Differential effects of upper gastrointestinal fill on milk ingestion and nipple attachment in the suckling rat

Milk intake and nipple attachment behaviors were studied in the natural suckling situation after gastrointestinal preloading. Rat pups, deprived of their dam for 9 hr at 1, 10, or 20 days of age, were preloaded by gavage with volumes ranging from 2 to 16% of their body weight and returned to suckle. Preloads of artificial bitch's milk decreased the intake of mother's milk in a volume-related manner at all ages. At 1 and 10 days 4% preloads decreased milk intake without affecting attachment behaviors; larger prelods of 8 and 16% decreased intake and reduced the incidence of attachment. At 20 days of age small and large preloads decreased both incidence of attachment and milk intake. Preloads up to 8% of body weight had no effect on latency to attach at any age. Complete subdiaphragmatic bilateral vagotomy increased milk intake of 7–9–day-old pups fed automatically through an anterior mouth cannula in a nonsuckling situation. Vagotomy combined with spinal cordotomy (T2-T3) resulted in a synergistic hyperphagia and massive distension of the upper GI tract. The results indicate that suckling rats can control their intake of mother's milk while remaining attached to a nipple as early as 1 day of age. The suppression of ingestion in response to GI filling appears to be mediated by visceral afferent activity. Conversely, attachment behaviors are less affected by GI fill. This suggests that ingestive behaviors and attachment behaviors have different controls during the 1st 10 days of postnatal development.     

Mercaptoacetate decreases latency to initiate independent ingestion,but not suckling,in preweanling rats

Altering oxidation of fatty acids by administration of mercaptoacetate (MA) or methyl palmoxirate has been demonstrated to stimulate independent ingestion in rat pups by 12 to 15 days of age. However, in other work, neither agent has been demonstrated to affect intake in the context of suckling. In the present experiments, we tested the effects of MA on both suckling and independent ingestive behavior by examining changes in latency to initiate suckling or independent ingestion and by measuring intake in short-term (30 min) and longer term (4 hr) suckling and independent ingestion tests in pups aged 12 to 15 days of age. The results demonstrated that (a) MA does not affect suckling intake in short- or longer term tests; (b) MA does not affect latency to attach to a nipple, most likely due to floor effects on latency to attach in control pups; (c) MA stimulates independent ingestion in short-term, but not longer terms tests; and (d) MA reduces the latency to initiate independent ingestion. These results suggest that administration of MA preferentially stimulates the initiation of independent ingestion over suckling, and that this response develops at an age just prior to the time at which independent ingestion is typically first expressed by rat pups.     

Electrical activity of septal neurones during suckling and the milk ejection reflex in the lactating rat

Summary Extracellular recordings from neurones in the lateral septum were performed in urethane anaesthetised lactating rats to study the eventual role of the septum in the control of suckling-induced oxytocin release. The connections of these neurones with the supraoptic nucleus, which contains cells secreting oxytocin, were assessed electrophysiologically by single pulse stimulation of the ipsilateral supraoptic nucleus. The neurones were thus classified into four categories: antidromically activated, orthodromically activated or inhibited, and unresponsive neurones. One hundred septal neurones were recorded in animals not exposed to suckling. A second group of 40 cells were analysed during suckling and one or more reflex milk ejections.The mean firing rates of each category of septal neurone did not differ significantly during suckling from the values observed in the absence of suckling. During suckling, almost all the recorded septal cells showed no significant alteration in their level of firing in relation to milk ejections. Two neurones presented an activation in the period between two milk ejections that seemed related to arousal. One neurone was clearly inhibited at the time of milk ejection.Our observations suggest that the septum does not represent an essential component of the pathways necessary for the milk ejection reflex induced by suckling, although it could exert an inhibitory action modulating either the intervals between two successive milk ejections, or the amount of oxytocin released.Supported by grant INSERM CRL 79.5.372.6     

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.     

The effects of engorgement with milk and of suckling on mammary blood flow in the rat

1. Mammary blood flow was estimated in rats by measuring the cardiac output and the proportion of it received by the mammary glands.2. When the young were removed on the 10th day of lactation the mammary glands began to fill with milk and mammary blood flow fell from 78 ml./min. 100 g tissue to 45 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g within 8 hr and decreased further to 34 ml./min. 100 g in the next 16 hr. These changes were associated with both a fall in cardiac output and a fall in the proportion of the cardiac output taken by the mammary glands.3. When the young were allowed to continue suckling, but milk removal was prevented by sealing the teat ducts with adhesive, more milk collected in the mammary glands within 8 hr and mammary blood flow was unchanged (74 ml./min. 100 g).4. In rats which had been separated from their young for 24 hr, milk was removed from the engorged glands by allowing the pups to suckle again. Mammary blood flow did not rise immediately following the removal of milk but only after 4 hr of suckling, and was associated largely with an increase in cardiac output.5. Upon resumption of suckling mammary blood flow was the same in emptied glands, and in full glands with the teats sealed.6. When the young were removed from 15-day lactating rats mammary blood flow after 24 hr was directly related to the volume of milk in the glands.7. It is concluded that the accumulation of milk in the mammary gland does not mechanically restrict the flow of blood through the tissue and that, in the rat, mammary blood flow and milk secretion are strongly dependent on a continually applied suckling stimulus.     

Rapid and robust olfactory conditioning with milk before suckling experience: promotion of nipple attachment in the newborn rat

An olfactory conditioning paradigm tested the hypothesis that newborn rats are able to learn about events associated with their first experience with milk as early as 3-5 hr after birth. Exposure to lemon odor (conditioned stimulus, [CS]) paired with intraoral milk infusions (unconditioned stimulus, [US]) resulted in strong conditioning: In the presence of the CS, sustained attachment occurred to an empty nipple as if it provided milk, whereas pups in control conditions showed little attachment. A single CS-US pairing was sufficient for strong conditioning, which was evident with a trace interval as long as 60 s. Conditioning was robust enough to promote attachment to a nipple providing saline, which is aversive to the newborn rat, and comparably strong conditioning occurred with sucrose or saccharin as the US. These findings suggest that olfactory conditioning has the potential to modify suckling behavior.     

Early vasculitis in the mercuric chloride induced Brown Norway rat model is neutrophil independent

In the Brown Norway rat, mercuric chloride (HgCl2) induces an autoimmune syndrome characterized by necrotizing vasculitis, predominantly affecting the caecum, and a polyclonal B-cell response. The time course of vasculitis is biphasic, with an alphabeta T-cell independent phase occurring within 24 h, and a T-cell and neutrophil dependent phase, maximal at two weeks. The pathogenesis of the early phase of vasculitis is unclear, and this study aims to examine the role of neutrophils. Rat neutrophils were depleted using cyclophosphamide. RP3, an antirat neutrophil monoclonal antibody, inhibited neutrophil leucocytosis but did not deplete neutrophils. Vasculitis was induced by subcutaneous HgCl2 injection. Serial measurements of peripheral blood leucocyte count were made. Rats were killed after 24 or 72 h. The macroscopic appearance of the caecum was scored by an experienced observer, and samples taken for histological examination. Caecums were excised and myeloperoxidase, a marker enzyme for neutrophil infiltration, assayed. Cyclophosphamide induced marked neutropaenia whereas RP3 inhibited the neutrophilia observed after HgCl2 injection. Vasculitis was present in both treated and control animals, with no significant differences in macroscopic or microscopic scores between the groups. Tissue myeloperoxidase activity was low in all animals and did not differ significantly between groups. The data do not support a role for neutrophils in the initial pathogenesis of vasculitis in this model.     

Facilitatory effect of antidromic stimulation on milk ejection-related activation of oxytocin neurons during suckling in the rat

Oxytocin neurons of the paraventricular nucleus were identified by their antidromic response to stimulation of the neurohypophysis and a short high-frequency burst of spikes displayed before reflex milk ejection in the urethane-anesthetized lactating rat. During suckling, the milk ejection-related bursts recurred at regular intervals. Stimulation of the neurohypophysis at 50-300 pulses/s for 0.5-6 s (current 1 mA) evoked additional burst(s) with the amplitude higher than those of spontaneously occurring ones. Stimulation with subthreshold current for antidromic response or constant-collision test could also facilitate the neurosecretory bursts. Possibly, oxytocin released within the magnocellular nuclei is responsible for these effects.     

Transitions in the dehydration-induced inhibition of milk intake in suckling rats

The effects of acute cellular or extracellular dehydration on milk intake via suckling were determined in rats 5, 10, 15, and 20 days of age. Milk was made available 10 times to each rat pup by the intravenous infusion of oxytocin to anesthetized test dams. Prior to 15 days of age, milk intake was not affected by either intracellular or extracellular dehydration. In rats 15 days and older, however, both forms of dehydration reduced milk intake at the nipple. Thus, dehydration starts to inhibit suckling behavior at the onset of weaning, when food and water are both taken directly from the environment by the developing animal.     

Relationship between litter size and the development of the rat brain during the suckling period

The brain of 23-day-old rats raised in small, large, and control litters is studied. In large litters, body weight is significantly lower and the relative mass of the brain is higher than in controls. In small litters, body weight of males and females is considerably higher, while the relative mass of the brain is lower; in females, the absolute mass of the brain and of the left hemisphere is significantly higher. In rats of both sexes from small litters the weight of body, brain, and hemisphere is higher than in rats from large litters. In addition, in males from small litters the neurons of layer V are larger. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 121, No. 5, pp. 582–584, May, 1996     

Preobesity in the infant OLETF rat: the role of suckling

The Otsuka Long Evans Tokushima Fatty (OLETF) rat model of obesity, which lacks CCK(1) receptors, has been extensively investigated over the last decade. We have recently focused on the early developmental stages of this strain, finding that OLETF pups are heavier than controls from birth and hyperphagic already from postnatal (PN) Day 2. OLETF mothers present differential maternal behavior patterns and increased nursing time and frequency, which might contribute to the preobese characteristics of the pups. The present investigation examined the pups' ability to gain weight from the nursing episodes. First, we measured the pups' weight gain from one nursing bout from their own dam. Next we examined the pups' weight gain after a feeding competition test with control pups from foster dams of both strains. OLETF pups gained more weight than controls from their own dams on PN Week 1 due to a higher suckling rate (and/or efficacy) and on PN Week 3 due to increased nursing time. When competing with control pups, OLETF pups gained significantly more weight after the same nursing bout, regardless of the strain of the mother. Body mass index (BMI) was significantly higher in OLETF pups compared to controls. The maternal parameters assessed from the experiment were latencies to pup retrieval and to nursing, and nursing duration; differences were only observed in nursing time. OLETF dams increased their average nursing time over the PN weeks, while control dams decreased their nursing time toward weaning. The results suggest an important contribution of OLETF pups toward their own preobese development.     

Maternal deprivation and prolonged suckling in the absence of milk alter the frequency and intensity of sucking responses in neonatal rat pups

Jaw-muscle EMG recordings reveal that 3–5-day-old rat pups suckling an anesthetized non-lactating dam emit one of three different sucking responses every 20–25 sec. The frequency and intensity of two of these responses increase if preceded by lengthy maternal deprivation and decrease during extended suckling in the absence of milk. Sucking in neonatal rats is pervasive, complex in structure, and unlike most other measures of suckling at this age, is affected by deprivation.     

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.     

Preobesity in the infant OLETF rat: The role of suckling

The Otsuka Long Evans Tokushima Fatty (OLETF) rat model of obesity, which lacks CCK₁ receptors, has been extensively investigated over the last decade. We have recently focused on the early developmental stages of this strain, finding that OLETF pups are heavier than controls from birth and hyperphagic already from postnatal (PN) Day 2. OLETF mothers present differential maternal behavior patterns and increased nursing time and frequency, which might contribute to the preobese characteristics of the pups. The present investigation examined the pups' ability to gain weight from the nursing episodes. First, we measured the pups' weight gain from one nursing bout from their own dam. Next we examined the pups' weight gain after a feeding competition test with control pups from foster dams of both strains. OLETF pups gained more weight than controls from their own dams on PN Week 1 due to a higher suckling rate (and/or efficacy) and on PN Week 3 due to increased nursing time. When competing with control pups, OLETF pups gained significantly more weight after the same nursing bout, regardless of the strain of the mother. Body mass index (BMI) was significantly higher in OLETF pups compared to controls. The maternal parameters assessed from the experiment were latencies to pup retrieval and to nursing, and nursing duration; differences were only observed in nursing time. OLETF dams increased their average nursing time over the PN weeks, while control dams decreased their nursing time toward weaning. The results suggest an important contribution of OLETF pups toward their own preobese development. © 2008 Wiley Periodicals, Inc. Dev Psychobiol 50: 202–202, 2008.     

Endogenous opioids and the first suckling episode in the rat

Endogenous activity at opioid receptors affects the appetitive behavior of Caesarean-delivered rat pups during presentation of a surrogate nipple that provides milk. Blockade of opioid receptors by peripheral injection of naloxone has no effect on responses evoked by the surrogate nipple. Similarly, blockade of caudal brain opioid receptors by injection of naloxone into the cisterna magna has no effect on the pup's behavior in response to the surrogate nipple. However, blockade of rostral opioid receptors by injection of naloxone into the cerebral ventricles increases the latency to the first oral grasp response, decreases total time on the nipple, and virtually eliminates ingestion of milk from the surrogate nipple (Experiment 1). Blockade of endogenous opioid activity does not affect responses to a nipple that provides distilled water (Experiment 2) or to an empty surrogate nipple (Experiment 3). These data indicate that during the initial suckling episode endogenous opioids in rostral brain regions affect the pup's behavioral responses to the nipple. The results are consistent with the hypothesis that milk engages opioid systems during the first suckling and that endogenous opioids play a role in early suckling. © 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 175–183, 1998     

Effects of gastric filling and vagotomy on ingestion, nipple attachment, and weight gain by suckling rats

The importance of gut signals for the short-term and long-term control of ingestion was investigated using rat pups in the natural suckling situation. Pups at 10 days of age were deprived of milk and their dam for 9 hr. Pyloric ligation, vagotomy, and preloading procedures were performed on the pups shortly before testing began. The initial latency to attach to a nipple and the incidence of attachment were recorded during the 2-hr suckling period. Milk intake and the weight of gastric contents were measured following the test. Pregastric stimulation together with natural or artificial gastric filling suppressed ingestion. Acute subdiaphragmatic truncal vagotomy enhanced ingestion under these conditions, and vagotomy alone resulted in controlled hyperphagia concomitant with exaggerated gastric filling. The data suggest that the stomach is one site where milk activates a suppression mechanism for ingestion. This mechanism is mediated primarily by the vagus nerve. The importance of nipple-attachment behavior is discussed in relation to ingestive behavior, and the question of satiety in suckling rats is raised. The chronic effects of truncal vagotomy on rat pups were also investigated, beginning with denervation at 10 days of age. The pups were challenged with deprivation and tested in the natural suckling situation on Day 20. Weaning occurred on Day 22, and the pups were sacrificed on Day 41, followed by measurements of body weight, body length, and gastric contents. The physical and behavioral changes that developed during the chronic phase of vagotomy included (1) permanent gastric distension following the first postsurgical bout of ingestion; (2) reduced intake of milk when the availability of milk was greatest during the suckling test; (3) increased resting behavior regardless of milk letdown conditions during the suckling test; (4) failure to gain weight at a normal rate; and (5) failure to grow at a normal rate. The data indicate that the vagus nerve is necessary for normal internal control of ingestion as well as normal gastric filling and emptying in the suckling rat. The inability of suckling and weanling pups to thrive in the chronic phase of vagotomy indicates that nonvagal internal controls of ingestion and digestion function adequately for survival, but they are not optimal for growth and maintenance of body weight in rats.     

Aluminum ingestion and behavior in the Long-Evans rat

A variety of behaviors was measured in adult Long-Evans male rats fed ground rat chow containing either no added aluminum, low aluminum (1500 mg/kg), moderate aluminum (2500 mg/kg), or high aluminum (3500 mg/kg). There were no effects of aluminum on either body weight or mouse killing. There was an inverse relationship between brain aluminum and open-field activity. Elevated brain aluminum was correlated with relatively poor performance on a single-trial passive-avoidance task and on a visual discrimination with reversal task.     

Intestinal absorption of ferritin in the suckling rat

Extensive absorption of ferritin was demonstrated in the proximal and distal ileum of 8–12 day-old rats. In some regions, ferritin droplets were distributed in the basal cytoplasm of the columnar epithelium and in the lamina propria one hour after feeding. Subsequently, iron-positive material was localized adjacent to vessels of the lamina propria and in macrophages. Absorption of lesser quantities was demonstrated in the duodenum.     

Milk lipid absorption and chylomicron production in the suckling rat

Structural correlates of milk lipid absorption and chylomicron production were studied in 10-day-old suckled rats. The gastric and duodenal contents and duodenal mucosae were examined with the light and electron microscopes. In the gastric lumen the milk lipid globule cores were smooth, circular and uniformly electron opaque. Many membranes and lamellar structures with a trilaminar and multilamellar appearance were adherent to the peripheries of the cores. In the central duodenal lumen the milk lipid globule cores were also smooth, circular and uniformly electron opaque. Very few milk lipid globules in the duodenal lumen showed adherent membranes or lamellae. Membrane fragments and lamellae were present in the lumen separate from the milk lipid globules. In the duodenal lumen between villi the milk lipid globules had multiple electron lucent indentations of the core. It is believed that the irregular peripheries of the milk lipid globule cores are the result of lipolysis within the duodenal lumen acting at the milk lipid globule surface. This lipolysis of triacylglycerol would produce amphiphilic lipids which may result in the electron lucent spaces at the milk lipid globule periphery. The absorptive epithelial cells along the length of the duodenal villus varied in structure relative to their position at the tip, middle, or base of the villus. Typical mid-villus epithelial cells contained lipid droplets averaging 0.3-μm diameter in the smooth and rough endoplasmic reticulum and in Golgi complexes in the apical cytoplasm. Villus tip and villus base cells contained large lipid droplets between 7–16 μm. Only a few 0.3-μm lipid droplets were present within these cells. These large lipid droplets appeared to be accumulations of triacylglycerol present in the apical cytoplasm associated with lamellar and membranous structures. Numerous chylomicrons were present between epithelial cells located in the middle region of the villus while significantly fewer chylomicrons were seen between epithelial cells at the tip and base of the villus. These observations suggest that the cells at the middle of the duodenal villus of suckling rats were more efficient in the production of chylomicron triacylglycerol derived from incoming milk triacylglycerol than cells at the tip and base of the villus.