Antinociceptive effect of systemically administered dipyrone (metamizol), magnesium chloride or both in a murine model of cancer.

BACKGROUND: Opioid effectiveness to treat cancer pain is often compromised by the development of tolerance and the occurrence of undesirable side effects, particularly during long-term treatment. Hence, the search for more efficient analgesics remains a necessity. The main goal of this study was to relieve neuropathic symptoms associated with tumour growth by administering the non-opioid analgesic dipyrone (DIP) alone or in combination with magnesium chloride (MgCl2 ), an adjuvant that blocks the NMDA receptor channel. METHODS: Mice were inoculated with a melanoma cell line (B16-BL6) in the left thigh and two protocols were used to evaluate the effect of DIP (270 mg/kg), MgCl2 (200 mg/kg), or the combination DIP-MgCl2 . In the therapeutic protocol the drugs, alone or combined, were administered once tumour had promoted increased nociception. In the preventive protocol, drugs were administered prior to the appearance of the primary tumour. Tumour growth was assessed with a caliper and nociception was determined using behavioural tests. RESULTS: DIP promoted antinociception only at the beginning of both protocols due to the development of tolerance. The combination DIP-MgCl2 improved the antinociceptive effect, avoiding tolerance and reducing tumour growth in the preventive treatment, more efficiently than each compound alone. CONCLUSIONS: These results suggest that DIP-MgCl2 may represent a safe, affordable and accessible option to reduce tumour growth and to treat cancer pain avoiding the risk of tolerance, without the typical complications of opioids agents, particularly when long-term treatment is required. SIGNIFICANCE: This study shows a non-opioid analgesic combined with an adjuvant as a therapeutic option to treat cancer pain. The avoidance of antinociceptive tolerance when repeated administration is required, as well as tumor growth reduction, are additional advantages to be considered.

Systemic changes following carrageenan-induced paw inflammation in rats.

OBJECTIVE AND DESIGN: Carrageenan-induced paw edema has been described as a local and acute inflammatory process. In fact, little is known about the time course and systemic changes following a carrageenan injection. In this study, we examine the systemic changes that follow carrageenan injection in the paw. METHODS: Acute inflammation was produced by subplantar injection of carrageenan in a hind paw of Sprague-Dawley rats. Saline was used in control rats. Paw volume was measured with a plethysmometer. The hot plate latency test was used to quantify antinociception. C-reactive protein (CRP) levels were measured with a sandwich enzyme immunoassay. Fibrinogen concentration was measured using the gravimetric method. Lung morphometric analysis was performed using an image processing package. Lungs and paws were also examined for tissue factor (TF) and proinflammatory cytokines expression by immunohistochemistry. RESULTS: We found diverse systemic changes including increased levels of acute phase proteins, such as CRP and fibrinogen, and a lung inflammatory process characterized by lung edema, fibrin deposition, and leukocyte infiltration. An elevated expression of TF, IL-6, IL-1ß, and TNFα, was observed in paw and lung tissue sections by immunohistochemical methods. CONCLUSION: This study provides new evidence that a local carrageenan injection induces a systemic response.

Metamizol, a non-opioid analgesic, acts via endocannabinoids in the PAG-RVM axis during inflammation in rats.

The most commonly used drugs against pain act by inhibiting the cyclooxygenases (COXs). Metamizol (dipyrone) inhibits the COXs and is widely used in Europe and Latin America as a non-opioid analgesic. One target of metamizol and other non-opioid analgesics is the periaqueductal grey matter (PAG), where they trigger descending inhibition of spinal nociceptive transmission. Also, cannabinoids exert an analgesic action at several structures in the peripheral and central nervous system, including the PAG. The present study investigates whether the antinociceptive action of metamizol in the lateral-ventrolateral (LVL) PAG during inflammation is related to endocannabinoids. In anaesthetized rats, unitary action potentials were recorded from spinal nociceptive neurons with receptive fields in the ipsilateral hind paw. Inflammation of the paw induced neuronal hyperexcitability, which was attenuated by intra-LVL-PAG microinjection of metamizol either at the beginning of inflammation or when hyperexcitability was fully established. In both cases, the antinociceptive effect of metamizol was reduced by a microinjection of AM251, an antagonist at the CB1 cannabinoid receptor, either into the LVL-PAG or into the rostral ventromedial medulla (RVM). The RVM is a downstream structure that funnels PAG-derived descending inhibition into the spinal cord. These results show that endocannabinoids and their CB1 receptor (1) contribute at the LVL-PAG to the antinociceptive effects of metamizol, and possibly other non-opioid analgesics; and (2) participate in the PAG-derived activation of RVM descending antinociceptive influences.

Spinal prostaglandins are involved in the development but not the maintenance of inflammation-induced spinal hyperexcitability.

Prostaglandins (PGs) are local mediators of several functions in the CNS. Both primary afferent neurons and intrinsic cells in the spinal cord produce PGs, with a marked upregulation during peripheral inflammation. Therefore, the significance of spinal PGs in the neuronal processing of mechanosensory information was herein investigated. In anesthetized rats, the discharges of spinal nociceptive neurons with input from the knee joint were extracellularly recorded. Topical administration of prostaglandin E(2) (PGE(2)) to the spinal cord facilitated the discharges and expanded the receptive field of dorsal horn neurons to innocuous and noxious pressure applied to the knee joint, the ankle, and the paw, thus mimicking inflammation-induced central sensitization. Conversely, topical administration of the PG synthesis inhibitor indomethacin to the spinal cord before and during development of knee joint inflammation attenuated the generation of inflammation-induced spinal neuronal hyperexcitability. However, after development of inflammation, the responses of spinal neurons to mechanical stimuli were only reduced by systemic indomethacin but not by indomethacin applied to the spinal cord. Thus, spinal PG synthesis is important for the induction and initial expression but not for the maintenance of spinal cord hyperexcitability. Spinal PGE(2) application facilitated dorsal horn neuronal firing elicited by ionophoretic delivery of NMDA, suggesting that an interaction of PGs and NMDA receptors may contribute to inflammation-induced central sensitization. However, after development of inflammation, spinal indomethacin failed to reduce responses to ionophoretic delivery of NMDA or AMPA, suggesting that such an interaction is not required for the maintenance of central sensitization.

Antinociception induced by PAG-microinjected dipyrone (metamizol) in rats: involvement of spinal endogenous opioids.

Dipyrone microinjection into the periaqueductal gray matter (PAG) elicits antinociception in rats by activating endogenous opioidergic circuits in PAG and the rostral ventromedial medulla. We have now found that endogenous opioids in the spinal cord are also involved. Responses of dorsal spinal neurons to noxious stimulation of a hindpaw were diminished (to 38-44%) by dipyrone microinjection (100 microg/0.5 microl) into the PAG. This was abolished by application of naloxone (50 microg/50 microl) to the spinal cord. The fact that dipyrone, a non-opioid analgesic, activates opioidergic circuits may be clinically important.

Prostaglandins and cyclooxygenases [correction of cycloxygenases] in the spinal cord.

The spinal cord is one of the sites where non-steroidal anti-inflammatory drugs (NSAIDs) act to produce analgesia and antinociception. Expression of cyclooxygenase(COX)-1 and COX-2 in the spinal cord and primary afferents suggests that NSAIDs act here by inhibiting the synthesis of prostaglandins (PGs). Basal release of PGD(2), PGE(2), PGF(2alpha) and PGI(2) occurs in the spinal cord and dorsal root ganglia. Prostaglandins then bind to G-protein-coupled receptors located in intrinsic spinal neurons (receptor types DP and EP2) and primary afferent neurons (EP1, EP3, EP4 and IP). Acute and chronic peripheral inflammation, interleukins and spinal cord injury increase the expression of COX-2 and release of PGE(2) and PGI(2). By activating the cAMP and protein kinase A pathway, PGs enhance tetrodotoxin-resistant sodium currents, inhibit voltage-dependent potassium currents and increase voltage-dependent calcium inflow in nociceptive afferents. This decreases firing threshold, increases firing rate and induces release of excitatory amino acids, substance P, calcitonin gene-related peptide (CGRP) and nitric oxide. Conversely, glutamate, substance P and CGRP increase PG release. Prostaglandins also facilitate membrane currents and release of substance P and CGRP induced by low pH, bradykinin and capsaicin. All this should enhance elicitation and synaptic transfer of pain signals in the spinal cord. Direct administration of PGs to the spinal cord causes hyperalgesia and allodynia, and some studies have shown an association between induction of COX-2, increased PG release and enhanced nociception. NSAIDs diminish both basal and enhanced PG release in the spinal cord. Correspondingly, spinal application of NSAIDs generally diminishes neuronal and behavioral responses to acute nociceptive stimulation, and always attenuates behavioral responses to persistent nociception. Spinal application of specific COX-2 inhibitors sometimes diminishes behavioral responses to persistent nociception.

Tolerance to repeated microinjection of morphine into the periaqueductal gray is associated with changes in the behavior of off- and on-cells in the rostral ventromedial medulla of rats.

Although the administration of opioids is the most effective treatment for pain, their efficacy is limited by the development of tolerance. The midbrain periaqueductal gray matter (PAG) participates in opioid analgesia and tolerance. Microinjection of morphine into PAG produces antinociception, probably through neurons in the rostral ventromedial medulla (RVM), namely through the activation of off-cells, which inhibit nociception, and the inhibition of on-cells, which facilitate nociception. After its repeated microinjection into the PAG morphine loses effectiveness. The present study sought to determine whether tolerance to PAG morphine administration is associated with changes in the behavior of RVM neurons. Morphine (0.5 microg/0.4 microl) or saline (0.4 microl) was microinjected into the ventrolateral PAG twice daily. Initially morphine caused a latency increase in the hot plate test (antinociception) but this effect disappeared by day 3 (tolerance). On day 4, each rat was anesthetized with halothane and recordings were made from off- and on-cells in the RVM, i.e. from neurons that decrease or increase their firing, respectively, just before a heat-elicited tail flick. In contrast to saline-pretreated rats, PAG microinjection of morphine in tolerant animals did not change the baseline activity of off- or on-cells, did not prevent the off-cell pause or the on-cell activation upon tail heating, and did not lengthen the tail flick latency. However, microinjection of kainic acid into the PAG (1) caused off-cells to become continuously active and on-cells to become silent, and (2) prevented the tail flick, i.e. exactly what morphine did before tolerance developed. These results demonstrate a correspondence between neuronal and behavioral measures of tolerance to PAG opioid administration, and suggest that tolerance is mediated by a change in opioid-sensitive neurons within the PAG.

How do we manage chronic pain?

Pain is an important symptom of acute damage and chronic inflammatory diseases such as rheumatoid arthritis. This chapter briefly summarizes the neuronal mechanisms of the peripheral and central sensitization of nociceptive neurones which are thought to be important in the generation and maintenance of inflammatory pain. Chronic pain in particular not only results from the neurobiological process of nociception, but is also influenced by psychological and social factors. The principles of current drug treatment are herein presented within the framework of neuroanatomy, neurophysiology and neuropharmacology, and options for the future are mentioned. A description is offered on how non-steroidal anti-inflammatory drugs and opioids interfere with peripheral and central pain mechanisms, and the rationale for using non-opioidergic and opioidergic analgesics is outlined. The importance of physical and psychosocial therapy is also addressed.

Opioid tolerance induced by metamizol (dipyrone) microinjections into the periaqueductal grey of rats.

Non-opioid analgesics have been shown to elicit antinociception by an action upon central nervous system structures, in addition to their well known action upon peripheral tissues. Microinjection of metamizol (dipyrone), a widely used nonopioid analgesic, into the periaqueductal grey matter (PAG) of rats activates pain-modulating systems in the nucleus raphe magnus and inhibits spinal nociceptive neurons and the tail-flick reflex. Since these effects involve an activation of endogenous opioidergic systems, the possibility that metamizol induces opioid tolerance was investigated. Microinjection of metamizol into the ventrolateral PAG in awake rats induced antinociception, as demonstrated in the heat-elicited tail flick and hot plate tests. When microinjected into the ventrolateral PAG twice daily for 2 days, metamizol induced tolerance, i.e. a progressive loss of its antinociceptive effect. In contrast to rats repeatedly microinjected with saline, metamizol-tolerant rats were also tolerant to morphine microinjection into the same PAG site, and displayed signs of opioid withdrawal upon systemic administration of naloxone. These and other results suggest that metamizol activates endogenous opioid systems and that nonopioid analgesics may, by an action upon the central nervous system, lead to opioid tolerance and the risk of opioid withdrawal.

Differential effects of calcitonin gene-related peptide and calcitonin gene-related peptide 8-37 upon responses to N-methyl-D-aspartate or (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate in spinal nociceptive neurons with knee joint input in the rat.

Calcitonin gene-related peptide is involved in the spinal processing of nociceptive input from the knee joint and in the generation and maintenance of joint inflammation-evoked hyperexcitability of spinal cord neurons. The present study examined whether this peptide influences the excitation of nociceptive spinal cord neurons by agonists at the N-methyl-D-aspartate and the non-N-methyl-D-aspartate [(R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate] receptors, both of which are essential for the excitation and hyperexcitability of spinal cord neurons. In anaesthetized rats extracellular recordings were made from dorsal horn neurons with knee input, and compounds were administered ionophoretically close to the neurons recorded. When calcitonin gene-related peptide was administered the responses of the neurons to the application of both N-methyl-D-aspartate and AMPA were increased. The coadministration of the antagonist calcitonin gene-related peptide 8-37 had no effect on the responses to N-methyl-D-aspartate, but it prevented the enhancement of the responses to N-methyl-D-aspartate by calcitonin gene-related peptide. By contrast, the administration of calcitonin gene-related peptide 8-37 enhanced the responses of the neurons to AMPA, and it did not antagonize but rather increased the effects of calcitonin gene-related peptide on these responses. The data suggest that the facilitatory role of calcitonin gene-related peptide on the development and maintenance of inflammation-evoked hyperexcitability is caused at least in part by the modulation of the activation of the dorsal horn neurons through their N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors. The different effects of calcitonin gene-related peptide 8-37 on the respones to N-methyl-D-aspartate and AMPA suggest that different intracellular pathways may facilitate the activation of N-methyl-D-aspartate and ionotropic non-N-methyl-D-aspartate receptors.

The antinociceptive effect of PAG-microinjected dipyrone in rats is mediated by endogenous opioids of the rostral ventromedical medulla.

Microinjection of non-opioid analgesics, such as dipyrone (DIP), into the periaqueductal gray matter (PAG) in rats causes an inhibition of nociceptive circuits in the spinal cord. We have herein investigated whether this effect is mediated by opioidergic mechanisms in the rostral ventromedial medulla (RVM), which is an important relay between the PAG and the spinal cord. The responses of spinal wide-dynamic-range neurons to noxious stimulation of their receptive field (RF) were inhibited by microinjection of DIP (100 microg/0.5 microl) into PAG. Subsequent microinjection of naloxone (NAL; 0.5 microg/0.5 microl) into RVM reversed this inhibition. The present and previous results suggest that non-opioid analgesics, as well as opiates, inhibit nociception by activating descending opioidergic mechanisms in PAG and RVM.

Effects of antagonists to high-threshold calcium channels upon spinal mechanisms of pain, hyperalgesia and allodynia.

High-threshold voltage-dependent calcium channels enable calcium ions to enter neurons upon depolarization and thereby influence synaptic mediator/receptor systems, membrane excitability levels, second and third messenger concentration, and gene expression. These phenomena underlie several processes including those of normal nociception and of hyperalgesia and allodynia. The present article deals with the role of spinal L-, N- and P/Q-type calcium channels in short-lasting nociception as well as in the hyperalgesia and allodynia elicited by chemical irritants of peripheral nociceptors, inflammatory and mechanical lesions of peripheral tissues, and lesions of peripheral nerves. The studies summarized herein are based on the spinal delivery of specific antagonists to high-threshold calcium channels, and reveal that blockade of L-type, P/Q-type and, particularly, N-type channels can prevent, attenuate, or both, subjective pain as well as primary and/or secondary hyperalgesia and allodynia in a variety of experimental and clinical conditions.

Assessment of insecticide resistance in five insect pests attacking field and vegetable crops in Nicaragua.

Field populations of Hypothenemus hampei (Ferrari), Plutella xylostella (L.), Spodoptera exigua (Hübner), Helicoverpa zea (Boddie) and Bemisia tabaci (Gennadius) were tested for resistance to several insecticides commonly used in Nicariagua. Assays were conducted to estimate the LD50s or LC50s and the corresponding resistance ratios. A diagnostic concentration was used to discriminate between susceptible and resistant strains of H. hampei. The tests with >6,000 H. hampei adults collected from six different sites indicate the absence of resistance to endosulfan. Resistance to cypermethrin, deltamethrin, chlorfluazuron, thiocyclam, and methamidophos was documented in six field populations of P. xylostella. High levels of resistance to cypermethrin and deltamethrin, but moderate levels of resistance to chlorpyriphos and methomyl, were also documented in two field populations of S. exigua. Moderate levels of resistance to cypermethrin, deltamethrin and chlorpyriphos were also documented in three field populations of H. zea. Moderate to high levels of resistance to bifenthrin, methamidophos and endosulfan were documented in four field populations of B. tabaci. The presence of significant correlations between LD50s or LC50s suggests the occurrence of cross-resistance or simultaneous selection for resistance by different insecticides with different modes of action. Our data could not differentiate between these two possibilities. Because insecticides will continue being used in Nicaragua, a resistance management program is urgently needed. The implementation of integrated pest management tactics must be accompanied by specific regulations for pesticide registration. In the future, pesticide registration regulations in Nicaragua should include periodic resistance monitoring. The mechanisms to cover the costs of resistance monitoring and resistance management should also be established.

Effects of omega-agatoxin IVA, a P-type calcium channel antagonist, on the development of spinal neuronal hyperexcitability caused by knee inflammation in rats.

Both N- and P-type high-threshold calcium channels are located presynaptically in the CNS and are involved in the release of transmitters. To investigate the importance of P-type calcium channels in the generation of inflammation-evoked hyperexcitability of spinal cord neurons, electrophysiological recordings were made from wide-dynamic-range neurons with input from the knee joint in the anesthetized rat. The responses of each neuron to innocuous and noxious pressure onto the knee and the ankle were continuously assessed before and during the development of an inflammation in the knee joint induced by the injections of K/C into the joint cavity. The specific antagonist at P-type calcium channels omega-agatoxin was administered into a 30-microl trough on the spinal cord surface above the recorded neuron. In most neurons the application of omega-agatoxin before induction of inflammation slightly enhanced the responses to pressure onto the knee and ankle or left them unchanged. Two different protocols were then followed. In the control group (13 rats) only Tyrode was administered to the spinal cord during and after induction of inflammation. In these neurons the responses to mechanical stimuli applied to both the inflamed knee and to the noninflamed ankle showed a significant increase over 4 h. In the experimental group (12 rats) omega-agatoxin was applied during knee injection and in five 15-min periods up to 180 min after kaolin. This prevented the increase of the neuronal responses to innocuous pressure onto the knee and to innocuous and noxious pressure onto the ankle; only the responses to noxious pressure onto the knee were significantly enhanced during development of inflammation. Thus the development of inflammation-evoked hyperexcitability was attenuated by omega-agatoxin, and this suggests that P-type calcium channels in the spinal cord are involved in the generation of inflammation-evoked hyperexcitability of spinal cord neurons. Finally, when omega-agatoxin was administered to the spinal cord 4 h after the kaolin injection, i.e., when inflammation-evoked hyperexcitability was fully established, the responses to innocuous and noxious pressure onto the knee were reduced by 20-30% on average. The shift in the effect of omega-agatoxin, from slight facilitation or no change of the responses before inflammation to inhibition in the state of hyperexcitability, indicates that P-type calcium channels are important for excitatory synaptic transmission involved in the maintenance of inflammation-evoked hyperexcitability.

Spinal application of omega-conotoxin GVIA, an N-type calcium channel antagonist, attenuates enhancement of dorsal spinal neuronal responses caused by intra-articular injection of mustard oil in the rat.

Administration of the N-type calcium channel antagonist omega-conotoxin GVIA to the spinal cord reduces spinal neuronal responses to innocuous and noxious pressure applied to the knee, both in rats with normal knees and in rats in which a knee inflammation has induced a state of hyperexcitability in spinal neurons (Neugebauer et al. 1996, J Neurophysiol 76: 3740-3749). In the present experiments we studied whether the development of hyperexcitability of spinal neurons induced by intra-articular injection of mustard oil, an excitant of C-fibres, can be influenced by spinal pretreatment with omega-conotoxin GVIA. In anaesthetized rats, responses of wide-dynamic-range neurons were recorded in the spinal dorsal horn when standardized stimulation with innocuous and noxious pressure was applied to the knee and ankle joints. Injection of mustard oil into the knee joint cavity caused an initial neuronal discharge followed by an early (peaking at about 15 min) and a late (after 60 min) facilitation of responses to innocuous and noxious stimulation of the knee. Responses to ankle stimulation showed only the late facilitation. When omega-conotoxin GVIA (20 microl, 1 microM) was applied into a small trough onto the spinal cord above the recording site the responses to articular stimulation were reduced. Furthermore, when mustard oil was injected while omega-conotoxin GVIA was on the spinal cord, the early increase in the neuronal responses to innocuous pressure on the knee and the late increase in responses to noxious pressure on the ankle were significantly smaller than those observed in rats not treated with omega-conotoxin GVIA; the drop in the responses to noxious pressure on the knee was not significant. Thus the spinal application of omega-conotoxin GVIA reduced but did not completely prevent the fast and slow development of neuronal hyperexcitability of spinal cord neurons produced by a prompt and strong excitation of afferent C-fibres. This suggests that N-type calcium channels are important for the development of spinal cord hyperexcitability.

PAG-microinjected dipyrone (metamizol) inhibits responses of spinal dorsal horn neurons to natural noxious stimulation in rats.

In addition to their well-known peripheral and spinal effects, non-steroidal antiinflammatory drugs (NSAIDs) are believed to diminish nociceptive responses by acting supraspinally and activating descending modulatory systems. We have herein investigated whether this descending action involves a depression of spinal sensory neurons. In rats under barbiturate anesthesia, responses of lumbar wide-dynamic-range neurons to a noxious clamp in their receptive fields were depressed to 46% of baseline value by the microinjection of 100 microg dipyrone (metamizol) into the periaqueductal gray matter (PAG). These results show that PAG application of NSAIDs activates descending systems which depress the excitation of spinal sensory neurons by natural noxious stimuli.

Omega-agatoxin IVA, a P-type calcium channel antagonist, reduces nociceptive processing in spinal cord neurons with input from the inflamed but not from the normal knee joint--an electrophysiological study in the rat in vivo.

High threshold voltage-dependent P- and Q-type calcium channels are involved in neurotransmitter release. In order to investigate the role of P- and Q-type calcium channels in the mechanosensory (nociceptive) processing in the spinal cord, their participation in the responses of spinal wide-dynamic-range neurons to innocuous and noxious mechanical stimulation of the knee and ankle joints was studied in 30 anaesthetized rats. The knee was either normal or acutely inflamed by kaolin/carrageenan. During the topical application of omega-agatoxin IVA (P-type channel antagonist, 0.1 microM) onto the dorsal surface of the spinal cord, the responses to innocuous and noxious pressure applied to the normal knee were increased to respectively 124 +/- 42% and 114 +/- 23% of predrug values (mean +/- SD, P < 0.05, 14 neurons). By contrast, in rats with an inflamed knee, the responses to innocuous and noxious pressure applied to the knee were reduced to respectively 72 +/- 19 and 73 +/- 22% of baseline (mean +/- SD, P < 0.01, 13 neurons). In the same neurons, omega-agatoxin IVA slightly increased the responses to pressure on the non-inflamed ankle whether the knee was normal or inflamed. Thus P-type calcium channels seem to acquire a predominant importance in the excitation of spinal cord neurons by mechanosensory input from inflamed tissue and hence in the generation of inflammatory pain. By contrast, the Q-type channel antagonist, omega-conotoxin MVIIC (1 or 100 microM), had no significant effect upon responses to innocuous or noxious pressure applied to either normal or inflamed knees (25 neurons).

Effects of N- and L-type calcium channel antagonists on the responses of nociceptive spinal cord neurons to mechanical stimulation of the normal and the inflamed knee joint.

1. The present study addresses the involvement of voltage-dependent calcium channels of the N and L type in the spinal processing of innocuous and noxious input from the knee joint, both under normal conditions and under inflammatory conditions in which spinal cord neurons become hyperexcitable. In 30 anesthetized rats, extracellular recordings were performed from single dorsal horn neurons in segments 1-4 of the lumbar spinal cord. All neurons had receptive fields in the ipsilateral knee joint. In 22 rats, an inflammation was induced in the ipsilateral knee joint by kaolin and carrageenan 4-16 h before the recordings. The antagonist at N-type calcium channels, omega-conotoxin GVIA (omega-CTx GVIA), was administered topically in solution to the dorsal surface of the spinal cord at the appropriate spinal segments in 6 rats with normal joints and in 12 rats with inflamed knee joints. The antagonist at L-type channels, nimodipine, was administered topically in 5 rats with normal joints and in 11 rats with inflamed knee joints. In another five rats with inflamed joints, antagonists at L-type calcium channels (diltiazem and nimodipine) and omega-CTx GVIA were administered ionophoretically with multibarrel electrodes close to the neurons recorded. 2. The topical administration of omega-CTx GVIA to the spinal cord reduced the responses to both innocuous and noxious pressure applied to the knee joint in a sample of 11 neurons with input from the normal joint and in a sample of 16 neurons with input from the inflamed joint (hyperexcitable neurons). The responses were decreased to approximately 65% of the predrug values within administration times of 30 min. A similar reduction of the responses to innocuous and noxious pressure was observed when omega-CTx GVIA was administered ionophoretically to nine hyperexcitable neurons. In neurons with input from the normal or the inflamed knee joint, the administration of omega-CTx GVIA led also to a reduction of the responses to innocuous and noxious pressure applied to the noninflamed ankle joint. 3. The topical administration of nimodipine decreased the responses to innocuous and noxious pressure applied to the knee in a sample of 9 neurons with input from the normal joint and in a sample of 16 neurons with input from the inflamed knee joint (hyperexcitable neurons). Within administration times of 30 min, the responses were reduced to approximately 70% of the predrug values. In hyperexcitable neurons, the responses to innocuous and noxious pressure applied to the knee were also decreased during ionophoretic administration of nimodipine (6 neurons) and diltiazem (9 neurons). When the noninflamed ankle was stimulated, the responses to innocuous pressure were reduced neither in neurons with input from the normal knee nor in neurons with input from the inflamed knee, but the responses of hyperexcitable neurons to noxious pressure onto the ankle were reduced. The ionophoretic administration of the agonist at the L-type calcium channel, S(-)-Bay K 8644, enhanced the responses to mechanical stimulation of the knee joint in all 14 hyperexcitable neurons tested. The effect of S(-)-Bay K 8644 was counteracted by both diltiazem (in 6 of 6 neurons) and nimodipine (in 5 of 5 neurons). 4. These data show that antagonists at both the N- and the L-type voltage-dependent calcium channels influence the spinal processing of input from the knee joint. The data suggest, therefore, that voltage-dependent calcium calcium channels of both the N and the L type are important for the sensory functions of the spinal cord. They are involved in the spinal processing of nonnociceptive as well as nociceptive mechanosensory input from the joint, both under normal and inflammatory conditions. The present results show in particular that N- and L-type channels are likely to be involved in the generation of pain evoked by noxious mechanical stimulation in normal tissue as well as in the mechanical hyperalgesia that is usually pres

Naloxone partial reversal of the antinociception produced by dipyrone microinjected into the periaqueductal gray of rats. Possible involvement of medullary off- and on-cells.

Medullary off- and on-cells have been proposed to inhibit and facilitate, respectively, nociceptive transmission. Upon heating the tail in lightly anesthetized rats, the tail flick (TF) reflex occurs only after off-cells decrease and on-cells increase their activity. Dipyrone (DIP) microinjection (100 micrograms/0.5 microliter) into the periaqueductal gray (PAG) caused retardation in the off-cell pause, on-cell burst and corresponding TF. This effect was partly reverted by naloxone given i.v. (l mg/kg) or microinjected into PAG (5 micrograms/0.5 microliter). These results suggest that endogenous opioids are partly responsible for the central antinociceptive action of DIP, and that such action involves medullary off- and on-cells.