The genetics of pain and pain inhibition

The present review summarizes the current state of knowledge about the genetics of pain-related phenomena and illustrates the scope and power of genetic approaches to the study of pain. We focus on work performed in our laboratories in Jastrzebiec, Poland; Portland, OR; and Los Angeles, which we feel demonstrates the continuing usefulness of classical genetic approaches, especially when used in combination with newly available molecular genetic techniques.

Opioid and nonopioid swim stress-induced analgesia: a parametric analysis in mice

Environmental stress causes the activation of two types of endogenous pain inhibitory systems in animals: opioid analgesia is antagonized by opiate receptor blockers (e.g., naloxone and naltrexone), whereas analgesia produced by nonopioid systems is insensitive to such antagonism. A large literature documents that the parameters of the laboratory stressor will determine the neurochemical identity of the resultant analgesia. In rats, low severity stressors produce opioid analgesia and higher severity stressors produce nonopioid analgesia. A recent parametric analysis of swim stress-induced analgesia (SSIA) in the female Quackenbush mouse, however, observed the opposite pattern. The present study is a parametric analysis of SSIA using a range of swim temperatures (15-38 degrees C), swim durations (45 s to 7 min), and genetic models [male Swiss-Webster mice, and mice selectively bred from this outbred strain for high (HA), low (LA), or control SSIA]. We find that in nonselected mice low severity swims (i.e., warm temperature, short duration) produce naloxone-sensitive opioid SSIA, whereas high severity swims (i.e., cold temperature, long duration) produce nonopioid SSIA. This pattern is also seen in HA mice displaying very high analgesic magnitudes, but not in LA mice displaying minimal SSIA. In the selectively bred mice, analgesia and hypothermia from forced swimming are positively correlated, but can be dissociated both genetically and neurochemically. Furthermore, swimming in body temperature (38 degrees C) water produces analgesia without concommitant hypothermia, and the increased magnitude of 38 degrees C SSIA displayed by HA mice over control levels is entirely opioid.

1,3-Di-o-tolylguanidine (DTG) differentially affects acute and tonic formalin pain: antagonism by rimcazole

The role of the sigma receptor in prolonged pain was examined by assessing the effects of 1,3,di-o-tolylguanidine (DTG), a selective sigma receptor ligand, on the formalin test in mice. Formalin injected subcutaneously into the hindpaw produces a biphasic pain response: an acute phase of short duration followed by a longer-lasting tonic phase. DTG (10 mg/kg, i.p.) potently reduced pain behavior in the acute phase but increased pain behavior in the tonic phase. Rimcazole (5 and 10 mg/kg, i.p.), a selective sigma receptor antagonist, blocked both the DTG-induced decrease and increase in pain behavior observed in the acute and tonic phases, respectively. These data support previous findings indicating a modulatory role for the sigma receptor in nociceptive processes, and suggest that this receptor differentially modulates acute vs. tonic pain.

Oligogenic determination of morphine analgesic magnitude: a genetic analysis of selectively bred mouse lines

Two ongoing selective breeding projects have produced mice that display divergent analgesic responses to morphine. These two projects have selected for similar phenotypes: high and low levorphanol analgesia (HAR/LAR lines; Portland, OR) and high and low swim stress-induced analgesia (HA/LA lines; Jastrzebiec, Poland). Evidence suggests genetic commonalities between mice of the two projects. Using a Mendelian breeding protocol, we have recently found that one or two genetic loci predominantly determine the high morphine analgesia exhibited by HA mice. In the present study we demonstrate that the differential morphine analgesia (5 mg/kg, i.p.) displayed by HAR and LAR mice is similarly oligogenic, predominantly determined by two unlinked loci. A complementation analysis, in which the analgesic responses to morphine of the recessive homozygotes of each project (HAR and HA) were compared to those of their hybrid offspring (HAR x HA), revealed that different genetic loci have been fixed in each project. An intriguing bimodal distribution was observed in the HAR x HA population: Some HAR x HA hybrids displayed greater morphine analgesia than either HAR or HA mice, whereas others displayed minimal analgesia. LAR x LA hybrids displayed less analgesia than either LAR or LA mice. The analgesic responses of HAR x LA and LAR x HA mice were comparable to those of their low-line parents. These findings indicate not only that different loci were responsible for producing high morphine responders in each selection project but that these distinct loci can interact synergistically to produce "superhigh" and "superlow" responders.

The analgesic response to stress: genetic and gender considerations

The brain contains neuronal circuits, activation of which by electrical stimulation or environmental stress causes analgesia. Both opioid and non-opioid forms of stimulus-induced analgesia exist, and are anatomically differentiated. Several transmitters have been postulated for non-opioid stimulus-induced analgesia, N-methyl-D-aspartic acid being a particularly likely candidate. In mice there are marked gender differences in the underlying neurochemical medication of stress-induced analgesia, the development of which is sensitive to the hormonal environment during early post-natal development and which changes with age in both sexes. Mice can be bred for a high or low analgesic response to stress and there is evidence that this is determined by a single gene. Operative pain, as a stressor, inhibits natural killer (NK) cell activity and influences the propensity to develop metastases when mice are inoculated with an experimental tumour after abdominal surgery. This can be influenced by peri-operative morphine in analgesic doses.

Antinociception following 1,3,-di-o-tolylguanidine, a selective sigma receptor ligand

The role of sigma receptors in antinociceptive processes remains equivocal, because previous sigma drugs also bind to PCP/NMDA and opiate receptors. The present study examined the antinociceptive effects of the high-affinity, sigma-selective ligand 1,3-di-o-tolylguanidine (DTG; 10, 15, and 20 mg/kg, IP) on tail-withdrawal latencies in mice. DTG produced significant but short-lived increases in withdrawal latencies at all dose levels. DTG also produced hypothermia, but this effect was dissociable from antinociception. The highly selective sigma ligand rimcazole (10 and 25 mg/kg, IP) antagonized DTG antinociception in a dose-dependent manner. The opiate antagonist naloxone and the PCP/NMDA antagonist MK-801 were, however, without effect. Haloperidol, which also binds to sigma receptors, increased withdrawal latencies but did not alter DTG antinociception. These data implicate sigma receptors as the site of DTG antinociception, and more generally support the distinction between sigma, opiate, and PCP/NMDA receptors.

The role of LGL/NK cells in surgery-induced promotion of metastasis and its attenuation by morphine

Painful stress such as surgery has been shown both to suppress immune function and to promote metastasis, although the degree to which alterations in immunity underlies the tumor-enhancing effects of surgery remains unclear. We recently reported that an experimental laparotomy results in a twofold increase in the number of lung metastases following iv injection of MADB106 tumor cells, a natural killer (NK)-sensitive mammary adenocarcinoma cell line, syngeneic to the Fischer 344 rats we studied. Further, the administration of an analgesic dose of morphine prevented these metastatic-enhancing effects of surgery. The aim of the present study was to investigate the role of NK cells in both the metastatic-enhancing effects of surgery and the attenuation of these effects by morphine. Using a simple 2 x 2 experimental design (surgery with anesthesia vs anesthesia only, and morphine vs vehicle), we found that surgery resulted in a decrease in both whole blood NK cytotoxic activity and number of circulating LGL/NK cells assessed 4 h postoperatively. In a second experiment involving an 18-h lung clearance assay, we used the mAb 3.2.3 to deplete rats of LGL/NK cells with the following rationale: if LGL/NK cells are necessary to mediate an event, then in their absence, that event should not occur. Normal and LGL/NK-depleted animals were assigned to the same four experimental groups, and radiolabeled MADB106 tumor cells were injected iv 4 h after surgery. In normal animals, there was a significant interaction between surgery and morphine such that morphine attenuated the surgery-induced increase in tumor cell retention without affecting tumor cell retention in the anesthesia groups. In the LGL/NK-depleted animals, however, although the tumor-enhancing effects of surgery remained evident, morphine did not mitigate this outcome. These results suggest that: (a) both LGL/NK cell activity and other factors independent of LGL/NK cells play a role in the surgery-induced increase in tumor cell retention; and (b) LGL/NK cells play a critical role in morphine's attenuating effects on this outcome. Finally, these results reinforce concern about the pathogenic consequences of unrelieved pain.

Mu-opiate receptor binding is up-regulated in mice selectively bred for high stress-induced analgesia

Pain perception and sensitivity to opiate analgesics strongly depend on genotype. Mice selectively bred for high (HA) and low (LA) swim stress-induced analgesia display markedly divergent morphine analgesia, a difference that appears to be determined by one or at the most two major genes. In an attempt to provide candidate genes mediating the supranormal analgesia displayed by HA mice, we performed mu-opiate receptor binding on 27th generation HA, LA, and control (C) mice using [3H]naloxone. HA mice were found to have significantly higher whole-brain receptor density (Bmax) than LA mice in whole brain homogenates; no significant difference in affinity (Kd) was observed. Quantitative autoradiography confirmed the line difference in whole-brain receptor binding. In the medial thalamus, a brain area implicated in ascending pathways of pain inhibition, HA mice were found to display significantly higher [3H]naloxone binding than C mice (a 64% increase) and LA mice (a 128% increase). No significant line differences were observed in any other brain locus. Thalamic mu receptors may therefore play an important role in a central 'volume control' mechanism of pain inhibition, and underlie individual differences in the responses of mice to opiate analgesic drugs.

Morphine fails to produce tolerance when administered in the presence of formalin pain in rats

The present study examined the development of tolerance to morphine analgesia under conditions in which morphine was administered in the presence or absence of pain induced by subcutaneous injection of 50 microliters of 2.5% formalin into the hind paw of rats. Animals were injected with morphine (25 mg/kg, i.p.) or saline for 3 consecutive days either in the presence of pain (10 min after formalin injection) or in the absence of pain (6 h prior to formalin injection). On the 4th day, tolerance to the analgesic effect of test doses of morphine (6 or 10 mg/kg) was assessed in the formalin and tail-flick tests, respectively. Significant tolerance in both tests was observed in animals receiving morphine in the absence of pain during the tolerance induction period, but not in animals receiving morphine in the presence of pain.

NMDA receptor antagonists, MK-801 and ACEA-1011, prevent the development of tonic pain following subcutaneous formalin

Subcutaneous injection of formalin produces a biphasic pain response: an early, transient phase followed by a late tonic phase. The present study examined the involvement of the N-methyl-D-aspartic acid (NMDA) receptor in the development of the late pain produced following subcutaneous injection of formalin into the hind paw in mice. Blockade of the NMDA receptor by its non-competitive antagonist, MK-801, prior to formalin injection, but not after, reduced pain during the late phase. Similarly, blockade of the NMDA receptor allosteric site by the novel glycine site antagonist, ACEA-1011, also reduced the pain response in the late phase. These results suggest that the development of the late phase of formalin pain is due to NMDA-mediated activity during the early phase.

Levorphanol and swim stress-induced analgesia in selectively bred mice: evidence for genetic commonalities

Two independent selective breeding programs have developed divergent lines of mice expressing either high and low swim stress-induced analgesia (HA/LA lines; Jastrzebiec, Poland) or high and low levorphanol analgesia (HAR/LAR lines; Portland, OR). In the present study, mice from both programs were tested for both levorphanol analgesia (2 mg/kg) and an opioid-mediated swim stress-induced analgesia (3 min swimming in 32 degrees C water) in the hot-plate test. Mice selected for high and low levorphanol analgesia displayed high and low swim stress-induced analgesia, respectively; mice selected for high and low swim stress-induced analgesia displayed high and low levorphanol analgesia, respectively. This pattern of correlated responses suggests a high degree of common genetic determination in opiate and swim stress-induced analgesia. These findings also suggest that individual differences in analgesic responsiveness to opiate drugs result from genetically determined individual differences in endogenous pain inhibitory mechanisms.

Antagonism of the non-opioid component of ethanol-induced analgesia by the NMDA receptor antagonist MK-801

Recent evidence from our laboratory suggests that the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) selectively antagonizes non-opioid (i.e. naloxone-insensitive) mechanisms of stress-induced analgesia in mice. For example, we have recently demonstrated that a low dose of MK-801 (0.075 mg/kg, i.p.) antagonizes the non-opioid component of a mixed opioid/non-opioid swim stress-induced analgesia (SSIA) resulting from forced swimming for 3 min in 20 degrees C water. Since ethanol-induced analgesia (EIA) has been found to be only partially attenuated by naloxone, we hypothesized that MK-801 would similarly block the non-opioid component of EIA. The effects of MK-801 and of the opioid receptor antagonist naloxone (10 mg/kg, i.p.) on analgesia produced by ethanol (2.5 g/kg in 20% vol/vol, i.p.) were studied in control mice and in mice selectively bred for high (HA) or low (LA) SSIA. HA mice showed significantly more, and LA mice significantly less, EIA than controls. Naloxone and MK-801 significantly attenuated EIA in control and HA mice, and in these lines the combined administration of both antagonists blocked EIA completely. In LA mice, which displayed very little EIA, naloxone but not MK-801 reversed EIA completely. These findings provide additional evidence for the role of the NMDA receptor in non-opioid mechanisms of analgesia. The finding that mice selectively bred for high and low SSIA, also display high and low EIA suggests common mediation of the effects of stress and ethanol on antinociceptive processes.

The specific N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 blocks U-50,488, but not morphine antinociception

The effect of the specific NMDA receptor antagonist MK-801 on antinociception produced by the kappa opiate receptor agonist U-50,488 and the mu receptor agonist morphine was assessed using the tail-flick test in rats. MK-801 (0.05 and 0.1 mg/kg) antagonized antinociception induced by all three doses of U-50,488 (7.5, 15 and 30 mg/kg), and potentiated antinociception induced by the lower (1 mg/kg) but not higher (5 mg/kg) dose of morphine. Naloxone at a dose of 1.0 but not 0.1 mg/kg blocked U-50,488 antinociception, indicating that MK-801 affects opiate antinociception. The present results are the first to suggest a critical role for the NMDA receptor in opiate antinociception involving the kappa receptor.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. II. Comparison across three swim-stress paradigms in selectively bred mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (dizocilpine, 0.075 mg/kg, i.p.) on swim-stress-induced analgesia (SSIA) were studied in control (C) mice and in mice selectively bred for high (HA) or low (LA) SSIA. In three consecutive experiments, animals were subjected to forced swimming at water temperature of 20 degrees C, 32 degrees C and 15 degrees C and the resulting analgesia (hot-plate test) was found to be mixed opioid/non-opioid, opioid and non-opioid, respectively, as a function of the degree of antagonism by naloxone (10 mg/kg, i.p.). The major finding of this study is that MK-801 attenuated 15 degrees C SSIA, against which naloxone was ineffective, but had no effect on 32 degrees C SSIA, which naloxone blocked completely. A combination of naloxone and MK-801 significantly attenuated 20 degrees C SSIA in C and HA mice and in HA mice this attenuation was significantly larger than that produced by either drug alone. Morphine analgesia (10 mg/kg, i.p.) was unaffected by MK-801. It is concluded that low doses of MK-801 selectively block non-opioid mechanisms of SSIA.

The NMDA receptor antagonist MK-801 prevents long-lasting non-associative morphine tolerance in the rat

Several studies have demonstrated that the N-methyl-D-aspartate (NMDA) antagonist MK-801 attenuates the development of morphine tolerance and withdrawal. These studies employed repeated morphine injections to induce tolerance, a procedure in which learning has been suggested to play a significant role in tolerance development. MK-801 has been reported to block some types of learning, and it is unclear, therefore, whether the effect of MK-801 on tolerance development is due to its antagonism of associative (learning) or non-associative factors. Moreover, previous studies have tested the effects of MK-801 on morphine tolerance only up to 48 h after its induction; yet morphine tolerance can persist for many months, and it is not known whether MK-801 can block long-lasting tolerance. In the present study, therefore, we adopted a model of morphine tolerance in which the involvement of learning is minimized by using a single injection of morphine in a sustained-release preparation, and we tested tolerance for up to 56 days. In the first experiment, simultaneously administering MK-801 (0.2 mg/kg, s.c.) and morphine (60 mg/kg, s.c.), each in a sustained-release preparation, abolished tolerance that lasted at least 12 days. Analgesia was measured in the hot-plate test following a test dose of morphine (15 mg/kg, i.p.). In the second experiment, delivering MK-801 and morphine as before, the duration of morphine-induced catalepsy and analgesia was prolonged. Nevertheless, 24 h later one symptom of naloxone-precipitated withdrawal was significantly attenuated in these same animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol increases tumor progression in rats: possible involvement of natural killer cells

The effects of acute and chronic ethanol administration on tumor progression and metastasis were studied in rat models of leukemia and breast cancer, respectively. Acute administration of 1.5-3.5 g of ethanol/kg body weight significantly reduced survival of rats injected with CRNK-16 leukemia cells in a dose-related manner. Acute administration of 2.5-3.5 g of ethanol/kg body weight, one hour before tumor inoculation, or chronic consumption of liquid diet containing ethanol for two weeks before and three weeks after tumor inoculation, significantly increased the number of lung metastases of MADB106 mammary adenocarcinoma. The ethanol-induced increase in the number of metastases was not correlated with plasma levels of corticosterone and was not altered by the opiate antagonist naltrexone. Incubation of spleen cells in vitro in the presence of ethanol, at concentrations comparable to those measured in the blood of ethanol-treated rats, significantly suppressed natural killer (NK) cell activity against MADB106 cells in a standard chromium-release assay and decreased the binding of effector to MADB106 tumor cells. However, neither acute nor chronic ethanol administration in vivo altered splenic NK activity against this tumor in the same in vitro assay, in which the ethanol would have been washed away. These results suggest that, in the presence of ethanol, tumor progression is facilitated. The possibility that this facilitation is related to ethanol-induced impairment of the normal tumoricidal interaction between NK and tumor cells is discussed.

Stress-induced analgesia prevents the development of the tonic, late phase of pain produced by subcutaneous formalin

Subcutaneous injection of formalin produces a biphasic pain response: a transient early phase followed by a tonic late phase. It has recently been suggested that development of the late phase depends upon the presence of the early one. In support of this suggestion, we now demonstrate that blocking the early phase by stress-induced analgesia prevents development of the late phase, whereas the same stressor given after the first phase does not. Both phases are manifested when stress-induced analgesia is blocked by the N-methyl-D-aspartate (NMDA) or opiate antagonists, MK-801 and naloxone.

Stimulation of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.

Delayed application of MK-801 attenuates development of morphine tolerance in rats

To investigate the possible involvement of enduring or delayed changes at the N-methyl-D-aspartic acid (NMDA) receptor in the mechanisms of morphine tolerance, rats were treated with the specific NMDA receptor antagonist, MK-801 (0.15 mg/kg) 2 h after morphine injection (20 mg/kg) during a 4-day induction period of tolerance. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by MK-801. These findings suggest that long-lasting or delayed changes at the NMDA receptor underlie the development of morphine tolerance. Moreover, because MK-801 was delivered 2 h after morphine and therefore could not serve as a cue for morphine administration, these findings indicate that the attenuating effect of MK-801 on the development of morphine tolerance is not attributable to state-dependent learning.

N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 blocks non-opioid stress-induced analgesia. I. Comparison of opiate receptor-deficient and opiate receptor-rich strains of mice

The effects of the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist MK-801 (0.075 mg/kg), and the specific opiate receptor antagonist naloxone (10 mg/kg), on swim stress-induced analgesia (SSIA) were studied in opiate receptor-deficient (CXBK) and opiate receptor-rich (CXBH) mice. Animals were subjected to forced swimming, and analgesia was assessed using the hot-plate test. In CXBK mice SSIA was blocked by MK-801 but was completely insensitive to naloxone. In CXBH mice SSIA was partially attenuated both by naloxone and MK-801, and it was nearly abolished by a combination of these drugs. Morphine analgesia (10 mg/kg) was abolished by naloxone but completely unaffected by MK-801 in CXBH mice. These findings suggest that the NMDA receptor is critically involved in the non-opioid component of SSIA.

Stress increases metastatic spread of a mammary tumor in rats: evidence for mediation by the immune system

Causal relationships among stress, immune suppression, and enhanced tumor development have often been suggested, but direct evidence is scant. We studied stress effects in Fischer 344 rats using a tumor model in which lung metastases of a syngeneic mammary tumor (MADB106) are controlled by natural killer (NK) cells. Animals exposed to acute stress showed a substantial decrease in NK cell cytotoxicity against this tumor in an in vitro assay and, when intravenously injected with this tumor, showed a twofold increase in surface lung metastases. The critical period during which stress increases metastases appears to be the same as that during which this tumor is known to be controlled by NK cells. These findings support the hypothesis that stress can facilitate the metastatic process via suppression of the immune system.

Excitatory amino acid antagonists (kynurenic acid and MK-801) attenuate the development of morphine tolerance in the rat

To investigate the possible role of excitatory amino acids (EAAs) in the mechanisms of morphine tolerance, rats were treated either with the wide-spectrum EAA antagonist, kynurenic acid (150 mg/kg), or the specific N-methyl-D-aspartic acid (NMDA) receptor antagonist. MK-801 (0.05 mg/kg), during a four-day induction period of morphine tolerance. Morphine was given once daily at a dose of 15 mg kg. On the fifth day rats were injected only with morphine (15 mg/kg), and analgesia was assessed using the hot-plate test. Morphine tolerance was significantly reduced by both EAA antagonists. Control experiments showed that at the same doses neither acute nor chronic administration of these antagonists affected morphine analgesia itself in a manner that can explain these findings. The possible involvement of EAAs in the mechanisms of morphine tolerance is discussed.

Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

The objective of the present study was to characterize stimulation-produced antinociception from the periaqueductal gray matter (PAG) in rats with unilateral hindlimb inflammation induced by an intraplantar injection of Freund's complete adjuvant. Rats were chronically implanted with a bipolar stimulating electrode in the PAG. Nociception was assessed using a paw pressure test. Prior to inflammation, PAG stimulation significantly increased paw pressure threshold in both paws compared to non-stimulated controls. Following inflammation, PAG stimulation inhibited nociception in the inflamed, but not the non-inflamed paw. Systemic administration of naloxone blocked antinociception from ventral, but not dorsal PAG stimulation sites. Intrathecal, but not subcutaneous, administration of quaternary naltrexone completely blocked stimulation-produced antinociception from the PAG. The known increased levels of endogenous opioids occurring in the spinal cord ipsilateral to the site of inflammation suggest a mechanism for the selective antinociceptive effect of ventral PAG stimulation seen for the inflamed paw.

Stimulation-produced analgesia in the mouse: evidence for laterality of opioid mediation

The effect of naloxone on periaqueductal gray matter (PAG) stimulation-produced analgesia (SPA) was examined in pentobarbital anesthetized Swiss-Webster mice. Electrodes were placed either in dorsolateral or ventrolateral PAG, and SPA threshold was assessed using the hind paw-flick test (paw withdrawal from radiant heat). SPA threshold did not differ between dorsal and ventral PAG, and naloxone equally attenuated SPA from both areas. SPA threshold for the paw contralateral to the stimulation site was half that for the ipsilateral paw. Elevation of SPA threshold by naloxone was greater for the contralateral than ipsilateral paw. Exposure to analgesic neck pinch prior to SPA almost completely abolished the antinociceptive effect of contralateral PAG stimulation without affecting SPA on the ipsilateral paw. This effect of pinch was itself reversed by prior naloxone administration. We suggest that the substrate of opioid mediated SPA from PAG in the mouse has principally a contralateral organization.

Intake of and preference for sweet solutions are attenuated in morphine-withdrawn rats

The hypothesis that intake of sweet solutions is partially controlled by endogenous opioid peptides was tested in 2 experiments that examined the effects of repetitive morphine administration and withdrawal on subsequent intake of and preference for saccharin solutions in rats. Experiment 1 established that 17 hr after morphine withdrawal, rats consumed less saccharin, but not less water, than did controls. The groups did not differ 8 days later. In Experiment 2, using a 2-bottle saccharin-preference test, rats exhibited a reduced preference to saccharin solutions (1, 3, 9, 30, or 60 mM) for 6 days after morphine withdrawal. The difference between the groups was most pronounced at the most preferred concentrations (9 and 30 mM). The results suggest that cross-tolerance occurs between morphine and the opioid-mediated hedonic effects of sweet solutions.

Stress-induced suppression of natural killer cell cytotoxicity in the rat: a naltrexone-insensitive paradigm

The suppression of natural killer (NK) cell cytotoxicity by footshock stress can be attenuated by opioid antagonists, implicating endogenous opioids in its mediation. A stress paradigm that induces NK suppression that is not blocked by the opioid antagonist naltrexone is reported. This stress paradigm is also shown to cause analgesia and elevated plasma corticosterone levels that are not attenuated by naltrexone. In the first experiment, a significant suppression of NK cell cytotoxicity after forced swimming was demonstrated in Fischer 344 rats treated with either saline or naltrexone, compared with nonstressed controls. Significantly higher corticosterone levels were evident in both stressed groups. In the second experiment, the same stress paradigm was shown to cause significant analgesia in the tail-flick test, whereas no differences were seen between groups pretreated with saline and naltrexone. It is concluded that opioids need not always be involved in the suppression of NK cell cytotoxicity by stress.

Genetic influences on brain stimulation-produced analgesia in mice: II. Correlation with brain opiate receptor concentration

The analgesic effect of electrical stimulation of the periaqueductal gray matter (PAG) was studied in 4 strains of mice: C57BL/6By (C57), BALB/cBy (BALB), CXBH, and CXBK. These strains are known to have high (CXBH), low (CXBK), and intermediate (C57 and BALB) concentrations of brain opiate receptors. The current intensity required for stimulation-produced analgesia (SPA) did not differ among strains. Naloxone attenuated SPA in CXBH, C57 and BALB mice, but was ineffective in the opiate receptor deficient CXBK mice. The results suggest that genetic differences in opiate receptor density can influence the degree to which opioid mechanisms are involved in SPA from the PAG.

Stress-induced suppression of natural killer cell cytotoxicity in the rat: A naltrexone-insensitive paradigm

The suppression of natural killer (NK) cell cytotoxicity by footshock stress can be attenuated by opioid antagonists, implicating endogenous opioids in its mediation. A stress paradigm that induces NK suppression that is not blocked by the opioid antagonist naltrexone is reported. This stress paradigm is also shown to cause analgesia and elevated plasma corticosterone levels that are not attenuated by naltrexone. In the first experiment, a significant suppression of NK cell cytotoxicity after forced swimming was demonstrated in Fischer 344 rats treated with either saline or naltrexone, compared with nonstressed controls. Significantly higher corticosterone levels were evident in both stressed groups. In the second experiment, the same stress paradigm was shown to cause significant analgesia in the tail-flick test, whereas no differences were seen between groups pretreated with saline and naltrexone. It is concluded that opioids need not always be involved in the suppression of NK cell cytotoxicity by stress.

Stimulation of the periaqueductal gray matter inhibits nociception at the supraspinal as well as spinal level

Stimulation of the periaqueductal gray matter (PAG) is known to modulate nociception at the spinal level. Several studies have suggested that nociception may also be modulated via ascending projections from the PAG. To study this hypothesis, the descending pathway was selectively disrupted immediately caudal to the PAG in 28 rats. Twenty-eight additional rats served as non-lesioned controls. All animals were chronically implanted with a stimulating electrode in the PAG, and antinociception was assessed using tests involving spinally and supraspinally mediated responses (tail-flick and hot-plate tests, respectively). Significantly fewer lesioned than non-lesioned rats showed stimulation-produced analgesia (SPA) in the tail-flick test (4 of 28 vs 14 of 28, respectively). In contrast, no significant difference in the incidence of SPA occurred between lesioned and non-lesioned rats in the hot-plate test. These findings demonstrate that nociception can be modulated at the supraspinal, as well as spinal, level.

Genetic influences on brain stimulation-produced analgesia in mice. I. Correlation with stress-induced analgesia

The analgesic effect of electrical stimulation of the periaqueductal gray matter (PAG) was studied in mice selectively bred for high and low stress-induced analgesia (HA and LA lines, respectively). The current intensity required for stimulation-produced analgesia (SPA) in LA mice was 5 times that for HA mice. Naloxone produced a 4-fold increase of SPA threshold in HA mice, but was ineffective in LA mice. These findings suggest that the differential responsiveness of these two lines to the analgesic effect of stress reflects a more general genetic modification of the efficacy and mechanism of their pain-inhibitory systems.

Suppression of natural killer cell activity by high-dose narcotic anesthesia in rats

Suppression of natural killer (NK) cell activity in the postoperative period has been reported in several clinical studies. Endogenous opioids and cerebral injection of morphine have been shown to suppress NK cell activity. Since high-dose opiates are commonly used in anesthetic practice, we sought to determine the effects of three narcotic agents on NK cell activity. Male rats were injected subcutaneously with morphine (30 mg/kg), fentanyl (0.3 mg/kg), or sufentanil (0.06 mg/kg). Three, 12, or 24 h later the cytotoxic activity of splenic NK cells was measured in a 4-h chromium-51 release assay using radiolabeled target cells. All three drugs significantly suppressed NK cytotoxicity at 3 h after administration; this effect was blocked by an opiate antagonist, naltrexone. Fentanyl and sufentanil also caused a significant suppression 12 h after drug administration. By 24 h NK activity of all groups returned to normal values. Interferon is known to augment NK cell activity. Therefore, in another experiment rats were given an interferon inducer, polyinosinic:polycytidylic acid (poly I:C), to determine if it would alter the effects of these narcotics on splenic NK activity. Poly I:C treatment increased NK cytotoxicity to above baseline; fentanyl in these animals reduced NK activity and brought it back to control levels. These findings suggest that clinically used high-dose narcotic anesthesia can suppress NK cytotoxic activity and that pretreatment with interferon can attenuate this suppression.

Characterization of stimulation-produced analgesia from the nucleus tractus solitarius in the rat

Electrical stimulation of the commissural region of the nucleus tractus solitarius (NTS) inhibits the tail-flick reflex evoked by noxious heat. This antinociception can be measured in the awake or pentobarbital anesthetized rat at current intensities that do not induce overt behavioral side effects. Glutamate microinjections into the NTS, but not immediately surrounding the NTS, also inhibit the tail-flick reflex, demonstrating that activation of NTS cell bodies, and not fibers of passage, mediates antinociception from this region. In contrast, morphine microinjections into the NTS have no effect on the tail-flick reflex in anesthetized rats. These findings provide further evidence that the NTS is involved in the modulation of nociception.

Natural killer cell activity in vasopressin-deficient rats (Brattleboro strain)

Several lines of evidence suggest that the neuropeptide vasopressin is involved in the regulation of the immune system. We explored this possibility by comparing the cytotoxic activity of natural killer (NK) cells in Brattleboro (DI) rats, which are homozygous for diabetes insipidus and lack vasopressin, and Long-Evans (LE) rats, the strain from which DI rats were derived. Additionally, we compared the effects of swim stress, morphine administration and vasopressin replacement on NK cell activity in these two strains. In DI rats, NK cell activity, determined by a standard 4-h chromium-release assay, was significantly higher than in LE rats. Both swim stress and morphine administration suppressed NK activity in DI and LE rats. There was no difference in the level of suppression between the two strains. Vasopressin replacement normalized water intake in DI rats, but had no significant effect on NK cell activity. DI rats exhibited lower plasma corticosterone levels, which were not elevated by vasopressin replacement. The results suggest that the lack of vasopressin in DI rats elevates baseline NK cell activity, probably via mechanisms that are secondary to the vasopressin deficiency (e.g. lower corticosterone levels). Neither vasopressin nor other hormones affected by vasopressin deficiency seem to be involved in the acute modulating effects of stress and morphine on NK cells.

Relationship between analgesia and cardiovascular changes induced by electrical stimulation of the mesencephalic periaqueductal gray matter in the rat

Analgesia and cardiovascular changes produced by electrical stimulation of the midbrain periaqueductal gray matter were examined in the lightly pentobarbital-anesthetized rat. The current intensity required to elicit analgesia was first determined, using the tail-flick test, after which the effects on arterial pressure and heart rate were recorded from stimulating at the same intensity. Intensity thresholds for decreases and/or increases in arterial pressure were also ascertained at the same sites. Although stimulation at the analgesia threshold produced increases in arterial pressure at more than 60% of the sites within the periaqueductal gray, decreases, no change, and mixed responses were also observed. Below the periaqueductal gray, increases in arterial pressure occurred at analgesia threshold for more than 70% of the sites studied, and no cardiovascular changes were found for 20% of the sites. Above the periaqueductal gray, no change and mixed responses were the predominant effects at analgesia threshold. A correlation across sites within the periaqueductal gray was found between the threshold for stimulation-produced analgesia and the threshold for a change in arterial pressure. No reliable alterations in heart rate were observed at any stimulation site. These results are in agreement with the existence of a common midbrain substrate for both the regulation of pain inhibition and cardiovascular function. However, they indicate that analgesia resulting from stimulation of the periaqueductal gray matter does not necessarily occur concurrently with an increase in arterial pressure.

Strain differences in the magnitude of swimming-induced analgesia in mice correlate with brain opiate receptor concentration

Swimming-induced analgesia was studied in 4 strains of mice differing in central opiate receptor density: C57BL/6By (C57), BALB/cBy (BALB/c), CXBK and CXBH. The degree of 'swim analgesia' significantly differed among strains in the order CXBH greater than BALB/c = C57 greater than CXBK. This order positively correlates with known differences in opiate receptor density in these strains. Naloxone reversed the analgesic effect of swimming in CXBH, C57 and BALB/c, but was ineffective in opiate receptor-deficient CXBK mice. These results suggest that genetic differences in central opiate receptor density influence the analgesic response to stressful stimuli.

Reduced saccharin preference in CXBK (opioid receptor-deficient) mice

The preference for sweet solutions in opioid receptor-deficient (CXBK) and control (C57BL/6By) mice was compared. CXBK and C57BL/6By (C57) mice were presented for 2 h/day with 2 tubes, one always containing water and the other containing either water or various concentrations of saccharin solution. Fifteen minutes before the drinking session, half of the mice in each strain were injected with naltrexone (0.2 mg/kg) and the other half with saline. Compared to C57 mice, CXBK mice had significantly lower saccharin preference. Naltrexone reduced the saccharin preference in both strains, almost completely abolishing preference in CXBK mice. The results support the hypothesis that brain opioid receptors are involved in mediating sweet palatability and suggest that genetic differences in opioid receptor density contribute to differences in the palatability of sweet solutions.

GABAergic modulation of the analgesic effects of morphine microinjected in the ventral periaqueductal gray matter of the rat

The possibility that GABAergic neurons in the ventral periaqueductal gray matter modulate the analgesic effects of morphine microinjected into this brain area was investigated in the rat. Microinjection of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin 3-ol (THIP) (0.4 microgram in 0.2 microliter), a GABA agonist, in the ventral periaqueductal gray matter significantly reversed the increase of tail-flick latency induced by a prior injection of morphine sulfate (4 micrograms in 0.2 microliter) at the same site. Conversely, microinjection in the same region of picrotoxin (10 ng in 0.2 microliter), a GABA antagonist, significantly potentiated the analgesic effect of the same dose of morphine. These results suggest the existence of GABAergic neurons that tonically inhibit periaqueductal gray output neurons involved in centrifugal pain inhibition. The analgesic effects of opiates may, at least in part, result from disinhibition of these GABAergic neurons.

Effects of a single administration of morphine or footshock stress on natural killer cell cytotoxicity

We previously reported that daily exposure for 4 days to an inescapable form of footshock stress, known to cause opioid-mediated analgesia, suppressed the cytotoxic activity of splenic natural killer (NK) cells in rats. Similarly, daily injection of high doses of morphine (greater than or equal to 30 mg/kg) for 4 days also suppressed splenic NK cell activity. We now report that a single exposure to the opioid form of footshock stress or a single high dose of morphine induces suppression of splenic NK cell cytotoxicity. This effect is evident 3 h after treatment, returning to normal by 24 h. Morphine-induced NK suppression is evident in both male and female rats, is blocked by the opiate antagonist naltrexone, and develops tolerance. Morphine-induced NK suppression is seen in cells derived simultaneously from the spleen, bone marrow, and peripheral blood, suggesting that this suppression does not result from a selective egress of NK cells from the spleen.

Site specificity in the development of tolerance to stimulation-produced analgesia from the periaqueductal gray matter of the rat

Pentobarbital-anesthetized rats were subjected to 21 min of continuous electrical stimulation of the caudal periaqueductal gray matter (PAG) at the current threshold for analgesia. Stimulation at ventral PAG sites supported analgesia for only 1 or 2 min in most animals. Stimulation at more dorsal PAG sites supported analgesia for the entire 21 min of stimulation. This demonstration of acute tolerance with continuous ventral, but not more dorsal, PAG stimulation corresponds well with previous evidence suggesting opioid mediation of analgesia from this brain region.

Diazepam dissociates the analgesic and aversive effects of periaqueductal gray stimulation in the rat

Electrical stimulation of the periaqueductal gray matter (PAG) of the rat can produce both analgesia and aversive reactions. To determine if these two effects can be dissociated, diazepam, a benzodiazepine, was administered to rats chronically implanted with electrodes in the PAG. The threshold for stimulation-produced analgesia or aversion, whichever was lowest, was determined before and after drug administration. Diazepam (1 mg/kg) attenuated stimulation-produced aversive reactions at 12 of 20 stimulation sites, allowing analgesia to be measured at the same threshold. Diazepam did not alter baseline pain sensitivity or thresholds for stimulation-produced analgesia. These results indicate that aversive reactions and analgesia from PAG stimulation can be pharmacologically dissociated.

Characterization of the analgesic effects of the benzodiazepine antagonist, Ro 15-1788

Three experiments were carried out in rats to evaluate the analgesic effects of the benzodiazepine antagonist, Ro 15-1788. Expt. 1 demonstrated a diazepam reversible analgesic effect of Ro 15-1788 using the tail-flick test. Expt. 2 analyzed the effects of Ro 15-1788 in the hot-plate test at various doses. Low doses of Ro 15-1788 proved analgesia 20 min after drug injection, whereas high doses had no analgesic effect on the 20 min test, but were effective on the 50 min test. Expt. 3 demonstrated that the opiate antagonist, naloxone, had no effect on analgesia produced by Ro 15-1788 in the hot-plate test. The analgesic effects of Ro 15-1788 may be attributable to its reported anxiogenic properties.

Barbiturate-induced inhibition of a spinal nociceptive reflex: role of GABA mechanisms and descending modulation

The present study investigated the effect of systemically administered pentobarbital on the tail-flick (TF) reflex in rats, the neurochemical mechanism of action and the role of descending influences. Pentobarbital produced a clear inhibition of the TF response. Systemic administration of naloxone did not significantly alter this effect, thus it appears to be independent of endogenous opioid systems. Complete spinal transection resulted in a marked potentiation of pentobarbital-induced TF inhibition, demonstrating a spinal locus of action. Moreover, this observation suggests the existence of a tonic descending excitatory influence, opposing the pentobarbital-produced depression of nociceptive transmission in the intact animal. Intrathecal administration of pentobarbital caused a much more pronounced TF inhibition in transected than in intact animals, lending further support to this hypothesis. To identify the neurochemical mechanisms involved in pentobarbital-produced antinociception, the gamma-aminobutyric acid (GABA) antagonists bicuculline and picrotoxinin were administered intrathecally in spinalized animals. Both substances caused an attenuation of the pentobarbital effect, demonstrating the involvement of GABAergic transmission. The proposed descending excitatory system may act either presynaptically and cause a decreased release of GABA into the synapse or postsynaptically via endogenous GABA antagonistic neurotransmitters, which may change the conformation of the GABA-barbiturate receptor complex.

Neurochemical identification of afferents onto spinomedullary neurons in the rat spinal cord central gray matter

The synaptic relationship between spinal cord central gray projection neurons and immunocytochemically identified afferents in the rat were examined at the light microscopic level using the combined techniques of retrogradely transported True blue and serotonin (5-HT), enkephalin (ENK), and substance P (SP) immunocytochemistry. At L4-L6, numerous retrogradely labeled neurons could be identified around the central canal after large bulbar injections of True blue. Of these projection neurons, 75% were apposted by 5-HT varicosities, 57% by ENK varicosities and 58% by SP varicosities. Hemisection of the spinal cord produced a marked reduction in the amount of 5-HT immunoreactivity and the number of putative 5-HT contacts observed on neurons of the spinal cord central gray. A small decrease in SP immunoreactivity and putative contacts was seen after dorsal rhizotomy. Neither rhizotomy nor hemisection produced discernable changes in ENK immunofluorescence. Based on the distributions of 5-HT, ENK and SP in the spinal cord, we suggest that a more precise delineation of lamina X in the rat can be made according to immunocytochemical rather than strictly morphological criteria.

Involvement of brain opiate receptors in the immune-suppressive effect of morphine

We previously reported that a single systemic injection of a high dose of morphine (greater than or equal to 20 mg/kg) transiently suppresses splenic natural killer cell cytotoxicity in rats. The present study examined the possibility that the immune-suppressive effect of morphine is mediated by opiate receptors in the brain. Supporting this hypothesis, we found that morphine (20 or 40 micrograms) injected into the lateral ventricle suppressed natural killer cell activity to the same degree as a systemic dose higher by three orders of magnitude. This effect was blocked by an opiate antagonist, naltrexone. Natural killer cell activity was unaffected by systemic administration of N-methyl morphine, a morphine analogue that does not cross the blood-brain barrier. These data implicate opiate receptors in the brain in morphine-induced suppression of natural killer cell cytotoxicity.

Morphine analgesia is potentiated in adult rats prenatally exposed to ethanol

Exposure to inescapable, intermittent footshock elicits an opioid-mediated stress-induced analgesia in rats. We have previously shown that this response is markedly potentiated in adult rats, prenatally exposed to ethanol. To further investigate our hypothesis that endogenous opioid pain-inhibitory systems are modified by prenatal ethanol exposure, we have measured the analgesic response to morphine, in vitro brain opiate receptor binding characteristics, and occupation of brain opiate receptors following systemic administration of morphine. Compared to controls, rats prenatally exposed to ethanol had significantly enhanced morphine analgesia. This enhancement, however, does not appear attributable to changes in number or affinity of mu or delta opiate receptors, or to altered occupation of receptors by morphine.