Olfactory contributions to dehydration-induced anorexia in weanling rats

Early life experience shapes the functional organization of stress-responsive visceral circuits

Emotions are closely tied to changes in autonomic (i.e., visceral motor) function, and interoceptive sensory feedback from body to brain exerts powerful modulatory control over motivation, affect, and stress responsiveness. This manuscript reviews evidence that early life experience can shape the structure and function of central visceral circuits that underlie behavioral and physiological responses to emotive and stressful events. The review begins with a general discussion of descending autonomic and ascending visceral sensory pathways within the brain, and then summarizes what is known about the postnatal development of these central visceral circuits in rats. Evidence is then presented to support the view that early life experience, particularly maternal care, can modify the developmental assembly and structure of these circuits in a way that impacts later stress responsiveness and emotional behavior. The review concludes by presenting a working hypothesis that endogenous cholecystokinin signaling and subsequent recruitment of gastric vagal sensory inputs to the caudal brainstem may be an important mechanism by which maternal care influences visceral circuit development in rat pups. Early life experience may contribute to meaningful individual differences in emotionality and stress responsiveness by shaping the postnatal developmental trajectory of central visceral circuits.

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.

Postnatal development of hypothalamic inputs to the dorsal vagal complex in rats

The hypothalamus is critically involved in energy homeostasis and is an appropriate focus for research investigating the central neural underpinnings of obesity, anorexia and normal food intake. However, little is known regarding pathways and mechanisms that convey relevant hypothalamic signals to the brainstem circuits that ultimately control ingestive behavior. This brief review highlights work investigating the postnatal development of hypothalamic inputs to the hindbrain dorsal vagal complex (DVC). Research findings indicate that these inputs are both structurally and functionally immature in newborn rats. The progressive postnatal maturation of descending projections to the DVC occurs in concert with newly emerging physiological and behavioral responses to osmotic dehydration, which inhibits gastric emptying and food intake in adult animals but not in neonates. The postnatal emergence of other intake controls might also reflect progressive engagement of DVC neural circuits, whose intrinsic components and output pathways are envisioned as being critical for initiating and terminating ingestive behavior.

The postnatal emergence of dehydration anorexia in rats is temporally associated with the emergence of dehydration-induced inhibition of gastric emptying

Osmotic dehydration produced by systemic hypertonic NaCl (HS) inhibits gastric motility and emptying and also inhibits feeding in adult rats. Conversely, in neonatal rats, dehydration does not inhibit feeding. The present study examined whether the postnatal emergence of dehydration anorexia is temporally associated with the emergence of dehydration-induced inhibition of gastric emptying. Rat pups 4 to 19 days old were injected subcutaneously with HS (0.75 M NaCl; 200 microL/10 g body weight). Control rats were injected with isotonic saline (0.15 M NaCl). Thirty minutes later, rats were given access to milk that could be lapped from paper towels on the floor of a warm testing chamber. Other HS-treated and control rats were given an intragastric load of 0.15 M NaCl (2% body weight) and then killed after 30 min to determine how much of the load had emptied from the stomach. Consistent with previous reports, HS-treated rats consumed significantly more milk than control rats from postnatal Day 4 (P4) through P11 but consumed significantly less milk than controls at P19. HS treatment did not affect gastric emptying of 0.15 M NaCl at P4 or P11. Conversely, HS treatment significantly inhibited gastric emptying at P19. These findings suggest that the hypophagic effects of dehydration develop in tandem with inhibitory effects on gastric motility and are consistent with the view that the full complement of mature homeostatic responses to plasma hyperosmolality requires coordinated activation of forebrain and hindbrain neural circuits that are only partially formed in neonatal rats.

Olfactory dysfunction in anorexia and bulimia nervosa

Fifty-five eating-disordered women and 16 normal controls participated in this study to determine whether olfactory function is altered in patients with food-restricting anorexia, anorexia with bulimic features, and bulimia nervosa. Olfactory function was assessed using the University of Pennsylvania Smell Identification Test and by determining phenyl ethyl alcohol odor detection thresholds. Only the very low-weight anorexics showed impairments in their identification and detection of odors. This group's olfactory function did not improve from admission to discharge despite significant weight gain. Although, overall, smoking had only a minor influence on olfactory function, the very low-weight anorexic smokers had the lowest scores of all subjects. Since higher-weight anorexics did not show such impairments, the results suggest that the severe and prolonged starvation experienced by the very low-weight anorexics caused or contributed to intractable deficits in the olfactory system and that these deficits are compounded by smoking.

Effects of physiological state on oral habituation in developing rats: cellular and extracellular dehydration

Hydrational state has been demonstrated to influence intake of various solutions in young rat pups. For instance, both cellular and extracellular dehydration produce an enhancement of intake in pups tested at 6 days of age. However, the behavioral mechanisms that result in increased intake following manipulations of hydrational state have been less extensively studied. The impact of hydrational state on behavioral responsiveness in young rat pups was examined by assessing the pattern of responding to a series of repeated oral infusions of diet. Pups were tested at 6, 12, or 18 days of age following either acute cellular dehydration produced by injection of 1 M NaCl or acute extracellular dehydration produced by injection of 10% polyethylene glycol (PEG). Oral responsiveness to a series of 30 brief infusions of one of four taste solutions (water, 10% sucrose, 0.135 M NaCl, or 1 M NaCl) was measured. Each infusion lasted 3 s and there was 1 min between infusions. The pattern of oral responding to solutions was affected by the developmental age of the pup, the hydrational state of the pup, and the solution offered, with the largest effects of dehydration observed in the youngest animals. In all conditions except one, pups habituated to repeated infusions. The exception was the failure of extracellularly dehydrated 6-day-old pups to display habituation to oral infusions of sucrose. These results suggest that, although intake is enhanced by both cellular and extracellular dehydration in very young pups, the behavioral changes responsible for the enhancement of intake after cellular dehydration are different from the behavioral changes resulting from extracellular dehydration. This dissociation of behavioral effects of dehydration in young pups demonstrates that intake measures alone may obscure subtle differences in behavior and argues for the utility of dissection of behavioral components in understanding the neural and physiological control of behavior.

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.

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.

Thirst-induced anorexias and the ontogeny of thirst in the rat

In previous work (Leshem, M., Boggan, B., and Epstein, A.N. (1988). The ontogeny of drinking evoked by activation of brain angiotensin in the rat pup. Dev. Psychobiol. Vol.21, pp. 63-75) we showed that thirst elicited by activation of the brain's renin-angiotensin system in the suckling becomes specific to water after 16 days of age. However, in the suckling, we did not find the anorexia that reportedly accompanies angiotensin-induced thirst in the adult. This suggests the existence of a further stage in the ontogeny of thirst. Therefore, the present study pursued the ontogeny of thirst and its effects on milk intake through prepubescence into adulthood. Experiment 1 revealed that intracranial renin does not cause an anorexia to milk in prepubescent or adult rats. Experiment 2 showed that the absence of anorexia is true of thirst induced by renin but not by cellular dehydration, although both dipsogens suppressed milk intake when rats also had water available. Experiment 3 confirmed that the preweanling shows anorexia to solid food, as does the adult. Together with other work, these findings suggest that the ontogeny of the thirsts aroused by renin or intracellular dehydration is complete before weaning.

The ontogeny of independent ingestion in mice: or, why won't infant mice feed?

Unlike infant rats, which show deprivation-related ingestion in several different test situations, infant mice appeared to be relatively unwilling to feed independently of suckling until 12 days of age. We tested mouse pups that were deprived (of food, water, suckling, and maternal care) for 1, 7, or 24 hr in ingestive tests in which a milk diet was spread on the floor of their test container (Experiment 1). Pups at 3, 6, and 9 days of age consumed small amounts of the diet and showed little increase in intake when deprivation was increased. In contrast (and like rat pups of all ages), mouse pups 12 and 15 days of age actively ingested the diet and increased their intake with increased deprivation. Six-day-old mouse pups were similarly unwilling to ingest a 5% sucrose solution, though 12-day-old pups showed deprivation-related intake (Experiment 2). Cellular dehydration (produced by hypertonic saline injection), a potent stimulus for ingestion in infant rats, did not stimulate ingestion in mice younger than 12 days of age (Experiment 3). Finally, when ingestion was tested with diet infusions made through oral cannulas, mouse pups at 6 and 9 days of age showed only a slight increase in intake with increased deprivation. However, by 12 days of age, pups' ingestion increased markedly with deprivation (Experiment 4). Thus, mouse pups seem to be very different from rat pups with respect to the early existence of ingestive systems. The neural substrates for the ingestive responses that subserve independent ingestion are only minimally present in infant mice or are somehow inhibited.

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

Because suckling behavior differs in many ways from later ingestive behavior, the development of feeding and drinking in rats is best studied apart from the normal suckling situation. Newborn rat pups, separated from their mothers, will actively ingest diet infused into their mouths or spread on the floor beneath them. Such "independent" ingestion resembles the ingestive behavior of adult animals, but it also undergoes developmental changes in organization and control during the pre- and post-weaning periods: When young, deprived pups are fed, they show generalized, non-directed behavioral excitement; but with increasing age, this generalized responding matures into directed and focused ingestive activity. Early independent ingestion depends on a warm test environment; but with development, other familiar environmental and social cues come to influence responding. The internal controls of ingestion also change. Only gastric distension and hydrational status seem to be involved in controlling intake volume during early ingestion, with other ingestive controls emerging later in development. Thus ingestion, independent of suckling from the mother, is a system undergoing revealing developmental changes. These changes offer opportunities for studying ingestion, its controls, and its neural basis at its simplest organizational stage in the newborn, and at higher levels of complexity as maturation adds new components to the feeding system.