Reversal of stress-induced analgesia by apomorphine,but not by amphetamine

Acute exposure to severe stressors induce profound analgesia as well as depleting catecholamine levels. The present study examined whether d-amphetamine and apomorphine, agents which increase catecholamine availability, would alter the analgesic effectiveness of cold-water swims (CWS) and 2-deoxy-D-glucose (2-DG) as measured by an operant liminal escape procedure. Two groups of 10 rats each were tested to determine alterations in liminal escape threshold functions following amphetamine at doses of 0.25, 0.5, 1,2 mg/kg and following apomorphine at doses of 0.025, 0.05, 0.1, 0.2 mg/kg. Half of the amphetamine and half of the apomorphine groups were tested across their respective dose ranges for the drug effects upon CWS analgesia. The remaining animals in each group received 2-DG (600 mg/kg IP) alone followed by 2-DG paired with each stimulant dose. No dose of amphetamine or apomorphine alone altered escape thresholds. While amphetamine produced slight potentiations of 2-DG analgesia at the two low doses, apomorphine at the 0.05 and 0.1 mg/kg doses returned CWS and 2-DG analgesia to within normal placebo values. These results provide indirect evidence for a role for brain norepinephrine and dopamine in stress-induced analgesia, and these data are discussed with respect to catecholamine involvement in pain-inhibitory processes.     

Stress and morphine analgesia: alterations following p-chlorophenylalanine

Recent studies have shown that while the analgesic responses induced by certain stressors appear to be related to morphine analgesia, the analgesic responses to other stressors do not. Para-chlorophenylalanine (PCPA), a potent tryptophan-hydroxylase inhibitor has been shown to decrease both basal pain thresholds and morphine analgesia on the flinch-jump test. To assess further the relationship between morphine and stress-induced analgesia, PCPA's effect upon the analgesic responses to cold-water swims, 2-deoxy-D-glucose, inescapable foot shock and morphine were determined using the flinch-jump and tail-flick tests. PCPA, which produced an 85% depletion of brain serotonin, significantly decreased jump thresholds while significantly increasing tail-flick latencies. Similarly, while morphine analgesia was decreased by PCPA on the flinch-jump test, it was not affected on the tail-flick test. The analgesic jump thresholds induced by cold-water swims and 2-deoxy-D-glucose as well as the increase tail-flick latencies induced by foot shock were unaffected by PCPA. These results are discussed in terms of PCPA's differential effects upon basal nociception and morphine analgesia and in terms of further dissociation between morphine and stress-induced analgesia.     

Antagonism of stress-induced analgesia by D-phenylalanine,an anti-enkephalinase

Methionine- and leucine-enkephalin produce mild and transient analgesic effects, presumably because of enzymatic degradation. Administration of high (250 mg/kg) doses of D-phenylalanine retards the degradation process and elicits analgesia which is reversed by naloxone and which summates with electroacupuncture analgesia. The present study evaluated D-phenylalanine's dose-dependent effects upon a non-opioid analgesic treatment, cold-water swims (CWS), and compared this with morphine. Following determination of flinch-jump baselines, three groups of rats recieved respectively either 25, 50 or 100 mg/kg of D-phenylalanine intraperitoneally in three conditions: alone, with CWS (2°C for 3.5 min), and with morphine (5 mg/kg, SC). Parallel controls with saline were also tested. Simultaneous exposure with each minimally analgesic dose of D-phenylalanine reduced significantly the analgesic, but not hypothermic effects of CWS. By contrast, morphine analgesia was unaffected by D-phenylalanine. These data provide further support that different pain-inhibitory systems mediate CWS and morphine analgesia and suggest that activation of one system is capable of exerting collateral inhibition upon the other.     

2-Deoxy-D-glucose analgesia: influences of opiate and non-opiate factors

Acute administration of 2-deoxy-D-glucose (2-DG), an antimetabolic glucose analogue induces a powerful analgesia which adapts following repeated administration. 2-DG analgesia displays significant cross-tolerance with morphine, and like morphine analgesia, is potentiated in hypophysectomized rats. The present study examined further the role of opiates in 2-DG analgesia by examining whether the opiate antagonist, naloxone, would affect 2-DG analgesia, and whether ineffective doses of 2-DG and morphine would interact in a synergistic fashion to induce analgesia. Nociceptive thresholds were measured by the flinch-jump test. Naloxone doses of 1, 5, 10 and 20 mg/kg were all ineffective in reducing significantly 2-DG (600 mg/kg) induced pain threshold elevations. Naloxone failed to attenuate 2-DG (350 mg/kg) analgesia whether administered before or after the 2-DG injection. On the other hand, simultaneous administration of sub-analgesic doses of 2-DG (200 mg/kg) and morphine (2.5 mg/kg) summated to produce significant analgesia. This, 2-DG analgesia is similar to opiates in its tolerant and summative actions, yet dissimilar in that naloxone is ineffective in reversing its effects.     

Potentiation of cold-water swim analgesia and hypothermia by clonidine

The analgesia induced by acute exposure to cold-water swims (CWS) covaries with levels of brain norepinephrine and is reduced by lesions placed in the locus coeruleus. In assessing whether alpha-noradrenergic receptor mechanisms mediated CWS analgesia, the first experiment found that clonidine pretreatment (500, 1000 micrograms/kg) elevated jump thresholds 60 min following injection. While clonidine (1000 micrograms/kg) paired with a 2 degrees C CWS potentiated CWS analgesia in a synergistic manner, additivity of analgesic effects was observed following pairing of clonidine (500 micrograms/kg) with a 2 degrees C CWS and pairing of clonidine (500 and 1000 micrograms/kg) with a 15 degrees C CWS. The second experiment showed that clonidine (500 micrograms/kg) paired with a 2 degrees C CWS enhanced CWS enhanced CWS analgesia on the tail-flick test. The third experiment indicated that while clonidine (500 and 1000 micrograms/kg) or CWS (2 degrees C) each produced hypothermia, pairing of these clonidine doses with CWS enhanced CWS hypothermia. These data are discussed in terms of the possible modulatory role that norepinephrine, and particularly its alpha-noradrenergic receptor subclass, plays in the full expression of CWS analgesia and hypothermia.     

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

Animals exposed to cold-water swims, rotation, inescapable shocks, abrupt food deprivation and other stressors display temporary analgesia. Since repeated exposures result in adaptation of this analgesia in much the same way that repeated administration of opiates results in tolerance, the possibility of cross-tolerance between cold-water stress-induced and morphine-induced analgesia was investigated. Flinch-jump thresholds were determined in ten experimental groups of six rats each. Three groups showed dose-dependent analgesia following single injections of morphine at 5, 10 and 15 mg/kg, respectively. A fourth group, subjected to a single cold-water swim at 2°C for 3.5 min, displayed analgesia comparable to that produced by 10 mg/kg of morphine. Groups subjected either to 14 daily cold-water swims or to 14 daily morphine injections at 10 mg/kg showed normal thresholds on the 14th day indicating that adaptation and tolerance had developed, respectively. The cross-over groups were exposed to either 13 days of cold-water swims followed by morphine or the reverse arrangement. Both groups showed profound analgesia instead of cross-tolerance, suggesting that a non-opiate neural mechanism may mediate stress-induced analgesia.     

Lack of cross-tolerance between morphine and autoanalgesia

The acquisition of autoanalgesia (behaviorally-induced antinociception) was investigated in morphine-tolerant and non-tolerant rats. Tolerance to morphine did not affect analgesia acutely-elicited by a brief (15 sec) schedule of footshock. Similarly, analgesia elicited by classically conditioned fear was not attenuated by morphine tolerance. These data suggest that endorphins may not be the principle mediators of autoanalgesic phenomena.     

Naloxazone and pain-inhibitory systems: evidence for a collateral inhibition model

The analgesic responses following morphine and cold-water swims (CWS) can be dissociated from each other. Indeed, certain manipulations in rats such as hypophysectomy or D-phenylalanine injections decrease CWS analgesia while increasing morphine analgesia. The present study examined the reciprocal notion, namely whether a manipulation that decreases morphine analgesia would increase CWS analgesia. Naloxazone, an opiate antagonist which selectively inhibits the high affinity binding site in a long-acting manner, was administered intracerebroventricularly and assessed for its effects upon morphine analgesia and CWS analgesia as measured by the jump test. While intracerebroventricular injections of naloxazone reduced morphine analgesia at 0.5 and 24 hr following microinjection, the same 50 μg dose significantly increased CWS analgesia at 0.5 hr after injection, suggesting a mechanism of collateral inhibition between opioid and non-opioid pain-inhibitory systems.     

Differential effects of hypophysectomy upon analgesia induced by two glucoprivic stressors and morphine

Pain threshold elevations induced in rats following acute exposure to stressful cold-water swims and to inescapable foot shocks are significantly attenuated by hypophysectomy. The present study investigated the effects of hypophysectomy upon the dose-dependent and time-dependent analgesia induced by morphine and by the glucoprivic agents, 2-deoxy-D-glucose (2-DG) and insulin. Two reflex pain tests, the tail-pinch and the flinch-jump were employed. In normal rats, insulin induced prolonged (180 min) analgesia at doses of 16 U/kg on the tail-pinch test and 256 U/kg on the flinch-jump test. However, the same agents induced small and brief pain threshold elevations in hypophysectomized animals. By contrast, though 2-DG increased both measures in both groups, its effects were more marked in hypophysectomized rats. Hypophysectomized rats also exhibited a potentiated analgesic effect on both tests following high doses of morphine. On the other hand, low doses of morphine transiently increased tail-pinch thresholds in normal, but not hypophysectomized subjects. These data provide further evidence of multiple pain-inhibitory mechanisms in which the pituitary plays a complex, but integral part.     

Neuroleptic and analgesic interactions upon pain and activity measures

Previous data in rats indicate that while dopamine receptor blockers like haloperidol (HAL) potentiate opiate analgesia, dopamine receptor stimulants like apomorphine reduce cold-water swim (CWS) and 2-deoxy-D-glucose (2-DG) analgesia. Yet recently, HAL and chlorpromazine (CBZ) have been shown to reduce heat and immobilization analgesia. To address these differences, the present study investigated whether HAL (10, 50, 100 microgram/kg) or CPZ (1, 3, 5 mg/kg) would potentiate or reduce the effects of morphine (MOR), CWS, 2-DG and chlordiazepoxide (CDP) upon analgesia and activity. While HAL increased jump thresholds in a dose-dependent manner, CPZ doses exerted erratic effects. MOR analgesia was potentiated by the two higher CPZ doses and by the highest HAL dose. 2-DG analgesia was potentiated by only the highest HAL dose while CDP analgesia was potentiated by the moderate CPZ dose. While all CPZ doses potentiated CWS-induced increases in jump thresholds, the lowest HAL dose reduced this effect. These effects are considered in terms of the analgesic manipulation and its magnitude of effect, the neuroleptic and its dose, the pain test, and possible concurrent effects upon activity.     

Social isolation: Effects on pain threshold and stress-induced analgesia

Individually housed DBA/2 mice showed higher pain thresholds than grouped mice. Stress-induced analgesia was evident in grouped but not in isolated mice. Since also morphine injections did not result in analgesic effects in isolated mice, it is suggested that social isolation results in an increased release of opioids which may produce a decreased sensitivity at the opiate receptor level. The role of endogenous opioids in relation to social isolation is discussed.     

Chlordiazepoxide antinociception: Cross-tolerance with opiates and with stress

Chlordiazepoxide (CDP) has been previously shown to possess antinociceptive properties that are resistant, except at high doses, to the opiate antagonist naloxone. The present study evaluated whether CDP's antinociceptive effects were subject to tolerance following repeated injections and whether cross-tolerance might develop between the antinociceptive action of CDP and that of either morphine or cold water swims. CDP increased flinch-jump thresholds following acute administration and exhibited tolerance following repeated injections. Neither morphine-tolerant nor cold water swim-adapted rats displayed an antinociceptive effect when tested with CDP. On the other hand, chronic pretreatment with CDP attenuated the antinociceptive effects of cold water swims, but did not produce any clear effect upon morphine analgesia.     

2-deoxy-D-glucose-induced decrements in operant and reflex pain thresholds

Acute exposure to many environmental stressors induces significant analgesia. The present study examined whether 2-deoxy-D-glucose (2-DG), an antimetabolic glucose analogue, which induces glucoprivation and peripheral sympathoadrenal discharge, would also produce analgesia as measured by either an operant liminal escape or a reflex tail-pinch procedure. In the liminal escape paradigm, 9 rats were tested at weekly intervals in 6 randomly selected testing conditions: 30 min pre-test injections of four 2-DG doses (100, 225, 350 and 700 mg/kg, IP) and 180 min pre-test injections of the 2 higher doses. Moderate analgesia occurred at the lower 2-DG doses 30 min after injection, while profound analgesia occurred at the higher doses. After 180 min, only the 700 mg/kg 2-DG dose produced moderate analgesia, which was further enhanced by food deprivation. Rats tested in the tail-pinch paradigm displayed a similar dose-dependent analgesia course. These results demonstrate that 2-DG decreases nociceptive sensitivity, possibly through stress-induced activation of an intrinsic pain-inhibitory system.     

Cross-sensitization and cross-tolerance between exogenous cannabinoid antinociception and endocannabinoid-mediated stress-induced analgesia

Footshock stress induces both endocannabinoid mobilization and antinociception. The present studies investigated behavioral plasticity in cannabinoid antinociceptive mechanisms following repeated activation using the tail-flick test. A secondary objective was to ascertain whether blockade of stress antinociception by the CB(1) antagonist rimonabant could be attributed to changes in locomotor activity. The cannabinoid agonist WIN55,212-2 induced hypoactivity in the open field relative to vehicle-treated controls. By contrast, rimonabant, administered at a dose that virtually eliminated endocannabinoid-mediated stress antinociception, failed to alter locomotor behavior (i.e. time resting, ambulatory counts, distance traveled) in rats subjected to the same stressor. Rats exposed acutely to footshock were hypersensitive to the antinociceptive effects of WIN55,212-2 and Delta(9)-tetrahydrocannabinol (Delta(9)-THC). The converse was also true; acute Delta(9)-THC and WIN55,212-2 administration potentiated stress antinociception, suggesting a bidirectional sensitization between endocannabinoid-mediated stress antinociception and exogenous cannabinoid antinociception. Stress antinociception was also attenuated following chronic relative to acute treatment with WIN55,212-2 or Delta(9)-THC. Repeated exposure to footshock (3 min/day for 15 days), however, failed to attenuate antinociception induced by either footshock stress or WIN55,212-2. Our results demonstrate that endocannabinoid-mediated stress antinociception cannot be attributed to motor suppression. Our results further identify a functional plasticity of the cannabinoid system in response to repeated activation. The existence of cross-sensitization between endocannabinoid-mediated stress antinociception and exogenous cannabinoid antinociception suggests that these phenomena are mediated by a common mechanism. The observation of stress-induced hypersensitivity to effects of exogenous cannabinoids may have clinical implications for understanding marijuana abuse liability in humans.     

Opiate and peptide interaction: effect of enkephalins on morphine analgesia

Interactions between the weakly analgesic enkephalins and morphine on morphine-induced analgesia were studied. Met-enkephalin exhibited morphine analgesia whereas Leu-enkephalin potentiated it. Both Met- and Leu-enkephalin, when tested alone, were not analgesic. The strongly analgesic FK33824 (Sandoz) compound, like Leu-enkephalin, also potentiated morphine analgesia. Tolerance developed to morphine analgesia but not to Met-enkephalin inhibition of morphine analgesia.     

Morphine analgesia in grouped and isolated rats

The effects of long-term isolation of young adult male rats on the analgesic effects of morphine were investigated. Isolated rats developed altered patterns of behavior, including muricidal behavior in some animals. Analgesic activity of morphine was assessed with both the tail compression and the hot plate methods. The results indicate that chronically isolated rats, whether developing muricidal behavior or not, show no alteration in either pain thresholds or in their response to morphine-induced analgesia.Supported in part by NATO Research Grant No. 719 and USPHS Grant DA 00376.Visiting Scientist from the Department of Pharmacology, Temple University School of Medicine, Philadelphia, Pa., U.S.A.     

Analgesia following intraventricular administration of 2-deoxy-D-gluocse

The glucose analogue, 2-deoxy-D-glucose (2-DG) elicits hyperphagic and analgesic responses in rats. The former response appears to be mediated by central processes since overeating is elicited following intraventricular administration of 2-DG at low (3.5 and 5.0 mg/kg) doses. The present study examined whether flinch-jump thresholds would increase 30, 90 and 180 min following intraventricular injections of 2-DG at low (3.5, 5.0 and 10.0 mg/kg) doses and compared these effects with systemically-applied 2-DG doses of 350 and 500 mg/kg. Intraventricular 2-DG administration increased jump thresholds for up to 180 min across all test doses. Flinch thresholds were also increased, but in a manner dissociated from jump thresholds. Animals with cannulae located near, but not in the lateral ventricle, displayed delayed analgesic effects. The magnitude of intraventricular 2-DG analgesia was not as potent as the 100-fold higher systemic injections. It appears that central mechanisms mediated intraventricular 2-DG analgesia at the low dose range since higher, systemic 2-DG doses have previously failed to increase flinch-jump thresholds.     

Enhancement of stress-induced analgesia in adrenalectomized mice: Its reversal by dexamethasone

Adrenalectomy significantly increased the level of analgesia induced by room temperature swimming in mice, as revealed by a hot-plate test. This augmentation of antinociceptive action of stress was abolished by dexamethasone pretreatment. Involvement of pituitary opioids in modulating post-stress pain sensitivity in mice is suggested.     

Social conflict analgesia: Studies on naloxone antagonism and morphine cross-tolerance in male DBA/2 mice

It has recently been reported that male mice exhibit pronounced analgesia in response to attack from aggressive conspecifics. Although several studies indicate that this reaction can be blocked by opiate antagonist pretreatment, unequivocal evidence of opioid involvement is very much more limited. In the present study, the phenomenon of conflict analgesia has been studied in male DBA/2 intruder mice following exposure to a criterion level of attack from aggressive BKW residents. Our findings indicate that this analgesia is (1) blocked and reversed by naloxone (2) unaltered by methyl naloxone, except at high doses (75 mg/kg) and (3) fully cross-tolerant with morphine. This profile confirms and extends earlier findings with B6AF₁ mice, indicating that the opioid mediation of this biologically-relevant form of environmental analgesia is not strain specific.     

Pavlovian control of cross-tolerance between pentobarbital and ethanol

Tolerance to several effects of a number of drugs has been shown to depend on Pavlovian conditioning processes. Experiment I extended the compensatory conditioning model (Siegel 1975) to tolerance to the hypothermic effect of pentobarbital (30 mg/kg). In Experiment I, rats that acquired hypothermic tolerance in one environment did not display tolerance when tested in an environment not previously associated with drug administration. In Experiment II, rats were made tolerant to the hypothermic effect of pentobarbital (30 mg/kg) and tested for cross-tolerance to ethanol (2.5 g/kg). Cross-tolerance was observed, but it was significantly reduced if the test was in an environment different from the one in which tolerance to pentobarbital was originally acquired. Thus, the compensatory conditioning model accounts for at least part of the tolerance and crosstolerance to the thermic effects of alcohol and pentobarbital. The physiological processes in the CNS underlying tolerance and cross-tolerance for these drugs, therefore, are controlled by associative processes.