Sensory determinants of suckling behavior in weanling rats

Neurobehavioral assessment of maternal odor in developing rat pups: implications for social buffering

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother's social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with 14C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.

Morphological analysis of regenerated bulbar fibers in relation to neonatal olfaction

It was revealed that regeneration of the lateral olfactory tract (LOT) occurred in developing rats and the regenerated olfactory system was functional 4 weeks after transection. The aim of this study was to determine the earliest onset of functional recovery in LOT-injured rats and to quantify regenerated nerve components with functional correlation. Neonatal rats on postnatal day (P) 2 were subjected to unilateral transection of the left LOT and underwent unilateral removal of the right olfactory bulb on P11. Functional recovery of the tract injury was assessed by the suckling capability, which can be achieved by olfaction. Suckling capability was observed on P12 in most neonatally LOT-transected pups. Rat pups were subjected to unilateral transection of the left LOT on P2, and received injections of biotinylated dextran amine (BDA) into the bilateral olfactory bulb on P5 to quantify normal and regenerated nerve components in the olfactory cortices at the level of the olfactory tubercle. BDA(+) areas and density indices of the olfactory cortices in the neonatally LOT-transected P12 pups were 11.05×10⁵μm² and 0.35 on the normal right side and 4.34×10⁵μm² and 0.21 on the transected left side. We concluded that functional recovery of the LOT-transected neonatal rats occurred as early as 10days after tract transection and that areas and densities of regenerated nerve components essential for functional recovery were approximately 40% and 60% of the age-matched normal values in the olfactory cortices at the level of the olfactory tubercle.

Determination of functionally essential neuronal population of the olfactory epithelium for nipple search and subsequent suckling behavior in newborn rats

Neuronal population of the olfactory epithelium required for nipple search and subsequent suckling behavior was examined in newborn rats. After unilateral ablation of the olfactory bulb, different concentrations of ZnSO(4) were contralaterally injected into the nasal cavity to produce varying degrees of neuronal degeneration in the olfactory epithelium. The ZnSO(4)-treatment resulted in two populations of pups. One exhibited suckling while the other did not, and intact olfactory receptor neurons were quantified immunohistochemically using an antibody for olfactory marker protein (OMP), a marker protein for olfactory receptor neurons. The total numbers of the OMP (+) cells in the ZnSO(4)-treated pups with suckling capability ranged between 2457 and 4615, whereas those in the ZnSO(4)-treated pups without suckling capability ranged between 112 and 2398. With the mean value (4969) of the total numbers of the OMP (+) cells of the normal/control pups assumed to represent 100%, the total numbers of the OMP (+) cells accounted for 49-93% in the suckling (+) group and 2-48% in the suckling (-) group. From these findings, we conclude that approximately 50% of neuronal population of the olfactory epithelium is a critical value to distinguish between the two groups with and without suckling capability in the unilateral olfactory system of newborn rats.

Sall3 is required for the terminal maturation of olfactory glomerular interneurons

Sall3 is a zinc finger containing putative transcription factor and a member of the Sall gene family. Members of the Sall gene family are highly expressed during development. Sall3-deficient mice die in the perinatal period because of dehydration and display alterations in palate formation and cranial nerve formation (Parrish et al. [2004] Mol Cell Biol 24:7102-7112). We examined the role of Sall3 in the development of the olfactory system. We determined that Sall3 is expressed by cells in the olfactory epithelium and olfactory bulb. Sall3 deficiency specifically alters formation of the glomerular layer. The glomerular layer was hypocellular, because of a decrease in the number of interneurons. The lateral ganglionic eminence and rostral migratory stream developed normally in Sall3-deficient animals, which suggests that Sall3 is not required for the initial specification of olfactory bulb interneurons. Fewer GAD65/67-, Pax6-, calretinin-, and calbindin-positive cells were detected in the glomerular layer, accompanied by an increase in cells positive for these markers in the granule cell layer. In addition, a complete absence of tyrosine hydroxylase expression was observed in the olfactory bulb in the absence of Sall3. However, expression of Nurr1, a marker of dopaminergic precursors, was maintained, indicating that dopaminergic precursors were present. Our data suggest that Sall3 is required for the terminal maturation of neurons destined for the glomerular layer.

Functional role of lingual nerve in breastfeeding

Functional role of lingual nerve in breastfeeding was investigated in rat pups during the suckling period. DiI, a postmortem neuronal tracer, was used to confirm the immature lingual nerve (LN) responsible for tongue sensation and resulted in successful fiber labeling anterogradely to the tongue, which showed different distribution patterns from fiber labeling derived from the hypoglossal nerve. Unilaterally LN-injured pups did not show suckling disturbance with absence of any shortening (P11 pups: 559+/-16s; 105% of the control value) in nipple attachment time and the survival rate remained high (P11: 100%). Bilaterally LN-injured pups showed suckling disturbance with marked shortening (P11 pups: 220+/-54 s; 42% of the control value) in nipple attachment time and a low survival rate (P1: 33%; P11: 41%). Bilaterally infraorbital nerve-injured or bilaterally bulbectomized pups did not show any nipple attachment at all and there were no survivors, confirming the crucial roles of upper lip sensation and olfaction in suckling. Based on these findings, we conclude that tongue sensation is very important, but not essential for suckling.

One-trial associative odor learning in neonatal mice

Behavior genetics studies in mice demand efficient training protocols for rapid phenotypic screening. However, the capacity of neonatal mice to form and retain associative memories has been difficult to study due to their limited sensorimotor capacities. The present study describes a method for robust, naturalistic associative learning in neonatal mice as young as 3 days old. After removal of the dam from the home cage for 2 h, preweanling CD-1 mice of ages 3, 5, and 10 days postnatal were conditioned to associate an arbitrary odorant with the suckling and milk delivery that ensued upon her return to the home cage. After a second maternal deprivation, neonates were tested on their acquired preference for that odorant. Neonates exhibited a learned preference for the conditioned odorant over a novel control odorant. No learning was observed without deprivation, that is, when the dam was removed only briefly for scenting. One-trial learning sufficed to show clear preferences for the conditioned odorant, although repeated training (three sessions over 8 days) significantly increased the expression of preference. The development of neonatal associative learning protocols requiring minimal human intervention is important for the behavioral phenotyping of mutant and transgenic strains, particularly those modeling developmental disorders.

Classical conditioning of responses to an artificial nipple in the rat fetus: mu and kappa opioid systems

The rat fetus exhibits oral grasp responses when presented with an artificial nipple in utero. Preexposure to milk produces a transient decrease in oral grasp responses. The effect of milk on oral grasping is mediated by endogenous activity in brain opioid systems. Specifically, milk triggers mu activity in rostral brain regions and kappa opioid activity in caudal brain regions to decrease oral grasping of the artificial nipple. Reexposure to the artificial nipple after it has been paired with milk during a classical conditioning trial results in a conditioned reduction in oral grasping that is evident when fetuses are reexposed to the nipple. The classically conditioned decrease in oral grasping is mediated by mu opioid activity in rostral brain regions and kappa opioid activity in caudal brain regions. Endogenous opioid systems, activated by exposure to milk and the nipple, may regulate the processing of sensory information during suckling in the rat.

Relative importance of experience, social facilitation, and availability of milk in weaning of rats

These studies investigated the role that social facilitation, availability of the dam, and milk play in the maintenance of suckling behavior. Beginning at Day 20, rat pups' suckling experiences were restricted to testing with an anesthetized dam. In the first experiment, nipple attachment was abandoned by about Day 25 in rats that were tested alone with an anesthetized dam for 1 hr per day. When tested in groups of four, nipple attachment was maintained until about Day 27. Increasing exposure to an anesthetized dam to 2 hr per day prolonged nipple attachment about another 4 days. In the second experiment, pups were given either one or two daily 1-hr attachment tests and tested with either an anesthetized dam or an anesthetized dam in which the milk letdown reflex was reinstated. Both increasing the daily exposure to an anesthetized dam and reinstating milk letdown significantly prolonged suckling. Pups given two daily exposures to an anesthetized, milk-laden dam attached until about Day 47, long past the normal age of weaning.

Behavioral interactions rather than milk availability determine decline in milk intake of weanling rats

We examined the relation between milk availability and milk intake during the period in which rat pups gradually abandon milk as a food source. The amount of milk produced by rat dams does not change from postpartum Day 15 to Day 20, but decreases thereafter and completely disappears around Day 30. In contrast, the amount of milk actually obtained by pups does begin to decline between Days 15 and 20. This decline in milk intake can be attenuated by integrating 20-day-old pups into 15-day-old litters. We concluded that pups do not begin to ingest less milk because of diminishing milk supplies. Rather, the decreased tendency of mothers to nurse older pups and the diminished tendency of older pups to extract available milk, together appear to underlie the decline in pups' milk consumption. Milk supplies decline after changes in behavioral interactions and may play an instrumental role in the eventual abandonment of suckling.

Olfactory influences on the ingestive behavior of infant rats

Twenty-four-hour deprived infant rats from 1 to 12 days of age were exposed to the odor of milk, either by itself or accompanied by oral infusions of water, sucrose, quinine, or milk (Experiment 1). Pups from 3 to 9 days of age became behaviorally active, and mouthed and probed in response to the odor of milk, even in the absence of oral infusions. Six- and 9-day-old pups also increased their intake of various test solutions in the presence of milk odor. This responsiveness to milk odor was not seen in 1-day-old pups, and waned by 12 days of age. Other stimulus odors that were presented in the same context (Experiment 2) failed to elicit such intense behavioral activation, indicating that there was some degree of specificity in the responsiveness to milk odor. Finally, 2 manipulations that influence pups' ingestion of milk, deprivation and ambient temperature, were found to influence pups' responses to milk odor (Experiment 3).

Nipple attachment and survival in neonatal olfactory bulbectomized rats

Seven-day-old rat pups were olfactory bulbectomized, cortical lesioned, or sham lesioned, tested for nipple attachment 24 hr after surgery, then housed with their dam and weighed daily for the next 7 days. On the basis of histological findings the bulbectomized animals were divided into two subgroups: Those with removal of all olfactory bulb tissue (n = 9) and those in which some cellular elements remained in the lesioned area (n = 7). There were no differences between sham and cortical lesioned groups for nipple attachment behavior or weight gain. The completely bulbectomized pups did not show nipple attachment, lost weight each day, became moribund and had to be sacrificed by the fifth postoperative day. Deficits in incompletely bulbectomized pups were somewhat less severe and each of these survived and maintained or gained weight in the latter part of the test period. These results suggest that suckling may be critically dependent upon olfaction in 8 day or older pups and that significant savings in this behavior may be mediated by small remnants of olfactory bulb tissue.

Control of suckling and feeding by methysergide in weaning albino rats: a determination of Y-maze preferences

To assess the effects of serotonergic receptor blockade on the weaning process, 20-, 25-, and 30-day-old albino rat pups were injected with methysergide and were permitted to choose between suckling and feeding in a Y maze. At every age, progressively larger doses of methysergide enhanced pup preferences for the suckling goal, increased the frequency of suckling when in the suckling goal, and decreased the frequency of feeding when in the feeding goal. These results, with the support of other findings, suggest that the weaning process may be, in part, the result of emerging or changing serotonergic neurological mechanisms.

Odor aversion learning by the rat fetus

Rat fetuses were exposed to an odor stimulus on day 20 of gestation via amniotic injection and then injected with LiCl. In a CER paradigm, 10 day old pups were trained to approach an anesthetized dam in a runway for suckling reinforcement. When running speeds had stabilized the odor stimulus experienced in-utero was introduced into the test chamber. This odor took on aversive properties as a function of its pairing with LiCl, as evidenced by a decrease in running speed on CER trials and increases in the number of trials that were terminated because pups failed to traverse the runway. These data indicate that the fetal rat is capable of odor aversion learning.

The role of perioral sensation in nipple attachment by weanling rat pups

Injecting .05 ml of 1% lidocaine into each vibrissal pad, or cutting the infraorbital nerves, abolished nipple attachment in weanling Wistar rat pups. Nipple attachment recovered following infraorbital section. Injecting the local anesthetic intraperitoneally, or into the region of the masseter muscles, did not disrupt attachment, indicating that the effect of the drug on suckling was specific to the site of injection and could not be attributed to systemic toxicity or paralysis of the masseter muscles. Performance on an olfactory-guided orientation task was not disrupted by lidocaine, indicating that the drug did not render pups anosmic. Tactile sensation in the vibrissal pads, rhinarium, and upper lip was abolished after injecting the drug into the vibrissal pads. Vibrissal movement was absent following injection of lidocaine into either the vibrissal pads or the region of the masseter muscles. Shaving the vibrissae did not disrupt nipple attachment. The results are interpreted as suggesting that the nipples' textural qualities elicit attachment in weanling pups.

Prenatal and postnatal determinants of the 1st suckling episode in albino rats

Conditions under which an odor could elicit the 1st nipple attachment in albino rats were investigated. In Experiment I rats exposed prenatally and immediately after birth to citral, a lemon scent, suckled the washed nipples of an anesthetized, parturient dam when the nipples had been scented with citral. Moreover, these rats did not suckle the normal, unwashed nipples of these dams. In Experiment II rats were exposed to citral either (a) in utero, (b) immediately after birth, (c) both pre- and postnatally, or (d) not at all. Only rats in Group (c) attached to washed, citral-scented nipples and did not suckle the normal unwashed nipples that elicited suckling in control rats. These findings suggest that prenatal and postnatal events can determine which olfactory stimuli elicit the newborn rat's 1st nipple attachment.

Suckling

Suckling is the only behavior that is common among mammals. In newborn albino rats it is originally elicited by amniotic fluid deposited by the mother during parturition. Subsequent suckling is stimulated by saliva deposited on the nipples by the infant rats. Internal controls over the volume of milk suckled do not appear until infant rats are about 2 weeks of age at which time gastric distension, milk, systemic dehydration, and intestinal hormone cholecystokinin suppress milk intake derived through suckling. The development of controls over suckling appetite appears to parallel that of consummatory control. Until about 2 weeks of age infant rats choose to suckle a nonlactating nipple with the same frequency as a lactating nipple. Thereafter, the lactating nipple is unanimously chosen. These studies suggest differences and commonalities in the suckling behavior of laboratory rats and other mammals.