Systemic morphine reduces the wind-up of trigeminal nociceptive neurons

Orofacial Pain in Cancer: Part I—Mechanisms

The mechanisms involved, and possible treatment targets, in orofacial pain due to cancer are poorly understood. The aim of the first of this two-part series is to review the involved pathophysiological mechanisms and explore their possible roles in the orofacial region. However, there is a lack of relevant research in the trigeminal region, and we have therefore applied data accumulated from experiments on cancer pain mechanisms in rodent spinal models. In the second part, we review the clinical presentation of cancer-associated orofacial pain at various stages: initial diagnosis, during therapy (chemo-, radiotherapy, surgery), and in the post-therapy period. In the present article, we provide a brief outline of trigeminal functional neuro-anatomy and pain-modulatory pathways. Tissue destruction by invasive tumors (or metastases) induces inflammation and nerve damage, with attendant acute pain. In some cases, chronic pain, involving inflammatory and neuropathic mechanisms, may ensue. Distant, painful effects of tumors include paraneoplastic neuropathic syndromes and effects secondary to the release of factors by the tumor (growth factors, cytokines, and enzymes). Additionally, pain is frequent in cancer management protocols (surgery, chemotherapy, and radiotherapy). Understanding the mechanisms involved in cancer-related orofacial pain will enhance patient management.

Pupil diameter during postanesthetic recovery is not influenced by postoperative pain, but by the intraoperative opioid treatment

STUDY OBJECTIVE

To investigate whether pupil diameter (PD) measured during scotopic conditions is influenced by pain in conscious patients in the early postoperative period.

DESIGN

Prospective, observational, cohort study.

SETTING

Single-center, postanesthesia care unit (PACU).

PATIENTS

Patients scheduled for a surgery during general anesthesia.

INTERVENTIONS

Baseline PD was measured the day before surgery. Patients were observed on admission to the PACU, immediately after extubation, during the different steps of analgesic intervention (demand, relief, plus intermediate measures when relevant), and either at discharge or 3 hours after admission.

MEASUREMENTS

PD, pain (numerical rating scale), and alertness (Observer's Assessment of Alertness/Sedation scale).

MAIN RESULTS

Of 103 patients enrolled, 80 required analgesia in the PACU and completed follow-up. Pain intensity evolved in line with expectations (temporary increase then relief), and alertness increased with time. PD increased from low mean values at admission to the PACU (40% of baseline) to a plateau throughout the rest of the study period (80% of baseline) and was not related to pain intensity. Multivariate analyses suggested that the factors influencing PD (or its value related to baseline) were time since extubation and the type of opioid (remifentanil, sufentanil, or sufentanil at high doses) administered during surgery.

CONCLUSIONS

Because of a residual effect of intraoperative opioids and a level of nociceptive stimulation lower than in surgical conditions, PD is not significantly influenced by early postoperative pain or pain relief.

Mu-opioidergic modulation differs in deep and superficial wide-dynamic range dorsal horn neurons in mice

The spinal cord dorsal horn is an important action site for morphine analgesia. Wide-dynamic range (WDR) neurons in the dorsal horn are essential to spinal pain transmission and show increased excitability after repetitive noxious drive (windup). In light of differences in mu-opioid receptor distribution and neurophysiological properties of WDR neurons between deep and superficial dorsal horn, we recorded extracellular single-unit activity of WDR neurons from deep (350-700 μm) and superficial (<350 μm) dorsal horn in C57BL/6 mice and compared their responses to spinal superfusion of morphine (0.5mM, 30 μl) and naloxone (1mM, 30 μl). The windup level to repetitive electrical stimulation of 1.0 Hz (16 pulses, suprathreshold for C-fiber activation, 2.0 ms) was significantly decreased by morphine in deep (n=8), but not superficial (n=11), WDR neurons. However, the steady C-component response to graded intra-cutaneous electrical stimuli (0.01-5.0 mA, 2 ms) was significantly depressed by morphine only in superficial neurons. In separate experiments, spinal administration of naloxone facilitated the development of windup to 0.2 Hz stimulation in deep (n=10), but not superficial (n=8), WDR neurons. Accordingly, morphine and naloxone modulation of neuronal activity may be related to a specific effect on neuronal sensitization/plasticity in deep WDR neurons, whereas morphine inhibition may depress acute noxious inputs to superficial WDR neurons. Our study suggests that mu-opioidergic modulation may be different in deep and superficial WDR neurons.

Nociceptive craniofacial muscle primary afferent neurons synapse in both the rostral and caudal brain stem

Limited information is available on muscle afferent neurons with fine fibers despite their presumed participation in musculoskeletal disorders, including temporomandibular disorders. To study these neurons, intracellular recordings were made from the central axons of slowly conducting muscle afferent neurons in anesthetized rats. After intraaxonal impalement, axons were characterized by masseter nerve stimulation, receptive field testing, muscle stretching and intramuscular injection of hypertonic saline. Intracellular recordings were made from 310 axons (conduction velocity: 6.5-60(M)/s, mean = 27.3(M)/s; following frequency: 27-250 Hz, mean = 110Hz). No neurons responded to cutaneous palpation or muscle stretching. Some axons (n = 34) were intracellularly stained with biotinamide. These neurons were classified as group II/III noxious mechanoreceptors because their mechanical threshold exceeded 15 mN, and conduction velocities ranged from 12 to 40.2(M)/s (mean = 25.3(M)/s). Two morphological types were recognized by using an object-based, three-dimensional colocalization methodology to locate synapses. One type (IIIHTM(Vp-Vc)) possessed axon collaterals that emerged along the entire main axon and synapsed in the trigeminal principal sensory nucleus and spinal trigeminal subnuclei oralis (Vo), interpolaris (Vi), and caudalis (Vc). A second type (IIIHTM(Vo-Vc)) possessed axon collaterals that synapsed only in caudal Vo, Vi, and Vc. Our previous studies show that muscle spindle afferent neurons are activated by innocuous stimuli and synapse in the rostral and caudal brain stem; here we demonstrate that nociceptive muscle mechanoreceptor afferent axons also synapse in rostral and caudal brain stem regions. Traditional dogma asserts that the most rostral trigeminal sensory complex exclusively processes innocuous somatosensory information, whereas caudal portions receive nociceptive sensory input; the data reported here do not support this paradigm.

Synergistic antinociception between zaprinast and morphine in the spinal cord of rats on the formalin test

BACKGROUND AND OBJECTIVE

The cyclic guanosine monophosphate level, which causes an antinociception, is increased in cells as a direct result of phosphodiesterase inhibition. This study used a nociceptive test to examine the nature of the pharmacological interaction between intrathecal zaprinast, a phosphodiesterase inhibitor, and morphine.

METHODS

Catheters were inserted into the intrathecal space through an incision in the atlantooccipital membrane of male Sprague-Dawley rats. As a nociceptive model, 50 microL of a 5% formalin solution was injected into the hind paw. After observing the effect of zaprinast (37, 111, 369 nmol) and morphine (1, 4, 10, 40 nmol) alone, the interactions of their combination were examined by an isobolographic analysis.

RESULTS

Intrathecal zaprinast (P < 0.05) and morphine (P < 0.05) dose-dependently suppressed the flinching observed during phase 1 and phase 2 in the formalin test. The ED50 values (95% confidence intervals) of zaprinast and morphine in phase 1 were 161.9 (87.9-298.3) and 11.6 nmol (4.8-27.9 nmol), respectively. The phase 2 ED50 values (95% confidence intervals) of zaprinast and morphine were 229.9 (142.5-370.9) and 3.9 nmol (1.9-7.6 nmol), respectively. Isobolographic analysis revealed a synergistic interaction after intrathecal delivery a zaprinast-morphine mixture in both phases. The ED50 values of (95% confidence intervals) zaprinast in the combination of zaprinast with morphine in phase 1 and phase 2 were 14.2 (4.9-40.6) and 10.4 nmol (3-35.9 nmol), respectively.

CONCLUSIONS

Intrathecal zaprinast and morphine are effective against acute pain and facilitated pain state. Zaprinast interacts synergistically with morphine.

Dose-dependent effects of morphine on experimentally induced cutaneous pain in healthy volunteers

This study examines the dose dependent analgesic effects of two doses of morphine and a single dose of alfentanil on experimentally induced cutaneous pain. In 16 healthy volunteers pain was induced by a skin burn injury and by continuous electrical skin stimulation. Mechanical pain thresholds (PT, von Frey filament), area of secondary hyperalgesia (SH) and 'wind-up like pain' upon repetitive stimulation (40-g load, 3Hz, 30s) were assessed. Analgesic effects on these pain parameters were tested at steady-state IV infusions of morphine, 50% (plasma concentration 15ng/ml) and 100% (plasma concentration 30ng/ml) of maximal tolerable dose to be given to healthy volunteers, and with an effective dose of alfentanil (plasma concentration 70ng/ml). All effects were compared to active placebo, midazolam infusion (20microg/kg for 10min). Alfentanil significantly diminished the SH area in the burn injury model as well as in the electrical pain model. Additionally, alfentanil increased PT several fold in both models. The high dose of morphine showed a similar analgesic response pattern as alfentanil even though the effects were only statistically significant in the electrical pain model. The low dose of morphine as well as placebo did not affect these pain parameters. 'Wind-up like pain' was not influenced by any of the given drugs. In conclusion, the present study clearly indicates dose dependent effects of morphine on experimentally induced cutaneous pain. The high dose of morphine (30ng/ml) was approximately equianalgesic to the administered alfentanil dose (70ng/ml).

Blockade of supraspinal 5-HT1A receptors potentiates the inhibitory effect of venlafaxine on wind-up activity in mononeuropathic rats

In mononeuropathic rats submitted to a C-fiber reflex responses paradigm, repeated administration (five successive injections every half-life) of 10 mg/kg, s.c. of venlafaxine, but not of 2.5 mg/kg, s.c., a mixed monoamine reuptake inhibitor with preferential inhibitory activity in 5-HT reuptake, induced a progressive reduction of spinal wind-up. Repeated co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 i.c.v. (50 microg/injection) significantly increased the effect of venlafaxine s.c., indicating that venlafaxine-induced inhibition of spinal wind-up in mononeuropathic rats is potentiated by blockade of central 5-HT1A receptors.

Venlafaxine-induced depression of wind-up activity in mononeuropathic rats is potentiated by inhibition of brain 5-HT1A receptor expression in vivo

Antinociceptive effects of inhibiting 5-HT1A receptor expression by intracerebroventricular administration of an antisense oligodeoxynucleotide were studied in mononeuropathic rats. A 7-day period of treatment with the antisense produced: (i) reduction of mechanical hyperalgesia in the neuropathic hindlimb starting from day 5 of treatment, (ii) decrease of the hypothermic effect of 8-OH-DPAT challenge on day 6 of treatment, and (iii) potentiation of the inhibitory effect of velafaxine on spinal wind-up activity on day 7 of treatment. Results suggest a counteracting role of somatodendritic 5-HT1A receptors of raphe nuclei neurons in the antinociceptive efficacy of antidepressants with serotonergic spectrum in neuropathic pain.

Beta-phenylethylamines and the isoquinoline alkaloids

This review covers beta-phenylethylamines and isoquinoline alkaloids and compounds derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2001 to June 2002 is reviewed, with 581 references cited.

Tender points as predictors of distress and the pharmacologic management of fibromyalgia syndrome

The object of this study was to determine the association between tender point pain ratings, tender point counts and distress in people with fibromyalgia and to review the pharmacotherapy of fibromyalgia. Demographic, psychosocial, and health status information was collected from 316 health maintenance organization members with fibromyalgia. A manual tender point exam was conducted. Tender point counts predicted 3.0%, and tender point severity ratings predicted 8.3%, of the variance in distress. Little difference was found between the variance predicted for physical versus psychologic distress. A principal components analysis of all measures produced four distinct factors: global-physical functioning, tender points, psychologic, and physical. Tender point pain ratings and counts predicted a small but significant amount of variance in distress. In addition, FMS involves at least four rather distinct factors, one of which is related to tender points. Pharmacotherapeutic management is provided on a patient-specific basis including pharmacokinetics, pharmacodynamic, pathophysiologic, and psychosocial needs designed and modulated for each individual patient.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.