Are the analgesic effects of social defeat mediated by benzodiazepine receptors?

Reversal of oxycodone and hydrocodone tolerance by diazepam

The Centers for Disease Control has declared opioid abuse to be an epidemic. Overdose deaths are largely assumed to be the result of excessive opioid consumption. In many of these cases, however, opioid abusers are often polydrug abusers. Benzodiazepines are one of the most commonly co-abused substances and pose a significant risk to opioid users. In 2016, the FDA required boxed warnings - the FDA's strongest warning - for prescription opioid analgesics and benzodiazepines about the serious risks associated with using these medications at the same time. The point of our studies was to evaluate the interactions between these two classes of drugs. We investigated whether diazepam adds to the depressant effects of opioids or do they alter the levels of tolerance to opioids. In the present study, we have found that the antinociceptive tolerance that developed to repeated administration of oxycodone was reversed by an acute dose of diazepam. Antinociceptive tolerance to hydrocodone was also reversed by acute injection of diazepam; however, a fourfold higher dose of diazepam was required when compared to reversal of oxycodone-induced tolerance. These doses of diazepam did not potentiate the acute antinociceptive effect of either opioid. The same dose of diazepam that reversed oxycodone antinociceptive tolerance also reversed oxycodone locomotor tolerance while having no potentiating effects. These studies show that diazepam does not potentiate the acute effect of prescription opioids but reverses the tolerance developed after chronic administration of the drugs.

Reversal of morphine analgesic tolerance by ethanol in the mouse

The chronic use of opioids in humans, accompanied by the development of tolerance, is a dangerous phenomenon in its own right. However, chronic opioid use is often made more dangerous by the coconsumption of other substances. It has been observed that the blood level of opioids in postmortem analyses of addicts, who consumed ethanol along with the opioid, was much less than that observed in individuals who died from opioids alone. This relationship between ethanol and opioids led us to investigate the hypothesis that ethanol alters tolerance to opioids. In the present study, we report that ethanol significantly and dose-dependently reduced the antinociceptive tolerance produced by morphine and the cross-tolerance between [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and morphine in the mouse tail-flick test. The reversal of morphine tolerance was partially blocked by both the gamma receptor blocker bicuculline and by the γ-aminobutyric acid (GABA)(B) receptor blocker phaclofen and the administration of both inhibitors completely reversed the effects of ethanol on morphine tolerance. Diazepam, like ethanol, decreased morphine tolerance. However, this inhibition was reversed by the GABA(A) antagonist bicuculline but not by the GABA(B) antagonist phaclofen. These findings have important implications for individuals who abuse opioids and ethanol as well as suggest a mechanism to reduce the amount of opioid needed in chronic pain treatment.

Influence of pharmacologically-induced experimental anxiety on nociception and antinociception in rats

Animal studies reveal that diverse environmental stimuli that generate anxiety-like behaviors also induce antinociception; conversely, clinical data show that pain perception is reduced under anxiolysis. This study was conducted to investigate the influence of pharmacologically induced-anxiety on nociception and antinociception. Experimental anxiety levels were measured using the rat burying behavior test. Diazepam (0, 0.5, 1.0 and 2.0 mg/kg, i.p.) or yohimbine (0, 0.5 and 1.0 mg/kg, i.p.) were used as anxiolytic or anxiogenic drugs, respectively. To evaluate the influence of different experimental anxiety levels on nociception, the pain-induced functional impairment in the rat (PIFIR model) was used. Nociception was induced by an intra-articular injection of 15% uric acid into the knee joint of the right hind limb. Diazepam or yohimbine were administered 15 min before uric acid and the ability of the rat to use the injured hind limb was recorded. To analyze the influence of different levels of anxiety on the antinociceptive effects produced by acetylsalicylic acid (0, 31, 100 and 310 mg/kg, p.o.); this analgesic was administered 3.5 h after uric acid. Fifteen min before diazepam (2.0 mg/kg) or yohimbine (1.0 mg/kg) were administered. We found that, in the burying behavior test, diazepam and yohimbine produced a dose-dependent decrease or augment in the cumulative time of burying, effects denoting reduced or increased experimental anxiety, respectively. Diazepam or yohimbine, administered alone, was unable to produce nociception. The results showed an influence of anxiety on nociception since a decreased (by diazepam) or increased (by yohimbine) experimental anxiety prevented nociception. Control experiments showed that acetylsalicylic acid did not modify experimental anxiety in the burying behavior test, but effectively reversed the nociception induced by uric acid (15%) in the PIFIR model. Such antinociceptive effect was unmodified by the anxiolytic or anxiogenic actions of diazepam or yohimbine. Data are discussed on the bases of clinical- and animal-studies revealing interactions between anxiety and nociception.

5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review

The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.

High intensity social conflict in the Swiss albino mouse induces analgesia modulated by 5-HT1A receptors

Social conflict between mice produces analgesia in the attacked mouse. Both the magnitude and type (opioid or nonopioid) of this analgesia have been related to attack intensity and strain of mouse. In the present study low intensity social conflict (7 bites) did not produce analgesia, whereas high intensity - 30 and 60 bites - interactions produced, respectively, short-lasting (5 min) and very short-lasting (1 min) analgesia in Swiss albino mice, when compared with nonaggressive interaction (0 bite). The 30 bites aggressive interaction induced analgesia (AIIA) was not affected by IP injection of either naloxone (5.0 and 7.5 mg/kg) or diazepam (0.5, 1.0, 2.0 and 4.0 mg/kg). However, this attack-induced analgesia was reduced after IP administration of the 5-HT1A agonists, gepirone (0.3 and 3.0 mg/kg) and BAY R 1531 (0.01 mg/kg). These results indicate that the analgesia induced by 30 bites social conflict in Swiss albino mice does not involve opioid and GABA-benzodiazepine (GABA-BZD) mechanisms. In addition, they suggest that high-intensity social conflict activates serotonergic pain modulatory systems that act through 5-HT1A receptors.

Stressor controllability, social interaction, and benzodiazepine systems

Effects of benzodiazepine receptor-active compounds on inescapable shock-produced changes in social interaction were studied in the rat. Inescapably shocked animals exhibited less social interaction in a novel situation than did escapably shocked or unshocked rats 24 h after shock. Administration of the selective benzodiazepine receptor antagonist flumazenil at the time of shock prevented the decrease in social interaction. Social interaction was unaffected by the same treatment at the time of measurement. Reduction in social interaction induced by inescapable stress endured for 48-72 h following stressor exposure but was absent 168 h after stress. It was subject to antagonist blockade at all measured time points. Stress-induced decreases in social interaction were also blocked by the benzodiazepine chlordiazepoxide given at the time of shock treatment. The receptor antagonist did not reverse this blockade. An inverse agonist, the beta-carboline FG 7142, administered in place of inescapable shock, produced an identical pattern of social interaction in a dose-dependent manner. The inverse agonist effect was also reversed by the antagonist. The results from antagonist, agonist, and inverse agonist treatments all suggest that an endogenous benzodiazepine receptor inverse agonist is released at the time of inescapable shock and is involved in producing the changes in social interaction subsequently measured.

Endogenous opioids: a proximate reward mechanism for kin selection?

The kin selection theory predicts that individuals would behave differently toward one another, depending on their genetic relatedness. Proximate mechanisms have been postulated to exist helping the individual to discriminate what is good or bad for him. Opioids have been discovered to be involved in the mediation of reinforcement, in particular they underlay social emotion. In this study it is shown that pain sensitivity decreased in male mice interacting with siblings following 2 months of separation; this analgesic response was antagonized by naloxone administration. Interaction with unknown and unrelated subjects did not change the nociceptive threshold. These results suggest that interacting with kin is an adaptive situation reinforced, at the neural level, by the release of endogenous opioids.

Anxiety enhancement in the murine elevated plus maze by immediate prior exposure to social stressors

Anxiety has been implicated in the acute nonopioid analgesic reaction seen in defeated mice. In the present study, behavioural responses to the elevated plus-maze test were examined in male DBA/2 mice immediately following defeat by an experienced aggressive conspecific. Compared to home-cage controls, defeat reduced total arm entries and rearing, although anxiety enhancement was indicated by decreases in percent open-arm entries and percent time spent on the open arms. These effects were accompanied by significant increases in nonexploratory behaviour (movement arrest and grooming) and risk assessment (closed arm returns, protected head dipping, and stretch-attend postures). This anxiogenic effect of social defeat was partially replicated in mice merely exposed to the scent of an aggressive male conspecific. The specificity of present findings to socially relevant stressors was confirmed by the general lack of effect on plus-maze behaviour of prior exposure to a novel cage or to interaction with a nonaggressive male. Present results are not only consistent with the anxiety hypothesis of defeat analgesia but also show that the elevated plus-maze test is sensitive to alterations in anxiety produced by ecologically relevant stimuli. Possible implications for pharmacological studies are discussed.

Effects of diazepam on behavioural and antinociceptive responses to the elevated plus-maze in male mice depend upon treatment regimen and prior maze experience

Recent studies have shown that brief exposure to an elevated plus-maze (EPM) produces non-opioid antinociception in male mice. The present experiments were designed to assess the effects of diazepam on this phenomenon. When acutely administered, low doses (0.5-1.0 mg/kg) of diazepam failed to produce an anxiolytic profile and exerted rather inconsistent effects on EPM-induced elevations in tail-flick latencies. In EPM-experienced mice, chronic treatment with higher doses of diazepam (2-4 mg/kg, 8 days) produced a weak anxiolytic action and inhibited the early phase of EPM antinociception only. However, in EPM-naive mice, 8-day diazepam pretreatment exerted a marked anxiolytic effect and completely eliminated the antinociceptive response to the maze. Together, these data support the view that anxiety is a key factor in certain forms of adaptive pain inhibition and suggest a possible mediational role for benzodiazepine receptors. Our findings also show that prior exposure to the EPM, rather than chronic handling/injection, greatly reduces the anti-anxiety effect of diazepam. Furthermore, since re-exposure to the maze, per se, decreased time spent on the open arms and central platform, a shift in behavioural baseline ("retest anxiogenesis") may have contributed to the weak behavioural effects of diazepam in test-experienced animals. Importantly, as chronic treatment with diazepam did not influence this anxiogenic-like retest profile, our data suggest that a single prior experience of the EPM may radically alter the nature of the anxiety reaction provoked by this test.

Effects of benzodiazepine receptor antagonist, flumazenil, on antinociceptive and behavioural responses to the elevated plus-maze in mice

Brief exposure to an elevated plus-maze has been shown to induce antinociception in male mice, a reaction that is not attenuated by manipulations of opiate receptors but which is fully blocked by diazepam. The present study examined the effects of the benzodiazepine receptor antagonist, flumazenil (5-20 mg/kg), on behavioural and antinociceptive responses to the elevated plus-maze in male DBA/2 mice. The results showed that, in the absence of an effect on total arm entries or rearing, flumazenil increased the time spent on the closed arms of the maze (an anxiogenic profile) and significantly enhanced antinociception induced by the elevated plus-maze. Data are discussed in relation to an "endogenous ligand theory" and it is concluded that the present findings are consistent with the proposed involvement of anxiety in at least certain forms of adaptive inhibition of pain.

Ethopharmacology of copulatory disorder induced by chronic social conflict in male mice

The present study was designed to investigate how the experience of fighting affects copulatory behavior in male mice and also to determine the effect of naloxone on the interaction between social conflict and copulatory disorder. To generate intraspecific fighting a resident-intruder paradigm was employed. Agonistic confrontations were terminated after 10 or 20 attack bites, and were repeated for 5 consecutive days. Twenty-four hours after the last confrontation test, both resident and intruder mice were tested with estrus females for 10 min. Compared to the control group without agonistic confrontation, intruder mice that had been attacked repeatedly showed a significant reduction of copulatory behavior. In contrast, attacking resident mice showed a significant increase in copulatory behavior. Pretreatment with naloxone (1 and 3 mg/kg, IP) prior to daily fighting failed to antagonize defeat-induced copulatory disorder. It would, therefore, appear that endogenous opioid mechanism may not participate in this phenomenon.

Modifications of social conflict-induced analgesic and activity responses in male mice receiving chronic opioid agonist and antagonist treatments

This study examined the effects of chronic (7 day) administrations of opioid agonists, via osmotic minipumps (20 micrograms/microliters/h, or 2 mg/kg/h for each agent) on: 1) nociception and activity, and 2) the analgesic and locomotor responses of subordinate male mice experiencing social conflict (aggression without defeat) and defeat in a "resident-intruder" paradigm. Chronic infusion of the mu opioid antagonist, naltrexone, resulted in a hypoanalgesic response and a decrease in basal locomotor activity on days 3-7 postimplantation which returned to the basal levels of saline-implanted control mice after termination of the infusions on day 9. Naltrexone reduced defeat-induced analgesia on the second day after implantation, but had no consistent effects on analgesia on test days 6 and 9 or on the aggression-induced (nondefeat) analgesia and increases in activity. The delta opioid antagonist ICI-154, 129, while having no significant effects on basal nociception or locomotor activity, augmented nondefeat-induced analgesia (day 2) and reduced the defeat-induced increases in activity (days 2 and 6). The mu agonist, levorphanol, resulted in a significant analgesia on the first two days after infusion, followed by the development of tolerance to the analgesic effects over days 3-7. On day 9, a hypoanalgesic response indicative of withdrawal was evident. Levorphanol also induced a marked decrease in locomotor activity over days 3-7 postimplantation, with no evidence of the development of tolerance or withdrawal following termination of infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

LY 171555-induced hyperdefensiveness in the mouse does not implicate benzodiazepine receptors

In naive mice the selective D2 agonist LY171555 dose-dependently (0.5-5 mg/kg) induces defensive responses toward non-aggressive conspecifics. In order to investigate possible anxiogenic properties of the D2 agonist, its behavioural effects were compared with those produced by the benzodiazepine receptor inverse agonist methyl-beta-carboline-3-carboxylate(beta-CCM) in the elevated plus maze and in social interactions with non-aggressive opponents. When tested in the elevated plus maze, mice injected with LY 171555 (0.005-1 mg/kg) showed no decrease either of the number of entries or of the time spent in the open arms. At 5 mg/kg an actual increase of these two measures was observed. By contrast, beta-CCM (1-3 mg/kg) dose-dependently decreased both the number of entries and the time spent in the open arms without altering locomotion. The effects of beta-CCM were antagonized by the benzodiazepine receptor antagonist RO 15-1788 (3 mg/kg) showing a selective involvement of benzodiazepine receptors in their modulation. On the other hand, beta-CCM, (1-3 mg/kg) did not produce significant effects on defensive behaviour of mice interacting with non-aggressive opponents and the defensive responses of mice treated with 1 mg/kg LY 171555 were not prevented by 5 mg/kg chlordiazepoxide. These results show that DA D2-mediated hyperdefensiveness and anxiety modulated by benzodiazepine receptors are unrelated phenomena and suggest that this behavioural response may represent a model of those forms of fear-related reaction that do not respond to benzodiazepine treatment.

Exposure to mosquitoes, Aedes togoi (Theo.), induces and augments opioid-mediated analgesia in mice

Mosquitoes and other biting flies are natural aversive stimuli commonly encountered by wild and domestic animals and by humans. We observed that male mice exposed for 30 min to a low density of female mosquitoes (Aedes togoi) displayed significant increases in nociceptive responses which were indicative of the induction of analgesia. This analgesia was blocked by the prototypic opiate antagonist naloxone (1.0 mg/kg). Exposure to a novel stimulus had no significant effect on nociception, whereas 30 min of restraint stress induced a naloxone-reversible analgesia qualitatively similar to that observed after exposure to mosquitoes. Confinement in a small chamber for 30 min also had a significant analgesic effect whose amplitude and duration were markedly potentiated by concurrent exposure to mosquitoes. These results show that exposure to mosquitoes, and likely other biting flies, both induces an opioid-mediated analgesia and augments the analgesic effects of other stressful stimuli.

Highly potent inhibitory effects of 5-HT3 receptor antagonist, GR38032F, on non-opioid defeat analgesia in male mice

Behavioural and pharmacological evidence indicates that non-opioid analgesia in defeated male mice is initiated by anxiety and that serotongergic (5-HT) substrates are implicated. In the present study, the effects of the novel putative 5-HT3 anxiolytic, GR38032F, on this form of adaptive inhibition of pain have been examined. The results showed that defeat analgesia was totally inhibited by 1 microgram/kg-1 mg/kg of GR38032F, with partial inhibition evident over the dose range of 0.0001-0.1 microgram/kg and loss of efficacy at smaller doses. These highly potent effects of GR38032F are consistent with its anxiolytic profile in animal models and cannot be accounted for by indirect actions on basal nociception. These findings point to a potentially important modulatory role for 5-HT3 receptor mechanisms in defeat analgesia and, more generally, provide further evidence for the involvement of 5-HT in the mediation of non-opioid forms of environmentally-induced antinociception.

Population differences in benzodiazepine sensitive male scent-induced analgesia in the deer mouse, Peromyscus maniculatus

We compared opioid and nonopioid involvement in the mediation of scent-induced analgesia in two populations of deer mice, Peromyscus maniculatus; P. m. artemisiae from a mainland region and P. m. angustus from a small marine island. Exposure to bedding taken from the soiled home cage of an isolated (dominant aggressive) male resident elicited a significant increase in the nociceptive responses of male deer mice from mixed sex pairs, with the island population of mice displaying significantly greater analgesic responses than the mainland animals. In the mainland population of mice, the large amplitude analgesia induced by the scent of a conspecific was insensitive to the opiate antagonist, naloxone, but could be blocked by either the benzodiazepine antagonist, Ro 15-1788, or agonist, diazepam. Exposure to the scent of individuals from the island population elicited a lower amplitude analgesia that was sensitive to both the opiate and benzodiazepine manipulations. In the island population, both the lower amplitude analgesia induced by the scent of a conspecific and the higher amplitude analgesic elicited by the scent of a mainland animal was blocked by naloxone and only partially reduced by the benzodiazepine manipulations. Bedding treated with the peppermint also induced analgesia, with the island mice displaying a markedly greater analgesic response than the mainland animals. In both populations of deer mice the peppermint-induced analgesia was blocked by naloxone and insensitive to the benzodiazepine manipulations. These findings are considered in terms of their possible ecological significance and relations to the differences in agonistic and social behaviors between island and mainland populations of deer mice and other small rodents.

Prevention of the analgesic consequences of social defeat in male mice by 5-HT1A anxiolytics, buspirone, gepirone and ipsapirone

Behavioural and pharmacological studies have suggested that anxiety may be an important factor in the initiation of non-opioid analgesia in defeated male mice. In the present study, the effects of three 5-HT1A anxiolytics (buspirone, ipsapirone and gepirone) on basal nociception and defeat analgesia were examined. Results show that the analgesic consequences of social defeat were potently blocked by all three compounds, with a rank-order potency (minimum effective doses) of ipsapirone (0.05 mg/kg) greater than gepirone (0.1 mg/kg) greater than buspirone (0.5 mg/kg). These inhibitory effects on defeat analgesia were observed in the absence of intrinsic activity on basal nociception (tail-flick assay). When administered alone, (-)pindolol produced biphasic effects on defeat analgesia with enhancement at 0.5 mg/kg and inhibition at 5.0 mg/kg. Lower doses of (-)pindolol (0.05 and 0.25 mg/kg) which did not affect defeat analgesia when administered alone, totally blocked the inhibitory effects of ipsapirone (0.5 mg/kg). Data are discussed in relation to the involvement of 5-HT1A receptor mechanisms in this adaptive form of pain inhibition.

5HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), inhibits non-opioid analgesia in defeated mice: influence of route of administration

Recent studies have suggested that anxiety may be an important factor in the non-opioid analgesic response to defeat in muroid rodents. In the present study, we have examined the influence of the 5-HT1A receptor agonist, 8-OH-DPAT, on basal nociception and defeat analgesia in male DBA/2 mice. Our results show that, while devoid of intrinsic activity on the mouse tail-flick assay, 8-OH-DPAT blocks the analgetic consequences of defeat. A ten-fold potency differential was observed as a function of route of injection, with minimum effective doses of 0.1 and 1.0 mg/kg for subcutaneous and intraperitoneal administration, respectively. Although further studies are required, these preliminary data support 5-HT1A receptor involvement in the mediation of this form of adaptive pain inhibition.

Effects of opiate agonists and antagonists on aggressive encounters and subsequent opioid-induced analgesia, activity and feeding responses in male mice

The effects of peripheral administration of the mu, kappa and sigma opiate agonists, levorphanol (1.0 mg/kg), U-50,488 (1.0 and 10.0 mg/kg), (+/-) SKF-10,047 (10.0 and 30.0 mg/kg), respectively, as well as the delta opiate antagonists, ICI-154,129 (10.0 mg/kg), and the prototypic antagonist, naloxone (1.0 mg/kg), on the agonistic behaviors and subsequent analgesic, locomotory and ingestive responses of subordinate mice were examined in a "resident-intruder" paradigm. The latter behaviors were examined in both defeated and nondefeated mice that had received an equivalent level of aggression. The mu and delta opiate antagonists decreased, while the mu, kappa, and sigma opiate agonists selectively increased aggressive behavior (number of bouts of aggressive interactions, number of bites to defeat, time to defeat). Both naloxone and the delta antagonist suppressed defeat- and aggression-induced activity and feeding, while only naloxone blocked the analgesic response. Levorphanol enhanced, U-50,488 had variable dose related effects, and SKF-10,047 decreased the defeat and aggressive-induced responses. These results indicate that various opioid systems and opiate receptors are differentially involved in the mediation of various components of the agonistic encounters and in the expression of the consequences of social conflict and defeat-induced opioid activation.

Potent inhibition of non-opioid defeat analgesia in male mice by benzodiazepine antagonist Ro15-3505

In male mice, defeat in social encounters is associated with an acute non-opioid analgesia, a reaction that may also be seen in response to the scent of a territorial conspecific. As this form of pain inhibition is blocked by diazepam and Ro15-1788, benzodiazepine receptor mediation has been proposed. To further test this hypothesis, the effects of a novel benzodiazepine receptor antagonist (Ro15-3505; 0.625-20 mg/kg) on basal nociception and defeat analgesia have been examined. Results show that, although devoid of intrinsic activity on the mouse tail-flick assay, Ro15-3505 totally blocks the analgesic consequences of defeat at doses above 1.25 mg/kg. Despite certain inconsistencies in the literature, present data provide further support for benzodiazepine receptor mediation of this ecologically-relevant form of pain inhibition.

Blockade of non-opioid analgesia in intruder mice by selective neuronal and non-neuronal benzodiazepine recognition site ligands

In male mice, the biologically significant experience of social defeat is associated with an acute non-opioid form of analgesia. Recent studies have shown that this reaction is sensitive to certain benzodiazepine receptor ligands but is unaffected by others. The present experiments were designed to assess the possibility that activity at "non-neuronal" benzodiazepine binding sites might account for this unusual pharmacological profile. Our results show that defeat analgesia was blocked by clonazepam (0.06-3 mg/kg), Ro05-4864 (2.5-20 mg/kg), Ro05-5115 (20 mg/kg), PK11195 (5-20 mg/kg) and PK14067 (10-20 mg/kg). Furthermore, when given in combination, subthreshold doses of PK11195 (2.5 mg/kg) and clonazepam (0.03 mg/kg) totally prevented defeat analgesia. All of these effects were observed in the absence of intrinsic activity on basal nociception. Together with earlier findings, current data imply that inhibition of defeat analgesia by ligands for neuronal and/or non-neuronal benzodiazepine recognition sites is most probably unrelated to their activity at these sites. Alternative explanations for the overall patterns of results are considered.

Emergence and development of stress-induced analgesia and concomitant behavioral changes in mice exposed to social conflict

Mice of the inbred strain DBA/2, when exposed to a social conflict, developed a low intensity, naloxone-insensitive analgesia after 15 bites, and a more pronounced naloxone-sensitive analgesia after 45 bites. The effective inhibition of the antinociceptive response following low and high number of bites by the alkylating opiate antagonist beta-chlornaltrexamine suggests participation of opioid mechanisms at both stress levels. Emergence of an increased tail-flick latency was indicated by the occurrence of defensive upright postures upon contact with the opponent, while animals displaying full analgesic response during the period of bite 31-45 increased their escape reactions without being in contact with the aggressor. Suppression of social conflict analgesia in mice by pretreatment with opiate antagonists facilitated the occurrence of these escape reactions. The display of panic escape responses is discussed in the context of increased fear and helplessness that developed under conditions of sustained attacks.

Effects of an endogenous anxiolytic adrenal steroid, alpha-THDOC, on non-opioid defeat analgesia in male mice: interaction with Ro15-1788

The effects of the putative anxiolytic adrenal steroid, 3α, 5α-tetrahydro deoxycorticosterone (α-THDOC), on non-opioid defeat analgesia in male mice were examined. Although devoid of intrinsic activity on the tail-flick assay, 5 mg/kg α-THDOC was found to significantly attenuate the analgesic consequences of defeat experience; lower (2.5 mg/kg) and higher (10-20 mg/kg) doses were ineffective. The β-isomer of THDOC also significantly reduced defeat analgesia, but only at the highest dose tested (20 mg/kg). Control experiments demonstrated that 5 per cent ethanol, which was employed as a vehicle for the steroids, was without effect per se on basal nociception and, over the limited dose range of 5-10 per cent, did not alter defeat analgesia. Finally, the benzodiazepine receptor antagonist, Ro15-1788 (5 mg/kg) failed to influence the inhibitory effects of either THDOC isomer. Results are discussed in relation to the potential modulation of this form of adaptive pain inhibition by adrenal steroids which influence the GABA receptor-coupled chloride ionophore.

Inhibitory influences of the adrenal steroid, 3 alpha, 5 alpha-tetrahydrodeoxycorticosterone [correction of tetrahydroxycorticosterone] on aggression and defeat-induced analgesia in mice

The effects of intraperitoneal administrations of the deoxycorticosterone metabolite, 5 alpha-pregnane-, 21 diol-20-one (3 alpha, 5 alpha-tetrahydrodeoxycorticosterone; alpha-THDOC) on the responses to aggression and defeat-induced analgesia were examined in subordinate intruder male mice in "resident-intruder" pairings. alpha-THDOC reduced in a dose-dependent mannter (1-20 mg/kg) the number of bites and time to obtain defeat in subordinate mice during the agonistic encounters, as well as attenuating defeat-induced analgesia. These inhibitory effects of alpha-THDOC were separate from its sedative actions at 20-30 mg/kg. In addition, the stereo-isomer, 3 beta-pregnane-3 alpha, 21 diol-20-one (20 mg/kg) had no significant effects on the agonistic encounters and defeat, indicating that the inhibitory effects of alpha-THDOC on agonistic interactions are stereospecific. Pretreatment with the benzodiazepine antagonist Ro 15-1788 (5 and 10 mg/kg) attenuated the inhibitory effects of alpha-THDOC on defeat-induced analgesia. Ro 15-1788 (5, 10 mg/kg) by itself, however, had minimal effects on these agonistic interactions and subsequent defeat-induced analgesia. These results indicate that the naturally occurring steroid, alpha-THDOC, has significant effects on responses to aggression and defeat-induced analgesia.

Brief exposure to a natural predator, the short-tailed weasel, induces benzodiazepine-sensitive analgesia in white-footed mice

Exposure to a natural predator, the short-tailed weasel, Mustela erminea, elicited significant increases in the nociceptive responses of wild male white-footed mice, Peromyscus leucopus. A short (30 sec), ecologically relevant, nonvisual exposure to a weasel elicited a relatively brief (15 min) analgesia that was insensitive to the opiate antagonist, naloxone (1.0 mg/kg), and was blocked by either pre- or post-exposure injections of the benzodiazepine antagonist, Ro15-1788 (10 mg/kg), or agonist, diazepam (4.0 mg/kg). A 5 min exposure to the weasel elicited an analgesic response of longer duration (15-30 min) that was sensitive to both naloxone and the benzodiazepine agonist and antagonist. A 15 min exposure to the weasel induced a higher amplitude analgesia that was of relatively long duration (45 min), blocked by naloxone, and insensitive to the benzodiazepine manipulations. Exposures of 5 and 15 min to a nonpredator, the European rabbit, Oryctolagus cuniculus, elicited low amplitude, naloxone-reversible analgesic responses that were unaffected by the benzodiazepine manipulations. Thirty-sec exposures to the rabbit had no significant effects on the nociceptive response of the rabbit. These results indicate that a brief, ecologically appropriate, exposure to a predator elicits a benzodiazepine-mediated analgesia while a more prolonged exposure to a predator induces opioid-mediated analgesia. These results show that the opioid and nonopioid distinction between the effects of long- and short-term laboratory stresses is also evident with a natural, ecologically relevant, stressor.

Social conflict analgesia: inhibition of early non-opioid component by diazepam or flumazepil fails to affect appearance of late opioid component

Two forms of analgesia (opioid and non-opioid) are associated with social conflict in mice. The non-opioid form is seen in response to the scent of an aggressive conspecific or defeat experience, whilst the opioid form occurs in response to extended conspecific attack. Recently, it has been reported that the non-opioid reaction is dose-dependently blocked by diazepam and by Flumazepil (Ro15-1788; a benzodiazepine receptor antagonist). In view of the temporal relationship between these two reactions, the present study was conducted to determine whether activation of non-opioid substrates is a necessary precursor to the development of opioid analgesia. Results indicate that inhibition of non-opioid analgesia by diazepam (2-4 mg/kg), or by Flumazepil (20-40 mg/kg), does not alter the opioid analgesic reaction to conspecific attack. Findings are discussed in relation to the presumed adaptive significance of these biologically-meaningful forms of pain inhibition.