Olfactory, thermal, and tactile controls of suckling in preauditory and previsual rats.

Determined exteroceptive control of nipple attachment in Sprague-Dawley rats and their litters by altering maternal sensory features. Preauditory and previsual pups, 2, 5, 8, and 11 days of age, were permitted to locate and attach to the nipples of their anesthetized mother after she had received one of the following treatments: (a) thermal--peripheral temperature was lowered to either 31 or 28°C; (b) tactile--the mother was shaved; (c) thermotactile--the mother was shaved and her skin temperature was lowered to 31°C; or (d) olfactory--all nipples were cleaned with methylene chloride and alcohol solvents. Only the olfactory treatment severely affected nipple attachment at all ages studied. Washing the nipples virtually eliminated suckling; painting them with a vacuum distillate of the nipple extract or of pup saliva reinstated suckling. The other manipulations either did not interfere with nipple attachment or did so at one age only. (42 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The role of experience in suckling maintenance in albino rats.

In Exp I nipple-attachment behavior in Sprague-Dawley albino rats was disrupted by raising rat pups in isolation from their mother and siblings on the 3rd–5th day after birth. In Exp II nipple attachment was maintained, however, in isolation-reared pups that received, on Days 3 and 4 postnatally, either 6 or 12 opportunities to search for, locate, and attach to the nipples of an anesthetized mother. Suckling remained severely disrupted on Day 5 in pups whose experiences on Days 3 and 4 were restricted to either nipple search alone or nipple attachment without previous search. Findings focus attention on the role of experience in suckling maintenance and suggest that in rats the suckling system is not fully specified at birth. (35 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Separation of opioid from nonopioid mediation of affect in neonatal rats: Nonopioid mechanisms mediate maternal contact influences.

Established a causal distinction in infant rats between opioid- (OP) and non-OP-mediated determinants of behavior. Contact influences were mediated by non-OP pathways, and gustatory influences were OP-mediated. Naltrexone (NTX) did not diminish quieting exerted by contact with an anesthetized dam but reversed the quieting effects of morphine in isolated Ss. Naloxone (NLX) did not affect the latencies with which nondeprived or 8-hr deprived Ss 9, 12, 15, and 18 days old attached to nipples (NPs) of anesthetized dams, nor did NLX cause systematic change in NP attachment in 10- and 18-day old Ss from the NP when milk letdown was induced by oxytocin. NLX did not reduce milk intake in Day-10 Ss that, while suckling, received milk via indwelling cannulae in the anterior portion of the lower jaw or in Ss that obtained milk directly from their awake mother. Milk intake was significantly reduced by NTX in Day-10 Ss that obtained milk by licking it off a saturated substrate. In Day-10 Ss, intake of milk delivered via cannulae was reduced by NLX in Ss that were either isolated, in contact with an anesthetized dam, or attached to her NPs. Behavioral influences of the tactile (and possibly olfactory) qualities of the dam were not mediated by OP systems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Experimental study of the first suckling episode: rat pups ingest fluids through a surrogate nipple

The present study reported a new technique in which pups ingested fluids from a surrogate nipple. Cesarean-delivered pups tested before suckling experience showed oral grasp responses and ingested milk from the surrogate nipple. Pups ingested equal amounts of distilled water and milk and rejected saline. After ingesting milk from the surrogate nipple, pups remained attached to an empty surrogate nipple, while pups exposed to distilled water or saline did not show sustained attachment. Brief experience with milk from an oral cannula or from a surrogate nipple elicited sustained attachment to an empty nipple. Pups ingesting milk from a surrogate nipple showed increased intake of water and saline from the nipple when tested subsequently. The surrogate nipple provides a new technique for experimental study of early suckling behavior. The results suggest that initial experiences with milk may reinforce components of early suckling behavior.     

Taste aversions to mother's milk: The age-related role of nursing in acquisition and expression of a learned association.

Seven experiments examined the development of taste aversion learning to novel cues contained in mother's milk in 176 laboratory and 377 Sprague-Dawley pups. Ss receiving distinctive milk by experimenter-delivered oral infusions followed by toxicosis formed an aversion to the dam's diet. Robust aversions were learned as early as Day 10 and were retained for at least 11 days. When the same distinctive milk was obtained directly from a foster mother through nursing, only weanling-age Ss formed an aversion. X-ray analysis of nipple location in the mouths of suckling Ss suggested that they receive milk at a similar tongue locus between the ages of 10 and 21 days; flavored milk was then delivered at specific time intervals in controlled quantities through tongue cannulas implanted at loci corresponding to the nipple position. Cannulated preweanling Ss that were attached to a nipple during mild delivery failed to associate the taste cue with illness, whereas both preweanlings off the nipple and weanlings on the nipple acquired aversions to the taste cue in the milk. Results suggest that pups of all ages are incapable of expressing a taste aversion in a nursing situation and that preweanling pups in particular are also deficient in acquiring aversions within a suckling context. The inability of preweanling pups to acquire taste aversions in a nursing situation appears to result from a failure to associate taste cues with illness rather than a failure to detect taste cues obtained from a nipple. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of feeding motor patterns in infant rats: An electromyographic analysis of suckling and chewing.

During mammalian ontogeny, there is a transition from suckling to the chewing of food. Is suckling a neuromuscular precursor to chewing, or are suckling and chewing independent systems? Electromyograms (EMGs) were recorded in rat pups of ages 6, 9, 12, 15, 18, and 21 days from the superficial masseter, anterior digastric, sternohyoideus, and genioglossus muscles during suckling and chewing. The EMG patterns of the 3 components of suckling behavior (nipple attachment, rhythmic sucking, and the stretch response) are distinctive from one another and reflect the musculoskeletal biomechanics of suckling. Chewing EMGs are present by 12 days of age and attain the adult pattern by 18–21 days of age. During nipple attachment, pups exhibit a motor pattern that is similar to that of adult chewing, but other aspects of suckling differ from chewing in some EMG features. Comparison of EMGs between behaviors and between ages allowed interpretation of the degree of continuity of muscular activity across the suckling-to-chewing transition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Suckling in newborn rats: eliminated by nipple lavage, reinstated by pup saliva

Chemical lavage of the nipples of anesthetized maternal rats virtually eliminated suckling by their 4- to 5-day-old pups. Normal suckling was immediately reinstated, however, by painting a vacuum distillate of the wash or of pup saliva onto the nipples. Thus, a substance necessary to direct and release suckling, possibly rat pup saliva, appears to coat the nipple surface.     

Suckling behavior and intake control in the developing rat pup.

Studied the behavioral development of suckling and intake control in 2 experiments with Charles River CD strain rat pups. Ss were observed at the initiation, during the course, and at the termination of suckling from their anesthetized mothers. Diet was delivered intraorally through a fine tongue cannula which enabled control of timing and volume. The control of diet intake and the behavior at termination of suckling showed correlated changes from 5 to 20 days of age. When deprived of suckling (and food and water) for 8 hr, 5- and 10-day-old Ss consumed large volumes of diet (10% of body weight or greater) and terminated suckling only in the presence of extreme gastrointestinal filling. These Ss were immediately lethargic and slept after intake termination. Five-day-old Ss persisted in reattaching to the nipple when manually stimulated; 10-day-old Ss eventually refused to reattach. In contrast, 20-day-old Ss consumed more moderate volumes of diet (5% of body weight). These Ss also remained awake for a period after feeding and engaged in the exploratory and grooming activities characteristic of adult rats at the termination of feeding. These observations demonstrate major changes in suckling behavior during development. They suggest that intake control processes shift from indirect to direct and become more effective and specifically food intake related in older pups. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nipple attachment in rats during the first 24 hours of life.

Investigated nipple attachment in 49 Purdue-Wistar laboratory rat pups during the 1st 24 hrs of life and identified determinants that influenced its expression. In Exp I, nipple attachment was shown to be age dependent: 1-hr-old pups exhibited a low percentage of attachment, and 12- and 24-hr-old pups exhibited high percentages when tested against an anesthetized lactator. Exps II–VI established the time course of nipple attachment as age related. Exps VII–X explored the behavioral mechanism responsible for the increase in attachment from Hr 1 to Hr 12. They showed that nipple experience, independent of milk acquisition, was the responsible factor for the increase. The findings suggest that learning is employed as a behavioral strategy by the newborn, and they reemphasize the value of using species-relevant measures in the assessment of behavioral development. (35 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Early weaning in the rat: Effects on whole-brain RNA and free amino acids.

Each of 67 CAW:CFE(SD)spf rat litters was reared with its mother or with its mother and a virgin female ("aunt"). At pup age 15 days, some litters were deprived of their mothers, some remained with mothers whose nipples were cauterized to prevent suckling, and some remained intact until 21 days when all pups were housed individually or in peer groups until tested at 56 days. While increases in whole-brain free aspartic acid were attributed to loss of opportunity for suckling since they occurred in pups without mothers or with cauterized mothers, decreases in brain RNA caused by deprivation were prevented by the presence of mother or aunt. Rearing with aunts increased brain weight and emotional reactivity. (27 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of amphetamine anorexia in rats: A behavioral analysis.

Conducted 4 experiments with 246 Sprague-Dawley rats to ascertain the effects of dextroamphetamine (DAM; 1 or 2 mg/kg) on the behaviors associated with feeding in weanling and preweanling rats. The 1st 2 experiments produced the following results: (a) In 5-day-old Ss, DAM increased speed of approach to an anesthetized mother; (b) at 15 days of age, DAM increased speed of approach to the mother and produced marked weight loss (relative to the weight of controls), slowed or reduced frequency of attachment to the nipple, and decreased time spent nursing; and (c) at 25 days of age, DAM disrupted all aspects of the feeding process except initial approach. Weight gain, time spent in contact with the mother, time spent nursing, and time attached to the nipple were all reduced by the drug. In Exps III and IV, milk was delivered to Ss at 5, 15, and 25 days of age through tongue cannulas, which allowed ingestion without suckling. Under these circumstances, DAM produced clear-cut anorexia in Ss of all 3 ages. It is concluded that in early development, the anorectic effects of DAM are overridden by the strong suckling response that is not disrupted by the drug. (17 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inhibition of suckling in weaning-age rats: A possible serotonergic mechanism.

Four experiments studied the nature, development, and specificity of serotonergic involvement in the control of suckling behavior in Sprague-Dawley rat pups from 10 to 35 days of age. During development, suckling normally declines after 10 days and is abandoned after 30 days. It was found that (a) methysergide, a serotonin (5-hydroxytryptamine, 5-HT) receptor blocker, reinstated suckling behavior in pups 15 days of age and older; (b) quipazine, a 5-HT receptor agonist, inhibited suckling of pups 10 days of age and older; (c) methysergide pretreatment blocked the quipazine inhibition of suckling; and (d) metergoline, another 5-HT blocker, also stimulated suckling, and fenfluramine, a 5-HT releaser, blocked suckling in deprived pups. Data support the hypothesis that a serotonergic inhibitory mechanism modulates the suckling of weaning-age rats. These pharmacological manipulations of 5-HT appear to alter specific components of suckling behavior involved in its initiation and maintenance but do not appear to alter a general hunger system. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Evidence for a transitional period for development of persistence in infant rats.

Conducted 4 experiments with a total of 188 rat pups to test the hypothesis that rat pups can learn persistence at age 11 days but cannot express this learning in their behavior until they are a few days older. Exp I used a space-trial procedure to investigate the relative reinforcing effects of milk suckling, dry suckling, and contact without suckling at ages 11 and 14 days. Exp II used the Exp I procedure, manipulating schedule of reward at 2 ages and under 2 of the reward conditions. Exp III manipulated the dry suckling reward schedule in 11- and 14-day-old pups. Exp IV manipulated partial and continuous reinforcement on Day 11 but tested for persistence on Day 14. Results suggest the existence of a traditional period in neonatal rats for the learning of persistence. Partial reinforcement training with suckling on an anesthetized dam as reward induced greater persistence in extinction of the approach response than did continuous reinforcement in rat pups 14 days old but not in 11-day-old pups. Other aspects of this period of development are discussed. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of warmth on newborn rats' motor activity and oral responsiveness to an artificial nipple.

Temperature is a powerful regulator of the behavior and physiology of newborn altricial animals. The effects of warmth on newborn rats' oral responsiveness to suckling stimuli and spontaneous motor activity in a thermoneutral environment were investigated. Newborn rat pups' oral grasp responses to an artificial nipple and overall motor activity were recorded for 18 min. Near-term pups were delivered by cesarean section so that their 1st experiences with suckling stimuli could be observed. Experimental pups were warmed for 15 s every 2 min; control pups were not warmed. Warmed pups grasped the nipple fewer times than the not-warmed pups. However, oral grasp durations became longer for the warmed pups but not for the not-warmed pups. Warmth increased pups' motor activity but only while the heat was applied. Warmth in a thermoneutral environment may promote longer nipple attachment during newborns' early feeding experiences. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Activation and odor conditioning of suckling behavior in 3-day-old albino rats.

Investigated circumstances under which a novel odor could elicit nipple attachment behavior in 3-day-old Sprague-Dawley rats. In Exp I, Ss suckled washed nipples scented with citral (a lemon odor) only if they either had received tactile stimulation (by stroking with a soft brush) or had been administered amphetamine in the presence of citral prior to the suckling test. Ss stimulated in citral's absence or only exposed to citral without stimulation failed to suckle such nipples. In Exp II, Ss stimulated in a benzaldehyde (an almond odor) ambience suckled washed nipples scented with benzaldehyde but not those with citral scent. The opposite held for those stimulated in a citral-rich environment. The stimulus conditions that supported this conditioning were investigated in Exp III. Simultaneously increasing citral concentration and raising ambient temperature markedly attenuated the phenomenon. Exp IV demonstrated that not all classes of stimulation produced conditioning. Caffeine, in a wide range of doses, did not allow citral to elicit suckling on washed nipples. Findings are discussed within a framework of higher-order conditioning and may provide a mechanism by which naturally occurring stimuli come to elicit the species- and age-typical behavior of suckling. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Self-administration of ethanol and saccharin in newborn rats: Effects on suckling plasticity.

Responsiveness to a surrogate nipple providing water, saccharin, 5% ethanol, or 10% ethanol was tested in newborn rats naive to suckling (3–5 hr old) on Postnatal Day (P) 0 and in older neonates with regular suckling experience on P1 or P2. At all ages, pups demonstrated greater nipple attachment for saccharin or 5% ethanol than for water. Intake of saccharin and 5% ethanol was high in newborns, far exceeding that of water. At P1 and P2, pups exhibited a preference for saccharin, but not for 5% ethanol. Preexposure to a nipple providing ethanol or saccharin (but not a nipple alone or fluids alone) increased subsequent responsiveness toward an empty surrogate nipple (no fluid), both 1 hr and 24 hr after preexposure. Although, during preexposure, pups responded most positively to the nipple providing saccharin, the longest time spent on an empty nipple was observed in pups preexposed to 10% ethanol. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Electromyographic patterns associated with nonnutritive sucking in 11–13-day-old rat pups.

Three experiments used an EMG technique to measure the frequency, duration, and intensity of nonnutritive sucking (NNS) in 56 11–13 day old Wistar pups. Ss were separated from their mothers for 2–6 or 20–24 hrs and then allowed to suckle an anesthetized dam for up to 3 hrs without receiving any milk. Jaw-muscle EMG and nipple detachments were recorded. EMG patterns representing 2 discrete modes of sucking were reliably discerned, as were changes in overall intensity of EMG. 20–24 hr separated Ss engaged in more frequent NNS bouts and bouts of longer duration than 2–6 hr separated Ss. Ss separated by 20–24 hrs also engaged in a mode of NNS not seen in 2–6 hr separated Ss. Overall EMG intensity was higher and frequency of nipple detachments was lower in Ss separated for 20–24 hrs. In both groups, but particularly the 20–24 hr Ss, frequency of some NNS patterns decreased as the length of the "dry" suckling increased. EMG intensity also decreased over time in both groups, and frequency of nipple detachments increased in 2–6 hr separated Ss. Data indicate that some aspects of NNS are highly labile and respond to variations of both deprivation from the mother and nutritive deprivation. (27 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Methysergide-induced nipple attachment depends on suckling experience in juvenile rats.

These studies were designed to investigate the links between pharmacological and behavioral procedures that facilitate suckling in weanling rats by assessing the effects of methysergide on nipple attachment behavior following experiential manipulations known to either promote or attenuate suckling. In the first experiment, methysergide failed to stimulate suckling in 25-day-old rats separated from their dam on Day 20, although it facilitated suckling in rats kept with the dam until either Day 24 or Day 25. In the second experiment, methysergide did not facilitate suckling in 35-day-old rats separated on Day 25, although rats separated on Day 30 or 34 were induced to suckle. In the third experiment, rats were housed with preweanling litters until Day 35 and then separated, housed with 25-day-old litters, or housed with preweanling litters for another 10 days. Rats in a fourth group remained with their dams until Day 35 and were then separated for 10 days. When tested at Day 45, nipple attachment was facilitated by methysergide only in the three groups that had received extended suckling experience. These results demonstrate that serotonergic inhibitory mechanisms modulate nipple attachment only when suckling experience is recent or extensive. (PsycINFO Database Record (c) 2010 APA, all rights reserved)