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

Epigenetic control of neurobehavioural plasticity: the role of neurotrophins

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are two neurotrophins involved in the differentiation, growth and maintenance of selected peripheral and central populations of neuronal cells, during development and at adulthood. Furthermore, neuronal activity enhances expression and action of these neurotrophins, modifying synaptic transmission and connectivity. Neurotrophin production has been shown to be experience-dependent. In particular, during early developmental phases, experiences such as maternal deprivation or exposure to an enriched environment markedly affect NGF and BDNF levels. At adulthood, psychosocial stress has been shown to markedly alter NGF and BDNF levels, both in plasma and selected brain areas, including the hypothalamus and hippocampus. These results have been extended to humans, showing that NGF levels are enhanced by emotional stress induced by parachute jumping. Overall, these findings suggest a role of neurotrophins as factors mediating both short- and long-term effects of experience on brain structure and function.

Mechanisms of the effects of adrenocorticotropic hormone on pain sensitivity in rats

Experiments on anaesthetized male Sprague-Dawley rats were performed to study the effects of adrenocorticotropic hormone (ACTH) on pain sensitivity. Systemic administration of ACTH to animals with normal hormone production induced rapidly developing (starting at 3 min) and prolonged (30 min) increases in pain response thresholds. Blockade of opiate receptors led to suppression of the initial stage of the analgesic effect of ACTH: the response was seen only from 15 to 30 min. In animals with deficient glucocorticoid production, the duration of the analgesic action of ACTH decreased to 15 min. Analgesia was completely eliminated by the combination of suppression of glucocorticoid production and blockade of opiate receptors. The analgesic effect of ACTH was mediated by two mechanisms: 1) a rapidly-acting (from 3 to 15 min) mechanism associated with opiate receptors and not related to glucocorticoids, and 2) a delayed (from 15 to 30 min) mechanism associated with glucocorticoids but not opiate receptors.

Psychosocial vs. "physical" stress situations in rodents and humans: role of neurotrophins

Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are well-studied polypeptide growth factors involved in the development and maintenance of specific peripheral and central populations of neuronal cells. In addition to its role as a neurotrophic agent, NGF controls very complex functions in vertebrate physiology. A variety of cells outside the nervous system are in fact able to synthesize NGF including epithelial cells, fibroblasts, lymphocytes, and macrophages. NGF target cells have been identified in the nervous, immune, and endocrine systems, suggesting that NGF may operate through multiple paths to ultimately regulate physiological homeostasis and behavioral coping. We used a mouse model of social stress to demonstrate that NGF levels increase both in plasma and in the hypothalamus following intermale aggressive interactions. The investigation has been extended to other species, including humans, to show that labour, lactation, and the anticipation of the first jump with a parachute also result in increased NGF plasma levels and in changes in the distribution of NGF receptors on lymphocytes. BDNF activation is caused by both physical and social stress events. The aim of this review is to (1) outline the current understanding of the roles of NGF and BDNF in stress-related physiological changes in vertebrates, in particular for physical vs. psychological stressors, which may activate both similar and different neurobiological pathways, and (2) summarize recent efforts to derive pharmacological strategies from the increasing body of BDNF and NGF neurobehavioral data.

Effects of social conflict on immune responses and E. coli growth within closed chambers in mice

Social conflict has been shown to affect the neuroendocrine stress response in rodents. The current study was designed to characterize the effects of social conflict on leukocyte subset distribution and function as well as in vivo bacterial growth. Male DBA/2 mice implanted or not implanted with a closed chamber containing Escherichia coli were repeatedly challenged by temporary placement in the territory of a dominant CF-1 mouse five times a day for 2 consecutive days. Nonstressed animals were similarly handled, but were not exposed to social conflict. Effects on immune responses and E. coli growth were analyzed 13 h after the last social conflict session. Social conflict alone was associated with an increase in plasma corticosterone concentration and decreases in thymocyte numbers and splenocyte ability to proliferate in vitro in the presence of lipopolysaccharide (p < 0.05). After social conflict, immature CD4+CD8+ thymocytes decreased, whereas mature T cells increased (p < 0.05). In the presence of E. coli, social conflict induced a significant increase in plasma concentration of interleukin-1beta, and a decrease in the number of thymocytes and the percentage of CD4+CD8+ T cells in the thymus (p < 0.05). In addition to the lymphocyte subpopulation changes observed with social conflict alone, the proportion of CD3+ and major histocompatibility complex (MHC) class II IAd+ cells were significantly higher in stressed mice implanted with a closed chamber containing E. coli (p < 0.05). Social conflict tended to favor E. coli growth in the closed chamber, indicating possible direct bacterial-neuroendocrine hormone interactions. Taken together, these results suggest that stress may modulate the host immune response by altering both bacterial growth and resistance to infection.

NGF regulatory role in stress and coping of rodents and humans

Nerve growth factor (NGF) is a polypeptide growth factor which exerts trophic and differentiative effects on specific peripheral and central populations of neurons. Recent data showing that various cellular types of the endocrine and immune systems are able to synthesize and release NGF have suggested that this neurotrophic factor may also play an important role in vertebrate physiologic homeostasis. Previous studies using a mouse model of aggressive behavior have shown that NGF levels increase in both plasma and the CNS following intermale agonistic encounters. More recently, we have extended this research area to include other species: in particular, humans. The data now available indicate that labour and lactation, or the occurrence of a stressful event such as the very first jump with a parachute causes in NGF plasma levels as well as changes in the distribution of NGF receptors on lymphocytes. This review aimed to outline the current understanding of NGF role in vertebrates in stress-related events.

Serum NGF levels increase during lactation and following maternal aggression in mice

In the present study, serum levels of nerve growth factor (NGF) were assessed in virgin and in lactating female CD-1 mice. In the case of the lactating females, NGF levels were assessed both under basal conditions and 60 and 180 min following a 10 min encounter with a male or a nonlactating female mouse. Basal serum NGF levels of lactating females were higher than those of virgin females but did not increase significantly above base after an aggressive encounter with a male or a female conspecific. Female intruders were attacked in a ritualized manner. In contrast, males received numerous bites to vulnerable regions of their body. A positive correlation was found between serum NGF levels and pattern of aggression in females confronting male conspecifics. Thus, in lactating mice, serum NGF levels following an aggressive encounter relate to the specific pattern of behavior the female uses to defend the offspring.

Endogenous analgesia in the chicken

Endogenous analgesia has been identified in mammals, but little is known about suppression of tonic pain following trauma or disease. Birds suffer from gouty arthritis which can be induced experimentally by intra-articular injection of sodium urate (SU) crystals. SU injection into the ankle joint of the chicken tested in cages resulted in pain-coping behaviour (one-legged standing, sitting) together with severe lameness. Birds kept and tested in large pens showed significantly less pain-coping behaviour, while birds tested in novel pens showed either complete analgesia or marked hypoalgesia, together with a significant reduction in lameness. Complete analgesia was observed during pre-laying behaviour. These results demonstrate a remarkable ability of birds to suppress such severe tonic pain as SU arthritis.

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.

Examination of the neuroendocrine basis for the social conflict-induced enhancement of immunity in mice

Intruder DBA/2 and C57BL/6 mice, which display different neuroendocrine responses to social conflict, evidence a pronounced increase in splenocyte phagocytosis of opsonized zymosan particles as a consequence of social conflict-induced stress. Interruption of the hypothalamic-pituitary-adrenal axis prior to social conflict results in an abrogation of the stress-induced enhancement of phagocytosis in DBA/2, but not in C57BL/6, mice. Administration of the opiate antagonists naloxone and naltrexone resulted in a potentiation of the stress-induced enhancement of phagocytosis in both strains. Similarly, administration of the alkylating antagonist beta-chlornaltrexamine which irreversibly blocks opioid binding sites potentiated the immune-enhancing effects of social conflict stress. Mitogen-induced T and B lymphocyte proliferation was unaffected by any of the experimental procedures with the exception of beta-chlornaltrexamine which suppressed activity equally in stressed and nonstressed groups. These results demonstrate the necessity of employing inbred murine strains in the dissection of the neuroendocrine pathways which govern stress-induced modulation of the immune system.

Social status and nerve growth factor serum levels after agonistic encounters in mice

Ten repeated daily interactions (20 min each) of the same pairs of isolated male mice produced a clear distinction between attacking (dominant) and defeated (subordinate) animals. The fighting level remained fairly constant over the 10 days. One hr after the end of the 10th session, the increase in serum NGF levels described previously (2) was significantly more marked in subordinate than in dominant mice. The mean level of serum NGF was correlated with the number of fighting episodes, particularly in the case of dominant individuals. Moreover, within-pair differences in NGF values were correlated with differences in locomotor activity between dominants and subordinates; this makes it possible that stimuli other than those produced by fighting per se may be responsible for the increase in circulating NGF. As is well known, the adrenal hypertrophy produced by fighting stress is more marked in subordinate than in dominant mice, while previous work has shown that stress of a nonpsychosocial kind does not elevate serum NGF levels. Therefore, the present data support the hypothesis that NGF release contributes to the modulation of adrenal function in a situation-specific fashion.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

NMDA receptor blockade in the periaqueductal grey prevents stress-induced analgesia in attacked mice

Microinjections of N-methyl-D-aspartate (NMDA, 0.1 and 1.0 nmol) into the periaqueductal grey (PAG) of the mouse resulted in potential antinociception. In a social conflict situation, attacked mice exhibited a marked analgesia that was prevented by prior injection of the competitive NMDA antagonist, AP-7 (2.0 nmol) or naloxone (6.0 nmol) into the PAG and also by i.p. injection of the non-competitive NMDA antagonist, MK-801 (33 nmol). These results demonstrate that NMDA receptors are involved in endogenous analgesic mechanisms activated by stress.

Place avoidance learning and stress-induced analgesia in the attacked mouse: role of endogenous opioids

In this study, mechanisms of pain inhibition (tail-flick test) and memory (place avoidance paradigm) were investigated in attacked, DBA/2 and C57BL/6, mice. During training, exposure of test animals to 10 or 30 bites by an aggressive, isolated ICR mouse situated in the dark half of a bright/dark conditioning box induced a significantly higher social conflict analgesia in DBA than in C57 mice. Naltrexone (0.5 and 2.0 mg/kg) reduced this response in DBA mice that received 30, but not 10, bites and was ineffective in C57 mice. This points to different, opioid versus naltrexone-insensitive nonopioid, analgesic mechanisms. During place choice testing in the same box 24 h later, DBA mice that had received 30, but not 10, bites showed a significant, naltrexone-reversible, avoidance of the attack place. No place avoidance learning was observed in C57 mice. The data provided unequivocal evidence that place avoidance learning was a result of associative conditioning, in that neither pairing nor social conflict per se significantly changed the preference for the dark side seen in experimentally naive DBA mice. Antagonism of place avoidance conditioning was observed regardless of whether testing was carried out in the drugged or undrugged state, excluding possible state-dependent effects as an explanation for the naltrexone-induced impairment. Individual correlational analysis in saline-injected, attacked DBA mice revealed a negative relationship between the analgesic state immediately after training and the avoidance of attack place during testing. In summary, the results suggest strain-dependent analgesic and learning mechanisms and indicate that endogenous opioids released in attacked DBA mice support pain inhibition and modulate the memorization of attack place by their analgesic effects, as well as by mechanisms independent of pain inhibitory systems.

Relationship between behavioral and nociceptive changes in attacked mice: effects of opiate antagonists

The relationship between analgesia and behavior during and after an aggressive encounter was investigated in saline- and opiate antagonist-treated DBA mice. A low number of bites induced an analgesia that was reversed by beta-chlornaltrexamine but not by naloxone, and that correlated positively with increased displays of defensive upright and immobility upon contact with the opponent. Extended attacks induced a naloxone-sensitive analgesia that was linked to a delayed occurrence of "panic" escape behavior. In the post-conflict phase, the degree of immobility and analgesia correlated positively in attacked mice. Naltrexone prevented this analgesia and lowered immobility. Endogenous opioids released during social conflict may induce analgesia and immobility in DBA mice.

Naltrexone-reversible pain suppression in the isolated attacking mouse

Fighting pairs of isolated DBA/2 mice showed a significant increase in tail-flick response latencies independent of whether opponents were losing or winning the combat. The effect lasted less than 10 min in both animals. Elevated pain thresholds were also found in isolates that attacked a nonaggressive conspecific, and were prevented by naltrexone (0.2 mg/kg), while a larger dose (1.0 mg/kg) inhibited the attack behavior. A small increase in pain threshold was observed after exposure of isolates to the test box alone, while isolation per se had no effect on baseline tail-flick latencies. The data demonstrate that endogenous pain suppressing systems are activated during attack and suggest that this opioid-mediated antinociception is a correlate of the isolation syndrome, reflecting enhanced arousal of the attacking animal.