Appetitive conditioning in neonatal rats: conditioned ingestive responding to stimuli paired with oral infusions of milk

Neonatal Suckling, Oxytocin, and Early Infant Attachment to the Mother

The neuropeptide oxytocin (OT) promotes maternal care and social affiliation in adults but its importance in infant attachment still remains unknown. True animal models of infant attachment are extremely rare, and the sheep (in complement to non-human primates) is one of the few that provides the opportunity to investigate its neuroendocrinological basis. In the lamb, access to the udder has strong rewarding properties for the establishment of a preferential relationship with the mother. Therefore, the present study explored the possible involvement of OT through its release during close social contacts with the mother. The first experiment revealed that lambs having free access to the udder from birth onward developed, by 12 h of age, a clear preference for their mothers over another maternal ewe. Delaying access to the udder for six, four or even only 2 h starting at birth, by covering the ewe's udder, resulted in the lack of such a preference without affecting general activity. These effects persisted in most cases at 24 h but by 72 h of age a bond with the mother was clearly expressed. Experiment two showed that social interactions with the mother were followed by a release of OT in the plasma when lambs had the possibility to suckle. Non-nutritive interactions were without effects. Preliminary data on two subjects suggested that OT might also increase in the cerebrospinal fluid after suckling. Finally, in the third experiment, oral administration of a non-peptide OT receptor antagonist (L-368-899, Merck) over the first 4 h after birth led to decreased exploration of the mother's body compared to lambs receiving saline, and impaired the expression of a preference for the mother at 24 h. The effects were no longer observed at 48 h. Our findings demonstrate that both delayed access to the mother's udder and OT receptor antagonist alter the onset of mother preference in newborn lambs. This suggests that central OT facilitates the development of filial attachment through its release during suckling.

Survival implications of the development of behavioural responsiveness and awareness in different groups of mammalian young

This paper focuses on the development of behaviours that are critical for the survival of newborn and juvenile mammals of veterinary and wider biological interest. It provides an updated, integrated and comparative analysis of how postnatal maturation of sensory, motor and perceptual capacities support and constrain behavioural interactions between mammalian young and the mother, any littermates and the environment. Young that are neurologically exceptionally immature, moderately immature and mature at birth are compared, and include, for example, marsupial joeys, rodent pups and ruminant offspring. Mothers in these three groups exhibit distinctive patterns of birthing and postnatal care behaviours. To secure survival of the young, maternal care must compensate for behavioural inadequacies imposed by the limited sensory capacities the young possess at each stage. These sensory capacities develop in a predictable sequence in most mammals such that before birth the sequence progresses to an extent that parallels the degree of neurological maturity reached at birth. The extent of neurological maturity is likewise reflected in how long it takes after birth for the necessary brain circuit connectivity to develop sufficiently to support cortically based cognitive modulation of behaviour. This takes several months, days-to-weeks or minutes-to-hours in young that are, respectively, neurologically exceptionally immature, moderately immature, or mature at birth. Once achieved, cognitive awareness confers a high degree of behavioural flexibility that allows the young to respond more effectively to the unpredictability of their postnatal environments. It is shown that the onset of this cognitively based flexibility in the young of each group coincides with their first exposure to a variable environment that requires such behavioural flexibility.

Saccharin pre-exposure enhances appetitive flavor learning in pre-weanling rats

In adult rats, data suggest that consumption of sweet tastes that do not deliver anticipated caloric consequences using high-intensity, non-caloric sweeteners, such as saccharin, interferes with learned relations that may contribute to energy balance. The goal of the present study was to assess the development of learning about sweet taste and calories by assessing whether pre-exposure to saccharin solutions reduces cue competition in pre-weanling rats. The results demonstrated that rats pre-exposed to saccharin and then trained with a novel grape flavor paired with a glucose-sweetened solution consumed more of the novel grape flavor presented alone than rats that had been pre-exposed to saccharin and given the grape flavor paired with water alone. No differences in intake of the novel grape flavor were observed in groups given pre-exposure to water or glucose solutions. Thus, by 15 days of age, rats appear to have established an association between sweet tastes and calories, and this association can be weakened by exposure to saccharin.

Pheromone-induced odor learning modifies Fos expression in the newborn rabbit brain

Associative learning contributes crucially to adjust the behavior of neonates to the permanently changing environment. In the European rabbit, the mammary pheromone (MP) excreted in milk triggers sucking behavior in newborns, and additionally promotes very rapid learning of initially neutral odor cues. Such stimuli become then as active as the MP itself to elicit the orocephalic motor responses involved in suckling. In this context, the rabbit is an interesting model to address the question of brain circuits early engaged by learning and memory. Here, we evaluated the brain activation (olfactory bulb and central regions) induced in 4-day-old pups by an odorant (ethyl acetoacetate, EAA) after single pairing with the MP and its subsequent acquired ability to elicit sucking-related behavior (conditioned group) or after mere exposure to EAA alone (unconditioned group). The brain-wide mapping of c-Fos expression was used to compare neural activation patterns in both groups. Evidence of high immunostaining to odorant EAA occurred in the mitral+granule cells layer of the main olfactory bulb in pups previously exposed to EAA in association with the MP. These pups also showed higher expression of Fos in the piriform cortex, the hypothalamic lateral preoptic area and the amygdala (cortical and basal nuclei). Thus, MP-induced odor learning induces rapid brain modifications in rabbit neonates. The cerebral framework supporting the acquisition appears however different compared to the circuit involved in the processing of the MP itself.

Post-oral and perioral stimulations during nursing enhance appetitive olfactory memory in neonatal rabbits

Nursing-suckling interactions facilitate olfactory learning in newborns as long as suckling and the olfactory stimulus temporally overlap. We tested the hypothesis that olfactory preferences would develop even with a long delay between odor presentation and nursing. Thyme was presented to 2-day-old rabbit pups by placing an odorized plate 2 cm above their nest box. Duration and time of nursing were controlled and occurred before, during, or after odor presentation. Controls were not nursed. When exposed to the odor for 15 min, control pups preferred thyme to a novel odor in a 2-choice test immediately after exposure but not 3 and 22 h later. When pups were nursed immediately before thyme exposure or during exposure, they preferred the familiar odorant until 22 h later. Identically, when nursing occurred 30 min before odor exposure, a preference for thyme was maintained up to 22 h. This was not observed when nursing occurred 60 min before odor presentation. We concluded that enhancement of olfactory memory occurs in neonates during nursing but also after post-oral stimulation by postprandial internal state.

Approach memory turns to avoidance memory with age

Ontogenetic modification of an early memory is relatively poorly understood. And an important question is whether the memory output is more determined by the age at acquisition or at retention? Here we explore the expression of odor-shock conditioning in the rat pup. Acquisition at post-natal day 6 (P6) leads to an approach response and at post-natal day 12 (P12) to an avoidance response when the retention test is 24h later. In both cases, anisomycin injected immediately post-acquisition induced a retrograde amnesia. Controls show that, in either case, short-term memory measured 4h after acquisition is not impaired and that anisomycin given after a 4h delay has no effect. Thus, at the two ages, memory involves a consolidation process. The main result is the spontaneous reversal of the conditioned response from approach acquired at P6 to avoidance when tested at P13. This phenomenon is robust as it is observed in three conditions. Moreover, amnesia induced at P6 is maintained at P13. Results are discussed in terms of maturation and/or competition of the memory traces.

An appraisal of the strengths and weaknesses of newborn and juvenile rat models for researching gastrointestinal development

Research on the impact of bioactive compounds on the development and functional maturation of the gastrointestinal (GI) tract using newborn and juvenile rats has greatly contributed to the knowledge of GI physiology and to the improved clinical management of both premature and full-term newborns. Of the animal models available, two types have been described for use with young rats – maintenance models and substitution models. Maintenance models are those in which the young are reared with the dam and therefore benefit from continuation of natural nutrition and maternal care. Substitution models are those in which the young are reared in the absence of the dam using artificially formulated milk delivered by various means into specific GI sites. In this review, we describe these models and their operation, and discuss the strengths and weaknesses of each. Attention is also given to questions of scientific validity and some animal welfare issues raised by the use of these models.

Regulation of c-Fos gene expression in the rat olfactory bulb during olfactory learning

An immunohistochemical method was used to study the expression of transcription factor c-Fos in the mid part of the main olfactory bulbs in 18-day-old rats after training to seek their mothers using an olfactory orientation cue (propionic acid) in a Y-maze. On the next day, rat pups demonstrated a significant preference for the propionic acid odor in behavioral tests, as compared with control animals trained to the skill without the olfactory orientation cue and pups familiarized with the odor without forming any association between it and any kind of reinforcement. The propionic acid odor evoked an insignificant increase in c-Fos expression, predominantly in the granular layer of the dorsomedial area of the olfactory bulb. Training in a maze lacking the odor signal evoked diffuse activation of c-Fos, in both the mitral and granular layers of all areas of the olfactory bulb. Training with the olfactory orientation cue was also accompanied by an increase in c-Fos expression to a level significantly exceeding that seen after odor-free training, in the granular layer of the dorsomedial area, i.e., the area in which the unreinforced odor acting alone induced insignificant activation of expression. These data provide evidence that, at the level of the regulation of neuronal transcription, associative learning is manifest as summation of the effects of a new neutral stimulus and excitation initiated by the motivational state. In the olfactory bulb, this summation is seen in areas on which non-specific centrifugal excitation and specific afferentation from the signal odor converge.

Development of learned flavor preferences

Rats, like humans, are born with only a few innate flavor preferences and aversions. Preferences retain great plasticity throughout the lifespan because they are sensitive to modification by experience. From an early age, rats can rapidly learn to prefer or avoid a flavor (conditioned stimulus, CS) that is associated with a positive or negative unconditioned stimulus (US). The US may be the mother's milk, social or thermotactile stimulation, or other food-related stimuli. Flavor-flavor learning occurs when the CS flavor is mixed with a naturally preferred (e.g., sweet) or avoided (e.g., bitter) US flavor. Flavor preferences and aversions are also produced by USs that have postoral positive (e.g., nutritious) or negative (e.g., toxic) actions. These types of learning appear to involve different behavioral and neural mechanisms as indicated by differences in conditioned responses, effective temporal parameters, resistance to extinction, and neurochemical mechanisms. New evidence indicates that flavor-nutrient preference learning can occur before weaning and influence food selection after weaning. Flavor conditioning not only affects food choice, but can also significantly increase food acceptance, that is, total consumption. Thus, from an early age, learning processes shape the feeding behavior of animals. While primarily serving an adaptive function, learning may play a role in biasing individuals towards excessive intake and weight gain.

Ontogeny of ingestive behavior

Review of the ontogeny of the controls of independent ingestion reveals that some of the direct and indirect controls of meal size identified in adult rats function in the first three postnatal weeks. The controls appear sequentially and some of them change their potency after they emerge. Indirect controls exerted by metabolism and adiposity do not emerge until the fourth postnatal week or later in the postweaning period. Recent experiments in rats with monogenic obesities involving the leptin and cholecystokinin receptors have demonstrated the usefulness of independent ingestion in the detection of the earliest expression of hyperphagia. Although much remains to be learned about the normal controls of independent ingestion, it is clear that it provides relevant information about the development of normal and abnormal controls of meal size in rodents that is useful for translational research into the controls of meal size in normal and obese children.

Artificial rearing alters the response of rats to natural and drug-mediated rewards

Artificial rearing (AR) of infant rats permits precise control over key features of the early environment without maternal influence. The present study examined the behavioral response of AR rats towards natural and drug-mediated rewards, as well as their exploratory and affective behaviors. Adolescent AR rats showed increased preference for sucrose consumption relative to chow and demonstrated greater activity in the open field and in the elevated plus-maze compared to maternally reared (MR) rats. With respect to measures of emotionality, AR rats showed enhanced avoidance of the open arms of the plus-maze, indicating increased anxiety, but they did not differ from MR rats in exploring the center of the open field. Adult AR rats displayed a stronger conditioned response to morphine in a place preference test. These findings support the potential of the AR model to contribute to understanding the role of early experience in the development of behavioral motivation.

Flavor preferences conditioned by postingestive effects of nutrients in preweanling rats

The purpose of these experiments was to determine if preweanling rats, like adults, are capable of learning to associate an arbitrary flavor with the postingestive effects of nutrients, and then demonstrate a preference for that flavor after weaning. In Experiment 1, preweanlings were trained daily from postnatal day (P) 16 through P19 with intraoral (IO) infusions of a grape or cherry flavor (CS+) mixed with 20% glucose (US), and the opposite flavor (CS-) mixed with 0.05% saccharin. After weaning, rats were given a 4-h two-bottle choice between the CS+ and CS- flavors both presented in 0.05% saccharin. Rats preferred the flavor previously paired with glucose. In Experiment 2 using similar methods, rats learned to prefer a flavor (CS+G) paired with a glucose US over a flavor (CS+S) paired with a sweeter but less nutritive sucrose US, indicating involvement of postingestive reinforcement. In Experiment 3 preweanling rats with IO and intragastric (IG) catheters were trained with a CS+ flavor paired with IG nutrient infusion, and a CS- flavor paired with no IG infusion. These rats showed no flavor preference 3 days after weaning, but did significantly prefer the CS+ flavor over the CS- flavor 10 days after weaning. Together these experiments demonstrate that neural mechanisms for flavor-nutrient associations are developed before weaning, allowing young rats to learn associations between arbitrary flavors and nutritive consequences. Thus nutrient conditioning may be one way that early experience (such as flavors from the maternal diet transmitted in milk) influences later dietary preferences.

Learning during the newborn's first meal: special resistance to retroactive interference

At their first postnatal meal, 3-hour-old rats learned an association between an odor and a sweet or bitter taste. Retention after a long interval or after associative interference was compared to that of 1-day-old rats. Despite equivalent and negligible effect of the long retention interval, contrary to infantile amnesia, newborns differed strikingly from 1-day-olds in susceptibility to associative interference. When lemon odor predicted saccharin in the first episode but quinine in the second, 1-day-olds had strong retroactive interference, but the newborn's first memory was unaffected by the second. The results were identical when the first memory was a lemon-quinine association and the second a lemon-saccharin association. It is uncertain whether this special robustness of memories associated with the first postnatal meal is best understood in terms of cognitive primacy or neurochemical and physiological consequences of the birth process.

Establishment of a preference by the newborn lamb for its mother: the role of opioids

Mother-young relationships in sheep are characterized by individual recognition and a rapidly developing exclusive bond. The authors examined the role of opioids in establishment of the lamb's preference for its mother. Newborn lambs received the opioid receptor antagonist naltrexone (0.0, 1.5, or 3.0 mg/kg i.p.), and lambs were tested at 24 hr and 48 hr of age. At 24 hr, controls spent significantly more time near the mother than near an alien ewe; no significant difference was obtained for the naltrexone-treated groups. The effect of naltrexone persisted at 48 hr. No other significant behavioral difference was observed. Results support the hypothesis that opioids mediate the establishment of mother preference and the view that positive affect associated with social attachment and maternal care may be modulated by opioids.

Sucking, not milk, is important for the rapid learning of nipple-search odors in newborn rabbits

Newborn rabbits are only nursed once a day for 3 to 4 min and are completely dependent on a pheromone on the mother's ventrum for the release of nipple-search behavior and sucking. However, if the mother's ventrum is perfumed, pups can be conditioned in just one 3-min nursing session to respond with nipple searching to the novel odorant. To define more precisely the reinforcing properties of the nursing situation supporting such rapid learning, odor conditioning was conducted in independent groups of 2-day-old pups after successively eliminating potential reinforcing stimuli such as the doe's behavior, milk ingestion, nipple-search behavior, or sucking nipples. All experimental groups showed significant conditioning compared to control groups subjected to similar treatments on unscented does, with the strength of conditioning indicating that behavior of the doe, milk consumption, or arousal during performance of the search behavior were not major reinforcers, but rather the opportunity to suck nipples. Thus, this study suggests intraoral stimulation associated with sucking to be an important reinforcer in this paradigm, as for early olfactory learning in other young mammals.

Classical conditioning in the rat fetus: involvement of mu and kappa opioid systems in the conditioned response

When the Embryonic Day 20 (E20) rat fetus is given a conditioning trial involving a paired presentation of an artificial nipple (the conditioned stimulus; CS) with an intraoral infusion of milk (the unconditioned stimulus; US), it shows evidence of classical conditioning when again exposed to the CS during a test trial. Specifically, the fetus shows fewer oral grasp responses (the conditioned response; CR) when continuously presented with the artificial nipple. The present study further investigated this classically conditioned reduction in oral grasping. Separate experiments (a) examined the time course of the reduction in oral grasping (Experiment 1), (b) characterized the time course of mu opioid (Experiment 2) and kappa opioid (Experiment 3) involvement in the CR, and (c) described changes in fetal behavior (Experiment 4) associated with mu and kappa opioid effects on responding to the artificial nipple. The results are discussed in terms of opioid involvement in establishing and maintaining early suckling behavior.

The presence of the dam affects the efficiency of gentling and feeding on the early establishment of the stockperson-lamb relationship

This experiment investigates how the maternal presence influences the effect of additional human contact in early age on the reaction of lambs to their stockperson. Forty twin-born lambs were involved in this experiment during their first 4 days of life. Ten pairs of twins were reared artificially from 12h of age. One of each litter (AF, n=10) received 6.5+/-0.7 sessions of 30min of separation from the twin (with a wire fence) with 5min of gentling and feeding (suckling from a bottle and from a bucket fitted with a rubber teat). The other twin was not treated. Ten pairs of twins were reared with their dam and received 6.6+/-0.7 sessions of treatment. One twin (MAF, n=10) received the same treatment as AF. The other twin (M, n=10) was separated for 30min from the dam and had no human contact. From the age of 70+/-7h, lambs were tested in a social isolation test (alone for 1min, with the familiar stockman for 2min, alone again for 1min), in a Preference test (2min) between an unfamiliar maternal ewe and the familiar stockman, and, for the AF lambs only, in a Preference test (2min) between their familiar and an unfamiliar stockman. Eight AF lambs learned to suck on their own from the bucket of milk by the end of the experiment compared to only one MAF (P<0.001). AF lambs approached the human more (P<0.01), vocalised less (P<0.01) and walked less (P<0.01) during the social isolation test than animals reared with their mother (M and MAF). AF did not show any preference between the stockman and the unfamiliar maternal ewe while M and MAF lambs chose the ewe (P<0.05). AF lambs discriminated the familiar from an unfamiliar stockman only if they had learned to suck from the bucket during the treatment. Nevertheless MAF lambs vocalised less than M (P<0.05) in the presence of the stockman during the social isolation test, indicating a possible reduction of isolation distress. These results show that artificially reared lambs are preferentially motivated to interact with a familiar human after only a few days of contact. Moreover, they highlight the difficulty in using a feeding reward to improve the human-lamb relationship when lambs are reared permanently with their dams. However, the results suggest that early gentling improves the human-animal relationship whatever the maternal environment.

Olfactory conditioning facilitates diet transition in human infants

We evaluated whether Pavlovian conditioning methods could be used to increase the ingestion of non-preferred solutions by formula-fed human infants. In baseline measures, 5-7 month old infants sucked less frequently and consumed less water than regular formula. During a 3-day olfactory conditioning period, parents placed a small scented disk, the conditioned stimulus, on the rim of their infants' formula bottle at every feeding. Following this training, infants' responses to water were tested when their water bottles had a disk scented with the training odor, a novel odor, or no odor. Infants tested with the training odor sucked more frequently and consumed significantly more water than they had at baseline. Infants tested with no odor or a novel odor consumed water at or below baseline levels. These data demonstrate that olfactory conditioning can be used to enhance ingestion in infants and suggest that such methods may be useful for infants experiencing difficulty when making transitions from one diet to another.

Conditioned changes in appetitive and consummatory responses to flavors paired with oral or nutrient reinforcement among adult rats

Recent studies of the behavioral organization of conditioned flavor preferences have suggested the involvement of at least two separate learning systems-an appetitive response system sensitive to the oral hedonic properties of the reinforcer, and a consummatory response system sensitive to its nutrient properties. However, these prior studies were conducted with weanling rats, that differ from adults in terms of their prior experience with food, their learning competencies, and the peculiar ontogenetic constraints on their behavior. It is, therefore, unknown whether flavor preference behaviors are similarly organized in adult rats. In this experiment, adult rats were trained to associate a specific CS flavor with either the sweet taste or the postingestive nutrient effects of sucrose. Conditioned appetitive orienting and consummatory oral responding to the CS flavors were then measured. Unlike weanling rats, adult rats exhibited both conditioned appetitive behavior and conditioned consummatory behavior in response a CS that was previously paired with either oral hedonic or nutrient reinforcement. These results suggest a set of important developmental changes in the neurobehavioral mechanisms of flavor preference learning in the postweaning period.

Olfactory stimulation enhances light-induced phase shifts in free-running activity rhythms and Fos expression in the suprachiasmatic nucleus

There is evidence to suggest that the olfactory and circadian systems are linked, functionally, and that olfactory stimuli can modulate circadian rhythms in mammals. Furthermore, olfactory bulb removal can alter free-running rhythms in animals housed in constant darkness and can attenuate the effect of social stimuli on photic entrainment of circadian rhythms. The mechanisms through which olfactory stimuli influence circadian rhythms are not known. One possibility is that olfactory stimuli influence circadian rhythms by modulating the activity of the circadian clock located in the hypothalamic suprachiasmatic nucleus. To study this, we assessed the effect of olfactory stimulation on free-running rhythms and on photic resetting of the circadian clock in rats using phase shifts in wheel-running rhythms and expression of the transcription factor Fos in the suprachiasmatic nucleus. We found that brief exposure to an olfactory stimulus, cedar wood essence, in the subjective day or subjective night had no effect on either free-running rhythms or Fos expression in the suprachiasmatic nucleus, but that when presented in combination with light, the odor dramatically enhanced light-induced phase shifts and Fos expression in the suprachiasmatic nucleus. Olfactory stimulation alone induced Fos expression in several structures that innervate the suprachiasmatic nucleus, pointing to ways by which stimulus information transmitted in the olfactory pathways could gain access to the suprachiasmatic nucleus to modulate photic resetting. These findings, showing that clock resetting by light can be facilitated by olfactory stimulation, point to a mechanism by which olfactory cues can modulate entrainment of circadian rhythms.

Olfactory alliesthesia in human neonates: prandial state and stimulus familiarity modulate facial and autonomic responses to milk odors

This study examines the effects of a shift in the motivational state (from hunger to satiety) of human neonates on their behavioral and autonomic responsiveness to artificial and food-related odors as a function of stimulus familiarity. In Experiment 1, videotaped facial movements and autonomic (respiration rate: RR, heart rate: HR) responses to five olfactory stimuli (familiar regular formula, unfamiliar regular formula, protein hydrolysate formula, vanillin, control) are recorded in 3-day-old neonates (n = 14) during episodes of irregular sleep. The infants are tested on average 50 min. before and after bottle feeding. RR discriminates the odor stimuli from the control stimulus, indicating clear olfactory detection. Furthermore, neonates react with higher HR change only when exposed to their familiar formula milk during the postprandial condition. The measurement of facial movements with the Baby-Facial Action Coding System indicates that disgust and aversive actions are more often evoked by the odor of regular formulas (familiar or unfamiliar) than by the other olfactory stimuli during the postprandial condition. In Experiment 2, untrained adult observers, presented with the videotapes of the infants' facial responses to the odors, are able to decode differential hedonic signals from the sender faces as a function of the infants' motivational states. The present findings are in line with the concept of olfactory alliesthesia as defined in adults.

Development of a preferential relationship with the mother by the newborn lamb: importance of the sucking activity

Lambs develop a preferential relationship with their mothers within 24 h after birth. In a first experiment, we attempted to determine whether neonatal sucking was a potent reinforcer in the establishment of this preference by temporarily covering the udders of ewes after parturition. Lambs were assigned to one of three treatments: they had free access to the udder (group 1) or were prevented from sucking either between birth and 6 h afterward (group 2) or between 12 and 18 h after birth (group 3). Measurements of cholecystokinin plasma levels at birth and at 6 and 18 h showed that all lambs ingested some colostrum when the udder was made accessible. When tested in a two-choice test situation at 24 h of age, lambs from group 1 spent significantly more time near their mothers than near the alien ewes, those from group 2 did not show any discrimination, and lambs from group 3 clearly preferred the alien ewes. At 2 days of age, lambs from groups 1 and 3 showed a strong preference for their mothers, whereas those from group 2 still displayed no preference. In a second experiment, we attempted to determine whether sucking also had a reinforcing value in the maintenance of this preference. Lambs were assigned to one of three treatments: they had free access to the udder (group 1) or were prevented from sucking for 6 h either at birth (group 2) or at the age of 3 days (group 3). Lambs from group 2 did not discriminate between their own and alien dams when tested at 24 h of age, unlike those of groups 1 and 3. When a second test was performed at 4 days of age, all the lambs showed a strong preference for their dams. We concluded that sucking has strong rewarding properties in the establishment of a preferential relationship with the mother by the lamb. On the other hand, the maintenance of this preference does not rely on sucking as strongly.

Classical conditioning in the fetal rat with a long delay between presentation of CS and US

A single paired presentation of the artificial nipple and milk results in classical conditioning of changes in perioral responsiveness in the E20 rat fetus. This classical conditioning is evidenced by a reduction in responding to perioral tactile stimulation. The results of Experiment 1 confirmed the specificity of milk as an unconditioned stimulus to support classical conditioning. Experiment 2 demonstrated that single-trial classical conditioning with the artificial nipple CS and milk US was possible with a delay of 30 s between nipple and milk presentations. Further, measurements of fetal motor behavior during the delay between CS and US presentations indicated that a single 15-s presentation of the artificial nipple increased movements of the mouth for 30 s after removal of the artificial nipple. Experiment 3 demonstrated that three exposures to the artificial nipple prolonged the expression of mouthing for up to 120 s and made possible single trial classical conditioning with a delay of 120 s between CS and US presentations. The capacity of the fetal CNS to maintain a "behavioral trace" for an ecologically important stimulus, such as the nipple, could have adaptive significance in the early development of motivated behavior.

Behavioral components of milk-induced activation in neonatal rat pups

Neonatal rat pups exhibit a complex constellation of behaviors in response to a variety of salient stimuli such as the odor of milk or maternal saliva, stroking with a soft brush, electrical brain stimulation, and intraoral infusions of milk. Although psychobiologists have used the term "behavioral activation" to refer to such behavioral displays, the exact nature of "behavioral activation" and its underlying neural substrates have yet to be elucidated. This study was undertaken to characterize "behavioral activation" quantitatively to describe and define this apparently global pattern of response in terms of possible underlying components. Principal components analysis suggested that "behavioral activation" may be comprised of separable ingestive, exploratory, and locomotor behavioral "assemblies."

Effects of altered olfactory experiences on the development of infant rats' responses to odors

A "natural" olfactory learning paradigm was used to assess the effects of an altered perinatal olfactory environment on the development of odor-elicited behavior in young rats. Infant rats (from 3 to 12 days of age) reared by dams fed a eucalyptol-adulterated diet responded to the odor of eucalyptol with high levels of activity, mouthing, and probing, and also demonstrated a marked preference for that odor in a two-choice situation (Experiment 1). The effects were observed only in pups whose dam was fed eucalyptol-adulterated diet and were not observed in pups exposed to nonmaternal sources of odor. The intensity of behavioral activity elicited by eucalyptol odor varied, to some extent, with the concentration of the eucalyptol in the dam's diet during prenatal exposure (Experiment 2). Cross-fostering revealed that these effects were due almost entirely to the pups' postnatal exposure to the eucalyptol odor (Experiment 3). These findings indicate that novel odors specifically associated with the mother can gain control over orientation and ingestion-related behaviors.

Learning of suckling odors by newborn rabbits declines with age and suckling experience

After being nursed just once for 4-5 min by an artificially scented doe, newborn rabbits show the full pattern of nipple-search behavior when placed on a fur scented with the same odor as their mother. It was the aim of the present study to test whether such rapid learning is dependent on age. In Experiment 1 in which normally raised pups were nursed by a scented doe either on Days 1, 3 or 5, conditionability was found to decline markedly by postnatal Day 5. In Experiment 2 conditionability was maintained in pups deprived of suckling experience by bottle feeding from Days 1-4, but not in hand-raised pups allowed to search on a doe for 4 min daily without milk intake. Possible mechanisms underlying the decline in conditionability to suckling odors, as well as the potential functional significance of this early learning for immediate postnatal and later life are discussed.

Neuroethology of olfactory preference development

Young mammals come to approach the odor of their mother, a response that facilitates their survival during early life. Young rats induce a cascade of events in their mother to induce the emission of her odor. The pups increase circulating prolactin levels, which increases food intake and the emission of large quantities of cecotrophe containing the maternal odor. This odor is synthesized by the action of cecal microorganisms and changes with maternal diet. The diet-dependence of the odor requires the pups to acquire their attraction to the odor postnatally. The acquisition of this preference occurs when an odor is paired with the tactile stimulation that pups receive during maternal care. The action of the tactile stimulation appears to be mediated by noradrenaline. The development of this type of olfactory attraction is accompanied by changes in the regions of the olfactory bulb that are responsive to the attractive odor. Metabolic, anatomical, and neurophysiological changes in response to the attractive odor emerge in such regions of the bulb after early olfactory preference training.

Organization of the stretch response to milk in the rat fetus

Intraoral infusion of milk reliably elicits a stretch response in the rat fetus. The fetal stretch response appears isomorphic to the stereotypic behavior exhibited postnatally by suckling rat pups during milk ejection. Using a procedure that permits direct fetal observation, milk infusions were delivered to rat fetuses on Day 20 or 21 of gestation. Infusion of milk promoted systematic changes in fetal behavior that culminated in expression of the stretch response nearly 3 min later. Interruption of this behavioral sequence prevented stretching, suggesting that a period of behavioral reorganization after milk infusion is necessary but not sufficient for the stretch response to occur. The behavioral sequence initiated by the fetal rat's first exposure to milk is remarkably similar to the behavior exhibited by experienced infant rats at the nipple.

Opioid mediation of odor preferences induced by sugar and fat in 6-day-old rats

Intraoral infusions of sucrose, fat or polycose reduce ultrasonic vocalizations during isolation, and increase pain threshold in infant rats. These effects are naltrexone reversible. The present study determined whether these substances, when paired with an odor, caused a change in preference for that odor. In 6-day-old rats, pairing orange odor with intraoral infusions of sucrose or corn oil, but not polycose, water, mineral oil or 0.01% quinine hydrochloride, caused a substantial increase in preference for orange. Preference formation was blocked by systemic injection of naltrexone (0.25 mg/kg) prior to pairing orange with either sucrose or corn oil. Moreover, preference expression was prevented by naltrexone injection prior to testing. Thus certain substances thought to reduce stress in infant rats via endogenous opioid release can also cause preference for substances that predict their occurrence. Preference formation depends upon the availability of endogenous opioids. Preference expression reflects the conditioned stimulus causing opioid release.

The early presence of food-oriented appetitive behavior in developing rats

The appetitive behavior of 3- to 6-day-old rat pups was studied by testing their ability to direct their ingestive behavior to a restricted food source. We found that, from 3 days of age, pups were able to feed efficiently from such a source. More specifically, pups that were deprived of nutrition but not of maternal care as well as pups that were dehydrated ingested significantly more than nondeprived animals, and did so whether liquid diet was spread over the entire floor surface beneath them or restricted to a fraction of the floor surface. However, pups that had been nutritionally and maternally deprived were not able to direct their feeding. The general locomotor activation of pups in this latter group appeared to interfere with their ability to direct their behavior to the restricted source. These results indicate that from early ages, developing rats possess the appetitive competence to guide their behavior and suggest that previous findings of poorly directed behavior were a confound of the behavioral activation shown by pups tested in a state of maternal deprivation.

Rapid odor conditioning in newborn rabbits: amnesic effect of hypothermia

Rabbit pups are completely dependent on a maternal pheromone for the release of suckling behavior. However, if the mother is perfumed prior to nursing, pups will learn to respond to the novel odor with the characteristic nipple-search behavior in just one 3-4 min nursing episode. In a first investigation of the processes underlying this recently developed learning paradigm, time-dependent effects of hypothermia on retention of the task could be demonstrated. Thus pups whose whole body was cooled to a mouth temperature of 7 degrees C immediately after conditioning (n = 10) and tested 24 h later for 3 min on a perfumed fur did not differ significantly in their search response from naive, untreated controls (n = 10). In contrast, pups cooled 4 h after conditioning (n = 10) demonstrated clear retention of the learned response and searched as vigorously as conditioned but uncooled animals (n = 10). As pups of all groups demonstrated normal nipple-search behavior when tested on a lactating doe, the deficits associated with immediate cooling appear to have been specific to the learning task and time of treatment.

Conditioned activation of fetal behavior

Previous demonstrations of prenatal learning have relied upon experimental protocols involving training at one gestational age and testing at a later age, potentially confounding effects of experience and retention with developmental changes in the fetus's behavioral repertoire. In this study, a single-session paradigm, which involved the pairing of a neutral chemosensory stimulus (sucrose CS) with a second stimulus that activates behavior (lemon US), was used to assess the learning capacity of the rat fetus. Four CS-US pairings were effective in promoting behavioral activity in 20-day-old rat fetuses when they were reexposed to the sucrose CS alone. The functional expression of conditioned behavioral activation on day 20 of gestation occurs at a time when CNS structures, many of which are involved in sensory processing and learning in older animals, are still undergoing rapid differentiation in the rat fetus.

Norepinephrine-induced plasticity and one-trial olfactory learning in neonatal rats

The influence of norepinephrine (NE) on the acquisition of a conditioned odor preference and enhanced focal uptake of [14C]2-deoxyglucose (2-DG) within the olfactory bulb was assessed in neonatal rat pups. On postnatal day (PN) 6, pups were injected with either an NE receptor agonist (isoproterenol), NE receptor antagonist (propranolol or timolol), or saline before one-trial odor conditioning. The experimental conditioning group received a 10-min exposure to an odor (peppermint) and reinforcing tactile stimulation similar to that received from the dam. Control groups received only the odor, only the tactile stimulation, backwards presentation of the odor and tactile stimulation or neither of these stimuli. The next day, pups were either tested for an olfactory preference (Expts. 1 and 2) or assessed for differential olfactory bulb activity using the 2-DG technique (Expt. 3). The results indicate that early odor experience with either tactile stimulation or isoproterenol is sufficient to produce a learned behavioral preference and enhanced focal 2-DG uptake within the olfactory bulb. Moreover, an NE receptor blocker injected prior to training with odor and tactile stimulation blocks the acquisition of both behavioral preference and the enhanced 2-DG uptake. In Expt. 4, the effects of tactile stimulation and isoproterenol were further assessed. An odor paired with a moderate level of either of these stimuli produces learning. However, the simultaneous presentation of a moderate level of these stimuli paired with an odor does not result in an odor preference. An odor preference may be reinstated by simultaneous presentation of these stimuli, provided the level of each of these stimuli is too low to produce an odor preference when presented alone with an odor. These data suggest that exogenous NE and tactile stimulation are additive in their effect on learning. These results are discussed in terms of the neural mechanisms underlying reinforcement in infant rats.

Modified behavioral and olfactory bulb responses to maternal odors in preweanling rats

Rat pups acquire an attraction for maternal odors, which can vary with maternal diet. In the two experiments reported here, maternal diet was modified and both pup behavioral responses and pup olfactory bulb neural responses [( 14C]2-DG uptake) to maternal odors were examined. In experiment 1, pups were reared from birth to postnatal day 19 with either a dam fed normal rat chow or a dam fed a sucrose-based diet which suppressed her normal maternal odor. In experiment 2, pups were raised from birth to postnatal day 19 with either a dam fed the sucrose-based diet adulterated with peppermint, or the non-scented sucrose-based diet. Pups selectively expressed both a behavioral attraction and an enhanced olfactory bulb neural response to odors that they experienced in the nest.

Norepinephrine and learning-induced plasticity in infant rat olfactory system

Postnatal olfactory learning produces both a conditioned behavioral response and a modified olfactory bulb neural response to the learned odor. The present report describes the role of norepinephrine (NE) on both of these learned responses in neonatal rat pups. Pups received olfactory classical conditioning training from postnatal days (PN) 1-18. Training consisted of 18 trials with an intertrial interval of 24 hr. For the experimental group, a trial consisted of a pairing of unconditioned stimulus (UCS, stroking/tactile stimulation) and the conditioned stimulus (CS, odor). Control groups received either only the CS (Odor only) or only the UCS (Stroke only). Within each training condition, pups were injected with either the NE beta-receptor agonist isoproterenol (1, 20, or 4 mg/kg), the NE beta-receptor antagonist propranolol (10, 20, 40 mg/kg), or saline 30 min prior to training. On day 20, pups received one of the following tests: (1) behavioral conditioned responding, (2) injection with 14C-2-deoxyglucose (2-DG) and exposed to the CS odor, or (3) tested for olfactory bulb mitral/tufted cell single-unit responses to the CS odor. The results indicated that training with either: (1) Odor-Stroke-Saline, (2) Odor-Stroke-Isoproterenol-Propranolol, or (3) Odor only-Isoproterenol (2 mg/kg) was sufficient to produce a learned behavioral odor preference, enhanced uptake of 14C-2-DG in the odor-specific foci within the bulb, and a modified output signal from the bulb as measured by single-cell recordings of mitral/tufted cells. Moreover, propranolol injected prior to Odor-Stroke training blocked the acquisition of both the learned behavior and olfactory bulb responses. Thus, NE is sufficient and may be necessary for the acquisition of both learned olfactory behavior and olfactory bulb responses.

A nutritive control of independent ingestion in rat pups emerges by nine days of age

The emergence of controls of independent ingestion in rat pups was studied using nutritive (0.6 M glucose in saline or water) and vehicle gastric preloads. Two hours after preloading, ingestive responses were assessed in a 30-minute test of feeding from the floor. In 6-day-olds, all preloads had similar effects on consumption of a milk diet. In 9-day-olds, however, glucose-water preloads inhibited intake compared to water preloads. This inhibition was secondary to an effect on gastric emptying. Glucose preloads also inhibited intake in 12- and 15-day-olds. Behavioral observations indicated that feeding patterns were altered in response to glucose preloads in 9-day-olds. These results provide evidence for the emergence of a nutritive, postgastric control of independent ingestion between 6 and 9 days of age.

Anterior and posterior, but not cheek, intraoral cannulation procedures elevate serum corticosterone levels in neonatal rat pups

Implantation of intraoral cannulas is a procedure that has been typically assumed to be relatively unstressful in neonatal rat pups. To test this assumption, endocrine responses to such implantations were compared with those of other standard procedures. In Experiment 1, corticosterone and growth hormone (GH) levels were assessed in 4-day-old rat pups placed in an incubator for 15 or 60 min following either: no treatment, subcutaneous (sc) injection of 0.9% NaCl, anterior or posterior intraoral cannulation, ice anesthesia or ether anesthesia. Corticosterone levels were elevated relative to nontreated controls 15 min after all treatments except sc injection. These levels remained elevated after 60 min in both cannulation groups and the ice anesthesia group. In Experiment 2, the ability of ether anesthesia to reduce the hormonal response to the cannulation procedures was assessed in addition to examining the hormonal response to intraoral cannulations through the cheek in 4-day-old rat pups. Ether did not attenuate the corticosterone response to either anterior or posterior cannulations. Pups subjected to the cheek cannulation procedure did not exhibit any significant alterations in serum corticosterone levels when compared with nontreated control pups. GH levels were found to differentiate less among the various procedures than corticosterone levels, with GH levels generally being low in all groups, including nontreated control animals. These data suggest that a cheek placement is less stressful than anterior and posterior placements and may provide a viable alternative in studies necessitating the implantation of cannula into the buccal cavity during the early postnatal period.

Associative Processes in Early Olfactory Preference Acquisition: Neural and Behavioral Consequences

Acquisition of behavioral conditioned responding and learned odor preferences during olfactory classical conditioning in rat pups requires forward or simultaneous pairings of the conditioned stimulus (CS) and the unconditioned stimulus (US). Other temporal relationships between the CS and US do not usually result in learning. The present study examined the influence of this CS-US relationship upon the neural olfactory bulb modifications that are acquired during early classical conditioning. Wistar rat pups were trained from Postnatal Days (PN) 1-18 with either forward (odor overlapping temporally with reinforcing stroking) or backward (stroking followed by odor) CS-US pairings. On PN 19, pups received either a behavioral odor preference test to the odor CS or an injection of (14)C 2-DG and exposure to the odor CS, or olfactory bulb single unit responses were recorded in response to exposure to the odor CS. Only pups that received forward presentations of the CS and US exhibited both a preference for the CS and modified olfactory bulb neural responses to the CS. These results, then, suggest that the modified olfactory bulb neural responses acquired during classical conditioning are guided by the same temporal constraints as those which govern the acquisition of behavioral conditioned responses.

Reinforcers in infancy: classical conditioning using stroking or intra-oral infusions of milk as UCS

Six-day-old rats received 20 forward pairings of an odor-conditioned stimulus (CS) with one of two unconditioned stimuli (UCS); 1) intra-oral milk infusions or 2) stroking with a sable-hair brush. These UCS's produce a common general response of increased behavioral activity, but different specific behaviors. For each UCS, additional pups received backward pairings of the CS and UCS, random pairings of the CS and UCS, CS only, UCS only, or no stimuli. Four hours later, pups received a two-odor choice test to assess the development of an odor preference and a CS-only test to assess the acquisition of conditioned responding (CR). The results of the two-odor choice test indicated that for both UCS's only forward pairings of the CS and UCS resulted in an odor preference. Similarly, the CS-only test showed that only forward pairings of the CS and either UCS were effective in producing CR's; pups that received forward pairings exhibited increased behavioral activity during presentations of the CS, which is an unconditioned response (UCR) to both UCS's. Only the forward paired CS-milk UCS group exhibited increased mouthing and probing during the CS only test; these are UCR's that occur to milk infusions but infrequently to the stroking UCS. These results demonstrate the development of similar conditioned odor preferences using behaviorally activating UCS's, but CR's which are specific to the form of the UCR.

Changes in the categorization of appetitive and aversive events during postnatal development of the rat

Environmental stimuli are not clearly categorized into appetitive and aversive classes during the first postnatal week. Three- and 6-day-old rats are highly activated by nominally appetitive (milk infusions) and aversive (foot shock) stimuli. Both evoked the same generalized behavioral responses (rolling, curling). By 12 days of age, these stimuli were responded to differently; mouthing and probing occurred in response to milk while flinching and escape responses were observed to shock. The affective properties of mild shock were found to be hedonically opposite for 6- and 12-day-old pups. Six-day-olds showed an acquired preference for an odor paired with shock which increased with intensity (.1-.5 mA) similar to that observed with milk infusions. An acquired odor aversion was displayed by 12 days old using these shock parameters. These results suggest that young rat pups may not differentiate many appetitive and aversive events behaviorally or affectively until the second postnatal week.