Hans-Rudolf Frischknecht

Isolation-induced locomotor hyperactivity and hypoalgesia in rats are prevented by handling and reversed by resocialization

Differences in locomotor activity in the open field were found between individually and group-housed rats (isol greater than soc). Daily handling, initiated at postnatal day 1, was without effect in group-housed rats but prevented the isolation-induced hyperactivity. For tail-flick latency, strikingly similar differences (isol greater than soc; prevention by handling) have been observed. The isolation-induced aberrations in both locomotor reactivity in a novel environment and in pain sensitivity could be reversed by subsequent resocialization. This indicates that the altered sensitivities to external stimuli are caused by the environmental manipulation.

Defeat, learned submissiveness, and analgesia in mice: effect of genotype

Defeat-induced unconditioned and conditioned behaviors of C57BL/6 and DBA/2 mice were assessed in a social-learning paradigm. Upon bites, mice of the DBA strain reacted with significantly more escape reactions, while C57 mice showed more immobility, crouch, and defensive sideways and upright postures. Clear genotype-dependent patterns were also evident from the conditioned responses recorded 24 h after defeat. DBA mice displayed more escape and defensive sideways and upright postures upon contact with a nonaggressive partner mouse; in contrast, C57 mice reacted with more immobility and crouch. With an increasing number of bites the sum of learned responses increased in C57 mice while it decreased in mice of the DBA strain. This decrement was paralleled by an increase in the analgesic response measured on the hot plate in defeated DBA mice. The possible role of endogenous opioids in the genotype-dependent interaction of defeat-induced learned submissiveness and analgesia is discussed.

Opioids and behavior: genetic aspects

Three animal models, based on genetic differences in endogenous opioid peptides and opioid receptors, are described. Obese mice and rats, whose pituitary opioid content is elevated, may be used to investigate eating disorders. Recombinant inbred strains of mice, which differ in brain opioid receptors and analgesic responsiveness, can be used for study of opioid-and nonopioid-mediated mechanisms of pain inhibition. Individual reactivity to opioids can be examined in C57BL/6 and DBA/2 inbred strains of mice. A model that combines a variety of opioid effects is offered and suggests the existence of a genetically determined dissociation of opioid effects on locomotor activity and pain inhibition. In addition, stimulatory locomotor responses in the C57BL/6 reaction type are linked to a high risk of drug addiction and facilitatory effects on adaptive processes, while high analgesic potency in the DBA/2 reaction type is accompanied by a low proneness to drug abuse and amnesic properties of opioids.

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.

Learning of submissive behavior in mice: A new model

The experience of winning or loosing fights plays an important role in subsequent aggressive or submissive behaviors. In this study agonistic behavior of male mice was chosen to investigate learning mechanisms in the context of a biologically meaningful situation. An ICR mouse introduced into a group of five C57BL/6 mice was attacked by mice of high social status (Fighter, F), but not by lower ranking animals (Non-Fighter, NF). On this basis the following model was developed to study learning of submissive behavior. Day 1 (baseline trial): An ICR mouse was introduced to a single NF-C57 mouse. Few submissive behaviors (crouch) were observed in naive ICR mice upon contact with NF-C57 mice. Day 2 (learning trial): The same ICR mouse was defeated by an F-C57 mouse until it showed defensive upright posture upon approach. This criterion was reached after a mean latency of 3.5 min and after being exposed to a mean number of 14 bites. Day 3 (retest trial): The same pairs as on day 1 confronted each other. Without being attacked, the ICR mouse showed a significant increase of submissive behavior (crouch, defensive sideways and upright) upon mere contact with the NF-C57 mouse when compared to day 1 and to control mice on day 3. Controls, confronted on all three days with NF-C57 mice, showed no increase in submissive behaviors. The results are discussed in terms of acquisition, memory, retrieval and extinction of learned submissive behavior. It is suggested that the mechanisms underlying learning of submissive behavior include generalization of conditioning and specific extinction processes. The further use of the learning scheme to assess drug effects is illustrated.

Inhibition of morphine-induced analgesia and locomotor activity in strains of mice: A comparison of long-acting opiate antagonists

The long-acting opiate antagonistic potency of naloxazone (NXZ), beta-chlornaltrexamine (beta-CNA) and beta-funaltrexamine (beta-FNA) was compared using three inbred strains of mice, in which morphine induces either analgesia (DBA/2), locomotion (C57BL/6), or both responses (C3H/He). The antagonists were applied SC 24-120 hr before morphine (10 or 20 mg/kg, IP), followed by the tests after 30 min. The minimal dose which completely antagonized morphine-induced analgesia in DBA and locomotion in C57 mice during 24 hr were: for NXZ 50 and 100 mg/kg, for beta-CNA 0.8 and 6.2 mg/kg, for beta-FNA 1.6 and 12.5 mg/kg, respectively. beta-FNA and beta-CNA more potently blocked morphine-induced analgesia in DBA mice than the activity response in the C57 strain. In contrast, beta-FNA prevented morphine-induced locomotion at a lower dose (6.2 mg/kg) than analgesia (greater than 50 mg/kg) in C3H mice, while beta-CNA was equipotent (1.6 mg/kg). In general, beta-CNA turned out to be the most reactive compound, antagonizing morphine effects in low doses up to 120 hr. beta-FNA selectively antagonized either morphine-induced analgesia or locomotion, depending on the strain used. This suggests that a given morphine response might be caused by a genetically determined multiplicity of opiate receptor types and their mutual interactions.

Vasopressin impairs or enhances retention of learned submissive behavior in mice depending on the time of application

The effects of vasopressin on learning and memory were investigated in a paradigm using adaptive capabilities of interacting male mice. Test animals of the DBA/2 strain which were not submissive in a confrontation with a non-aggressive subordinate C57BL/6 mouse on day 1 (baseline), were defeated on day 2 (learning) by an aggressive dominant C57 mouse, and showed learned submissive behavior upon mere contact with a non-aggressive C57 mouse on day 3 (retest). Pretrial injections of lysine-vasopressin (0.01, 0.1 or 1.0 I.U., s.c.) 20 min before defeat on day 2 resulted in less submissive behavior on day 3 compared to controls, with 0.1 I.U. (equal to 370 ng) being the most effective dose. Post-trial injections of vasopressin (0.1 I.U.) immediately after defeat on day 2 significantly improved retention on day 3. Preretention injections of vasopressin (0.1 I.U.) 20 min before testing on day 3 significantly increased learned submissive behavior. The amnesic effect observed after pretrial injections of vasopressin was neither due to state dependency nor to an acquisition deficit, nor to antinociception. It is concluded that processing of the stressful experience of defeat is differently influenced by vasopressin given before or after training, resulting in an impaired or facilitated retention, respectively. Among the hypothetically discussed underlying mechanisms, one suggestion is that exogenous vasopressin interacts with an assumed discriminative stimulus function of endogenously released vasopressin. Another possibility might be that exogenous vasopressin interferes with the defeat-activated opioid peptide system.

Long-term analgesic reaction in attacked mice.

Four experiments were designed to characterize long-term analgesic (LTA) reaction in attacked mice. In Experiment 1 we showed that analgesic reaction in DBA mice, induced by the stress of being attacked (30 or 50 bites), is reinstated upon reexposure to seven bites 24 hr later. The magnitude of the LTA response depended on the level of analgesia on Day 1 and was smaller than the original response. In Experiment 2 we showed that LTA was prevented by naloxone or beta-chlornaltrexamine given before exposure (50 bites) on Day 1. Results of Experiment 3 revealed that naloxone or beta-chlornaltrexamine injected before reexposure to seven bites on Day 2 antagonized LTA measured 10 min, but not 1 min, after reexposure. In Experiment 4 we showed that morphine substituted for being attacked on Day 1 failed to produce LTA. We concluded that pain inhibitory mechanisms remain in a state of increased readiness for at least 24 hr after attack stress and that activation of opioid systems is necessary but not sufficient to produce LTA, a response that is only partly sensitive to opioid antagonists.

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

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

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.