Mu opioid receptors in the ventrolateral periaqueductal gray mediate stress-induced analgesia but not immobility in rat pups

Rat pups become immobile and analgesic when exposed to an adult male rat. The aim of this study was to determine whether these reactions are under the control of endogenous opioids and to determine the role of the midbrain periaqueductal gray (PAG), which mediates stress-induced immobility and analgesia in adult animals. In Experiment 1, 14-day-old rats were injected systemically with the general opioid receptor antagonist naltrexone (1 mg/kg), which blocked male-induced analgesia to thermal stimulation but did not affect immobility. In Experiment 2, the selective mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; 50 or 100 ng/200 nl) was microinjected into the ventrolateral and lateral PAG. CTOP suppressed male-induced analgesia when injected into the ventrolateral PAG. Male-induced immobility was not affected by CTOP. Male proximity therefore seems to induce analgesia in rat pups by releasing endogenous opioids that bind to mu opioid receptors in the ventrolateral PAG.     

Effects of undernutrition during suckling on novelty-induced analgesia in young and adult rats

The effects of undernutrition on novelty-induced analgesia were investigated in young and adult rats. Rats were undernourished by feeding their dams an 8% casein diet from birth until weaning (21 days of age). Rats were exposed to an open field (novelty) for 2 min and the nociception was measured by the tail-flick method. At adult age, only well-nourished rats presented novelty-induced analgesia, suggesting that early undernutrition abolishes this response. At 21 days of age, the exposure to the open field had no effect on nociception of both nutritional groups, suggesting that some developmental factor is necessary for the emergence of novelty-induced analgesia.     

Stress responses of adolescent male and female rats exposed repeatedly to cat odor stimuli,and long‐term enhancement of adult defensive behaviors

In order to characterize the short‐ and long‐term effects of repeated stressor exposure during adolescence, and to compare the effects of using two sources of cat odor as stressor stimuli, male and female adolescent rats (postnatal day (PND) ～ 38–46) were exposed on five occasions to either a control stimulus, a cloth stimulus containing cat hair/dander, or a section of cat collar previously worn by a cat. Relative to control stimulus exposure, activity was suppressed and defensive behavior enhanced during exposure to either cat odor stimulus (most pervasively in rats exposed to the collar). Only cloth‐exposed rats showed elevated levels of corticosterone (CORT), and only after repeated stressor exposure, but interestingly, rats exposed to the collar stimulus during adolescence continued to show increased behavioral indices of anxiety in adulthood. In this group, the time an individual spent in physical contact with a cagemate during the final adolescent exposure was negatively related to stress‐induced CORT output in adulthood, which suggests that greater use of social support during adolescent stress may facilitate adult behavioral coping, without necessitating increased CORT release. These findings demonstrate that adolescent male and female rats respond defensively to cat odor stimuli across repeated exposures and that exposure to such stressors during adolescence can augment adult anxiety‐like behavior in similar stressful conditions. These findings also suggest a potential role for social behavior during adolescent stressor exposure in mediating long‐term outcomes. © 2012 Wiley Periodicals, Inc. Dev Psychobiol 55: 551–567, 2013     

Early and Delayed Effects of Hypoxia during the Infantile Period on Behavioral and Hormonal Reactions of Rats

We studied the early and delayed effects of hypoxia during the infantile period on the behavioral reactions and corticosterone concentration in male rats. The elevation of corticosterone concentration, decrease in the immobility time (forced swimming test), and increase in the nociceptive response (formalin test) were observed in 7-day-old rats immediately after hypoxia. Adult animals exposed to hypoxia at the age of 7 days exhibited elevated basal corticosterone level and lengthened immobility time. Hypoxia had the same effect on plasma corticosterone concentration in 7-day-old and adult rats. Changes in corticosterone concentration after forced swimming were shown to differ in hypoxic animals and non-hypoxic specimens. Studying the dynamics of age-related variations in the test parameters will contribute to the understanding of pathogenetic mechanisms and development of new methods for pharmacological correction of postnatal changes in CNS after hypoxia during early ontogeny.     

The influence of age and experience on salt preference of the rat

The degree to which adult taste preferences can be influenced by early taste exposure is a matter of some debate. The present studies characterize the developmental course of saline preference during maturation in the rat and examine whether adult saline preference is influenced by postweaning exposure to saline solutions. Saline intake from weaning to adulthood was examined in Wistar rats allowed continuous access to demineralized water and either .0, .9, 1.8, or 2.7% sodium chloride (NaCl) w/v solutions. When rats were immature they expressed greater preference for saline solutions than they did as adults. It is hypothesized that the overconsumption of saline by young animals is related to lower sensitivity of the taste system to NaCl in the early postweaning period. Animals that had had continuous access to saline from weaning were compared to those that had not received saline exposure early in life, in order to determine whether extensive experience with salt solutions leads to alterations in adult salt preference. These studies failed to reveal any evidence that rearing condition affects adult salt preference; thus adult salt preference may be maturationally determined and relatively independent of postweaning environmental influences.     

Fear, risk assessment, and playfulness in the juvenile rat

The effects of predatory odors on play were assessed in juvenile rats. When rats were exposed directly to a collar previously worn by a cat, play was abolished and remained suppressed for up to 6 days. Providing rats with an opportunity to hide did not alter cat odor's ability to reduce their play. Rat play was also suppressed shortly after they were exposed to cat odor in their home cage, and a substantial amount of risk assessment behavior was present up to 24 hr later. Trimethylthiazoline, a component found in fox feces, only reduced play during exposure. These data suggest that predatory odor-induced reductions in play may provide a useful model for gaining insight into the consequences of fear and anxiety in young animals.     

Modulation of predatory odor processing following lesions to the dorsal premammillary nucleus

Previous studies have shown that electrolytic lesions of the dorsal premammillary nucleus (PMd) produce robust reductions in responsivity of rats to the presence of a live predator as well as to its odor, suggesting a critical role for the PMd in the modulation of defense. The present study investigated whether disruptions in defensive responding were specific to predators or if they may indicate a more general deficit in responding to pheromonal odors. Sexually naive male rats with bilateral ibotenic acid lesions of the PMd were exposed to the odor of a female rat in estrus as well as to the presence of cat odor, and, a live cat. PMd lesions produced a dramatic reduction in freezing and avoidance to the cat odor; and, reductions in freezing, enhanced activity and risk assessment to cat exposure. However, PMd lesions produced no changes in response to the presentation of the female odorant. These results confirm earlier findings of attenuation in defensiveness following electrolytic PMd lesions while extending these findings to suggest that the reduced defensiveness occurs specifically in response to predatory odors.     

Preference for natural odors in rat pups: Implications of a failure to replicate

The results of the present experiment indicate that very minor changes in the diet on which a rat dam is maintained can have profound effects on the attractiveness of her feces to her pups. Rat pups exhibited a strong preference for the odor of feces taken from dams maintained on one sample of Purina Rodent Laboratory Chow No. 5001, but no preference for the odor of the feces of dams maintained on a second sample of the same diet. These data point to the need for very great care in the design of experiments employing, as independent variables, stimuli which are not under direct experimenter control. The results also suggest that experience of a natural odor during ontogeny may not be sufficient for the development of a preference for that odor in young rats.     

‘When an old rat smells a cat’: A decline in defense-related, but not accessory olfactory, Fos expression in aged rats

Comparisons were made between young (3-6 months) and aged (20-30 months) Wistar rats on locomotor activity, emergence, social interaction and cat odor avoidance. Aged rats were less active and spent less time in the open field during the emergence test than younger rats. Older rats also showed fewer contacts with a novel conspecific in the social interaction test, although total duration of interaction did not differ. There were very few behavioral differences between male and female rats. Older rats were less reactive than younger rats in a test of cat odor avoidance. However, they expressed similar amounts of cat odor-induced Fos in the posterior accessory olfactory bulb, a critical region for processing the predator odor stimulus. Older rats had reduced Fos expression in several defense-related brain regions that are normally activated by predator odors such as the medial amygdala and dorsal premammillary nucleus. These results indicate that aged rats are less reactive than younger rats to predator odors due to decreased responsiveness in defense-related but not necessarily olfactory circuits.     

Prenatal stress effects on exploratory activity and stress-induced analgesia in rats.

Pregnant rats were exposed three times daily to immobilization stress during gestational Days 15-19. The behavior of their offspring was compared with the behavior of offspring from unstressed control mothers. Although the stress procedure decreased the weight gain of mothers during pregnancy, it slightly but significantly increased the weight of their offspring at birth and at weaning. On postnatal Day 10, prenatally stressed pups returned to their home cage more quickly than did prenatally unstressed control pups during a nest odor discrimination task, but no differences between groups in the number of correct responses were found. On postnatal Days 70-72, prenatally stressed offspring showed increased exploratory activity in a complex tunnel maze compared with control offspring. On postnatal Day 80, analgesia induced by stress (swimming for 3 min in cold water) was determined (tail flick latency). The degree of stress-induced analgesia was smaller in prenatally stressed rats than in control rats. These data suggest that the effects of prenatal stress on behavior are most clearly discernable when such animals are confronted with a novel or stressful situation.     

Morphine treatment and reproductive condition alter olfactory preferences for pup and adult male odors in female rats

Administration of morphine sulfate (MS) to pregnancy-terminated and postpartum lactating female rats disrupts both maternal behavior and postpartum aggression. Since the display of these behaviors may be heavily dependent on olfactory cues provided by the stimulus animals (rat pups and adult male rats, respectively), we examined whether MS was affecting the perception of the olfactory stimuli, and whether olfactory perception was modified by reproductive condition. In Experiment 1, lactating rats had their pups removed and were injected with MS (5.0 mg/kg, sc.) or saline. 60 min later they were placed into a two-choice apparatus, one side of which contained bedding soiled by neonates and the other clean bedding. Time spent on each side was recorded for a total of 5 min (300 s; chance = 150 s). Saline-treated mothers spent significantly more time on the pup-odor side, whereas MS-treated females spent significantly less. In Experiment 2, lactating females were treated with MS or saline and exposed to male odors (soiled bedding). MS significantly increased time spent on the side with male odors; when treated with saline, time spent was significantly reduced. Thus, in lactating rats. MS creates an aversion for pup odors while reducing the female's normal aversion toward male odors. In Experiment 3, ovariectomized (ovx) virgin females expressed neither an aversion nor a preference for the odor of pups following saline administration. After MS treatment, however, the virgins showed a distinct preference for pup odors. When exposed to male odors in Experiment 4 ovx virgins showed a marked preference for male odors after MS treatment, and neither a preference nor an aversion after saline. Experiment 5 examined pup odor preferences in intact virgins, early (Day 7), middle (Day 14), late-pregnant (Day 21), and prepartum (Day 22) rats. The pup odor preferences of virgin, Day 7, Day 14, and Day 21 pregnant rats were not different and generally were at chance levels. Day 22 pregnant females exhibited a marked preference for pup odors compared to chance levels, as well as compared to the other four groups. These findings suggest that opiates and endogenous opioids may regulate olfactory preferences and that alterations in this system may underlie normal behavioral changes toward conspecifics prepartum as well as during lactation.     

Development of water-immersion produced analgesia

Nociceptive and antinociceptive systems change dramatically during the first 10 days of life in rats. The present studies sought to determine whether exposure to water stress can produce analgesia during this period, and when in the ontogeny changes in magnitude and duration of such stress-induced analgesia occur. It was found that exposure to water reliably induced analgesia as early as 3 days postnatally. This analgesia increased in magnitude and duration at the age of 17–20 days, when supra-spinal descending inhibition develops, and depended on the temperature of the water and duration of exposure. © 1995 John Wiley & Sons, Inc.     

Alterations in opioid system of the rat brain after cat odor exposure

The effect of cat odor exposure was studied on morphine-induced increase of exploratory behavior and on the expression of opioid genes in forebrain structures of male Wistar rats. Treatment with morphine (1 mg/kg) induced a significant increase in exploratory behavior in an unfamiliar environment in rats. Previous exposure of animals to cat odor completely abolished this stimulating action of mu-opioid receptor agonist on exploratory activity. Cat odor exposure induced a significant increase in the expression of pro-opio-melanocortin (POMC) and mu-opioid receptor (MOR) genes in the brain structures related to anxiety and motivation. This study clearly demonstrates that cat odor exposure increases the activity of opioid system in rat forebrain structures.     

Predator and restraint stress during gestation facilitates pilocarpine-induced seizures in prepubertal rats

Stress during gestation can result in early and long-term developmental aberrations. This study aimed to assess the impact of prenatal restraint or predator stress on pilocarpine-induced epileptic behavior. Pregnant rats were exposed to stressors on gestational days 15, 16, and 17. Restraint stress consisted of daily restraint of the dam. During predator stress, caged rats were exposed to a cat in a cage. On postnatal day 25, male pups were injected with pilocarpine and the behavior of each rat was observed. Prenatal stress led to low birth weight and increased blood corticosterone levels. Both stressors significantly potentiated pilocarpine-induced seizures. Predator-stressed pups exhibited significantly severe tonic-clonic seizures compared with restraint-stressed animals. These data emphasize the impact of prenatal stress on fetal growth, and neural and endocrine function. The results also suggest that psychosocial stressors have a greater impact on neural and endocrine function than physical stressors do.     

Postnatal development of circadian rhythms in disaccharidase activities in rat small intestine.

The circadian rhythms in the activities of maltase and lactase of the small intestine were examined at various stages of postnatal development in rats. When the rats were fed ad libitum, no circadian change in the enzyme activities was found during the suckling period or the weaning period. However, several days after the time of weaning, the enzymes showed the same circadian changes as in adult rats with higher activities at night. After weaning, when the rats were fed only during the daytime, the phase of the enzyme rhythm shifted about half a day, and the highest activity was observed around feeding time. However, during the suckling period, no circadian rhythm in the enzyme acvities was found, even when the rats were allowed to feed only during a restricted time of the day.     

Development of defensive behavior and conditioning to cat odor in the rat

Laboratory rats show a range of defensive behaviors, including freezing, avoidance, and risk assessment upon exposure to cat odor, an unconditioned but highly effective threat stimulus. This study examined defensive behaviors, and the rapid conditioning to context plus cue, of these behaviors, in 18-, 26-, and 38-day-old male and female rats exposed to cat odor. Rats were placed individually in a runway with a cloth covered (control or saturated with cat fur/skin odor) block for a 10-min trial. On the following day, a similar trial involved an odorless block. On the odor exposure day, rats of all ages showed less contact with the odor block than with the control block. The 26- and 38-day-old rats, but not the 18-day-old rats, also showed locomotor suppression, more avoidance of the area where the odor block was located, and more risk assessment than no-odor controls. On a test of conditioned behavior 24 h following exposure, 26- and 38-day-old rats exhibited defensive behavior including avoidance and reduction of locomotion while 18-day-old pups did not.     

Hormonal control of the perception of the olfactory signals which facilitate lordosis behavior in the male rat

This study was designed to evaluate in the male rat the hormonal requirements for the facilitation of feminine behavior by the odor of male urine. Wistar rats from the WI and WII strains in our colony were orchidectomized (ORCH) as adults. A first group was given a single dose of 75 micrograms estradiol benzoate (EB) and tested for lordosis behavior 48 hr later. Exposure to the odor of male urine by 9 +/- 1 hr before the behavioral session did not increase the number of animals showing lordosis behavior as compared to non exposed controls. A second group of WI rats was given 0.5 micrograms EB every day for 4 to 8 days. A similar number of animals displayed lordosis behavior irrespective of whether they were exposed to the odor of urine before testing. A third group of WI rats was injected with 75 micrograms EB and 1 mg progesterone (P) 39 hr apart. Exposure to the odor of urine during estrogen treatment remained ineffective but significantly increased the number of animals showing lordosis behavior when performed at the time of P injection. A last group of WII rats was given 25 micrograms EB and 100 micrograms or 150 micrograms P 39 hr apart. Although uncapable as such to facilitate lordosis behavior the dose of 100 micrograms P rendered the animals responsive to the odor of urine. It was concluded that (1) the perception by feminized males of olfactory signals from the male was dependent on P; (2) an interaction between hormonal and sensory mechanisms was involved in the facilitation of lordosis behavior in the male rat.     

Non-specific olfactory aversion induced by intrabulbar infusion of the GABA(A) receptor antagonist bicuculline in young rats

On postnatal day 12, young rats show an aversion to an odor to which they had been exposed along with presentations of foot shock on postnatal day 11. The acquisition of this aversive learning involves and requires disinhibition of the mitral/tufted cells induced by centrifugal noradrenergic activation during somatosensory stimulation. This olfactory learning is established only for the odor to which the rat has been exposed during conditioning. Infusion of the GABA(A) receptor antagonist bicuculline at a high dose (2.0 nmol/each olfactory bulb) into the olfactory bulb in the presence of an odor is capable of developing olfactory aversive responses without somatosensory stimulation in young rats. The purpose of this study is to characterize the properties of bicuculline-induced aversive responses. In contrast to the odor specificity of aversive learning produced by odor-shock conditioning, bicuculline-induced aversive responses lack odor specificity. Namely, bicuculline infusion in the presence of a citral odor results, in a dose-dependent manner, in subsequent aversive responses to strange odors (benzaldehyde and vanillin) that have never been presented. Moreover, bicuculline infusion alone is sufficient to produce dose-dependent aversive responses to strange odors (citral, benzaldehyde and geraniol).From these results we suggest that disinhibition of mitral/tufted cells from granule cells by bicuculline infusion makes young rats aversive to strange odors non-specifically, as if the rats had learned the odor aversion as a result of odor exposure paired with foot shock. Different mechanisms of disinhibition of the mitral/tufted cells may underlie both the pharmacological manipulation and noradrenergic activation by somatosensory stimulation.     

Behavioral correlates of hippocampal type 2 theta in the rat

Previous research has indicated that there are two types of theta activity which can be recorded from the hippocampus in most rodents. One type (type 1) of theta is correlated with voluntary movements while the second (type 2) occurs during immobility. A third waveform termed LIA (Large Amplitude Irregular Activity) occurs during more automatic behaviors, e.g., grooming, shivering or immobility. In the rabbit and guinea pig type 2 theta can readily be elicited in unrestrained animals. However, type 2 theta is rarely seen in the unrestrained rat. The purpose of the present study was to determine the stimulus conditions that are necessary for the elicitation of type 2 theta in the rat. In the present study 20 unrestrained rats were observed in the presence of a cat or a ferret. Movements by the cat or ferret produced theta in immobile rats. Sensory stimuli which did not ordinarily produce type 2 theta would reliably do so in the presence of the cat or ferret. It is hypothesized that type 2 theta in the unrestrained immobile rat occurs during sensory processing but only when the animal is in a high state of "arousal."     

Influence of neonatal handling on blood pressure,locomotor activity,and preweanling heart rate in spontaneously hypertensive and Wistar Kyoto rats

This experiment tested the hypothesis that increased stimulation early in development would (a) alter developmental changes in heart rate and behavioral reactivity and (b) affect the level at which blood pressure was regulated in adulthood. For this purpose, the effects of daily handling and maternal separation (3 min per day) on both behavioral and cardiovascular measures were examined in spontaneously hypertensive (SHR) and normotensive control Wistar Kyoto (WKY) rats. Prior to weaning, elevated heart rates in pups handled during the first postnatal week were most pronounced among 4-week-old prehypertensive SHR pups. Early handling affected behavior observed during openfield testing similarly in young adult rats of the SHR and WKY strains (e.g., increased locomotor activity on the first day of testing). In female rats of the WKY strain, early handling resulted in a lower baseline blood pressure; the blood pressure; the blood pressure of SHR rats was not affected by increased stimulation in infancy. Examination of longitudinal data yielded no support for a direct association between behavioral reactivity or preweaning heart rate and high blood pressure. These findings demonstrate the influence of both early environmental conditions and genetic factors on maturation within the cardiovascular system and suggest that genetic models of pathological conditions may provide a productive means of examining environmentally shaped aspects of individual differences in physiological regulation.