Effects of orochemical stimulation on postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Early-life influences of low-calorie sweetener consumption on sugar taste

Children and adolescents are the highest consumers of added sugars, particularly from sugar-sweetened beverages (SSB). Regular consumption of SSB early in life induces a variety of negative consequences on health that can last into adulthood. Low-calorie sweeteners (LCS) are increasingly used as an alternative to added sugars because they provide a sweet sensation without adding calories to the diet. However, the long-term effects of early-life consumption of LCS are not well understood. Considering LCS engage at least one of the same taste receptors as sugars and potentially modulate cellular mechanisms of glucose transport and metabolism, it is especially important to understand how early-life LCS consumption impacts intake of and regulatory responses to caloric sugars. In our recent study, we found that habitual intake of LCS during the juvenile-adolescence period significantly changed how rats responded to sugar later in life. Here, we review evidence that LCS and sugars are sensed via common and distinct gustatory pathways, and then discuss the implications this has for shaping sugar-associated appetitive, consummatory, and physiological responses. Ultimately, the review highlights the diverse gaps in knowledge that will be necessary to fill to understand the consequences of regular LCS consumption during important phases of development.

Deconstructing the function of maternal stimulation in offspring development: Insights from the artificial rearing model in rats

This article is part of a Special Issue on "Parental Care". Maternal behavior has an important function in stimulating adequate growth and development of the young. Several approaches have been used in primates and rodents to deconstruct and examine the influence of specific components of maternal stimulation on offspring development. These approaches include observational studies of typical mother-infant interactions and studies of the effects of intermittent or complete deprivation of maternal contact. In this review, we focus on one unique approach using rats that enables the complete control of maternal variables by means of rearing rat pups artificially without contact with the mother or litter, while maintaining stable nutrition, temperature and exposure to stressful stimuli. This artificial rearing model permits the removal and controlled replacement of relevant maternal and litter stimuli and has contributed valuable insights regarding the influence of these stimuli on various developmental outcomes. It also enables the analysis of factors implicated in social isolation itself and their long-term influence. We provide an overview of the effects of artificial rearing on behavior, physiology, and neurobiology, including the influence of replacing maternal tactile stimulation and littermate contact on these outcomes. We then discuss the relevance of these effects in terms of the maternal role in regulating different aspects of offspring development and implications for human research. We emphasize that artificial rearing of rats does not lead to a global insult of nervous system development, making this paradigm useful in investigating specific developmental effects associated with maternal stimulation.

Impact of chorda tympani nerve injury on cell survival, axon maintenance, and morphology of the chorda tympani nerve terminal field in the nucleus of the solitary tract

Chorda tympani nerve transection (CTX) has been useful to study the relationship between nerve and taste buds in fungiform papillae. This work demonstrated that the morphological integrity of taste buds depends on their innervation. Considerable research focused on the effects of CTX on peripheral gustatory structures, but much less research has focused on the central effects. Here, we explored how CTX affects ganglion cell survival, maintenance of injured peripheral axons, and the chorda tympani nerve terminal field organization in the nucleus of the solitary tract (NTS). After CTX in adult rats, the chorda tympani nerve was labeled with biotinylated dextran amine at 3, 7, 14, 30, and 60 days post-CTX to allow visualization of the terminal field associated with peripheral processes. There was a significant and persistent reduction of the labeled chorda tympani nerve terminal field volume and density in the NTS following CTX. Compared with controls, the volume of the labeled terminal field was not altered at 3 or 7 days post-CTX; however, it was significantly reduced by 44% and by 63% at 30 and 60 days post-CTX, respectively. Changes in the density of labeled terminal field in the NTS paralleled the terminal field volume results. The dramatic decrease in labeled terminal field size post-CTX cannot be explained by a loss of geniculate ganglion neurons or degeneration of central axons. Instead, the function and/or maintenance of the peripheral axonal process appear to be affected. These new results have implications for long-term functional and behavioral alterations.

Orosensory deprivation alters taste-elicited c-Fos expression in the parabrachial nucleus of neonatal rats

In the present study we examined the effects of neonatal orosensory deprivation on taste-elicited gustatory activity in the rat parabrachial nucleus (PBN) using the functional anatomical marker c-Fos. Animals in three groups (GG, GO and GM) received gastric cannula implantation surgery on postnatal day 9 (P9). Animals in the fourth group (MR) did not receive any surgery. GG rats were fed by infusion of artificial milk directly into the stomach. GO rats were fed by intraoral infusion of artificial milk. GM and MR rats were reared by their mother with free access to mother's milk, water and rat chow. Rats from all groups were similar in body weight and length by P21. On P21 rats in all groups were intraorally presented with 0.5M sucrose solution and the brains were extracted and processed for c-Fos immunohistochemistry. Taste-elicited c-Fos expression in both the gustatory waist area, and the external lateral subnucleus of the PBN in rats in the GG group was significantly more robust than in the other three groups. These findings suggest a substantial alteration in orosensory-evoked neuronal response in this nucleus, due to sensory or motor deprivation during a critical developmental stage.

Postnatal reorganization of primary afferent terminal fields in the rat gustatory brainstem is determined by prenatal dietary history

Dietary manipulation has been used as an experimental strategy to gain insight into the normal development of the gustatory system. Institution of a diet low in sodium chloride (NaCl) from embryonic day 3 (E3) to E12 (E3-E12 sodium-restricted rats) yields dramatically enlarged terminal fields of the chorda tympani (CT), greater superficial petrosal (GSP), and glossopharyngeal (IX) nerves in the rostral pole of the nucleus of the solitary tract (NTS) at adulthood. To examine how this early, limited dietary manipulation affects postnatal terminal field development, we used a triple anterograde nerve label procedure at postnatal day 15 (P15), P25, P35, and > or =P40 (adults) in two groups: rats fed a commercial diet replete in sodium (controls) and E3-E12 sodium-restricted rats. Results showed an age-related decrease in terminal field volumes of all three nerves during normal development. In contrast, E3-E12 sodium-restricted rats displayed age-related increases of the CT and IX terminal fields, with the terminal field volume of the GSP remaining unchanged throughout development. NTS volume did not grow after P15; therefore, alterations in terminal field volumes are not due to parallel alterations in the size of the NTS. Our data suggest that the age-related decrease in terminal fields observed in controls may reflect activity-dependent pruning of afferent terminals, whereas terminal field increases seen in E3-E12 sodium-restricted rats may reflect cellular/molecular differences in the NTS induced predominantly by activity-independent mechanisms. These findings predict a significant difference in the development of neural coding and sensory-guided behaviors between E3-E12 sodium-restricted rats and controls.

Modifications of gustatory nerve synapses onto nucleus of the solitary tract neurons induced by dietary sodium-restriction during development

The terminal fields of nerves carrying gustatory information to the rat brainstem show a remarkable amount of expansion in the nucleus of the solitary tract (NTS) as a result of early dietary sodium restriction. However, the extent to which these axonal changes represent corresponding changes in synapses is not known. To identify the synaptic characteristics that accompany the terminal field expansion, the greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves were labeled in rats fed a sodium-restricted diet during pre- and postnatal development. The morphology of these nerve terminals within the NTS region where the terminal fields of all three nerves overlap was evaluated by transmission electron microscopy. Compared to data from control rats, CT axons were the most profoundly affected. The density of CT arbors and synapses quadrupled as a result of the near life-long dietary manipulation. In contrast, axon and synapse densities of GSP and IX nerves were not modified in sodium-restricted rats. Furthermore, compared to controls, CT terminals displayed more instances of contacts with postsynaptic dendritic protrusions and IX terminals synapsed more frequently with dendritic shafts. Thus, dietary sodium restriction throughout pre- and postnatal development had differential effects on the synaptic organization of the three nerves in the NTS. These anatomical changes may underlie the impact of sensory restriction during development on the functional processing of taste information and taste-related behaviors.

Hypothetical anatomical model to describe the aberrant gag reflex observed in a clinical population of orally deprived children

In this 'clinical conundrum', we propose a hypothetical anatomical model to explain the abnormal gag reflex that is consistently observed in a clinical population of children experiencing feeding delays. This model is based on the presence of 'transient' connections formed during the normal development of autonomic brainstem circuitry involving the nucleus tractus solitarius (NTS). We propose that, as a result of normal feeding and swallowing, the activity of these transient fibers typically diminishes shortly after birth. In children who are orally deprived during infancy, these transient connections persist and the aberrant gag reflex is maintained into childhood. The most critical feature of the proposed model is the idea that swallowing during feeding initiates the retraction of the tactile 'transient' input to NTS. In the NICU feeding clinics, it has been suggested that triggering the gag reflex in neonates by tactile stimulation of non-oral body areas and anterior portions of the mouth directly or indirectly may contribute to oral feeding delays. To the contrary, we propose an anatomical model to suggest that oral feeding delays and lack of swallowing food, when experienced by neonates, actually contribute to the development of the aberrant gag reflex observed in later developmental stages.

Gustatory terminal field organization and developmental plasticity in the nucleus of the solitary tract revealed through triple-fluorescence labeling

Early dietary sodium restriction has profound influences on the organization of the gustatory brainstem. However, the anatomical relationships among multiple gustatory nerve inputs have not been examined. Through the use of triple-fluorescence labeling and confocal laser microscopy, terminal fields of the greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves were visualized concurrently in the nucleus of the solitary tract (NTS) of developmentally sodium-restricted and control rats. Dietary sodium restriction during pre- and postnatal development resulted in a twofold increase in the volume of both the CT and the IX nerve terminal fields but did not affect the volume of the GSP terminal field. In controls, these nerve terminal fields overlapped considerably. The dietary manipulation significantly increased the overlapping zones among terminal fields, resulting in an extension of CT and IX fields past their normal boundaries. The differences in terminal field volumes were exaggerated when expressed relative to the respective NTS volumes. Furthermore, increased terminal field volumes could not be attributed to an increase in the number of afferents because ganglion cell counts did not differ between groups. Taken together, selective increases in terminal field volume and ensuing overlap among terminal fields suggest an increased convergence of these gustatory nerve terminals onto neurons in the NTS. The genesis of such convergence is likely related to disruption of cellular and molecular mechanisms during the development of individual terminal fields, the consequences of which have implications for corresponding functional and behavioral alterations.

Age-related decrease of the chorda tympani nerve terminal field in the nucleus of the solitary tract is prevented by dietary sodium restriction during development

Institution of a low-NaCl diet beginning at embryonic day 3 and continued throughout pre- and postnatal development has widespread effects on the neuroanatomical organization of the first gustatory relay in the nucleus of the solitary tract. To determine when these effects are expressed postnatally, the terminal field of the chorda tympani nerve was compared between sodium-restricted and sodium-replete rats at postnatal days 15-17, postnatal days 25-27, postnatal days 35-37, and adults. Total terminal fields were significantly larger in postnatal days 35-37 and adult sodium-restricted rats compared with aged-matched controls. The group-related differences appear related more to a remodeling of the terminal field in the dorsal zone of the terminal field in controls. Specifically, the terminal field volume in the dorsal zone in controls decreased dramatically from postnatal days 25-27 to postnatal days 35-37 and then again from postnatal days 35-37 to adulthood. In contrast, the fields did not change during development in sodium-restricted rats. These findings suggest that remodeling of the chorda tympani field occurs in controls at about the developmental period of taste response maturation. The lack of remodeling in sodium-restricted rats may be explained by a corresponding lack of functional response development to sodium salts. These results also illustrate the specificity and extent of how early dietary manipulations shape the developing brainstem.

Neural plasticity in the gustatory system

Sensory systems adapt to changing environmental influences by coordinated alterations in structure and function. These alterations are referred to as plastic changes. The gustatory system displays numerous plastic changes even in receptor cells. This review focuses on the plasticity of gustatory structures through the first synaptic relay in the brain. Unlike other sensory systems, there is a remarkable amount of environmentally induced changes in these peripheral-most neural structures. The most consistent and largest changes occur to stimuli that also impact on homeostatic systems, especially when the environmental manipulation is instituted during early development.

Alterations in geniculate ganglion proteins following fungiform receptor damage

Previous anatomical studies in rat have shown that damage produced to fungiform receptors of the anterior tongue at postnatal age 2 (P2) alters the growth and ramification of primary gustatory axons in the rostral nucleus of the solitary tract (NST). Studies employing artificial rearing (AR) procedures, which functionally deprive rat pups of orochemical stimulation during critical periods of postnatal life, produce similar alterations in the development of primary gustatory axons in the NST. Therefore, orochemical stimulation during rat's early postnatal life is necessary for normal development of primary gustatory axons in the rostral NST. One hypothesis concerning receptor-damage effects and AR effects is that receptor damage during critical periods of development may alter the regulation (i.e. transcription/translation) and/or distribution (i.e. transport) of proteins in geniculate ganglion neurons, thereby affecting growth of primary gustatory axons in the rostral NST. Specific aims of the present experiments were to comprehensively examine electrophoretic profiles of geniculate ganglion proteins following P2 receptor damage and late (> P40) receptor damage. Results show that concentrations of particular geniculate ganglion proteins are differentially altered following P2 receptor damage and late receptor damage, and that early receptor damage and late receptor damage produces distinct effects on the electrophoretic profiles of particular classes of proteins. Between the ages of P7-P38, P2 receptor damage lowers ganglion concentration of an acidic membrane glycoprotein designated as A1, with an apparent M(r) of 64-67 kDa and a pI of 4.8-5.2 P2 receptor damage also lowers ganglion concentrations of GAP-43. P2 receptor damage produces transient decreases in ganglion concentrations of NF-160, NF-200, and 8 additional acidic proteins. Three of these proteins may correspond to peripheral nerve sheath proteins analyzed in previous studies of the sciatic nerve, and one of these proteins may correspond to a 24 kDa growth-associated protein characterized in regenerating optic nerve. The time-course for changes observed in ganglion proteins following P2 damage was consistent with that observed for normal anatomical development of primary gustatory axons in both the lingual epithelium and NST. Receptor damage produced at P40 and later yielded different patterns of changes in geniculate ganglion proteins. Late receptor damage produced a transient increase in ganglion concentrations of NF-160, NF-200, GAP-43 and four additional acidic proteins within the 29-57 kDa M(r) range. Late receptor damage also produced a transient decrease in the concentrations of protein A1 and a 30 kDa protein that was not affected by P2 damage. Therefore, proteins that were preferentially affected by P2 damage may be involved in the regulation of initial axonal growth within the lingual epithelium and NST, as opposed to the structural repair or maintenance of extant axons. Relationships between normal anatomical development in peripheral and central components of primary gustatory axons are discussed in relation to availability of particular cytoskeletal and growth-associated proteins.