What we know and don't know about the development of independent ingestion in rats

Gastrointestinal Interoception in Eating Disorders: Charting a New Path

PURPOSE OF REVIEW

Abnormal interoception has been consistently observed across eating disorders despite limited inclusion in diagnostic conceptualization. Using the alimentary tract as well as recent developments in interoceptive neuroscience and predictive processing as a guide, the current review summarizes evidence of gastrointestinal interoceptive dysfunction in eating disorders.

RECENT FINDINGS

Eating is a complex process that begins well before and ends well after food consumption. Abnormal prediction and prediction-error signals may occur at any stage, resulting in aberrant gastrointestinal interoception and dysregulated gut sensations in eating disorders. Several interoceptive technologies have recently become available that can be paired with computational modeling and clinical interventions to yield new insights into eating disorder pathophysiology. Illuminating the neurobiology of gastrointestinal interoception in eating disorders requires a new generation of studies combining experimental probes of gut physiology with computational modeling. The application of such techniques within clinical trials frameworks may yield new tools and treatments with transdiagnostic relevance.

Maternal high-fat diet increases independent feeding in pre-weanling rat pups

In laboratory settings, the adult offspring of rodent dams that are maintained on high-fat diet (HFD) before conception and/or during pregnancy/lactation display an increased incidence of obese phenotypic markers, including increased body weight and adiposity, reduced leptin sensitivity, and impaired glucose tolerance. In rat pups raised by dams consuming HFD, these obese markers emerge during the first postnatal week. Since the week-old offspring of HFD dams consume excess amounts of milk during experimental tests of independent feeding (i.e., intake away from the dam), we hypothesized that maternal diet affects suckling and/or independent ingestion by pups in the home-cage environment. In the present study, this hypothesis was tested by conducting detailed analyses of ingestive behaviors expressed by pups in the home cage. Pups raised by dams consuming HFD displayed an earlier onset of independent feeding and more amounts of calorie intake from solid food during the third postnatal week compared to pups raised by dams consuming regular chow, with no diet-related differences in suckling behavior. Independent ingestion by pups in both diet groups was most frequently observed after nursing, with offspring of HFD dams engaged more frequently in post-nursing independent feeding episodes compared to offspring of chow-fed dams, particularly when the prior nursing episode was nutritive (i.e., including milk receipt by pups). We conclude that early-life exposure to HFD enhances the facilitative effect of nutritive suckling on independent feeding in pups, promoting increased caloric intake from solid food in the home-cage environment.

Neonatal Food Restriction Induces Hypoplasia in Developing Facial Motor Neurons of Rats

The effects of neonatal food restriction upon the dendritic development of facial nucleus (FN) motor neurons of Wistar rats were analyzed. Rats neonatally underfed by daily (12 h) mother-litter separation in an incubator from 5-30 days after birth exhibited, in brain stem Golgi-Cox sections, significant reductions in the number and extension of stellate, triangular and bipolar FN neuronal dendritic prolongations with negligible effects upon perikarya measurements. Data suggest that in the underfed newborn, the ability of FN neurons to establish synaptic contacts with afferent fibers is reduced, which then interferes with their capacities for the integration and triggering of nerve impulses to modulate facial motor expression in response to sensory cues.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Dopamine signaling in the dorsal striatum is essential for motivated behaviors: lessons from dopamine-deficient mice

Genetically engineered mice that lack tyrosine hydroxylase in all dopaminergic neurons become hypoactive and aphagic, and they starve by 4 weeks of age. However, they can be rescued by daily treatment with l-dopa, which restores activity and feeding for about 10 hours. Thus, these mice can be examined in both dopamine-depleted and dopamine-replete states. A series of behavioral experiments lead to the primary conclusion that in the dopamine-depleted state these mice are not motivated to engage in goal-directed behaviors. Nevertheless, they still have a preference for sucrose, they can learn the location of food rewards, and they can form a conditioned-place preference for drugs. Dopamine signaling can be restored to the striatum by several different viral gene-therapy procedures. Restoring dopamine signaling selectively to the dorsal striatum is sufficient to allow feeding, locomotion, and reward-based learning. The rescued mice appear to have normal motivation to engage in all goal-directed behaviors that have been tested. The results suggest that dopamine facilitates the output from dorsal striatum, which provides a permissive signal allowing feeding and other goal-directed behaviors.

Ontogeny of hypothalamic-hindbrain feeding control circuits

Ingestive behavior is a complex product of distributed central control systems that respond to a diverse array of internal and external sensory stimuli. Relatively little is known regarding the pathways and mechanisms by which relevant signals are conveyed to the neural circuits that ultimately control ingestive motor output. This report summarizes findings regarding the postnatal development of descending hypothalamic inputs to the hindbrain dorsal vagal complex (DVC). Evidence accumulated primarily in rats indicates that descending neural projections from the hypothalamus to the DVC are both structurally and functionally immature at birth. The progressive postnatal maturation of these projections occurs in parallel with newly emerging physiological and behavioral responsiveness to treatments and stimuli that affect food intake in adults. Thus, the postnatal emergence of new feeding controls may reflect the emerging access of these controls to DVC neural circuits.

Perinatal overfeeding in rats results in increased levels of plasma leptin but unchanged cerebrospinal leptin in adulthood

OBJECTIVE

To study the effect of perinatal programming and overfeeding on the hypothalamic control mechanisms of food intake in adult rats.

DESIGN

Neonatal programming effects on body weight, food intake, central and peripheral leptin levels, hypothalamic neuropeptides, leptin receptors and central leptin responsiveness in adult rats.

MEASUREMENTS

Plasma and cerebrospinal fluid (CSF) leptin levels were analyzed using radioimmunoassay. Neuropeptide mRNA levels were analyzed using in situ hybridization. Leptin receptor mRNA levels were analyzed using reverse transcriptase-polymerase chain reaction.

RESULTS

Perinatally overfed rats growing up in small litters (SL) maintain their obese and hyperleptinemic phenotype in adulthood. However, leptin levels in CSF are abnormally low considering the plasmatic hyperleptinemia. In contrast to the already reported changes in perinatally overfed juvenile rats, perinatally overfed adult rats did not show any alteration in the expression of leptin receptor isoforms and evaluated neuropeptides. Moreover, SL adult rats showed a normal sensitivity regarding the inhibitory effect of intracerebroventricular leptin administration on food intake.

CONCLUSION

Perinatal overfeeding does not induce alterations in either the anorectic response to central leptin administration or expression of leptin receptors and neuropeptides in adulthood. The leptin resistance to peripheral leptin in SL adult rats may be related to impaired leptin transport across the blood-brain barrier.

Taste differentiation in the context of suckling and independent, adultlike ingestive behavior

The present study compared intake of sweet (saccharin), bitter (quinine), and neutral (water) tastants available either in the context of suckling behavior through a surrogate nipple or independent adultlike feeding through an intraoral cheek cannula in 3-hr-old newborn rats lacking any suckling experience and 24-hr-old rats with regular experience with the dam's nipple. The new technique of online monitoring of fluid flow was applied for assessment of the temporal patterns of ingestion. Newborn and 1-day-old rats tested in the context of suckling behavior showed extremely low intake of quinine, moderate intake of water, and high intake of saccharin. In the same way, they demonstrated low intake of quinine and high intake of saccharin in the context of independent feeding, but intake of water was also high and comparable to that of saccharin. Suckling rats attained higher efficacy of fluid extraction from nipple than pups drinking from cannula. The differential responsiveness to quinine, saccharin, and water in suckling rats was also manifested through attachment behavior, with pups spending less time on the nipple providing quinine and more time on the nipple with saccharin than on the nipple providing water. These results suggest that neonates show taste differentiation as early as 3 hr after birth, and that this taste differentiation is more pronounced in the context of suckling behavior than in the context of adultlike, independent ingestion.

A possible role of neuropeptide Y, agouti-related protein and leptin receptor isoforms in hypothalamic programming by perinatal feeding in the rat

AIM/HYPOTHESIS

Perinatal overfeeding predisposes humans and rats to obesity and diabetes in later life. One classical model for studying the effect of early feeding is manipulation of the size of rat litters. Rats growing up in small litters gain more weight than rats growing up in normal-sized litters. Interestingly, these obese rats maintain this phenotype in adulthood. Conversely, rats raised in large litters show a delay in growth and a decrease in body weight. The aim of this work was to assess the hypothalamic control mechanisms of food intake regulated by perinatal feeding.

METHODS

Leptin levels were analysed using RIA. Leptin receptor mRNA levels were analysed using RT-PCR. Neuropeptide mRNA levels were analysed using in situ hybridisation.

RESULTS

Perinatally overfed neonatal male rats exhibited hyperleptinaemia and a decrease in hypothalamic mRNA levels of the long isoform of the leptin receptor (OB-Rb), explaining their leptin resistance. Moreover, this obese model showed an increase in the mRNA expression of cocaine- and amphetamine-regulated transcript, neuropeptide Y and agouti-related protein in the hypothalamic arcuate nucleus (ARC). In contrast, perinatally underfed neonatal male rats with hypoleptinaemia showed an increase in hypothalamic mRNA of the short isoforms of the leptin receptor. Furthermore, they exhibited an increase in expression of neuropeptide Y and agouti-related protein in the ARC.

CONCLUSIONS/INTERPRETATION

Rats overfed during early postnatal life show a leptin-resistant state mediated by down-regulation of the hypothalamic OB-Rb. These data, together with the increased expression of neuropeptide Y and agouti-related protein in specific neurons in the ARC, might indicate the existence of regulated programming in this nucleus and may provide a new aetiopathogenic concept in susceptibility to obesity.

The effect of leptin treatment on the development of obesity in overfed suckling Wistar rats

OBJECTIVE

To identify the role of hyperleptinaemia in mediating the effects of early postnatal overfeeding in a rat strain known to be prone to manipulations of the early environment which result in predispositions for obesity and associated metabolic and cardiovascular disturbance in later life.

DESIGN

Wistar rats were reared in normal litters (NL, 10--12 pups) or small litters (SL, four pups) from postnatal day 3 and killed for determination of body composition and plasma leptin and insulin concentrations on day 7 or day 21 after having been treated with recombinant leptin (2 x 50 (pmol/g)/day) or saline from day 1.

RESULTS

Rearing in SL doubled the body fat content and plasma leptin levels in comparison to NL pups by 21 days of age. Under leptin-treatment throughout suckling age, NL pups remained leptin responsive, ie the difference in body fat content was progressively reduced relative to the controls. Until 7 days of age, despite the body fat content of untreated SL pups being 2-fold higher and their plasma leptin level 7-fold higher than that of NL pups, leptin treatment caused the same percentage decreases in body fat in SL than in NL pups. But in contrast to NL pups, the SL pups became leptin resistant thereafter. Plasma insulin levels in 7-day-old leptin-treated SL pups were 3-fold higher than in untreated littermates and 5-fold higher than in the NL groups.

CONCLUSION

Prophylactic leptin treatment does not prevent hyperinsulinaemia and excessive fat deposition in SL pups. On the other hand, selective hyperleptinaemia during suckling age does not trigger leptin resistance and obesity in NL pups. Rather than hyperleptinaemia per se, other factors associated with early postnatal overnutrition, for example, the concurrent hyperinsulinaemia, seem to play a pivotal role for the development of leptin-resistance and life-long obesity risk in SL rats.

Milk delivery schedules and stomach preloading alter patterns of suckling behavior by newborn rats on a surrogate nipple

Newborn rat pups tested before suckling experience attached to and ingested milk from the surrogate nipple. Time attached to the nipple and amount of milk ingested depended on the schedule of milk infusion through the nipple. More frequent milk infusions resulted in more frequent disengagements from the nipple during the test, less time attached to the nipple, and less body weight gain. The initial patterns of attachment behavior--continuous or intermittent--were reproduced later when rats were tested on the surrogate nipple. Preloading of the stomach with milk effectively altered both attachment and ingestion from the nipple, whereas preloading with the same amount of water had no effect on suckling behavior. The data suggest that newborn rats flexibly adjust their attachment behavior to peculiarities of milk delivery through the surrogate nipple and reproduce the initial attachment pattern when reexposed to the surrogate nipple.

Oxytocin is elevated in plasma of 10-day-old rats following gastric distension

In adult rats, oxytocin (OT) has been shown to reduce the intake of both food and fluids, and oxytocinergic cells are activated by gastric distension and administration of the intestinal peptide cholecystokinin (CCK-8). These and other findings indicate that OT can play a role in inhibiting ingestion under some conditions. A previous study has shown, however, that oxytocinergic cells are unresponsive to CCK-8 in 2-day-old rats. We report here that OT is elevated in the plasma of 10-day-old rats after induction of gastric distension with both mother's milk and saline. These results indicate that the vagal-hypothalamic axis becomes mature between 2- and 10-days of age in infant rats.

Functional properties of jaw and tongue muscles in rats fed a liquid diet after being weaned

Decreased masticatory demands due to liquid or soft diets cause a reduction in the growth of craniofacial bones and in the development of feeding musculature, but the effects on masticatory function and jaw/tongue muscle activities are unclear. The present study was undertaken to test the hypotheses that a liquid diet feeding after weaning affects the critical-period programming of mastication and the motor performances of jaw and tongue muscles. Thirty-six male Wistar rats were divided into two equals groups at weaning and fed either a solid (solid-diet group) or a liquid (liquid-diet group) diet until they reached 50 days of age. Electromyograms (EMG) of the masseter, medial pterygoid, temporalis, anterior digastric, styloglossus, and genioglossus were recorded while animals were naturally ingesting ordinary pellets, apple cubes, and a liquid diet. It was found that: (1) a more irregular chewing rhythm, a shorter chewing sequence, and a longer chewing cycle were found in the liquid-diet group, but there were no differences observed during lapping/licking between the two groups; (2) during the chewing cycles, the EMG onset of each muscle in relation to that of the masseter in the liquid-diet group was similar to that in the lapping/licking cycles in both groups; (3) the activities of jaw elevators (masseter, medial pterygoid, and temporalis) during the chewing cycles were significantly higher in the liquid-diet group; and (4) the increase in the EMG activities of jaw elevators during pellet chewing compared with apple cube chewing was significantly weaker in the liquid-diet group, whereas such an enhancement was found simultaneously in the styloglossus in the solid-diet group, and in the anterior digastric in the liquid-diet group. These findings verify that: (1) the motor output of jaw and tongue muscles may be altered in rats fed a liquid diet after being weaned; (2) the feeding of a liquid diet to rats after being weaned may obstruct the functional transition from suckling to mastication; and (3) jaw elevators that develop without motor learning of mastication are inefficiency when performing functionally.

Neurotoxicology and amino acid intake during development: the case of threonine

The development of the central nervous system is highly dependent on an adequate supply of nutrients. In particular, protein and amino acid availability is of major concern during gestation and in early postnatal life. Numerous data have been published on some amino acids directly involved in brain functions as neurotransmitters or indirectly as precursors of neurotransmitters, but scant information is available on the possible consequences of hyperthreoninemia, a phenomenon repeatedly noted in clinical reports. The results of neurochemical and behavioral studies in the developing rat suggest that despite numerous possible effects of threonine on brain constituents, moderate hyperthreoninemia does not impair markedly the development of the central nervous system.

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."

Effect of threonine on the behavioural development of the rat

Rats received different levels of threonine (Thr), one, 1.7 and four times the normal dietary intake, from conception to adulthood. The mothers were fed the experimental diets before and during pregnancy. Their offspring received a daily oral load of Thr or placebo until weaning. Thereafter, the juveniles were fed the same diet as their mothers. Morphologic development, ingestive behaviour, homing, and locomotion were observed before weaning. Exploration and spontaneous alternation were studied thereafter. Animals exposed during gestation to 1.7 times the normal Thr intake consumed more food during the test of independent ingestion. Grooming showed inconsistent variations between days 12 and 29 in pups fed 1.7 times the normal Thr intake. Rats performed equally well on the other behavioural tasks independently of the dietary treatment. We conclude that Thr intake as much as four times higher than the levels found in normal diets does not impair the behavioural ontogenesis of the rat.

Learning as adaptation of the infant

The present report advocates an adaptive, ecological approach to the study of learning in infants. Concepts of developmental niche and ontogenetic adaptation are applied to early mammalian development. Within this conceptual framework, it is asserted that learning cannot be fully understood separately from a behaving body; that learning is a dimension of behavior and physiology. The role of learning in the development of ingestive behavior, especially suckling and the transition to solid food, is used to illustrate the potential of studying learning in development. These considerations are offered as examples of an alternative approach to the empirical study of learning by infants. The approach advocated herein can be applied to clinical issues: developmental adaptations evolved in contexts that differ from our modern environments. Exposure to contexts or contingencies that are evolutionarily unexpected may inadvertently create pathology.

Effects of CCK-8 on ingestive behaviors of suckling and weanling rats

The present series of experiments was designed to determine the dose-response characteristics of the cholecystokinin suppression effect on rat pups ingesting through suckling and adult modes. Pups at 10-22 days of age were deprived of their dam and food for 6 hr. At test time, the subjects received intraperitoneal injections or intravenous infusions of CCK-8 at 2, 4, 8, or 16 micrograms/kg in saline, or the saline vehicle alone. The pups were permitted to ingest for 30 min immediately after the injection or during the infusion. Ingestion included taking milk from the dam by suckling or feeding from the substrate. Regardless of the route of administration, CCK-8 elicited a low level of suppression across all doses and all ages when pups ingested within the natural suckling situation. In marked contrast, pups ingesting from the substrate in the adult mode suppressed their intake of bovine light cream or chow pellets in a dose-related manner following injections of the peptide. The blockade of the CCK dose-related suppression effect during suckling indicates that a different set of mechanisms governs intake of milk from the mother. The results may be related to the level of arousal required to sustain motivated behaviors while pups engage in the different modes of ingestion.

Early and persistent abnormalities in rats with neonatally acquired Borna disease virus infection

Newborn rats inoculated with Borna disease virus (BDV) develop a persistent, tolerant nervous system infection (PTI-NB), with no signs of encephalitis or Borna disease. We measured body weight, body length, taste preferences, and spontaneous locomotor activity over a 4-month period in PTI-NB and control rats. PTI-NB rats had decreased weight and length but not detectable disturbances in growth hormone and insulin-like growth factor-1 biosynthesis as compared to control rats. In single bottle taste acceptance tests, PTI-NB rats did not differ from controls and drank normal amounts of all solutions. When offered a choice of solutions in two-bottle taste preference tests, PTI-NB rats exhibited a normal preference for saccharin and a normal aversion for quinine, but an exaggerated preference for saline. At 1 and 4 months of age, PTI-NB rats were significantly more active than normal rats, although only 1-month-old PTI-NB rats had increased daytime activity. Thus, even in the absence of encephalitis, BDV infection of the PTI-NB rat is associated with a number of physiological and behavioral abnormalities.

The pharmaco-ontogeny of the perifornical lateral hypothalamic beta 2-adrenergic and dopaminergic receptor systems mediating epinephrine- and dopamine-induced suppression of feeding in the rat

The functional ontogeny of beta 2-adrenergic and dopaminergic receptors in the perifornical lateral hypothalamus (PLH) that mediate adrenergic and dopaminergic suppression of feeding in rats was investigated. Rat pups, ranging in age from 2 to 15 days, were removed from their mothers and implanted with a brain cannula directed unilaterally at the PLH or a more rostral site lateral to the anterior nucleus of the hypothalamus. On the next day, following a 22-h period of food and water deprivation, each pup was implanted with an intra-oral cannula for oral infusion of milk that could be swallowed or rejected. Subsequently, each pup received an intracerebral injection of saline, or a single dose of epinephrine (EPI, 0.1-30.0 nmol), the beta 2-adrenergic receptor agonist salbutamol (1.0-30.0 nmol) or the dopaminergic receptor agonist apomorphine (1.0-30.0 nmol). Milk intake was then assessed following a 1-h period of infusion. The results showed significant dose-dependent suppression of milk intake in pups as young as 2 days of age in response to PLH injection of EPI, salbutamol and apomorphine. In contrast to its effectiveness in the PLH at 2 days of age, EPI failed to suppress milk intake at this age following injection into a more rostral site lateral to the anterior nucleus of the hypothalamus. Together, these findings suggest that both beta 2-adrenergic and dopaminergic receptors, mediating adrenergic and dopaminergic suppression of feeding, are functionally mature very early in the postnatal development of the rat. Moreover, consistent with evidence in adult rats, these catecholaminergic receptors in young pups appear to be located in the region of the PLH.

The pharmaco-ontogeny of the paraventricular alpha 2-noradrenergic receptor system mediating norepinephrine-induced feeding in the rat

The present study was undertaken to assess the functional ontogeny of alpha 2-noradrenergic receptors in the hypothalamic paraventricular nucleus (PVN) that mediate noradrenergic stimulation of feeding in the rat. Rat pups, ranging in age from 2 to 15 days, were removed from their mothers and implanted with a brain cannula directed unilaterally at the PVN or third ventricle. On the following day, each pup was implanted with an intra-oral cannula for oral infusion of milk or water that could be swallowed or rejected. Following a 1-h period of satiation, each pup received an intracerebral injection of saline, or a single dose of norepinephrine (NE, 0.01-100.0 nmol) or the alpha 2-noradrenergic receptor agonist clonidine (0.01-1.0 nmol). Milk or water intake was then assessed following a 1-h period of infusion. Injection of NE into the PVN and third ventricle significantly enhanced milk intake at 2 days of age. NE was 10-fold more potent in the PVN than in the ventricle. While paraventricular injections of NE stimulated milk and water intake equally at 2 days of age, NE produced a greater stimulation of milk than water intake at 15 days of age. Like NE, clonidine significantly enhanced milk intake at 2 days of age following injection into the PVN. Collectively, these findings suggest that alpha 2-noradrenergic receptors, mediating noradrenergic stimulation of feeding, are functionally mature very early in the postnatal development of the rat. Moreover, consistent with evidence in the adult rat, these findings indicate that alpha 2-noradrenergic receptors relevant to feeding are located in the vicinity of the PVN.

Weaning in the Norway rat: relation between suckling and milk, and suckling and independent ingestion

We examined in rat pups the relation between the decline of suckling and the emergence of independent ingestion, and the role of milk delivery conditional on nipple attachment in the maintenance of suckling. Fifteen-day-old litters were reared for either 5 or 10 days, and 20-day-old litters for 5 days by either a thelectomized (no nipples), ligated (nipples but no milk), or intact (nipples and milk) dam. Pups' food and water intakes were monitored daily, and their suckling, feeding, and drinking behaviors were videorecorded for 24 hr in the presence of their foster dam (Day 19 or 24) and for 24 hr in the presence of an intact, lactating dam (Day 20 or 25). There were no differences between treatment conditions with respect to either the onset or rate of increase of independent feeding or drinking. Pups reared by a thelectomized dam for 10 days displayed a pronounced, lasting depression of suckling. Twenty-five-day-old pups reared by a ligated dam displayed suckling levels comparable to those of control pups; in the presence of the ligated dam, however, their tendency to attach to a nipple was notably reduced. The implications of the findings for our understanding of the weaning process are discussed.

Infant rats depleted of brain dopamine as neonates exhibit normal independent ingestion

Weanling or adult rats that have sustained near-total depletions of striatal dopamine (DA) on postnatal Day 3 do not exhibit the severe ingestive deficits seen in comparably depleted adults. The present experiments demonstrated that these animals are capable of ingesting independently, via an intraoral cannula, as early as three days after the depletion. Both activational and ingestive responses to maternal deprivation and intracellular dehydration were virtually identical to the responses of control pups. However, these responses may not be entirely dopaminergically mediated at Day 6 since administration of haloperidol failed to attenuate these behaviors in normal pups. When tested on Day 12, DA-depleted pups continued to exhibit normal ingestive and activational behaviors, despite the fact that haloperidol now suppressed these behaviors in both groups of 12-day-old pups. These findings demonstrate that there is a developmental progression of dopaminergic involvement in the control of independent ingestion and the associated activational responses, yet a near-total depletion of striatal DA is not sufficient to impair the expression of these behaviors.

[Regulation of food intake]

Regulation of food intake is commonly treated as a negative feedback-loop. Hunger and/or appetite lead man and animals to ingest food. The subsequent meal-contingent activation of pre- and postabsorptive mechanisms then leads to satiety. The activation of oral and gastrointestinal chemo- and mechanoreceptors is important on the preabsorptive site. The gastrointestinal hormone cholecystokinin may also have a physiological satiety effect. Preabsorptive satiety mechanisms are influenced by the rate of gastrointestinal transit. The pancreatic hormone glucagon, which is released during meal taking, and various metabolites contribute to the postabsorptive regulation of food intake through activation of hepatic chemoreceptors, which are connected to the brain via predominantly vagal afferents. In addition, glucoreceptors in the brain, in particular in the nucleus of the solitary tract, contribute to food intake regulation by monitoring blood glucose concentration or, more specifically, glucose utilization. The nucleus of the solitary tract, which relays vagal afferents from gut and liver and also gustatory afferents, projects to the hypothalamus and to other forebrain structures. In this neural network the informations from the periphery are integrated by various neurotransmitters and neuropeptides, but the exact role of the substances involved is not fully understood yet. Body weight and, hence, body fat presumably affects feeding through modulation of a postabsorptive mechanism.

The ontogeny of opioid receptors mediating opiate-induced feeding in rats

Acute administration of naloxone to preweanling rats does not attenuate independent ingestion of milk until 14 days of age suggesting that the full expression of an endogenous opioid system(s), regulating feeding rats, is not complete prior to this age. The present study was undertaken to examine the functional ontogeny of opioid receptors mediating opiate-induced feeding in rats. Rat pups, satiated with milk, were given intraperitoneal injections of the opiate receptor agonist, morphine, and were allowed free access to milk. Morphine stimulated the intake of milk at 3, 5, 7, 14 and 21 days of age, within 2 hr of injection. A time-course analysis in 7-day-old pups showed greater enhancement of intake between hours 2 and 4, than between hours 0 and 2, for large doses of morphine (0.3 and 1.0 mg/kg) suggesting that morphine-induced behavioral depression, which was observed early in the test session, confounded intake at earlier hours. Administration of the opiate receptor antagonist, naltrexone, produced no effect on intake of its own, but blocked the stimulation of intake by morphine in 5-day-old pups confirming that the effect of morphine on the intake of milk was mediated by opioid receptors. Thus, while a functional endogenous opioid system(s), regulating feeding in rats, is not fully mature until 14 days postpartum, the present results suggest that opioid receptors mediating feeding are functional very early in the postnatal development of the rat.

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.

The effects of amphetamine and chlorpromazine on independent ingestion of milk in preweanling rats

To assess the effects of catecholaminergic drugs on independent feeding during development, preweanling rats were administered amphetamine (AMPH) or chlorpromazine (CPZ) and were allowed to ingest milk through anteriorly located intra-oral cannulas. In 1-hr milk-deprived rat pups, AMPH stimulated milk intake at 3, 7 and 10 days of age and suppressed intake at 15 days. In 22-hr-deprived pups, AMPH had no effect at 3, 7 and 10 days, but reliably suppressed intake at 15 days. CPZ stimulated intake in 3-, 10- and 15-day-old milk-satiated pups. In 22-hr-deprived pups, CPZ had no effect at 3 and 10 days, but stimulated intake at 15 days. While AMPH produced pronounced nonappetitive behavioral activation in conjunction with enhanced intake in 1-hr-deprived pups, AMPH-induced activation occurred without enhanced intake in 22-hr-derived pups. Thus, there was a dissociation between AMPH's effect on milk intake and AMPH-induced nonappetitive behavioral activation. Collectively, the present results support the following conclusions. First, a catecholaminergic system(s) that enhances independent feeding is present very early in postnatal development of the rat. Second, level of food deprivation is an important state-dependent variable when assessing the effects of AMPH and CPZ on independent feeding in preweanling rats.

Drinking behavior and motor function in rat pups depleted of brain dopamine during development

The ability of rat pups depleted of brain dopamine (DA) at either 3 or 15 days of age to increase fluid intake in response to intracellular dehydration was studied. Despite near-total depletions of striatal DA, animals depleted at either age ingested normal amounts of fluid even as soon as 3-7 days after incurring the brain damage. The effects of DA receptor blockade on ingestion and motor function were also studied. Haloperidol had markedly different effects as a function of the age at which the depletions were sustained. Weanlings that were depleted of DA at 3 days of age failed to exhibit the drug-induced adipsia, akinesia, and catalepsy seen in controls and animals that were depleted at 15 days of age. These findings suggest that the DA depletions were performed at different periods of brain organization. The resultant neural controls of ingestion and motor function are qualitatively different in the two groups of depleted animals. Rats depleted at 3 days of age utilize nondopaminergic mechanisms, whereas animals lesioned at 15 days of age continue to rely upon residual DA neurons for both behaviors.

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

Infant rats deprived of food, maternal care, and the opportunity to suckle display a dramatic behavioral activation and vigorously ingest when provided milk through oral cannulas. These experiments assessed which components of deprivation are important in producing these responses to milk. Nutritional deprivation alone, with or without the presence of an active maternal female, appears to be sufficient to produce ingestion. Behavioral activation, on the other hand, appears to require both nutritional deprivation and deprivation from a maternal female. The effect of maternal stimulation on later behavioral reactivity was not a function of the pups' opportunity to suckle. However, active maternal stimulation was more effective in preventing activation than was passive maternal stimulation (e.g., thermotactile and olfactory stimulation). Stimulation provided by an active, nonlactating mother was effective in preventing behavioral activation, but the effect was short-lived, lasting only 2 hr after the pup was removed from the mother's care. This series of studies thus reveals that identified components of maternal separation have dissociable effects on appetitively motivated behaviors in infant rats.