Undernutrition during the brain growth period of the rat significantly delays the development of processes mediating Pavlovian trace conditioning

Pavlovian trace conditioning procedures allow one to assess the ability of animals to associate events that are temporally separated because the conditioned stimulus terminates prior to the occurrence of the unconditioned stimulus. We report that undernutrition during Postnatal Days 2-18 significantly delays the development of trace conditioning to a visual stimulus. Previously undernourished 20-day-old rats conditioned when no temporal interval separated the termination of the CS and US onset (0-s trace interval). However, it was not until the undernourished pups were 30 days old that they conditioned when the trace interval was 10 or 30 s. In contrast, control pups only 20 days old were able to condition when a 10-s trace interval separated the CS and US events, and 25-day-old control pups conditioned when the interval was either 10 or 30 s. These results suggest that undernutrition delays the development of processes that enable the rat to sustain a representation of a visual CS during the trace interval.     

Behavioral responses of juvenile rats (Rattus norvegicus) to neonates after infusion of maternal blood plasma

Individual 18- or 30-day-old male and female Wistar rats (juveniles) were continuously exposed to 3-8-day-old pups for 5 days (sensitization) after intravenous (iv) infusion of maternal blood plasma (PL group) or 5% dextrose and water (IC group) through chronically implanted cannulas and were compared with nonhandled littermate controls (NHC group). The results showed complex interactions with sex and age of the animals tested, depending upon which maternal behaviors were examined. First, plasma from parturient females selectively increased retrieving behavior in 30-day-olds, and it was more powerful in females than in males. Second, on the first day, prior to sensitization, both PL and IC 30-day-olds showed higher frequencies of retrieving, crouching, anogenital licking, and contact than did nonhandled juveniles. Third, 30-day-olds that received maternal blood plasma continued to retrieve pups, with a latency peak for retrieving occurring at 72 hours, suggesting a long-term effect of plasma not seen in control animals. Fourth, 18-day-old subjects did not generally show effects of any of our manipulations, but they did show more nest building and contact behaviors than did 30-day-old subjects. Last, we found that the iv cannulation and infusion procedures led to a persistent dissociation of retrieving behavior from play in older animals and an integration of retrieving with crouching and licking regardless of whether maternal plasma or control infusion was administered. Retrieving behavior was associated with play behaviors (charge and pounce) in younger juveniles and in older, nonhandled control animals irrespective of treatment. For these animals, no integration of retrieving occurred with other maternal behaviors.     

The effect of rearing environments on the contrafreeloading phenomenon in rats

Eight naive rats were reared in enriched or impoverished environments for 39 days after weaning and then lived in operant chambers, in which they could obtain food pellets freely or by lever pressing, for 25 or 30 days. The animals raised in an impoverished environment acquired the bar-press response quickly when placed in the operant chambers and maintained a preference for obtaining food via bar pressing. Animals raised in an enriched environment did not learn to lever press, as demonstrated by low levels of responding and the lack of bar pressing when free food was subsequently removed. It was concluded that restricting animals' postweaning environments facilitated learning in a choice situation, probably because of increased activity levels. The results are interpreted in relation to previous studies on rearing environments and on contrafreeloading.     

Chronic exposure to a novel odor increases pups' vocalizations, maternal care, and alters dopaminergic functioning in developing mice

This study was designed to assess the stress effect of manipulation of the olfactory environment in developing mice. In a first experiment it was found that mouse pups could be stressed (as measured by an increase in ultrasonic calls) by removing the litter from the dam for 15 min/day for the first 14 days of life and exposing them to a novel odor (clean bedding). This stress procedure also produced a long-term modification in maternal behavior. The stress response (ultrasounds) and the modification of maternal behavior were prevented by providing the litter with home cage bedding during maternal separation. In a second experiment it was demonstrated that early stress influenced apomorphine-induced wall climbing behavior in 15-day-old mice, suggesting stress-induced alterations in the dopaminergic system. Pups exposed to clean bedding during infancy exhibited more wall climbing behavior than pups never separated from the mother. Moreover, preventing the early stress response during mother-offspring separation, by providing pups with home cage bedding, eliminated the increase in apomorphine-induced wall climbing. Taken together these results suggest that olfactory cues are decisive in characterizing stressful situations inducing both immediate and long-lasting effects in mouse pups.     

Rat gestation during space flight: Outcomes for dams and their offspring born after return to earth

Sprague-Dawley rats were studied to learn whether gestation in the near-zero gravity, high radiation environment of space impacts selected mammalian postnatal events. Ten rats spent days nine to twenty of pregnancy aboard the space shuttle orbiterAtlantis (STS-66). Their movement, was studied shortly after return to Earth; subsequently, several of their offspring were cross-fostered and examined through postnatal day 81 (P81) for whole body growth and somatic motor development. Values for the flight animals were compared to ground-based control groups. Relative to controls, the pregnant flight rats showed a marked paucity of locomotion during the first few hours after returning to Earth. There was greater likelihood of perinatal morbidity for the offspring of flight dams when compared to the control groups. Whole body weight of surviving offspring, averaged for each group separately, showed typical sigmoidal growth curves when plotted against postnatal age. The flight group for our study had a larger ratio of female to male pups, and that was sufficient to account for the lower average daily weight gained by the flight animals when compared to the control groups. Walking was universally achieved by P13 and preceded eye opening which was complete in all pups by P17. Thus, both of these developmental horizons were attained on schedule in the flight as well as the control rats. Characteristic changes were observed in hind limb step length and gait width as the pups grew. These patterns occurred at the same time in each group of rats. Therefore, prenatal space flight from days nine to twenty of gestation did not interfere with the establishment of normal patterns for hind paw placement during walking.     

Human choice on concurrent variable-interval variable-ratio schedules.

Each of 5 adult male humans sat in a 4 degrees C room where they could warm themselves by illuminating six heat lamps for 10-second periods according to a concurrent variable-interval variable-ratio schedule. Left-button presses on a response panel switched between the schedules and started a 2-second changeover delay. Right-button presses illuminated the heat lamps if assigned by the associated schedule and if the changeover delay had timed out. Panel lights identified the schedule in effect and each effective right-button press. A discrimination procedure--either a multiple variable-interval variable-ratio schedule or the presentation of each schedule individually on alternate days--preceded exposure to the choice procedure for some subjects. For subjects not exposed to a discrimination procedure prior to exposure to choice, or if such exposure failed to result in higher rates to the ratio than to the interval schedule, relative response rates matched relative reinforcement rates. However, if subjects responded at higher rates to the ratio schedule than to the interval schedule during a prior discrimination procedure, relative rates on a subsequent choice procedure deviated from matching in the direction of reinforcement-rate maximizing. In eight of 11 conditions, choice appeared to be governed by maximizing processes. In all cases, human concurrent ratio-interval performances differed from those of nonhumans in that matching was never obtained with local ratio-interval rate differences.     

Effects of transections to the vomeronasal nerves or to the main olfactory bulbs on the initiation and long-term retention of maternal behavior in primiparous rats.

While the onset of maternal behavior at parturition is mediated by hormones, the maintenance of maternal behavior during the first few postpartum weeks depends on experiences acquired while the dam interacts with pups (Rosenblatt, 1990). In fact, if female rats are permitted as little as 2 h or maternal experience within 36 h after Cesarean delivery, they exhibit heightened maternal behavior during maternal induction tests 10 days later; in contrast, dams separated from young at the time of Cesarean delivery and not permitted a maternal experience fail to respond maternally in tests 10 days later (Orpen & Fleming, 1987). In this study we investigated the role of chemosensory input through the vomeronasal and main olfactory systems in this maternal experience effect. Six groups of primiparous females were tested for maternal behavior to foster pups presented 9-10 days after Cesarean delivery: three groups were permitted to interact with pups for a 2-h period 36 h after Cesarean delivery; and three groups were separated from pups until testing and were given no maternal experience. Within each experience condition, one group sustained bilateral section of the vomeronasal nerves, one sustained bilateral coronal cuts through the midsection of the main olfactory bulbs, and one group sustained small medial olfactory bulb cuts. The results showed that animals sustaining vomeronasal or olfactory transections, regardless of experience condition, exhibited significantly reduced latencies to maternal behavior in maternal induction tests. However, these chemosensory disruptions did not prevent an additional facilitation of maternal behavior produced by a prior maternal experience.     

Cortisol and behavioral responses to separation in mother and infant guinea pigs

In Experiment 1, lactating guinea pigs 3 days postpartum exhibited an elevation of plasma levels of cortisol when their litters were taken from the home cage and placed out of auditory range for 30 min. If pups were left within auditory range during the period of separation, or if the litter was disturbed but not removed, no increase in plasma cortisol levels was observed. In Experiment 2, 11/12- and 18/19-day-old guinea pig pups placed alone in a novel environment for 30 min displayed levels of plasma cortisol and vocalizations that were greater than those of pups tested in the same environment but with their mother. Pups tested with littermates but not with their mother exhibited plasma cortisol levels that were as great as those of pups tested alone. Pups tested with littermates vocalized much less than pups tested alone but more than pups tested with their mother. These results indicate that brief mother-infant separation can activate the pituitary-adrenal system in mother as well as infant guinea pigs and they provide further evidence for the existence of a reciprocal mother-infant attachment in this species.     

Avoidance of heat by rats: Effects of thermal context on rapidity of extinction

Rats were trained to avoid radiant heat in a one-way, jump-up apparatus. The animals that could avoid heat responded more frequently and more rapidly than did yoked controls that received heat, but for whom no avoidance contingency was deliberately arranged. Control animals trained with light, rather than heat, did not learn to avoid. Using this jump-up avoidance, rats were trained to avoid heat in a room maintained at 27°C. They were then given a series of extinction trials, with no heat presentations, with the temperature maintained at either 27 or 7°C. Placing the rats in a cold environment facilitated extinction of heat avoidance behavior, but only in animals that had previously experienced heat while they were in the cold. Prior heat experience administered in a warm, rather than cold, context was not sufficient to potentiate the effect of ambient temperature on avoidance extinction. Implications of these results for theories of avoidance learning are discussed.     

Effect of postweaning social experience on response of Siberian hamsters (Phodopus sungorus sungorus) toward young

Between 18 and 50 days of age, Siberian hamsters were housed either with same-sex littermates, opposite-sex littermates, opposite-sex littermates and parents, or opposite-sex littermates, parents, and a younger litter. When adults, these animals were presented with two 3-8-day-old pups for 8 hr, and their responses were recorded. Rearing conditions did not affect behavior of females. Males housed with female littermates in the absence of a younger litter showed fewer pup attacks and more nesting with pups than did males housed only with other males. Thus, housing with females in the early postweaning period may influence males' later (adult) responses toward pups. Only among hamsters housed with same-sex littermates did males and females differ in their response to pups, a result emphasizing that sex differences in behavior may depend on early social rearing.     

Ontogeny of second-order odor-aversion conditioning in neonatal rats

In Experimental 1, rat pups 1, 3, 5, or 7 days old received a Phase 1 S1-US (Stimulus 1-unconditioned stimulus) pairing calculated to establish a first-order conditioned aversion to S1 (lemon scent). On the next day each pup received a Phase 2 S2-S1 pairing designed to establish a second-order aversion to S2 (orange scent). Pups at all ages displayed a first-order aversion to S1, but only pups given Phase 2 training when 6 or 8 days old displayed a second-order aversion to S2. Experiment 2 revealed that pups that received the S1-US pairing when 3 days old acquired an aversion to S2 if they were 6 days old when they received the S2-S1 pairing but not if they received the pairing when 4 days old. In Experiment 3, two Phase 2 treatments were compared. One was identical to that employed in Experiments 1 and 2: Pups 4 days old received a sequential exposure to S2 and S1. Another set of pups received a modified treatment: They were exposed first to S2 and then jointly to both S2 and S1. Only pups in the latter set displayed an aversion to S2. It is suggested that these results reflect developmental differences in the memory processes mediating the rat's experience with olfactory stimulation.     

Maternal responsiveness to infant Norway rat (Rattus norvegicus) ultrasonic vocalizations during the maternal behavior cycle and after steroid and experiential induction regimens

When removed from the nest and placed in a cool environment, Norway rat (Rattus norvegicus) pups emit ultrasonic vocalizations that can elicit maternal search behavior. The authors examined the behavior of pregnant dams, mothers, and virgin females during exposure to a pup that was either warm and silent or cool and vocalizing. Results indicate potentiated maternal reactions to a vocalizing pup: Mothers approached and maintained proximal orientation to a vocalizing pup far more than did virgin females. Elevated levels of proximal orientation appeared within hours of birth, increased during the 1st week postpartum, and declined by the time of weaning. Estrogen plus progesterone administration facilitated virgin females' proximal orientation toward vocalizing pups, whereas prolonged exposure to pups in the absence of hormones was without effect, suggesting that the ontogeny of the maternal response is regulated, at least in part, by maternal hormones.     

Short- and long-term behavioral effects of neonatal exposure to bombesin.

Subcutaneous (sc) administration of the tetradecapeptide bombesin (BN) (1-10 mg/kg) elicited grooming in rat pups of 1-10 days of age and the magnitude of this response decreased as a function of age. The form of grooming induced was qualitatively different from that seen following central injection of BN to adult rats. Subchronic neonatal exposure to BN (5 or 10 mg/kg; sc, twice daily for the first 8 postnatal days) had no effect on subsequent adult behavior displayed under mildly stressful or novel conditions, in the open field or in an elevated plus maze. However, both saline and the high dose of BN (10 mg/kg) pretreatments increased adult sensitivity to central BN (0.1 micrograms; icv) as compared to noninjected but neonatally handled controls or those rats neonatally pretreated with the lower dose of BN (5 mg/kg). This was best demonstrated by increases in scratching activity at the 0.1-micrograms dose of icv BN. Neonatal pretreatments had no effect on later adult sensitivity to BN injected intraperitoneally (ip). These data indicate that BN receptors in the rat central nervous system are pharmacologically functional from an early stage in ontogeny. Systems utilizing BN-like peptides are, to a degree, plastic early in ontogeny and altered adult sensitivity to BN icv can be achieved via subchronic exposure to BN during infancy. Endogenous BN-based mechanisms did not appear to play a role in the development and/or expression of behavior(s) elicited under mildly stressful or novel conditions.     

Spatial assimilation effects in sequential movements: effects of parameter value switching and practice organization

In Experiment 1, the author extended earlier work by investigating spatial assimilations in sequential aiming movements when participants were able to preplan only the 1st movement of a 2-movement sequence. Right-handed participants (N = 20) aged 18-22 years tried unimanual rapid lever reversals of 20 degrees and 60 degrees with an intermovement interval of 2.5 s. Following the 1st movement, participants made a same-distance movement, different-distance movement, or no movement in a randomly determined order. Participants overshot the short-distance target and undershot the long-distance target for both movements in the sequence, but the errors were greater when the 2nd movement differed from the 1st one. In Experiment 2, right-handed participants (N = 20) demonstrated greater assimilation effects after random practice than after blocked practice of both same-distances (20 degrees -20 degrees and 60 degrees -60 degrees ) and different-distances (20 degrees -60 degrees and 60 degrees -20 degrees ) sequences, although spatial errors were greater in different-distances conditions than in same-distances conditions. Overall, the experiments showed that parameter-value switching and practice organization are 2 major sources of spatial inaccuracy in sequential aiming movements.     

Motor Programming

In many activities, the human being must quickly decide on the response to be produced following a change in the environment. In some of these situations, the limb that the individual chooses to carry out a response seems to be a significant element in performance. Thus, if the individual carries out the response with the limb closest to the target, the performance can improve because it will take less time to achieve the goal. However, it seems that in these situations, the human being does not take this decrease in movement time into consideration and that the response is carried out with the dominant hand. Why is this so? It may be because the reaction is faster when there doesn’t have to be a choice as to which limb will carry out the response. The goal of this study was to check this possibility. In order to do so, the subjects performed a two choice reaction-time task. For this task, some subjects knew beforehand which hand they had to use to carry out the response while other subjects were unaware of this fact. The results of two experiments indicate that the choice of the limb which is to carry out the response requires no particular delay when the movement to be produced is externally guided.     

Proximal versus distal cue utilization in spatial navigation: the role of visual acuity?

Proximal versus distal cue use in the Morris water maze is a widely accepted strategy for the dissociation of various problems affecting spatial navigation in rats such as aging, head trauma, lesions, and pharmacological or hormonal agents. Of the limited number of ontogenetic rat studies conducted, the majority have approached the problem of preweanling spatial navigation through a similar proximal-distal dissociation. An implicit assumption among all of these studies has been that the animal's visual system is sufficient to permit robust spatial navigation. We challenged this assumption and have addressed the role of visual acuity in spatial navigation in the preweanling Fischer 344-N rat by training animals to locate a visible (proximal) or hidden (distal) platform using double or null extramaze cues within the testing environment. All pups demonstrated improved performance across training, but animals presented with a visible platform, regardless of extramaze cues, simultaneously reached asymptotic performance levels; animals presented with a hidden platform, dependent upon location of extramaze cues, differentially reached asymptotic performance levels. Probe trial performance, defined by quadrant time and platform crossings, revealed that distal-double-cue pups demonstrated spatial navigational ability superior to that of the remaining groups. These results suggest that a pup's ability to spatially navigate a hidden platform is dependent on not only its response repertoire and task parameters, but also its visual acuity, as determined by the extramaze cue location within the testing environment. The standard hidden versus visible platform dissociation may not be a satisfactory strategy for the control of potential sensory deficits.     

Motor programming: does the choice of the limb which is to carry out the response imply a delay?

In many activities, the human being must quickly decide on the response to be produced following a change in the environment. In some of these situations, the limb that the individual chooses to carry out a response seems to be a significant element in performance. Thus, if the individual carries out the response with the limb closest to the target, the performance can improve because it will take less time to achieve the goal. However, it seems that in these situations, the human being does not take this decrease in movement time into consideration and that the response is carried out with the dominant hand. Why is this so? It may be because the reaction is faster when there doesn't have to be a choice as to which limb will carry out the response. The goal of this study was to check this possibility. In order to do so, the subjects performed a two choice reaction-time task. For this task, some subjects knew beforehand which hand they had to use to carry out the response while other subjects were unaware of this fact. The results of two experiments indicated that the choice of the limb which is to carry out the response requires no particular delay when the movement to be produced is externally guided.     

The development of aversive responses to quinine in hyperthyroid rats

Normal and hyperthyroid rat pups received intraoral infusions of 0.25% quinine hydrochloride at 8, 10, 12, 14, or 16 days of age. Chin scraping and paw treading, aversive behaviors that adult rats show to quinine, emerged at 12 to 14 days in normal animals and were accelerated in hyperthyroid animals. Once these behaviors emerged, the hyperthyroid pups showed levels of both paw treading and chin scraping that were typical of controls several days older. Hyperthyroid and control pups did not differ in their intake of quinine at ages at which there were marked differences in chin scraping and paw treading. Other types of responses that pups show to aversive stimuli, such as paw treading. Other types of responses were not affected by the thyroid treatment. These findings suggest that the appearance of chin scraping and paw treading at 12-14 days of age in normal animals results from maturation of motor systems underlying these behaviors, and that this maturation can be accelerated by treatment with thyroid hormones.     

Inhibition of suckling in weaning-age rats: a possible serotonergic mechanism.

The nature, development, and specificity of serotonergic involvement in the control of suckling behavior in rat pups from 10 to 35 days of age were studied. During development, suckling normally declines after 10 days and is abandoned after 30 days. It was found that (a) methysergide, a serotonin (5-HT) receptor blocker, reinstated suckling behavior in pups 15 days of age and older but had no effect on the suckling of 10-day old pups, (b) quipazine, a 5-HT receptor agonist, inhibited suckling of pups 10 days of age and older, (c) methysergide pretreatment blocked the quipazine inhibition of suckling, and (d) metergoline, another 5-HT blocker, also stimulated suckling, and fenfluramine, a 5-HT releaser, blocked suckling in deprived pups. Together, these data support the hypothesis that a serotonergic inhibitory mechanism modulates the suckling of weaning-age rats. These pharmacological manipulations of 5-HT appear to alter specific components of suckling behavior involved in its initiation and maintenance but do not appear to alter a general hunger system.