The effect of pituitary-adrenal function in the modulation of pain sensitivity in the rat

Organizational influence of the postnatal testosterone surge on the circadian rhythm of core body temperature of adult male rats

The suprachiasmatic nucleus (SCN) of the hypothalamus coordinates physiological and behavioral circadian rhythms such as activity, body temperature, and hormone secretion. Circadian rhythms coordinated by the SCN often show sex differences arising from both organizational and activational effects of gonadal hormones. In males, little is known about the organizational role of testosterone on the circadian regulation of core body temperature (CBT) in adulthood. To explore this, we castrated or sham-operated male rats on the day of birth, and at 4 months of age, implanted them with transmitters that measured CBT rhythms under a 12:12 light/dark cycle. This study revealed a significantly earlier rise in CBT during the light phase in neonatally castrated males. Subsequently, we found that treating neonatally castrated males with testosterone propionate (TP) in adulthood did not reverse the effect of neonatal castration, thus indicating an organizational role for testosterone. In contrast, a single injection of TP at the time of neonatal surgery, to mimic the postnatal surge of testosterone, coupled with TP treatment in adulthood, normalized the circadian rise in CBT. In a final study we examined CBT circadian rhythms in intact adult male and female rats and detected no differences in the rise of CBT during the light phase, although there was a greater overall elevation in female CBT. Together, results of these studies reveal an early organizational role of testosterone in males on the timing of the circadian rise of CBT, a difference that does not appear to reflect "defeminization".

Compulsive targeted self-injurious behaviour in humans with neuropathic pain: a counterpart of animal autotomy? Four case reports and literature review

Four cases of compulsive self-injurious behaviour (SIB) with variable degrees of tissue damage targeted to the painful body part are reported in humans with neuropathic pain. Review of human literature revealed several cases, primarily after central nervous system (CNS) lesions, during which non-psychotic verbally communicating humans (mostly with intact mental status) target specifically the painful part which is usually analgesic or hypoalgesic. In few instances, however, the involved part is not only sentient but also hyperalgesic in part or as a whole. The act is characterized by uncontrollable urge and compulsion, aggravated under conditions of stress, isolation, confusion or depression, and occasionally occurring in patients with personality disorders, ongoing drug abuse and pre-existing compulsive habits (i.e., habitual nail biting or picking). It fails to be deterred by the appearance of the injured part, social mores or even the experience of pain. Successful treatment of underlying painful dysesthesiae with specific medications, neurostimulation or surgery has resulted in marked improvement of dysesthesiae accompanied by wound healing in several cases. The four presented cases and the human literature experience provide evidence that compulsive targeted SIB in humans with neuropathic pain and painful dysesthesiae is consistent with the concept that animal autotomy may result from chronic neuropathic pain after experimental peripheral or CNS lesions.

Blockade of the polyamine site of NMDA receptors produces antinociception and enhances the effect of morphine, in mice

The possible effect of ifenprodil--a potent antagonist at the polyamine site of the NMDA receptor complex--on nociceptive threshold and morphine analgesia was investigated in mice. In the hot plate test, the intraperitoneal (i.p.) injection of ifenprodil significantly prolonged the reaction time of mice at the dose of 30 mg/kg, and increased the analgesic effect of morphine. In the phenylquinone writhing test, ifenprodil reduced the number of abdominal constrictions of mice starting from the dose of 2.5 mg/kg i.p., and increased the effect of morphine. The effect of ifenprodil on pain threshold was prevented by naloxone. Moreover, ifenprodil antagonized the pain threshold-reducing effect of alpha-melanocyte-stimulating hormone (0.05 microgram/mouse, intracerebroventricularly). These data show that blockade of the polyamine site of the NMDA receptor complex produces analgesia and increases the analgesic effect of morphine.

Effects of hypophysectomy and adrenalectomy on naloxone-induced analgesia

Experiment 1 demonstrated that pairings of the opiate antagonist, naloxone, with a heated floor came to induce analgesia, as indexed by the latencies with which rats licked their paws. This analgesia appears to be neurally mediated because it is unaffected by either hypophysectomy (experiment 2) or adrenalectomy (experiment 3). However, there was evidence for a pituitary involvement, as its removal potentiated the analgesic effect accruing from naloxone-stressor pairings.

Octopus automutilation syndrome

This paper describes an automutilation syndrome (OAS) in three species of captive octopuses, Octopus dolfleini, O. bimaculoides, and O. maya, characterized by external arm and mantle lesions. Three clinical patterns in nine animals had similar and characteristic gross and histopathologic features. Axial nerve or brachial artery lesions were observed in six of the nine cases and vascular lesions were seen in two of eight cases with mantle ulcerations. A relationship between automutilation in the octopus and dysesthesias due to neural or vascular pathology is proposed.

The relationship between cardiovascular and pain regulatory systems

An increasing amount of anatomical, physiological, and pharmacological evidence suggest that pain inhibitory circuitry is linked with cardiovascular regulatory systems in man and laboratory animals. Induction of hypertension in rats by different methods (mineralocorticoid treatment, stenosis of renal artery, or social deprivation) is associated with reduced responsiveness to noxious thermal stimuli (hot-plate) or to noxious mechanical stimuli (paw pressure). Genetically hypertension-prone rats derived from the SABRA strain and spontaneously hypertensive rats derived from Wistar/Kyoto strain also display a similar hypoalgesia. Acute increases in blood pressure are associated with reduced sensitivity to painful stimuli. Additionally, the interaction between blood pressure and pain perception has also been supported by the demonstration that various experimental interventions that diminish the magnitude of hypertension also attenuate the hypoalgesia. Recent clinical findings are also in agreement with the laboratory animal findings since sensory and pain thresholds have been shown to be significantly higher in unmedicated essential hypertensive subjects compared to normotensive controls. Thus, the human data corroborate animal data and suggest that a relation between blood pressure and pain sensitivity is likely to be a general phenomenon. It is unlikely that damage to peripheral pain fibers caused by a change in blood pressure contributes to the observed hypoalgesia. Naloxone, which has no effect on blood pressure, returns the pain sensitivity to normal levels. Behavioral tests (open field and motor activity cage) of normotensive and of renal and genetically (SBH and SHR) hypertensive rats exclude the possibility of a general motor deficit in hypertensive rats. Endogenous opioid peptides in central and peripheral nervous systems as well as in endocrine organs are implicated, although non-opioid mechanisms are also evident. Activation of baroreceptor afferents by acute or chronic increases in arterial or venous blood pressure may play an important role in the somatosensory responses associated with the increase in blood pressure. Coordinated cardiovascular-pain regulatory responses may be part of an adaptive mechanism that helps the body to face stressful events.

Dysesthesias and self-mutilation in humans and subhumans: a review of clinical and experimental studies

The chronic deafferentation syndrome includes a complex pattern of abnormal self-directed behavior and a stress response. Subhuman self-mutilation is a secondary consequence of the chronic deafferentation syndrome. The evidence indicates that the chronic deafferentation syndrome in subhumans is a valid model for the induced and the spontaneous dysesthesias in humans. Objective criteria for the definition of subhuman dysesthesias have been derived from independent sources of evidence, in neurally intact subjects; those criteria are then found to match the subhuman syndrome of deafferentation. Support for the validity of the inference of subhuman dysesthesias derives from the parallels with the various facts of the human dysesthesias. The credibility of this argument is significantly strengthened by reports of morphological and excitatory physiological abnormalities, in central somatosensory structures, in response to deafferentation. There is no independent subhuman evidence in support of alternate interpretations of the deafferentation syndrome, and those interpretations seem to be inadequate in several aspects. Doubts concerning the validity of this animal model have been allayed by reports of dysesthesias in humans with spinal posterior rhizotomies or ganglionectomies, and also those with congenital analgesia. Moreover, the occurrence of this syndrome in hypoalgesic areas as a consequence of anterolateral cordotomy in monkeys, can best be interpreted as a reflection of dysesthesias. This syndrome is released by neuropathological or neurosurgical lesions in the peripheral or central nervous system; lesions which involve small caliber peripheral afferents or the spinothalamic tract. Variability in the release of this syndrome has been associated with several different factors. So far, the chronic syndrome is intractable. Evidence relates the abnormalities of this syndrome to pathophysiological foci in central relays of the somatosensory system, and suggests that the chronic abnormalities of this syndrome can be sustained at brain levels.

Genetic differences in avoidance behavior: cardiovascular activity, pain sensitivity and stress-induced analgesia

Four experiments are reported which examine cardiovascular activity, pain sensitivity and stress-induced analgesia in rats selectively bred for differences in shuttlebox avoidance behavior. The results of Experiments 1 and 2 indicate that the two genetic lines differ in basal pain sensitivity, as measured by the hot-plate test. This difference in pain sensitivity appears not to be mediated by endogenous opioids, because it was not altered by pretreatment with a large dose of naloxone. In Experiments 1, 3 and 4 tail-flick tests of basal pain sensitivity failed to reveal line differences. Basal and stress levels of cardiovascular activity also showed no differences between the lines. In Experiment 3, LA but not HA animals showed profound stress-induced analgesia which was not blocked by a large dose of naltrexone. In Experiment 4, both LA and HA animals showed stress-induced analgesia, perhaps because the procedure of this experiment permitted conditioning mechanisms to contribute to the analgesia. Differential genetic selection for avoidance behavior also selected for differential pain sensitivity and some forms of non-opioid stress-induced analgesia but without concomitant selection for differential cardiovascular activity.

Development of cerebral methionine-enkephalinergic neurons in rats: some difference in Wistar-Kyoto rats and spontaneously hypertensive rats

Development of methionine-enkephalin (ME) and ME receptor binding in the embryonic and neonatal rat cerebral nuclei was defined quantitatively by immunocytofluorescent and microautoradiographic methods. On embryonic days 14 and 18, ME was localized in 10 of 83 nuclei particularly in the n. amygdaloideus centralis, n. periventricularis, n. supraopticus, n. interpeduncularis, n. suprachiasmaticus, n. arcuatus and n. ambiguus. ME receptor binding was distributed in the former 4 nuclei on embryonic day 14 and additional 5 nuclei on embryonic day 18. At the day of birth both levels markedly increased in these nuclei (2-3 times) and abruptly emerged in 65 out of 73 nuclei in which ME neurons were not detectable in embryonic nuclei. The abrupt appearance in most nuclei at birth probably regulates nociceptive and non-nociceptive stimuli at birth and in the neonatal period. Both levels attained their maximum at the postnatal day 7 and gradually declined thereafter. Among the perinatal period examined, ME immunoreactivity in the tractus spinalis nervi trigemini and ME receptor binding in the n. tractus spinalis nervi trigemini were markedly lower in both SHR neonates than in corresponding Wistar-Kyoto rats. Lowered levels in both areas of neonatal SHR may be involved in central hyperreactivity and preganglionic sympathetic activation of young SHR.

The effect of pituitary removal on pain regulation in the rat

The effects of hypophysectomy (HX) on pain regulation in basal and in various stressful situations were investigated in the rat. Pain sensitivity was assessed by measuring the thresholds of 3 nociceptive reactions (tail withdrawal, vocalization, vocalization afterdischarge) following electrical stimulation of the tail. The completeness of HX and the integrity of hypothalamus were verified in each HX rat. (1) Baseline pain thresholds were lower in HX rats than in sham-operated animals; (2) naloxone (Nx) hyperalgesia was only slightly altered by HX; (3) different types of stress induced different types of changes in nociception i.e. analgesia or hyperalgesia. The influence of HX varied according to the stress: it increased hyperalgesia, reduced analgesia, or had no effect at all. These results indicate that in the rat: (i) the pituitary participates in the regulation of basal pain sensitivity, probably through analgesic factors; (ii) Nx hyperalgesia results essentially from an antagonism of endogenous opioids originating in the CNS and not in the pituitary; and (iii) the pain regulatory processes engaged in adaptation to stressful stimuli involve the CNS and the pituitary in variable proportions depending upon the nature of the stress.

High versus low intensity acupuncture analgesia for treatment of chronic pain: effects on platelet serotonin

The 26 chronic pain patients were tested in a baseline plus cross-over design. Half of the subjects were first treated with high intensity acupuncture; then they were treated with low intensity acupuncture. For the other 13 subjects the treatment order was reversed. In the first treatment sequence subjects reported lower pain estimates and engaging in more activities of daily living during treatment with high intensity acupuncture--but not with low intensity acupuncture. In addition, under high intensity acupuncture (i.e. with low pain levels), subjects had higher levels of platelet serotonin; this last finding is consistent with recent research which implicates central serotonin in pain control. The results of the second treatment sequence were ambiguous.

The role of endorphins in stress: evidence and speculations

Several lines of evidence suggest that the endogenous opioid peptides endorphins may play a role in the defensive response of the organism to stress. The present paper summarizes these findings as well as evidence linking endorphins to the anterior pituitary polypeptide hormone adrenocorticotropin (ACTH). Evidence is presented that endorphins may function as trophic hormones in peripheral target organs such as the adrenal medulla and the pancreas. As such they may be part of the physiological mechanisms that mediate adrenaline and glucagon release in response to stress. Endorphins (enkephalins) are also suggested to play a role in the control of the pituitary gland during stress. In such capacity they may act as hormone-releasing or inhibiting factors. Finally, endorphins appear to play a role in the behavioral concomitants of stress. In such capacity endorphins are suggested to function as modulators of neural systems that mediate the elaboration and expression of the reactive/affective components of stress. Speculations on the mode of interaction between endorphins and ACTH in the global response to stress are discussed.

Enhanced analgesic effects of stress following chronic administration of naltrexone in rats

Chronic administration of the long acting opiate antagonist naltrexone potentiated the analgesic effects of foot-shock stress in the hot-plate test in rats. No changes in pain responsiveness were noted in naltrexone-treated rats that were not subjected to the foot-shock treatment. The results suggest that chronic opiate receptor may lead to the development of supersensitivity in endogenous opiate systems that mediate the analgesic effects of stress.