Behaviors of Swiss-Webster and C57/BL/6N sin mice in a fear/defense test battery

When confronted by an approaching threat stimulus (experimenter or laboratory rat), Swiss-Webster mice show initial flight, followed by freezing and defensive vocalization and biting, the latter only when escape is blocked. These defense patterns resemble those of the wild rat, suggesting that mice of this strain do not show the reductions in flight and defensive threat/attack that are typical of laboratory rats. C57/BL/6N Sin strain mice showed fewer avoidances to an approaching predator, as well as reduced vocalization and defensive biting, a pattern more similar to that of laboratory rats. As with rats, female mice appeared to be more defensive to a predator. They showed greater reactivity to dorsal contact and more frequent defensive biting and jump attacks than males of the same strains. These patterns of defensive behaviors suggest that, although strain differences in defense are substantial, laboratory mice are suitable for, and may offer several advantages in, the study of the genetic, endocrine, and pharmacological basis of antipredator defense. © 1995 Wiley-Liss, Inc.     

Facilitation of Sodium Aversion Learning in Sodium-Deprived Rats

Two experiments with rats explored the effects of sodium deprivation induced by furosemide injections upon acquisition of taste aversion to sodium chloride (NaCl). In Experiment 1, rats under either sodium deprivation or a balanced nutrition condition were given access to a limited amount of NaCl solution prior to poisoning. When all rats were tested under sodium repletion, the previously sodium-deprived rats consumed less NaCl than did the nondeprived rats. This finding was replicated in Experiment 2A in which ingestion of a compound solution of NaCl and hydrochloric acid (HCl) was followed by poisoning. Consumption of HCl, however, showed the opposite pattern: the sodium-deprived rats drank more HCl than did the nondeprived rats, a result that was replicated in Experiment 2B. These results suggest that sodium deprivation strengthens the salience of NaCl, thereby facilitating acquisition of aversion to this taste and strongly overshadowing that to a simultaneously presented taste.     

Immobility as an avoidance response, and its disruption by drugs

Much of the available literature on avoidance behavior is based on responses which require the animal to run, lever-press, or to make some active response to avoid noxious stimulation. The purpose of Experiment I reported in this paper was to determine whether animals can learn to sit or stand motionless in order to escape or avoid electric shock. Five experimental rats were given escape-avoidance training, while five yoked control animals received electric shocks without any response-related contingency. It was shown that an immobility avoidance response, as distinct from the unconditioned “freezing” response to shock, can be trained. The results of Experiment II (30 rats) revealed that this response is more readily acquired at higher shock intensities than at lower ones, provided escape by jumping is prevented at the high shock intensities. The effects of six doses of each of three drugs on the immobility avoidance response were studied in Experiment III (13 rats). Methylphenidate, chlorpromazine, and imipramine all produced a decrement in the immobility response, but the pattern and amount of the effects of the three drugs were quite different, one from the other. The implications of these findings for a general theory of avoidance behavior and for drug screening are discussed.     

Group rearing abolishes hyperdefensiveness induced in weanling rats by lateral septal or medial accumbens lesions but not by medial hypothalamic lesions

Lesions of the medial hypothalamus, medial accumbens, or septum were made in 21- to 25-day-old male hooded rats. Half of the animals in each group were subsequently reared in groups and the other half in isolation. When tested for defensiveness toward the experimenter at 31, 34, and 37 days postoperatively, rats with medial hypothalamic lesions were most hyperdefensive toward the experimenter if reared in isolation but were significantly more defensive than sham-lesioned animals even when reared in groups. Rats with septal lesions were significantly more defensive than sham-lesioned animals only when reared in isolation while rats with medial accumbens lesions were not different from controls whether reared individually or in groups. These results suggest that the medial hypothalamus may have a special importance in determining temperament since the hyperdefensiveness that results from interference with its functioning is resistant to experiential remediation.     

Ontogeny of sex differences in defensive burying behavior in rats: effect of social isolation

The aim of the present experiments was to clarify sex differences in socio-developmental factors that affected defense behavior in rats. Sex differences in the defensive burying behavior of rats, and related social factors, were explored in three developmental stages: juvenile, puberty, and adult; 30, 50, and 80 days of age, respectively. The duration of burying, digging into bedding material, stretch-attend postures, and crouch/freezing were measured in a shock-prod test. For males, the duration of burying was longer in the juvenile and pubertal stages than in adulthood. For females, no age differences in the duration of burying were found. Males showed longer burying durations than females in both the juvenile and pubertal stages. For both sexes, the highest duration of digging was found in the juvenile stage, and females showed longer durations of digging than males. Both male and female rats isolated during the juvenile stage, from 26 to 40 days of age, showed smaller durations of burying behavior compared to pair-reared rats. This effect of juvenile isolation was maintained among both adult males and females even when they were returned to pair rearing after isolation. Isolation during adulthood, from 66 to 80 days of age, increased burying behavior in males, but decreased it in females. The durations of digging, stretch-attend postures, and crouch/freezing were not affected by isolation. The decrease in defensive burying and its increase resulting from isolation in adult male rats, suggest that the emergence of adult-like social relationships in males suppressed the duration of burying. Male and female rats isolated during the juvenile stage maintained lower levels of burying, suggesting that social experience as juveniles is important for the emergence of defensive burying behavior.     

Rat defensive behavior: burying noxious food.

In Experiment 1, rats living in chambers containing bedding material were injected with a toxicosis-producing dose of lithium chloride shortly after their initial taste of sweetened condensed milk. They consumed no additional milk and used the bedding to bury the spout through which the milk had been delivered, although they did not bury a concurrently available water spout. In another control condition, rats did not bury a spout containing a novel solution (saccharin) not paired with toxicosis. In Experiment 2, rats did not bury a milk spout until milk consumption was followed by toxicosis. In Experiment 3, rats buried a spout containing Tabasco pepper sauce but not a concurrently available water spout. Thus, burying the food source appears to be an integral component of the rat's defensive reaction to noxious food.     

Influence of dorsolateral periaqueductal grey (dlPAG) lesions on contextual conditioning with massed and distributed shock

Three experiments investigated the conditions under which electrolytic lesions of the dorsolateral periaqueductal grey (dlPAG) facilitate conditioned defensive freezing in the rat ( Rattus norvegicus ). Experiment 1 found that dlPAG lesions placed before context-shock pairings facilitated conditioned defensive freezing with massed but not distributed shock. No such effect was found in Experiment 2, when the lesions were placed after context-shock pairings. Experiment 3 found that dlPAG lesions facilitated subsequent conditioning with massed but not a single shock. In addition, no differences in sensitivity to thermal or shock pain were evident in lesioned and unlesioned rats. Taken together, these results are consistent with the suggestion that dlPAG activation interferes with the processing of contextual cues during association formation.     

Tolerance of pain as a measure of fear

Fear thresholds were measured in four experiments by exposing rats to electric shock in order to determine the maximal intensity rats would tolerate rather than enter a fear-arousing box and/or stop freezing. Increasing fear raised these thresholds. They were greater for rats having to escape shock to a fear-arousing box than for rats having to escape shock and fear to a neutral box. The forgetting functions for the latter two groups differed: the first group yielded a monotonic decay function, whereas the second group yielded an inverted U-shaped function. These thresholds decreased as a function of an avoidance learning procedure. Rats that had to escape shock to a fear arousing box did not do so immediately, although they had stopped freezing. An avoidance-avoidance conflict explanation for immobility was not found to be valid. A theoretical formulation based on the following two hypotheses was suggested to explain these results: the fear-aroused freezing (immobility) is an unlearned response; finding a way to escape the source of fear starts another unlearned response, withdrawal.     

Defensive reactions of “wild-type” and “Domesticated” wild rats to approach and contact by a threat stimulus

Selective breeding of wild rats over many generations on the basis of low or high defensive threat and attack to human approach and contact has produced highly polarized “domesticated” and “wild-type” animals. Because the selection procedure selectively involves these two defense patterns, and these clearly differ in the two groups, it is of interest to determine if other, nonselected, defensive behaviors to threat stimuli also change. “Domesticated” and “wild-type” rats of the thirty-fifth generation were run in a fear defense test battery (F/DTB) to systematically evaluate defensive behaviors to a variety of present threat stimuli. “Domesticated” rats showed reduced avoidance and slower flight speed to an approaching experimenter, reduced jump/startle response to handelap and dorsal contact, less vocalization and boxing to vibrissae stimulation or to an anesthetized conspecific, and reduced defensiveness to an attempted pickup by the experimenter. These results indicate that selective bi-directional breeding for defensive threat and attack to human approach and contact produces group differences in a variety of defensive behaviors, and in defensiveness to stimuli other than those on which the selection was based. © 1994 Wiley-Liss, Inc.     

Species-specific danger signals, endogenous opioid analgesia, and defensive behavior

The effects of handling stimuli and stress odors on species-specific defensive behavior and pain sensitivity were examined in rats. Animals not adapted to handling had longer jump latencies on the hot plate test of pain sensitivity than those with extensive handling experience. In a postshock freezing test, naltrexone enhanced defensive freezing relative to saline controls in nonadapted animals. However, naltrexone produced no such effect in rats that were adapted to handling. These two studies indicate that the handling procedure triggered an endogenous opioid analgesic response in rats not adapted to handling. Experiment 3 showed that a similar naltrexone-reversible opioid analgesia can be triggered by stress odors. Naltrexone, when compared to saline, enhanced postshock freezing in the presence of conspecific stress odors, but not in their absence. In Experiment 4, stress odors and nonadapted handling were able to activate defensive freezing directly, when tested in compound but not in isolation. The studies are consistent with the view that stress odors and handling stimuli are danger signals that activate endogenous opioid analgesia as well as defensive behavior, suggesting that analgesia is a component of the rat's defensive behavior system.     

Isolation-induced facilitation of male sexual behavior in mice

Sexual performance of male mice housed individually or in groups of 3 or 12 was compared. Experiment 1 examined naive males presented at weekly intervals with ovariectomized, estrogen-primed, progesterone-treated females. Performance in isolates was consistently superior and reached an asymptote that was twice that of grouped animals. Reversal of housing conditions reversed performance. Experiment 2 varied intervals of isolation among subjects, finding facilitation at several intervals. Experiment 3 compared animals under different population densities. Density did not alter the effects of isolation and grouping. In all experiments, additional tests with target males indicated that aggressive and sexual performance were moderately correlated and responded similarly to parametric manipulations. These results parallel and extend studies of isolation-induced aggression.     

The effects of atropine and adrenergic antagonists on behavioral arousal in rats

Two experiments investigated the mechanism for changes in measures of behavioral arousal inhibition in rats following administration of atropine. In Experiment 1, 40-day-old rats were given administrations of atropine sulfate, the alpha-, beta-adrenergic blocker labetalol, or both. The drugs, either alone or in combination, increased transport response intensity, whereas both together increased dorsal immobility durations. In Experiment 2, rats were given atropine, the beta-adrenergic antagonist propranolol, the alpha-adrenergic antagonist phentolamine, or a combination of two of the drugs. Propranolol blocked atropine-induced increases in transport response, and phentolamine was without effect. Phentolamine, when combined with atropine, increased dorsal immobility durations. Results are discussed with respect to aspects common to both transport response and dorsal immobility.     

The Effects of Atropine and Adrenergic Antagonists on Behavioral Arousal in Rats

Two experiments investigated the mechanism for changes in measures of behavioral arousal inhibition in rats following administration of atropine. In Experiment 1, 40-day-old rats were given administrations of atropine sulfate, the α-, β-adrenergic blocker labetalol, or both. The drugs, either alone or in combination, increased transport response intensity, whereas both together increased dorsal immobility durations. In Experiment 2, rats were given atropine, the β-adrenergic antagonist propranolol, the α-adrenergic antagonist phentolamine, or a combination of two of the drugs. Propranolol blocked atropine-induced increases in transport response, and phentolamine was without effect. Phentolamine, when combined with atropine, increased dorsal immobility durations. Results are discussed with respect to aspects common to both transport response and dorsal immobility.     

When Illnesses or Accidents Befall Others: The Role of Gender in Defensive Distancing

Two experiments were designed to investigate the proposition that men engage in greater defensive distancing than do women. In Experiment 1, we tested this hypothesis by having male and female participants (predominantly White, from working class backgrounds) distance themselves from a person with an illness or medical condition. In Experiment 2, we tested this hypothesis by having male and female participants distance themselves from a person involved in a mild or severe accident. We also attempted to replicate the finding that people distance themselves more over time from a person with a serious illness. As predicted, men engaged in greater defensive distancing than did women. We did not find that participants distanced themselves more over time from a person with a serious illness. Implications of these findings are discussed.     

Socially mediated fear reduction in rodents: Distraction,communication, or mere presence?

Two studies investigated why the presence of conspecifics reduces open-field fear. In Experiment 1 rats paired with another experimentally naive rat were not less fearful than alone controls. However, rats paired with a partner who had been preexposed to the open field exhibited fewer fear responses than alone controls. Since preexposed companions appeared to initiate more social interactions than nonpreexposed companions, it was argued that the preexposure effect could be the result of preexposed companions distracting actor rats from threatening stimuli. Experiment 2 tested this explanation by pairing socially sated or socially deprived rats with a socially sated or socially deprived companion. In accord with a distraction hypothesis, pairing two socially deprived rats produced the greatest fear reduction effects and also led to more social interaction than pairing socially sated rats. Alternative explanations involving conditioning and communication are discussed.     

Control of instrumental behavior by deprivation stimuli

In Experiment 1, rats were given one trial per day in a straight alley under food deprivation on half of the trials and under water deprivation on the other half. Wet mash was available in the goal box under food deprivation for Group H and under water deprivation for Group T, the other deprivation being nonrewarded for each group. After 15--18 trials both groups ran significantly faster on their rewarded than on their nonrewarded deprivation days. A third group showed that random variation of alley color retarded formation of the discrimination. A fourth group was run in a conditional discrimination in which under food deprivation wet mash was available in a black alley, nonreward in a white alley, and vice versa under water deprivation. This group took 114 trials to begin running significantly faster in their rewarded than in their nonrewarded alley under each deprivation. In Experiment 2, it was shown that prior learning about deprivation cues "blocks" learning about alley color when alley color is subsequently presented in compound with the deprivation cue but that when both alley color and deprivation cues are relevant from the start of training, the rat learns about both cues. It is suggested that previous studies have underestimated the importance of deprivation cues by using conditional discrimination designs, choice measures rather than speeds, and parameters that are not optimal for discrimination learning.     

Deprivation and satiation: The interrelations between food and wheel running

Two experiments were designed to assess whether depriving rats of food would increase the reinforcement effectiveness of wheel running (Experiment 1) and whether satiation for wheel running would decrease the reinforcement effectiveness of food (Experiment 2). In Experiment 1, a progressive-ratio schedule was used to measure the reinforcement effectiveness of wheel running when rats were deprived or not deprived of food. Completion of a fixed number of lever presses released a brake on a running wheel for 60 s, and the response requirement was systematically increased until the rat stopped pressing or until 8 hr had elapsed. The ratio value reached (and the total number of lever presses) was an inverted-U function of food deprivation (percentage body weight). In Experiment 2, when wheel running preceded test sessions, fewer food-reinforced lever presses were maintained by the progressive-ratio schedule, and responding occurred at a lower rate on a variable-interval schedule. An interpretation of these results is that deprivation or satiation with respect to one event (such as food) alters the reinforcement effectiveness of a different event (such as access to wheel running).     

Effects of food deprivation upon cue utilization as measured by novelty-incentive

A series of three experiments was done to test the hypothesis that high levels of food deprivation would adversely affect cue utilization from a complex stimulus goal as tested by its novelty-incentive value when that goal was later opposed to food for hungry subjects in a T-maze. It was found that the hunger drive level under which the male rats had originally experienced the complex stimulus goal determined its later incentive value, whether the original experience was in a latent learning type II situation (Experiment I, 20 subjects), a drive-shift situation (Experiment II, 40 subjects), or a free exploration situation (Experiment III, 30 subjects). In each experiment, having first experienced the complex goal under low levels of deprivation significantly decreased the frequency of choices of that goal in a later test relative to the performance of the more deprived animals. The data was interpreted as indicating that utilization of cues, in the sense of input and possibly retention of information, was hindered by the higher levels of deprivation.     

Associative vs topographical accounts of the immediate shock-freezing deficit in rats: Implications for the response selection rules governing species-specific defensive reactions

Rats were placed in a novel chamber and were given a single shock, either immediately upon placement in the chamber or after a 2-m delay. During a postshock observation period, delayed shock animals froze while immediate shock rats did not (Experiments 1–5). Additionally, delayed shock animals showed an elevation in defecation, relative to unshocked controls, while immediate shock animals did not (Experiment 1). Freezing and elevated defecation were first found with a 9-s delay between placement and shock, and both responses increased linearly with increasing delays up to at least 81 s (Experiment 2). Immediate shock rats did not show escape-related behaviors when potential escape routes were available (Experiments 3–4). A 2-m preexposure to the chamber, that was separated in time from shock, did not alleviate the immediate shock deficit (Experiments 4–5). The immediate shock deficit was also apparent in a test given 24 h after shock receipt (Experiment 6). If immediate shock rats were later given delayed shock they began to freeze but showed no carryover effects of their prior shock experience (Experiment 6). It was concluded that the immediate shock deficit reflects a deficit in association formation between contextual stimuli and shock rather than a difference in defensive behavioral topography. It also appears that freezing is the dominant conditional response to shock-associated stimuli even though freezing is not unconditionally elicited by the shock itself.     

Functions of the vibrissae in the defensive and aggressive behavior of the rat

Removal of mystacial vibrissae for strange rats placed in an established rat colony produced reliable decrements in defensive boxing for these animals, and corresponding increases in freezing. In a subsequent experiment, removal of the vibrissae of the attacking colony males did not change the attack behavior of the vibrissectomized animals. These results indicate that the vibrissae are involved in defensive boxing behavior, but play no essential role in the elicitation or maintenance of conspecific attack in the rat.