Antinociception in the rat induced by a cold environment

Heat abatement during the pre-weaning period: effects on lying behavior and disbudding-related responses of male Holstein calves

Strategies to abate heat stress are seldom adopted for pre-weaned dairy calves and little is known about their effects on behavior and pain sensitivity of youngstock. Our objectives were to determine the effects of heat stress abatement on lying behavior and disbudding-related pain sensitivity, wound healing, and change in intake. Male Holstein calves (n = 60; 0 to 68 d of age) were assigned randomly at birth (d 0) to 1 of 3 treatments: hutch outdoors with 50% of its area covered with plywood (control = 20), hutch in a barn with no cooling (SH = 21), and hutch in a barn with ceiling fans (SHF = 19). Calves were fitted with lying-behavior loggers on the hind leg from d 1 to 68. On d 32 ± 8 (±SD), we disbudded calves using hot iron, 30 min after cornual nerves were blocked with lidocaine. Immediately before (0 h), and at 1, 2, and 3 h after disbudding, we evaluated calves for mechanical nociceptive threshold (MNT) and head (ear flick, head shake, head rubbing) and somatic (tail flicking, foot stamping, restlessness) behaviors. On d 1, 3, 7, and 14 after disbudding, we evaluated the MNT and, on d 7 and 14, we evaluated wound healing (1 = crust, 5 = exudate). We calculated the relative change in milk solids and starter intake from d 0 to 6 relative to disbudding compared with the average of the 72 h preceding the procedure. The lying time was 0.6 h/d greater for the SHF treatment compared with the SH treatment. The control treatment resulted in 3.2 and 4.1 more lying bouts per day than the SH and SHF treatments, respectively; consequently, the control treatment resulted in lying-bout duration 7.7 and 10.9 min/event shorter than the SH and SHF treatments, respectively. We did not detect an effect of treatment on the number of disbudding-related head and somatic behaviors and MNT. The odds of calves having abnormal wound was 3.5 and 3.2 times greater for the control treatment compared with the SH and SHF treatments, respectively. We did not detect an effect of treatment on the relative change in intake of milk solids and starter. Heat abatement improves the welfare of pre-weaned dairy calves and may hasten healing.

Heat hypersensitivity at a site proximal to a surgically induced osteochondral fracture in horses

OBJECTIVE

To evaluate thermal nociceptive thresholds (TNTs) before and after inducing a standardized radiocarpal bone osteochondral fracture (OCF) in horses.

STUDY DESIGN

Prospective, controlled, randomized, masked study.

ANIMALS

A group of 10 Thoroughbred fillies aged 2 years.

METHODS

Skin temperature and TNTs were measured on the skin over the triceps brachii muscle in both the thoracic limbs before (week 0) and weekly (weeks 1-8) after unilateral arthroscopic induction of a radiocarpal OCF (n = 4) or sham surgery (n = 6) followed by a standardized exercise programme. The contralateral, non-operated thoracic limb was used as a control within each horse. Percentage thermal excursion (%TE) defined as %TE = 100 ∗ (TNT - skin temperature)/(cut-off temperature - skin temperature) was calculated. Data were analysed with a mixed-effects model followed by Dunnett's and Tukey's tests for within and between-limbs comparisons, respectively; p < 0.05 was considered significant.

RESULTS

Skin temperature in the control limb of OCF horses was significantly higher at week 7 than at week 0 (p = 0.0125). At week 1, TNTs and %TE values in operated limbs of OCF horses were significantly reduced compared with their baseline values at week 0 (p ≤ 0.0153) and their values in contralateral control limbs (p ≤ 0.0024) and operated limbs of sham-operated horses (p ≤ 0.0162). At week 2, TNTs and %TE values in operated limbs of OCF horses remained significantly reduced compared with values in operated limbs of sham-operated horses (p ≤ 0.0248).

CONCLUSIONS AND CLINICAL RELEVANCE

Creation of an OCF in a radiocarpal bone induced transitory (<2 weeks) ipsilateral heat hypersensitivity proximal to the surgery site (skin over the triceps brachii muscle) in horses. Surgically induced OCF may cause somatosensory abnormalities consistent with secondary thermal hyperalgesia.

Role of afferent pathways of heat and cold in body temperature regulation

The detection of surface and internal temperatures is achieved by axons terminating at lamina I of the spinal dorsal horn, otherwise approached only by nociceptive afferents. Recent advances in thermal physiology research have disclosed that temperature-sensitive ion channels belonging to the "transient receptor potential" family exist in the peripheral sensory neurons and in the brain. Thermosensory, nociceptive and polymodal afferents project to different thalamic nuclei, and specific pathways to the insular cortex evoke the conscious experience of thermal sensation. The posterior insular region represents discriminative thermal sensation, while the largest correlation with subjective ratings of temperature is located in the orbitofrontal and anterior insular cortex. The insular cortex forms an integrative part of the limbic system and is closely tied with the hypothalamus, the amygdala, the anterior cingulate cortex and the orbitofrontal cortex and emerges as the main coordinator of behavioral, autonomic and endocrine responses to both non-noxious and noxious thermal stimuli. The firing rate of warm and cold receptors is not altered by pyrogens. A strong correlation between the onset of fever and production of superoxide by macrophages following the injection of pyrogens implicates reactive oxygen species as elicitors of fever, a hypothesis strengthened by the observation that oxygen radical scavengers or thiol reductants act as antipyretics. Oxidative stress appears to be sensed by the brain and a likely structure for its detection may be the redox-sensitive site of the N-methyl-D-aspartate (NMDA) receptor for glutamate, in that oxidation of this site causes fever while its reduction lowers body temperature, effects which are abrogated by specific NMDA receptor blockers.

The lower pole of the earlobe is an alternative site for painless blood sampling in the self-assessment of blood glucose concentrations

OBJECTIVES

Measurement of blood glucose is essential for better control of diabetes mellitus. The pain associated with repeated blood sampling is a significant problem, and a less painful technique would be advantageous. This study was conducted to establish a less painful method of blood sampling for monitoring of blood glucose concentrations.

METHODS

Fourteen healthy doctors and nurses at the age of 22 to 32 years were enrolled into this study. The earlobe was divided into 20 areas. Each area was punctured with a fine needle and the pain was assessed by the visual analog scale (VAS). Then, localized cooling or warming was applied before puncture and the pain was assessed by the VAS.

RESULTS

The VAS tended to be lower toward the lower part of the earlobe. The VAS in the uppermost area was 53.1 +/- 19.1 mm and 28.9 +/- 16.8 mm in the lowest part of the earlobe. The VAS was higher in the uppermost area than in the lowest area in six of the seven volunteers (p=0.028). We did not find significant differences in the VAS and blood glucose levels between the control sampling and the warming, or cooling sampling. The localized warming shortens the time to obtain blood by 4.4 seconds (p=0.0426).

CONCLUSION

The area of the ear, but not localized cooling or warming, significantly affects the pain in sampling blood from the earlobe. Blood sampling for glucose tests are recommended to be obtained at the lower pole of the earlobe.

Changes in analgesia-producing mechanism of repeated cold stress loading in mice

Functional changes in opioid receptors involved in analgesia of repeated cold stress (RCS)-loaded mice were investigated. The antinociceptive potency of morphine (4 mg/kg, PO) was not affected in normal mice by norbinaltorphimine (10 mg/kg, SC), but treatment with this agent resulted in a lower level of morphine-induced antinociception in RCS-loaded animals. The antinociceptive activity of U-50488H (3 mg/kg, SC) was increased in RCS-loaded mice. In contrast to hypersensitivity to U-50488H (1 and 10 microg, IT) noted in RCS-loaded mice, the antinociception induced by DAMGO (0.1 and 1 microg, ICV) was reduced compared to that of normal animals. Diazepam (1 mg/kg/day SC) was given during RCS loading, and this agent prevented the development of hyperalgesia and the decrease in the antinociceptive activity of DAMGO (1 microg, ICV) in RCS-loaded mice, but there was no effect on the enhancement of the antinociceptive potency of U-50488H (10 microg, IT). These results indicate that the RCS-loaded mice were hyposensitive to supraspinal mu-opioid receptor-mediated antinociception, whereas their antinociceptive activities through kappa-opioid receptor in the spinal cord were increased. Hypofunction of the supraspinal mu-opioid receptor due to anxiety may explain the mechanism involved in the lowering of the nociceptive threshold in RCS-loaded animals.

Stimulation of human thalamus for pain relief: possible modulatory circuits revealed by positron emission tomography

Stimulation of human thalamus for pain relief: possible modulatory circuits revealed by positron emission tomography. J. Neurophysiol. 80: 3326-3330, 1998. Stimulation of the somatosensory thalamus was used for more than 2 decades to treat chronic pain in the human. However, despite clinical reports of successful results, little is known about the actual mechanisms mediating this form of stimulation-produced analgesia. To reveal possible neuronal pathways evoked by thalamic stimulation, we measured regional changes in cerebral blood flow (rCBF) in five patients who received successful long-term relief of chronic pain with somatosensory thalamic stimulation. Positron emission tomography during thalamic stimulation revealed significant activation of the thalamus in the region of the stimulating electrodes as well as activation of the insular cortex ipsilateral to the thalamic electrodes (contralateral to the patients' clinical pain). For these patients, thalamic stimulation also evoked paresthesiae that included thermal sensations in addition to tingling sensations. Results of this study indicate that in some cases somatosensory thalamic stimulation may activate a thalamocortical pain modulation circuit that involves thermal pathways. These results are consistent with other recent reports suggesting that activation of thermal pathways may contribute to modulation of nociceptive information.

Effects of repeated cold stress on feeding, avoidance behavior, and pain-related nerve fiber activity

The specific alternation of rhythm in temperature (SART), which is defined as rapid and frequent changes in the environmental temperature several times within the course of a day, produces abnormalities in behavior such as hyperphagia and in sensory sensation such as hyperalgesia. As the first step toward understanding the mechanisms of these abnormalities, we studied the effects or SART stress on ingestive behavior. During the light and dark phases, the animals' food intake increased, but their body weight gain decreased. In addition, diurnal variation in body weight also decreased. Next, we examined the behavioral and electrophysiological effects of SART stress on avoidance behavior by studying the rat's avoidance of a noxious stimulus in the form of a footshock. The rats demonstrated hyperreactivity; the delay in escaping the footshock was decreased by SART stress. The excitability of C-fiber activity, which responds to mechanical and thermal stimuli to a single saphenous nerve, was not changed by SART stress. This suggests that the hyperreactivity in footshock avoidance and the hyperalgesia in pain response induced by SART stress are based on excessive emotionality.

Functional imaging of an illusion of pain

Touching warm and cool bars that are spatially interlaced produces a painful burning sensation resembling that caused by intense, noxious cold. We demonstrated previously that this thermal grill illusion can be explained as an unmasking phenomenon that reveals the central inhibition of pain by thermosensory integration. In order to localize this unmasking in the human brain, we have used positron emission tomography (PET) to compare the cortical activation patterns evoked by the thermal grill and by cool, warm, noxious cold and noxious heat stimuli. The thermal grill illusion produces activation in the anterior cingulate cortex, whereas its component warm and cool stimuli do not. This area is also activated by noxious heat or cold. Thus, increased activity in the anterior cingulate cortex appears to be selectively associated with the perception of thermal pain. Disruption of thermosensory and pain integration may account for the central pain syndrome that can occur after stroke damage.

Fine structural organization of spinothalamic and trigeminothalamic lamina I terminations in the nucleus submedius of the cat

We examined lamina I trigemino- and spinothalamic tract (TSTT) terminals labeled with Phaseolus vulgaris leucoagglutinin in the nucleus submedius (Sm), a nociceptive relay in the cat's thalamus. Volume-rendered (three-dimensional) reconstructions of ten lamina I TSTT terminals identified with light and electron microscopy were built from serial ultrathin sections by computer, which enabled the overall structures of the terminal complexes to be characterized in detail. Two fundamentally different terminations were observed: compact clusters of numerous boutons, which predominate in the dense focus of a lamina I terminal field in the Sm, and boutons-of-passage, which are present throughout the terminal field and predominate in its periphery. Reconstructions of cluster terminations reveal that all boutons of each cluster make synaptic contact with protrusions and branch points on a single dendrite and involve presynaptic dendrites (PSDs) in triadic arrangements, providing a basis for the secure relay of sensory information. In contrast, reconstructions show that boutons-of-passage are generally characterized by simple contacts with PSDs, indicating an ascending inhibitory lamina I influence. These different synaptic arrangements are consistent with physiological evidence indicating that the morphologically distinct nociceptive-specific and thermoreceptive-(cold)-specific lamina I TSTT neurons terminate differently within the Sm. Thus, a suitable structural substrate exists in the cat's Sm for the inhibitory effect of cold on nociception, a behavioral and physiological phenomenon of fundamental significance. We conclude that the Sm is more than a simple relay for nociception, and that it may be an integrative comparator of ascending modality-selective information that arrives from neurons in lamina I.

Endogenous analgesia in the chicken

Endogenous analgesia has been identified in mammals, but little is known about suppression of tonic pain following trauma or disease. Birds suffer from gouty arthritis which can be induced experimentally by intra-articular injection of sodium urate (SU) crystals. SU injection into the ankle joint of the chicken tested in cages resulted in pain-coping behaviour (one-legged standing, sitting) together with severe lameness. Birds kept and tested in large pens showed significantly less pain-coping behaviour, while birds tested in novel pens showed either complete analgesia or marked hypoalgesia, together with a significant reduction in lameness. Complete analgesia was observed during pre-laying behaviour. These results demonstrate a remarkable ability of birds to suppress such severe tonic pain as SU arthritis.

The thermal grill illusion: unmasking the burn of cold pain

In Thunberg's thermal grill illusion, first demonstrated in 1896, a sensation of strong, often painful heat is elicited by touching interlaced warm and cool bars to the skin. Neurophysiological recordings from two classes of ascending spinothalamic tract neurons that are sensitive to innocuous or noxious cold showed differential responses to the grill. On the basis of these results, a simple model of central disinhibition, or unmasking, predicted a quantitative correspondence between grill-evoked pain and cold-evoked pain, which was verified psychophysically. This integration of pain and temperature can explain the thermal grill illusion and the burning sensation of cold pain and may also provide a basis for the cold-evoked, burning pain of the classic thalamic pain syndrome.

Antinociceptive effects of intrathecal administration of alpha-adrenoceptor antagonists and clonidine in the formalin test in the mouse

The hypothesis that the descending noradrenergic system tonically inhibits nociception at the spinal level was investigated, using the formalin test in mice. The alpha-adrenoceptor agonist clonidine (0.46 and 0.92 microgram), injected intrathecally, significantly reduced licking activity in both the early and late phase of the test. The alpha 1-antagonist prazosin (3.75, 7.5 and 15 micrograms) and the alpha 2-antagonist yohimbine (7.5 micrograms) also significantly reduced licking activity in both phases. The smaller doses of yohimbine (1.87 and 3.75 micrograms) induced an insignificant reduction of licking in the early phase. Except for the largest doses of clonidine (0.92 microgram), the drugs used had no effect on the general level of activity and motor performance. These results support previous findings that increased noradrenergic activity in the spinal cord inhibits nociception, however, this inhibition seems not to be tonically active. The mechanisms of the antinociceptive actions of alpha-antagonists are not clear.

Morphine alters the firing of cold-receptive neurons in the superficial dorsal horn of the medulla in the rat

Effects of morphine (1-3 mg/kg, i.v.) were tested on the innocuous cold-receptive input in the superficial dorsal horn of the medulla. The static activity of most cold-receptive (cold-specific) neurons (12/16) was reduced, whereas an enhancement (4/16) was observed in the remaining neurons. Naloxone (200 micrograms/kg, i.v.) reversed, partially or completely, the effects of morphine in 9/12 cold-receptive neurons, and enhanced the static activity of some cold-receptive neurons. Static activity, at different adapting temperatures, during a warming (10 degrees C-->40 degrees C) and a cooling (40 degrees C-->10 degrees C) sequence at steps of 5 degrees C was reduced by morphine. The effects of morphine were also tested on the static as well as the dynamic responses of 9 cold-receptive neurons. The effects of morphine on the dynamic responses were not dependent on the static firing frequency. Morphine produced similar effects, excitatory or inhibitory, on the static as well as the dynamic responses of 7/9 neurons whether the static firing frequency was high (17-33 Hz) or low (< 12 Hz). However, morphine effects on static and dynamic responses were different in the remaining 2 neurons (high static firing frequency). We suggest that the predominantly inhibitory effect of morphine on the innocuous cold receptive input in the medullary dorsal horn may explain the inhibitory effect on the perception of cooling stimuli by systemic morphine in behavioral studies.

Effects of intrathecal antibodies to substance P, calcitonin gene-related peptide and galanin on repeated cold stress-induced hyperalgesia: comparison with carrageenan-induced hyperalgesia

Rats exposed to a cold environment (4 degrees C) for 30 min every 1 h during the day and at night show a gradual decrease in the nociceptive threshold for pressure stimulation. Such hyperalgesia, referred to as repeated cold stress (RCS)-induced hyperalgesia, is stable for at least 4 h and maintained for 3 days only by exposing to cold overnight; thus, no adaptation to RCS is apparent. Hyperalgesia gradually returns over 4 days after cold exposure ceases. To determine whether three neuropeptides, substance P (SP), calcitonin gene-related peptide (CGRP) and galanin (GAL), which are present in the superficial dorsal horn including primary afferent terminals, would be responsible for RCS-induced hyperalgesia, we examined the effects of intrathecal injections of their antibodies (used as inhibitors of neuropeptide-mediated synaptic transmission) on the nociceptive threshold of RCS rats, and compared this with the antibody effect on carrageenan-induced hyperalgesia. An intrathecal injection of anti-SP antibody significantly inhibited the hyperalgesia of RCS rats as well as carrageenan-induced hyperalgesia, and slightly increased the nociceptive threshold of non-RCS rats. Anti-CGRP antibody produced an improvement in the hyperalgesia of RCS rats as well as carrageenan-induced hyperalgesia without having an effect on the nociceptive threshold of non-RCS rats. Although anti-GAL antibody significantly inhibited carrageenan-induced hyperalgesia, it did not affect the nociceptive threshold of RCS and non-RCS rats. The present results suggest that enhancement of synaptic transmission mediated by SP and CGRP, but not GAL, in the spinal dorsal horn is, at least in part, involved in RCS-induced hyperalgesia.

Morphine enhances the activity of thermoreceptive cold-specific lamina I spinothalamic neurons in the cat

The possibility that morphine might differentially affect spinal neurons responsive to small-diameter thermoreceptive-specific afferents was tested. Systemic morphine enhanced a portion or all of the stimulus-response function of 7 of 9 lamina I spinothalamic tract cells specifically sensitive to cold applied to the glabrous hindpaw in the cat. This result contrasts strongly with the predominant inhibition of nociceptive lamina I neurons by morphine.

The formalin test in mice: the influence of ambient temperature

The ambient temperature had a confounding influence on the licking response in the formalin test. No effect was demonstrated in the early phase. In the late phase, the licking activity was much lower at 20 degrees C than at 25 degrees C. Both the intensity and the duration of the response were increased by increasing the ambient temperature from 20 degrees C to 28 degrees C. 5,6-Dihydroxytryptamine lesions of descending serotonergic pathways induced an increase in the nociceptive response at an ambient temperature of 20 degrees C, while the response was no different from control values at 25 degrees C. Differences in paw skin temperature may explain these temperature dependent effects. It was concluded that control of the ambient temperature is necessary to obtain reliable results in the formalin test, late phase.