Stress-produced analgesia and morphine-produced analgesia: lack of cross-tolerance

Organismic variables and pain inhibition: roles of gender and aging

Multiple pain-inhibitory systems dependent upon both opioid and nonopioid mechanisms of action have been identified, particularly in the rodent. The experimental subject has typically been the young, adult male rat, and generalizations concerning these systems have been made from this subject pool. This review focuses upon the roles of two organismic factors, aging and gender, in the modulation of analgesic processes. Using an array of age cohorts (4, 9, 14, 19, 24 months), these data illustrate that aging produces differential decrements in the analgesic responses following morphine, different parameters of footshock, continuous cold-water swims (CCWS: a nonopioid stressor), intermittent cold-water swims (ICWS: an opioid stressor) and 2-deoxy-D-glucose (a mixed opioid/nonopioid stressor). In contrast, neither beta-endorphin nor food deprivation analgesia is affected by aging. This review identifies that CCWS and ICWS analgesia are sensitive to gender differences, gonadectomy differences and steroid replacement differences such that females display less analgesia than males, gonadectomy reduces both analgesic responses, and that testosterone is most effective in reinstating gonadectomy-induced analgesic deficits. These data are considered in terms of therapeutic implications for the organismic variables under study as well as for the conceptual and methodological modifications that must be made in studying intrinsic pain inhibition.

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.

Lack of effect of adrenal denervation on analgesia elicited by continuous and intermittent cold water swim in the rat

The present experiment was conducted to determine if continuous and/or intermittent cold water swim analgesia is dependent upon a factor released from the adrenal medullae. Male Sprague-Dawley rats (300-350 g) underwent either bilateral adrenal denervation or sham surgery and two weeks later were exposed to continuous (3.5 m) or intermittent (10 s in, 10 s out for 6 min) swim in water at 4 degrees C. Adrenal denervation failed to affect either intermittent or continuous cold water swim analgesia. It was concluded that both types of cold water swim analgesia are independent of adrenal medullary function.

Gender determinants of opioid mediation of swim analgesia in rats

Continuous cold-water swims (CCWS) and intermittent cold-water swims (ICWS) elicit respective nonopioid and opioid analgesic responses in adult male rats. The present experiment evaluated whether gender differences were observed in naloxone's (14 mg/kg, SC) ability to alter differentially CCWS and ICWS analgesia on the tail-flick and jump tests in age-matched and weight-matched intact rats and in gonadectomized rats. CCWS analgesia was unaffected by naloxone on either test in age-matched males and females. Naloxone significantly reduced ICWS analgesia on the tail-flick (45%) and jump (37%) tests in intact males, but not age-matched females. Naloxone significantly reversed ICWS analgesia in weight-matched males on the tail-flick (1-14 mg/kg, 30-32%) and jump (14 mg/kg, 31%) tests. Naloxone also significantly reduced ICWS analgesia on the tail-flick (32%) and jump (41%) tests in castrated males, but not ovariectomized females. Changes in swim hypothermia could not account for the above effects. These data indicate gender differences in naloxone's differential modulation of swim analgesia, and reflect further differences in pain-inhibitory responses as a function of gender.

Analgesia or hyperalgesia following stress correlates with emotional behavior in rats

Analgesia following exposure to a variety of noxious and non-noxious stressors is well documented and is commonly referred to as stress-induced analgesia. Hyperalgesia following stress has also been reported. The present study shows that a mild stressor (15 min of vibration) produced increased tail-flick latencies (TFL) in some rats, but decreased latencies in other rats. The results of the individual subjects were reproduced in a later session: the rats showing increased TFL on day 1, responded with increased TFL on day 2. Rats showing decreased TFL on day 1 responded with decreased TFL on day 2. Whichever reaction occurred, analgesia or hyperalgesia, this correlated with the animal's behavior during the stress procedure. Analgesia was produced in quiet rats and hyperalgesia in hyperemotional ones. Various peripheral nerve stimulation procedures producing hyperemotional reactions also resulted in lowering of the pain threshold. The results of the present study show behavioral modulation of pain mechanisms.

Noradrenergic mechanisms in mediation of stress-induced hyperalgesia in rats

Exposure to a mild stressor (15 min of vibration) produced analgesia in some rats and hyperalgesia in other rats from the same batch treated in the same way. Rats which responded with decreased tail-flick latencies (TFL) showed signs of hyperemotionality during the stress procedure. Stress-induced hyperalgesia was abolished by the administration of diazepam (2.5 mg/kg i.p.) and clonidine (25 micrograms/kg i.p.). It is suggested that the reversal of hyperalgesia was due to the anxiolytic properties of the drugs. Yohimbine, an alpha 2-adrenoceptor antagonist (5 mg/kg i.p.), antagonized the effect of clonidine. The influence of clonidine on stress-induced hyperalgesia may be mediated by alpha 2-adrenoceptors.

Day-night rhythms of opioid and non-opioid stress-induced analgesia: differential inhibitory effects of exposure to magnetic fields

Day-night rhythms occurred in the naloxone-reversible (1.0 mg/kg), warm (opioid) and naloxone-insensitive, cold (non-opioid) swim stress-induced analgesia displayed by CF-1 mice. Maximum antinociceptive responses were evident at night, with the cold stress having significantly greater day- and night-time analgesic effects than the warm stress. An exposure for 30 min to a 0.5 Hz rotating magnetic field (1.5-90 gauss) reduced both the warm and cold stress-induced analgesia, with the magnetic stimuli having significantly greater inhibitory effects at night and on the opioid-induced responses. These results indicate that exposure to oscillating magnetic fields can significantly, and differentially, alter both opioid and non-opioid stress-induced analgesia and their day-night rhythms.

Responsivity to pain in rats changed by the ingestion of flavored water

Sprague-Dawley male rats drank flavored water and subsequent pain responsivity was assessed using a hot plate first after isotonic saline and later after either 2.75 (Experiment 1) or 2.5 (Experiment 2) mg/kg of morphine hydrochloride was injected. A 48-h exposure to any one of several different flavors resulted in a reduction of the analgesic effects of morphine as shown by an attenuation of the increased latency to paw lick caused by the morphine. This effect was independent of the amount consumed. A 26-h exposure to a flavor decreased pain reactivity before and after morphine as shown by an overall increase in latency to paw lick. This effect was not influenced by whether the normally preferred flavor was made aversive by a previous pairing with lithium chloride-induced illness. These findings are consistent with the idea that many flavors, independently of palatability and amount consumed, increase the release and utilization of endogenous opioids.

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.

Evidence for opioid and non-opioid forms of stress-induced analgesia in the snail, Cepaea nemoralis

Exposure to either cold or warm stress increased the thermal nociceptive thresholds of the terrestrial snail, Cepaea nemoralis. The warm stress-induced 'analgesia' was blocked by the prototypic opiate antagonist, naloxone, and the delta-opiate antagonist, ICI 154,129, and was suppressed by a 24-h pretreatment with the irreversible opiate antagonist, beta-funaltrexamine (B-FNA). In contrast, cold stress-induced analgesia was unaffected by either naloxone, ICI 154,129 or B-FNA. These results indicate that this mollusc displays both opioid and non-opioid forms of stress-induced analgesia in a manner analogous to that reported for mammals. These findings suggest an early evolutionary development and phylogenetic continuity of opioid and non-opioid mediated stress responses to aversive environmental stimuli.

Swim stress reduces chronic pain in mice through an opioid mechanism

Chronic nociception has been studied in male mice by means of the formalin test in which forelimb motor behaviour is scored after subcutaneous formalin injection. The rating remained above 2.0 for 30 min after the injection (scale range 0-3). The magnitude of the nociception has been compared with that reported in other animal types. Mice are more sensitive than rats, cats and monkeys. The stress of a swim of 3 min has been found to reduce nociception by up to 25%. This analgesia is wholly opioid in nature, being abolished by a moderate dose of naloxone (1 mg/kg).

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.

Multiple opiate receptors and pharmacological response to morphine in rats maintained on diets differing in protein concentration

1. The influence of diets differing in protein concentration on the characteristics of mu, delta, and kappa-opiate receptors and on the analgesic and hyperthermic responses to morphine was examined in rats. Three groups of male Sprague-Dawley rats were maintained for four weeks on isocaloric diet containing either 4, 20 or 50% protein. 2. The animals maintained on 4% protein diet weighed 92 +/- 2% of the initial weight at the end of the fourth week, whereas animals maintained on 20% and 50% protein diet weighed 222 +/- 2% and 221 +/- 2%, respectively. The average food intake per 100 g body weight on day 1 of the study in 4, 20 and 50% protein diet group was 5.0 +/- 2.2 g, 10.4 +/- 1.4 g, and 10.0 +/- 1.2 g, respectively. This difference in food intake was not observed during rest of the period of the study. 3. Water intake was higher for the animals maintained on diet containing 50% protein as compared to the other two groups. 4. The analgesic and the hyperthermic response to morphine varied in direct relation to the concentration of protein in the diet. The concentration of morphine in the brain and plasma of animals maintained on the three diets following challenge dose of morphine did not differ. Similarly the Bmax and Kd values for the binding of [3H]naltrexone, [3H]D-Ser2-Thr6-leucine enkephalin and [3H]ethylketocyclazocine to brain membranes prepared from rats kept on 4, 20 and 50% protein concentration did not differ. 5. It is concluded that the diet differing in protein concentration can alter the responses to morphine, and that such altered effects can not be accounted for by the changes in the distribution of morphine in brain and plasma or to the changes in the characteristics of the mu, delta and kappa opiate receptors in the brain.

MIF-1 and Tyr-MIF-1 antagonize morphine and opioid but not non-opioid stress-induced analgesia in the snail, Cepaea nemoralis

The effects of prolyl-leucyl-glycinamide (MIF-1, PLG), tyrosine-prolyl-leucyl-glycinamide (Tyr-MIF-1, YPLG) and naloxone on morphine and warm and cold stress-induced increases in the latency of the thermal (40 degrees C hot plate) avoidance behaviors of the terrestrial snail, Cepaea nemoralis, were examined. All three substances blocked the morphine- and warm stress-induced opioid analgesia, while having no effects on non-opioid cold stress-induced analgesia. Tyr-MIF-1 had a significantly greater inhibitory effect than MIF-1. These results indicate that MIF-1 and Tyr-MIF-1 antagonize the antinociceptive effects of exogenous opiates and opioid-mediated analgesia in snails in a manner analogous to that described for mammals. This raises the possibility of an evolutionary conservation of functional opioid antagonists.

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.

Arcuate nucleus lesions reduce opioid stress-induced analgesia (SIA) and enhance non-opioid SIA in rats

When rats were tested more than two weeks following surgery, lesions of the medial basal hypothalamus centered on the arcuate nucleus enhanced a form of foot-shock stress-induced analgesia (SIA) that was not blocked by injections of the opiate receptor blocker, naltrexone (6 mg/kg;). These arcuate nucleus lesions reduced the SIA produced by the same stressor when similar rats were tested 3-4 days following surgery. Finally, when similar rats were tested more than 2 weeks following surgery these lesions reduced a different form of SIA that was blocked by naltrexone. There were no effects of the lesions or naltrexone on baseline pain reactivity in any of the experiments. We suggest that arcuate nucleus lesions disrupt a system important for the elaboration of opiate-mediated SIA (Expt. 4), perhaps by damaging the brain's beta-endorphin system. In response to damage to this opioid analgesic system, we hypothesize that the damaged brain initiates time-dependent compensatory changes in an undamaged non-opioid analgesic system, resulting in enhanced non-opiate-mediated SIA.

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.

Age-related decrements in morphine analgesia: a parametric analysis

Although age-related reductions in levels of opiate receptors and endogenous opioid peptides have been observed in rats, effects of aging upon basal pain thresholds and morphine analgesia have not indicated clear results. The present study evaluated the dose-dependent (1,2.5, 5 and 10 mg/kg, SC) and time-dependent (30, 60, 90, 180 min) properties of morphine analgesia on two nociceptive measures (tail-flick latencies and jump thresholds) across five age cohorts of rats (4, 9, 14, 19 and 24-months of age). To ascertain whether any changes were the result of specific alterations in pain inhibition or an overall shift in opiate responses, effects upon morphine-induced hyperthermia were also evaluated. Age-related effects upon morphine analgesia on the tail-flick test exhibited a biphasic pattern with the three older age groups displaying significant decreases in morphine analgesia 30 min after injection and significant increases in morphine analgesia 180 min after injection. Age-related effects upon morphine analgesia on the jump test revealed equivilent reductions in analgesic magnitude across doses for the three older cohorts. In contrast, morphine hyperthermia displayed small inconsistent changes across cohorts. The reductions in morphine analgesia in older animals could not be attributed to changes in either baseline pain thresholds or delayed peripheral absorption and/or clearance of the drug. Rather, the reductions in morphine analgesia in older animals complement the recent findings of similar age-related reductions in the analgesic responses induced by exposure to several environmental stressors.

Opioid and non-opioid stress analgesia from cold water swim: importance of stress severity

We have previously reported that stress severity plays an important role in determining the neurochemical basis of stress-induced analgesia from inescapable footshock. Increasing severity (duration or intensity of continuous footshock) causes a shift in mediation of the resultant analgesia from opioid to non-opioid. In this study, we find that stress severity plays a similar role in analgesia from cold water swim. More severe swims (longer duration or lower water temperature) produce stress analgesia insensitive to the opiate antagonist, naltrexone, whereas less severe swims produce analgesia significantly attenuated by this drug.

Age-related decrements in the analgesic response to cold-water swims

Among the compromised physiological responses affected by aging is an impaired ability to thermoregulate in a cold environment. Since acute exposure to cold-water swims (CWS) produces both analgesic and hypothermic responses in young adult rats, the present study examined whether systematic variations in these responses occurred as a function of age. Separate cohorts of 4, 9, 14, 19 and 24-month old female rats received a no-swim condition and a 2 degrees C swim for 3.5 min with tail-flick latencies, jump thresholds and core body temperature assessed 30, 60 and 90 min later. The order of conditions was counterbalanced with an interval of four days between conditions. While the four younger cohorts displayed similar CWS analgesia on the tail-flick test, the 24-month cohort failed to display CWS analgesia on this measure. Age-related differences in CWS analgesia on the jump test appeared more gradual with the three older cohorts displaying significant attenuations relative to the 9-month group, and the oldest cohort displaying significant attenuations relative to the 4-month group. In contrast, the hypothermic effect of CWS was significantly potentiated in the three oldest cohorts, with a progressive inability to cope with thermoregulation observed as a function of age. The observed decrements in the analgesic response to CWS as a function of age appear to represent a change in (a) the ability of the animal to perceive the CWS stimulus as stressful (b) an endogenous pain-inhibitory system and/or (c) an endogenous pain transmission system that is independent of thermoregulatory mechanisms.

Use of hypothermia for general anesthesia in preweanling rodents

Documentation and rationale are provided for the use of deep hypothermia alone for anesthesia during surgical manipulations in young preweanling rodents and moderate hypothermia as an adjunct to other anesthesia in older preweanling animals. Techniques and applications for the procedure are also described.

Potentiation of cold-water swim analgesia by acute, but not chronic desipramine administration

Like other stress responses, cold-water swim (CWS) analgesia can be altered by changes in norepinephrine (NE) availability. While clonidine pretreatment potentiates CWS analgesia, lesions placed in the noradrenergic locus coeruleus reduce this response. Desipramine (DMI) can alter both the availability and receptor function of catecholamines, particularly NE: while both acute and chronic DMI treatments decrease NE reuptake, subsensitivity of beta-adrenergic receptors occurs only after chronic DMI treatment. The present study examined whether acute and chronic DMI treatments differentially alter CWS analgesia as measured by the jump test, CWS hypothermia and basal jump thresholds. The first experiment determined that pretreatment at either 24, 5 and 1 hr or only at 1 hr with DMI doses of 20 and 5 but not 1 mg/kg potentiated CWS analgesia. The second experiment found that chronic DMI pretreatment at a dose of 10 mg/kg administered twice daily over seven days failed to alter CWS analgesia at 1, 24, 48 or 72 hr thereafter. Neither CWS hypothermia nor basal jump thresholds were affected by the acute or chronic DMI injection regimens. The selective potentiation of CWS analgesia by acute DMI pretreatment is discussed in terms of the differential actions of acute and chronic injection regimens upon NE availability, receptor function, and adaptation processes.

Endogenous opiates and stress. Behavioral activation elicited in the rat and effects of naltrexone, diprenorphine and morphine

Acute exposure to intense (95dB) white noise may selectively increase the open field behavior of rats. Previous studies have indicated that this effect was mediated in part by endogenous opioid neuropeptides. Three experiments were carried out to characterize further and more specifically the stress-activation syndrome, on behavioral and pharmacological grounds. The time course within which blockade of opiates by naltrexone was effective behaviorally was examined. A second experiment examined the issue of cross-tolerance with opiates, by using dependent subjects, while a final experiment examined involvement of enkephalins, by assessing the effects of the drug diprenorphine upon stressed-behavior. Blockade with naltrexone was achieved with a small dose of the drug, and had a behavioral effect restricted to the initial 6 min of a 12-min test. Morphine-dependent rats proved not to be tolerant behaviorally to the activating effect of noise. Finally, diprenorphine did not mimic the effects of naltrexone. It produced no clear changes in the measures at any dose tested.

Sensitization or tolerance to morphine effects after repeated stresses

Rats were subjected to prolonged footshock, intensive acoustic stress, cold water swim and restraint over a period of 10 days. The analgesic and thermoregulatory properties of morphine (2, 4 and 8 mg/kg, sc.) were tested on the 11th day. Analgesia assessment was performed by means of hot-plate (HP) and tail-flick (TF) tests, and body temperature (Tb) changes was measured. Prolonged footshock and acoustic stress increased the sensitivity to morphine, while repeated restraint lessened morphine's effect. Cold water swim caused ambiguous consequences, facilitated the effect of a small dose of morphine, but reduced that of a large dose. It was concluded that the sensory components of the stressful exposure determine the effects of repeated stress on morphine sensitivity. Whereas painful interventions led to sensitization, and non-painful procedures result in tolerance to morphine's effects. The finding that analgesic and thermoregulatory effects of morphine were simultaneously enhanced supports the contention that the mechanism of sensitization to opiates involves a site where pathways mediating opiate analgesia and thermoregulatory effects converge.

Naloxone injections into the periaqueductal grey area and arcuate nucleus block analgesia in defeated mice

In a situation of social conflict, mice that are defeated by an opponent exhibit a marked analgesia. Microinjections of naloxone (1 or 10 micrograms) into the periaqueductal grey area (PAG) or into the region of the arcuate nucleus prior to the defeat prevented the emergence of analgesia. Microinjections of morphine (5 micrograms) into these sites had previously been shown to produce profound analgesia. Mice whose adrenals were removed rapidly developed analgesia when attacked by a stimulus animal. Injection of naloxone into PAG also antagonized defeat-induced analgesia in adrenalectomized mice. These observations indicate that sites and processes in the brain rather than in the periphery are responsible for the development of analgesia in mice that are subjected to social defeat.

Stress analgesia: effects of PCPA, yohimbine, and naloxone

Evidence suggests that morphine analgesia depends on the integrity of monoaminergic transmitter systems. Some forms of stress analgesia seem to be related to morphine analgesia, while others are not. To assess whether opioid and non-opioid stress analgesia differ in their reliance on monoamine systems, the effects of parachlorophenylalanine (PCPA) and yohimbine on analgesia produced by prolonged intermittent and brief continuous footshock were examined on the hotplate test. The interaction of adrenergic and endorphinergic activity with serotonergic mechanisms following these stressors was also investigated by testing the effects of yohimbine and naloxone on rats with prior PCPA treatment. Yohimbine alone significantly reduced baseline hotplate latencies, while PCPA and naloxone did not. The two stressors differed in the effects produced by both naloxone and PCPA. Naloxone significantly reversed stress analgesia in the prolonged stress condition, but not the brief stress condition. PCPA significantly enhanced the antinociceptive effect of brief continuous shock, while leaving the response to prolonged intermittent shock unaffected. In contrast, yohimbine blocked the analgesic effects of prolonged stress. For the brief stress condition, naloxone reversed the elevated thresholds elicited in PCPA treated rats. Naloxone also reversed stress analgesia for PCPA treated rats exposed to prolonged intermittent stress. Yohimbine lowered the responses of PCPA treated rats subjected to brief continuous shock. These results support an interactive model of stress analgesia dependent upon serotonergic, adrenergic, and endorphinergic transmitter systems.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Comparisons between warm and cold water swim stress in mice

The following experiments evaluated the effects of warm- or cold-water swim stress on tail-flick latencies (TFL) in mice. To first determine the appropriate control group, the TFL's of dry-vs-dunked mice were compared. Dry mice had significantly shorter TFL's than dunked mice, implying that the dampness of the mouse's tail contributed to the increase in the TFL. Therefore, dunked mice were used as the relevant control for the swum mice. Cold water swimming (2 degrees C) produced a significant increase in the TFL; this was not blocked by the opiate antagonist naloxone (3 mg/kg sc) or potentiated by the enkephalinase inhibitor thiorphan (100 mg/kg sc). Warm water swimming (32 degrees C) up to 3 min produced an inconsistent effect on TFL's, implying that the effects were at the threshold of detectability. Naloxone attenuated and thiorphan modestly potentiated the effects of warm water swimming on TFL's. This suggests that warm water swim stress-induced increases in mouse TFL's may involve opioid pathways, whereas cold water swim stress-induced changes in mice TFL's appear not to be opioid mediated.

Intermittent cold water stress-analgesia in rats: cross-tolerance to morphine

Continuous cold water swim ( CCWS , 3.5 min, 2 degrees C) induces a non-opiate type of analgesia since 14 mg/kg of naltrexone or 20 mg/kg of naloxone only partially antagonize this stress-induced analgesia (SIA) and since there is no cross-tolerance between CCWS and morphine-analgesia. Intermittent cold water swim ( ICWS ) analgesia is significantly antagonized by naltrexone (14 mg/kg). These studies suggested that CCWS -analgesia is mediated by non-opioid systems, while ICWS -analgesia acts through a system that also mediates morphine analgesia. The hypothesis that ICWS -analgesia shares a common opioid pathway with morphine-analgesia, but not with CCWS -analgesia, was further tested by cross-tolerance studies in rats. The results showed a complete cross-tolerance to morphine analgesia in ICWS -tolerant animals,, but no cross-tolerance in animals. This suggests that morphine- and ICWS -analgesia partially share a common pathway, ICWS acting probably at levels "downstream" from the opiate-sensitive site, while CCWS induces analgesia by acting on a different system which is not mediated by opioids.

REM sleep deprivation decreases the antinociceptive property of enkephalinase-inhibition, morphine and cold-water-swim

Rats treated with phosphoramidon (an enkephalinase-inhibitor 250 micrograms, i.c.v.), morphine (20 micrograms i.c.v.) or subjected to cold-water-swim (CWS, animals forced to swim in water at 5 degrees C for 5 min) showed consistent analgesia. The antinociceptive effect of phosphoramidon, morphine and CWS was antagonised by REM sleep deprivation (REMSD). It is suggested that normal duration of REM sleep is of importance for the anti-nociceptive activity of endogenous and exogenous opiates.

Prevention of stress-induced analgesia by substance P

It has been shown that a variety of stressful procedures, such as immobilization and footshock, can induce a significant degree of analgesia in mice. In addition, it has been shown that for some, but not all, stressful treatments, the analgesic effect is mediated via endogenous opioids. This report describes the effects of substance P, administered systemically, on both opioid-mediated immobilization-induced analgesia and non-opioid footshock-induced analgesia. Substance P completely blocked the opioid-mediated form of stress-induced analgesia while having no effect on the non-opioid form. Exogenous substance P appears to interact with endogenous opioid pain-suppressing systems.

Changes in morphine self-administration after exposure to ionizing radiation: evidence for the involvement of endorphins

Recent findings have implicated endogenous opiates in radiation-induced behavioral change. The present experiment further investigated this hypothesis by observing alterations in morphine self-administration after irradiation. Under the presumption that the release of endogenous opiates would decrease the need for exogenously supplied morphine, we hypothesized that after radiation exposure morphine-experienced mice would self-administer less of the opiate. C57BL/6J mice had continuous access to two drinking flasks which contained either water or morphine in saccharine water. Irradiated mice drank significantly less morphine than did sham-irradiated controls. This decrease was naloxone-reversible and could not be entirely attributed to a generalized radiogenic hypodipsia or taste aversion. These results are consistent with the hypothesis that radiation-induced behavioral changes may be due, in part, to the fluctuations of endogenous opiates.

Cholinergic mechanisms of analgesia produced by physostigmine, morphine and cold water swimming

This study concerns the cholinergic involvement in three experimental procedures which produce analgesia. Rats were given one of seven treatments: saline (1.0 ml/kg, i.p.); morphine sulfate (3.5, 6.0 or 9.0 mg/kg, i.p.); physostigmine salicylate (0.65 mg/kg, i.p.); warm water swim (3.5 min at 28 degrees C); and cold water swim (3.5 min at 2 degrees C). Each rat was tested on a hot plate (59.1 degrees C) once prior to and 30 min after treatment. Immediately after the last test the rats were killed with focussed microwave radiation. Levels of acetylcholine (ACh) and choline (Ch) in six brain areas (brain stem, cerebral cortex, hippocampus, midbrain, cerebellum and striatum) were analyzed by gas chromatograph-mass spectrometer. Morphine (9.0 mg/kg), physostigmine and cold water swimming caused significant analgesia. Morphine elevated the levels of ACh in the cerebellum and striatum, cold water swimming--in the cerebellum, striatum and cortex, and physostigmine--in the striatum and hippocampus. Levels of choline were elevated by morphine in the cerebellum, cortex and hippocampus, while cold water swimming elevated levels of choline in the cerebellum, cortex, striatum and hippocampus. Physostigmine did not change levels of choline in any of the brain areas studied. These data suggest that the analgetic effects of morphine or cold water swimming may be mediated by components of the cholinergic system that differ from those involved in the analgetic effects of physostigmine.

Endogenous opiates mediate radiogenic behavioral change

Exposure of C57BL/6J mice to ionizing radiation caused stereotypical locomotor hyperactivity similar to that produced by morphine. Naloxone administration prevented this radiation-induced behavioral activation. These results support the hypothesis that endorphins are involved in some aspects of radiogenic behavioral change.

Spinal cord monoamines modulate the antinociceptive effects of vaginal stimulation in rats

Perispinal administration (into the lumbar intrathecal space) of phentolamine (40 micrograms), an alpha-adrenergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 39.7% (measured by tail flick latency) and 57.1% (measured by vocalization threshold) as compared to controls. Perispinal administration of methysergide (10 micrograms), a serotoninergic receptor blocking agent, reduced the analgesic effect of vaginal stimulation by 48.5% (measured by vocalization threshold), although it did not significantly affect the tail flick measure. In a separate experiment, vaginal stimulation activated the release of norepinephrine and serotonin into a superfusate of the spinal cord. During vaginal stimulation, levels of norepinephrine and serotonin increased about 2-fold above resting levels. These findings indicate that vaginal stimulation releases norepinephrine and serotonin into the spinal cord, thereby exerting an analgesic effect.

Opioid and non-opioid mechanisms of stress analgesia: lack of cross-tolerance between stressors

Qualitatively different analgesic responses can be evoked in rats by exposure to prolonged, intermittent or brief, continuous footshock stress. These two forms of stress analgesia appear to be mediated by opioid and nonopioid pain-inhibitory substrates, respectively. The present study confirms our previous observation that tolerance develops to only the opioid form of stress analgesia and shows that cross-tolerance does not occur between the opioid and nonopioid forms. These data provide further evidence that independent mechanisms underlie opioid and nonopioid stress analgesia.

Capsaicin treatment and stress-induced analgesia

Capsaicin modulates animal pain perception, increasing chemosensitive and pressure thresholds following systemic administration, increasing thermal thresholds following intrathecal administration, and decreasing electric shock thresholds following intracerebroventicular (ICV) administration. Since morphine analgesia is decreased in a dose-dependent manner following ICV capsaicin, the present study examined whether ICV injections of capsaicin (0, 25, 50, 100 micrograms) would alter other analgesic responses as well. Experiment 1 demonstrated that the analgesic response to a 450 mg/kg dose of 2-deoxy-D-glucose was significantly reduced by the 25 and 50, but not the 100 micrograms capsaicin dose. Further, while analgesia induced by cold-water swims (CWS) in a 2 degrees C bath was significantly attenuated by the 25 micrograms capsaicin dose, the entire dose range eliminated analgesia induced by CWS in a 15 degrees C bath. Experiment 2 indicated that the capsaicin-induced alterations in CWS analgesia were not attributable to parallel changes in CWS hypothermia. Experiment 3 demonstrated that capsaicin failed to alter both the non-opioid analgesic response induced by 20 inescapable foot shocks (FS) and the opioid analgesic response induced by 80 FS. These data are discussed in terms of the similarities to and/or dissimilarities from capsaicin-induced effects upon morphine analgesia.