Microglia density decreases in the rat rostral nucleus of the solitary tract across development and increases in an age-dependent manner following denervation.

Microglia are critical for developmental pruning and immune response to injury, and are implicated in facilitating neural plasticity. The rodent gustatory system is highly plastic, particularly during development, and outcomes following nerve injury are more severe in developing animals. The mechanisms underlying developmental plasticity in the taste system are largely unknown, making microglia an attractive candidate. To better elucidate microglia's role in the taste system, we examined these cells in the rostral nucleus of the solitary tract (rNTS) during normal development and following transection of the chorda tympani taste nerve (CTX). Rats aged 5, 10, 25, or 50days received unilateral CTX or no surgery and were sacrificed four days later. Brain tissue was stained for Iba1 or CD68, and both the density and morphology of microglia were assessed on the intact and transected sides of the rNTS. We found that the intact rNTS of neonatal rats (9-14days) shows a high density of microglia, most of which appear reactive. By 29days of age, microglia density significantly decreased to levels not significantly different from adults and microglia morphology had matured, with most cells appearing ramified. CD68-negative microglia density increased following CTX and was most pronounced for juvenile and adult rats. Our results show that microglia density is highest during times of normal gustatory afferent pruning. Furthermore, the quantity of the microglia response is higher in the mature system than in neonates. These findings link increased microglia presence with instances of normal developmental and injury induced alterations in the rNTS.

Selective Deletion of Sodium Salt Taste during Development Leads to Expanded Terminal Fields of Gustatory Nerves in the Adult Mouse Nucleus of the Solitary Tract.

Neuronal activity plays a key role in the development of sensory circuits in the mammalian brain. In the gustatory system, experimental manipulations now exist, through genetic manipulations of specific taste transduction processes, to examine how specific taste qualities (i.e., basic tastes) impact the functional and structural development of gustatory circuits. Here, we used a mouse knock-out model in which the transduction component used to discriminate sodium salts from other taste stimuli was deleted in taste bud cells throughout development. We used this model to test the hypothesis that the lack of activity elicited by sodium salt taste impacts the terminal field organization of nerves that carry taste information from taste buds to the nucleus of the solitary tract (NST) in the medulla. The glossopharyngeal, chorda tympani, and greater superficial petrosal nerves were labeled to examine their terminal fields in adult control mice and in adult mice in which the α-subunit of the epithelial sodium channel was conditionally deleted in taste buds (αENaC knockout). The terminal fields of all three nerves in the NST were up to 2.7 times greater in αENaC knock-out mice compared with the respective field volumes in control mice. The shapes of the fields were similar between the two groups; however, the density and spread of labels were greater in αENaC knock-out mice. Overall, our results show that disruption of the afferent taste signal to sodium salts disrupts the normal age-dependent "pruning" of all terminal fields, which could lead to alterations in sensory coding and taste-related behaviors. SIGNIFICANCE STATEMENT: Neural activity plays a major role in the development of sensory circuits in the mammalian brain. To date, there has been no direct test of whether taste-elicited neural activity has a role in shaping central gustatory circuits. However, recently developed genetic tools now allow an assessment of how specific taste stimuli, in this case sodium salt taste, play a role in the maturation of the terminal fields in the mouse brainstem. We found that the specific deletion of sodium salt taste during development produced terminal fields in adults that were dramatically larger than in control mice, demonstrating for the first time that sodium salt taste-elicited activity is necessary for the normal maturation of gustatory inputs into the brain.

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats.

This study was designed to investigate brain connections among chemosensitive areas in newborn rats. Rhodamine beads were injected unilaterally into the locus coeruleus (LC) or into the caudal part of the nucleus tractus solitarius (cNTS) in Sprague-Dawley rat pups (P7-P10). Rhodamine-labeled neurons were patched in brainstem slices to study their electrophysiological responses to hypercapnia and to determine if chemosensitive neurons are communicating between LC and cNTS regions. After 7-10 days, retrograde labeling was observed in numerous areas of the brainstem, including many chemosensitive regions, such as the contralateral LC, cNTS and medullary raphe. Whole-cell patch clamp was done in cNTS. In 4 of 5 retrogradely labeled cNTS neurons that projected to the LC, firing rate increased in response to hypercapnic acidosis (15% CO2), even in synaptic blockade medium (SNB) (high Mg(2+)/low Ca(2+)). In contrast, 2 of 3 retrogradely labeled LC neurons that projected to cNTS had reduced firing rate in response to hypercapnic acidosis, both in the presence and absence of SNB. Extensive anatomical connections among chemosensitive brainstem regions in newborn rats were found and at least for the LC and cNTS, the connections involve some CO2-sensitive neurons. Such anatomical and functional coupling suggests a complex central respiratory control network, such as seen in adult rats, is already largely present in neonatal rats by at least day P7-P10. Since the NTS and the LC play a major role in memory consolidation, our results may also contribute to the understanding of the development of memory consolidation.

Abrupt changes in pentobarbital sensitivity in preBötzinger complex region, hypoglossal motor nucleus, nucleus tractus solitarius, and cortex during rat transitional period (P10-P15).

On postnatal days P10-P15 in rat medulla, neurotransmitter receptor subunit composition shifts toward a more mature phenotype. Since medullary GABAARs regulate cardiorespiratory function, abrupt alterations in GABAergic synaptic inhibition could disrupt homeostasis. We hypothesized that GABAARs on medullary neurons become more resistant to positive allosteric modulation during P10-P15. Medullary and cortical slices from P10 to P20 rats were used to record spontaneous action potentials in pre-Botzinger Complex (preBötC-region), hypoglossal (XII) motor nucleus, nucleus tractus solitarius (NTS), and cortex during exposure to pentobarbital (positive allosteric modulator of GABAARs). On P14, pentobarbital resistance abruptly increased in preBötC-region and decreased in NTS, but these changes in pentobarbital resistance were not present on P15. Pentobarbital resistance decreased in XII motor nucleus during P11-P15 with a nadir at P14. Abrupt changes in pentobarbital resistance indicate changes in GABAergic receptor composition and function that may compensate for potential increased GABAergic inhibition and respiratory depression that occurs during this key developmental transitional period.

5α-Reduced neurosteroids sex-dependently reverse central prenatal programming of neuroendocrine stress responses in rats.

Maternal social stress during late pregnancy programs hypothalamo-pituitary-adrenal (HPA) axis hyper-responsiveness to stressors, such that adult prenatally stressed (PNS) offspring display exaggerated HPA axis responses to a physical stressor (systemic interleukin-1ß; IL-1ß) in adulthood, compared with controls. IL-1ß acts via a noradrenergic relay from the nucleus tractus solitarii (NTS) to corticotropin releasing hormone neurons in the paraventricular nucleus (PVN). Neurosteroids can reduce HPA axis responses, so allopregnanolone and 3ß-androstanediol (3ß-diol; 5α-reduced metabolites of progesterone and testosterone, respectively) were given subacutely (over 24 h) to PNS rats to seek reversal of the "programmed" hyper-responsive HPA phenotype. Allopregnanolone attenuated ACTH responses to IL-1ß (500 ng/kg, i.v.) in PNS females, but not in PNS males. However, 3ß-diol normalized HPA axis responses to IL-1ß in PNS males. Impaired testosterone and progesterone metabolism or increased secretion in PNS rats was indicated by greater plasma testosterone and progesterone concentrations in male and female PNS rats, respectively. Deficits in central neurosteroid production were indicated by reduced 5α-reductase mRNA levels in both male and female PNS offspring in the NTS, and in the PVN in males. In PNS females, adenovirus-mediated gene transfer was used to upregulate expression of 5α-reductase and 3α-hydroxysteroid dehydrogenase mRNAs in the NTS, and this normalized hyperactive HPA axis responses to IL-1ß. Thus, downregulation of neurosteroid production in the brain may underlie HPA axis hyper-responsiveness in prenatally programmed offspring, and administration of 5α-reduced steroids acutely to PNS rats overrides programming of hyperactive HPA axis responses to immune challenge in a sex-dependent manner.

Developmental study of the distribution of hypoxia-induced factor-1 alpha and microtubule-associated protein 2 in children's brainstem: comparison between controls and cases with signs of perinatal hypoxia.

Perinatal asphyxia and hypoxia are common causes of morbidity in neonates. Prenatal birth associated with hypoxemia often results in several disorders because of the lack of oxygen in the brain. Survival rates from perinatal hypoxia have improved, but appropriate treatments for recovery are still limited, with great impact on patients, their families, society in general and health systems. The aim of this work is to contribute to a better understanding of the cellular mechanisms underlying the brainstem responses to hypoxia. For this purpose, distributions of two proteins, hypoxia-inducible factor-1 alpha (HIF-1α) and microtubule-associated protein 2 (MAP-2) were analyzed in brainstems of 11 children, four of them showing neuropathological evidence of brain hypoxia. They were included in control or hypoxic groups, and then in several subgroups according to their age. Immunohistochemical labeling for these proteins revealed only cell bodies containing HIF-1α, and both cell bodies and fibers positive for MAP-2 in the children's brainstems. The distribution of HIF-1α was more restricted than that of MAP-2, and it can be suggested that the expression of HIF-1α increased with age. The distribution pattern of MAP-2 in the medulla oblongata could be more due to age-related changes than to a response to hypoxic damage, whereas in the pons several regions, such as the nucleus ambiguus or the solitary nucleus, showed different immunolabeling patterns in controls and hypoxic cases. The distribution patterns of these two proteins suggest that some brainstem regions, such as the reticular formation or the central gray, could be less affected by conditions of hypoxia.

Effect of hyperoxic exposure during early development on neurotrophin expression in the carotid body and nucleus tractus solitarii.

Synaptic activity can modify expression of neurotrophins, which influence the development of neuronal circuits. In the newborn rat, early hyperoxia silences the synaptic activity and input from the carotid body, impairing the development and function of chemoreceptors. The purpose of this study was to determine whether early hyperoxic exposure, sufficient to induce hypoplasia of the carotid body and decrease the number of chemoafferents, would also modify neurotrophin expression within the nucleus tractus solitarii (nTS). Rat pups were exposed to hyperoxia (fraction of inspired oxygen 0.60) or normoxia until 7 or 14 days of postnatal development (PND). In the carotid body, hyperoxia decreased brain-derived neurotrophic factor (BDNF) protein expression by 93% (P = 0.04) after a 7-day exposure, followed by a decrease in retrogradely labeled chemoafferents by 55% (P = 0.004) within the petrosal ganglion at 14 days. Return to normoxia for 1 wk after a 14-day hyperoxic exposure did not reverse this effect. In the nTS, hyperoxia for 7 days: 1) decreased BDNF gene expression by 67% and protein expression by 18%; 2) attenuated upregulation of BDNF mRNA levels in response to acute hypoxia; and 3) upregulated p75 neurotrophic receptor, truncated tropomyosin kinase B (inactive receptor), and cleaved caspase-3. These effects were not observed in the locus coeruleus (LC). Hyperoxia for 14 days also decreased tyrosine hydroxylase levels by 18% (P = 0.04) in nTS but not in the LC. In conclusion, hyperoxic exposure during early PND reduces neurotrophin levels in the carotid body and the nTS and shifts the balance of neurotrophic support from prosurvival to proapoptotic in the nTS, the primary brain stem site for central integration of sensory and autonomic inputs.

Postnatal development of chorda tympani axons in the rat nucleus of the solitary tract.

The chorda tympani nerve (CT), one of three nerves that convey gustatory information to the nucleus of the solitary tract (NTS), displays terminal field reorganization after postnatal day 15 in the rat. Aiming to gain insight into mechanisms of this phenomenon, CT axon projection field and terminal morphology in NTS subdivisions were examined using tract tracing, light microscopy, and immunoelectron microscopy at four postnatal ages: P15, P25, P35, and adult. The CT axons that innervated NTS rostrolateral subdivision both in the adult and in P15 rats were morphologically distinct from those that innervated the rostrocentral, gustatory subdivision. In both subdivisions, CT terminals reached morphological maturity before P15. Rostrolateral, but not rostrocentral axons, went through substantial axonal branch elimination after P15. Rostrocentral CT synapses, however, redistribute onto postsynaptic targets in the following weeks. CT terminal preference for GABAergic postsynaptic targets was drastically reduced after P15. Furthermore, CT synapses became a smaller component of the total synaptic input to the rostrocentral NTS after P35. The results underlined that CT axons in rostrocentral and rostrolateral subdivisions represent two distinct populations of CT input, displaying different morphological properties and structural reorganization mechanisms during postnatal development.

Glia determine the course of brain-derived neurotrophic factor-mediated dendritogenesis and provide a soluble inhibitory cue to dendritic growth in the brainstem.

Cardiorespiratory control neurons in the brainstem nucleus tractus solitarius (NTS) undergo dramatic expansion of dendritic arbors during the early postnatal period, when functional remodeling takes place within the NTS circuitry. However, the underlying molecular mechanisms of morphological maturation of NTS neurons are largely unknown. Our previous studies point to the neurotrophin brain-derived neurotrophic factor (BDNF), which is abundantly expressed by NTS-projecting primary sensory neurons, as a candidate mediator of NTS dendritogenesis. In the current study, we used neonatal rat NTS neurons in vitro to examine the role of BDNF in the dendritic development of neurochemically identified subpopulations of NTS neurons. In the presence of abundant glia, BDNF promoted NTS dendritic outgrowth and complexity, with the magnitude of the BDNF effect dependent on neuronal phenotype. Surprisingly, BDNF switched from promoting to inhibiting NTS dendritogenesis upon glia depletion. Moreover, glia depletion alone led to a significant increase in NTS dendritic outgrowth. Consistent with this result, astrocyte-conditioned medium (ACM), which promoted hippocampal dendritogenesis, inhibited dendritic growth of NTS neurons. The latter effect was abolished by heat-inactivation of ACM, pointing to a diffusible astrocyte-derived negative regulator of NTS dendritic growth. Together, these data demonstrate a role for BDNF in the postnatal development of NTS neurons, and reveal novel effects of glia on this process. Moreover, previously documented dramatic increases in NTS glial proliferation in victims of sudden infant death syndrome (SIDS) underscore the importance of our findings and the need to better understand the role of glia and their interactions with BDNF during NTS circuit maturation. Furthermore, while it has previously been demonstrated that the specific effects of BDNF on dendritic growth are context-dependent, the role of glia in this process is unknown. Thus, our data carry important implications for mechanisms of dendritogenesis likely beyond the NTS.

α2-Adrenergic receptor distribution and density within the nucleus tractus solitarii of normotensive and hypertensive rats during development.

The nucleus tractus solitarii (NTS), located in the brainstem, is one of the main nuclei responsible for integrating different signals in order to originate a specific and orchestrated autonomic response. Antihypertensive drugs are well known to stimulate alpha(2)-adrenoceptor (alpha(2R)) in brainstem cardiovascular regions to induce reduction in blood pressure. Because alpha(2R) impairment is present in several models of hypertension, the aim of the present study was to investigate the distribution and density of alpha(2R) binding within the NTS of Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats during development (1,15,30 and 90 day-old) by an in vitro autoradiographical study. The NTS shows heterogeneous distribution of alpha(2R) in dorsomedial/dorsolateral, subpostremal and medial/intermediate subnuclei. Alpha(2R) increased from rostral to caudal dorsomedial/dorsolateral subnuclei in 30 and 90 day-old SHR but not in WKY. Alpha(2R) decreased from rostral to caudal subpostremal subnucleus in 15, 30 and 90 day-old SHR but not in WKY. Medial/intermediate subnuclei did not show any changes in alpha(2R) according to NTS levels. Furthermore, alpha(2R) are decreased in SHR as compared with WKY in all NTS subnuclei and in different ages. Surprisingly, alpha(2R) impairment was also found in pre-hypertensive stages, specifically in subpostremal subnucleus of 15 day-old rats. Finally, alpha(2R) decrease from 1 to 90 day-old rats in all subnuclei analyzed. This decrease is different between strains in rostral dorsomedial/dorsolateral and caudal subpostremal subnuclei within the NTS. In summary, our results highlight the importance of alpha(2R) distribution within the NTS regarding the neural control of blood pressure and the development of hypertension.

Pre- and postnatal differences in membrane, action potential, and ion channel properties of rostral nucleus of the solitary tract neurons.

There is little known about the prenatal development of the rostral nucleus of the solitary tract (rNST) neurons in rodents or the factors that influence circuit formation. With morphological and electrophysiological techniques in vitro, we investigated differences in the biophysical properties of rNST neurons in pre- and postnatal rats from embryonic day 14 (E14) through postnatal day 20. Developmental changes in passive membrane and action potential (AP) properties and the emergence and maturation of ion channels important in neuron function were characterized. Morphological maturation of rNST neurons parallels changes in passive membrane properties. Mean soma size, dendritic branch points, neurite endings, and neurite length all increase prenatally. whereas neuron resting membrane potential, input resistance, and time constant decrease. Dendritic spines, on the other hand, develop after birth. AP discharge patterns alter in pre- and postnatal stages. At E14, neurons generated a single TTX-sensitive, voltage-gated Na(+) AP when depolarized; a higher discharge rate appeared at older stages. AP amplitude, half-width, and rise and fall times all change during development. Responses to current injection revealed a number of voltage-gated conductances in embryonic rNST, including a hyperpolarization-activated inward current and a low-threshold Ca(2+) current that initiated Ca(2+) spikes. A hyperpolarization-activated, transient outward potassium current was also present in the developing neurons. Although the properties of these channels change during development, they are present before synapses form and therefore, can contribute to initial establishment of neural circuits, as well as to the changing electrophysiological properties in developing rNST neurons.

Chorda tympani nerve terminal field maturation and maintenance is severely altered following changes to gustatory nerve input to the nucleus of the solitary tract.

Neural competition among multiple inputs can affect the refinement and maintenance of terminal fields in sensory systems. In the rat gustatory system, the chorda tympani, greater superficial petrosal, and glossopharyngeal nerves have distinct but overlapping terminal fields in the first central relay, the nucleus of the solitary tract. This overlap is largest at early postnatal ages followed by a significant refinement and pruning of the fields over a 3 week period, suggesting that competitive mechanisms underlie the pruning. Here, we manipulated the putative competitive interactions among the three nerves by sectioning the greater superficial petrosal and glossopharyngeal nerves at postnatal day 15 (P15), P25, or at adulthood, while leaving the chorda tympani nerve intact. The terminal field of the chorda tympani nerve was assessed 35 d following nerve sections, a period before the sectioned nerves functionally regenerated. Regardless of the age when the nerves were cut, the chorda tympani nerve terminal field expanded to a volume four times larger than sham controls. Terminal field density measurements revealed that the expanded terminal field was similar to P15 control rats. Thus, it appears that the chorda tympani nerve terminal field defaults to its early postnatal field size and shape when the nerves with overlapping fields are cut, and this anatomical plasticity is retained into adulthood. These findings not only demonstrate the dramatic and lifelong plasticity in the central gustatory system, but also suggest that corresponding changes in functional and taste-related behaviors will accompany injury-induced changes in brainstem circuits.

Perinatal development of inhibitory synapses in the nucleus tractus solitarii of the rat.

The nucleus tractus solitarii (NTS) plays a key role in the central control of the autonomic nervous system. In adult rats, both GABA and glycine are used as inhibitory neurotransmitter in the NTS. Using a quantitative morphological approach, we have investigated the perinatal development of inhibitory synapses in the NTS. The density of both inhibitory axon terminals and synapses increased from embryonic day 20 until the end of the second postnatal week (postnatal day 14). Before birth, only GABAergic axon terminals developed and their number increased during the first postnatal week. Mixed GABA/glycine axon terminals appeared at birth and their number increased during the first postnatal week. This suggests the development of a mixed GABA/glycine inhibition in parallel to pure GABA inhibition. However, whereas GABAergic axon terminals were distributed throughout the NTS, mixed GABA/glycine axon terminals were strictly located in the lateral part of the NTS. Established at birth, this specific topography remained in the adult rat. From birth, GABA(A) receptors, glycine receptors and gephyrin were clustered in inhibitory synapses throughout the NTS, revealing a neurotransmitter-receptor mismatch within the medial part of the NTS. Together these results suggest that NTS inhibitory networks develop and mature until postnatal day 14. Developmental changes in NTS synaptic inhibition may play an important role in shaping neural network activity during a time of maturation of autonomic functions. The first two postnatal weeks could represent a critical period where the impact of the environment influences the physiological phenotypes of adult rats.

House-dust mite allergen and ozone exposure decreases histamine H3 receptors in the brainstem respiratory nuclei.

Allergic airway diseases in children are a common and a growing health problem. Changes in the central nervous system (CNS) have been implicated in contributing to some of the symptoms. We hypothesized that airway allergic diseases are associated with altered histamine H3 receptor expression in the nucleus tractus solitarius (NTS) and caudal spinal trigeminal nucleus, where lung/airway and nasal sensory afferents terminate, respectively. Immunohistochemistry for histamine H3 receptors was performed on brainstem sections containing the NTS and the caudal spinal trigeminal nucleus from 6- and 12-month-old rhesus monkeys who had been exposed for 5 months to house dust mite allergen (HDMA)+O3 or to filtered air (FA). While histamine H3 receptors were found exclusively in astrocytes in the caudal spinal trigeminal nucleus, they were localized to both neuronal terminals and processes in the NTS. HDMA+O3 exposure significantly decreased histamine H3 receptor immunoreactivity in the NTS at 6 months and in the caudal spinal trigeminal nucleus at 12 months of age. In conclusion, exposing young primates to HDMA+O3 changed histamine H3 receptor expression in CNS pathways involving lung and nasal afferent nerves in an age-related manner. Histamine H3 receptors may be a therapeutic target for allergic asthma and rhinitis in children.

Glutamatergic neurotransmission in the nucleus tractus solitarii: structural and functional characteristics.

Glutamate is the main excitatory transmitter in the central nervous system. As such, it plays a major role in transmitting and processing visceral sensory information within the nucleus tractus solitarii (NTS). Here, we review current knowledge on NTS glutamatergic transmission. We describe the main organizational features of NTS glutamatergic synapses as determined by work performed during the last decade using antibodies against glutamate receptors and transporters proteins. In light of these recent neuronatomical findings, we discuss some functional properties of developing and adult NTS glutamatergic synapses.

5-HT2A receptors are concentrated in regions of the human infant medulla involved in respiratory and autonomic control.

The serotonergic (5-HT) system in the human medulla oblongata is well-recognized to play an important role in the regulation of respiratory and autonomic function. In this study, using both immunocytochemistry (n=5) and tissue section autoradiography with the radioligand (125)I-1-(2,5-dimethoxy-4-iodo-phenyl)2-aminopropane (n=7), we examine the normative development and distribution of the 5-HT(2A) receptor in the human medulla during the last part of gestation and first postnatal year when dramatic changes are known to occur in respiratory and autonomic control, in part mediated by the 5-HT(2A) receptor. High 5-HT(2A) receptor binding was observed in the dorsal motor nucleus of the vagus (preganglionic parasympathetic output) and hypoglossal nucleus (airway patency); intermediate binding was present in the nucleus of the solitary tract (visceral sensory input), gigantocellularis, intermediate reticular zone, and paragigantocellularis lateralis. Negligible binding was present in the raphé obscurus and arcuate nucleus. The pattern of 5-HT(2A) immunoreactivity paralleled that of binding density. By 15 gestational weeks, the relative distribution of the 5-HT(2A) receptor was similar to that in infancy. In all nuclei sampled, 5-HT(2A) receptor binding increased with age, with significant increases in the hypoglossal nucleus (p=0.027), principal inferior olive (p=0.044), and medial accessory olive (0.038). Thus, 5-HT(2A) receptors are concentrated in regions involved in autonomic and respiratory control in the human infant medulla, and their developmental profile changes over the first year of life in the hypoglossal nucleus critical to airway patency and the inferior olivary complex essential to cerebellar function.

Developmental changes in brainstem neurons regulating lower airway caliber.

Premature infants are at risk for lower airway obstruction; however, maturation of reflex pathways regulating lower airway patency is inadequately studied. We hypothesized that postnatal maturation causes developmental change in brainstem efferent airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) that project to the airways and in pulmonary afferent fibers that terminate in the nucleus tractus solitarius (NTS). Ferrets aged 7, 14, 21, and 42 d received intrapulmonary injection of cholera toxin (CT)-beta subunit, a transganglionic retrograde tracer. Five days later, their brainstem was processed for dual immunolabeling of CT-beta and the cholinergic marker, choline acetyl transferase. CT-beta-labeled AVPNs and CT-beta-labeled afferent fiber optical density (OD) were analyzed. There was a significantly higher CT-beta-labeled cell number within the rNA at the youngest compared with older ages. All efferent CT-beta-labeled cells expressed choline acetyl transferase. OD of CT-beta-labeled afferent fibers was also higher at 7 d compared with 14 d. We conclude that the number of efferent AVPNs and afferent fiber OD both diminish over the second postnatal week. We speculate that exposure to injurious agents in early postnatal life may inhibit natural remodeling and thereby enhance later vulnerability to airway hyperreactivity.

Postnatal development of axosomatic synapses in the rat nucleus tractus solitarius: dorsal and ventral subnuclei differences.

Inhibitory axosomatic synapses could effectively suppress the excitability of postsynaptic cells. It is important to examine the development of inhibitory axosomatic synapses to understand the maturation of information processing. The caudal nucleus tractus solitarius (cNTS), which regulates the autonomic system, consists of several subnuclei. In the present study, development of axosomatic synapses in the dorsal and ventral subnuclei was examined by electron microscopy. In dorsal subnuclei, the percentage of GAD-positive terminals on the somata, the percentage of small cell somata with synapses and axosomatic synapse density drastically decreased from postnatal day (P) 5 to P10. In ventral subnuclei, the percentage of GAD-positive terminals on the soma, the percentage of small or large cell somata with synapses and axosomatic synapse density were maintained or increased from P5 to P10. Thus, decrease of inhibitory axosomatic synapses in dorsal subnuclei might facilitate maturation of fine receptive areas for peripheral inputs, while increase of inhibitory axosomatic synapses in ventral subnuclei might facilitate the establishment of an effective regulation system for cNTS output.

Development of chemosensitivity in neurons from the nucleus tractus solitarii (NTS) of neonatal rats.

We studied the development of chemosensitivity during the neonatal period in rat nucleus tractus solitarii (NTS) neurons. We determined the percentage of neurons activated by hypercapnia (15% CO(2)) and assessed the magnitude of the response by calculating the chemosensitivity index (CI). There were no differences in the percentage of neurons that were inhibited (9%) or activated (44.8%) by hypercapnia or in the magnitude of the activated response (CI 164+/-4.9%) in NTS neurons from neonatal rats of all ages. To assess the degree of intrinsic chemosensitivity in these neurons we used chemical synaptic block medium and the gap junction blocker carbenoxolone. Chemical synaptic block medium slightly decreased basal firing rate but did not affect the percentage of NTS neurons that responded to hypercapnia at any neonatal age. However, in neonates aged

The effects of dietary protein restriction on chorda tympani nerve taste responses and terminal field organization.

Prenatal dietary sodium restriction produces profound developmental effects on rat functional taste responses and formation of neural circuits in the brainstem. Converging evidence indicates that the underlying mechanisms for these effects are related to a compromised nutritional state and not to direct stimulus-receptor interactions. We explored whether early malnourishment produces similar functional and structural effects to those seen following dietary sodium restriction by using a protein deficient, sodium replete diet. To determine if early dietary protein-restriction affects the development of the peripheral gustatory system, multi-fiber neurophysiological recordings were made from the chorda tympani nerve and anterograde track tracing of the chorda tympani nerve into the nucleus of the solitary tract (NTS) was accomplished in rats fed a protein-restricted or a control diet (6% and 20%, respectively). The dietary regimens began on embryonic day 7 and continued until rats were used for neurophysiological recordings (postnatal days (P) 35-50) or for chorda tympani terminal field labeling (P40-50). Responses to a concentration series of NaCl, sodium acetate, KCl, and to 0.50 M sucrose, 0.03 M quinine-HCl, and 0.01 N HCl revealed attenuated responses (30-60%) to sodium-specific stimuli in rats fed the 6% protein diet compared with those fed the 20% protein diet. Responses to all other stimuli were similar between groups. Terminal field volumes were nearly twofold larger in protein-restricted rats compared with controls, with the differences located primarily in the dorsal-caudal zone of the terminal field. These results are similar to the results seen previously in rats fed a sodium-restricted diet throughout pre- and postnatal development, suggesting that dietary sodium- and protein-restriction share similar mechanisms in altering gustatory development.