopic dual immunocytochemical localization of SP and the R1 subunit of the NMDA receptor (NMDAR1) in this region to determine the cellular basis for interactions between SP and NMDA receptor ligands. Of 971 profiles immunolabeled for NMDAR1, 40% were dendrites and the remainder were primarily unmyelinated axons and astrocytic processes. In dendrites, NMDAR1-like immunoreactivity (NMDAR1-LI) was associated with synaptic and non-synaptic portions of the plasma membrane, as well as intracellular membranes including smooth endoplasmic reticulum. These NMDAR1-labeled dendrites received synaptic input from unlabeled terminals and from terminals containing SP and/or NMDAR1-LI and they occasionally (25/389) also contained SP. In contrast, of 540 SP-immunoreactive profiles, 60% were axon terminals and the majority (252/324) of these SP-labeled terminals were presynaptic to NMDAR1-containing dendrites. These results provide anatomical evidence that the synergistic nociceptive effects of SP and NMDA ligands are attributed mainly to dual modulation of the activity of single dendritic targets in the dorsal horn of the spinal cord. They also suggest that activation of NMDA receptors may also play a role in the modulation of SP neurons, presynaptic release of SP or other neurotransmitters, and in glial function in the dorsal horn.

Time course of degenerative and regenerative changes in the dorsal horn in a rat model of peripheral neuropathy

The time course of histochemical changes in the dorsal horn of rats subjected to an experimental peripheral neuropathy has been examined. Qualitative and quantitative analyses of the changes in dorsal horn staining were made for soybean agglutinin (SBA)-binding glycoconjugates, the soluble lectins RL-14.5 and RL-29, the growth-associated protein (GAP)-43, and the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP). These analyses were made at various time points after chronic constriction of the sciatic nerve. Quantitative analysis indicated that staining density increased in the normal territories stained for SBA-binding glycoconjugates, RL-14.5, RL-29, and GAP-43 on the neuropathic side compared with the control side. In addition, there was an extension of the territories stained for SBA-binding glycoconjugates and RL-29 ipsilateral to the injury. The peak increases occurred at 14 or 28 days, followed by a decrease toward control levels by 70 days. In contrast, the staining density for SP in the ipsilateral dorsal horn decreased at 3 and 5 days and reached a peak decrease at 14 days. Then, the staining for SP returned toward control values. The staining for CGRP was unchanged at all time points examined. Dorsal rhizotomies ipsilateral to the nerve injury in neuropathic rats indicated that the increases in staining were attributable to changes in primary afferent neurons. These data suggest that peripheral neuropathy causes complex degenerative and regenerative changes in the central branches of primary afferent neurons. The associated synaptic reorganization may contribute to the sensory abnormalities that accompany peripheral neuropathy.

Afferent volley patterns and the spinal release of immunoreactive substance P in the dorsal horn of the anaesthetized spinal cat

Microprobes bearing immobilized antibodies to the C-terminus of substance P were used to measure release of this neuropeptide in the spinal cord of the anaesthetized spinal cat in response to peripheral nerve stimulation. Release of substance P was just detectable in laminae I, II with 150 stimuli (0.5 Hz, 5 min) and was near maximal with 300 stimuli. Using two periods of stimulation of 10 min separated by 15 min, greater levels of substance P were detected during the second period. Fifteen to 25 min after two periods of peripheral nerve stimulation levels of substance P detected by microprobes were still elevated above those present prior to stimulation. Stimulation with bursts of three impulses when delivering a fixed number of stimuli resulted in detection of increased levels of substance P at sites adjacent to the areas of maximal release. The results suggest that maximal release of substance P from the central terminals of primary afferent fibres occurs with relatively few impulses and at low frequencies in agreement with what is known of release from the peripheral terminals of these fibres.

Bi-directional effects of intrathecal NMDA and substance P on rat dorsal horn neuronal responses

Intrathecal NMDA (5 ng) facilitated wind-up but not C-fibre evoked responses, whereas 50 ng facilitated C-fibre and A delta-fibre evoked responses but not wind-up of convergent dorsal horn neurones in the halothane anaesthetized rat. Higher doses of NMDA and SP (10-500 ng) were without effect. Co-administered SP (10 ng) with NMDA (5 ng) facilitated A delta-fibre evoked responses and wind-up. Excitatory amino acid and peptide interactions are discussed.

Release of immunoreactive galanin in the spinal cord of rats with ankle inflammation: studies with antibody microprobes

Antibody microprobes bearing antibodies to the carboxy-terminus of rat galanin were inserted into the spinal cords of anaesthetized normal rats and those in which ankle inflammation had been induced by the unilateral subcutaneous injection of Freund's adjuvant four to six days previously. In normal rats, a basal presence of immunoreactive galanin was detected in the dorsal horn. Similar levels of immunoreactive galanin were found in the dorsal horn of both sides of the spinal cord in animals with unilateral ankle inflammation. Flexing the ankle or compressing the foot in normal rats failed to alter levels of immunoreactive galanin detected by microprobes. In animals with ankle inflammation, prolonged periods of ankle flexion did release immunoreactive galanin in the ipsilateral dorsal horn. Subsequent noxious ankle compression in these animals did not increase but rather decreased immunoreactive galanin in the dorsal horn to below basal levels. The reason for this decrease is unknown but it may represent an inhibition of release or a depletion of spinal stores of galanin.

C fiber generates a slow Na+ spike in the frog sciatic nerve

The present study was carried out to examine the properties of A and C fibers in the bullfrog sciatic nerves by applying several agents through perfusing solutions between stimulating and recording electrodes. The compound action potentials (CAPs) of A beta and A delta fibers were tetrodotoxin (TTX)-sensitive and were abolished in Na(+)-free solution. However, C fiber CAP was TTX-insensitive although CAP disappeared in Na(+)-free solution. Moreover, C fiber CAP was not blocked by Ca2+ channel blockers and its chronaxy (2 ms) and conduction velocity (0.70 m/s) indicate that the time constant of C fiber CAP is relatively large (2.88 ms). These suggest that a slow Na+ channel, which is TTX-resistant, contributes to C fiber action potentials.