Analgesia induced by exposure to a novel environment in rats: effect of concurrent and post-training stressful stimulation

Exposure of rats for 2 min to a new environment (a 50 X 25 X 25-cm box) induced a mild analgesia measured by the tail-flick method. Additional stressful stimuli (0.5 mA, 1.5-s footshocks, light flashes, or tones) presented during the 2-min exposure did not alter the analgesia. However, the postexposure presentation of light flashes or tones, for either 10 s or 2 min, while the animals were alone in a waiting cage, prevented the analgesic response. Similarly, placing the subjects with their conspecifics in the home cage for 2 min after the exposure prevented the analgesic response. The data suggest that the analgesia may represent a physiological correlate of novelty and that the response can be impaired by post-training treatments.     

The effect of a contraceptive theme on response to erotica

Undergraduate students (60 males, 60 females) viewed a sexually explicit movie in which a couple engaged in intercourse. Different themes were introduced by prefacing the film with information varying the filmed couple's apparent marital status (single, married) and contraceptive practices (no information, no contraception, contraception via the pill). After viewing the movie, subjects filled out a questionnaire assessing their own affective reactions plus their perceptions and evaluations of the couple. Male subjects reported the greatest positive affect when the film couple was described as single and using contraception, but they also perceived the stimulus couple as caring the least for one another when they were using contraceptives. In contrast, females perceived contraceptive use as positive evidence of mutual caring. These sex differences in reacting to contraceptive use were discussed in terms of the relatively greater tendency of males to separate love and sex than is true for females.Work on this experiment was supported by a research grant (SOC 74-15254) from the National Science Foundation. The authors wish to thank Elizabeth Rice Allgeier, William A. Fisher, and Richard Knox for their comments and suggestions.     

Acute exposure to a novel stressor further reduces the habituated corticosterone response to restraint in rats

The present study sought to identify dishabituation of the hypothalamic-pituitary-adrenal(HPA) axis response to different psychological stressors. Young adult male Sprague Dawley rats were exposed to five, 1 h sessions of restraint stress on five consecutive days. On the sixth day, and 2 h before additional exposure to restraint, animals were subjected to 30 min of a small (27cm square), elevated open field stressor (pedestal), which served as the dishabituating stimulus. We predicted HPA axis response dishabituation in chronically restrained rats exposed to the novel pedestal. Rats which underwent five days of restraint stress showed significantly blunted plasma corticosterone levels to restraint (habituation) as compared to restraint-nai've rats. However, rats which underwent five sessions of restraint responded with an enhanced habituation response when confronted with restraint shortly after exposure to the novel pedestal. Instead of HPA axis response dishabituation, we observed enhanced habituation. Subsequent experiments determined that a 1.25 mgkg corticosterone injection could substitute for pedestal exposure to produce enhanced restraint habituation.Combined treatment with both the glucocorticoid receptor antagonist RU40555 (30 mgkg)and the mineralocorticoid receptor antagonist RU283 18 (50 mgkg) blocked the expression of enhanced habituation after pedestal exposure. Thus, the delayed corticosterone negative feedback produced by novel stress led to enhanced expression of corticosterone response habituation.     

Prolactin in rats is attenuated by conspecific touch in a novel environment

Prolactin is released in response to physical and psychological stress, the latter of which involves an animal’s interpretation of its environment as potentially dangerous. This interpretation might be altered by social buffering, as defined by the presence of a social partner. In support of this claim, I previously found that the presence of a conspecific during exposure to the open field caused lower levels of prolactin in juvenile rats than did exposure to the open field alone. Because testing with a conspecific allowed the animals to touch, physical contact may have been necessary for the reduction of prolactin. To test this hypothesis, juvenile male and female rats in the present study were exposed to the open field (1) alone, (2) with a conspecific separated by a Plexiglas partition, or (3) with a conspecific without a partition. Touch was necessary to reduce stress as measured by prolactin levels.     

Effects of the interval between exposure to a novel environment and the occurrence of shock on the freezing responses of rats

Rats were exposed to a novel environment (E1) at time T1, given a footshock at time T2, and tested for freezing in E1 or in a second environment (E2). The function relating freezing to the T1-T2 interval among rats tested in E1 was an inverted U-shape. Rats exposed to short T1-T2 intervals displayed just as much freezing in E2 as in E1, whereas rats exposed to longer intervals froze less in E2 than in E1. These differences between the freezing responses in E1 and in E2 were obtained when the T1-T2 intervals were varied, but time spent in the shocked E1 was equated. Rats given two shocks in E1 differentiated between E1 and E2 when the initial shock occurred some time after exposure to E1, but not when the initial shock was presented shortly after that exposure. Rats shocked some time after exposure to E1 on Day 1 and shortly after exposure to that environment on Day 2 differentiated between E1 and E2 more than did rats exposed to the reverse sequence of T1-T2 intervals. The results were attributed to the formation of a network of connections among the E1 cues in rats exposed to moderate or long T1-T2 intervals, and to an impairment in the formation of this network as a result of the conditioning of a subset of cues in rats exposed to short T1-T2 intervals.     

The effect of a masked stimulus on the response to the masking stimulus

Four experiments are reported in which the subjects had to respond to a target that masked a preceding prime via metacontrast masking. In one part of Experiment l, the subjects discriminated the target's shape (square or diamond) by a motor-choice reaction, and in another part they had to respond with a simple reaction. The prime was neutral (circular) with respect to the target's shape. The data showed a facilitation effect. In both tasks the reaction time was reduced by the masked prime. However, the reduction was more pronounced with simple reaction than with choice reaction. In the other experiments, additional primes were used with the same angular shapes as the targets. In Experiments 2 and 3, after discriminating the target's shape by a choice reaction, the subjects had to judge the prime's shape in a signal-detection task. While neither the d' value for discriminating the angular primes from the circular ones (Exp. 2) nor the d' value for distinguishing between the angular primes (Exp. 3) was different from zero, the choice-reaction data showed a congruency effect. With a congruent prime (i.e., a prime that had the same shape as the target), the reaction times were reduced. With an incongruent prime, the reaction times grew. In Experiment 4 the errors were investigated. The facilitation effect was present in the RT, but not in the number of errors, whereas the congruency effect was present in the number, but not in the RT of errors.While the facilitation effect can be attributed either to an unspecific activation by the masked prime or to an influence of the prime on attentional processes, the congruency effect can be explained by the assumption that the masked prime directly activates the specific response, which corresponds to the prime's shape.     

The effect of home or novel environment on the facilitation of passive avoidance by post-training ethanol

Passive avoidance behavior of mice is improved when mice are injected with ethanol immediately after footshock training. Further study has shown that avoidance can be affected by ethanol injections given within 1 h, but not at 90 or 180 min, after training. The present study was conducted to investigate the possibility that events which occur in the homecage during this sensitive period may influence the effect of ethanol on subsequent avoidance. Male Swiss-Webster mice were housed either singly in a novel environment for 90 min or returned to their (group) homecage following one-trial, step-through, passive avoidance training (0.1 mA footshock) and intraperitoneal injection of 3.0 g/kg ethanol (15% v/v) or saline. As in previous studies, when ethanol-treated mice were returned to their homecage, avoidance was significantly increased at 24 h compared to the behavior of saline-treated mice. However, when mice were isolated in the novel environment for 90 min immediately following treatment, the memory facilitating effects of ethanol were not observed. The avoidance behavior of mice injected with saline was the same regardless of their post-training environment. Also, the number of mice (6 or 10) housed per homecage did not significantly influence the effects of ethanol or post-training environment on avoidance. These findings indicate that environmental factors may interact with the effects of ethanol to modify avoidance behavior. The possible influence of variables such as aggression, thermoregulation, and behavioral arousal on the effects of ethanol in this paradigm are discussed.     

Responses of dominant and subordinate male rats to the odors of male and female conspecifics

The resident-intruder paradigm was used to examine the effects of social dominance and individual recognition on odor preferences and urine-marking in male rats. Resident males were significantly more aggressive than intruders and spent more time investigating the odors of familiar intruders. Resident males urine-marked most over the odors of females and familiar intruders while intruders marked least over the odor of the familiar resident. Intruders did not avoid investigating nor marking over the odors of familiar resident males or other conspecifics. These results suggest that individual odors of male rats may be more salient than a general odor of dominance, and that the dominant males increase their investigation and marking over the odors of familiar subordinates but not unfamiliar subordinates. The importance of olfactory learning during aggressive interactions is discussed.     

The effect of a pre-shock signal on a free-operant avoidance response

After 25 free-operant avoidance training sessions, a 1-min signal followed by a brief shock was presented on the average of once every 4 min. During the signal, the avoidance schedule was suspended (20 sessions). Response rates during the signal were markedly reduced. Shock rates during non-signalled periods increased. Fifteen additional sessions were given during which the signal was presented without shock. Response rates during signalled periods were greater than previously observed during signalled periods, indicating that signalled shock had suppressive control over a previously acquired avoidance response rate.     

The effect of worry on cardiovascular response to phobic imagery

Cardiovascular response to phobic exposures has been shown to reflect the degree of emotional processing of phobic material and relates in important ways to outcome in exposure therapy. The present study investigated the effect of worry on such processing. Speech anxious subjects visualized a phobic scene for 10 trials. Three groups differed according to whether they engaged in relaxed, neutral, or worrisome thinking just prior to each scene presentation. Although the three types of thinking had no effect on cardiovascular activity during the thinking periods, subjects in the worry group displayed significantly less heart rate response to the phobic image than those in the neutral condition who had significantly less response than the relaxed condition. However, the worry group showed significantly greater subjective fear to the images than the neutral condition, with the relaxed group near to the level of the worry group and not significantly different from either of the other two conditions. The results suggest that worry may inhibit emotional processing and thus preserve cognitive/affective fear structure, and are discussed in terms of Gray's (Behavioral and Brain Sciences, 5, 469-534, 1982) theory of the behavioral inhibition system and semantic satiation effects in cognitive psychology.     

Resurgence of response sequences during extinction in rats shows a primacy effect

Rats were trained to emit a series of three-response sequences to a criterion (i.e., more than 80% of all emitted sequences correct over five successive sessions). Each rat was trained on a series of different, three-response sequences. After the final three-response sequence was acquired, two extinction tests were administered, and the three-response sequences that re-emerged during these extinction tests were noted. Resurgence effects during extinction were observed; that is, the previously trained sequences were emitted. These resurgence effects followed an orderly pattern, which involved a primacy effect. The rats initially emitted the immediately previously trained response, but then started to emit the response sequence they first were trained to emit. Thus, resurgence behavior during extinction can be an orderly function of previous training history. These results replicate those previously obtained with human subjects.