Some determinants of milk ingestion in suckling rats.

Eight experiments were conducted to examine several possible determinants and controls of milk intake in 10-day-old preweanling Sprague-Dawley rats. Results indicate that while the amount of milk the mother provides is a major determinant of milk consumption, pups will individually adjust their intake in response to certain treatments. Deprived pups ingested more milk than nondeprived littermate controls if given access to enough milk. Intragastric intubations of skimmed milk or distilled water reduced milk intake; heavy cream or isotonic saline did not. Injections of polyethylene glycol or formalin, which produced hypovolemia, increased milk consumption, and hypertonic saline decreased it. Results indicate that preweanling rats are behaviorally responsive to certain changes in body fluids and suggest that milk intake involves more than an invariant reflex. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of preference for nutritive over nonnutritive suckling in albino rats.

Investigated nutritive vs nonnutritive suckling in 48 10–21 day old Sprague-Dawley rat pups in a spatial discrimination task. Ss preferred nutritive to nonnutritive suckling on their anesthetized mother at l7 and 21 days of age, but no preference was apparent in 10- and 12-day-old Ss. Rearing in isolation from the mother during Days 11–26 did not impair the development of preference for nutritive suckling in 16 17-day-old Ss. Thus, the appetitive component of suckling of infant rats, like the consummatory component, appears to come under the direct control of nutritional factors at the start of the weaning period. (21 ref) (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)     

Ontogeny of defensive reactions in Norway rats.

In 3 experiments, 219 Sprague-Dawley albino rats of several ages were presented with stimuli (a caged domestic cat, a footshock, and a suddenly moving object) known to be aversive to adults and disruptive of behavior in mature animals. 20-day-old Ss were relatively unaffected by these events, while Ss aged 30 days and older tended to reduce their locomotion and freeze upon the presentation of these cues. Data are consistent with R. C. Bolles's (1970, 1971) hypothesis that shock-elicited responses are innate defensive reactions. It is suggested that the inefficient passive avoidance learning in juvenile rats may result from their deficit in shock-induced freezing. (41 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)     

Dehydration inhibits suckling behavior in weanling rats.

Determined the relation between dehydration and suckling behavior in 567 Sprague-Dawley weanling rats 15, 20, and 23 days of age. After 15 days of age, intra- and extracellular dehydration sharply reduced both the number of Ss that suckled and the amount of milk consumed. Rehydration returned both behaviors to control levels. Thus, during the weaning period, the internal determinants of suckling are not homologous with those of drinking, but are more homologous with those governing feeding. (29 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)     

Ontogeny of eyeblink conditioned response timing in rats.

Eyeblink conditioned response (CR) timing was assessed in adult and infant rats. In Experiment 1, adult rats were trained with a 150-ms tone conditioned stimulus (CS) paired with a periorbital shock unconditioned stimulus (US; presented at 200- or 500-ms interstimulus intervals [ISIs]). The rats acquired CRs with 2 distinct peaks that occurred just before the US onset times. Experiments 2 and 3 examined developmental changes in CR timing in pups trained on Postnatal Days 24-26 or 32-34. Experiment 3 used a delay conditioning procedure in which the tone CS continued throughout the ISIs. Pups of both ages exhibited robust conditioning. However, there were age-related increases in the percentage of double-peaked CRs and in CR timing precision. Ontogenetic changes in eyeblink CR timing may be related to developmental changes in cerebellar cortical or hippocampal function. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Oropharyngeal control of ingestion in rats: Acquisition of sham-drinking patterns.

Conducted 3 experiments with female Sprague-Dawley rats with esophageal fistulas (3 Ss in Exp I, 3 in Exp II, and 4 in Exp III). Offered a concentrated glucose solution, Ss sham drank relatively small amounts on the 1st session, whether or not such a solution was familiar. Intake rose to high levels over ensuing sham-drinking sessions, showing that the initial small volumes were not attributable to fixed properties of the solution itself. The gradual acquisition of copious sham drinking was not affected by previous "practice" at sham drinking other commodities; it was not simply an alteration in motor habits. The gradual acquisition of the sham-drinking pattern did not occur with more dilute solutions; in response to these, sham drinking was copious and continuous from the onset. The following conclusions are made: (a) Rats must learn to respond, by continuous drinking, to the absence of the postingestive inhibition normally produced by concentrated solutions. (b) No such learning is required in the case of more dilute solutions. This is further evidence that controlling factors with differing properties operate at different levels even of the single commodity, glucose in solution. (17 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)     

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)     

Intestinal uptake of uridine in suckling rats: mechanism and ontogeny

Nucleosides, essential substrates for a variety of intracellular metabolic reactions, are obtained from dietary and endogenous sources. Nucleotides (which dephosphorylate to nucleosides prior to intestinal absorption) are present in milk and have trophic effects on the developing gastrointestinal tract. The mechanism of transport of nucleosides in the developing intestine of suckling rats is unknown. To address this issue, we therefore examined uridine uptake in rat everted intestinal sacs. In suckling rats (15-17 days old), tissue uptake of low (5-microM) and high (60 microM) concentrations of [3H]-uridine was linear for up to 2 min of incubation. Initial rate of uptake of [3H]-uridine was (i) not significantly different in the jejunum and the ileum; (ii) greater in the presence of Na+, than other cations; (iii) saturable as a function of concentration with a Vmax of 21,044 +/- 2,302 pmol/g tissue wet wt/30 sec and an apparent Km of 33.8 +/- 10.1 microM; (iv) inhibited by high concentration (500 microM) of unlabeled uridine and other nucleosides; (v) temperature-dependent; (vi) energy-dependent; and (vii) pH-sensitive. Developmental maturation was associated with a progressive decrease in the Vmax of the uridine transport process (21,044 +/- 2,302, 14,651 +/- 1,679, and 8,461 +/- 1,369 pmol/g tissue wet wt/30 sec for suckling, weanling, and adult rats, respectively) and a progressive increase in the apparent Km of the uptake system (33.8 +/- 10.1, 55.6 +/- 13.1, and 61.7 +/- 14.5 microM for suckling, weanling, and adult rats, respectively). We concluded that uptake of uridine by the developing intestine of suckling rats involves a carrier-mediated system, which is energy- and temperature-dependent, and requires extracellular sodium. Furthermore, the uptake process was found to undergo clear ontogenic changes with maturation.     

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)     

Effect of dopamine-depleting brain lesions on suckling and weaning in rats.

Rats given large dopamine (DA)-depleting brain lesions as adults exhibit severe impairments in ingestive behavior and sensorimotor function. In contrast to these well-known effects, the present 2 studies showed that virtually complete destruction of central dopaminergic neurons produced no such dysfunctions when it occurred in neonates (Sprague-Dawley). Ss continued to suckle and grow, albeit somewhat more slowly, and they could be weaned readily when they were 27 days old. Although most brain-damaged Ss did not survive weaning when they were 18 days old (controls exhibited no difficulty), this failure appeared to be the consequence of their reduced body weight and related inability to maintain body temperature in a relatively cool environment (22°C). Such premature weaning occurred more successfully when growth was stimulated by rearing brain-damaged pups in small litters or when ambient temperatures were raised to 31°C to minimize heat loss. Results demonstrate that the effects of near-total DA-depleting brain lesions are considerably less severe when they occur in infants than when they occur in adults, and, consequently, they reveal a capacity for neural plasticity during development that is no longer present at maturity. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nutritive and nonnutritive determinants of milk intake of suckling rats.

Determined the factors that enhanced milk intake during deprivation in albino rats 15, 20, and 25 days of age. Ss were subjected, for 8 hrs, to 1 of 6 regimens. Results show that intake at Day 15 was reduced by the opportunity to suckle, independent of receiving a milk load. This same trend was apparent, although not as strong, among Day 20 Ss. By Day 25, nonnutritive suckling during the privation period no longer attenuated milk intake, although preloads did, whether or not they were paired with nonnutritive suckling. Thus, suckling in Ss became increasingly freed from oral demands and more responsive to the nutritive consequences. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned aversions and their memories in 5-day-old rats during suckling.

In 4 experiments, 216 5-day-old Sprague-Dawley rat pups equipped with tongue cannulae placed either 2 mm rostral or 4–6 mm caudal to the intermolar eminence received sweet or salty solutions while suckling. This ingestion was followed by intraperitoneal injections of .3 M LiCl or isotonic saline. Ss with anterior cannulae took in considerably less fluid than controls when tested 5 or 16 days later. A series of control groups demonstrated that this acquired aversion was associative in nature. Ss with posterior cannulae did not form the association. It is suggested that the failure of 5-day-old Ss with posterior cannulae to form associations while suckling was not due to the prevention of conditioning by the act of suckling but instead resulted from the failure of the fluid to reach anterior taste receptors when injected into the posterior oropharynx, where the nipple normally empties its contents. Findings are discussed in terms of the transfer of information obtained during suckling to feeding and drinking during and after weaning. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cell kinetics in the small intestine of suckling rats. I. Influence of hypophysectomy

Thyroid activity, pituitary and serum thyrotrophic potency in response to the administration of clomid, sexovid, PGE1 and PGF2alpha, were studied in H. fossilis. Hightened thyroidal activity and CR (conversion ratio of PB 131I in blood serum in relation to total serum 131I uptake) were noticed a week after clomid (150 microgram/fish/day) and sexovid (150 microgram/fish/day) treatment. Clomid and sexovid also elevated the serum thyrotrophic potency although pituitary TSH level was unaffected. It is evident from the results that clomid and sexovid either act via hypothalamus or directly over pituitary to increase TSH secretion followed by increased thyroid activity. PGE1 and PGF2alpha (100 microgram/fish/day, each) administration increased thyroidal 131I uptake but failed to stimulate hormone output from thyroid gland. Increased TSH level in blood and decreased level of TSH in pituitary was observed in response to the above prostaglandins. It seems that PGE1 and PGF2alpha inhibit thyroid hormone secretion like anti-thyroid drugs triggering the release of TSH into blood.     

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)     

Ontogeny and interdependence of genetically selected behaviors in rats: Avoidance response and open field.

Investigated in 2 experiments, ontogeny and interdependence of genetically selected avoidance and open-field behaviors in 6 genetic lines (e.g., MNR/Har/Lu, RCA/Lu), involving 1,018 rats, and 3 developmental models (longitudinal, cross-sectional, and generational). In selected lines, behaviors varied with age (either 21-, 35-, 70-, or 100 days of age), depending upon the genetic selection; in the control (random bred) line, no such effects were found. Between the lines selected for high (RHA) and low (RLA) rates of 2-way active avoidance, ontogenetic differences in the selected behavior emerged as Ss grew older, whereas between the lines selected for high (MR) and low (MNR) frequency of open-field defecation, the differences in the selected behavior were present at all stages of development. In Ss' unconditioned escape response to footshock, the differences were present at all stages. The relationship between avoidance response and open-field defecation (emotional reactivity) was not linear but suggestive of a curvilinear inverted-–U function. (31 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogeny of behavioral arousal in rats: effect of maternal and sibling presence

Spontaneous locomotor activity of 5- to 30-day-old rats housed either alone or in the presence of various components of the normal nest and litter stimuli was recorded by means of time-lapse videography. Developing animals observed alone showed a sharp increase in total daily locomotor activity from Day 5 to Day 15 followed by a rapid decline from Day 15 to Day 30. Individual animals observed in the context of the normal litter environment showed an entirely different pattern of development. They were relatively inactive during the first 15 days of life and then began a gradual increase of activity which continued for the next 15 days. The heightened activity characteristic of the isolated 15-day old inhibited by the presence of four siblings, an anesthetized lactating female, or an anesthetized adult male rat. Thermal conditioning, including heating of the floor to approximately nest temperature or the presence of a heated tube in one corner of the test apparatus, did not inhibit the heightened activity. These results question the generality of the ontogenetic sequence of excitation and inhibition proposed by Campbell and his associates - at least a portion of the heightened activity seem around 15 days of age was the result of isolation distress, not merely maturational changes in the brain.