Masayoshi Tsuruoka

Bilateral lesions in the area of the nucleus locus coeruleus affect the development of hyperalgesia during carrageenan-induced inflammation

We have compared the development of hyperalgesia following induction of unilateral inflammation between rats with bilateral lesions in the area of the locus coeruleus (LC) and rats with a sham operation. Unilateral inflammation was produced by the subcutaneous injection of carrageenan (6 mg in 0.15 ml saline). Prior to carrageenan injection, paw withdrawal latency (PWL) to thermal stimuli did not differ between the LC-lesioned and the sham-operated groups. After carrageenan injection, in the inflamed paw, the decreased PWLs of LC-lesioned rats were significantly shorter than those of sham-operated rats. This difference in PWL between the two groups was not observed in non-inflamed paws and in paws receiving an injection of saline. The result suggests that the area of the LC affects the development of hyperalgesia during peripheral inflammation.

Descending modulation from the region of the locus coeruleus on nociceptive sensitivity in a rat model of inflammatory hyperalgesia.

The aim of the present study was to evaluate the action of the descending modulation system from the locus coeruleus (LC) in a rat model of unilateral hyperalgesic inflammation. Unilateral hindlimb inflammation was produced by a subcutaneous injection of carrageenan (6 mg in 0.15 ml saline). One week before testing, rats received bilateral lesions of the LC using anodal current. Nociception was assessed by measuring withdrawal of the paw from a noxious thermal stimulus. Four hours after carrageenan injection, paw withdrawal latencies (PWLs) in the inflamed paw of the LC-lesioned rats were significantly shorter than those of the sham-operated rats. This difference in PWL between the two groups was not observed at 7 days, whereas edema and hyperalgesia still remained in the inflamed paw. At 4 h, systemic naloxone produced a further decrease of the PWL in the LC-lesioned rats but not in the sham-operated rats. These results suggest that inflammation-induced activation of the descending modulation system from the LC occurs in only the acute phase of inflammation and that a decrease in the extent of the development of hyperalgesia in the acute phase of inflammation might depend on the interaction between the descending modulation system from the LC and the opioid inhibitory system.

Inhibitors of G-proteins and protein kinases reduce the sensitization to mechanical stimulation and the desensitization to heat of spinothalamic tract neurons induced by intradermal injection of capsaicin in the primate

Abstract Intradermal injection of capsaicin results in sensitization of spinothalamic tract cells to brushing and pressure applied to the cutaneous receptive field in anesthetized monkeys. A significant increase in background activity also occurs immediately after capsaicin injection that lasts for at least 2 h. A 40–50% decrease in the response to noxious heat stimuli is also observed following capsaicin injection. This study investigated the spinal role of second messengers by extracellularly recording from spinothalamic tract cells and delivering inhibitors of second messenger pathways to the spinal cord by microdialysis. Blockade of protein kinases with the general protein kinase inhibitor, H7 (5.0 mM, n = 6), reduced the sensitization of the cells to brush and pressure. Blockade of protein kinase C with NPC15437 (10.0 mM, n = 10) reduced the increased background activity and the increased responses to brush. Blockade of protein kinase A with H89 (0.01 mM, n = 9) was most effective. H89 reduced the background activity, the increased responses to brush and press, and reversed the decreased response to noxious heat stimuli. Blockade of G-proteins with the general G-protein inhibitor, GDP-β-S (1.0 mM, n = 9), reduced the background activity and the responses to brush and pressure without affecting the decreased response to heat. Thus, multiple intracellular messengers appear to be involved in the processing of central sensitization induced by activation of C-fibers following intradermal injection of capsaicin.

Capsaicin-induced sensitization of primate spinothalamic tract cells is prevented by a protein kinase C inhibitor

Protein kinase C (PKC) has been shown to be involved in nociceptive transmission in the spinal cord. This study tested the hypothesis that induction of central sensitization in the dorsal horn by an intradermal capsaicin injection involves activation of PKC. A PKC inhibitor (NPC15437) was infused through a microdialysis fiber into the spinal cord prior to capsaicin injection. The responses of spinothalamic tract (STT) cells were recorded before and after infusion of NPC15437, and after injection of capsaicin. STT cells show an increased background activity and increased responses to innocuous stimuli following capsaicin injection while responses to heat are decreased. Spinal infusion of the PKC inhibitor, NPC15437, had no effect on background activity or responses to peripherally applied stimuli prior to capsaicin injection. However, NPC15437 prevented the sensitization of cells to weak mechanical stimuli (brush and pressure) that occurs following capsaicin injection. NPC15437 had no effect on the increased background activity or decreased responses to heat stimuli induced by capsaicin injection, suggesting alternative mechanisms for these responses. These data suggest that PKC is important for the development of central sensitization to peripheral mechanical stimuli.

Unilateral hindpaw inflammation induces bilateral activation of the locus coeruleus and the nucleus subcoeruleus in the rat

Several lines of evidence have shown that unilateral hindpaw inflammation produces activation of the locus coeruleus (LC) and the nucleus subcoeruleus (SC), resulting in descending modulation of nociceptive processing in the dorsal horn. However, it is unclear if the LC/SC is activated unilaterally or bilaterally following the development of unilateral hindpaw inflammation. The present study was designed to clarify this question. For the induction of unilateral hindpaw inflammation, lambda carrageenan (2.0mg in 0.15ml saline) was injected subcutaneously into the plantar surface of the left hindpaw. Four hours after carrageenan injection, in the LC/SC both ipsilateral and contralateral to the inflamed paw, the number of Fos-positive cells increased significantly in carrageenan-injected rats when compared to vehicle (saline)-injected and untreated control rats. The Fos expression in the LC/SC was equivalent bilaterally in the carrageenan-injected rats, as well as in vehicle-injected and untreated control rats. For nociceptive testing, the paw withdrawal latency, which measures cutaneous hyperalgesia in response to thermal stimuli, was determined in rats receiving a unilateral lesion of the LC/SC either ipsilateral or contralateral to the inflamed paw. Two and a half hours after the induction of inflammation, in both groups of rats with unilateral lesion, paw withdrawal latencies decreased significantly in the LC/SC-lesioned rats. However, there was no significant difference in paw withdrawal latencies between the LC/SC-lesioned rats and sham-operated rats, indicating that unilateral activation of the LC/SC is sufficient for modulating nociceptive processing in the dorsal horn. These results suggest that unilateral hindpaw inflammation induces bilateral activation of the LC/SC.

Coeruleospinal inhibition of nociceptive processing in the dorsal horn during unilateral hindpaw inflammation in the rat

&NA; Behavioral and neurochemical studies have shown that the coeruleospinal modulation system is activated by peripheral inflammation, and that this modulation system is active in only the dorsal horn ipsilateral, but not in the dorsal horn contralateral, to the site of inflammation; the present study was designed to confirm electrophysiologically this previous finding. Extracellular recordings from dorsal horn neurons were continued for at least 4 h after the induction of inflammation. Unilateral hindpaw inflammation was produced by a subcutaneous injection of carrageenan (2 mg in 0.15 ml saline). Background activity and responses to noxious heating were compared between rats receiving bilateral lesions in the locus coeruleus/subcoeruleus (LC/SC) and non‐operated control rats. In neurons located in the dorsal horn ipsilateral to the inflamed paw, prior to inflammation, there was no significant difference in either the background activity or the heat‐evoked response in neurons in LC/SC‐lesioned compared to LC/SC‐intact rats. Four hours after the induction of inflammation, there was a significant increase in both the background activity and heat‐evoked response in neurons in LC/SC‐lesioned compared to LC/SC‐intact rats. In neurons located in the dorsal horn contralateral to the inflamed paw, 4 h after inflammation, no significant increase in either the background activity or the heat‐evoked response in neurons in LC/SC‐lesioned rats was observed, as well as in the case before inflammation. These results suggest that the coeruleospinal modulation system is active in only the dorsal horn ipsilateral, but not in the dorsal horn contralateral, to the site of inflammation during the development of unilateral hindpaw inflammation.

Coeruleotrigeminal inhibition of nociceptive processing in the rat trigeminal subnucleus caudalis

It has been accepted that the descending system from the nucleus locus coeruleus (LC)/nucleus subcoeruleus (SC) plays a significant role in spinal nociceptive processing. The present study was designed to examine modulation of nociceptive processing in the caudal part of the trigeminal sensory nuclear complex, the trigeminal subnucleus caudalis which is generally considered to be involved in the relay of oral-facial nociceptive information. Experiments were performed on anesthetized Sprague-Dawley rats. The site of LC/SC stimulation was confirmed by histology using potassium ferrocyanide to produce a Prussian blue reaction product marking the iron deposited from the stimulating electrode tip. Only data from rats which had electrode placements in the LC/SC were used. Electrical stimulation was delivered at a stimulus intensity below 100 microA in the present study. Stimulation at sites inside the LC/SC produced a reduction of both spontaneous activity and responses of subnucleus caudalis neurons to somatic input, especially nociceptive input. Increasing stimulation frequency in the LC/SC resulted in an increase in inhibitory effects on nociceptive responses of subnucleus caudalis neurons. At three of nine sites outside the LC/SC, electrical stimulation was effective on descending inhibition. A significant difference in the inhibitory effects was observed when the inhibitory effects were compared between sites of stimulation inside the LC/SC and three effective sites of stimulation outside the LC/SC. These findings suggest that nociceptive processing in the subnucleus caudalis is under the control of the descending modulation system from the LC/SC. To understand the effects of repetitive stimulation with high frequency on fine unmyelinated LC/SC fibers, the existence of recurrent collateral excitation in the LC/SC may be considered.

Spinal pathways mediating coeruleospinal antinociception in the rat

In a previous study, we showed in rats that axons of some locus coeruleus/subcoeruleus (LC/SC) neurons involved in coeruleospinal modulation of nociception descend through the ipsilateral side of the spinal cord and cross the midline at spinal segmental levels. The present study was designed to investigate a possible spinal pathway of these descending axons from the LC/SC. Extracellular recordings were made from the left dorsal horn with a carbon filament electrode (4-6 M(omega)). To block impulses from the LC/SC which descend through spinal pathways ipsilateral to the recording sites, a hemisection of the spinal cord ipsilateral to the recording sites was performed at the C2 level with fine forceps in all rats tested. In these rats, responses of dorsal horn neurons to noxious heat (53 degrees C) applied to receptive fields were inhibited during electrical stimulation (100 microA, 100 Hz, 0.1 ms pulses) of the LC/SC. The transection of the dorsolateral funiculus contralateral to the recording sites did not affect LC/SC stimulation-produced inhibition. Following transection of the ventrolateral funiculus (VLF) contralateral to the recording sites, LC/SC stimulation failed to inhibit heat-evoked responses. These results suggest that interruption of descending inhibition from the LC/SC produced by the VLF transections is due to the blockage of axons descending in the ventrolateral quadrant of the spinal cord, but not in the dorsolateral quadrant.

Neurochemical evidence for inflammation-induced activation of the coeruleospinal modulation system in the rat

By using the microdialysis technique, the concentration of noradrenaline (NA) in the dorsal horn during unilateral hindpaw inflammation was compared between rats receiving bilateral lesions of the locus coeruleus (LC) and non-operated control rats. Bilateral lesions of the LC were made using an anodal current one week before testing. Unilateral hindpaw inflammation was produced by a subcutaneous injection of carrageenan (6 mg in 0.15 ml saline). Under conditions of sodium pentobarbital anesthesia, the microdialysis probe was inserted into the dorsal horn either ipsilateral or contralateral to the site of inflammation. The NA concentration in the dialysate was measured by high-performance liquid chromatography with electrochemical detection. Prior to carrageenan injection, the NA level (baseline level) did not differ between the LC-lesioned and the non-operated groups. After carrageenan injection, in the non-operated rats, the NA level increased significantly compared to the baseline level only in the dorsal horn ipsilateral to the site of inflammation, but not in the dorsal horn contralateral to the site of inflammation. An increase of the NA level was not observed in the LC-lesioned rats and in rats receiving an injection of saline. The result suggests that unilateral hindpaw inflammation produces excitation of descending NA-containing neurons from the LC, resulting in an increase of the NA level in the dorsal horn ipsilateral to the site of inflammation.

Persistent hindpaw inflammation produces coeruleospinal antinociception in the non-inflamed forepaw of rats.

In a rat model of unilateral hindpaw inflammation, it is unclear whether the coeruleospinal modulation system is active at spinal segments distant from the inflamed plantar region, such as the cervical segments. To clarify this query, in the present study we measured paw withdrawal latency (PWL) to thermal stimuli on four paws (both forepaws and both hindpaws) following induction of inflammation and compared PWLs between rats with bilateral lesions of the locus coeruleus/subcoeruleus (LC/SC) and rats with sham operation. Unilateral hindpaw inflammation was produced by a subcutaneous injection of carrageenan (2 mg in 0.15 ml saline). Prior to carrageenan injection, in all four paws, PWLs did not differ between the LC/SC-lesioned and the sham-operated rats. Four hours after carrageenan injection, PWLs in the inflamed left hindpaw decreased significantly in both the LC/SC-lesioned and the sham-operated rats. The decreased PWLs of the LC/SC-lesioned group were significantly shorter than those of the sham-operated group. These phenomena which were observed in the inflamed left hindpaw were also observed in the non-inflamed left forepaws. In the right forepaws and the right hindpaws, no significant change in PWL was observed between before and 4 h after injection in both the sham-operated and the LC/SC-lesioned rats. These results suggest that unilateral hindpaw inflammation activates the coeruleospinal modulation system and that this modulation system is active not only at the lumbar segments but also at the cervical level where spinal segments are distant from the inflamed plantar region.