Possible explanation for analgesia mediated by direct spinal effect of morphine

Anger Inhibition and Pain Modulation

BACKGROUND

The tendency to inhibit anger (anger-in) is associated with increased pain. This relationship may be explained by the negative affectivity hypothesis (anger-in increases negative affect that increases pain). Alternatively, it may be explained by the cognitive resource hypothesis (inhibiting anger limits attentional resources for pain modulation).

METHODS

A well-validated picture-viewing paradigm was used in 98 healthy, pain-free individuals who were low or high on anger-in to study the effects of anger-in on emotional modulation of pain and attentional modulation of pain. Painful electrocutaneous stimulations were delivered during and in between pictures to evoke pain and the nociceptive flexion reflex (NFR; a physiological correlate of spinal nociception). Subjective and physiological measures of valence (ratings, facial/corrugator electromyogram) and arousal (ratings, skin conductance) were used to assess reactivity to pictures and emotional inhibition in the high anger-in group.

RESULTS

The high anger-in group reported less unpleasantness, showed less facial displays of negative affect in response to unpleasant pictures, and reported greater arousal to the pleasant pictures. Despite this, both groups experienced similar emotional modulation of pain/NFR. By contrast, the high anger-in group did not show attentional modulation of pain.

CONCLUSIONS

These findings support the cognitive resource hypothesis and suggest that overuse of emotional inhibition in high anger-in individuals could contribute to cognitive resource deficits that in turn contribute to pain risk. Moreover, anger-in likely influenced pain processing predominantly via supraspinal (e.g., cortico-cortical) mechanisms because only pain, but not NFR, was associated with anger-in.

Conditioned Pain Modulation in Sexual Assault Survivors

Sexual assault (SA) is associated with an increased risk of chronic pain, but the mechanisms for this relationship are poorly understood. To explore whether disrupted descending inhibition is involved, this study used a conditioned pain modulation task to study the inhibition of pain and the nociceptive flexion reflex (NFR; a correlate of spinal nociception) in 32 pain-free SA survivors. This group was compared with 32 pain-free, trauma-exposed persons without SA and a group of 40 pain-free persons who reported no trauma exposure. Conditioned pain modulation was assessed from painful electric stimulations (test stimulus) delivered to the ankle before, during, and after participants submerged their hand in painful 10°C water (conditioning stimulus). Pain ratings and NFR were assessed in response to test stimuli. All groups demonstrated significant inhibition of pain during conditioned pain modulation. However, only the no trauma exposure group demonstrated significant inhibition of NFR. The persons without SA group showed no inhibition of NFR, whereas the SA group showed significant facilitation of the NFR. These findings suggest that trauma exposure may impair inhibitory cerebrospinal circuits, but that SA may specifically promote facilitation of spinal nociception. Perspective: This study suggests that trauma exposure disrupts the cerebrospinal inhibition of spinal nociception, but that exposure to SA further promotes chronic pain risk by facilitating spinal nociception. This finding help may help to elucidate the pain risk mechanisms in trauma survivors.

The Influence of Placebo Analgesia Manipulations on Pain Report, the Nociceptive Flexion Reflex, and Autonomic Responses to Pain

Expectations for pain relief and experience/conditioning are psychological factors that contribute to placebo analgesia, yet few studies have studied the physiological mechanisms underlying their effects. This study randomized 133 participants to 4 groups: an expectation only (E-only) group, a conditioning only (C-only) group, an expectation plus conditioning (E+C) group, and a natural history (NH) control group. Painful electric stimulations were delivered before and after an inert cream was applied to the site of stimulation. Pain-related outcomes (pain ratings, nociceptive flexion reflex [NFR], skin conductance response, and heart rate acceleration) were recorded after each stimulation. NFR (a measure of spinal nociception) assessed if placebo analgesia inhibited spinal processing of pain. E+C was the only manipulation that significantly inhibited pain and skin conductance response. Surprisingly, NFR was facilitated in the E+C and E-only groups. No effects were noted for C-only. Mediation analysis suggested 2 descending processes were engaged during E+C that influenced spinal nociception: 1) descending facilitation and 2) descending inhibition that was also responsible for pain reduction. These results suggest that E+C manipulations produce the strongest analgesia and have a complex influence on spinal nociception involving both inhibitory and facilitatory processes. PERSPECTIVE: This study assessed whether placebo analgesia manipulations that include expectations, conditioning, or both modulate the NFR (measure of spinal nociception). Only the manipulation that involved expectations and conditioning inhibited pain, but both expectation manipulations facilitated NFR. This suggests a complex modulation of spinal neurons by placebo manipulations.

Perspective graphics as a means of portraying the distribution of radiolabelled ligands in the spinal cord--a pilot study using intrathecally administered 3H morphine

Tritium-labelled morphine sulphate was injected into the lumbar (L4-5) subarachnoid space of an adult male baboon. Three hours after injection, the animal was sacrificed. Using quantitative light microscopic autoradiographic mapping techniques, contour and perspective diagrams were prepared that described the position of radiolabel and by inference the distribution of morphine binding sites within the spinal cord. High concentrations of 3H was found in the medial regions of laminae I, II (substantia gelatinosa) and III of the dorsal horns. Smaller, but significant levels were seen bilaterally in the spinal anterolateral quadrant. Minimal 3H activity was seen in the remainder of the spinal cord with the lowest level being recorded in the spinal canal. Perspective graphics proved a precise and attractive method for locating the position and quantifying the concentration of radiolabel in baboon spinal cord.

Spinal opiate modulation of cardiovascular reflexes in the exercising dog

The possible role of spinal opiate receptors on hind limb muscle afferents, participating in arterial blood pressure and heart rate responses to ischemic exercise, were investigated following lumbosacral intrathecal injection of morphine and naloxone in chronically instrumented dogs. Morphine markedly attenuated these responses. Naloxone had no effect alone but blocked the morphine responses. Thus, spinal opiate receptors may modulate ascending afferent information mediating cardiovascular reflexes from ischemic muscle.

Intrathecal morphine for pelvic and sacral pain caused by cancer

Ten patients underwent placement of a lumbar subarachnoid catheter attached to a subcutaneous reservoir allowing daily injection of intrathecal morphine sulfate by family members. Sacral and pelvic pain were fairly well controlled. Neck and leg pain and pain mediated by the celiac plexus were not as well controlled by the intrathecal injection of morphine. The only significant complication was collection of cerebrospinal fluid around three reservoirs. Using this technique, narcotic concentration and frequency of administration can be easily altered to provide a safe but effective dose. Lumbar administration appears to block pain from lumbar and sacral dermatomes without the sedating and mood-altering effects of systemic narcotics.

Pain, nociception and spinal opioid receptors

Opioid peptides derived from proenkephalin and prodynorphin are differentially distributed in the spinal cord. Proenkephalin peptides are preferentially located in the sacral portion of the cord while prodynorphin peptides are concentrated in the cervical spinal cord. Mu opioid receptor are highly concentrated in superficial layers of the dorsal horn in all the spinal cord. Delta opioid receptor are more diffusely distributed in the gray matter of the spinal cord. These sites are principally located in cervical and thoracic portions of the spinal cord. Kappa opioid receptors are highly concentrated in the superficial layers of the lumbo-sacral spinal cord. Its density decreased in the upper levels of the spinal cord. It appears that mu opioid receptors are indifferentially activated by thermal, pressure and visceral nociceptive inputs. Delta receptors are more likely to be involved in thermal nociception while kappa opioid binding sites are associated to visceral pain nociceptive inputs.

Morphine and postoperative rewarming in critically ill patients

Morphine sulfate (MSO4) has been demonstrated to attenuate the stress response. MSO4 might be useful in minimizing the stress associated with the perioperative period, particularly that due to awakening from anesthesia and rewarming. Two groups of critically ill patients who developed hypothermia (35.8 degrees C) during a surgical procedure were studied. The control group was observed during routine medical management. Group II received 1 or 4 mg/kg MSO4 followed by an infusion of 0.2 or 0.5 mg/kg/hr. During the postoperative rewarming period the control group patients demonstrated a major increase in metabolic demand and myocardial work. In group II patients the infusion of MSO4 resulted in a lower metabolic rate. This was associated with a significantly longer rewarming time and a significant reduction in shivering, heat loss, heart rate, mean arterial pressure, and rate-pressure product. Infusion of MSO4 in critically ill patients during the perioperative period suppressed metabolic demands and myocardial work while preserving cardiovascular function.

Extradural versus intramuscular diamorphine. A controlled study of analgesic and adverse effects in the postoperative period

The effects of diamorphine hydrochloride 0.1 mg/kg, given either extradurally or intramuscularly for postoperative analgesia were compared in two randomised double-blind studies involving 39 patients undergoing thoracotomy and major gynaecological surgery. Assessments were made at fixed intervals after the administration of diamorphine and consisted of the measurement of pain or analgesic effect. Segmental, sympathetic and any adverse effects were sought. There was no significant difference in the quality of analgesia between the two groups in either trial. Extradural diamorphine provided safe and effective analgesia of rapid onset, with no specific undesirable side-effects. In both studies, analgesia was more prolonged following extradural administration. The relative proportion of spinal binding may be increased after extradural administration and this may be reflected in the prolonged analgesia observed.

Experimental cutaneous pain thresholds and tolerance in clinical analgesia with epidural morphine

Ten patients experiencing significant analgesia from repeated low-dose morphine injections via an indwelling epidural catheter were studied. One group (n = 5) with acute, postoperative pain was tested for changes in experimental cutaneous pain thresholds with and without clinical morphine analgesia. Three of these patients received intravenous naloxone. The analgesia was reversed in two. There was no significant alteration in any cutaneous modality, including pain, although the postoperative deep pain was relieved. This discrepancy might be explained by the existence of separate subpopulations of opiate receptors at the spinal cord level, which differentiate nociceptive input from the skin and from deep body structures. The other group (n = 5) of cancer pain patients with a permanent epidural catheter was monitored for long-term changes in clinical pain. Signs of tolerance were not seen with an observation period up to 6 weeks.