Postnatal changes in electrophysiological properties of rat nucleus tractus solitarii neurons

Perinatal high-fat diet exposure alters oxytocin and corticotropin releasing factor inputs onto vagal neurocircuits controlling gastric motility

Perinatal high-fat diet (pHFD) exposure alters the development of vagal neurocircuits that control gastrointestinal (GI) motility and reduce stress resiliency in offspring. Descending oxytocin (OXT; prototypical anti-stress peptide) and corticotropin releasing factor (CRF; prototypical stress peptide) inputs from the paraventricular nucleus (PVN) of the hypothalamus to the dorsal motor nucleus of the vagus (DMV) modulate the GI stress response. How these descending inputs, and their associated changes to GI motility and stress responses, are altered following pHFD exposure are, however, unknown. The present study used retrograde neuronal tracing experiments, cerebrospinal fluid extraction, in vivo recordings of gastric tone, motility and gastric emptying rates, and in vitro electrophysiological recordings from brainstem slice preparations to investigate the hypothesis that pHFD alters descending PVN-DMV inputs and dysregulates vagal brain-gut responses to stress. Compared to controls, rats exposed to pHFD had slower gastric emptying rates and did not respond to acute stress with the expected delay in gastric emptying. Neuronal tracing experiments demonstrated that pHFD reduced the number of PVNOXT neurons that project to the DMV, but increased PVNCRF neurons. Both in vitro electrophysiology recordings of DMV neurons and in vivo recordings of gastric motility and tone demonstrated that, following pHFD, PVNCRF -DMV projections were tonically active, and that pharmacological antagonism of brainstem CRF1 receptors restored the appropriate gastric response to brainstem OXT application. These results suggest that pHFD exposure disrupts descending PVN-DMV inputs, leading to a dysregulated vagal brain-gut response to stress. KEY POINTS: Maternal high-fat diet exposure is associated with gastric dysregulation and stress sensitivity in offspring. The present study demonstrates that perinatal high-fat diet exposure downregulates hypothalamic-vagal oxytocin (OXT) inputs but upregulates hypothalamic-vagal corticotropin releasing factor (CRF) inputs. Both in vitro and in vivo studies demonstrated that, following perinatal high-fat diet, CRF receptors were tonically active at NTS-DMV synapses, and that pharmacological antagonism of these receptors restored the appropriate gastric response to OXT. The current study suggests that perinatal high-fat diet exposure disrupts descending PVN-DMV inputs, leading to a dysregulated vagal brain-gut response to stress.

Refinement of Active and Passive Membrane Properties of Layer V Pyramidal Neurons in Rat Primary Motor Cortex During Postnatal Development

Achieving the distinctive complex behaviors of adult mammals requires the development of a great variety of specialized neural circuits. Although the development of these circuits begins during the embryonic stage, they remain immature at birth, requiring a postnatal maturation process to achieve these complex tasks. Understanding how the neuronal membrane properties and circuits change during development is the first step to understand their transition into efficient ones. Thus, using whole cell patch clamp recordings, we have studied the changes in the electrophysiological properties of layer V pyramidal neurons of the rat primary motor cortex during postnatal development. Among all the parameters studied, only the voltage threshold was established at birth and, although some of the changes occurred mainly during the second postnatal week, other properties such as membrane potential, capacitance, duration of the post-hyperpolarization phase or the maximum firing rate were not defined until the beginning of adulthood. Those modifications lead to a decrease in neuronal excitability and to an increase in the working range in young adult neurons, allowing more sensitive and accurate responses. This maturation process, that involves an increase in neuronal size and changes in ionic conductances, seems to be influenced by the neuronal type and by the task that neurons perform as inferred from the comparison with other pyramidal and motor neuron populations.

Perinatal high-fat diet alters development of GABAA receptor subunits in dorsal motor nucleus of vagus

Perinatal high-fat diet (pHFD) exposure increases the inhibition of dorsal motor nucleus of the vagus (DMV) neurons, potentially contributing to the dysregulation of gastric functions. The aim of this study was to test the hypothesis that pHFD increases the inhibition of DMV neurons by disrupting GABAA receptor subunit development. In vivo gastric recordings were made from adult anesthetized Sprague-Dawley rats fed a control or pHFD (14 or 60% kcal from fat, respectively) from embryonic day 13 (E13) to postnatal day 42 (P42), and response to brainstem microinjection of benzodiazepines was assessed. Whole cell patch clamp recordings from DMV neurons assessed the functional expression of GABAA α subunits, whereas mRNA and protein expression were measured via qPCR and Western blotting, respectively. pHFD decreased basal antrum and corpus motility, whereas brainstem microinjection of L838,417 (positive allosteric modulator of α2/3 subunit-containing GABAA receptors) produced a larger decrease in gastric tone and motility. GABAergic miniature inhibitory postsynaptic currents in pHFD DMV neurons were responsive to L838,417 throughout development, unlike control DMV neurons, which were responsive only at early postnatal timepoints. Brainstem mRNA and protein expression of the GABAA α1,2, and 3 subunits, however, did not differ between control and pHFD rats. This study suggests that pHFD exposure arrests the development of synaptic GABAA α2/3 receptor subunits on DMV neurons and that functional synaptic expression is maintained into adulthood, although cellular localization may differ. The tonic activation of slower GABAA α2/3 subunit-containing receptors implies that such developmental changes may contribute to the observed decreased gastric motility. NEW & NOTEWORTHY Vagal neurocircuits involved in the control of gastric functions, satiation, and food intake are subject to significant developmental regulation postnatally, with immature GABAA receptors expressing slower α2/3-subunits, whereas mature GABAA receptor express faster α1-subunits. After perinatal high-fat diet exposure, this developmental regulation of dorsal motor nucleus of the vagus (DMV) neurons is disrupted, increasing their tonic GABAergic inhibition, decreasing efferent output, and potentially decreasing gastric motility.

Postnatal Increases in Axonal Conduction Velocity of an Identified Drosophila Interneuron Require Fast Sodium, L-Type Calcium and Shaker Potassium Channels

During early postnatal life, speed up of signal propagation through many central and peripheral neurons has been associated with an increase in axon diameter or/and myelination. Especially in unmyelinated axons postnatal adjustments of axonal membrane conductances is potentially a third mechanism but solid evidence is lacking. Here, we show that axonal action potential (AP) conduction velocity in the Drosophila giant fiber (GF) interneuron, which is required for fast long-distance signal conduction through the escape circuit, is increased by 80% during the first day of adult life. Genetic manipulations indicate that this postnatal increase in AP conduction velocity in the unmyelinated GF axon is likely owed to adjustments of ion channel expression or properties rather than axon diameter increases. Specifically, targeted RNAi knock-down of either Para fast voltage-gated sodium, Shaker potassium (Kv1 homologue), or surprisingly, L-type like calcium channels counteracts postnatal increases in GF axonal conduction velocity. By contrast, the calcium-dependent potassium channel Slowpoke (BK) is not essential for postnatal speeding, although it also significantly increases conduction velocity. Therefore, we identified multiple ion channels that function to support fast axonal AP conduction velocity, but only a subset of these are regulated during early postnatal life to maximize conduction velocity. Despite its large diameter (∼7 µm) and postnatal regulation of multiple ionic conductances, mature GF axonal conduction velocity is still 20-60 times slower than that of vertebrate Aβ sensory axons and α motoneurons, thus unraveling the limits of long-range information transfer speed through invertebrate circuits.

Ventilatory and chemoreceptor responses to hypercapnia in neonatal rats chronically exposed to moderate hyperoxia

Rats reared in hyperoxia hypoventilate in normoxia and exhibit progressive blunting of the hypoxic ventilatory response, changes which are at least partially attributed to abnormal carotid body development. Since the carotid body also responds to changes in arterial CO₂/pH, we tested the hypothesis that developmental hyperoxia would attenuate the hypercapnic ventilatory response (HCVR) of neonatal rats by blunting peripheral and/or central chemoreceptor responses to hypercapnic challenges. Rats were reared in 21% O₂ (Control) or 60% O₂ (Hyperoxia) until studied at 4, 6-7, or 13-14days of age. Hyperoxia rats had significantly reduced single-unit carotid chemoafferent responses to 15% CO₂ at all ages; CO₂ sensitivity recovered within 7days after return to room air. Hypercapnic responses of CO₂-sensitive neurons of the caudal nucleus tractus solitarius (cNTS) were unaffected by chronic hyperoxia, but there was evidence for a small decrease in neuronal excitability. There was also evidence for augmented excitatory synaptic input to cNTS neurons within brainstem slices. Steady-state ventilatory responses to 4% and 8% CO₂ were unaffected by developmental hyperoxia in all three age groups, but ventilation increased more slowly during the normocapnia-to-hypercapnia transition in 4-day-old Hyperoxia rats. We conclude that developmental hyperoxia impairs carotid body chemosensitivity to hypercapnia, and this may compromise protective ventilatory reflexes during dynamic respiratory challenges in newborn rats. Impaired carotid body function has less of an impact on the HCVR in older rats, potentially reflecting compensatory plasticity within the CNS.

Developmental regulation of inhibitory synaptic currents in the dorsal motor nucleus of the vagus in the rat

Prior immunohistochemical studies have demonstrated that at early postnatal time points, central vagal neurons receive both glycinergic and GABAergic inhibitory inputs. Functional studies have demonstrated, however, that adult vagal efferent motoneurons receive only inhibitory GABAergic synaptic inputs, suggesting loss of glycinergic inhibitory neurotransmission during postnatal development. The purpose of the present study was to test the hypothesis that the loss of glycinergic inhibitory synapses occurs in the immediate postnatal period. Whole cell patch-clamp recordings were made from dorsal motor nucleus of the vagus (DMV) neurons from postnatal days 1-30, and the effects of the GABA_A receptor antagonist bicuculline (1-10 μM) and the glycine receptor antagonist strychnine (1 μM) on miniature inhibitory postsynaptic current (mIPSC) properties were examined. While the baseline frequency of mIPSCs was not altered by maturation, perfusion with bicuculline either abolished mIPSCs altogether or decreased mIPSC frequency and decay constant in the majority of neurons at all time points. In contrast, while strychnine had no effect on mIPSC frequency, its actions to increase current decay time declined during postnatal maturation. These data suggest that in early postnatal development, DMV neurons receive both GABAergic and glycinergic synaptic inputs. Glycinergic neurotransmission appears to decline by the second postnatal week, and adult neurons receive principally GABAergic inhibitory inputs. Disruption of this developmental switch from GABA-glycine to purely GABAergic transmission in response to early life events may, therefore, lead to adverse consequences in vagal efferent control of visceral functions.

Exposure to a high fat diet during the perinatal period alters vagal motoneurone excitability, even in the absence of obesity

KEY POINTS

Obesity is recognized as being multifactorial in origin, involving both genetic and environmental factors. The perinatal period is known to be critically important in the development of neural circuits responsible for energy homeostasis and the integration of autonomic reflexes. Diet-induced obesity alters the biophysical, pharmacological and morphological properties of vagal neurocircuits regulating upper gastrointestinal tract functions, including satiety. Less information is available, however, regarding the effects of a high fat diet (HFD) itself on the properties of vagal neurocircuits. The present study was designed to test the hypothesis that exposure to a HFD during the perinatal period alters the electrophysiological, pharmacological and morphological properties of vagal efferent motoneurones innervating the stomach. Our data indicate that perinatal HFD decreases the excitability of gastric-projecting dorsal motor nucleus neurones and dysregulates neurotransmitter release from synaptic inputs and that these alterations occur prior to the development of obesity. These findings represent the first direct evidence that exposure to a HFD modulates the processing of central vagal neurocircuits even in the absence of obesity. The perinatal period is critically important to the development of autonomic neural circuits responsible for energy homeostasis. Vagal neurocircuits are vital to the regulation of upper gastrointestinal functions, including satiety. Diet-induced obesity modulates the excitability and responsiveness of both peripheral vagal afferents and central vagal efferents but less information is available regarding the effects of diet per se on vagal neurocircuit functions. The aims of this study were to investigate whether perinatal exposure to a high fat diet (HFD) dysregulated dorsal motor nucleus of the vagus (DMV) neurones, prior to the development of obesity. Whole cell patch clamp recordings were made from gastric-projecting DMV neurones in thin brainstem slices from rats that were exposed to either a control diet or HFD from pregnancy day 13. Our data demonstrate that following perinatal HFD: (i) DMV neurones had decreased excitability and input resistance with a reduced ability to fire action potentials; (ii) the proportion of DMV neurones excited by cholecystokinin (CCK) was unaltered but the proportion of neurones in which CCK increased excitatory glutamatergic synaptic inputs was reduced; (iii) the tonic activation of presynaptic group II metabotropic glutamate receptors on inhibitory nerve terminals was attenuated, allowing modulation of GABAergic synaptic transmission; and (iv) the size and dendritic arborization of gastric-projecting DMV neurones was increased. These results suggest that perinatal HFD exposure compromises the excitability and responsiveness of gastric-projecting DMV neurones, even in the absence of obesity, suggesting that attenuation of vago-vagal reflex signalling may precede the development of obesity.

Kv4 channels underlie A-currents with highly variable inactivation time courses but homogeneous other gating properties in the nucleus tractus solitarii

In the nucleus of the tractus solitarii (NTS), a large proportion of neurones express transient A-type potassium currents (I KA) having deep influence on the fidelity of the synaptic transmission of the visceral primary afferent inputs to second-order neurones. Up to now, the strong impact of I KA within the NTS was considered to result exclusively from its variation in amplitude, and its molecular correlate(s) remained unknown. In order to identify which Kv channels underlie I KA in NTS neurones, the gating properties and the pharmacology of this current were determined using whole cell patch clamp recordings in slices. Complementary information was brought by immunohistochemistry. Strikingly, two neurone subpopulations characterized by fast or slow inactivation time courses (respectively about 50 and 200 ms) were discriminated. Both characteristics matched those of the Kv4 channel subfamily. The other gating properties, also matching the Kv4 channel ones, were homogeneous through the NTS. The activation and inactivation occurred at membrane potentials around the threshold for generating action potentials, and the time course of recovery from inactivation was rapid. Pharmacologically, I KA in NTS neurones was found to be resistant to tetraethylammonium (TEA), sea anemone toxin blood-depressing substance (BDS) and dendrotoxin (DTX), whereas Androctonus mauretanicus mauretanicus toxin 3 (AmmTX3), a scorpion toxin of the α-KTX 15 family that has been shown to block all the members of the Kv4 family, inhibited 80 % of I KA irrespectively of its inactivation time course. Finally, immunohistochemistry data suggested that, among the Kv4 channel subfamily, Kv4.3 is the prevalent subunit expressed in the NTS.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Nesfatin-1 influences the excitability of neurons in the nucleus of the solitary tract and regulates cardiovascular function

Nesfatin-1 has been identified as one of the most potent centrally acting anorexigenic peptides, and it has also been shown to play important roles in the control of cardiovascular function. In situ hybridization and immunohistochemical studies have revealed the expression of nesfatin-1 throughout the brain and, in particular, in the medullary autonomic gateway known as the nucleus of the solitary tract (NTS). The present study was thus undertaken to explore the cellular correlates and functional roles of nesfatin-1 actions in the medial NTS (mNTS). Using current-clamp electrophysiology recordings from mNTS neurons in slice preparation, we show that bath-applied nesfatin-1 directly influences the excitability of the majority of mNTS neurons by eliciting either depolarizing (42%, mean: 7.8 ± 0.8 mV) or hyperpolarizing (21%, mean: −8. 2 ± 1.0 mV) responses. These responses were observed in all electrophysiologically defined cell types in the NTS and were site specific and concentration dependent. Furthermore, post hoc single cell reverse transcriptase polymerase reaction revealed a depolarizing action of nesfatin-1 on NPY and nucleobindin-2-expressing mNTS neurons. We have also correlated these actions of nesfatin-1 on neuronal membrane potential with physiological outcomes, using in vivo microinjection techniques to demonstrate that nesfatin-1 microinjected into the mNTS induces significant increases in both blood pressure (mean AUC = 3354.1 ± 750.7 mmHg·s, n = 6) and heart rate (mean AUC = 164.8 ± 78.5 beats, n = 6) in rats. Our results provide critical insight into the circuitry and physiology involved in the profound effects of nesfatin-1 and highlight the NTS as a key structure mediating these autonomic actions.

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.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

Specific involvement of gonadal hormones in the functional maturation of growth hormone releasing hormone (GHRH) neurons

Growth hormone (GH) is the key hormone involved in the regulation of growth and metabolism, two functions that are highly modulated during infancy. GH secretion, controlled mainly by GH releasing hormone (GHRH), has a characteristic pattern during postnatal development that results in peaks of blood concentration at birth and puberty. A detailed knowledge of the electrophysiology of the GHRH neurons is necessary to understand the mechanisms regulating postnatal GH secretion. Here, we describe the unique postnatal development of the electrophysiological properties of GHRH neurons and their regulation by gonadal hormones. Using GHRH-eGFP mice, we demonstrate that already at birth, GHRH neurons receive numerous synaptic inputs and fire large and fast action potentials (APs), consistent with effective GH secretion. Concomitant with the GH secretion peak occurring at puberty, these neurons display modifications of synaptic input properties, decrease in AP duration, and increase in a transient voltage-dependant potassium current. Furthermore, the modulation of both the AP duration and voltage-dependent potassium current are specifically controlled by gonadal hormones because gonadectomy prevented the maturation of these active properties and hormonal treatment restored it. Thus, GHRH neurons undergo specific developmental modulations of their electrical properties over the first six postnatal weeks, in accordance with hormonal demand. Our results highlight the importance of the interaction between the somatotrope and gonadotrope axes during the establishment of adapted neuroendocrine functions.

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.

Sensory afferent and hypoxia-mediated activation of nucleus tractus solitarius neurons that project to the rostral ventrolateral medulla

The nucleus tractus solitarius (nTS) of the brainstem receives sensory afferent inputs, processes that information, and sends projections to a variety of brain regions responsible for influencing autonomic and respiratory output. The nTS sends direct projections to the rostral ventrolateral medulla (RVLM), an area important for cardiorespiratory reflexes and homeostasis. Since the net reflex effect of nTS processing ultimately depends on the properties of output neurons, we determined the characteristics of these RVLM-projecting nTS neurons using electrophysiological and immunohistochemical techniques. RVLM-projecting nTS neurons were identified by retrograde tracers. Patch clamp analysis in the horizontal brainstem nTS slice demonstrated that RVLM-projecting nTS cells exhibit constant latency solitary tract evoked excitatory postsynaptic currents (EPSCs), suggesting they receive strong monosynaptic contacts from visceral afferents. Three distinct patterns of action potential firing, associated with different underlying potassium currents, were observed in RVLM-projecting cells. Following activation of the chemoreflex in conscious animals by 3 h of acute hypoxia, 11.2+/-1.9% of the RVLM-projecting nTS neurons were activated, as indicated by positive Fos-immunoreactivity. Very few RVLM-projecting nTS cells were catecholaminergic. Taken together, these data suggest that RVLM projecting nTS neurons receive strong monosynaptic inputs from sensory afferents and a subpopulation participates in the chemoreflex pathway.

Mixed GABA-glycine synapses delineate a specific topography in the nucleus tractus solitarii of adult rat

Using combined morphological and electrophysiological approaches, we have determined the composition of inhibitory synapses of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibres. Immunohistochemical experiments demonstrate that, in adult rat, GABA axon terminals are present throughout the NTS while mixed GABA-glycine axon terminals are strictly located to the lateral part of the NTS within subnuclei surrounding the tractus solitarius. Purely glycine axon terminals are rare in the lateral part of the NTS and hardly detected in its medial part. Electrophysiological experiments confirm the predominance of GABA inhibition throughout the NTS and demonstrate the existence of a dual inhibition involving the co-release of GABA and glycine restricted to the lateral part of NTS. Since GABA(A) and glycine receptors are co-expressed postsynaptically in virtually all the inhibitory axon terminals throughout the NTS, it suggests that the inhibition phenotype relies on the characteristics of the axon terminals. Our results also demonstrate that glycine is mostly associated with GABA within axon terminals and raise the possibility of a dynamic regulation of GABA/glycine release at the presynaptic level. Our data provide new information for understanding the mechanisms involved in the processing of visceral information by the central nervous system in adult animals.

Postnatal emergence of synaptic plasticity associated with dynamic adaptation of the respiratory motor pattern

The shape of the three-phase respiratory motor pattern (inspiration, postinspiration, late expiration) is controlled by a central pattern generator (CPG) located in the ponto-medullary brainstem. Synaptic interactions between and within specific sub-compartments of the CPG are subject of intensive research. This review addresses the neural control of postinspiratory activity as the essential determinant of inspiratory/expiratory phase duration. The generation of the postinspiratory phase depends on synaptic interaction between neurones of the nucleus tractus solitarii (NTS), which relay afferent inputs from pulmonary stretch receptors, and the pontine Kölliker-Fuse nucleus (KF) as integral parts of the CPG. Both regions undergo significant changes during the first three postnatal weeks in rodents. Developmental changes in glutamatergic synaptic functions and its modulation by brain-derived neurotrophic factor may have implications in synaptic plasticity within the NTS/KF axis. We propose that dependent on these developmental changes, the CPG becomes permissive for short- and long-term plasticity associated with environmental, metabolic and behavioural adaptation of the breathing pattern.

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 Collapse

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MESH Headings

• Age Factors
• Analysis of Variance
• Animals
• Animals, Newborn
• Carbenoxolone/pharmacology
• Carbon Dioxide/pharmacology
• Chemoreceptor Cells/physiology
• Drug Interactions
• Electric Stimulation
• Hypercapnia/physiopathology
• In Vitro Techniques
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Patch-Clamp Techniques/methods
• Rats
• Rats, Sprague-Dawley
• Solitary Nucleus/cytology
• Solitary Nucleus/growth & development

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Grants

• R01 HL056683-08 NHLBI NIH HHS
• HL-56683 NHLBI NIH HHS
• R01 HL056683-10 NHLBI NIH HHS
• R01 HL056683-09A1 NHLBI NIH HHS
• R01 HL056683-11 NHLBI NIH HHS
• R01 HL056683 NHLBI NIH HHS

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Affiliation(s)

Susan C. Conrad Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Nicole L. Nichols Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Nick A. Ritucci Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435
Jay B. Dean Department of Molecular Pharmacology and Physiology, University of South Florida College of Medicine, 12901 Bruce B. Downs Boulevard, Tampa, FL 33612
Robert W. Putnam Department of Neuroscience, Cell Biology and Physiology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435

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Adiponectin acts in the nucleus of the solitary tract to decrease blood pressure by modulating the excitability of neuropeptide Y neurons

Adiponectin is an adipocyte derived hormone which acts in the CNS to control autonomic function, energy and cardiovascular homeostasis. Two 7-transmembrane domain receptors, AdipoR1 and AdipoR2, expressed in the hypothalamus and brainstem mediate the actions of adiponectin. The medulla's nucleus of the solitary tract (NTS) is the primary viscerosensory integration site and an important nucleus in the regulation of cardiovascular function. Here we show the localization of both AdipoR1 and AdipoR2 mRNA in the NTS. We have investigated the consequences of receptor activation in response to exogenous application of adiponectin on cardiovascular (blood pressure and heart rate monitoring in vivo), and single neuron (whole cell current-clamp recordings in vitro) function. Microinjection of adiponectin in the medial NTS (mNTS) at the level of the area postrema resulted in a decrease in BP (mean AUC= -2055+/-648.1, n=5, mean maximum effect: -11.7+/-3.6 mm Hg) while similar commissural NTS (cNTS) microinjections were without effect. Patch clamp recordings from NTS neurons in a medullary slice preparation showed rapid (within 200 s of application) reversible (usually within 1000 s following washout) effects of adiponectin on the membrane potential of 62% of mNTS neurons tested (38/61). In 34% (n=21) of mNTS neurons adiponectin induced a depolarization of membrane potential (6.8+/-0.9 mV), while the remainder of mNTS cells influenced by adiponectin (n=17) hyperpolarized in response to this adipokine (-5.4+/-0.7 mV). Post-hoc single cell RT-PCR (ssRT-PCR) analysis of neurons showed that the majority of NPY mRNA positive mNTS neurons were depolarized by adiponectin (7/11), while 4 of these depolarized cells were also GAD67 positive. The results presented in this study suggest adiponectin acts in the NTS to control BP and suggest that such effects may occur as a direct result of the ability of this adipokine to modulate the excitability of discrete groups of neurons in the NTS. These studies identify the mNTS as a new CNS site which adiponectin may act to influence central autonomic processing.

Brain-derived neurotrophic factor in arterial baroreceptor pathways: implications for activity-dependent plasticity at baroafferent synapses

Functional characteristics of the arterial baroreceptor reflex change throughout ontogenesis, including perinatal adjustments of the reflex gain and adult resetting during hypertension. However, the cellular mechanisms that underlie these functional changes are not completely understood. Here, we provide evidence that brain-derived neurotrophic factor (BDNF), a neurotrophin with a well-established role in activity-dependent neuronal plasticity, is abundantly expressed in vivo by a large subset of developing and adult rat baroreceptor afferents. Immunoreactivity to BDNF is present in the cell bodies of baroafferent neurons in the nodose ganglion, their central projections in the solitary tract, and terminal-like structures in the lower brainstem nucleus tractus solitarius. Using ELISA in situ combined with electrical field stimulation, we show that native BDNF is released from cultured newborn nodose ganglion neurons in response to patterns that mimic the in vivo activity of baroreceptor afferents. In particular, high-frequency bursting patterns of baroreceptor firing, which are known to evoke plastic changes at baroreceptor synapses, are significantly more effective at releasing BDNF than tonic patterns of the same average frequency. Together, our study indicates that BDNF expressed by first-order baroreceptor neurons is a likely mediator of both developmental and post-developmental modifications at first-order synapses in arterial baroreceptor pathways.

Polysialylated-neural cell adhesion molecule (PSA-NCAM) in the human trigeminal ganglion and brainstem at prenatal and adult ages

Background

The polysialylated neuronal cell adhesion molecule (PSA-NCAM) is considered a marker of developing and migrating neurons and of synaptogenesis in the immature vertebrate nervous system. However, it persists in the mature normal brain in some regions which retain a capability for morphofunctional reorganization throughout life. With the aim of providing information relevant to the potential for dynamic changes of specific neuronal populations in man, this study analyses the immunohistochemical occurrence of PSA-NCAM in the human trigeminal ganglion (TG) and brainstem neuronal populations at prenatal and adult age.

Results

Western blot analysis in human and rat hippocampus supports the specificity of the anti-PSA-NCAM antibody and the immunodetectability of the molecule in postmortem tissue. Immunohistochemical staining for PSA-NCAM occurs in TG and several brainstem regions during prenatal life and in adulthood. As a general rule, it appears as a surface staining suggestive of membrane labelling on neuronal perikarya and proximal processes, and as filamentous and dot-like elements in the neuropil. In the TG, PSA-NCAM is localized to neuronal perikarya, nerve fibres, pericellular networks, and satellite and Schwann cells; further, cytoplasmic perikaryal staining and positive pericellular fibre networks are detectable with higher frequency in adult than in newborn tissue. In the adult tissue, positive neurons are mostly small- and medium-sized, and amount to about 6% of the total ganglionic population. In the brainstem, PSA-NCAM is mainly distributed at the level of the medulla oblongata and pons and appears scarce in the mesencephalon. Immunoreactivity also occurs in discretely localized glial structures. At all ages examined, PSA-NCAM occurs in the spinal trigeminal nucleus, solitary nuclear complex, vestibular and cochlear nuclei, reticular formation nuclei, and most of the precerebellar nuclei. In specimens of different age, the distribution pattern remains fairly steady, whereas the density of immunoreactive structures and the staining intensity may change and are usually higher in newborn than in adult specimens.

Conclusion

The results obtained show that, in man, the expression of PSA-NCAM in selective populations of central and peripheral neurons occurs not only during prenatal life, but also in adulthood. They support the concept of an involvement of this molecule in the structural and functional neural plasticity throughout life. In particular, the localization of PSA-NCAM in TG primary sensory neurons likely to be involved in the transmission of protopathic stimuli suggests the possible participation of this molecule in the processing of the relevant sensory neurotransmission.

Dual modulation of synaptic transmission in the nucleus tractus solitarius by prostaglandin E2 synthesized downstream of IL-1beta

The activation of the innate immune system induces the production of blood-borne proinflammatory cytokines like interleukin-1beta (IL-1beta), which in turn triggers brain-mediated adaptative responses referred to as sickness behaviour. These responses involve the modulation of neural networks in key regions of the brain. The nucleus tractus solitarius (NTS) of the brainstem is a key nucleus for immune-to-brain signalling. It is the main site of termination of vagal afferents and is adjacent to the area postrema, a circumventricular organ allowing blood-borne action of circulating IL-1beta. Although it is well described that IL-1beta activates cerebral endothelial and glial cells, it is still unknown if and how IL-1beta or downstream-synthesized molecules impact NTS synaptic function. In this study we report that IL-1beta did not modulate NTS synaptic transmission per se, whereas prostaglandin E(2) (PGE(2)), which is produced downstream of IL-1beta, produced opposite effects on spontaneous and evoked release. On the one hand, PGE(2) facilitated glutamatergic transmission between local NTS neurons by enhancing the frequency of spontaneous excitatory postsynaptic currents through a presynaptic receptor different from the classical EP1-4 subtypes. On the other hand, PGE(2) also depressed evoked excitatory input from vagal afferent terminals through presynaptic EP3 receptors coupled to G-proteins linked to adenylyl cyclase and protein kinase A activity. Our data show that IL-1beta-induced PGE(2) can modulate evoked and spontaneous release in the NTS differentially through different mechanisms. These data unravel some molecular mechanisms by which innate immune stimuli could signal to, and be integrated within, the brainstem to produce central adaptative responses.

Visceral afferents directly activate catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 neurons are catecholamine (CA) neurons within the solitary tract nucleus (NTS) that influence many homeostatic functions, including cardiovascular reflexes, food intake, and stress. Because NTS is a major interface between sensory visceral afferents and the CNS, NTS CA neurons are ideally suited to coordinate complex responses by their projections to multiple brain regions. To test how NTS CA neurons process visceral afferent information carried by solitary tract (ST) afferents, we identified CA neurons using transgenic mice expressing TH-EGFP (enhanced green fluorescent protein under the control of the tyrosine hydroxylase promoter) and recorded synaptic responses to ST activation in horizontal slices. ST shocks evoked large-amplitude, short-latency, glutamatergic EPSCs (ST-EPSCs) in 90% of NTS CA neurons. Within neurons, ST-EPSCs had constant latency, rarely failed, and depressed substantially at high ST frequencies, indicating that NTS CA neurons receive direct monosynaptic connections from afferent terminals. NTS CA neurons received direct ST inputs from only one or two afferent fibers, with one-half also receiving smaller amplitude indirect inputs. Up to 90% of ST shocks evoked action potentials in NTS CA neurons. However, transmission of sensory afferent information through NTS CA neurons critically depended on the expression of an A-type potassium current (I(KA)), which when active attenuated ST-activated action potentials to a 37% success rate. The satiety peptide, cholecystokinin, presynaptically facilitated glutamate transmission in one-half of NTS CA neurons. Thus, NTS CA neurons are directly driven by visceral afferents with output being modulated by presynaptic peptide receptors and postsynaptic potassium channels.

Laryngeal apnea in rat pups: effects of age and body temperature

In neonatal mammals of many species, including human infants, apnea and other reflex responses frequently arise from stimulation of laryngeal receptors by ingested or regurgitated liquids. These reflexes, mediated by afferents in the superior laryngeal nerves (SLNs), are collectively known as the laryngeal chemoreflex (LCR) and are suspected to be responsible for some cases of the sudden infant death syndrome (SIDS). The LCR is strongly enhanced by mild increases in body temperature in decerebrate piglets, a finding that is of interest because SIDS victims are often found in overheated environments. Because of the experimental advantages of studying reflex development and mechanisms in neonatal rodents, we have developed methods for eliciting laryngeal apnea in anesthetized rat pups and have examined the influence of mild hyperthermia in animals ranging in age from 3 to 21 days. We found that apnea and respiratory disruption, elicited either by intralaryngeal water or by electrical stimulation of the SLN, occurred at all ages studied. Raising body temperature by 2–3°C prolonged the respiratory disturbance in response to either stimulus. This effect of hyperthermia was prominent in the youngest animals and diminished with age. We conclude that many studies of the LCR restricted to larger neonatal animals in the past can be performed in infant rodents using appropriate methods. Moreover, the developmental changes in the LCR and in the thermal modulation of the LCR seem to follow different temporal profiles, implying that distinct neurophysiological processes may mediate the LCR and thermal prolongation of the LCR.

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.

Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of -61.8 mV. The input resistance and time constant diminished from 82.9-53.1 M omega and from 9.4-4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1-5 days postnatally). The motoneurons' rheobase gradually decayed from 0.29-0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3-1.2 ms, and the medium afterhyperpolarization from 184.8-94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.

Neonatal maturation of the hypercapnic ventilatory response and central neural CO2 chemosensitivity

The ventilatory response to CO2 changes as a function of neonatal development. In rats, a ventilatory response to CO2 is present in the first 5 days of life, but this ventilatory response to CO2 wanes and reaches its lowest point around postnatal day 8. Subsequently, the ventilatory response to CO2 rises towards adult levels. Similar patterns in the ventilatory response to CO2 are seen in some other species, although some animals do not exhibit all of these phases. Different developmental patterns of the ventilatory response to CO2 may be related to the state of development of the animal at birth. The triphasic pattern of responsiveness (early decline, a nadir, and subsequent achievement of adult levels of responsiveness) may arise from the development of several processes, including central neural mechanisms, gas exchange, the neuromuscular junction, respiratory muscles and respiratory mechanics. We only discuss central neural mechanisms here, including altered CO2 sensitivity of neurons among the various sites of central CO2 chemosensitivity, changes in astrocytic function during development, the maturation of electrical and chemical synaptic mechanisms (both inhibitory and excitatory mechanisms) or changes in the integration of chemosensory information originating from peripheral and multiple central CO2 chemosensory sites. Among these central processes, the maturation of synaptic mechanisms seems most important and the relative maturation of synaptic processes may also determine how plastic the response to CO2 is at any particular age.

Somatic vs. dendritic responses to hypercapnia in chemosensitive locus coeruleus neurons from neonatal rats

Cardiorespiratory control is mediated in part by central chemosensitive neurons that respond to increased CO(2) (hypercapnia). Activation of these neurons is thought to involve hypercapnia-induced decreases in intracellular pH (pH(i)). All previous measurements of hypercapnia-induced pH(i) changes in chemosensitive neurons have been obtained from the soma, but chemosensitive signaling could be initiated in the dendrites of these neurons. In this study, membrane potential (V(m)) and pH(i) were measured simultaneously in chemosensitive locus coeruleus (LC) neurons from neonatal rat brain stem slices using whole cell pipettes and the pH-sensitive fluorescent dye pyranine. We measured pH(i) from the soma as well as from primary dendrites to a distance 160 mum from the edge of the soma. Hypercapnia [15% CO(2), external pH (pH(o)) 7.00; control, 5% CO(2), pH(o) 7.45] resulted in an acidification of similar magnitude in dendrites and soma ( approximately 0.26 pH unit), but acidification was faster in the more distal regions of the dendrites. Neither the dendrites nor the soma exhibited pH(i) recovery during hypercapnia-induced acidification; but both regions contained pH-regulating transporters, because they exhibited pH(i) recovery from an NH(4)Cl prepulse-induced acidification (at constant pH(o) 7.45). Exposure of a portion of the dendrites to hypercapnic solution did not increase the firing rate, but exposing the soma to hypercapnic solution resulted in a near-maximal increase in firing rate. These data show that while the pH(i) response to hypercapnia is similar in the dendrites and soma, somatic exposure to hypercapnia plays a major role in the activation of chemosensitive LC neurons from neonatal rats.

Transient potassium conductances protect nucleus tractus solitarius neurons from NMDA induced excitotoxic plateau depolarizations

Ischemic insults, followed by excessive accumulation of extracellular glutamate, destroy most, but not all, neurons in affected area(s) of the central nervous system (CNS). Characterization of the unique properties of cells resistant to such excitotoxic challenge may identify novel preventive/therapeutic strategies to reduce cell death. We have previously reported that transient potassium conductances expressed in magnocellular neurons of the paraventricular nucleus protect these cells from excitotoxic cell death. In the present study, in vitro patch-clamp recording techniques were used to assess the roles of similar potassium conductances in protecting delayed excitation (DE) neurons of the nucleus tractus solitarius (NTS) from over-excitation after N-methyl-d-aspartate (NMDA) receptor activation. DE neurons show a reduced sensitivity (compared to NTS neurons which lack these potassium conductances) to NMDA receptor activation which protects against long duration plateau depolarizations (LDPDs). We identify two types of transient K(+) conductances (I(A) and I(D)), which contribute to the rapid repolarization of the membrane after a strong depolarization, and show that inhibition of these currents with 4-aminopyridine increases neuronal excitability after NMDA receptor activation such that DE cells now respond with LDPDs. In contrast, lower concentrations of 4-AP (100 mM) which inhibit only the I(D) have no effect on NMDA induced depolarization. These results suggest that the reduced sensitivity of DE neurons in NTS to NMDA receptor activation is the result of the large transient potassium conductance I(A) expressed in these neurons, and identify this as a common mechanism protecting against NMDA receptor mediated excitotoxicity in both PVN and NTS neurons.

Secretin depolarizes nucleus tractus solitarius neurons through activation of a nonselective cationic conductance

The recent suggestion that secretin may be useful in treating autism and schizophrenia has begun to focus attention on the mechanisms underlying this gut-brain peptide's actions in the central nervous system (CNS). In vitro autoradiographic localization of125I-secretin binding sites in rat brain shows the highest binding density in the nucleus tractus solitarius (NTS). Recent evidence suggests that intravenous infusion of secretin causes fos activation in NTS, a relay station playing important roles in the central regulation of autonomic functions. In this study, whole cell patch-clamp recordings were obtained from 127 NTS neurons in rat medullary slices. The mean resting membrane potential of these neurons was -54.7 ± 0.3 mV, the mean input resistance was 3.7 ± 0.2 GΩ, and the action potential amplitude of these neurons was always >70 mV. Current-clamp studies showed that bath application of secretin depolarized the majority (80.8%; 42/52) of NTS neurons tested, whereas the remaining cells were either unaffected (17.3%; 9/52) or hyperpolarized (1.9%; 1/52). These depolarizing effects were maintained in the presence of 5 μM TTX and found to be concentration dependent from 10-12to 10-7M. Using voltage-clamp techniques, we also identified modulatory actions of secretin on specific ion channels. Our results demonstrate that while secretin is without effect on net whole cell potassium currents, it activates a nonselective cationic conductance (NSCC). These results show that NTS neurons are activated by secretin as a consequence of activation of a NSCC and support the emerging view that secretin can act as a neuropeptide within the CNS.

N-methyl-d-aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

Excitatory effects of orexin-A on nucleus tractus solitarius neurons are mediated by phospholipase C and protein kinase C

Orexin (ORX)-A is a 33-amino acid peptide with demonstrated roles in the regulation of energy metabolism, autonomic control, and sleep. Orexin receptors (OXRs), OX1R and OX2R, and immunoreactive axons are present in the nucleus tractus solitarius (NTS). We demonstrated previously that bath application of ORX-A depolarizes NTS neurons through activation of a nonselective cationic conductance (NSCC) and inhibition of a sustained potassium current (IK). The present study examined the signaling pathways underlying the excitatory effects of ORX-A on NTS neurons using whole-cell patch-clamp recording techniques. Inclusion of guanosine 5'-O-(2-thiodiphosphate) in the internal pipette solution abolished the effects of ORX-A, confirming that the actions of ORX-A are mediated by G-protein-coupled receptors. The responses of ORX-A were also blocked by a phospholipase C (PLC) inhibitor, D609, and by a nonselective protein kinase (PK) inhibitor, H7, demonstrating the involvement of PLC and protein kinases. However, PKA appears not to play a role, because the depolarizing effects of ORX-A were still observed when the PKA inhibitor peptide (6-22) was included in the pipette solution, and bath application of 8-bromo-cAMP (a PKA agonist) was without effect on NTS neurons. In contrast, 12-O-tetradecanoylphorbol-13-acetate (a PKC agonist) depolarized NTS neurons, and bisindolylmaleimide (BIS), a PKC inhibitor, abolished the depolarizing effects of ORX-A. Finally, voltage-clamp experiments demonstrated that BIS also blocked the activation of NSCC and inhibition of IK by ORX-A in NTS neurons. These results therefore show that the excitatory effects of ORX-A on NTS neurons are mediated through activation of the PLC-PKC-NSCC and -IK signaling pathways, which probably result from OXR-coupled activation of Gq.

A-, T-, and H-type currents shape intrinsic firing of developing rat abducens motoneurons

During postnatal development, profound changes take place in the excitability of nerve cells, including modification in the distribution and properties of receptor-operated channels and changes in the density and nature of voltage-gated channels. We studied here the firing properties of abducens motoneurons (aMns) in transverse brainstem slices from postnatal day (P) 1-13 rats. Recordings were made from aMNs in the whole-cell configuration of the patch-clamp technique. Two main types of aMn could be distinguished according to their firing profile during prolonged depolarizations. Both types were identified as aMns by their fluorescence following retrograde labelling with the lipophilic carbocyanine DiI in the rectus lateralis muscle. The first type (BaMns) exhibited a burst of action potentials (APs) followed by an adaptation of discharge and were encountered in approximately 70 % of aMns. Their discharge profile resembled that of adult aMns and was encountered in all aMns after P9. BaMns exhibited a hyperpolarization-induced rebound potential that was blocked by low concentrations of Ni2+ or by Ca2+-free external solution. This current had the properties of the T-type current. Action potentials of BaMns showed a complex afterhyperpolarization (AHP). An inward rectification was evidenced following hyperpolarization and was blocked by external application of caesium or ZD7288, indicating the presence of the hyperpolarization-activated cationic current (IH). Blocking the IH current almost doubled the input resistance of BaMns. The second class of aMns (DaMns) displayed a delayed excitation that was mediated by A-type K+ currents and was observed only between P4 and P9. DaMns exhibited immature characteristics: an action potential with a simple AHP, a linear current-voltage relation and a large input resistance. The number of aMns remained unchanged when both types were present (P5-P6) and later in development when only BaMns were encountered (P19), suggesting that DaMns mature into BaMns during postnatal development. We conclude that aMns display profound reorganization in their intrinsic excitability during postnatal development.

Orexin-A depolarizes nucleus tractus solitarius neurons through effects on nonselective cationic and K+ conductances

The nucleus tractus solitarius (NTS) plays central roles in a number of autonomic functions including cardiovascular control. Orexin (ORX)-A is a 33-amino-acid peptide implicated in the central regulation of energy metabolism, sleep, and the cardiovascular system. Studies demonstrate the presence of ORX-immunoreactive axons and both OX(1)R (orexin receptor) and OX(2)R mRNA within NTS. In this study, whole cell patch-clamp recordings were obtained from NTS neurons in rat medullary slices. Current-clamp studies showed that bath application of various concentrations of ORX-A depolarized 90.7% (78 of 86) of neurons tested while the remaining cells were either unaffected or showed small hyperpolarizations in response to peptide administration. Depolarizing effects were maintained in the presence of 5 microM TTX, and were concentration dependent. Using voltage-clamp techniques, we also identified modulatory actions of ORX-A on specific ion channels. Our results demonstrate that not only does ORX-A inhibit a specific potassium conductance (the sustained K(+) current) in NTS neurons, but it also activates a nonselective cationic conductance (NSCC). These data suggest that ORX-A effects on central cardiovascular control may result from direct actions on NTS neurons and also highlight the ability of this peptide to influence neuronal excitability as a consequence of concurrent modulation of multiple ion channels.

Dual effects of NMDA receptor activation on polysialylated neural cell adhesion molecule expression during brainstem postnatal development

Here we show a dual role of N-methyl-d-aspartate receptor (NMDAR) activation in controlling polysialylated neural cell adhesion molecule (PSA-NCAM) dynamic expression in the dorsal vagal complex (DVC), a gateway for many primary afferent fibres. In this structure the overall expression of PSA-NCAM decreases during the first 2 weeks after birth to persist only at synapses in the adult. Electrical stimulation of the vagal afferents causes a rapid increase of PSA-NCAM expression both in vivo and in acute slices before postnatal day (P) 14 whereas a similar stimulation induces a decrease after P15. Inhibition of NMDAR activity in vitro completely prevented these changes. These regulations depend on calmodulin activation and cGMP production at all stages. By contrast, blockade of neuronal nitric oxide synthase (nNOS) prevented these changes only after P10 in agreement with its late expression in the DVC. The pivotal role of NMDAR is also supported by the observation that chronic blockade induces a dramatic decrease in PSA-NCAM expression.

NMDA receptor and nitric oxide synthase activation regulate polysialylated neural cell adhesion molecule expression in adult brainstem synapses

Here we report that synapses in the adult dorsal vagal complex, a gateway for many primary afferent fibers, express a high level of the polysialylated neural cell adhesion molecule (PSA-NCAM). We show that electrical stimulation of the vagal afferents causes a rapid decrease of PSA-NCAM expression both in vivo and in acute slices. Inhibition of NMDA receptor activity completely prevented the decrease. Blockade of calmodulin activation, neuronal nitric oxide (NO) synthase, or soluble guanylyl cyclase and chelation of extracellular NO mimicked this inhibition. Our data provide a mechanistic framework for understanding how activity-linked stimulation of the NMDA-NO-cGMP pathway induces rapid changes in PSA-NCAM expression, which may be associated with long-term depression.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

Correlation between electrophysiological and morphological characteristics during maturation of rat supraoptic neurons

The neurohypophysial peptides oxytocin (OT) and vasopressin (AVP) are well known for their role in reproductive functions and fluid balance regulation, respectively. During development, these peptides are thought to act as trophic factors on both peripheral and central structures. However, despite this early developmental function, the maturation of their secreting neurons remains poorly investigated. In this study, we have characterized the electrical and morphological characteristics displayed by OT and AVP supraoptic (SO) neurons between embryonic day 21 and postnatal day 20. Transient changes in passive membrane properties, correlated with a transient increase in the dendritic arborization, were observed at the beginning of the second postnatal week (PW2). The action potential matured mostly during PW1 and its threshold progressively hyperpolarized in parallel with the resting membrane potential. During PW1, SO neurons displayed unique characteristics with a low-threshold Ca(2+)-dependent depolarizing potential and a prominent hyperpolarization-activated current (I(h) ). This latter is involved in a depolarizing sag during hyperpolarization and an after hyperpolarizing potential following a depolarization. During this period, maintaining E(Cl) unchanged by the use of gramicidin-perforated patch recordings revealed excitatory GABAergic potentials, that became inhibitory during PW2, whilst glutamatergic potential appeared. The electrical activity was very erratic in young neurons and progressively differentiated in the typical firing observed in mature neurons (tonic and phasic for OT and AVP neurons, respectively) during PW2--3. These results show that the development of electrical properties of SO neurons is correlated with the maturation of their dendritic arborization.

Postnatal differentiation of local networks in the nucleus of the tractus solitarius

Whole-cell voltage-clamp recordings from rat brain slice preparation were used to investigate a possible developmental change in the patterns of synaptic interactions among the nucleus tractus solitarii neurons by analysing spontaneous postsynaptic current activity. Three types of patterns of spontaneous postsynaptic current activity were distinguished in the nucleus tractus solitarii neurons which showed high activities in terms of current frequency and amplitude. The first type was characterized by the presence in an individual cell of high frequencies and large amplitudes of both spontaneous glutamatergic and GABAergic postsynaptic currents, and observed exclusively in postnatal day 0-7 rats. The second and third types of cells showed predominant either inhibitory or excitatory postsynaptic currents, respectively. After postnatal day 5, nucleus tractus solitarii neurons with high background activity were shown to differentiate into either the second or the third type, with the latter of about 70% in the adult caudal/intermediate nucleus tractus solitarii. Axon collaterals of some medium to large neurons seemed to be decreased by pruning during postnatal development. The early postnatal differentiation of background synaptic activity observed in the nucleus tractus solitarii presumably reflects the local network reorganization and may be related to maturational changes in cardiovascular and respiratory functions.

Synapse formation and spontaneous activity in rat brainstem neurons in primary culture

The correlation between synaptogenesis and onset of spontaneous action potentials was assessed in rat brainstem cells up to 29 days in primary culture. Cells exhibited different stages of maturation followed by electron microscopy and patch clamp recordings. Terminal boutons with no preferential orientation of presynaptic vesicles appeared after 2 days in culture. After 5 days, preferential orientation of presynaptic vesicles and thickening of postsynaptic membranes were observed. The spontaneous discharge of action potentials, single or bursting, was observed after 7 days in vitro. This was followed by the expression of a 128-pS K(+) channel starting at 13 days in vitro. A 69-pS K(+) channel was also present throughout the duration of the cultures. These results suggest that spontaneous discharge of action potentials does not occur before synapses are formed and K(+) channel types develop differentially in brainstem neurons in vitro.

Postnatal development of rat nucleus tractus solitarius neurons: morphological and electrophysiological evidence

Postnatal development of neurons in the caudal nucleus tractus solitarii of rats was studied using the Golgi-Cox technique and whole-cell recordings. Two cell classes were defined on the basis of somatic and dendritic morphology. Elongated neurons have two thick primary dendrites originating from the long axis of the soma. The primary dendrites, tapering distally, give rise to one to four secondary dendrites. Multipolar neurons have pyramidal somas. Extending from each apex of the cell body was a long primary dendrite, which gave rise to a variable number of secondary dendrites. The relative proportion of the two classes was rather constant from birth to adulthood. During the first two postnatal weeks, dendritic length and area of influence increase, but neuronal geometry is not altered in either class. Dendritic appendages appear by postnatal day 5, reach a peak at postnatal day P12 and then almost disappear in adult neurons. Combined intracellular injection of neurobiotin and whole-cell recordings indicate that morphological alteration of caudal nucleus tractus solitarius neurons occurs in parallel with changes in passive properties and spike characteristics. However, the firing pattern of discharge is not correlated with morphology. The physiological significance of these results is discussed.

Electrophysiological and morphological characterization of cytochemically-defined neurons in the caudal nucleus of tractus solitarius of the rat

Morphological and electrophysiological properties of calbindin D-28k-, GABA- and dopamine-beta-hydroxylase-immunopositive neurons were investigated in the caudal nucleus of tractus solitarius of rats, using a patch-clamp whole-cell recording combined with intracellular staining and immunocytochemistry. Calbindin D-28K- and GABA-positive neurons had a small cell body (10.9+/-0.3 microm in diameter) and were distributed throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that calbindin- and GABA-positive neurons formed distinct subpopulations. Calbindin- and GABA-positive neurons double stained for biocytin showed extensive axon collaterals within the nucleus of tractus solitarius and some calbindin-positive, but not GABA-positive neurons, had also projection axons leaving the nucleus of tractus solitarius. Dopamine-beta-hydroxylase-immunopositive neurons had a small (10.8+/-0.3 microm) or large (17.2+/-0.4 microm) cell body. Neurons with a small cell body were observed in the dorsomedial nucleus at the level of the area postrema, and in the area postrema, while neurons with a large cell body were observed in the medial nucleus throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that almost all small dopamine-beta-hydroxylase-positive neurons were also immunoreactive for calbindin, while large dopamine-beta-hydroxylase-positive neurons were not. Double staining for dopamine-beta-hydroxylase and biocytin showed that neurons with a small cell body had moderate axon collaterals. On the contrary, neurons with a large cell body had few, if any, axon collaterals and a projection axon which could leave the nucleus of tractus solitarius. Following stimulation of the tractus solitarius, all neurons with a small cell body exhibited a polysynaptic excitatory response (type I neurons), while dopamine-beta-hydroxylase-immunopositive neurons with a large cell body exhibited a monosynaptic excitatory response (type II neurons) or an excitatory followed by an inhibitory response (type III neurons). Spontaneous and evoked excitatory postsynaptic currents of (type I neurons) calbindin- or GABA-positive neurons were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Spontaneous and evoked inhibitory postsynaptic currents of type III neurons were reversibly blocked by bicuculline. Type II neurons showed no spontaneous excitatory nor inhibitory postsynaptic currents. It was concluded that the three kinds of chemically-defined neurons formed distinct neuronal subpopulations in the caudal nucleus of tractus solitarius in terms of synaptic responses and morphological characteristics such as cell size and axonal trajectory.

Postnatal development of synaptophysin immunoreactivity in the rat nucleus tractus solitarii and caudal ventrolateral medulla

Synaptophysin (SY) is a major integral membrane protein of small synaptic vesicles. In the present study, SY immunohistochemistry was used to investigate the postnatal development of the rat nucleus tractus solitarii (NTS) and nucleus ambiguus/ventrolateral medulla (NA/VLM). Whatever the age of the animal, SY immunoreactivity showed a typical pattern of punctate staining reminiscent of presynaptic terminal labeling. In the NTS and the NA/VLM, SY immunoreactive puncta were few at birth and increased in number during the first postnatal days. These changes were quantified by measuring the volumetric fraction occupied by SY immunoreactive puncta at various postnatal ages. Using volumetric fraction data, an index of the total volume occupied SY immunoreactivity in each region was then calculated. Between birth and adulthood, this index increased by 6-fold in the NTS and by 7-fold in the NA/VLM, suggesting that most of the synaptic development of these regions occurs postnatally.

Vagally evoked synaptic currents in the immature rat nucleus tractus solitarii in an intact in vitro preparation

1. Whole-cell voltage-clamp recordings in an in vitro brainstem-cranial nerve explant preparation were used to assess the local circuitry activated by vagal input to nucleus tractus solitarii (NTS) neurones in immature rats. 2. All neurones that responded to vagal stimulation displayed EPSCs of relatively constant latency. Approximately 50 % of these also demonstrated variable-latency IPSCs, and approximately 31 % also displayed variable-latency EPSCs to vagal stimulation. All neurones also had spontaneous EPSCs and IPSCs. 3. Evoked and spontaneous EPSCs reversed near 0 mV and were blocked by the glutamate AMPA/kainate receptor antagonists 6,7-nitroquinoxaline-2,3-dione (DNQX) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at rest. Evoked EPSCs had rapid rise times (< 1 s) and decayed monoexponentially (tau = 2. 04 +/- 0.03 ms) at potentials near rest. 4. At holding potentials positive to approximately -50 mV, a slow EPSC could be evoked in the presence of DNQX or CNQX. This current peaked at holding potentials near -25 mV and was blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5). It was therefore probably due to activation of NMDA receptors by vagal afferent fibres. 5. Fast IPSCs reversed near -70 mV and were blocked by the GABAA receptor antagonist bicuculline. In addition, bicuculline enhanced excitatory responses to vagal stimulation and increased spontaneous EPSC frequency. Antagonists to AMPA/kainate receptors reversibly blocked stimulus-associated IPSCs and also decreased the frequency of spontaneous IPSCs. 6. These findings suggest that glutamate mediates synaptic transmission from the vagus nerve to neurones in the immature NTS by acting at non-NMDA and NMDA receptors. NTS neurones may also receive glutamatergic and GABAergic synaptic input from local neurones that can be activated by vagal input and/or regulated by amino acid inputs from other brainstem neurones.1. Whole-cell voltage-clamp recordings in an in vitro brainstem-cranial nerve explant preparation were used to assess the local circuitry activated by vagal input to nucleus tractus solitarii (NTS) neurones in immature rats.