Analgesia produced by injection of lidocaine into the lateral hypothalamus

The Dopaminergic System in the Ventral Tegmental Area Contributes to Morphine Analgesia and Tolerance

Morphine has a strong analgesic effect and is suitable for various types of pain, so it is widely used. But long-term usage of morphine can lead to drug tolerance, which limits its clinical application. The complex mechanisms underlying the development of morphine analgesia into tolerance involve multiple nuclei in the brain. Recent studies reveal the signaling at the cellular and molecular levels as well as neural circuits contributing to morphine analgesia and tolerance in the ventral tegmental area (VTA), which is traditionally considered a critical center of opioid reward and addiction. Existing studies show that dopamine receptors and μ-opioid receptors participate in morphine tolerance through the altered activities of dopaminergic and/or non-dopaminergic neurons in the VTA. Several neural circuits related to the VTA are also involved in the regulation of morphine analgesia and the development of drug tolerance. Reviewing specific cellular and molecular targets and related neural circuits may provide novel precautionary strategies for morphine tolerance.

Antinociceptive tolerance to NSAIDs in the anterior cingulate cortex is mediated via endogenous opioid mechanism

BACKGROUND

In the past decade several studies have reported that in some brain areas, particularly, in the midbrain periaqueductal gray matter, rostral ventro-medial medulla, central nucleus of amygdala, nucleus raphe magnus, and dorsal hippocampus, microinjections of non-steroidal anti-inflammatory drugs (NSAIDs) induce antinociception with distinct development of tolerance. Given this evidence, in this study we investigated the development of tolerance to the analgesic effects of NSAIDs diclofenac, ketorolac and xefocam microinjected into the rostral part of anterior cingulate cortex (ACC) in rats.

METHODS

Male Wistar experimental and control (saline) rats were implanted with a guide cannula in the ACC and tested for antinociception following microinjection of NSAIDs into the ACC in the tail-flick (TF) and hot plate (HP) tests. Repeated measures of analysis of variance with post-hoc Tukey-Kramer multiple comparison tests were used for statistical evaluations.

RESULTS

Treatment with each NSAID significantly enhanced the TF and HP latencies on the first day, followed by a progressive decrease in the analgesic effect over a 4-day period, i.e., developed tolerance. Pretreatment with an opioid antagonist naloxone completely prevented the analgesic effects of the three NSAIDs in both behavioral assays.

CONCLUSIONS

These findings support the concept that the development of tolerance to the antinociceptive effects of NSAIDs is mediated via an endogenous opioid system possibly involving descending pain modulatory systems.

Is Hippocampus Susceptible to Antinociceptive Tolerance to NSAIDs Like the Periaqueductal Grey?

Emotional distress is the most undesirable feature of painful experience. Numerous studies have demonstrated the important role of the limbic system in the affective-motivational component of pain. The purpose of this paper was to examine whether microinjection of nonsteroidal anti-inflammatory drugs (NSAIDs), Clodifen, Ketorolac, and Xefocam, into the dorsal hippocampus (DH) leads to the development of antinociceptive tolerance in male rats. We found that microinjection of these NSAIDs into the DH induces antinociception as revealed by a latency increase in the tail-flick (TF) and hot plate (HP) tests compared to controls treated with saline into the DH. Subsequent tests on consecutive three days, however, showed that the antinociceptive effect of NSAIDs progressively decreased, suggesting tolerance developed to this effect of NSAIDs. Both pretreatment and posttreatment with the opioid antagonist naloxone into the DH significantly reduced the antinociceptive effect of NSAIDs in both pain models. Our data indicate that microinjection of NSAIDs into the DH induces antinociception which is mediated via the opioid system and exhibits tolerance.

Depressive-like history alters persistent pain behavior in rats: Opposite contribution of frontal cortex and amygdala implied

Numerous studies have shown that pain perception is strongly influenced by depression. However, very few studies have examined whether pain perception is altered in the remission period of depression, and what role the fronto-limbic circuits may play in the behavioral changes associated with remission. Using an unpredictable chronic mild stress (UCMS) animal model of depression, the present study investigated pain-related behaviors in rats with prior exposure to a UCMS stimulus. The γ-aminobutyric acid (GABA)A receptor agonist muscimol was microinjected bilaterally into the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) to examine the modulation of pain by these brain regions in the recovery state. Rats with a depression-like history displayed increased ongoing pain behavior in the formalin test, although their thermal pain thresholds were unchanged. Intra-BLA muscimol during the recovery phase dramatically decreased formalin-induced pain behavior and also significantly increased rats' sucrose preference. By contrast, in the mPFC, muscimol produced the opposite effect, suggesting different, perhaps opposing, roles of the BLA and mPFC in mediating the influence of prior UCMS exposure on pain perception. Taken together, these results demonstrated that a depressive experience may cause long-term alterations in limbic circuit excitability and thus lead to long-lasting changes in pain perception.

Adult-age inflammatory pain experience enhances long-term pain vigilance in rats

BACKGROUND

Previous animal studies have illustrated a modulatory effect of neonatal pain experience on subsequent pain-related behaviors. However, the relationship between chronic pain status in adulthood and future pain perception remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

In the current study, we investigated the effects of inflammatory pain experience on subsequent formalin-evoked pain behaviors and fear conditioning induced by noxious stimulation in adult rats. Our results demonstrated an increase of the second but not the first phase of formalin-induced pain behaviors in animals with a history of inflammatory pain that have recovered. Similarly, rats with persistent pain experience displayed facilitated acquisition and prolonged retention of pain-related conditioning. These effects of prior pain experience on subsequent behavior were prevented by repeated morphine administration at an early stage of inflammatory pain.

CONCLUSIONS/SIGNIFICANCE

These results suggest that chronic pain diseases, if not properly and promptly treated, may have a long-lasting impact on processing and perception of environmental threats. This may increase the susceptibility of patients to subsequent pain-related disorders, even when chronic pain develops in adulthood. These data highlight the importance of treatment of chronic pain at an early stage.

Effect of lidocaine administration at the nucleus locus coeruleus level on lateral hypothalamus-induced antinociception in the rat

Several lines of evidence have shown that stimulation or inactivation of lateral hypothalamus (LH) produces antinociception. In this study, we assessed the role of nucleus locus coeruleus (LC) in antinociceptive response induced by LH stimulation or inactivation in the rat. The cholinergic agonist carbachol (125 nmol/0.5 microl saline) or lidocaine (2%; 0.5 microl) was unilaterally microinjected into the LH with the LC inactivation concurrently. Antinociceptive responses were obtained by tail-flick test and represented as maximal possible effect (MPE) at 5, 10, 15, 20, 30 and 60 min after drug administration. The results showed that microinjection of carbachol into the LH significantly induced antinociception at 5 and 10 min (p<0.001). This effect was significantly blocked by microinjection of lidocaine into the LC. On the other hand, microinjection of lidocaine into LH-induced antinociception at 5 (p<0.01) and 10 (p<0.05) min after administration. However, inactivation of the LC following the LH inactivation increased MPE at 5 min after injection. These findings support the conclusion that antinociception produced by LH stimulation or inactivation involves two separate mechanisms. It seems that analgesic response induced by LH stimulation is mediated in part by the subsequent activation of spinally projecting noradrenergic neurons in the LC cell group.

[Effect of music therapy among hospitalized patients with chronic low back pain: a controlled, randomized trial]

OBJECTIVE

To evaluate the influence of music therapy in hospitalized patients with chronic low back pain.

METHODS

A controlled, randomized study (N = 65). During a stationary rehabilitation stay of 12 days, 65 patients with low back pain were randomized to receive on alternate months standardized physical therapy plus 4 music therapy sessions between day 1 and day 5 (intervention group; N = 33) or standardized physical therapy alone (control group; N =32). Scores for pain (as measured on a visual analogue scale [VAS]), disability (Oswestry index) and anxiety and depression (as measured on the hospital anxiety and depression scale [HAD]) were collected on day 1, 5 and 12. Pain intensity was also evaluated on a VAS just before and after music therapy sessions.

RESULTS

Introduced music therapy sessions during a stationary rehabilitation stay in patients with chronic low back pain reduce pain (-2.0+/-2.7 vs -1.8+/-2.6) but not significantly. However, music therapy significantly (p < 0.01) reduced disability as measured on the Owestry index between day 1 and day 5 (-11.8+/-17.8 vs -2.5+/-9.4), anxiety (-3.5+/-3.7 vs -0.9+/-2.7) and depression (-2.1+/-3.0 vs 0.6+/-2.4). The immediate effect on pain intensity (VAS score) was confirmed (p < 0.001).

CONCLUSION

Our results confirmed the effectiveness of music therapy for hospitalized patients with chronic low back pain. Music therapy can be a useful complementary treatment in chronic pain and associated anxiety-depression and behavioural consequences.

Relaxation and music reduce pain following intestinal surgery

Three nonpharmacological nursing interventions, relaxation, chosen music, and their combination, were tested for pain relief following intestinal (INT) surgery in a randomized clinical trial. The 167 patients were randomly assigned to one of three intervention groups or control and were tested during ambulation and rest on postoperative days 1 and 2. Pain sensation and distress were measured with visual analog scales (VAS). Multivariate analysis of covariance showed significantly less post-test pain in the intervention groups than in the control group on both days after rest and at three of six ambulation post-tests (p = .024-.001), resulting in 16-40% less pain. Mixed effects after ambulation were due to the large variation in pain and difficulty relaxing while returning to bed; but post hoc explorations showed effects for those with high and low pain. These interventions are recommended along with analgesics for greater postoperative relief without additional side effects.

Effect of VMH lesion on sucrose-Fed nociceptive responses

An initial analgesia followed by hyperalgesia to phasic noxious stimuli occurs after ingestion of sucrose ad libitum. However, the mechanism underlying hyperalgesia is not known. The present study was designed to explore the role of VMH in the mediation of the hyperalgesic effect of sucrose ingestion. Adult male albino rats received sucrose solution (20% p.o.) in addition to laboratory food pellets and tap water ad libitum. Their behavioural responses to various phasic and tonic noxious stimuli were recorded after 6, 12 and 48 h during pre and post-sucrose fed states in both the control and VMH lesion groups of rats. Sucrose feeding to control rats significantly reduced the tail flick latency (TFL) and threshold of vocalization during stimulus (SV) and after discharge (VA) indicating hyperalgesia, while the threshold of tail flick remained unaffected. The average pain rating during the formalin test (tonic pain) decreased significantly indicating analgesia. VMH lesion decreased the latency (mean +/- SD) for tail flick (11.26 +/- 4.65 from 15.61 +/- 5.12 s), threshold (median) for tail flick (0.04 from 0.08 mA), vocalization during stimulus (0.05 from 0.1 mA) and vocalization after discharge (0.15 from 0.2 mA), while the tonic pain rating increased, thereby suggesting a hyperalgesic state. However, sucrose feeding to lesioned rats neither potentiated nor attenuated their hyperalgesia. The results suggest that sucrose feeding for 6-48 h ad libitum produces hyperalgesia to phasic noxious and analgesia to tonic noxious stimuli, while VMH lesion produces hyperalgesia to both phasic and tonic noxious stimuli. Secondly, sucrose ingestion by VMH lesion rats does not affect their responses to pain, suggesting the possible role of VMH in the mediation of sucrose-fed nociceptive responses.

Effects of electrical stimulation of thalamic nucleus submedius and periaqueductal gray on the visceral nociceptive responses of spinal dorsal horn neurons in the rat

Electrical stimulation of the nucleus submedius (Sm) has been shown to suppress the viscerosomatic reflex (VSR), which is evoked by colorectal distension (CRD). We have examined the effects of focal electrical stimulation (0.3 ms, 50 Hz, 100 microA, 10 s) of the Sm and the periaqueductal gray (PAG) on the excitatory responses evoked by CRD in spinal dorsal horn neurons within the L6-S1 region in the urethane-anesthetized Wistar rats. Extracellular recordings were made from 32 spinal excitatory CRD responses. All of these neurons were convergent neurons with cutaneous receptive fields. The majority of the neurons (27/32) were wide dynamic range (WDR) neurons (responding to noxious and non-noxious cutaneous stimuli) while the remaining five neurons were nociceptive specific (NS) neurons (responding only to noxious cutaneous stimuli). The effects of electrical stimulation applied to 28 sites within the Sm were assessed for spinal neurons. Electrical stimulation in seven sites within the Sm (25%) inhibited the CRD excitatory response of dorsal horn neurons, while in two sites (7%) the same stimulation yielded facilitation. Electrical stimulation in the majority of the sites in the Sm (19/28, 68%) did not affect spinal excitatory CRD responses. On the other hand, electrical stimulation of the PAG clearly inhibited 20 of 22 (90%) CRD excitatory responses. These results suggest that the majority of Sm neurons may suppress VSR activity at a supraspinal reflex center rather than via a descending inhibition of spinal visceral nociceptive transmission, as is the case for the PAG.

A role of periaqueductal grey NMDA receptors in mediating formalin-induced pain in the rat

In the present study we examined the role of periaqueductal grey (PAG) N-methyl-D-aspartate (NMDA) receptors in the perception of tonic and phasic pain. Under sodium pentobarbital anesthesia rats were implanted unilaterally with a guide cannula aimed at the PAG. Following a 7-14 day recovery period rats received an infusion of the NMDA antagonist, 2-amino-5-phosponopentanoic acid (AP5), or saline into the PAG. Five minutes after the infusion of AP5 rats were tested for analgesia in the formalin test, or in the hotplate test. AP5 injections into the PAG reduced pain in the formalin test, but not the hotplate test. These data show that NMDA receptors within the PAG are involved in the perception of tonic, inescapable pain as measured in the formalin test, but not phasic, escapable pain as measured in the hotplate test.

Comparison of the effects of nucleus tractus solitarius and ventral medial medulla lesions on illness-induced and subcutaneous formalin-induced hyperalgesias

We have previously demonstrated that illness-inducing agents (lipopolysaccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formalin) induce hyperalgesia by similar pathways. The present series of experiments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocellularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). Illness-induced hyperalgesia is abolished by unilateral (left) NTS lesions, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias induced by illness agents and by inflammatory agents are mediated by similar but not identical pathways. They also illustrate that neural structures have the capacity for opposed actions, in that both the NTS and NRM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structures involved in pain modulation.

Morphine analgesia in the formalin test: evidence for forebrain and midbrain sites of action

A mapping study was performed to determine where in the rat brain morphine acts to produce analgesia in the formalin test, which is an animal model of prolonged pain associated with tissue injury. A single dose (5 nmol) of morphine was bilaterally microinjected into a wide range of brain areas throughout the midbrain and forebrain. Strong analgesia was elicited from the posterior hypothalamic area, the periaqueductal gray and ventral tegmental area. Other sites from which analgesia was elicited were the nucleus accumbens and a few sites in the retrorubral field and caudate-putamen. Analgesia from the periaqueductal gray or nucleus accumbens was accompanied by decreased locomotor activity and catalepsy, whereas analgesia from the posterior hypothalamic area or ventral tegmentum was accompanied by a noticeable increase in locomotor activity and rearing. Morphine into various thalamic nuclei had no effect. These results indicate that the primary sites of action of morphine in the formalin test are probably the posterior hypothalamic area and periaqueductal gray, with an additional contribution from regions innervated by tegmental dopamine cells.

On the general theory of neural circuitry

A general theory of neural circuitry is proposed wherein neural impulses travel in a continuous circuit from the brain to the extremities and back to the brain. At the extremities the impulse may be modified by the environment there. At the spinal column the return signal is compared with the outgoing signal and the appropriate motoneuronal 'reflex' signal is generated if the difference is sufficiently large. In the thalamus the return signal is again compared with the outgoing signal and the difference between the two generates a sensory impulse which is sent to the cortical regions of the brain for comparison with stored patterns from similar signals of past experience. This theory allows for an explanation of feelings of pain and pleasure, pain remote from an area of trauma, phantom limb pain and the relationship between sensory impulses and motor impulses. New approaches to reducing pain are suggested by this theory.

The formalin test: an evaluation of the method

The formalin test for nociception, which is predominantly used with rats and mice, involves moderate, continuous pain generated by injured tissue. In this way it differs from most traditional tests of nociception which rely upon brief stimuli of threshold intensity. In this article we describe the main features of the formalin test, including the characteristics of the stimulus and how changes in nociceptive behaviour may be measured and interpreted. The response to formalin shows an early and a late phase. The early phase seems to be caused predominantly by C-fibre activation due to the peripheral stimulus, while the late phase appears to be dependent on the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord. These functional changes seem to be initiated by the C-fibre barrage during the early phase. In mice, the behavioural response in the late phase depends on the ambient temperature. We argue that the peripheral tissue temperature as well as other factors influencing the peripheral inflammation may affect the response, possibly confounding the results obtained with the test. Furthermore, we discuss the methods of recording the response and the value of observing more than one aspect of behaviour. Scoring of several behavioural variables provides a means of assessing motor or sensorimotor function as possible causes for changes in behaviour. In conclusion, the formalin test is a valuable addition to the battery of methods available to study nociception.

Systemic injections of alpha-1 adrenergic agonists produce antinociception in the formalin test

The role of alpha 1 receptors in antinociception was investigated in the formalin test, a well established test of tonic pain. The effect of systemic injections of selective alpha 1-adrenergic agonists (phenylephrine and methoxamine), a mixed alpha agonist selective for alpha 2 receptors (ST-91), and 2 adrenergic antagonists (prazosin and idazoxan) was measured in groups of Long-Evans rats. All agonists tested produced significant antinociception in this test. Dose-response curves for each agonist were statistically parallel and equally efficacious (100% antinociception). Prior injection of 0.15 mg/kg prazosin (an alpha 1 antagonist) completely antagonized the antinociception produced by either an ED50 or a maximally effective dose of each agonist tested. Idazoxan (0.5 mg/kg), an alpha 2 antagonist, was without effect on the antinociception produced by phenylephrine or methoxamine. ST-91 produced significant antinociception in the presence of idazoxan although the response was different from that obtained with ST-91 alone. The observed antinociception in the formalin test was not due to drug-induced changes in peripheral inflammation as measured using plethysmometry. Moreover, none of the drugs tested produced significant changes in coordinated motor behavior (accelerated rotarod test) at doses that produced significant analgesia (ED50). We conclude that alpha 1 receptors contribute significantly to adrenergic analgesia in the formalin test by an undefined action on sensory processing mechanisms.

Temporal processes of formalin pain: differential role of the cingulum bundle, fornix pathway and medial bulboreticular formation

Subcutaneous injection of formalin produces a characteristic biphasic pain response. An early phase develops in the first 5 min after injection; the pain then decreases for 10-15 min, followed by a gradual rise to a stable plateau that lasts about 1 h. Rats were injected with 1 microliter of 2% lidocaine or saline into the anterior cingulum bundle at 0 (immediately), 10 or 30 min prior to formalin injection, or 10, 20 or 30 min after formalin injection, and tested for analgesia in the late phase of the formalin test, 30-70 min after formalin injection. A time-dependent increase in analgesia was obtained when lidocaine was injected into the cingulum at periods ranging from 10 to 30 min after formalin injection, reflecting an anaesthetic duration of less than 20 min. When lidocaine was injected 0 or 10 min prior to formalin injection, a time-dependent increase in analgesia in the late phase was again observed. In these groups, lidocaine should have blocked cingulum activity during the early but not the late phase. The role of the fornix pathway and the medial bulboreticular formation in mediating formalin pain was also examined. Lidocaine produced analgesia in the late phase when injected into the fornix prior to formalin injection but had no effect when administered after it. In contrast, when lidocaine was injected into the medial bulboreticular formation it produced analgesia in the late phase when administered after formalin injection, but not prior to it. Taken together, these results suggest that the late pain response to formalin is in part dependent upon plasticity in the central nervous system which occurs during the transient early phase.

Analgesia produced by lidocaine microinjection into the dentate gyrus

The local anesthetic lidocaine was injected into the dentate gyrus (DG) of alert, unrestrained rats 10 min prior to investigation within the formalin test. Regional anesthesia of the DG resulted in a reduction of pain scores when administered contralateral to the site of subcutaneous formalin injection. The analgesic effect was evident 30-50 min after central infusion. These results provide evidence of the involvement of the hippocampal formation (HF) in pain perception.

A psychophysical comparison of sensory and affective responses to four modalities of experimental pain

It is generally accepted that the sensory and affective components of pain may be differentially associated with various acute and chronic diseases, and that some treatment regimens are best directed toward certain aspects of the pain experience. In addition, experimental animal models have been described that presume to assess either the sensory-discriminative aspects of phasic pain or the affective responses associated with tonic pain. The present psychophysical experiment directly compares the perceived intensity and unpleasantness of sensations evoked by four types of experimental noxious stimuli: contact heat, electric shock, ischemic exercise, and cold-pressor pain. A novel pain measurement technique is described that incorporates unbounded magnitude-estimation/category scales; this technique allows precise ratio responses, while minimizing within- and between-subject variability. We observe that, relative to the perceived intensity of the individual stimuli, subjects consistently differentiate among the degrees of unpleasantness evoked by the four stimulus modalities. Ischemic exercise and cold-pressor pain evoke higher estimates of unpleasantness, and thus may better mimic the pain of chronic disease. The relative unpleasantness produced by contact heat is significantly less than that of the other modalities tested, and therefore contact heat stimuli may be ideally suited for assessing sensory-discriminative aspects of pain perception. Possible neurophysiological mechanisms underlying the observed differences in perceived unpleasantness are discussed in relation to the growing body of literature concerning tonic and phasic pain stimuli.

Phantom limbs and the concept of a neuromatrix

The phenomenon of a phantom limb is a common experience after a limb has been amputated or its sensory roots have been destroyed. A complete break of the spinal cord also often leads to a phantom body below the level of the break. Furthermore, a phantom of the breast, the penis, or of other innervated body parts is reported after surgical removal of the structure. A substantial number of children who are born without a limb feel a phantom of the missing part, suggesting that the neural network, or 'neuromatrix', that subserves body sensation has a genetically determined substrate that is modified by sensory experience.

Analgesia produced by injection of lidocaine into the anterior cingulum bundle of the rat

The role of cingulum in the perception of tonic and phasic pain was examined by injecting lidocaine, a local anesthetic, into the anterior cingulum bundle of the rat. A cannula was stereotaxically implanted into the anterior cingulum on one side in anesthetized rats. Seven to 10 days after surgery, the rats were infused with 1 microliter of 2% lidocaine in saline or saline alone into the anterior cingulum bundle immediately prior to testing for analgesia in the formalin or foot-flick test. Injection of lidocaine into the anterior cingulum bundle produced a significant reduction in formalin pain scores, but had no effect on foot-flick latencies. The analgesia in the formalin test persisted for the entire 40 min observation period. No analgesia in the formalin test was observed in rats that received injections of saline into the cingulum, lidocaine into the cingulum 90 min prior to testing or lidocaine into the cingulate cortex. These data suggest that the cingulum is involved in the perception of tonic pain but not phasic pain, and support previous studies which indicate that different neural mechanisms underlie different types of pain. Furthermore, these findings suggest that the cingulum is involved in the affective component of pain and that interruption of cingulum activity with lidocaine can produce analgesia.