Maternal and nutritional contributions to infant rats' activational responses to ingestion

Early perturbations to fluid homeostasis alter development of hypothalamic feeding circuits with context-specific changes in ingestive behavior

Drinking and feeding are tightly coordinated homeostatic events and the paraventricular nucleus of the hypothalamus (PVH) represents a possible node of neural integration for signals related to energy and fluid homeostasis. We used TRAP2;Ai14 and Fos labeling to visualize neurons in the PVH and median preoptic nucleus (MEPO) responding to both water deprivation and hunger. Moreover, we determined that structural and functional development of dehydration-sensitive inputs to the PVH from the MEPO precedes those of agouti-related peptide (AgRP) neurons, which convey hunger signals and are known to be developmentally programmed by nutrition. We also determined that osmotic hyperstimulation of neonatal mice led to enhanced AgRP inputs to the PVH in adulthood, as well as disruptions to ingestive behaviors during high-fat diet feeding and dehydration-anorexia. Thus, development of feeding circuits is impacted not only by nutritional signals, but also by early perturbations to fluid homeostasis with context-specific consequences for coordination of ingestive behavior.

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.

Demanding pups improve maternal behavioral impairments in sensitized and haloperidol-treated lactating female rats

The impairments in the maternal behavior of ovariectomized sensitized females, relative to lactating dams, resemble those deficits found in lactating females after treatment with the D1/D2 DA receptor antagonist haloperidol, which interferes with maternal motivation. Therefore, it could be speculated that these behavioral deficits found in sensitized females and haloperidol-treated dams are due to a reduced motivation to interact with pups. In support of this hypothesis, we have found that both sensitized and haloperidol-treated lactating females exhibited remarkably similar impairments in the expression of all active maternal behaviors relative to lactating dams. Furthermore, these deficits were overridden when they were allowed to interact with 12h-isolated pups (demanding pups). Interestingly, lactating dams also improved their maternal behavior in the presence of demanding pups, and clearly chose demanding more than non-demanding pups in a preference paradigm. These data support the idea that the behavioral deficits of sensitized and haloperidol-treated lactating females are due to a reduced behavioral activation in response to the incentive cues from pups compared to lactating dams, and not because of a motor inability to express maternal behavior. These findings ultimately suggest that pups modulate the activity of DA system involved in the regulation of maternal behavior.

Guidance and differentiation of maternally directed orienting behaviors: probing the origins of filial attachment

Newborn rats seek and attain contact with their mothers by orienting and primitive locomotor behaviors called maternally-directed orienting behaviors (MDOBs). By 2 days of age MDOBs are elicited specifically by maternal features and are heightened by a period of acute separation from the mother. This behavioral regulation by the mother's presence suggests an early form of filial attachment. To probe for other evidence of filial attachment, two aspects of the controls of MDOBs were investigated: (1) whether MDOBs are goal-corrected in relation to the dam's position and (2) whether the performance of MDOBs can be separated from that of suckling. It was found that, during the first 2 days after birth, MDOBs maintained the pup's ventrum-to-ventrum contact with the dam regardless of her position, suggesting that they are goal-corrected. Evidence was not found for independent controls of nipple grasping (NG), the behavior that initiates suckling, and MDOBs during the first two postnatal days.

Maternally directed orienting behaviors of newborn rats

We observed an organized repertoire of behaviors performed by newborns of the laboratory rat when placed under suspended artificial surfaces simulating various properties of the awake dam's ventrum. Pups traveled and wriggled, turned on their sides and upside down, ventroflexed and probed into the surface while on side or upside down, and audibly barked, all in a state of heightened behavioral arousal. Within 48 to 72 hr after birth, these behaviors generally increased with the addition to the simulation of more maternal features, including the texture of the dam's ventral fur and odor, separately and in combination. In addition, the behaviors were intensified by 18 hours of maternal deprivation. The possible functional relations among the components of this repertoire, which we refer to as "maternally directed orienting and proximity-seeking behaviors," and between these behaviors and nipple attachment and suckling, are discussed.

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.

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.