Empathy for Pain: Insula Inactivation and Systemic Treatment With Midazolam Reverses the Hyperalgesia Induced by Cohabitation With a Pair in Chronic Pain Condition

Empathy for pain is the ability to perceive and understand the pain in the other individual. Recent studies suggested that rodents have this social ability. GABAergic system has receptors in the brain structures involved in emotional processes as well as in the insular cortex. This area has been described as an important key in modulation of pain and empathy. The present study has investigated the role of insula and its Benzodiazepine-GABAA system on social modulation of pain induced by cohabiting with a mouse submitted to sciatic nerve constriction, a neuropathic pain model. The insular cortex function was assessed by the structure inactivation (Experiments 1 and 2); the role of GABA system was evaluated by systemic treatment of midazolam (MDZ 0.5, 1, and 2 mg/kg) (Experiment 3); and the role of GABAA receptors of insula were studied by bilateral MDZ (3 and 30 nmol/0.1 μl) microinjections in the structure (Experiment 4). Male Swiss mice were housed in groups or dyads. On dyads, after 14 days of cohabitation they were divided into two groups: cagemate nerve constriction and cagemate sham (CS). After 14 days of familiarity, cagemates were evaluated on the writhing test. For group-housed, insula inactivation did not change nociception. For dyad-housed, cohabiting with a mouse in chronic pain increased the nociceptive response and the insula inactivation has reverted this response. Systemic MDZ attenuated nociception and intra-insula MDZ did not alter it. Our results suggest that cohabitation with a pair in chronic pain induces hypernociception, insula possibly modulates this response and the GABA system is also possibly involved, but not its insular mechanisms.

No man is an island. A personal tribute to Bob Blanchard and ethoexperimental approaches to the study of behaviour

I first met Bob Blanchard at an international conference in Paris some 40 years ago. We collaborated intensively during the late 1980s/early 1990s on the ethopharmacology of antipredator defence in wild and laboratory rats, and remained good friends until his untimely passing in November 2013. Bob will undoubtedly be remembered as one of the most influential behavioural neuroscientists of the 20th century and, with Caroline, the most eloquent advocate of ethoexperimental approaches to the study of behaviour. In this brief trip down memory lane, I describe when and where Bob and I first met and how, over a lengthy period, he directly and indirectly helped shape my own research career. His profound influence in this regard is illustrated by reference to not only our collaborative research on antipredator behaviour but also my other work on the ethopharmacology of agonistic behaviour, social conflict analgesia, anxiety, and appetite. The element common to all of this work has been ethoexperimental analysis and, for teaching me the true value of this approach, I shall always remain indebted to the big man. Literally and figuratively, Bob was most certainly larger than life.

Morphine attenuates antipredator ultrasonic vocalizations in mixed-sex rat colonies

Mixed-sex groups of laboratory rats living in a visible burrow system (VBS) emit 18-27 kHz ultrasound and retreat to the burrow when a cat is placed in the open area of the VBS. The total duration of ultrasonic vocalizations was reliably reduced by pretreatment with 5 mg/kg morphine. In a subsequent study using male-female colony pairs, presentation of a cat to individual rats in the absence of their colony mate indicated significant gender differences in base frequency, degree of emission, and characteristics of pulses elicited. Specifically, females showed a greater number and duration of vocalizations, of higher frequency (kHz), and with shorter individual pulse durations than males. In the same study, morphine (5 mg/kg) produced a general decrease in the level of ultrasonic emissions in both sexes, reduced the mean base frequency (kHz), and increased the mean duration of individual pulses. These data suggest that endogenous opioid mechanisms may be involved in the mediation of ultrasonic vocalization in response to a predator, and are discussed with reference to known involvement of such systems in defensive responding.

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.

"Paradoxical" effects of morphine on antipredator defense reactions in wild and laboratory rats

In a Fear/Defense Test Battery, measuring defensive reactions to a present, approaching and contacting predator, the highest dose of morphine tested (7.5 mg/kg) reliably reduced vocalization to dorsal contact, to vibrissae stimulation, and to an anesthetized conspecific in laboratory-bred wild R. norvegicus. Except for a dose-dependent reduction in flinch/jump reactions to dorsal contact (taps), other defensive behaviors (flight, freezing, etc.) were not reliably altered by morphine treatment (0, 1.0, 2.5, 7.5 mg/kg). Vocalization responses to vibrissae stimulation in wild-trapped R. rattus were reliably increased following naloxone (1.0 and 10.0 mg/kg) administration, lending support for opiate receptor involvement in the mediation of defensive vocalization. In the Anxiety/Defense Test Battery, measuring defensive reactions to situations associated with a predator (cat) or with cat odor, laboratory rats showed no decrease in defensive behavior with morphine (0, 1.0, 5.0 mg/kg). In direct contrast to the above findings, the effects of morphine treatment in this test battery suggested a generalized increase in defensiveness to noncontacting and nonpainful threat stimuli. These effects included a decrease in time spent near the cat compartment, with a complementary increase in time spent at maximum distance, a decrease in transits between these sections, an increase in crouching, and a decrease in grooming and rearing. This pattern of results suggests that morphine may have two opposing effects on defensive behavior, a generalized enhancement, together with a more specific reduction of responses to tactile or painful stimulation. A very widespread pattern of reliable sex or sex x drug effects in the Anxiety/Defense Test Battery was in good agreement with previous reports of sex differences in these tests, with females generally more defensive than males. Consonant with previous findings, no reliable sex differences were found with the Fear/Defense Test Battery, although several values approached an acceptable level of statistical significance.

The calls of murine predators activate endogenous analgesia mechanisms in laboratory mice

In view of the suggested role of endogenous analgesia mechanisms as an antipredator defense mechanism, the effects on nociception of exposure to the calls of various murine predatory and nonpredatory species were assessed. Data revealed that the calls of the Tawny Owl, Barn Owl and Common Gull all induced significant analgesia following exposure to 2 min of birdsong. Time course analysis revealed the analgesia induced by the Tawny Owl call to have a duration in excess of 40 min while the Barn Owl and Gull call-induced analgesias were much shorter lasting (approximately 10 min or less). Five mg/kg naloxone was found to attenuate the analgesia induced by the Tawny and Barn Owls but not the Common Gull. Together, these data suggest that brief exposure to the calls of night-hunting, aerial predators activate endogenous opioid-mediated analgesia mechanisms in mice.

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.

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.

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.

Hyperalgesia following agonistic encounters in DBA/2 intruder mice is not associated with recuperative behaviours

Recent findings concerning the time-course of opioid-mediated social conflict analgesia have revealed the existence of two distinct nociceptive phases. Analgesia lasting between 30-45 minutes postattack is evident in the early phase whilst hyperalgesia is observed in the late phase at around 70-75 min postattack. This pattern of nociception has similarities with that envisaged in the Perceptual-Defensive-Recuperative (PDR) model of fear and pain and is predictive of defense/fear reactions being seen whilst animals are analgesic with recuperative behaviours becoming evident in the hyperalgesic phase. Detailed ethological analysis was conducted on DBA/2 mice exposed to opioid-activating attack parameters in a resident-intruder paradigm and tested in the presence of a nonaggressive conspecific. During the analgesic phase, at 10 min postagonistic encounter, a profile was observed consisting largely of increased measures of static, self-directed, possibly recuperative, behaviours (immobile crouch and autogrooming). In contrast, during the hyperalgesic phase, at 70 minutes postencounter, there was evidence of increased environmental exploration, attending to and avoiding nonaggressive conspecifics but no increases in autogrooming or any other behaviours that could be viewed as being recuperative. It is concluded that under present test conditions, using DBA/2 mice exposed to opioid activating attack parameters, predictions based on the PDR model of fear and pain are not supported.

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.

Plasticity of behavioural response to repeated injection of glutamate in cuneiform area of rat

Whereas a single microinjection of L-glutamate (10 nmol) into the cuneiform area of rats gives freezing, a second or third injection (delivered at 4-min intervals to the same site)can produce fast running. To examine whether this plasticity of response was caused by a simple increase in the amount of glutamate present, 30 nmol of glutamate were given in a single injection. In 93% of sites in the cuneiform area this procedure gave only freezing, although subsequent testing with repeated injections produced fast running in 53% of these sites. Thus, response potentiation to glutamate appears to require repeated stimulation, and may therefore be related to processes underlying the natural conditioning of defensive responses.

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.

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.