Mesial prefrontal cortical lesions and timidity in rats. I. Reactivity to aversive stimuli

Two experiments were conducted to assess the degree to which mesial prefrontal cortical (MFC) lesions in male rats alter the behavioral manifestations of fear (timidity). The first experiment gauged the reactivity to the threat of electroshock. MFC lesions did not alter the incidence of freezing in a box associated with shock. MFC and sham-lesioned subjects made few errors when they were required to run from an open field into a small compartment to avoid shock, but displayed the classical MFC deficit when forced to run in the opposite direction. MFC lesions also increased the latency to enter an open field when the field was novel or aversive, but not when it was familiar. In a second experiment, it was found that the classical MFC hoarding deficit occurred when rats had to traverse an illuminated but not a dark runway to reach food pellets. Similarly, burrowing was normal when subjects had to burrow to escape from an open field, but not when they had to enter an open field to burrow. It is concluded that damage restricted to MFC enhances timidity, and that this effect accounts for several behavioral alterations that contribute to the MFC lesion syndrome.     

Mesial prefrontal cortical lesions and timidity in rats. III. Behavior in a semi-natural environment

Behavior of adult male rats with and without lesions in mesial prefrontal cortex (MFC) was studied in a large (72 m2) semi-natural environment (SNE) equipped with numerous wooden nest boxes. The experiment was conducted in 2 partial replicates, one in summer, one in mid-winter. Subjects were introduced to this environment 11 days after surgery, in groups. In the first replication, control and lesioned rats were subjected to mild food deprivation, beginning two weeks after introduction into the SNE. In the second replication, rats were first restricted to separate areas of the SNE, then later allowed full run of the environment. This procedure was then repeated. Summarizing the findings of both replicates, MFC rats were more likely to be found inside nest boxes than controls, changed nest boxes less often, and were less likely to obtain or hoard food from the daily feeding sessions, or to hoard and use nesting material. MFC rats were lower in social rank than controls, and lost less weight during the initial two weeks in replicate one, perhaps due to their relative inactivity. These differences waned over time when MFC rats were competing only within their group, but not when the brain damaged rats were forced to compete with intact controls. It is concluded that the timidity previously described in studies of MFC rats in a laboratory setting (Holson, Holson and Walker) has ethological validity, in that it is also evident in an SNE. The implications of these findings for an understanding of the evolution and function of prefrontal cortex are briefly discussed.     

Effects of olfactory bulb removal on fear responses and passive avoidance in the rat

Following complete bulbectomies, male hooded rats showed an increase in irritability and difficulty of handling, but a decrease in timidity or fear responses. After rats had learned to drink in an open field, a cat was confined in the center, and fear was defined by the behavior of controls, viz., almost total suppression of drinking and long periods of freezing, broken by brief bursts of high-speed activity. Bulb animals could not have differed more radically. Bulbs showed neither freezing nor suppression of drinking. The present results could not be attributed to differences in shock reactivity; nor could they be attributed to differences in learning or retention. Bulb animals also showed an impairment on the acquisition of step-down passive avoidance but no difference in step-downs after acquisition. These results argue against an impairment of response inhibition. The present study suggests that bulbectomy increases irritability and decreases timidity.     

Gender and early environmental influences on activity,overresponsiveness, and exploration

One hundred eighty-five rats reared in either an enriched or restricted environment were tested during adulthood to determine the influence of gender and rearing environment on 3 related response characteristics, activity, overresponsiveness, and exploratory behavior. Eight experiments were performed. It was found that although females are more active than males, rearing environment does not influence behavior in the running wheel or open field. When tested in a complex compartmentalized open field, females in general and restricted rats are significantly more active than enriched males. In addition, over subsequent testing, restricted animals are increasingly responsive and fail to habituate to the testing stimuli. When tested and retested for maze learning ability, males excel over females; enriched rats maintain their ability to outperform restricted rats, although both groups had previously learned the problem. Moreover, enriched rats demonstrate a greater tendency to explore and make irrelevant section entries on a maze that is problem free. Restricted rats, specifically, and females generally, have difficulty suppressing a learned repetitious pattern of rewarded responding when it is subsequently punished; restricted rats were deficient in the ability to passively avoid or escape noxious stimuli. These experiments, as well as supporting evidence in the literature, indicate that rats reared in a restricted environment develop a limited behavioral repertoire which is characterized by a generalized tendency to overrespond, a propensity towards perseverating in repetitious patterns of limited and circumscribed responding, and a failure to habituate to repeated contact with novel stimuli.     

Adrenocortical, beta-endorphin and behavioral responses to graded stressors in differentially reared rats

Isolation rearing has long been suspected to alter hormonal and behavioral responses to stress. Two experiments were conducted to test the hypothesis that isolates are more timid or fearful than socially reared rats when exposed to novel test environments. In both, isolate response to 3 graded stressors was compared to that of socially-reared rats. In the first experiment, animals were handled, shocked or not treated prior to testing to produce three levels of conditioned fear. They were then tested on four paradigms previously shown sensitive to conditioned fear: open field activity, emergence latency, auditory startle, and latency to accept food from the experimenter. In the second experiment, rats were given a 0-, 5- or 20-min forced swim, then sacrificed for analysis of plasma corticosterone and pituitary and hypothalamic beta-endorphin. It was found that isolates showed little evidence of enhanced behavioral timidity, although rearing effects were seen on all 4 behavioral measures. Plasma corticosterone levels increased in a graded fashion over the course of the forced swim, but there was no effect of rearing conditions. While there were no effects of rearing or stress on hypothalamic beta-endorphin, pituitary beta-endorphin content was lower in females than in males, and isolate males had lower pituitary endorphin than social males. In summary, these experiments provide no evidence that isolation rearing produces a primary, global increase in fearfulness, but identify several behavioral and hormonal differences associated with differential housing in rats.     

The behavior of vasopressin-deficient rats (Brattleboro strain)

Seven Brattleboro rats homozygous for diabetes insipidus (DI) and seven normal Long-Evans (LE) rats were tested on a neuropsychological test battery comprised of the following tasks: time-spent-eating in two adaptation boxes, time-to-emerge into an open field, adaptation to a T-maze, contingently reinforced T-maze alternation, olfactory and visual discrimination, runway learning, approach-avoidance conflict, step-through passive avoidance, prod burying, and stress-induced interference. It was found that DI rats adapted more slowly than LE rats to novel environments (e.g., adaptation box, T-maze, and runway), and DI rats were slower to emerge into an open field. However, DI rats performed as well as LE rats on all other tasks. These results suggest that DI rats have altered temperamental dispositions (timidity or cautiousness), normal working and reference memory, and similar susceptibility (compared to LE rats) to the interfering effects of inescapable stress.     

Variability and discrimination reversal learning in the open field following septal lesions in rats.

Response variability on a 10 position horizontal response task and a position-habit discrimination followed by 2 reversals was examined in groups of rats with septal damage and operated control rats in the open field. As revealed by the number of positions with responses and the one position with the most responses, the rats with septal damage were reliably less variable in responding than the controls. In addition, the rats with septal damage made a reliably greater number of responses during extinction of the response. In the second experiment, groups of animals were tested for discrimination learning in a 2 part experiment using both a free-ranging and restricted open field. In both parts of the experiment, the rats with septal lesions learned the discrimination and both reversals faster than the controls. Animals tested in the restricted field learned the discrimination slightly faster than those tested in the free-ranging field. The superior discrimination learning for the rats with septal lesions was discussed as a lesion-induced increase in the incentive value of positive reward, producing shorter observed response latencies, resulting in a shorter delay of reinforcement.     

Persistent neonatal Borna disease virus (BDV) infection of the brain causes chronic emotional abnormalities in adult rats.

Neonatal Borna disease virus (BDV) brain infection results in selective developmental damage to the hippocampal dentate gyrus and the cerebellum. When mature, neonatally BDV-infected rats show extreme locomotor hyperactivity and reduced freezing behavior in novel environments. Traditional interpretation of both of these behavioral abnormalities would suggest decreased anxiety in infected rats compared to normal animals. However, it also possible that the locomotor hyperactivity in infected rats reflects higher rather than reduced anxiety, and is the result of increased escape responses to aversive stimuli. The present experiments were undertaken to test a hypothesis about elevated anxiety in neonatally BDV-infected adult Lewis rats by studying their species-specific fear-related responses. Compared to normal subjects, BDV-infected rats exhibited locomotor hyperactivity and elevated defecation in a highly aversive, brightly lit open field. As expected, in a less aversive, dimly lit open field, uninfected controls increased ambulation, whereas infected rats significantly decreased locomotor activity and defecation. Unlike uninfected rats, BDV-infected rats exhibited an attenuated freezing response immediately after loud auditory stimuli. On the contrary, immediate freezing responses following footshock were comparable in the two groups of animals indicating an intact ability to freeze in BDV-infected rats. Despite a decreased baseline startle responsiveness, BDV-infected rats demonstrated increased sensitization of the startle response by preceding footshocks, suggesting a tendency toward elevated escape responses. Compared to normal subjects, BDV-infected rats showed decreased conditional freezing and elevated conditional defecation response in the context previously paired with aversive stimulation indicating sparing of an autonomic component of fear conditioning. The findings indicate that neonatally BDV-infected adult rats are hyperreactive to aversive stimuli, possibly as a result of chronic emotional abnormalities.     

Stress-induced responses and open-field behavior in estrous and nonestrous mice

Three experiments were performed to compare the reactions of estrous and nonestrous female CD-1 mice to stressful stimuli. These experiments revealed no effect of estrous state on (a) response latencies to thermal pain in the mouse hot-place test, (b) immediate, active responses to electric foot shock, or (c) open field ambulation under no stress condition or under stress conditions provided by foot shock or bright light. Estrous mice were found to defecate less in a small open field, and to ambulate more in the center of a large open field, than nonestrous mice, but neither of these effects interacted with an effect of stress condition. An incidental finding was a diurnal effect on response latencies in the mouse hot-place test. These experiments, combined with previous experiments with female CD-1 mice, support the conclusion that estrus results in decreased avoidance of potentially dangerous or stressful places, but does not affect the animal's reactivity to unavoidable stressful stimuli.     

Neurological reactivity during medial prefrontal cortex stimulation: Effects of self-stimulation experience

Electrical stimulation of the medial prefrontal cortex (MFC) induced a moderate behavioral suppression in a series of neurological tests that included measures of (1) open field activity (2) righting and climbing reflexes (3) somatosensory reactivity and (4) approach-withdrawal responsivity to positive and negative stimuli. The same animals were trained to lever press for medial prefrontal cortex stimulation and after several days of self-stimulation were again tested for neurological reactivity during MFC stimulation. No habituation or lessening of behavioral suppression was evident as a result of self-stimulation experience. Therefore behavioral suppression, by itself, cannot account for the slow acquisition of MFC self-stimulation. Other data suggest that the stimulation interferes with the learning of complex operant responses and/or that the reward value of the stimulation is increased as a result of repetition.     

Taste reactivity in rats following lesions of the zona incerta or amygdala

Rats with amygdala lesions (n=12) and rats with zona incerta lesions (n=12) were compared to normal rats (n=12) in their response to various taste stimuli. Five concentrations each of sucrose, sodium chloride, quinine hydrochooride, and hydrochloric acid were presented to all animals in single bottle tests during a schedule of restricted fluid access. Results indicated that both groups of rats with lesions had significantly lower baseline water consumption than normal rats. Using taste consumption measures expressed as a percent of water baseline, it was found that zona incerta lesions did not significantly alter normal taste reactivity. Animals with zona incerta lesions did fail to show normal habituation to a 0.0001 M quinine solution over repeated presentations. Lesions of the amygdala resulted in an increase in consumption of the acid solutions; consumption of sucrose, quinine, and sodium chloride was normal across concentrations. Amygdala damaged rats failed to show normal neophobia when first presented with the 0.0001 M quinine solution. It was concluded that taste response functions of rats with lesions of the zona incerta or amygdala remain essentially normal following surgery but that subtle changes in reactivity do occur, particularly with regard to neophobia.     

Head-dipping in rats with superior collicular,medial frontal cortical and hippocampal lesions

When tested in a modified “holeboard” apparatus, hooded rats with deep superior collicular lesions were hyperactive, reared less than controls and spent a great deal of time at the apparatus perimeter. They failed to explore holes in the floor and frequently fell into them. Those with lesions restricted to superficial laminae showed reduced rearing, and preference for the perimeter but were normally active and explored floor holes as frequently as controls. The frontal cortical group failed to differ from controls on any measure. Hippocampals exhibited reduced intersession habituation of locomotor activity. It was concluded that superior colliculus, hippocampus and medial frontal cortex have separate functional roles. Differences between deep and superficial collicular lesions effects might arise because deep lesions disrupt both motor integration and attention while superficial lesions disrupt only the latter. Alternatively, since superficial lesions spare lateral tissue subserving attention to stimuli in lower portions of the visual field, they may produce a less global attentional deficiency which has less impact upon motor function.     

Influence of perinatal malnutrition on adult physiological and behavioral reactivity in rats

A series of experiments examined the behavioral and pituitary-adrenal response to novelty of perinatally malnourished rats tested as adults after nutritional rehabilitation begun at weaning. Neither the behavioral measures of ambulation, rearing and defecation, nor the plasma corticosterone response to a brief exposure to an open field differentiated the previously malnourished subjects from controls. Similar to controls, previously malnourished subjects were also capable of displaying a graded corticoid elevation to environments increasingly different from the home cage. However, exploratory behavior, as measured by head-dip frequency and duration in the hole-board, was reduced in the previously malnourished rats. Although latency and amount of fluid consumed in a novel environment did not differ, previously malnourished rats were unable to use the cues associated with a consummatory behavior to modulate the pituitary-adrenal response to novelty. Thus, perinatal malnutrition does not influence either the behavioral or physiological activational response to novel stimulation but appears to alter the ability of the animal to use a consummatory behavior to modulate this response.     

Evidence that electrolytic median raphe lesions increase locomotion but not exploration

Electrolytic lesions of the median raphe nucleus were found to increase locomotion but decrease rearing in the open field. Additionally, these lesions reduced the amount of time that rats spent sniffing at a novel object placed in the open field on their first encounter with it. In a test of exploratory behavior in a T-maze, median raphe lesions eliminated the preference for entering a novel arm displayed by sham operated animals. These results suggest that, although median raphe lesions increase locomotion, they may actually decrease exploration.     

Conditioned taste aversion induced by estradiol pellets.

In two experiments, ovariectomized rats were given a novel diet prior to the implantation of a fused pellet of estradiol (E2 pellet). In short-term (3 weeks) ovariectomized rats, the suppression of food intake induced by estrogen was not affected by the introduction of the novel diet. However, a sensitive two-choice preference test revealed that subjects implanted with the E2 pellet had a lesser preference for the novel diet than controls implanted with the vehicle pellet. In long-term (18 weeks) ovariectomized rats, implantation of the E2 pellet had a large effect on the consumption of the novel diet. Intake was reduced to less than 1 g in all subjects on Days 3-7 after E2 pellet implantation. A subsequent two-choice preference test indicated the presence of a strong aversion to the novel diet in the estradiol-treated rats relative to the controls. These experiments show that estradiol can induce conditioned taste aversions that have either no effect on intake or totally suppress food intake, depending upon postovariectomy time.     

Effects of lesions to the glutamatergic afferents to the nucleus accumbens in the modulation of reactivity to spatial and non-spatial novelty in mice.

The purpose of this study was to compare the effects of selective lesions of the three main sources of limbic afferents to the nucleus accumbens-fornix, prelimbic cortex and amygdala-with those induced by N-methyl-D-aspartate receptor blockage in this structure, in a non-associative task designed to estimate the ability of rodents to encode spatial and non-spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). Focal administrations of the competitive N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonopentanoic acid (0.1 microg/side) induced a selective impairment in the ability of mice to react to the spatial change. Lesions to the different structures affect the response of mice to spatial and non-spatial novelty in different ways. In particular, while fornix lesions induced a decrease in re-exploration of the displaced objects, prelimbic cortex lesions enhanced the exploration of both displaced and non-displaced objects. Finally, the basolateral amygdala lesions did not induce any impairment in the detection of the displaced objects but decreased the latencies to approach novel objects. It is concluded that N-methyl-D-aspartate receptor blockage in the nucleus accumbens subsumes the effects of the three lesions. Some hypotheses on the role of glutamatergic transmission in the accumbens on information processing are briefly discussed.     

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.     

Task dependent rate of recovery following serial posterior hypothalamic lesions

Forty-two male rats received one-stage or sequential unilateral (30 days between sides) lesions of the posterior hypothalamus and were tested 1, 3 and 5 weeks later on a swimming task and in the open field. Subjects with serial lesions were comparable to sham-operated rats in swimming ability at each observation point. Rats with one-stage lesions, in contrast, showed pronounced swimming deficits (time under water, body angle, hindlimb kicks/sec) at each test session, although some improvement was noted over time. With respect to open field activity, subjects with serial lesions did not demonstrate significant recovery until the third postoperative week, while rats with one-stage lesions were impaired at each test session. These data show that serial lesions of the posterior hypothalamus may be associated with more rapid recovery of function than one-stage lesions, but that the time course for such recovery may vary with the specific ability being evaluated.     

Neonatal handling reverses behavioral abnormalities induced in rats by prenatal stress

The aim of this study was to see whether neonatal handling can reverse the behavioral deficits induced in rats by unpredictable prenatal stress. Rats (24) were exposed to random noise and light stress (RS) on a random basis throughout pregnancy. Half the litters of RS and control (C) dams were subjected to neonatal handling (NH), 3' daily for 21 days. Behavioral measures, open field in both sexes and plus maze test in females only were conducted at 1.5-2 months, and spatial lateralization by amphetamine-induced rotation, in females at 3 months. RS caused significant increases in emotionality and timidity (higher incidence of defecation and less time in open arms of plus maze), and a change in directional bias towards the left. NH completely reversed all these behavioral abnormalities in RS rats but had little effect in C. It is concluded that NH can influence postnatal development of brain organisation in the opposite direction to that induced by prenatal stress.     

Early postnatal deprivation of active sleep with desipramine or zimeldine impairs later behavioural reactivity to auditory stimuli in rats

To examine the functional significance of early postnatal active sleep for the development of behavioural reactivity to auditory stimuli, rat pups were daily injected i.p. from the 7th to the 18th postnatal days with 5 mg kg-1 (6.6 mmol l-1) desipramine or 25 mg kg-1 (12.2 mmol l-1) zimeldine. Sleep-wake behaviour was recorded with a static-charge-sensitive bed (SCSB) method. Both desipramine and zimeldine suppressed the percentage of active sleep relative to the total recording time throughout the treatment period. In addition, these drugs increased the percentage of quiet state and waking. At the age of 38 days the zimeldine-treated rats showed more motor activity in the open field than the controls. At the age of 39 and 78 days all rat groups behaved similarly in the open field. Startle measures and motor activation, provoked by auditory stimulation, were determined by the SCSB method when the rats were 4 months of age. Auditory stimuli, consisting of a series of ten clicks, induced a greater number of startles as well as strong movement responses in the control rats than in the desipramine- or zimeldine-treated rats. The number of small movement responses did not differ between the rat groups. These findings indicate that early postnatal active sleep and the monoaminergic systems regulating it may be important for the normal development of neuronal circuitry associated with later reactivity to auditory stimuli.