Benzodiazepine ligands, nociception and 'defeat' analgesia in male mice

Stress-induced analgesia

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

The antinociceptive effects of midazolam on three different types of nociception in mice

Antinociceptive effects of systemically administered midazolam remain controversial. The present study was performed to investigate its antinociceptive effects on different types of nociception in mice. Four different doses of midazolam (1, 3, 10, and 30 mg/kg) were administered intraperitoneally (i.p.). Saline was used as a control. The hot plate test, tail pressure test, acetic acid writhing test, the running wheel test, and the balance beam test were performed following the drug administration. In the hot plate test and tail pressure test, i.p. midazolam produced significant antinociceptive effects with the 50% effective dose (ED(50)) of 3.46 mg/kg [confidence interval (CI), 1.99 - 6.01 mg/kg] and 3.52 mg/kg (CI, 2.77 - 4.47 mg/kg), respectively. In the acetic acid writhing test, i.p. midazolam also produced significant antinociceptive effects. In the running wheel test, no mice stopped running after saline or midazolam at 1, 3, or 10 mg/kg, but all mice stopped running 30 and 45 min after i.p. administration of midazolam at 30 mg/kg. In the balance beam test, 30 min after i.p. administration of saline or midazolam at 1, 3, and 10 mg/kg, all mice were able to stay on the beam for 90 s, none of them could with midazolam at 30 mg/kg. In conclusion, systemically administered midazolam had antinociceptive effects on acute thermal, acute mechanical, and acute inflammatory-induced nociception in mice. The antinociceptive potency of midazolam was the same for both acute thermal-induced nociception and mechanical-induced nociception.

Reflex sympathetic activity after intravenous administration of midazolam in anesthetized cats

BACKGROUND

Although intrathecal midazolam has been reported to produce antinociceptive effects mediated by gamma-aminobutyric acid type A-benzodiazepine receptor complexes in the spinal cord, the effects of systemic midazolam on nociception remain unclear. We performed this study to examine the effects of IV-administered midazolam on somatosympathetic Adelta and C reflex discharges in brain-intact cats and decerebrate cats (with transection at midbrain level).

METHODS

Somatosympathetic Adelta and C reflexes were elicited in the inferior cardiac sympathetic nerve by electrical stimulation of myelinated (Adelta) and unmyelinated (C) afferent fibers of the superficial peroneal nerve in 28 mature cats. After control somatosympathetic reflex responses were obtained, midazolam was administered IV to four groups of randomly allocated cats as follows: brain-intact cats at a dose of 0.03 mg/kg, brain-intact cats at a dose of 0.1 mg/kg, brain-intact cats at a dose of 0.5 mg/kg, and decerebrate cats at a dose of 0.1 mg/kg.

RESULTS

C reflex discharges were significantly augmented at the dose of 0.03 mg/kg and significantly depressed at the dose of 0.1 and 0.5 mg/kg in brain-intact cats. C reflex discharges were also significantly depressed at the dose of 0.1 mg/kg in decerebrate cats.

CONCLUSIONS

We have demonstrated that IV midazolam produces dose-related effects on somatosympathetic reflex discharges. The clinical implication of these findings is that the effect of midazolam on nociception depends on its dosage. It also appears that the infra-midbrain region plays a major role in mediating the depressive effects of midazolam on somatosympathetic C reflex discharges.

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.

Influence of inflammatory nociception on the anxiolytic-like effect of diazepam and buspirone in rats

RATIONALE

The effect of anxiety on nociception has been evaluated but not that of nociception on anxiety.

OBJECTIVE

The study was conducted to analyse the influence of nociception on basal levels of anxiety-like behaviour and on the action of anxiolytic drugs.

METHODS

Nociception was induced by an intra-articular injection of uric acid at 3.75 or 7.5%. Experimental anxiety was determined in the rat burying behaviour and the elevated plus maze tests. To separate specific anxiety-related drug actions, a spontaneous ambulatory test was included. The anxiolytics, buspirone (2.5 and 5.0 mg/kg, i.p.) and diazepam (0.5, 1.0 and 2.0 mg/kg, i.p.), were used.

RESULTS

In the nociception test, the pain-induced functional impairment rat model, uric acid at 3.75 and 7.5% had an effect of around 35 and 75%, respectively. Uric acid (UA) at the lower dose (3.75%) lacked an effect on burying behaviour but significantly increased the time spent and number of entries to the open arms; the higher UA dose (7.5%) produced a significant increase in the time spent and number of entries to the open arms and a statistically significant reduction in cumulative burying. Diazepam and buspirone produced a clear dose-dependent reduction in cumulative burying. In the plus maze, diazepam also induced an increase in the time spent and number of entries to the open arms. In the burying behaviour test, rats with a mild level of nociception (uric acid at 7.5%) were insensitive to the anxiolytic-like effect of these anxiolytic drugs. In the plus maze test, the anxiolytic-like effect of diazepam (1.0 mg/kg) was blocked under both levels of nociception.

CONCLUSIONS

These data demonstrate that nociception modifies the response to anxiolytic drugs. The role of factors with anxiogenic properties produced during inflammation, which may modify diazepam and buspirone effects, is discussed.

Male and female C57BL/6 mice respond differently to diazepam challenge in avoidance learning tasks

Benzodiazepines (BZ) impair learning and memory performance of animals. The goal of this study was to examine sex differences in the effects of diazepam on learning and memory of C57BL/6 mice in avoidance paradigms. Male and female C57BL/6 mice were tested in the one-way active avoidance, step-down passive avoidance, and foot-shock pain threshold tasks, following administration of vehicle or diazepam (1 mg/kg). No substantial sex or drug effects on the threshold of the pain response to shock were found. There were no significant differences in avoidance performance between vehicle-treated male and female mice while 1 mg/kg of diazepam produced opposite effects on performance of males and females in both tasks. Diazepam-treated females learned faster in the active avoidance task and showed stronger retention in the passive avoidance task. In contrast, diazepam impaired learning of males in the active avoidance task and had no effect on their performance in the passive avoidance task. Diazepam-induced impairment in males was not due to higher sensitivity to the sedative effect of diazepam as females were more sedated than males on the first trial of the passive avoidance task. Our data showed that sedative and amnesic effects of BZs are not tightly linked. This study also suggests that cognitive effects of BZs in rodents could be sex dependent and highlight the importance of using both sexes in studies on behavioral effects of psychoactive drugs.

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.

Behavioral effects of diazepam in the murine plus-maze: flumazenil antagonism of enhanced head dipping but not the disinhibition of open-arm avoidance

Although it is widely believed that benzodiazepines reduce anxiety through positive allosteric modulation of the GABA(A)-chloride channel complex, this is not the only mechanism through which agents of this class can modify CNS function. Furthermore, a significant number of reports of apparent flumazenil blockade of diazepam anxiolysis in animal models have paid limited attention to possible intrinsic behavioral actions of the antagonist per se. In the present study, ethological methods were employed to assess in detail the effects of diazepam, flumazenil, and their combination on the behavior of male DBA/2 mice in the elevated plus-maze paradigm. In two experiments, diazepam (1.5 mg/kg) alone reduced open-arm avoidance and increased head dipping, whereas flumazenil (10-40 mg/kg) alone was without significant behavioral effect. However, with the sole exception of head dipping, prior administration of flumazenil (10 and 40 mg/kg) failed to block the behavioral effects of diazepam under present test conditions. These findings imply that the anxiolytic effects of diazepam in the mouse plus-maze are not mediated through flumazenil-sensitive benzodiazepine receptors and that alternate mechanisms must be considered.

Effects of chlordiazepoxide on opioid-induced antinociception and respiratory depression in restrained rats

This study investigates the influence of possible stress due to housing in Bolman cages on antinociception and on respiratory depression following opioid administration. To evaluate the functional role of this stressor and to modulate it, rats were subcutaneously pretreated with the anxiolytic chlordiazepoxide (CDP; 10 mg/kg) or saline (SAL) before the immobilization in the Bolman cages and before the intravenous administration of small doses of morphine (MOR), sufentanil (SUF), or vehicle (VEH). Antinociception, respiratory impairment and stress were evaluated by means of the tail-flick latency, blood gas analysis, and serum corticosterone (CS), adrenocorticotropic hormone (ACTH), and prolactin (PRL) determinations. The results demonstrated that 10 mg/kg CDP did not alter the antinociceptive effects of low doses of morphine and sufentanil. CDP pretreatment differentially affected the various blood gas parameters. Compared to vehicle pretreatment, there was a larger decrease in PaO2 following MOR and SUF in the CDP-pretreated rats. The effects were most pronounced at the lowest doses of both opioids. A CDP potentiation was also observed for the short-lasting raises in PaCO2 with the lowest concentrations of the opioids. At higher concentrations of the opioids, CDP was without any effect. With regard to the stress hormones, immobilization and an intravenous injection resulted in increases in CS and PRL in both CDP- and VEH-pretreated rats. ACTH did not change in these controls. SUF prevented the CS raises independent of a CDP pretreatment, while ACTH only increased in the SUF plus CDP groups, pointing to a stress-reducing effect of SUF. Also, MOR without CDP prevented the increases in CS, but the opioid intrinsically increased ACTH. These results indicate that restraint in Bolman cages in the present setup, with animals recovering for several hours in these cages after being equipped with an arterial catheter, is stressful but without any significant effect on the opioid-induced antinociception. Pretreatment with an anxiolytic benzodiazepine only minimally affected the outcome of the opioids on respiratory depression and pointed to a stress-reducing effect of low doses of the opioids, especially sufentanil.

Identification of a sex-specific quantitative trait locus mediating nonopioid stress-induced analgesia in female mice

It is increasingly appreciated that the sexes differ in their perception of noxious stimuli and in their responsivity to exogenous and endogenous analgesic manipulations. We previously reported the existence of qualitative sex differences in the neurochemical mediation of nonopioid (i.e., naloxone-insensitive) stress-induced analgesia (SIA) produced by forced swims and suggested that female mice possess a sex-specific SIA mechanism. This female-specific system is now known to be estrogen-dependent, to be ontogenetically organized, and to vary with reproductive status; however, its neurochemical identity remains obscure. In an attempt to identify candidate genes underlying SIA in both sexes, we performed a two-phase quantitative trait locus (QTL) mapping experiment using the BXD/Ty recombinant inbred (RI) set derived from DBA/2J (D2) and C57BL/6J (B6) inbred mouse strains and (B6xD2)F2 hybrid mice derived from these same progenitors. All mice were subjected to 3 min forced swims in 15 degrees C water; nociceptive sensitivity on the 54 degrees C hot-plate assay was assessed immediately before and 2 min after cessation of the swim. We report the localization of a QTL statistically associated with SIA magnitude [p = 0.00000012; logarithm of the odds (LOD) = 6.1] in female mice only. This female-specific QTL, which we name Fsia1, is located on chromosome 8 at 52-84 cM from the centromere and accounts for 17-26% of the overall trait variance in this sex. The present data provide further evidence of the existence of a female-specific SIA mechanism and highlight the important role of both genetic background and gender in the inhibition of pain.

Learning and novelty induced increase of central benzodiazepine receptors from chick forebrain, in a food discrimination task

Young chicks were trained to discriminate food grains from inedible pebbles. On Day 1 and Day 2 of the task, latency to peck, and number of pecks were scored and the forebrain [3H]flunitrazepam receptor binding was also determined at 0 and 30 min after an 8-min training session. Compared with quiet controls, the receptor density increased 46%, 30 min after the training session on Day 1. Compared with chicks that had learned the discrimination and were merely repeating already learned behavior on Day 2, the receptor increased more than 46%. Since chicks that had learned the discrimination had a higher behavioral activity, we interpret that the learning of a new task is itself responsible in addition to stress for the receptor density increase. Stressful factors accompanying the learning task as handling and novelty increased 17% the receptor density, 30 min after a training session without food, compared with quiet controls. However, receptor density did not increase in chicks repeating the same housing conditions, suggesting that chicks were habituated to handling and novelty on Day 2. Differences in receptor density were not observed between quiet controls and experimental groups, at 0 min after the training session, indicating that changes were time dependent. In all cases the affinity remained unchanged. Our results suggest that, the GABA(A) receptor (i) is involved in early stages of memory formation and in stress adaptive responses, and (ii) is modulated by new non-repetitive environmental conditions.

Animal models of anxiety: an ethological perspective

In the field of anxiety research, animal models are used as screening tools in the search for compounds with therapeutic potential and as simulations for research on mechanism underlying emotional behaviour. However, a solely pharmacological approach to the validation of such tests has resulted in distinct problems with their applicability to systems other than those involving the benzodiazepine/GABAA receptor complex. In this context, recent developments in our understanding of mammalian defensive behaviour have not only prompted the development of new models but also attempts to refine existing ones. The present review focuses on the application of ethological techniques to one of the most widely used animal models of anxiety, the elevated plus-maze paradigm. This fresh approach to an established test has revealed a hitherto unrecognized multidimensionality to plus-maze behaviour and, as it yields comprehensive behavioural profiles, has many advantages over conventional methodology. This assertion is supported by reference to recent work on the effects of diverse manipulations including psychosocial stress, benzodiazepines, GABA receptor ligands, neurosteroids, 5-HT1A receptor ligands, and panicolytic/panicogenic agents. On the basis of this review, it is suggested that other models of anxiety may well benefit from greater attention to behavioural detail.

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.

On mouse pups and their lactating dams: behavioral consequences of early exposure to oxazepam and interacting factors

Behavioral analysis in animal models appears to be a valuable and sensitive tool for detecting subtle alterations in CNS function, which can be produced by early exposure to small perturbations of sensory experience, hormonal milieu, or exposure to psychotropic agents devoid of major teratogenic potential. Concerning anxiolytics, the more recent work in mice, which is here summarized, was carried out by putting the emphasis on changes in naturally occurring species-typical social responses as a function of early exposure to benzodiazepines. For adult females, on the behavior expressed during the early postpartum period, whereas for infant subjects, on the ontogenetic stage of the establishment of social bonding. Critical issues such as the choice of fostering procedure and rearing conditions are also addressed. Evidences of relationships between changes in social milieu taking place during early rearing-i.e., when dramatic transitions in the neurochemical target system occur- and the adult behavioral response to challenges with BDZ agents are presented. These data strengthen the notion that the modes of reaction of adult animals to the joint influence of physiological and environmental (stimulus) variables are under the influence of events in early ontogenesis. Therefore, a better understanding of the mechanisms-as unveiled by an appropriate use of drug tools-that mediate such a plasticity might have considerable psychobiological and clinical-therapeutical relevance.

Ethopharmacological analysis of the effects of putative 'anxiogenic' agents in the mouse elevated plus-maze

The literature on the effects of anxiety-provoking agents in humans and animals is replete with inconsistent and contradictory findings as well as data that may have alternate explanations. To further our understanding in this area, ethological methods were used to assess in detail the effects of four putative anxiogenic agents in the murine elevated plus-maze test. Compounds studied were FG 7142 (1.25-10.0 mg/kg), pentylenetetrazol (PTZ; 1.875-30.0 mg/kg), isoproterenol (0.125-1.0 mg/kg), and sodium lactate (32.75-262.0 mg/kg). FG 7142 produced an anxiogenic-like profile at 10 mg/kg, an effect that could not be attributed to seizure activity or nonspecific behavioural suppression. PTZ exerted biphasic effects, with low doses (1.875-3.75 mg/kg) producing anxiolytic-like effects and high doses (20.0-30.0 mg/kg) anxiogenic-like effects. With the exception of the highest dose tested, which radically disrupted behavior, these effects of PTZ were also seen to be behaviorally specific. Although some minor behavioural changes were evident with sodium lactate and isoproterenol, neither compound altered anxiety-related measures under present test conditions. Data are discussed in relation to distinctions between anxiety and panic, and the nature of anxiety expressed in and detected by animal models.

The benzodiazepine receptor antagonists flumazenil and CGS8216 block the enhancement of fear conditioning and interference with escape behavior produced by inescapable shock

Prior work has suggested that the mediation of the behavioral effects of inescapable shock (IS) might involve release of an endogenous beta-carboline-like ligand at the dorsal raphe nucleus (DRN) that binds to the benzodiazepine (BZ) recognition site on the GABAA complex, thereby disinhibiting the DRN. This was tested by microinjection of the BZ receptor antagonists flumazenil and CGS8216 in the region of the DRN, either before IS or before later behavioral testing. Both compounds blocked subsequent enhancement of fear conditioning and interference with shuttlebox escape when administered before IS, but had no effect when given before testing. In addition, flumazenil did not alter the behavior of escapably shocked subjects.

Antinociceptive activity of midazolam in sheep

The purpose of this study was to examine the effects of midazolam on the nociceptive threshold responses in sheep. The intravenous administration of midazolam (0.1-0.3 mg/kg) produced a significant dose-dependent elevation of the mechanical and thermal nociceptive thresholds. The intravenous administration of flumazenil (20 micrograms/kg) markedly attenuated the antinociceptive activity of midazolam in the mechanical nociceptive test, whereas intravenous naloxone (0.2 mg/kg) had no significant effect on midazolam-mediated analgesia. The intrathecal administration of midazolam (1 mg), via chronically implanted cervical subarachnoid catheters, produced a significant elevation in the mechanical threshold responses. These results indicate that midazolam has antinociceptive actions in the sheep and suggest that this effect is, at least partially, mediated at the spinal level.

Bilateral lesions of the amygdala attenuate analgesia induced by diverse environmental challenges

This study was designed to evaluate the role of the amygdala, particularly its central nucleus, in the induction of analgesia elicited by environmental challenges. Rats with large, radiofrequency lesions centered in the central nucleus were found to display significantly attenuated analgesic responses to three different challenges: cat exposure, acute footshock, and re-exposure to an environment associated with footshock. These findings show that the amygdala plays an important role in the elicitation of analgesia by each of the environmental challenges tested. Since the amygdala has been shown to play a critical role in fear, these findings suggest that the analgesia elicited by these challenges involves a substantial fear component. Moreover, the finding that amygdala lesions significantly reduced the analgesia elicited by a non-noxious unconditional stimulus (cat exposure) strongly suggests that these lesions disrupt the expression of analgesia rather than producing a learning impairment. And finally, the findings of this study support the suggestion that fear-elicited analgesia is triggered by activation of a projection from amygdala to periaqueductal gray which forms one component of an integrated 'defensive behavioral system.'

Effect of chronic maternal diazepam treatment on the development of stress-induced antinociception in young rats

The use and abuse of benzodiazepines is widespread and we have begun to address whether maternal exposure to these drugs influences the development of opioid systems. We have studied the effect of maternal diazepam treatment on stress-induced antinociception in the neonatal offspring. Diazepam (1 or 10 mg/kg) was administered twice daily to mothers from conception. Pre- and postweanling rat pups were assessed for opioid-mediated stress-induced antinociception by 3-min swimming and measuring nociception using the tail immersion test. In preweanling rats there was stress-induced antinociception in both vehicle- and diazepam-treated animals but in diazepam-treated groups (1 and 10 mg/kg) this was insensitive to reversal by the opioid antagonist naloxone, suggesting that nonopioid systems are operating this response. In postweanling rats a similar insensitivity to naloxone was observed in 1 mg/kg diazepam-treated groups; with 10 mg/kg diazepam there was no significant antinociception. The results suggest that maternal diazepam treatment interferes with the development of stress-mediated responses and that part of this toxicity is due to actions on opioid systems in the CNS.

Modulation of the in vivo antibody response by a benzodiazepine inverse agonist (DMCM) administered centrally or peripherally

Exposure to stressors can result in changes in immune function. Although there is increasing information concerning the peripheral hormonal and neural mediators of stress-induced changes in immune function, there is little information concerning the central nervous system mechanisms that lead to the peripheral changes. The following experiments examined the possible involvement of the benzodiazepine-GABAA-chloride complex in modulation of the in vivo antibody response. Rats were given either peripheral or intracerebroventricular injections of methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), a drug that has been shown to act at the benzodiazepine-GABAA complex and produces a behavioral state similar to anxiety. Rats were then immunized with keyhole limpet hemocyanin (KLH) and serum levels of KLH-specific antibody were measured for 2 weeks after immunization. Both peripheral and central administration of DMCM modulated the in vivo antibody response. The dose-response relationship of DMCM and changes in antibody levels was nonmonotonic, with high doses resulting in an increase in serum antibody levels and moderate doses resulting in a decrease in serum antibody levels. A possible role of the benzodiazepine-GABAA system in stress-induced immunomodulation is discussed.

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.

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.

Naloxone-induced hypoalgesia: lack of involvement of the GABA-benzodiazepine receptor complex

Previous evidence has demonstrated that repeated daily administration of the opiate receptor antagonist naloxone prior to assessment of pain sensitivity provokes the development of a nonopioid form of hypoalgesia. The present experiments assessed whether the GABA-benzodiazepine receptor complex may be involved in the mediation of this effect. Male Wistar rats were administered 10 mg/kg naloxone prior to hot-plate tests (48.5 degrees C) for pain sensitivity for 8 consecutive days. Control animals were administered saline prior to, and naloxone 2-4 h after, assessment of pain reactivity. Beginning on the fourth or fifth day of this regimen, animals tested under the influence of naloxone displayed longer paw-lick latencies than controls. Preadministration of the GABAA agonist muscimol (1.0-5.0 mg/kg) and GABAA antagonist bicuculline (0.25-1.0 mg/kg) failed to affect paw-lick latencies in naloxone-tested and control rats. The GABAB receptor agonist baclofen (1.0-5.0 mg/kg) and the benzodiazepine receptor agonist diazepam (1.0-5.0 mg/kg) both elevated paw-lick latencies to the same degree in both groups of animals. These results suggest that the GABA-benzodiazepine receptor complex is not involved in the mediation of naloxone-induced hypoalgesia.

Sexual segregation in infancy and bi-directional benzodiazepine effects on hot-plate response and neophobia in adult mice

In the present experiment, the hypothesis that rearing animals in conditions of sexual segregation in infancy (ISS) would affect their adult behavioral reactivity to drug or environmental challenges was tested. Outbred Swiss CD-1 mouse litters were reduced at birth to six pups according to three conditions: MM (all males), MF (sex-balanced composition), and FF (all females). At weaning (day 21), all mice were rehoused in unisexual groups. At adulthood (day 70), animals were challenged either with BDZ agonist chlordiazepoxide (CDP at 2.5- or 5.0-mg/kg dose) or BDZ receptor partial inverse agonist Ro 15-3505 (RO at 3-, 10-, or 30-mg/kg dose) and assessed in sequence for pain reactivity in a hot-plate apparatus (set at 55 +/- 1 degrees C), for locomotor activity in a Varimex apparatus, and finally for neophobia level by measuring the latency to first approach a novel object. As concerns the hot-plate test, lick latency was significantly shortened in MF females receiving CDP (5.0 mg/kg), while RO was either ineffective in MF females or induced a prominent dose-dependent analgesia in FF females. Activity was decreased by CDP (2.5 mg/kg) and enhanced by RO (3.0 mg/kg). For latency to approach a novel object, males as a whole exhibited shorter times than females. Mixed-sex animals of both sexes were less fearful, being also more explorative than their corresponding unisexually reared groups. In particular, MF males receiving either a 5.0-mg/kg CDP dose or a 3.0-mg/kg RO dose explored the object more often than MM males.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA and behavior: the role of receptor subtypes

The discovery of different GABA receptor subtypes has stimulated research relating this neurotransmitter to a variety of behavioral functions and clinical disorders. The development of new and specific GABAergic compounds has made it possible to try to identify the specific functions of these receptors. The purpose of the present review is to evaluate the data regarding the functions of the GABA receptor subtypes in different behaviors such as motor function, reproduction, learning and memory, and aggressive-defensive behaviors. A description of GABAergic functions (stress, peripheral effects, thermoregulation) that might directly or indirectly affect behavior is also included. The possible involvement of GABA in different neurological and psychiatric disorders is also discussed. Although much research has been done trying to identify the possible role of GABA in different behaviors, the role of receptor subtypes has only recently attracted attention, and only preliminary data are available at present. It is therefore evident that still much work has to be done before a clear picture of the behavioral significance of these receptor subtypes can be obtained. Nevertheless, existing data are sufficient to justify the prediction that GABAergic agents, in the near future, will be much used in the field of behavioral pharmacology. It is hoped that the present review will contribute to this. Some specific suggestions concerning the most efficient way to pursue future research are also made.

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.

Benzodiazepines and pain: effects of midazolam on the activities of nociceptive non-specific dorsal horn neurons in the rat spinal cord

The high density of GABA-benzodiazepine receptors in the superficial dorsal horn suggests a possible involvement of benzodiazepines (BZs) in the modulation of spinal pain processes. In this electrophysiological study we have examined the effects of midazolam (MZ), a water-soluble short-acting BZ, on the activities of 57 nociceptive non-specific dorsal horn cells, one in each animal. Recordings were performed at lumbar level in unanesthetized decerebrate spinal rats before and following intravenous injection of MZ (1, 2 or 5 mg/kg). The spontaneous activity was weakly and significantly but not dose dependently reduced by MZ. For the total neuronal population MZ induced no significant effect on C-fiber evoked responses, whatever the dose used. More precise analysis shows that for 45/55 neurons the responses were slightly depressed, but this effect was not dose dependent. In contrast, A delta-fiber evoked responses were markedly and dose dependently depressed. These effects of MZ were reversed by intravenous administration of the antagonist flumazenil (FZ). Despite the fact that MZ displays a very weak effect on responses due to C-fiber stimulation, the possible involvement of BZs in the modulation of nociceptive transmission at the level of the dorsal horn is discussed on the basis of clinical and experimental findings, taking into account the role of GABAergic mechanisms in sensory events.

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.

Differential effects of novel ligands for 5-HT receptor subtypes on nonopioid defensive analgesia in male mice

The effects of a number of 5-HT receptor ligands were examined on nonopioid defensive analgesia in male DBA/2 mice. MDL 73005EF (0.05-1.0 mg/kg), a selective 5-HT1A receptor agonist, potently and dose-dependently inhibited the analgesic consequences of social defeat. CGS 12066B (0.5-10.0 mg/kg) and MK-212 (0.3-10.0 mg/kg), selective agonists for 5-HT1B and 5-HT1C sites, respectively, failed to influence this particular form of adaptive pain inhibition. Two 5-HT2/1C receptor antagonists, ritanserin (0.05-10.0 mg/kg) and ICI 169.369 (0.3-10.0 mg/kg), were also devoid of specific effects upon defensive analgesia. Both ritanserin and ICI 169,369 were found to have intrinsic analgetic efficacy and to induce behavioural changes indicative of increased defensiveness. These data, together with previous findings, confirm the specific involvement of 5-HT1A receptor mechanisms in the analgesic consequences of social defeat in male mice. Results are discussed in relation to the role of anxiety in adaptive pain inhibition.

Effects of the benzodiazepine receptor inverse agonist, DMCM, on the behaviour of mice: an ethopharmacological study

Ethopharmacological procedures and a two-compartment black and white test box were used to examine behavioural effects produced by the benzodiazepine receptor inverse agonist, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), when given by intraperitoneal injection (0.4 and 1.0 mg/kg) to pair-housed adult CD1 male mice. Control mice received injections of the solvent. Behaviour in the light-dark box was examined at 30 min after the injection and behaviour during encounters with an untreated group-housed male, in a neutral cage, was then assessed by ethological procedures. In dominant mice, aggressive behaviour was significantly reduced and the ratio of flight, relative to aggression received, was significantly increased by DMCM at 0.4 mg/kg. At 1 mg/kg but not 0.4 mg/kg, DMCM decreased time spent by dominant mice in the light compartment of the test box. In both dominant and subordinate mice, flight was increased by DMCM at 1 mg/kg to a level close to statistical significance. Treatment with DMCM had no other detectable effect on the behaviour of subordinate animals. It is suggested that anxiogenic activity of this compound might induce a shift of agonistic behaviour from aggression to "fear-induced flight".

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.

Effects of diazepam on nociception in rats

Acute i.p. injection of diazepam (1 mg/kg) resulted in a moderate increase in the tail-flick latency in rats. Tolerance to this diazepam effect developed after 10 days of diazepam treatment (1 mg kg-1 day-1). The benzodiazepine antagonist Ro 15-3505 only partially reversed the effect of diazepam on nociception. Naloxone (5 mg/kg i.p.) failed to affect the effect of diazepam on nociception, while the kappa antagonist MR 2266 fully antagonized the diazepam-induced increase of the tail-flick latency. Diazepam injected intracerebroventricularly (1, 5, 20 micrograms/rat) did not alter basal nociceptive threshold, however, diazepam injected intrathecally (20 micrograms/rat) prolonged the tail-flick latency. Furthermore, intracerebroventricular injection of muscimol partially antagonized the i.p. diazepam-induced increase of the tail-flick latency. These results suggest that benzodiazepine receptor sites are partially involved in the effect of diazepam on nociception and indicate that an indirect kappa-opioid-receptor-mediated mechanism may be involved. The anatomical site of diazepam action on tail-flick latency seems to be at the spinal level. Descending axons to the spinal cord from brain areas reached by intracerebroventricular injection of muscimol seem to modulate the effect of diazepam effect on nociception.

Experience influences environmental modulation of function at the benzodiazepine (BZD)/GABA receptor chloride channel complex

Function of the benzodiazepine (BZD)/GABA receptor chloride channel complex was selectively altered by specific aspects of an environmental challenge, i.e. encountering a stranger in a familiar versus an unfamiliar environment. Chloride (Cl-) enhancement of [3H]flunitrazepam [( 3H]flu) binding was facilitated in rats tested in an unfamiliar environment relative to that in rats tested in a familiar environment. Basal [3H]flu binding (binding in the absence of NaCl) also was greater in rats tested in the unfamiliar environment than in rats tested in the familiar environment, and Scatchard analysis of [3H]flu binding indicated that increased [3H]flu binding in the unfamiliar environment reflected an increase in both binding affinity and maximal binding capacity. In addition, both the sensitivity of [3H]flu binding to Cl- and the affinity of BZD recognition sites were decreased in handled control rats relative to non-handled control rats as well as to environmentally-challenged (prehandled) rats, suggesting that the experience of daily handling as well as familiarization with the environment modulates function at the BZD/GABA receptor complex. GABA-mediated 36Cl- uptake was facilitated by testing in either the familiar or unfamiliar environment relative to that measured in non-handled control rats. Thus, changes in GABA-gated chloride channel function may reflect a more fundamental response of this complex to challenging situations. These findings suggest that components of the BZD/GABA receptor complex are differentially influenced by specific aspects of an environmental challenge. Furthermore, function at the BZD recognition site/chloride channel component of this receptor complex is influenced by both repeated and single exposure to specific environments.

An ethological model for the study of activation and interaction of pain, memory and defensive systems in the attacked mouse. Role of endogenous opioids

The present work reviews neurochemical, physiological and behavioral data recorded from the attacked mouse and integrates them into a model of coping mechanisms during social conflict. More specifically, the possible relationships between systems of pain, memory and defense are presented, with special emphasis on the role of endogenous opioid peptides (EOPs). In recipients of attack, decreased beta-endorphin-like immunoreactivity and changes in opiate and benzodiazepine binding characteristics are found in structures of the brain defensive system. EOPs mediate the social conflict-induced increase of dopamine synthesis in the periaqueductal grey and frontal cortex. Social conflict analgesia in attacked mice is under the control of central opioid and nonopioid (e.g., benzodiazepine, glutamate) mechanisms, and is modified by experience (e.g., long-term analgesic reaction; tolerance). EOPs and pain-inhibitory mechanisms participate in the organization of behavioral defense, recuperative behavior and the memory of attack experience. The data are considered in relation to the perceptual-defensive-recuperative model of fear and pain, forwarded by Bolles and Fanselow.

The influence of stress on convulsive parameters in the mouse

Mice exposed to the stress of conspecific aggression for 10 min showed shorter latencies to convulsions induced by pentylenetetrazol but not by pilocarpine. This effect was short lived and was not influenced by pretreatment with naltrexone (5 mg/kg, SC). The onset of pilocarpine-induced convulsions in stressed mice was reduced by the opioid antagonist. Aggression stress did not change the incidence, duration or severity of convulsions triggered by the chemoconvulsants or electroshock. The results differ widely from those obtained using other stressogenic models such as cold-restraint or swim stress. This suggests that alterations of convulsive parameters and the involvement of opioid mechanisms in their mediation are critically dependent on the characteristics of the stressogenic procedure employed.

8-OH-DPAT specifically enhances feeding behaviour in mice: evidence from behavioural competition

The behavioural specificity of the hyperphagic effects of 8-OH-DPAT is a controversial issue. The present study addressed this question through the introduction of behavioural competition. Feeding behaviour in male mice was assessed under both basal (free-feeding) and social conflict conditions. Since, in the latter condition, defence and escape are prepotent responses, elicitation of feeding would be indicative of a specific treatment effect on mechanisms controlling food intake. Results showed that 8-OH-DPAT enhanced basal feeding duration (at doses of 0.05-0.50 mg/kg) and also elicited feeding in intruder mice during encounters with aggressive resident conspecifics (at doses of 0.10-0.50 mg/kg). As the 5-HT3 antagonist GR38032F (1.0-2.0 mg/kg) enhanced feeding only under basal conditions, the effect of 8-OH-DPAT cannot readily be attributed to anxiety reduction. Finally, diazepam (1.0-2.0 mg/kg) produced a similar profile to that of 8-OH-DPAT, suggesting that the hyperphagic effects of the 5-HT1A agonist are not pharmacologically specific.

Antinociceptive effects of elevated plus-maze exposure: influence of opiate receptor manipulations

It has been suggested that anxiety may be a critical factor in certain forms of non-opioid environmental analgesia. In the present study, 5-min exposure to the elevated plus-maze test of anxiety (EPM) induced a mild, though enduring, elevation in tail-flick latencies in male mice. Pretreatment with the opiate antagonist naltrexone (0.1-10.0 mg/kg) failed to block EPM-induced antinociception: indeed, the highest dose actually enhanced the response. This effect could not be attributed to intrinsic analgetic activity of naltrexone. Rather, analysis of EPM behaviours suggested that it may have been secondary to an anxiogenic effect of the compound. The involvement of non-opioid substrates in the form of pain inhibition was further supported by the failure of chronic morphine treatment (7 days; 7.5 mg/kg) to alter either the antinociceptive or behavioural response to EPM exposure. Irrespective of treatment history, mice showed a retest EPM profile of enhanced anxiety, with tail-flick data suggesting a major contribution of anticipatory factors. Several important methodological variables are discussed and findings are contrasted with parallel studies on non-opioid defeat analgesia.

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.

Day-night rhythms of shoaling behavior in goldfish: opioid and pineal involvement

Intracerebroventricular (ICV) administration of the opioid peptide, beta-endorphin (5.0 pg/g) to goldfish, Carassius auratus, significantly increased the cohesiveness and duration of shoaling ('bout' length) in shoals of five fish, as well as decreasing the latency of shoal formation in response to an external disturbance, while a higher dose of beta-endorphin (15 pg/g) decreased shoaling. There were day-night rhythms in shoaling and in the extent of the facilitatory effects of beta-endorphin (5.0 pg/g) on shoaling behavior, the fish displaying significantly greater shoaling responses in the day than during the night. The facilitatory effects of the low dose of beta-endorphin were blocked by systemic administration of naloxone (1.0 mg/kg), while ICV administrations of naloxone (1.0 pg/g) decreased daytime shoaling behavior. Removal of the pineal gland disrupted the day-night rhythm of shoaling, reducing daytime levels of shoaling. In addition, pinealectomy reduced the stimulatory effects of beta-endorphin (5.0 pg/g) on shoaling, and attenuated the day-night rhythms in the effects on beta-endorphin on shoaling. These results suggest that both opioid systems and the pineal gland influence shoaling behavior and the expression of its day-night rhythm in goldfish.

Beta-endorphin-like immunoreactivity levels in the hypothalamus, the periaqueductal grey and the pituitary of the DBA mouse: determination by ELISA and relationship to nociception

The present paper describes the development and application of an enzyme-linked immunosorbent assay (ELISA) for the assessment of beta-endorphin-like immunoreactivity (beta-ELIR) level in the hypothalamus, the periaqueductal grey (PAG) and the pituitary of DBA/2 mice that were subjected to mild social stress (aggressive confrontation). After confrontation these subjects showed elevated tail-flick latencies (TFL) when compared to controls, a finding that indicates stress-induced analgesia (SIA). A positive correlation was found between individual TFLs and beta-ELIR levels in the PAG but not in the hypothalamus and the pituitary. These results suggest that individual baseline PAG beta-ELIR levels may be taken as a predictor of high degrees of stress-induced analgesia.

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.