Analgesia induced by cold-water stress: attenuation following hypophysectomy

Early Intervention of Cold-Water Swimming on Functional Recovery and Spinal Pain Modulation Following Brachial Plexus Avulsion in Rats

Brachial plexus avulsion (BPA) causes peripheral nerve injury complications with motor and sensory dysfunction of the upper limb. Growing evidence has shown an active role played by cold-water swimming (CWS) in alleviating peripheral neuropathic pain and functional recovery. This study examined whether CWS could promote functional recovery and pain modulation through the reduction of neuroinflammation and microglial overactivation in dorsal horn neurons at the early-stage of BPA. After BPA surgery was performed on rats, they were assigned to CWS or sham training for 5 min twice a day for two weeks. Functional behavioral responses were tested before and after BPA surgery, and each week during training. Results after the two-week training program showed significant improvements in BPA-induced motor and sensory loss (p < 0.05), lower inflammatory cell infiltration, and vacuole formation in injured nerves among the BPA-CWS group. Moreover, BPA significantly increased the expression of SP and IBA1 in dorsal horn neurons (p < 0.05), whereas CWS prevented their overexpression in the BPA-CWS group. The present findings evidenced beneficial rehabilitative effects of CWS on functional recovery and pain modulation at early-stage BPA. The beneficial effects are partially related to inflammatory suppression and spinal modulation. The synergistic role of CWS combined with other management approaches merits further investigation.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Exodontia-induced muscular hypofunction by itself or associated to chronic stress impairs masseter muscle morphology and its mitochondrial function

Stress is associated with orofacial pain sensitivity and is qualified as a temporomandibular disorder risk factor. During stressful periods, painful thresholds of masticatory muscles in individuals suffering muscle facial pain are significantly lower than in controls, but the exact physiologic mechanism underlying this relation remains unclear. Our hypothesis is that chronic unpredictable stress and masticatory hypofunction induce morphologic and metabolic masseter muscle changes in rats. For test this hypothesis, adult Wistar rats were submitted to chronic unpredictable stress and/or exodontia of left molars and the left masseter muscle was removed for analysis. The parameters evaluated included ultrastructure, oxidative level, metabolism activity and morphological analysis in this muscle. Our data show by histological analysis, that stress and exodontia promoted a variation on diameters and also angled contours in masseter fibers. The masticatory hypofunction increased oxidative metabolism as well as decreased reactive species of oxygen in masseter muscle. The ultrastructural analysis of muscle fibers showed disruption of the sarcoplasmic reticulum cisterns in certain regions of the fiber in stress group, and the disappearance of the sarcoplasmic reticulum membrane in group with association of stress and exodontia. Our findings clarify mechanisms by which chronic stress and masticatory hypofunction might be involved in the pathophysiology of muscular dysfunctions. Masticatory hypofunction influenced oxidative stress and induced oxidative metabolism on masseter muscle, as well as altered its fiber morphology. Chronic stress presented malefic effect on masseter morphology at micro and ultra structurally. When both stimuli were applied, there were atrophic fibers and a complete mitochondrial derangement.

Impact of stress, fear and anxiety on the nociceptive responses of larval zebrafish

Both adult and larval zebrafish have been demonstrated to show behavioural responses to noxious stimulation but also to potentially stress- and fear or anxiety- eliciting situations. The pain or nociceptive response can be altered and modulated by these situations in adult fish through a mechanism called stress-induced analgesia. However, this phenomenon has not been described in larval fish yet. Therefore, this study explores the behavioural changes in larval zebrafish after noxious stimulation and exposure to challenges that can trigger a stress, fear or anxiety reaction. Five-day post fertilization zebrafish were exposed to either a stressor (air emersion), a predatory fear cue (alarm substance) or an anxiogenic (caffeine) alone or prior to immersion in acetic acid 0.1%. Pre- and post-stimulation behaviour (swimming velocity and time spent active) was recorded using a novel tracking software in 25 fish at once. Results show that larvae reduced both velocity and activity after exposure to the air emersion and alarm substance challenges and that these changes were attenuated using etomidate and diazepam, respectively. Exposure to acetic acid decreased velocity and activity as well, whereas air emersion and alarm substance inhibited these responses, showing no differences between pre- and post-stimulation. Therefore, we hypothesize that an antinociceptive mechanism, activated by stress and/or fear, occur in 5dpf zebrafish, which could have prevented the larvae to display the characteristic responses to pain.

Influence of hypophysectomy, ovariectomy and gonadectomy on postoperative hypersensitivity in rats

Surgical procedures lead to profound and sustained (up to 1–2 weeks) activation of the pituitary gland, resulting in changes in endocrine function. Questions remain on whether activation of the pituitary influences the threshold and development time-course of postoperative pain. To address these questions, we evaluated postoperative hypersensitivity in female and male rats with ablated pituitary and gonadal hormone productions via hypophysectomy, ovariectomy and gonadectomy, respectively. Plantar incision, a model of acute postoperative pain, or sham operation was performed on rat hind paws. Hypophysectomy, ovariectomy and gonadectomy were achieved by surgical disconnection of pituitary, ovaries and testicles, respectively. Postoperative thermal and mechanical hypersensitivity were monitored for 7 days post incision. Hypophysectomy on female and male rats produced statistically similar thermal and mechanical postoperative hypersensitivity thresholds and time-courses as compared to intact estrous female and male rats. Moreover, ovariectomy and gonadectomy did not significantly change postoperative hypersensitivity observed in control female and male animals. Our experiments demonstrate that hypophysectomy, ovariectomy and gonadectomy do not significantly impact postoperative hypersensitivity observed in normal female and male animals. These data suggest that surgery-induced changes in the endocrine system via activation of pituitary and subsequently gonadal tissues have little impact on the threshold and development of postoperative pain in female and male rats.

Stress-induced analgesia and endogenous opioid peptides: the importance of stress duration

Stress is known to elicit pain relief, a phenomenon referred to as stress-induced analgesia. Based on stress parameters, opioid and non-opioid intrinsic pain inhibitory systems can be activated. In the present study, we assessed whether changing the duration of stress would affect the involvement of endogenous opioids in antinociception elicited by swim in warm water (32 °C), known to be opioid-mediated. Using mice lacking beta-endorphin, enkephalins or dynorphins and their respective wild-type littermates, we assessed the role of each opioid peptide in antinociception induced by a short (3 min) vs. long (15 min) swim. Mice were tested for baseline hot plate latency, exposed to swim (3 or 15 min) in warm water (32 °C) and then tested for antinociception at 5, 15 and 30 min. Our results revealed that both swim paradigms induced significant antinociception in wild-type mice. However, the short swim failed to induce antinociception in beta-endorphin-deficient mice, illustrating that beta-endorphin is important in this form of stress-induced antinociception. On the other hand, antinociception elicited by the long swim was only slightly reduced in beta-endorphin-deficient mice despite pretreatment with naloxone, a non-selective opioid receptor antagonist, significantly attenuated the antinociception elicited by the long swim. Nevertheless, a delayed hyperalgesic response developed in mice lacking beta-endorphin following exposure to either swim paradigm. On the other hand, mice lacking enkephalins or dynorphins and their respective wild-type littermates expressed a comparable antinociceptive response and did not exhibit the delayed hyperalgesic response. Together, our results suggest that the endogenous opioid peptide beta-endorphin not only mediates antinociception induced by the short swim but also prevents the delayed hyperalgesic response elicited by either swim paradigm.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Increased stress-induced analgesia in mice lacking the proneuropeptide convertase PC2

Many neuropeptides involved in pain perception are generated by endoproteolytic cleavages of their precursor proteins by the proprotein convertases PC1 and PC2. To investigate the role of PC2 in nociception and analgesia, we tested wild-type and PC2-null mice for their responses to mechanical and thermal nociceptive stimuli, before and after a short swim in cold or warm water. Basal responses and responses after a cold swim were similar between the two groups. However, after a short forced swim in warm water, PC2-null mice were significantly less responsive to the stimuli than wild-type mice, an indication of increased opioid-mediated stress-induced analgesia. The enhanced analgesia in PC2-null mice may be caused by an accumulation of opioid precursor processing intermediates with potent analgesic effects, or by loss of anti-opioid peptides.

The effects of restraint stress on nociceptive responses induced by formalin injected in rat's TMJ

It has been reported that stress can alter nociception from superficial tissues, such as skin and subcutaneous region. However, the influence of stress on an experimental deep nociception model is not understood. In this study, the temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute and chronic restraint stress on nociceptive responses in rats. Animals were initially submitted to one session of acute restraint stress (1 h) or exposed to chronic stress (40 days-1 h/day). Then, animals were killed immediately to collect blood for hormonal determinations by radioimmunoassay, or submitted to the TMJ formalin test to evaluate nociception. Rats submitted to acute restraint presented a performance similar to unstressed controls in the TMJ formalin test, whereas chronically stressed rats showed an increase in nociceptive responses. After 40 days of restraint, morphine was injected i.p. (1, 5 mg/kg or saline). The stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. These findings suggest that repeated stress can produce hyperalgesia, which is, at least in part, due to alterations in the activity of opioid systems. This model may help elucidate the underlying neural mechanisms that mediate the effects of repeated stress on orofacial pain.

Differential effects of stress on escape and reflex responses to nociceptive thermal stimuli in the rat

Acute stress has been shown to increase latencies of nociceptive reflexes, and this effect is considered evidence for stress-induced analgesia. However, tests for nociception that rely on motivated operant escape assess cerebral processing of pain and could be modulated independent of reflex responses. We therefore compared the effects of an acute stressor (restraint) on escape responses and lick/guard reflexes to stimulation of the paws by a thermally regulated floor. Testing sessions included a pre-test exposure to 36 degrees C, followed by a test trial in which either escape from 44 or 36 degrees C or reflex responses to 44 degrees C were observed. Behavioral responses to stress were assessed during a three day period, with baseline testing on day 1, post-stress or control testing on day 2, and evaluation of long-term stress effects on day 3. On day 2, half the animals received 15 min of restraint stress, followed by 15-min pre-test and test trials. Licking and guarding responses to thermal stimulation during 44 degrees C test trials were significantly reduced by restraint stress, confirming previously reported stress effects on nociceptive reflexes. In contrast, learned escape responses to the same thermal stimulus were significantly enhanced after stress. The increase in operant sensitivity suggests that acute restraint, a form of psychological stress, produces hyperalgesia for a level of thermal stimulation that preferentially activates C nociceptors. These results are discussed in relation to studies involving physical or psychological forms of stress, different nociceptive stimuli, and assessment strategies used to evaluate thermal pain sensitivity.

Cytogenetic comparison of the sensitivity to mutagens in mice selected for high (HA) and low (LA) swim stress-induced analgesia

Sensitivity to mutagens was studied in mouse lines selectively bred for high analgesia (HA) and for low analgesia (LA) induced by 3-min swimming in 20 degrees C water. Apart from pain-related traits HA mice also manifest, as compared to the LA line, higher emotionality in various behavioural tests, and cope worse with the hypothermic challenge of swimming in cold water. In the present study HA mice appeared more susceptible to the mutagenic effect of whole-body gamma-radiation and mitomycin-C injection. Both treatments caused higher frequencies of chromosomal aberrations and micronucleus in bone marrow cells in the HA than in the LA line. The results are discussed in terms of a genetic correlation between animals' susceptibility to environmental stressors and the mechanism of mutagenesis. As shown by our recent molecular study, the selection for magnitude of swim analgesia has differentiated the parental outbred population into two distinct genotypes characterised by specific minisatellite and microsatellite sequences for each line, which may be genetic markers of particular traits. We conceive that the breeding strategy, along with the differentiation of stress-related phenomena, has altered the frequencies of genes controlling DNA repair in each line.

Analgesia in selectively bred mice exposed to cold in helium/oxygen atmosphere

In order to evaluate the stressing role of swim hypothermia in producing swim stress-induced analgesia (SSIA), we examined whether a mere decrease in the animals' core temperature without swimming would be sufficient to elicit analgesia. The subjects were Swiss-Webster mice selectively bred for 37 and 40 generations for divergent magnitudes of SSIA. High (HA) and low analgesia (LA) mice were exposed for 15 min to temperatures in the range between -5 and +20 degrees C in 79% He/21% O2 (Heliox) atmosphere. The Heliox exposure produced ambient temperature-dependent hypothermia and analgesia, as assessed with a hot-plate test (56 degrees C). The post-Heliox analgesia was of much higher magnitude in HA than in LA mice. The steeper slope of regression of the magnitude of analgesia upon hypothermia in HA mice indicates that these mice are far more sensitive to the analgesic effect of hypothermia than LA mice. Naltrexone HCl (10 mg/kg i.p.) attenuated analgesia in ambient temperature-dependent manner in HA, but not in LA mice. In view of the apparent similarity of Heliox-induced analgesia and SSIA we suggest that hypothermia is a powerful component of swim stress to induce SSIA in the mouse.

Repeated cold water swim produces delayed nociceptive responses, but not analgesia, for tonic pain in the rat

Earlier studies have demonstrated that cold water swim (CWS) produces stress-induced analgesia in tests of brief, phasic pain and produces a delayed nociceptive response (DNR) for more prolonged tonic pain. The present study reports the effect of repeated CWS on tonic pain, as measured by the formalin test. One group of rats was exposed to a 3.5-min swim in 2 degrees C water immediately prior to the formalin injection, to a 1.5-min swim at 50 min, and to another 1.5-min swim at 100 min postformalin injection. Compared to the no-swim control group, subjects which received repeated CWS had dramatically altered formalin pain responses. Formalin responses began just over 3 h postformalin injection, peaked at 4 h, and were still present at 5 h. Inspection of individual responses revealed a substantial degree of variability in the onset of responses, although the magnitude and duration of the formalin pain response remained at the same levels as those of control subjects. The lack of a decrease in the magnitude and duration of the delayed formalin responses indicates that repeated CWS does not produce analgesia for tonic pain. The period of stress, therefore, produces pain suppression but not loss of the mechanisms that subsequently underlie the pain. Earlier controls have ruled out peripheral mechanisms (such as retention of the formalin in the paw tissue). Rather, a memory mechanism appears to have been indicated and it is not lost, but persists until it can be manifested. Further research is needed to study the mechanisms responsible for the DNR.

Blockade of tolerance to stress-induced analgesia by MK-801 in mice

The N-methyl-D-aspartate (NMDA) receptor has been implicated in mechanisms of tolerance to morphine-induced analgesia. The present study examined the role of the NMDA receptor in the development of tolerance to stress-induced analgesia (SIA). In the first experiment, mice were exposed to a stressor (a 3-min forced swim in water maintained at 32 degrees C) once daily for 15 consecutive days. Analgesia was measured 2 min after stress on the first and last day using the hot-plate test. To examine the role of the NMDA receptor in the development of tolerance to SIA mice were treated daily with the non-competitive NMDA receptor antagonist, MK-801, 15 min before swimming. Pretreatment with MK-801 was found to block both analgesia and tolerance. In a second experiment, to examine whether SIA and tolerance to SIA are mediated by similar or different mechanisms, mice were injected daily with MK-801 after analgesia had dissipated (1 h following swim). Tolerance to SIA was blocked by delayed injections of MK-801. These results suggest that the NMDA receptor is involved in mechanisms of tolerance to SIA, independent of its role in analgesia.

Relationship between hypothalamic-pituitary-adrenal activity and blockade of tolerance to morphine analgesia by pain: a strain comparison

We previously reported that morphine fails to produce analgesic tolerance when administered in the presence of formalin-induced pain. The hypothalamic-pituitary-adrenal (HPA) axis is known to respond to stressful stimuli, including pain. To examine whether the blockade of tolerance by pain is related to HPA activity, we assessed the development of tolerance to morphine analgesia in an inbred strain of rats that lack typical stress-induces HPA responses (Lewis strain). Lewis rats lack typical stress-induced activation of corticotropin-releasing hormone, adrenocorticotropin hormone and glucocorticoids. Female Lewis rats were injected with morphine (20 mg/kg, i.p.) or saline for 4 consecutive days in the presence or absence of pain induced by injection of formalin into the hind-paw. The analgesic effect of morphine (5, 10 or 20 mg/kg, i.p.) was then measured in the tail-flick test 24 h after tolerance induction. Inbred female Fischer rats, which show significant stress-induced HPA activity, were used for comparison. Analgesic tolerance was produced in both strains when morphine was delivered in the absence of pain. However, the presence of pain during morphine administration prevented the development of analgesic tolerance in Fischer, but not in Lewis, rats. The differential of pain on the development of tolerance to morphine analgesia are suggested to be related to genetically determined differences in stress-induced HPA activity.

The response pattern of noradrenaline release to repeated stress in the hypothalamic paraventricular nucleus differs according to the form of stress in rats

The effect of two repeated forms of stress, manual restraint and tail-pinch, on noradrenaline (NA) release in the hypothalamic paraventricular nucleus of the rat was examined by intracerebral microdialysis. Manual restraint significantly increased NA release, but the stimulatory effect gradually declined when the stress was repeated at intervals of 120 min. High K+ induced a great increase in NA release even when manual restraint produced no significant effect on NA release. In contrast, tail-pinch significantly increased NA release to a greater extent than manual restraint, and the increase in NA release did not change when the stress was repeated three times at intervals of 120 min. These results suggest that desensitization of NA neurons ending in the hypothalamic paraventricular nucleus occurs in response to painless stress, such as manual restraint, whereas no attenuation of NA release is caused by repeated stress accompanied by pain, such as tail-pinch.

A permissive role of corticosterone in an opioid form of stress-induced analgesia: blockade of opiate analgesia is not due to stress-induced hormone release

The 100 inescapable tail-shock paradigm produces three sequential analgesic states as the number of shocks increases: an early opioid analgesia (after 2 shocks) that is attenuated by systemic naltrexone, a middle analgesia (after 5-40 shocks) that is unaffected by systemic naltrexone, and a late opioid analgesia (after 80-100 shocks) that is attenuated by systemic naltrexone. In order to determine whether the absence of adrenal hormones would affect any of these analgesias, we tested adrenalectomized (ADX) versus sham-operated control rats 2 weeks post-surgery. Pain threshold was assessed using the tail-flick (TF) test. ADX attenuated both the early (2 shock) and late (80-100 shock) opiate analgesias and failed to reduce the naltrexone-insensitive analgesia after 5-40 shocks. We demonstrated that a loss of adrenomedullary catecholamines does not underlie the ADX-induced attenuation of opioid analgesia since sympathetic blockade using systemic chlorisondamine (6 mg/kg) failed to reduce analgesia at any point in the shock session. It was further shown that stress levels of adrenal hormones are not critical since (a) analgesia was unaffected when animals were tested 48 h after ADX, (b) 2 shocks do not produce a surge in corticosterone (CORT) over and above levels observed in animals restrained and TF tested in preparation for shock, and (c) basal CORT replacement in drinking water fully restored analgesia in ADX rats. These experiments demonstrate that basal CORT, rather than adrenomedullary substances, is critical to the expression of analgesia. The function of CORT here is not linked to a shock-induced surge of the steroid. CORT appears to play a permissive role in the expression of analgesia. Potential effects of the absence of corticosteroids on neurotransmitter biosynthesis important in analgesia production are discussed.

Site-specific modulation of morphine and swim-induced antinociception following thyrotropin-releasing hormone in the rat periaqueductal gray

Central administration of thyrotropin-releasing hormone (TRH) produces a short-lived antinociceptive response in rats, and also modulates opioid and non-opioid forms of antinociception. Given the presence of TRH cells, fibers and receptors in the periaqueductal gray (PAG), the present study examined the effects of TRH administered into the PAG upon antinociception following either continuous cold-water swims (CCWS, 2 degrees C for 3.5 min) or morphine (0.1-2.5 micrograms) administered into the PAG on the tail-flick and jump tests, and measured changes in core body temperatures as well. Histological examination revealed two groups in which anterior PAG placements were found rostral to the dorsal raphe nucleus, and posterior PAG placements which were at the level of this nucleus. TRH produced brief (5-15 min) but significant increases in latencies and thresholds without altering body temperature in both anterior and posterior PAG placements. Whereas TRH in anterior PAG placements dose dependently (0.1-10 micrograms) decreased CCWS antinociception on both tests, TRH in posterior PAG placements significantly increased CCWS antinociception on the jump test. TRH in both placements reduced the magnitude of CCWS hypothermia. TRH significantly potentiated the magnitude and duration of both morphine antinociception and hyperthermia in both anterior and posterior PAG placements, and shifted mesencephalic morphine's antinociceptive dose-response curve significantly to the left. These data are discussed in terms of the role of the PAG in opioid and non-opioid forms of stress-induced antinociception as well as morphine antinociception, and in terms of the roles of TRH and anterior PAG placements as potential candidates for a collateral inhibition model of antinociceptive responses.

Modulation of cellular immune responses by cold water swim stress in the rat

Alterations of cell-mediated immune responses in the rat produced by 5-day (one 3-min stress session each day for 5 days) and 1-day (three 3-min stress sessions within 12 h) cold water stress administration were investigated. Mitogenic responses to concanavalin A (Con A) and lipopolysaccharide (LPS), interleukin-2 (IL-2) production, CD4+/CD8+ T-cell ratios, and natural killer (NK) cell activity of blood and spleen lymphocytes were increased by the 5-day cold water stress. Responses to Con A and LPS, IL-2 production, and CD4+ and CD8+ percentages of blood and spleen lymphocytes were decreased by the 1-day cold water stress. Corticosterone levels were increased by both the 1-day and 5-day cold water stress. Cold water stress, as a natural stressor, may have its own unique pattern of neuroendocrine changes because of the accompanying body temperature variations that may influence immune functions.

Characterization of pituitary mediation of stress-induced antinociception in rats

Antinociception, induced by continuous cold-water swims (CCWS) and certain parameters of inescapable foot shock, is reduced in hypophysectomized rats receiving supplements of corticosterone and l-thyroxine. To assess which lobe of the pituitary gland is involved in this effect, the first experiment compared the effects of total hypophysectomy and posterior lobectomy in supplemented rats upon CCWS antinociception on the tail-flick and jump tests and upon continuous inescapable foot shock antinociception on the tail-flick test. Total hypophysectomy, but not posterior lobectomy, significantly reduced CCWS antinociception on both tests in supplemented rats relative to sham surgery. Both total and posterior hypophysectomy either reduced or potentiated foot shock antinociception as functions of shock intensity or duration of exposure in supplemented rats. To assess whether hormonal supplementation is necessary for the observed effects, the second experiment examined CCWS antinociception in sham-operated and hypophysectomized rats that received either no hormonal supplements or corticosterone and/or l-thyroxine. These regimens failed to alter CCWS antinociception in sham-operated rats. Treatment of hypophysectomized rats with corticosterone and l-thyroxine either separately or together significantly reduced CCWS antinociception. In contrast, if hypophysectomized rats did not receive supplements, CCWS antinociception was significantly potentiated relative to sham-operated controls. These effects could not be attributed to treatment-induced changes in either body weight or CCWS hypothermia. These data suggest that the anterior lobe of the pituitary gland and adrenal cortex are involved in the mediation and/or maintenance of CCWS antinociception.

Prolonged inflammatory pain modifies corticotropin-releasing factor-induced opioid peptide release in the hypothalamus

The influence of prolonged pain upon hypothalamic opioid peptide release in vitro was examined in rats subjected to Freund's adjuvant (FA)-induced unilateral inflammation of the hindlimb. Basal release of enkephalin (ENK) but not beta-endorphin (END) or dynorphin (DYN) was increased 10 days following FA treatment. Superfusion of corticotropin-releasing factor (CRF; 10(-8) M) stimulated the release of opioid peptides in control hypothalami. CRF, however, failed to modify beta-END and DYN release in hypothalami of FA-treated rats, whereas ENK release was markedly reduced. In contrast, KCl-stimulated opioid peptide release did not differ between FA and control hypothalami. These data demonstrate that prolonged inflammatory pain alters the responsiveness of hypothalamic opioid systems to CRF. It is suggested that this effect is mediated at the level of the CRF neuron or its receptor.

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.

Organization of peptidergic and catecholaminergic efferents from the nucleus of the solitary tract to the rat amygdala

Previous studies have focused on the role of the central nucleus of the amygdala (CeA) in cardiovascular and other amygdaloid functions. The combined retrograde tracing/immunohistochemical method was used to test for the presence of enkephalin, neurotensin, neuropeptide Y, and catecholamine neurons within the nucleus of the solitary tract that send efferents to the CeA. After injections of retrograde tracer into the CeA, retrogradely labeled neurons were observed within the caudal, medial nucleus of the solitary tract. Most CeA-projecting neurons were located ipsilaterally within the medial nucleus of the solitary tract at the level of the area postrema. Retrogradely labeled enkephalin- and neurotensin-immunoreactive neurons were found within the medial nucleus of the solitary tract at this level, while retrogradely labeled neuropeptide Y-immunoreactive neurons were found within the medial nucleus of the solitary tract rostral to the area postrema. About 60-74% of CeA-projecting cells were also immunoreactive for tyrosine hydroxylase. Approximately 9% of retrogradely neurons were phenylethanolamine-N-methyltransferase immunoreactive. The results provide evidence that within the nucleus of the solitary tract, peptidergic CeA-projecting neurons have a topographic distribution. In addition, noradrenergic neurons within the A2 group, rather than adrenergic neurons of the C2 group, provide the bulk of catecholaminergic input to the CeA from the nucleus of the solitary tract. Cell counts indicate that each of these peptides may be colocalized (to varying extents) within catecholamine-producing neurons. Also the catecholaminergic and enkephalinergic contribution to the ascending pathway from the nucleus of the solitary tract to the CeA distinguishes it neurochemically from the descending pathway. Thus, although there are afferent and efferent connections between the nucleus of the solitary tract and CeA, their peptidergic/neurotransmitter connections are not necessarily reciprocal. Input from nucleus of the solitary tract peptidergic and catecholaminergic neurons to the CeA may be important in the etiology of a number of pathophysiological conditions including hypertension, gastric ulcers, and schizophrenia.

An opiate mechanism involved in conditioned analgesia influences forced swim-induced immobility

The modulatory effect of conditioned opiate analgesia on immobility during a forced swim situation was studied. Animals submitted to inescapable shock (IS) were exposed 6 days later to a similar or different shock application context and, immediately after, tested in either hot plate test or forced swim test. A conditioned analgesia was observed only on animals submitted to the shock context. This conditioned analgesia was blocked by naloxone administration injected either before IS or before context exposure. In the same way, animals exposed to shock context and immediately forced to swim showed an increase in the immobility time, which was sensitive to naloxone injection before IS as well as before context exposure. These results and additional data referring to naloxone effect on inactivity during IS are discussed in terms of the possible role of endogenous opiate in analogous behavior expressed during different aversive experiences.

Is spinal cord dorsolateral funiculus involved in hypoalgesia induced by counter-irritation?

Several previous studies have demonstrated that, depending upon the behavioral test used, counter-irritation (i.e. the pain-relieving effects of pain elicited from heterotopic body areas) can produce hypoalgesia. In the present study, behavioral responses were elicited in the rat by increasing calibrated pressure applied to a hindpaw (Randall-Selitto test; 'passive' stimulus) and were studied before and after a subcutaneous formalin injection ('active' stimulus). The vocalization threshold to the pressure was clearly increased after injection of the algogenic solution either in the forepaw or in the cheek. Using this vocalization threshold, the counter-irritation-produced hypoalgesia was generally unchanged by unilateral dorsolateral funiculus (DLF) lesions. Following bilateral DLF lesions, hypoalgesia was decreased when formalin was injected in the forepaw, but was unaffected when the algogen was injected in the cheek. The present results partly contrast with previous papers from our group, where it has been assumed that the DLF is mainly involved in the neural circuitry subserving diffuse noxious inhibitory controls (DNIC), which have been considered as one possible neurophysiological basis for counter-irritation phenomena. They are discussed with reference to various hypotheses, including DNIC, as explanations for counter-irritation.

Antagonism of morphine analgesia by nonopioid cold-water swim analgesia: direct evidence for collateral inhibition

The demonstrated existence of opioid and nonopioid forms of pain control has raised questions as to how they interact. Previous indirect evidence suggests that activation of one system inhibited the activation of the other. The present study assessed this directly using morphine as an opiate form of analgesia and continuous cold-water swims (CCWS, 4 degrees C, 2 min) as the nonopioid form. A significant reduction in morphine (8 mg/kg, SC) analgesia on the tail-flick test was observed if rats were acutely exposed to CCWS immediately prior to morphine administration. The inability of naloxone (10 mg/kg, SC) to reduce CCWS analgesia verified its nonopioid nature. The antagonism of morphine (3 mg/kg, SC) analgesia was greater following preexposure to 2 min of CCWS than 1 min of CCWS. CCWS was also more effective in antagonizing analgesia induced by the 3 mg/kg than the 8 mg/kg dose of morphine. The antagonism of morphine analgesia by CCWS was dependent upon the temporal patterning of stimulus presentation: exposure to CCWS 20 or 60 min prior to morphine failed to alter subsequent morphine analgesia. A significant reduction in analgesia induced by intraperitoneal administration of morphine (10 mg/kg) was also observed when CCWS was presented immediately prior to injection, suggesting that pharmacokinetic factors such as altered drug absorbance by CCWS-induced vasoconstriction do not appear to explain these effects. These data provide direct support for the existence of collateral inhibitory mechanisms activated by CCWS and morphine, and suggests that these opioid and nonopioid forms of analgesia do not function synergistically, but instead involve some form of hierarchical order.

The role of pituitary beta-endorphin in mediating corticotropin-releasing factor-induced antinociception

The hypothesis that blood-borne beta-endorphin modulates nociception was examined with corticotropin-releasing factor (CRF) as a potent and selective agent to stimulate its release from the pituitary gland. Intravenously administered CRF produced a dose-related antinociception in rats as determined by measuring paw-lick latencies on a 50 degrees C hot plate. A dose of 25 nmol/kg of CRF was comparable in both magnitude and duration of antinociception to a 7,500 nmol/kg (= 2.5 mg/kg) dose of morphine sulfate. The antinociceptive effect of CRF was blocked by both hypophysectomy and dexamethasone pretreatment, suggesting that it was mediated by hormone release from the anterior pituitary corticotrophs. Furthermore, the effect of CRF was antagonized by 1) naltrexone, 2) naltrexone methyl bromide, and 3) passive immunization with anti-beta-endorphin antiserum. Together, these data support the hypothesis that opiate-active, beta-endorphin, released by pituitary corticotrophs, participates in the physiological modulation of nociception in rats.

Antinociception in the rat induced by a cold environment

Rats placed in a cold environment (4 degrees C) for 2 h had a sustained increase in tail flick latency (TFL) as well as an increase in tail pinch latency (TPch) that was often biphasic with an early peak response at 15 min and a later, often higher, peak at 2 h. Plasma beta-endorphin levels after a modest increase at 5 min (24%) declined throughout the remaining time in the cold. The long-acting opioid antagonist naltrexone had no effect on TFL increases but led to greater increases in TPch (P less than 0.04). In morphine-tolerant rats TFL response was the same as in controls but TPch increases were greater (P less than 0.04). Rats exposed to 2 h of cold for 17 or 18 consecutive days generally developed tolerance to the analgesia of cold, i.e. TFL and TPch increases were diminished; however, the response to morphine on day 18 was the same as in rats never exposed to cold. Adrenalectomy and hypophysectomy led to significantly smaller increases in TFL (P less than 0.02 and P less than 0.001, respectively). The TPch response in contrast, was greater in adrenalectomized (P less than 0.001) and the same in hypophysectomized rats compared to sham controls. An opioid kappa receptor antagonist (Mr 1452) given prior to cold reduced both TFL and TPch response during the first hour. Thus the analgesia induced by cold appeared to shift from an early possibly kappa opioid to a later non-opioid form. The TFL effects seemed to be under hormonal influence while the TPch were not.

Dissociation of opioid and nonopioid analgesic responses following adult monosodium glutamate pretreatment

Neonatal administration of monosodium glutamate (MSG: 2-4 mg/g, SC) selectively destroys circumventricular organs, especially the arcuate nucleus and median eminence of the hypothalamus, and also attenuates both nonopioid (continuous cold-water swim: CCWS) and opioid (morphine) analgesia when rats are tested as adults. The present study evaluated whether administration of MSG (1-6 g/kg, SC) or its equiosmotic control (2.37 M NaCl) to adult rats altered either basal nociception on the tail-flick and jump tests or analgesia following morphine (5 mg/kg, SC) or CCWS (2 degrees C for 3.5 min). MSG treatment dose-dependently produced small but significant increases in basal nociceptive thresholds in adult rats. Morphine analgesia was significantly reduced on both tests following pretreatment with MSG (30-32%) and hypertonic NaCl (17-25%). In contrast, MSG (55-247%), but not NaCl pretreatment potentiated both nonopioid CCWS analgesia on both tests and CCWS hypothermia. These data are discussed in terms of differential neonatal and adult MSG effects, dissociations between opioid and nonopioid pain-inhibition, and the role of MSG in altering adaptive mechanisms to environmental stressors.

Stress induced analgesia plays an adaptive role in the organization of behavioral responding

The data on ability of stressful or noxious stimuli to suppress the perception of pain was reviewed. The focus of this review has been the attempt to demonstrate that the emergent "Stress Induced Analgesia" (SIA) plays an adaptive role in the modulation of behavioral responding by organisms during periods of threat or danger to the organism. We have reviewed the experimental paradigms that normally used in these studies which point to the fact that the variables inducing SIA need not be actually aversive or even stressful. We also reviewed the data on the mechanisms of SIA and suggested that both opioid and nonopioid mechanisms are involved in the mediation of SIA and that these mechanisms are at least semiindependent and subject to differential conditioning. Finally, we have described a series of experiments carried out in our laboratory where the induction of SIA interacted with behavioral performance in an inverted U shape function, low levels of stress facilitated responding and learning while high levels disrupted responding. We argued that taken together, the effects of SIA seem to be highly adaptive in that it allowed animals to deal with a dangerous and threatening situation in a manner which increased the organism's chance of survival.

Analgesia induced by continuous versus intermittent cold water swim in the rat: differential effects of intrathecal administration of phentolamine and methysergide

Continuous cold water swim produces analgesia that is partially mediated by a noradrenergic mechanism, but is independent of both serotonergic and opioid systems. On the other hand, intermittent cold water swim elicits analgesia which is partly mediated by an opioid mechanism; the contribution of the monoamines to the production of this analgesia is not known. Therefore, the present study was done to determine whether intermittent cold water swim is also mediated by noradrenergic and/or serotonergic substrates. Prior to either continuous (3.5 min) or intermittent (10 sec in, 10 sec out for 6 min) cold water (4 degrees C) swim, male Sprague-Dawley rats (225-250 g) were administered either the noradrenergic receptor blocker phentolamine (30 micrograms), the serotonergic blocker methysergide (30 micrograms) or artificial cerebrospinal fluid to the fifth lumbar vertebral spinal level via chronic intrathecal catheters. Phentolamine significantly attenuated the analgesia resulting from both continuous and intermittent cold water swim. Methysergide attenuated intermittent cold water swim analgesia, but was without effect on continuous cold water swim analgesia. Phentolamine, but not methysergide, also attenuated continuous footshock- (2.5 mA for 3 min) induced analgesia. The similarity between the effects of phentolamine and methysergide on continuous footshock and continuous cold water swim analgesia suggests that the effects of these drugs on cold water swim analgesia are not attributable to changes in thermoregulation. These results suggest that a spinal noradrenergic mechanism is involved in the mediation of both forms of cold water swim analgesia, whereas a spinal serotonergic mechanism is involved in only intermittent cold water swim analgesia.

Potentiation of opioid and nonopioid forms of swim analgesia by cimetidine

Antagonism of the H-2 receptor with cimetidine and other histaminergic receptor antagonists has been used to differentiate nonopioid and opioid forms of footshock analgesia which are mediated by neural mechanisms. Cimetidine reduces nonopioid footshock analgesia while potentiating an opioid form of this analgesia. The present study examined whether cimetidine altered the nonopioid, neurohormonal analgesia induced by either continuous cold-water swims (CCWS: 2 degrees C for 3.5 min) or the opioid analgesia induced by intermittent cold-water swims (ICWS: 2 degrees C, 18 10-sec swims, 18 10-sec recovery periods). Vehicle or cimetidine (10, 50, 100 mg/kg) injections were administered alone or paired with either CCWS or ICWS; tail-flick latencies, jump thresholds and core body temperatures were then measured. Cimetidine (100 mg/kg) significantly potentiated CCWS and ICWS analgesia and hypothermia, while having minimal effects upon basal thresholds. Lower cimetidine doses produced transitory effects on these measures. These data demonstrate dissociations between neural and neurohormonal forms of nonopioid analgesia following cimetidine treatment. The latter effect may be attributed to changes in stress responsiveness or thermoregulation rather than pain inhibition.

Gender-specific and gonadectomy-specific effects upon swim analgesia: role of steroid replacement therapy

Both gender-specific and gonadectomy-specific effects have been observed for the analgesic responses following continuous and intermittent cold-water swims (CCWS and ICWS respectively): female rats display significantly less analgesia than males, and gonadectomized rats display significantly less analgesia than sham-operated controls. The present study evaluated the effects of steroid replacement therapy with testosterone propionate (TP: 2 mg/kg, SC) upon CCWS and ICWS analgesia on the tail-flick and jump tests and hypothermia in sham-operated or gonadectomized male and female rats. Thirty days following surgery, rats received either no treatment, a sesame oil vehicle or TP for 14 days prior to, and then during testing. Relative to the no treatment condition, repeated vehicle injections in sham-operated rats eliminated the gender-specific, but did not affect the gonadectomy-specific effects upon CCWS and ICWS analgesia. TP reversed the deficits in CCWS and ICWS analgesia observed in both castrated and ovariectomized rats on both pain tests. TP only potentiated CCWS analgesia in sham-operated males on the tail-flick test. TP potentiated CCWS and ICWS hypothermia in gonadectomized rats and in male sham-operated rats. These data indicate that gonadal steroids play a major modulatory role in the etiology of swim analgesia, and that the observed gender effects are sensitive to possible adaptational variables.

Corticotropin-releasing factor (CRF) produces analgesia in humans and rats

The analgesic activity of corticotropin releasing factor (CRF) was determined in a clinical model and in the rat hot plate test. Patients administered CRF reported significantly less postoperative pain than patients pretreated with placebo. In rats, injection of CRF resulted in a significant analgesia which was comparable in both intensity and duration to a 300 times greater molar dose of morphine. These findings suggest that endogenous CRF may play a physiologic role in modulating pain when released under conditions of stress.

Neuromodulatory effects of TRH upon swim and cholinergic analgesia

In addition to short-acting analgesic actions by itself and modulation of analgesic responses induced by endogenous opioids and neurotensin, central administration of thyrotropin-releasing hormone (TRH) potentiates footshock analgesia. The present study evaluated the effects of TRH upon the neurohormonally-mediated though nonopioid analgesia induced by swims in rats. Intracerebroventricular TRH (10 and 50 micrograms) dose-dependently potentiated swim (21, 15, 2 degrees C baths) analgesia on the tail-flick test, an effect which was not due to the hypothermic or basal pain threshold changes. Intravenous (8 mg/kg) TRH potentiated swim (21 degrees C) analgesia; the 600:1 difference in potency between routes strongly suggests central sites of neuromodulatory action. Intracerebroventricular diketopiperazine (50 micrograms), a TRH metabolite, and RX77368 (50 micrograms), a TRH analogue, also potentiated swim (21 degrees C) analgesia, effects also independent of hypothermia and basal reactivity to pain. Finally, given the excitatory interaction between TRH and acetylcholine as well as the cholinergic involvement in swim analgesia, intracerebroventricular TRH potentiated pilocarpine (10 mg/kg, IP) analgesia.

Modulation of gender-specific effects upon swim analgesia in gonadectomized rats

Gender-specific effects have been observed for continuous and intermittent cold-water swim (CCWS and ICWS respectively) analgesia: analgesic magnitudes following CCWS and ICWS are significantly smaller in female rats than in age-matched and weight-matched male rats. The present study evaluated the role of gonadal status in these gender-specific effects by examining CCWS and ICWS analgesia, hypothermia and activity in intact and gonadectomized rats. Following confirmation of the original gender-specific effects on the tail-flick and jump tests, it was found that both castration and ovariectomy significantly reduced CCWS and ICWS analgesia. Indeed, castrated males displayed similar magnitudes of analgesia as intact females. The more marked hypothermia observed in intact females indicated that this variable failed to account for the analgesic gender-specific effects. The reduced hypothermia following gonadectomy also failed to account for the analgesic changes. The increased activity during ICWS, but not CCWS following gonadectomy also did not account for the analgesic changes. These data suggest that gonadal steroids normally appear to facilitate these stress-related analgesic responses.

Adrenalectomy and dexamethasone differentially affect postswim antinociception in mice selectively bred for high and low stress-induced analgesia

The effects of dexamethasone and naloxone on analgesia induced by swimming (3 min, 20 degrees C) were studied in the 6th and 7th generations of adrenalectomized and intact mice selectively bred for high (HA) and low (LA) postswim analgesia. Swim-induced analgesia in intact HA animals was significantly reduced by naloxone and dexamethasone while in LA mice these two compounds were ineffective. Naloxone ability to reverse adrenalectomy-caused swim analgesia increase was much greater in HA than in LA mice. In both intact and adrenalectomized HA animals dexamethasone and naloxone decreased postswim analgesia to the level observed in LA mice. It is suggested that selective breeding for high and low swim analgesia modified the extent of pituitary-adrenal axis involvement in the generation of stress-induced analgesia.

Reduction in cold-water swim analgesia following hypothalamic paraventricular nucleus lesions

The analgesic response following cold-water swims (CWS) has been shown to be mediated through neurohormonal mechanisms and independently of opioid systems. The hypothalamic paraventricular nucleus (PVN) appears to be important in autonomic, hypophysial and medial-basal hypothalamic function. The present study examined whether lesions placed in the PVN in rats would alter CWS analgesia on the tail-flick test. Animals with lesions placed in the PVN displayed significant reductions in analgesic magnitude 30 (66%) and 60 (54%) min following CWS relative to sham-treated rats without alterations in baseline latencies. In contrast, CWS analgesia was not altered in animals with lesions placed dorsal and/or lateral to the PVN. These data are discussed in terms of the roles of PVN projections to the median eminence and brainstem/spinal structures as well as roles for neuropeptides in the PVN.

The analgesia produced by food deprivation in 4-month old, 14-month old, and 24-month old rats

The analgesia produced by 24 hr of food deprivation was examined in 4-mo, 14-mo, and 24-mo old rats. To assess opioid and hormonal involvement in food deprivation induced analgesia, different groups of rats from each age group were injected with naltrexone (7 mg/kg), dexamethasone (0.4 mg/kg), or equivolume saline. Results revealed that food deprivation produced an equivalent analgesic response in each saline-treated age group. Also, naltrexone and dexamethasone were equally potent in blocking food deprivation induced analgesia in each age group. These results demonstrated that food deprivation activates an endogenous opioid-mediated analgesic system that involves hormonal factors and that this system does not change in function with increasing age.

Effects of muscarinic receptor antagonism upon two forms of stress-induced analgesia

The present study assessed in rats the effects of muscarinic receptor antagonism upon analgesia induced by cold-water swims (CWS: 2 degrees C for 3.5 min) and 2-deoxy-D-glucose (2DG: 600 mg/kg). First, CWS analgesia was significantly reduced 30 min after the swim by scopolamine (0.01 and 0.1 mg/kg) and methylscopolamine (10 mg/kg) pretreatment, and was eliminated 60 min after the swim by scopolamine (0.01-10 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment. In contrast, scopolamine potentiated CWS hypothermia. Second, while scopolamine (1 mg/kg) and methylscopolamine (1,10 mg/kg) pretreatment prolonged 2DG analgesia, both antagonists dose-dependently reduced 2DG hyperphagia. Third, the changes in analgesic and hypothermic stress responses were not due to baseline shifts in jump thresholds or body temperatures. However the dose-dependent reductions by scopolamine and methylscopolamine in baseline food intake and 2DG hyperphagia were significantly correlated. Fourth, the dose-dependent reduction by scopolamine and methylscopolamine of pilocarpine analgesia differed in pattern from the other analgesic effects, suggesting heterogeneity in muscarinic receptor modulation of different analgesic responses.

Dissociated effects of apomorphine on various nociceptive responses in mice

The effects of increasing doses of apomorphine were studied in mice in six nociceptive tests: (1) withdrawal of the tail immersed in hot water, (2) vocalization induced by the electrical stimulation of the tail, (3) tail flick using a radiant beam, (4) withdrawal of a tail-clip, (5) writhing induced by the i.p. injection of phenylbenzoquinone, (6) forepaw licking and jump latencies on a hot plate. Only in the tests (5) and (6), an apparent analgesia was obtained. Differences were observed between tests (5) and (6), as to: (i) the apparent relative effectiveness of (-)sulpiride (more effective in test [5] than in test [6]), (ii) the naloxone-induced modifications of apomorphine effects (whereas naloxone antagonized apomorphine effects in test [6], it did not in test [5]), (iii) the decrease of apomorphine-induced responses by prevention of hypothermia (in test [6] but not in test [5]). These data suggest that APO-induced increased jump latencies are at least partly related with hypothermia and endogenous opioid systems, whereas APO effect on the writhing test depends on the stimulation of dopamine receptors particularly sensitive to sulpiride and is independent from body temperature and opioidergic transmissions.