Effect of maternal deprivation on milk intake in normal and bilaterally facial nerve-injured developing rats

Neural Deletion of Glucose Transporter Isoform 3 Creates Distinct Postnatal and Adult Neurobehavioral Phenotypes

We created a neural-specific conditional murine glut3 (Slc2A3) deletion (glut3 ^{flox/flox/nestin-Cre+}) to examine the effect of a lack of Glut3 on neurodevelopment. Compared with age-matched glut3 ^flox/flox = WT and heterozygotes (glut3 ^{flox/+/nestin-Cre+}), we found that a >90% reduction in male and female brain Glut3 occurred by postnatal day 15 (PN15) in glut3 ^{flox/flox/nestin-Cre+} This genetic manipulation caused a diminution in brain weight and cortical thickness at PN15, a reduced number of dendritic spines, and fewer ultrasonic vocalizations. Patch-clamp recordings of cortical pyramidal neurons revealed increased frequency of bicuculline-induced paroxysmal discharges as well as reduced latency, attesting to a functional synaptic and cortical hyperexcitability. Concomitant stunting with lower glucose concentrations despite increased milk intake shortened the lifespan, failing rescue by a ketogenic diet. This led to creating glut3 ^{flox/flox/CaMK2α-Cre+} mice lacking Glut3 in the adult male limbic system. These mice had normal lifespan, displayed reduced IPSCs in cortical pyramidal neurons, less anxiety/fear, and lowered spatial memory and motor abilities but heightened exploratory and social responses. These distinct postnatal and adult phenotypes, based upon whether glut3 gene is globally or restrictively absent, have implications for humans who carry copy number variations and present with neurodevelopmental disorders.SIGNIFICANCE STATEMENT Lack of the key brain-specific glucose transporter 3 gene found in neurons during early postnatal life results in significant stunting, a reduction in dendritic spines found on neuronal processes and brain size, heightened neuronal excitability, along with a shortened lifespan. When occurring in the adult and limited to the limbic system alone, lack of this gene in neurons reduces the fear of spatial exploration and socialization but does not affect the lifespan. These features are distinct heralding differences between postnatal and adult phenotypes based upon whether the same gene is globally or restrictively lacking. These findings have implications for humans who carry copy number variations pertinent to this gene and have been described to present with neurodevelopmental disorders.

Wired for eating: how is an active feeding circuitry established in the postnatal brain?

From birth, mammals have to find food and maximize caloric intake to ensure growth and survival. Suckling must be initiated quickly after birth and then maintained and controlled until weaning. It is a complex process involving interactions between sensory and motor neuronal pathways. Meanwhile, the control of food intake and energy homeostasis is progressively established via the development of hypothalamic circuits. The development of these circuits is influenced by hormonal and nutritional signals and can be disturbed in a variety of developmental disorders leading to long-term metabolic, behavioral and cognitive dysfunctions. This review summarizes our current knowledge of the neuronal circuits involved in early postnatal feeding processes.

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.

Early leptin intervention reverses perturbed energy balance regulating hypothalamic neuropeptides in the pre- and postnatal calorie-restricted female rat offspring

Pre- and postnatal calorie restriction is associated with postnatal growth restriction, reduced circulating leptin concentrations, and perturbed energy balance. Hypothalamic regulation of energy balance demonstrates enhanced orexigenic (NPY, AgRP) and diminished anorexigenic (POMC, CART) neuropeptide expression (PN21), setting the stage for subsequent development of obesity in female Sprague-Dawley rats. Leptin replenishment during the early postnatal period (PN2-PN8) led to reversal of the hypothalamic orexigenic:anorexigenic neuropeptide ratio at PN21 by reducing only the orexigenic (NPY, AgRP), without affecting the anorexigenic (POMC, CART) neuropeptide expression. This hypothalamic effect was mediated via enhanced leptin receptor (ObRb) signaling that involved increased pSTAT3/STAT3 but reduced PTP1B. This was further confirmed by an increase in body weight at PN21 in response to intracerebroventricular administration of antisense ObRb oligonucleotides (PN2-PN8). The change in the hypothalamic neuropeptide balance in response to leptin administration was associated with increased oxygen consumption, carbon dioxide production, and physical activity, which resulted in increased milk intake (PN14) with no change in body weight. This is in contrast to the reduction in milk intake with no effect on energy expenditure and physical activity observed in controls. We conclude that pre- and postnatal calorie restriction perturbs hypothalamic neuropeptide regulation of energy balance, setting the stage for hyperphagia and reduced energy expenditure, hallmarks of obesity. Leptin in turn reverses this phenotype by increasing hypothalamic ObRb signaling (sensitivity) and affecting only the orexigenic arm of the neuropeptide balance.

Quantitative analysis of survival of hypoglossal neurons in neonatally nerve-injured rats: Correlation with milk intake

INTRODUCTION

Tongue movement innervated by the hypoglossal (XII) nerve is essential for the survival of neonatal rats. The pups with bilateral XII nerve resection failed to suckle milk and did not survive, and the pups with unilateral XII nerve resection showed disturbed suckling capability and lower survival rates. The present study was performed to investigate the relation between neuronal population and milk intake of developing rats that had received various degrees of crush injuries to the unilateral XII nerve during the neonatal period.

METHODS

The right XII nerve of postnatal day 1 (P1) pups was crushed and milk intake was estimated at 3 days and 6 days after the nerve injury. As nerve injury at the neonatal stage results in death of axotomized neurons, varying degrees of crushing was estimated by the number of survived motor neurons.

RESULTS

In nerve-crushed rats, the populations of XII motor neurons and amounts of milk intake were reduced in a varied manner. Statistically significant positive correlations were observed between increasing XII neuron survival and increasing milk intake at 3 (r=0.62) and 6 (r=0.71) days after the nerve injury.

CONCLUSION

The results indicate that there is a strong relationship between the number of XII motor neurons and the amount of milk intake in neonatally XII nerve-injured rats.

Pre- and postnatal calorie restriction perturbs early hypothalamic neuropeptide and energy balance

Energy balance is regulated by circulating leptin concentrations and hypothalamic leptin receptor (ObRb) signaling via STAT3 but is inhibited by SOCS3 and PTP1B. Leptin signaling enhances anorexigenic neuropeptides and receptor (POMC, MC3-R, MC4-R) activation while suppressing orexigenic neuropeptides (NPY, AgRP). We investigated in a sex-specific manner the early (PN2) and late (PN21) postnatal hypothalamic mechanisms in response to intrauterine (IUGR), postnatal (PNGR), and combined (IPGR) calorie and growth restriction. At PN2, both male and female IUGR were hypoleptinemic, but hypothalamic leptin signaling in females was activated as seen by enhanced STAT3. In addition, increased SOCS3 and PTP1B supported early initiation of leptin resistance in females that led to elevated AgRP but diminished MC3-R and MC4-R. In contrast, males demonstrated leptin sensitivity seen as a reduction in PTP1B and MC3-R and MC4-R with no effect on neuropeptide expression. At PN21, with adequate postnatal caloric intake, a sex-specific dichotomy in leptin concentrations was seen in IUGR, with euleptinemia in males indicative of persisting leptin sensitivity and hyperleptinemia in females consistent with leptin resistance, both with normal hypothalamic ObRb signaling, neuropeptides, and energy balance. In contrast, superimposition of PNGR upon IUGR (IPGR) led to diminished leptin concentrations with enhanced PTP1B and an imbalance in arcuate nuclear NPY/AgRP and POMC expression that favored exponential hyperphagia and diminished energy expenditure postweaning. We conclude that IUGR results in sex-specific leptin resistance observed mainly in females, whereas PNGR and IPGR abolish this sex-specificity, setting the stage for acquiring obesity after weaning.

Neural plasticity of neonatal hypoglossal nerve for effective suckling

The adaptive movement of the tongue after unilateral lesion of the hypoglossal (XII) nerve during the early postnatal days is essential for recovery of milk intake. The present study investigated the basic mechanisms underlying such adaptation, focusing on the neural plasticity that allows effective suckling. After resection of the ipsilateral XII nerve on P1, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlolate (DiI), a postmortem neuronal tracer, was applied to the contralateral uninjured XII nerve on P4 and P7. DiI-labeled fibers were traced successfully within the tongue and showed gradually increased extension over the XII nerve-injured side in the central core portion of the denervated tongue between P4 and P7. Systematic neuroanatomic experiments showed that contralateral axonal sprouting occurred as early as 1 day after nerve injury (P2), and that such axonal sprouting occurred exclusively from the medial branch of the XII nerve responsible for tongue protrusion, an essential movement for suckling. These findings provide direct evidence of functional neural plasticity that allows effective suckling in XII nerve-injured newborns with suckling disturbance.

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.

Differential effects of hypoglossal and facial nerve injuries on survival and growth of rats at different developmental stages

The hypoglossal (XII) nerve is made up of functionally different nerve branches: the medial branch related to protrusion of the tongue and the lateral branch related to its retraction. The present study was performed to determine the effects of facial (VII) and XII nerve injuries on the survival and growth of rats in which the unilateral or bilateral VII and XII nerve components (main trunk, XII-trunk; medial branch, XII-med; lateral branch, XII-lat) had been resected at different developmental stages. In the suckling period, unilateral as well as bilateral injuries in the XII-trunk or XII-med nerve produced disturbed milk intake, lower survival rates and growth retardation in the nerve-injured rats. In the transition and mastication periods, only bilateral injury in the XII-trunk or XII-med nerve produced disturbed food intake followed by lower survival rates and growth retardation in those animals. The unilateral XII-lat nerve injury did not have significant effects on milk and food intake, whereas the bilateral injury caused disturbance in milk intake especially at the early neonatal stage. The unilateral VII nerve injury at the early neonatal stage caused deteriorating effects on food intake resulting in lower survival rate and severe growth retardation in the nerve-injured rats. The results indicate that the survival and growth of XII and VII nerve-resected rats differ considerably depending on the nerves injured and the developmental ages of the animals at the time of nerve insult.