Activity correlations between on-like and off-like cells of the rostral ventromedial medulla and simultaneously recorded wide-dynamic-range neurons of the spinal dorsal horn in rats

Considerable evidence supports the notion that on- and off-cells of the rostral ventromedial medulla (RVM) facilitate and depress, respectively, spinal nociceptive transmission. This notion stems from a covariation of on- or off-cell activities and spinal nocifensive reflexes. Such covariation could theoretically be due to their independently responding to a common source, or to an RVM-derived modulation of ventral horn neurons. Here, we tested whether on- and off-cells indeed modulate spinal nociceptive neurons. In deeply anesthetized rats, unitary recordings were simultaneously made from an RVM on-like or off-like cell and a spinal nociceptive neuron that shared a receptive field (RF) at a hind paw. Action potential firing in RVM/spinal neuron pairs was highly correlated, positively for on-like cells and negatively for off-like cells, both during ongoing activity and during application of calibrated noxious pressure to the RF. Microinjection of morphine into RVM induced a correlated decrease in on-like cell/spinal neuron ongoing activity and response to noxious stimulation. RVM morphine induced changes in off-like cell activity that were not correlated with spinal neuronal activity. These results suggest that on-cells exert a positive modulation upon spinal nociceptive neurons, upstream to ventral horn circuits and plausibly at the origin of nociceptive information that eventually reaches the cerebral cortex. On-cells may in this manner contribute to inflammation- and neuropathy-induced increases in withdrawal reflexes. Most significantly, on-cell modulation of nociceptive neurons may be a key factor in clinical pain conditions such as hyperalgesia and allodynia.

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 antinociceptive effects of cyclooxygenase inhibition during inflammation: Involvement of prostaglandins and endocannabinoids

Both cyclooxygenase-1 and -2 are expressed in the spinal cord, and the spinal COX product prostaglandin E(2) (PGE(2)) contributes to the generation of central sensitization upon peripheral inflammation. Vice versa spinal COX inhibition is considered an important mechanism of antihyperalgesic pain treatment. Recently, however, COX-2 was shown to be also involved in the metabolism of endocannabinoids. Because endocannabinoids can have analgesic actions it is conceivable that inhibition of spinal COX produces analgesia not only by inhibition of PG synthesis but also by inhibition of endocannabinoid breakdown. In the present study, we recorded from spinal cord neurons with input from the inflamed knee joint and we measured the spinal release of PGE(2) and the endocannabinoid 2-arachidonoyl glycerol (2-AG) in vivo, using the same stimulation procedures. COX inhibitors were applied spinally. Selective COX-1, selective COX-2 and non-selective COX inhibitors attenuated the generation of spinal hyperexcitability when applied before and during development of inflammation but, when inflammation and spinal hyperexcitability were established, only selective COX-2 inhibitors reversed spinal hyperexcitability. During established inflammation all COX inhibitors reduced release of spinal PGE(2) almost equally but only the COX-2 inhibitor prevented breakdown of 2-AG. The reversal of spinal hyperexcitability by COX-2 inhibitors was prevented or partially reversed by AM-251, an antagonist at the cannabinoid-1 receptor. We conclude that inhibition of spinal COX-2 not only reduces PG production but also endocannabinoid breakdown and provide evidence that reversal of inflammation-evoked spinal hyperexcitability by COX-2 inhibitors is more related to endocannabinoidergic mechanisms than to inhibition of spinal PG synthesis.

Spinal endocannabinoids and CB1 receptors mediate C-fiber-induced heterosynaptic pain sensitization

Diminished synaptic inhibition in the spinal dorsal horn is a major contributor to chronic pain. Pathways that reduce synaptic inhibition in inflammatory and neuropathic pain states have been identified, but central hyperalgesia and diminished dorsal horn synaptic inhibition also occur in the absence of inflammation or neuropathy, solely triggered by intense nociceptive (C-fiber) input to the spinal dorsal horn. We found that endocannabinoids, produced upon strong nociceptive stimulation, activated type 1 cannabinoid (CB1) receptors on inhibitory dorsal horn neurons to reduce the synaptic release of gamma-aminobutyric acid and glycine and thus rendered nociceptive neurons excitable by nonpainful stimuli. Our results suggest that spinal endocannabinoids and CB1 receptors on inhibitory dorsal horn interneurons act as mediators of heterosynaptic pain sensitization and play an unexpected role in dorsal horn pain-controlling circuits.

Joint pain

Both inflammatory and degenerative diseases of joints are major causes of chronic pain. This overview addresses the clinical problem of joint pain, the nociceptive system of the joint, the mechanisms of peripheral and central sensitization during joint inflammation and long term changes during chronic joint inflammation. While the nature of inflammatory pain is obvious the nature and site of origin of osteoarthritic pain is less clear. However, in both pathological conditions mechanical hyperalgesia is the major pain problem, and indeed, both joint nociceptors and spinal nociceptive neurons with joint input show pronounced sensitization for mechanical stimulation. Molecular mechanisms of mechanical sensitization of joint nociceptors are addressed with an emphasis on cytokines, and molecular mechanisms of central sensitization include data on the role of excitatory amino acids, neuropeptides and spinal prostaglandins. The overview will also address long-term changes of pain-related behavior, response properties of neurons and receptor expression in chronic animal models of arthritis.

Critical role of the rostral ventromedial medulla in early spinal events leading to chronic constriction injury neuropathy in rats

Neuropathic pain is a major clinical problem, and several animal models have been developed to investigate its mechanisms and its treatment. In this report, the role of the rostral ventromedial medulla (RVM) in the early events of the chronic constriction injury (CCI) model was investigated in behavioral and electrophysiological experiments. Placing the 4 CCI ligatures around the sciatic nerve induced large discharges and residual ongoing activity in spinal nociceptive neurons. Two weeks after CCI ligation, the rats showed behavioral hyperalgesia and allodynia as well as increased ongoing activity and responsiveness of spinal nociceptive neurons to innocuous and noxious stimuli. Blockade of excitatory synapses in the RVM by a kynurenate microinjection (2 nmol in 0.5 muL) 5 minutes before placement of the sciatic ligatures had no immediate effect on spinal neuronal activity but largely prevented the activation of spinal neurons. In kynurenate microinjected rats, behavioral hyperalgesia and allodynia developed slowly and incompletely, which corresponded with an incompletely developed hyperexcitability of spinal neurons. To the best of our knowledge, these results show for the first time that the initial response to nerve damage requires facilitation from the RVM.

PERSPECTIVE

The present and previous findings indicate that descending facilitation from brainstem nuclei critically contributes to the spinal hyperexcitability that underlies neuropathic pain. The present results indicate that this contribution begins at the very moment the nerve is damaged and should be prevented and treated accordingly.

A nonopioid analgesic acts upon the PAG-RVM axis to reverse inflammatory hyperalgesia

Metamizol (dipyrone) and other nonsteroidal anti-inflammatory drugs (NSAIDs) induce antinociception by acting upon peripheral tissues and upon central nervous system structures, notably the periaqueductal grey matter (PAG) and the spinal cord. Inflammation-induced hyperalgesia is prevented by spinal application of NSAIDs before the inflammation, but once central sensitization is established the spinal effect of NSAIDs is uncertain. The present study examines whether the action upon the PAG contributes to the attenuation of inflammation-induced spinal hyperalgesia by NSAIDs. In deeply anaesthetized rats, responses of spinal multireceptive neurons to mechanical stimulation of the ipsilateral paw and leg were recorded. An inflammation in the paw was induced with carrageenan. Fifty minutes later, neuronal responses to innocuous and noxious stimulation had, respectively, increased to 206 and 304% for paw, and 160 and 190% for leg. When metamizol (150 microg in 0.5 microL) was microinjected into PAG before the inflammation, neuronal hyperexcitability was delayed for approximately 60 min and was much reduced by 215 min. More interestingly, microinjection of metamizol into PAG when hyperexcitability was fully developed depressed neuronal responses down to baseline for approximately 1 h. The effect of PAG metamizol was reversed by microinjection of a GABA(A) agonist into the rostral ventromedial medulla (RVM), which indicates that RVM relays the metamizol effect from PAG onto the spinal cord. These results suggest that, upon clinical administration of NSAIDs, a joint action upon PAG and spinal cord contributes to preventing the development of hyperalgesia but it is mainly the action upon PAG which contributes to reducing fully established hyperalgesia.

Antinociception induced by intravenous dipyrone (metamizol) upon dorsal horn neurons: Involvement of endogenous opioids at the periaqueductal gray matter, the nucleus raphe magnus, and the spinal cord in rats

Microinjection of dipyrone (metamizol) into the periaqueductal gray matter (PAG) in rats causes antinociception. This is mediated by endogenous opioidergic circuits located in the PAG itself, in the nucleus raphe magnus and adjacent structures, and in the spinal cord. The clinical relevance of these findings, however, is unclear. Therefore, in the present study, dipyrone was administered intravenously, and the involvement of endogenous opioidergic circuits in the so-induced antinociception was investigated. In rats, responses of dorsal spinal wide-dynamic range neurons to mechanical noxious stimulation of a hindpaw were strongly inhibited by intravenous dipyrone (200 mg/kg). This effect was abolished by microinjection of naloxone (0.5 microg/0.5 microl) into the ventrolateral and lateral PAG or into the nucleus raphe magnus or by direct application of naloxone (50 microg/50 microl) onto the spinal cord surface above the recorded neuron. These results show that dipyrone, a non-opioid analgesic with widespread use in Europe and Latin America, when administered in a clinically relevant fashion causes antinociception by activating endogenous opioidergic circuits along the descending pain control system.

Descending control of persistent pain: inhibitory or facilitatory?

The periaqueductal gray matter (PAG) and the nucleus raphe magnus and adjacent structures of the rostral ventromedial medulla (RVM), with their projections to the spinal dorsal horn, constitute the "efferent channel" of a pain-control system that "descends" from the brain onto the spinal cord. Considerable evidence has recently emerged regarding participation of this system in persistent pain conditions such as inflammation and neuropathy. Herein, this evidence is reviewed and organized to support the idea that persistent nociception simultaneously triggers descending facilitation and inhibition. In models of inflammation, descending inhibition predominates over facilitation in pain circuits with input from the inflamed tissue, and thus attenuates primary hyperalgesia, while descending facilitation predominates over inhibition in pain circuits with input from neighboring tissues, and thus facilitates secondary hyperalgesia. Both descending facilitation and inhibition mainly stem from RVM. The formalin-induced primary hyperalgesia, although considered a model for inflammation, is mainly facilitated from RVM. Also, formalin-induced secondary hyperalgesia is facilitated by RVM. Again, formalin triggers a concomitant but concealed descending inhibition. The (primary) hyperalgesia and allodynia of the neuropathic syndrome are also facilitated from RVM. Simultaneously, there is an inhibition of secondary neuronal pools that is partly supported from the PAG. Because in all these models of peripheral damage descending facilitation and inhibition are triggered simultaneously, it will be important to elucidate why inhibition predominates in some neuronal pools and facilitation in others. Therapies that enhance descending inhibition and/or attenuate descending facilitation are furthermore an important target for research in the future.

Involvement of cholecystokinin in the opioid tolerance induced by dipyrone (metamizol) microinjections into the periaqueductal gray matter of rats

The analgesic effect of non-steroidal anti-inflammatory drugs (NSAIDs) is partly due to an action upon the periaqueductal gray matter (PAG), which triggers the descending pain control system and thus inhibits nociceptive transmission. This action of NSAIDs engages endogenous opioids at the PAG, the nucleus raphe magnus and the spinal cord. Repeated administration of NSAIDs such as dipyrone (metamizol) and acetylsalicylate thus induces tolerance to these compounds and cross-tolerance to morphine. Since cholecystokinin plays a key role in opioid tolerance, the present study in rats investigated whether PAG cholecystokinin is also responsible for tolerance to PAG-microinjected dipyrone. Microinjection of cholecystokinin (1 ng/0.5 microl) into PAG blocked the antinociceptive effect of a subsequent microinjection of dipyrone (150 microg/0.5 microl) into the same site, as evaluated by the tail flick and hot plate tests. Microinjection of proglumide (0.4 microg/0.5 microl), a non-selective cholecystokinin antagonist, into PAG prevented the development of tolerance to subsequent microinjections of dipyrone, as well as cross-tolerance to microinjection of morphine (5 microg/0.5 microl) into the same site. In rats tolerant to PAG dipyrone, a PAG microinjection of proglumide restored the antinociceptive effect of a subsequent microinjection of dipyrone or morphine. These results suggest that PAG-microinjected dipyrone triggers and/or potentiates local opioidergic circuits leading to descending inhibition of nociception, on the one hand, and to a local antiopioid action by cholecystokinin, on the other. Reiteration of these events would then result in an enhancement of cholecystokinin's antiopioid action and thus tolerance to opioids and dipyrone in the PAG.

Induction of opioid tolerance by lysine-acetylsalicylate in rats

The analgesic effect of non-steroidal antiinflammatory drugs (NSAIDs) is due to their action upon the peripheral damaged tissues, the spinal cord, and brain stem structures of the 'descending pain-control system' such as the periaqueductal gray matter (PAG) and the nucleus raphe magnus (NRM). The NSAID dipyrone (metamizol) has been shown to engage opioidergic circuits at the PAG, the NRM and the spinal cord, but it is unknown whether this can be generalized to typical NSAIDs and to systemic administration. In the present study lysine-acetylsalicylate (LASA), an injectable form of the prototypical NSAID aspirin, was microinjected into the PAG (100 microg/0.5 microl) in freely moving rats to induce inhibition of tail flick and hot plate responses. This antinociception was reverted by naloxone (1 mg/kg i.p.). PAG microinjection of LASA twice daily for three days induced tolerance to LASA (i.e. a progressive loss of effectiveness) and cross-tolerance to PAG-microinjected morphine (5 microg/0.5 microl). The antinociceptive effect of systemically administered LASA (300 mg/kg i.p., equivalent to the 1000 mg analgesic dose for humans) was also abolished by naloxone. Intraperitoneal injection of LASA twice daily induced tolerance to LASA and cross-tolerance to i.p. morphine (1 or 5 mg/kg). LASA-tolerant rats showed opioid withdrawal signs when injected with naloxone. These findings support the notion that the contribution of the PAG and downstream pain-control structures to the analgesic effect of NSAIDs involves opioidergic mechanisms, and suggest that repeated therapeutic administration of NSAIDs may induce tolerance, cross-tolerance to opiates, and susceptibility to a withdrawal syndrome.

To the descending pain-control system in rats, inflammation-induced primary and secondary hyperalgesia are two different things

The periaqueductal gray matter and the rostral ventromedial medulla (RVM), with its projections to the spinal dorsal horn, constitute the efferent channel of the 'descending pain-control system'. Noxious stimulation of a peripheral tissue causes more pain if this tissue is inflamed (primary hyperalgesia). In such cases, stimulation of neighboring but uninflamed tissues also becomes more painful (secondary hyperalgesia). In animal models of inflammation, the descending pain-control system sends down, simultaneously, inhibitory and facilitatory influences, but inhibition predominates for primary hyperalgesia while facilitation predominates for secondary hyperalgesia. Descending inhibition and facilitation during peripheral inflammation are due not only to previously existing descending modulation, but also to inflammation-induced changes in RVM which involve receptors for NMDA, AMPA, cholecystokinin and neurotensin, as well as synthesis of enkephalins and nitric oxide.

Involvement of local cholecystokinin in the tolerance induced by morphine microinjections into the periaqueductal gray of rats

The ventrolateral periaqueductal gray (PAG) is a key structure for the development of opioid tolerance. An increased activity of 'anti-opioids' like cholecystokinin (CCK) has been proposed as a possible mechanism for opioid tolerance. The present study evaluates the role of PAG-located CCK in the opioid tolerance induced by repeated microinjections of morphine (MOR) into PAG. Male rats were implanted with chronic guide cannulae aimed at the PAG. Microinjection of MOR (0.5 microg in 0.5 microl) into PAG caused antinociception as quantified with the tail flick and the hot plate tests. When MOR microinjection was repeated twice daily, the antinociceptive effect disappeared within 2 days (tolerance). However, if each MOR microinjection was preceded (within 15 min) by a microinjection of the non-selective CCK receptor antagonist proglumide (PRO), (0.4 microg in 0.5 microl) into the same PAG site, the microinjections of MOR always produced antinociception and did not induce tolerance. If PRO microinjections were suspended, subsequent MOR microinjections induced tolerance. In MOR-tolerant rats, a single PRO microinjection into the same PAG site was enough to restore the antinociceptive effect of MOR. On the other hand, if CCK (1 ng in 0.5 microl) was microinjected into PAG, then MOR microinjection administered 15 min later into the same PAG site did not elicit antinociception. These results show that CCK has anti-opioid activity in PAG and that tolerance to MOR in PAG can be prevented or reversed if CCK receptors are blocked with PRO. Finally, opioid tolerance induced by repeated systemic MOR injections (5mg/kg intraperitoneal ) was reversed by a single microinjection of PRO into PAG. This emphasizes the central importance of PAG in the MOR/CCK interactions that lead to opioid tolerance.

Opioidergic effects of nonopioid analgesics on the central nervous system

1. The analgesic effect of nonsteroidal anti-inflammatory drugs (NSAIDs) is partly due to the fact that they act upon the periaqueductal gray matter (PAG) and the rostral ventromedial medulla of the brain stem and thus activate the descending pain-control system, which inhibits nociceptive transmission at the spinal dorsal horn. 2. The analgesic action of dipyrone (metamizol) and of lysine-acetylsalicylate (LASA), two well-known NSAIDs. whether microinjected into the PAG or given systemically, can be reverted by naloxone. Repeated administration of dipyrone or LASA induces tolerance to their antinociceptive effect, with cross-tolerance to morphine, and a withdrawal syndrome upon naloxone administration. Dipyrone tolerance can be reverted by proglumide, a cholecystokinin antagonist. 3. These findings reveal a close association between the central action of NSAIDs and endogenous opioids.

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.

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.

Encoding of noxious stimulus intensity by putative pain modulating neurons in the rostral ventromedial medulla and by simultaneously recorded nociceptive neurons in the spinal dorsal horn of rats

Neurons in the nucleus raphe magnus and adjacent structures of the rostral ventromedial medulla (RVM) are involved in the control of nociceptive transmission. In the RVM the so-called on-cells are excited, and the so-called off-cells are inhibited, by noxious stimuli applied almost anywhere on the body surface, thus showing that they receive information from spinal and trigeminal nociceptive neurons. In deeply anesthetized rats, recordings were made from RVM neurons that resembled on- and off-cells (herein called putative on- and off-cells) in order to investigate (1) how they encode the intensity of thermal noxious stimuli (46--56 degrees C) applied to a hindpaw, and (2) how their encoding properties relate to those of simultaneously recorded spinal neurons. In 49 of 98 cases, a graded increase in the stimulus temperature caused a monotonic decrease in the response latency of putative on-cells, putative off-cells and spinal neurons, while the response discharge rate monotonically increased for putative on-cells and spinal neurons and decreased for putative off-cells. In the majority of simultaneous recordings of RVM and spinal neurons, the latency and discharge rate of the putative on- or off-cell were highly correlated with the latency and discharge rate of the spinal neuron, and the stimulus/response slopes were similar. These results show that putative on- and off-cells can encode the stimulus intensity in terms of response latency and discharge rate, and suggest that such encoding closely reflects spinal neuronal encoding. This may be relevant for the transmission and modulation of pain information by RVM neurons.

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.

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.

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.

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.

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.

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 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.

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).

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.

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.

Fractal analysis of spinal dorsal horn neuron discharges by means of sequential fractal dimension D

The present study describes a new method for converting a typical point process, such as a train of neuronal action potentials (spikes), into a planar curve which is then processed by means of a fast algorithm to calculate and display the fractal dimension D values of each of a sequence of blocks having an equal and preselectable number of interspike intervals, hence the term sequential fractal dimension D (SFD). This method is fast, does not require special computing facilities, and provides a continuous, high temporal resolution display of the neuronal discharge complexity along the course of spontaneous activity or event relating changes. The method affords insight into short duration changes in neuronal behaviour in a way independent of its discharge rate. SFD analysis of spike trains from spinal dorsal horn neurons suggests that the neuronal response to a given stimulus can be expressed as changes in the discharge pattern complexity, thus revealing a novel sensory coding strategy.

Anti-nociception induced by systemic or PAG-microinjected lysine-acetylsalicylate in rats. Effects on tail-flick related activity of medullary off- and on-cells

Previous experiments using metamizol have shown that this non-steroidal anti-inflammatory drug (NSAID) produces a central anti-nociceptive effect probably through neural substrates that also support the analgesic effects of opiates, such as the periaqueductal grey matter (PAG) and the off- and on-cells of the rostral ventromedial medulla (RVM). Off- and on-cells have been postulated to respectively inhibit and facilitate nociceptive transmission, since the heat-elicited tail flick reflex (TF) occurs only after off-cells have decreased (pause), and on-cells, have increased (burst) their activity. The aim of the present study was to examine whether the effect of metamizol upon TF and off- and on-cells responses could be generalized to other NSAIDs such as, in this case, lysine-acetylsalicylate (LASA). Fifty-nine off- and on-cells of the RVM were recorded in lightly anaesthetized rats. Systemic administration (200 and 300 mg/kg) or PAG microinjection (30, 50 and 100 micrograms) of LASA caused retardation of the heat-elicited off-cell pause, on-cell burst and the corresponding TF. Neuronal responses and TF retained their mutual time relationship but shifted simultaneously toward longer latencies. This anti-nociceptive effect of LASA was dose-dependent, present 5 min after administration and reached a maximum in 30 min for both administration methods. These data confirm that analgesics typically defined as peripherally-acting, such as metamizol and LASA in this study, may also have an anti-nociceptive effect by acting directly upon PAG, and suggest that this central effect involves the RVM off- and on-cells.

Medullary on-cell activity during tail-flick inhibition produced by heterotopic noxious stimulation

Reflex responses and neuronal excitation elicited by noxious stimuli applied to a given body site can be inhibited by application of noxious stimulation to another, even distant body region. Such heterotopic noxious stimulation (HNS) has been proposed to act via 'diffuse noxious inhibitory controls' (DNIC) which involve supraspinal components. The so-called on-cells of the rostral ventromedial medulla (RVM) in rats are thought to facilitate nociceptive transmission. Experimental manipulations that inhibit on-cells also inhibit withdrawal reflexes and nociceptive thalamic responses. In the present study on-cell activity was recorded in relation to the tail flick (TF) elicited by noxious heat applied to the tail both before and during either immersion of a paw in water above 56 degrees C or application of strong pinch to various body regions. Such HNS elicited strong activation of on-cells, followed by depression even when HNS continued. When this depression was intense, tail-heating failed to elicit vigorous on-cell firing, and TF was retarded or abolished. These results are compatible with the hypothesis that antinociception elicited by HNS involves depression of on-cell firing and hence lack of facilitation of nociceptive transmission.

Putative role of medullary off- and on-cells in the antinociception produced by dipyrone (metamizol) administered systemically or microinjected into PAG

Recent investigations have shown that non-steroidal antiinflammatory drugs (NSAIDs) may exert an antinociceptive effect when administered at or within the central nervous system (CNS). This might be due to the engagement of CNS substrates that support the analgesic effects of opiates, including the periaqueductal gray matter (PAG) and the rostral ventromedial medulla (RVM). The off- and on-cells of the RVM have been proposed to inhibit and facilitate, respectively, nociceptive transmission. Accordingly, upon heating of a rat's tail the tail-flick (TF) reflex occurs only after off-cells have decreased, and on-cells have increased, their activity. In the present study, i.v. administration (200 and 400 mg/kg) or PAG microinjection (25, 50, 100 and 250 micrograms) of dipyrone (metamizol) to lightly anesthetized rats caused a dose-related retardation of the heat-elicited off-cell pause, on-cell discharge and corresponding TF. Neuronal response and TF retained their mutual time relationship but shifted pari passu toward longer latencies. This antinociception was apparent already 5 min post-injection and reached a maximum in 50-60 min for i.v. administration and 30-35 min for PAG microinjection. These results confirm other authors' findings of the direct antinociceptive action of NSAIDs upon PAG, and provide the first evidence for a plausible involvement of RVM off- and on-cells in such antinociceptive effect.

Medullary on- and off-cell responses precede both segmental and thalamic responses to tail heating

Medullary on- and off-cell responses to tail heating were studied in lightly anesthetized rats. In 10 animals the electromyographic (EMG) activation of tail muscles was recorded simultaneously with on- or off-cells. The on-cell burst or the off-cell pause always preceded segmental EMG activation by about 0.5 sec. In turn, EMG activation preceded visible tail flick by about 0.09 sec. In 13 other animals ventrobasal thalamic unitary responses were recorded simultaneously with on- and off-cell responses. On-cell bursts or off-cell pauses always preceded thalamic responses by about 0.4 sec. These results support the notion that on- and off-cells play a regulatory role in both segmental and ascending nociceptive transmission.

Tooth pulp stimulation advances both medullary off-cell pause and tail flick

It has been postulated that the so-called off-cells of nucleus raphe magnus and adjacent structures in the rat are the output elements of a system which inhibits nociceptive transmission at the spinal cord. Off-cells stop firing about 0.4 s before the tail flick reflex (TF) elicited by the application of noxious heat to the tail. When continuous off-cell activity is induced by either morphine injection or periaqueductal gray stimulation, the TF is delayed. The present results show that electrical stimulation of the tooth pulp (TP) causes the off-cells to stop firing. Furthermore, when TP is stimulated during tail heating and before the expected time for TF, off-cells stop firing earlier and the TF occurs also earlier. This supports the notion that off-cells inhibit nociceptive transmission.

Diameters and terminal patterns of retinofugal axons in their target areas: an HRP study in two teleosts (Sebastiscus and Navodon)

Studies in various vertebrate classes, particularly amphibians and mammals, have revealed that retinal ganglion cells with different functional properties project by means of axons of correspondingly different diameters onto specific target regions. Whether a similar pattern exists in teleosts is partly investigated in the present study. HRP was injected into the optic nerve of Sebastiscus and Navodon. The calibers of intraretinal HRP-labeled axons were classed as fine (ca. 0.8 micron), medium (ca. 1.3 micron), and coarse (ca. 2.5 microns). The calibers of HRP-labeled retinofugal axons were then determined in their target areas, and these can be summarized as follows: Optic hypothalamus: fine, medium. Lateral geniculate nucleus: fine. Dorsolateral thalamic nucleus: fine, medium. Area pretectalis: fine. Nucleus of the posterior commissure: fine, medium. Area ventralis lateralis, contralateral: fine, medium, coarse; ipsilateral: coarse. Optic tectum, stratum opticum: fine, medium; stratum fibrosum et griseum superficiale: fine, medium, coarse, segregated in sublayers; stratum album centrale: fine, medium, coarse. Therefore, fine fibers were found to reach all target areas except the ipsilateral area ventralis lateralis, and these were the only fibers found in the lateral geniculate nucleus, area pretectalis, and stratum griseum centrale of the optic tectum. Coarse fibers, on the other hand, were found only in the area ventralis lateralis and the optic tectum (stratum fibrosum et griseum superficiale and stratum album centrale). Terminal patterns of these fibers were also studied. Most fine fibers take tortuous courses giving off a few branches and terminate with many varicosities, and medium and coarse fibers give off several finer branches and terminate with bulbous swellings. The physiological significance of these findings is discussed. In addition, retrogradely labeled (retinopetal) cells were found in the olfactory bulb and the area ventralis pars ventralis of the telencephalon, as well as in the preoptic area and the dorsolateral thalamic nucleus.

Tail-flick related activity in medullospinal neurons

Using the classification system of Fields et al. 131 neurons in the rostral ventromedial medulla (RVM) of lightly anesthetized rats were divided into 3 groups according to their response during tail-flick (TF) testing: those with an abrupt increase in activity prior to TF (on-cells); those with a sudden pause in activity prior to TF (off-cells); those with no change in activity prior to TF (neutral cells). Collision testing was performed using a cervical spinal cord stimulating electrode to determine whether these neurons projected to the cord. Conduction velocities were determined for all cord-projecting neurons. All 3 cell types projected to the cord and approximately 38% of cord-projecting neurons were flick-related (off-or on-cells). All projecting neurons were within or immediately adjacent to the nucleus raphe magnus. The mean conduction velocity of on-cell axons (17.7 m/s) was significantly greater than that of off-cell axons (10.7 m/s) and neutral cell axons (12.4 m/s). Conduction velocities for all cells were within the range for myelinated axons. These findings support the hypothesis that off-and on-cells in the RVM play a significant role in pain modulation at the spinal cord level.

Midbrain stimulation inhibits tail-flick only at currents sufficient to excite rostral medullary neurons

The effects of midbrain electrical stimulation on the activity of tail-flick (TF) related neurons in the rostral ventromedial medulla (RVM) were studied. Neurons whose activity either decreased (off-cells) or increased (on-cells) immediately prior to TF were examined. Of 31 off- and on-cells, 26 (84%) showed increased activity during midbrain stimulation sufficient to suppress the TF. Furthermore, in 21 of these cells, the threshold for activation was identical to the threshold for TF suppression, and in the other 5 cells the threshold difference was less than or equal to 5 microA. This study provides evidence that off-and on-cells in the RVM mediate the antinociceptive actions of midbrain stimulation.