Postnatal development of the parabrachial gustatory zone in rat: dendritic morphology and mitochondrial enzyme activity

Feeding circuit development and early-life influences on future feeding behaviour

A wide range of maternal exposures - undernutrition, obesity, diabetes, stress and infection - are associated with an increased risk of metabolic disease in offspring. Developmental influences can cause persistent structural changes in hypothalamic circuits regulating food intake in the service of energy balance. The physiological relevance of these alterations has been called into question because maternal impacts on daily caloric intake do not persist to adulthood. Recent behavioural and epidemiological studies in humans provide evidence that the relative contribution of appetitive traits related to satiety, reward and the emotional aspects of food intake regulation changes across the lifespan. This Opinion article outlines a neurodevelopmental framework to explore the possibility that crosstalk between developing circuits regulating different modalities of food intake shapes future behavioural responses to environmental challenges.

The Kölliker-Fuse nucleus: a review of animal studies and the implications for cranial nerve function in humans

To review the scientific literature on the relationship between Kölliker-Fuse nucleus (KF) and cranial nerve function in animal models, with view to evaluating the potential role of KF maturation in explaining age-related normal physiologic parameters and developmental and acquired impairment of cranial nerve function in humans. Medical databases (Medline and PubMed). Studies investigating evidence of KF activity responsible for a specific cranial nerve function that were based on manipulation of KF activity or the use of neural markers were included. Twenty studies were identified that involved the trigeminal (6 studies), vagus (9), and hypoglossal nerves (5). These pertained specifically to a role of the KF in mediating the dive reflex, laryngeal adductor control, swallowing function and upper airway tone. The KF acts as a mediator of a number of important functions that relate primarily to laryngeal closure, upper airway tone and swallowing. These areas are characterized by a variety of disorders that may present to the otolaryngologist, and hence the importance of understanding the role played by the KF in maintaining normal function.

Developmental specification of metabolic circuitry

The hypothalamus contains a core circuitry that communicates with the brainstem and spinal cord to regulate energy balance. Because metabolic phenotype is influenced by environmental variables during perinatal development, it is important to understand how these neural pathways form in order to identify key signaling pathways that are responsible for metabolic programming. Recent progress in defining gene expression events that direct early patterning and cellular specification of the hypothalamus, as well as advances in our understanding of hormonal control of central neuroendocrine pathways, suggest several key regulatory nodes that may represent targets for metabolic programming of brain structure and function. This review focuses on components of central circuitry known to regulate various aspects of energy balance and summarizes what is known about their developmental neurobiology within the context of metabolic programming.

Morphology and connectivity of parabrachial and cortical inputs to gustatory thalamus in rats

The ventroposterior medialis parvocellularis (VPMpc) nucleus of the thalamus, the thalamic relay nucleus for gustatory sensation, receives primary input from the parabrachial nucleus, and projects to the insular cortex. To reveal the unique properties of the gustatory thalamus in comparison with archetypical sensory relay nuclei, this study examines the morphology of synaptic circuitry in the VPMpc, focusing on parabrachiothalamic driver input and corticothalamic feedback. Anterogradely visualized parabrachiothalamic fibers in the VPMpc bear large swellings. At electron microscope resolution, parabrachiothalamic axons are myelinated and make large boutons, forming multiple asymmetric, adherent, and perforated synapses onto large-caliber dendrites and dendrite initial segments. Labeled boutons contain dense-core vesicles, and they resemble a population of terminals within the VPMpc containing calcitonin gene-related peptide. As is typical of primary inputs to other thalamic nuclei, parabrachiothalamic terminals are over five times larger than other inputs, while constituting only 2% of all synapses. Glomeruli and triadic arrangements, characteristic features of other sensory thalamic nuclei, are not encountered. As revealed by anterograde tracer injections into the insular cortex, corticothalamic projections in the VPMpc form a dense network of fine fibers bearing small boutons. Corticothalamic terminals within the VPMpc were also observed to synapse on cells that were retrogradely filled from the same injections. The results constitute an initial survey describing unique anatomical properties of the rodent gustatory thalamus.

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.

Excitatory and inhibitory synaptic function in the rostral nucleus of the solitary tract in embryonic rat

The embryonic development of synapses in the rostral nucleus of the solitary tract (rNST) was investigated in rat to determine when synapses begin to function. Using a brain slice preparation we studied appearance of synaptic receptors on second order rNST neurons and investigated the development of postsynaptic responses elicited by afferent nerve stimulation. Prenatal excitatory and inhibitory synaptic responses were recorded as early as E14. Glutamatergic and GABAergic postsynaptic responses were detected as early as E16. Both NMDA and AMPA receptors contributed to glutamatergic postsynaptic responses. GABAergic postsynaptic responses resulted primarily from activation of GABA(A) receptors. However, functional GABA(C) receptors were also demonstrated. A glycinergic postsynaptic response was not found although functional glycine receptors were demonstrated at E16. Solitary tract (ST) stimulation-evoked EPSCs, first detected at E16, were eliminated by glutamate receptor antagonists. ST-evoked IPSPs, also detected at E16, were eliminated by GABA(A) receptor antagonist. Thus, considerable prenatal development of rNST synaptic connections occurs and this will ensure postnatal function of central taste processing circuits.

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

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

Ontogeny of hypothalamic-hindbrain feeding control circuits

Ingestive behavior is a complex product of distributed central control systems that respond to a diverse array of internal and external sensory stimuli. Relatively little is known regarding the pathways and mechanisms by which relevant signals are conveyed to the neural circuits that ultimately control ingestive motor output. This report summarizes findings regarding the postnatal development of descending hypothalamic inputs to the hindbrain dorsal vagal complex (DVC). Evidence accumulated primarily in rats indicates that descending neural projections from the hypothalamus to the DVC are both structurally and functionally immature at birth. The progressive postnatal maturation of these projections occurs in parallel with newly emerging physiological and behavioral responsiveness to treatments and stimuli that affect food intake in adults. Thus, the postnatal emergence of new feeding controls may reflect the emerging access of these controls to DVC neural circuits.

Development of adaptive behaviour of the respiratory network: implications for the pontine Kolliker-Fuse nucleus

Breathing is constantly modulated by afferent sensory inputs in order to adapt to changes in behaviour and environment. The pontine respiratory group, in particular the Kolliker-Fuse nucleus, might be a key structure for adaptive behaviours of the respiratory network. Here, we review the anatomical connectivity of the Kolliker-Fuse nucleus with primary sensory structures and with the medullary respiratory centres and focus on the importance of pontine and medullary postinspiratory neurones in the mediation of respiratory reflexes. Furthermore, we will summarise recent findings from our group regarding ontogenetic changes of respiratory reflexes (e.g., the diving response) and provide evidence that immaturity of the Kolliker-Fuse nucleus might account in neonates for a lack of plasticity in sensory evoked modulations of respiratory activity. We propose that a subpopulation of neurones within the Kolliker-Fuse nucleus represent command neurones for sensory processing which are capable of initiating adaptive behaviour in the respiratory network. Recent data from our laboratory suggest that these command neurones undergo substantial postnatal maturation.

Postnatal development of hypothalamic inputs to the dorsal vagal complex in rats

The hypothalamus is critically involved in energy homeostasis and is an appropriate focus for research investigating the central neural underpinnings of obesity, anorexia and normal food intake. However, little is known regarding pathways and mechanisms that convey relevant hypothalamic signals to the brainstem circuits that ultimately control ingestive behavior. This brief review highlights work investigating the postnatal development of hypothalamic inputs to the hindbrain dorsal vagal complex (DVC). Research findings indicate that these inputs are both structurally and functionally immature in newborn rats. The progressive postnatal maturation of descending projections to the DVC occurs in concert with newly emerging physiological and behavioral responses to osmotic dehydration, which inhibits gastric emptying and food intake in adult animals but not in neonates. The postnatal emergence of other intake controls might also reflect progressive engagement of DVC neural circuits, whose intrinsic components and output pathways are envisioned as being critical for initiating and terminating ingestive behavior.

Cooling lesions of the lateral parabrachial nucleus during LiCl activation block acquisition of conditioned taste avoidance in male rats

Lesions of the lateral parabrachial nucleus (lPBN) disrupt acquisition of LiCl-induced conditioned taste avoidance. Animals with lesions in this area also fail to exhibit taste neophobia. This raises the possibility that an inability of rats to recognize the taste solution as novel contributes to the deficit in taste avoidance learning. If this is the case, then one would expect conditioned taste avoidance not to be disrupted if the lPBN is functional during taste processing but not during LiCl processing. The first three experiments demonstrated that cooling was a viable method by which to temporarily inactivate the lPBN. Measurement of neural temperature during cooling indicated that the lPBN was cooled to temperatures that have been shown to block synaptic transmission but not axonal transmission. Cooling the lPBN itself induced a conditioned avoidance to a sucrose solution but this avoidance was abolished by exposure to daily cooling for 1 week prior to acquisition. In experiment 4, all animals were preexposed to lPBN cooling for 1 week. Those rats that received cooling lesions during a period that started immediately after sucrose solution consumption and extended through the peak effectiveness of LiCl failed to acquire a taste avoidance. These results fail to support the hypothesis that the deficit in taste avoidance learning after permanent lesions of the lPBN is due to an inability of lesioned animals to recognize the taste as novel. They are consistent with the hypothesis that this neural area processes ascending information about LiCl.

The cytoarchitecture of the adult human parabrachial nucleus: a Nissl and Golgi study

The parabrachial nucleus (PBN) plays important roles in numerous autonomic functions and in pain modulation. In different animal species, three main regions of the PBN have been identified: the m-PB, the l-PB, and the Kolliker-Fuse nucleus (KF). The KF has not been identified in humans. The present study used Nissl and Golgi-Cox material and morphoquantitative methods to investigate the cytoarchitectural organization of the adult human PBN, paying particular attention to neuronal features endowed with functional significance, i. e. the arborization of the neurons. The PBN neuron population is made up of elements which are heterogeneous in size, shape and dendritic arborization, and grouped into two regions, the lateral and medial PBN (l- and m-PB). It has been suggested that some large sized neurons located in the ventral region of the m-PB might be the counterpart of the KF. In the m-PB the fusiform neurons are the most numerous cells; in the l-PB the multipolar neurons prevail, and are particularly numerous in the dorsal l-PB. Since the dendritic arborization is generally the main target of afferent projections to a neuron, it is possible that the l-PB, and in particular its dorsal region, might be the main site for the endings of afferences to the human PBN.

Parabrachial nucleus projections to midline and intralaminar thalamic nuclei of the rat

The projections from the parabrachial nucleus to the midline and intralaminar thalamic nuclei were examined in the rat. Stereotaxic injections of the retrograde tracer cholera toxin-beta (CTb) were made in each of the intralaminar nuclei of the dorsal thalamus (the lateral parafascicular, medial parafascicular, oval paracentral, central lateral, paracentral, and central medial nuclei), as well as the midline thalamic nuclei (the paraventricular, intermediodorsal, mediodorsal, paratenial, rhomboid, reuniens, parvicellular part of the ventral posterior, and caudal ventral medial nuclei). The retrograde cell body labeling pattern within the parabrachial subnuclei was then analyzed. The paracentral thalamic nucleus received an input only from the internal lateral parabrachial subnucleus. However, this subnucleus also projected to all the other intralaminar thalamic nuclei, except for the central lateral thalamic nucleus, which received no parabrachial afferent inputs. The external lateral parabrachial subnucleus projected to the lateral parafascicular, reuniens, central medial, parvicellular part of the ventral posterior, and caudal ventromedial thalamic nuclei. Following CTb injections in the paraventricular thalamic nucleus, retrogradely labeled cells were found in the central lateral, dorsal lateral, and external lateral parabrachial subnuclei. The medial and ventral lateral parabrachial subnuclei projected to the oval paracentral, parafascicular, and rhomboid thalamic nuclei. Finally, the waist area of the parabrachial nucleus was densely labeled after CTb injections in the parvicellular part of the ventral posterior thalamic nucleus. Nociceptive, visceral, and gustatory signals may reach specific cortical and other forebrain sites via this parabrachial-thalamic pathway.

The neural code for taste in the brain stem: response profiles

In the study of the neural code for taste, two theories have dominated the literature: the across neuron pattern (ANP), and the labeled line theories. Both of these theories are based on the observations that taste cells are multisensitive across a variety of different taste stimuli. Given a fixed array of taste stimuli, a cell's particular set of sensitivities defines its response profile. The characteristics of response profiles are the basis of both major theories of coding. In reviewing the literature, it is apparent that response profiles are an expression of a complex interplay of excitatory and inhibitory inputs that derive from cells with a wide variety of sensitivity patterns. These observations suggest that, in the absence of inhibition, taste cells might be potentially responsive to all taste stimuli. Several studies also suggest that response profiles can be influenced by the taste context, defined as the taste stimulus presented just before or simultaneously with another, under which they are recorded. A theory, called dynamic coding, was proposed to account for context dependency of taste response profiles. In this theory, those cells that are unaffected by taste context would provide the signal, i.e., the information-containing portion of the ANP, and those cells whose responses are context dependent would provide noise, i.e., less stimulus specific information. When singular taste stimuli are presented, noise cells would provide amplification of the signal, and when complex mixtures are presented, the responses of the noise cells would be suppressed (depending on the particular combination of tastants), and the ratio of signal to noise would be enhanced.

Developmental sodium restriction and gustatory afferent terminal field organization in the parabrachial nucleus

Dietary sodium restriction instituted early in prenatal development produces physiological, anatomical, and functional changes in the gustatory system. For example, a rearrangement of the chorda tympani nerve terminal field within the nucleus of the solitary tract (NST) is observed in rats sodium restricted during development. The altered pattern of the chorda tympani nerve innervation within the nucleus of the solitary tract remains even after dietary sodium is restored in the diet at adulthood. In light of these observations, the terminal fields of second-order projections from the nucleus of the solitary tract to the parabrachial nucleus (PBn) were examined. To determine the possible rearrangements of the second-order projections, the rostral pole of the NST in control, restricted, and repleted rats was injected with the fluorescent tracer Fluoro-Ruby and the terminal fields in the parabrachial nucleus were analyzed. Results show no differences in the size or topography of the parabrachial nucleus terminal field among control, restricted, and repleted rats. These results suggest that the terminal field of second-order gustatory neurons is resistant to dietary sodium restriction during development. The apparent target-dependent effects may relate to differences in the developmental processes along the gustatory pathway.

Differences in the intrinsic membrane characteristics of parabrachial nucleus neurons processing gustatory and visceral information

Whole-cell current-clamp recordings were made from neurons in the rat parabrachial nucleus (PBN) in three rostro-caudal brain slices. During recording the neurons were located in one of four quadrants of the PBN. Successful recordings were obtained from neurons in three of these quadrants termed the dorsolateral (DL), dorsomedial (DM) and ventromedial (VM) quadrants. Recordings were made of the intrinsic membrane properties and repetitive discharge characteristics of 58 neurons in the DL, 60 neurons in the DM, and 54 neurons in the VM-quadrants. The input resistance of the neurons in the DL quadrant was significantly lower and the membrane time constant significantly shorter than that of the neurons in the DM- and VM-quadrants. The mean action potential duration of the VM-quadrant neurons was significantly longer than that of both DL- and DM-quadrant neurons. The discharge frequency in response to a 1500 ms 100 pA current pulse of the DL quadrant neurons was significantly lower than that of the neurons in the other two quadrants. The latency of action potential initiation following a 100 pA depolarizing current pulse was significantly longer for DL quadrant neurons compared to neurons in the other two quadrants. Neurons were divided into groups based on their response to a long depolarizing current pulse immediately preceded by a hyperpolarizing current pulse. In all three rostro-caudal slices of the PBN, the largest populations of neurons were in Group II and Group III. The results demonstrate that neurons in different locations in the PBN have different membrane and repetitive discharge properties. These different PBN locations receive inputs from the visceral and gustatory regions of the NST. It is possible therefore that the differences in properties of the PBN neurons may relate to the type of sensory information that they process.

Target pioneering and early morphology of the murine chorda tympani

Many studies demonstrate that differentiation of certain sensory receptors during development is induced by their nerve supply. Thus the navigational accuracy of pioneering fibres to their targets is crucial to this process. The special gustatory elements of the facial and glossopharyngeal nerves are used extensively as model systems in this field. We examined the chorda tympani, the gustatory component of the facial nerve, to determine the precise time course of its development in mice. The transganglionic fluorescent tracer DiI was injected into the anterior aspect of the mandibular arch of fixed embryos aged between 30 and 50 somites (E10-E12). It was allowed to diffuse retrogradely via the geniculate ganglion to the brainstem for 4 wk, before the distribution of DiI was determined using confocal laser scanning microscopy. Geniculate ganglion cells were first labelled at the 34 somite stage (E10). Pioneering chorda tympani fibres that arise from these cells passed peripherally and followed an oblique course as they grew towards the mandibular arch. At the 36 somite stage (E10.5), the peripheral component followed an intricate postspiracular course and passed anteriorly to arch over the primitive tympanic cavity, en route to the lingual epithelium. From the 36 to 50 somite stages (E10.5-E12), it consistently traced in the fashion of a 'U' bend. The central fascicle also traced at the 36 somite stage (E10.5) and just made contact with the brainstem. At the 40 somite stage (E11), the central fibres clearly chose a route of descent into the spinal trigeminal tract and branched into the solitary tract. Pioneering chorda tympani fibres contact the lingual epithelium when the target is primordial. The lingual epithelium may be a source of a neurotropic factor that attracts peripheral chorda tympani fibres to the sites of putative papillae. However, the chorda tympani is probably not a vital influence on the subsequent differentiation of gustatory papillae, since the papillae are elaborated 5 d later at E15 in murine embryos. The early morphology of the nerve is true to the amniote vertebrate phenotype.

In vitro study of afferent synaptic transmission in the rostral gustatory zone of the rat nucleus of the solitary tract

The synaptic responses of rostral nucleus of the solitary tract (rNST) neurons to electrical stimulation of the solitary tract (ST) fibers were investigated using whole-cell recordings in brain slices of adult rat medulla. Most neurons of the rNST (47%) responded to stimulation of the ST with excitatory postsynaptic potentials (EPSPs), 28% responded with mixed excitatory and inhibitory postsynaptic potentials (PSPs) and 25% responded with inhibitory postsynaptic potentials (IPSPs). The estimated reversal potentials for the EPSPs (EEPSP) was -7 mV and for the IPSPs (EIPSP) was -69 mV. The glutamate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) acting at the alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptor, either reduced or blocked all EPSPs tested. D-2-Amino-5-phosphonovalerate (APV), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, also reduced the amplitude of the EPSPs. These results suggest that glutamate is released following stimulation of afferent fibers in the ST and acts on both AMPA/kainate and NMDA glutamate receptors. The IPSPs result from release of gamma-aminobutyric acid (GABA) since superfusion of the GABAA receptor antagonist, bicuculline reversibly blocked the IPSPs. The GABAB receptor antagonist, phaclofen, also reduced the IPSP components in some neurons, indicating that both GABAA and GABAB receptors are involved in inhibitory transmission in the rNST. When the morphology of the recorded neurons was examined by filling the neurons with biocytin and reconstructing the neurons, each morphological type of rNST neuron responded with excitatory and inhibitory PSPs following stimulation of the ST.

Morphology and dendritic domains of neurons in the lateral parabrachial nucleus of the rat

The present study provides a description of the dendritic morphology and the dendritic domains of neurons in the lateral parabrachial nucleus (PB) of the rat. The cells were intracellularly stained in vitro with Lucifer yellow. A subpopulation of these cells was characterized beforehand as neurons projecting to the amygdaloid complex by retrograde transport with rhodamine beads. With respect to their dendritic arborization, different types of "spatially" organized PB neurons were discriminated. One major cell type in the external lateral PB (PBel) is characterized by long, elongated dendritic trees that are preferentially oriented parallel to the superior cerebellar peduncle. The majority of their dendrites appears to respect subnuclear boundaries, yet their distal dendrites often exceed the limits of the PBel to encroach upon adjacent subnuclei located dorsally and ventrolaterally to the PBel. Another prominent cell type in the PBel has fairly small and locally restricted dendritic trees that are also elongated, running with their main axis from ventrolateral to dorsomedial. The dendrites of the majority of these neurons apparently stay within the confines of the PBel. A distinct group of neurons is found in the ventral portion of the PBel. The majority of their dendrites is mediolaterally oriented and not confined to the PBel subnucleus. In addition, we found a smaller number of neurons scattered within the lateral PB whose dendrites do not show a preferential orientation but travel across subnuclear boundaries into several different PB subnuclei. Our data show that the dendrites of a large proportion of neurons in the lateral PB either stay within the confines of a particular subnucleus or slightly extend across subnuclear limits. In any case, they appear to match with terminal territories of afferent axons and, thus, maintain the functional specificity of inputs by their relay through the PB. In contrast, PB neurons that extend their dendrites across subnuclear boundaries or known terminal territories are likely to receive inputs of different qualities from a variety of sources and therefore transmit a more general, integrated signal to the forebrain.

Postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Previous studies have examined pre- and postsynaptic development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST). This region of the NST is innervated by primary gustatory axons arising from the facial-intermediate nerve. However, a large portion of the gustatory NST is innervated by axons arising from the glossopharyngeal nerve, and although the time course for development of N.VII recipient zones has been defined development of glossopharyngeal afferent terminal fields has not been examined. Moreover, the time course for development of projection neurons located postsynaptic to gustatory afferent axons has not been examined in any portion of the NST. The objectives of the present study were to 1) define the time course for development of N.VII and N.IX terminal fields and 2) examine temporal relationships between development of afferent terminal fields and development of projection neurons located postsynaptic to gustatory afferent axons. To this end, triple fluorescent labeling procedures were used to simultaneously visualize developing axons and projection neurons. Results show that afferent terminal fields develop along the rostrocaudal axis of the NST. Axons of the N.VII terminal field are present in the rostral NST at P1 and develop to approximately P25. Axons and terminal endings of N.IX do not enter the NST until approximately P9-P10, and these terminal fields develop within the intermediate NST until approximately P45. Many NST neurons destined to project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN), do not possess axonal connections with the PBN during the first 2-3 weeks of postnatal life. As afferent terminal fields develop, these neurons establish connections with the PBN between the ages of approximately P7 and P45-P60. The delay between afferent terminal field development and development of PBN projection neurons in the N.VII terminal field is approximately 3 weeks. The delay between pre- and postsynaptic development in the N.IX terminal field is approximately 1 week. Potential relationships between pre- and postsynaptic development are discussed, in addition to relationships between anatomical development in the NST and the emergence of taste-guided behaviors.

Effects of early postnatal receptor damage on dendritic development in gustatory recipient zones of the rostral nucleus of the solitary tract

The rostral gustatory zone of the nucleus of the solitary tract (NST) exhibits extensive anatomical development during the first 3 weeks of postnatal life, and this development requires the presence of intact gustatory receptors during a critical period. We have previously shown that unilateral damage induced to fungiform papillae of the anterior tongue at postnatal day 2 (P2) alters normal migration and ramification of chorda tympani (CT) axons in the rostral NST. In addition to alterations of axonal development, P2 receptor damage decreases the intraneuronal distance between neurons that project axons to the second-order central gustatory relay, located in the caudal parabrachial nucleus (PBN). This observation suggested that P2 receptor damage may alter both axonal development and dendritic development in the rostral gustatory NST. The present study evaluated potential changes in dendritic development of PBN projection neurons following either P2 or P10 receptor damage. Morphological studies were first conducted to quantitatively define somatic characteristics of neurons that project axons to the PBN. Independent experiments used fluorescent labeling combined with subsequent Golgi-impregnation to study dendritic architecture of identified PBN projection neurons. Results confirmed that P2 receptor damage alters dendritic development of PBN projection neurons located in CT terminal fields. Anterior tongue receptor damage at P2 (1) reduces planar length of first- and second-order dendritic branches, (2) reduces the mean number of second-order branches per neuron, and (3) reduces the density of spine processes on second-order dendritic branches. A critical period exists for these effects, similar to that reported for axonal development, insofar as P2 receptor damage alters dendritic development of PBN projection neurons, whereas P10 receptor damage does not. Dendrites of identified PBN projection neurons located in regions of the NST that receive primary afferent axons from the glossopharyngeal nerve are not affected by anterior tongue damage at P2. These results show that early postnatal receptor damage influences both pre- and postsynaptic development in the rostral gustatory NST. These anatomical changes are undoubtedly related to alterations in taste-guided behaviors that are observed following P2 receptor damage.

The ascending gustatory pathway: a Golgi analysis of the medial and lateral parabrachial complex in the adult hamster

This study examined the somal areas, dendritic features and orientations of neurons within taste responsive regions of the parabrachial complex, including the "waist" area that spans the brachium conjunctivum. The data were compared with those of a Golgi study of the gustatory zone of the nucleus of the solitary tract. Both fusiform and multipolar neurons were identified. Fusiform neurons had elongated somata that average 205 microns2 (range: 128-281 microns2) and generally possessed bipolar primary dendrites. Multipolar neurons had a stellate appearance and somal areas that averaged 230 microns2 (range: 109-443 microns2). These multipolar neurons possessed significantly more primary dendrites than fusiform neurons (4.0 versus 2.9 primary dendrites). Fusiform neurons were uncommon in the medial and lateral regions of the parabrachial complex but predominated in the solitary nucleus. Parabrachial neurons were usually larger and possessed more complex higher-order dendritic arborizations than solitary neurons. Computer-generated three-dimensional rotational analyses failed to demonstrate the strong orientation specificity in parabrachial neurons that characterizes gustatory solitary neurons. These Golgi studies described for the first time the morphological features of pontine neurons that could possibly receive ascending gustatory projections, and the morphological differences between neurons that receive direct peripheral input from taste receptors and the pontine targets of such neurons.

Effects of early postnatal receptor damage on development of gustatory recipient zones within the nucleus of the solitary tract

The temporal correspondence between neuroanatomical and neurophysiological development of peripheral and central gustatory neurons has suggested that morphological development of the first-order central gustatory relay, located in the rostral nucleus of the solitary tract (NST), may be dependent on afferent input from peripheral gustatory pathways. The objective of the present study was to determine the effects of perinatal receptor damage on development of gustatory recipient zones within the rostral and intermediate NST. Results show that damage induced to fungiform receptors of the anterior tongue at postnatal day 2 (P2) alters normal development of NST terminal fields associated with the chorda tympani nerve (CT) and greater superficial nerve (GSP), and that alterations in the CT/GSP terminal field persist in adulthood after peripheral gustatory receptors have regenerated. Damage induced to fungiform receptors at P2 does not alter the normal development of glossopharyngeal terminal fields in the intermediate NST. Receptor damage produced at P10 and P20 is without effect on normal development of the CT/GSP terminal field. Thus, fungiform receptor damage at P2 produces specific alterations in the development of NST terminal fields that receive projections from the facial-intermediate nerve, and receptor damage effects are only obtained during a critical period of postnatal development. P2 receptor damage has the overall effect of eliminating caudally directed migration of CT/GSP axons to additional projection neurons that establish connections with the second-order central gustatory relay located in the parabrachial nucleus (PBN). Behavioral studies were conducted to determine the functional consequences of early receptor damage. Results from behavioral studies show that bilateral damage to fungiform papillae at P2 alters normal adult preferences to low and intermediate concentrations of NaCl and sucrose tastes, yet aversions to citric acid and quinine HCl are not obviously affected. Therefore, anatomical alterations in the CT/GSP terminal field produced by P2 receptor damage are accompanied by specific changes in adult taste preference responses.

Connections of the parabrachial nucleus with the nucleus of the solitary tract and the medullary reticular formation in the rat

We examined the subnuclear organization of projections to the parabrachial nucleus (PB) from the nucleus of the solitary tract (NTS), area postrema, and medullary reticular formation in the rat by using the anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase conjugate and anterograde tracing with Phaseolus vulgaris-leucoagglutinin. Different functional regions of the NTS/area postrema complex and medullary reticular formation were found to innervate largely nonoverlapping zones in the PB. The general visceral part of the NTS, including the medial, parvicellular, intermediate, and commissural NTS subnuclei and the core of the area postrema, projects to restricted terminal zones in the inner portion of the external lateral PB, the central and dorsal lateral PB subnuclei, and the "waist" area. The dorsomedial NTS subnucleus and the rim of the area postrema specifically innervate the outer portion of the external lateral PB subnucleus. In addition, the medial NTS innervates the caudal lateral part of the external medial PB subnucleus. The respiratory part of the NTS, comprising the ventrolateral, intermediate, and caudal commissural subnuclei, is reciprocally connected with the Kölliker-Fuse nucleus, and with the far lateral parts of the dorsal and central lateral PB subnuclei. There is also a patchy projection to the caudal lateral part of the external medial PB subnucleus from the ventrolateral NTS. The rostral, gustatory part of the NTS projects mainly to the caudal medial parts of the PB complex, including the "waist" area, as well as more rostrally to parts of the medial, external medial, ventral, and central lateral PB subnuclei. The connections of different portions of the medullary reticular formation with the PB complex reflect the same patterns of organization, but are reciprocal. The periambiguus region is reciprocally connected with the same PB subnuclei as the ventrolateral NTS; the rostral ventrolateral reticular nucleus with the same PB subnuclei as both the ventrolateral (respiratory) and medial (general visceral) NTS; and the parvicellular reticular area, adjacent to the rostral NTS, with parts of the central and ventral lateral and the medial PB subnuclei that also receive rostral (gustatory) NTS input. In addition, the rostral ventrolateral reticular nucleus and the parvicellular reticular formation have more extensive connections with parts of the rostral PB and the subjacent reticular formation that receive little if any NTS input. The PB contains a series of topographically complex terminal domains reflecting the functional organization of its afferent sources in the NTS and medullary reticular formation.

Behavioral reactions to gustatory stimuli in young chicks (Gallus gallus domesticus)

Freely-moving, posthatch chicks were individually presented 2 concentrations each of quinine, citric acid, fructose, sucrose, sodium saccharin, and distilled water and their behavioral reactions were videotaped and analyzed. Already during the first posthatch day distinct rejection responses to quinine and citric acid could be recognized. Prolonged head shaking and beak clapping episodes were the most dominant features of these reactions. While responses to water and sweet stimuli could be interpreted as acceptance behaviors, the resolution was generally not fine enough to discriminate between reactions to the 2 different sweet concentrations of these stimuli or between them and water. When only water or sugar solutions were presented to other hatchlings in a single session, there was a suggestion of more definite acceptance behavior to some sweet stimuli as compared to water. It is concluded that the systems mediating aversive gustatory responses are present and functioning in posthatching chicks while acceptance responses, though present, are less discriminative among stimuli.

Postnatal development of protein P-38 ('synaptophysin') immunoreactivity in pontine and medullary gustatory zones of rat

Neurons located in gustatory zones of the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN) exhibit extensive morphological development during the first 5-7 weeks of rat's postnatal life. The present study examined the time course for development of protein P-38 ('synaptophysin'), a specific integral membrane glycoprotien of clear synaptic vesicles, in the rostral NST and PBN. Results show that volumetric fractions of P-38 immunoreactions increase in the NST from P1 to P10, and are adult-like in the NST after approximately P11. In contrast, volumetric fractions of P-38 immunoreactivity increase in the PBN between approximately P11 and P30. Because we have previously demonstrated that dendritic growth in the rostral gustatory NST precedes dendritic growth in the PBN, these results confirm that maturation of both presynaptic and postsynaptic constituents of brainstem gustatory relays develop in a temporally sequential manner. Relationships between neurological development of brainstem gustatory relays and the ontogeny of taste-guided responses are discussed.

Postnatal development of the rostral solitary nucleus in rat: dendritic morphology and mitochondrial enzyme activity

Morphological and metabolic development of the gustatory zone of the rostral nucleus of the solitary tract (NST) was examined in rat. Transganglionic transport of horseradish peroxidase (HRP) was used to visualize the organization of gustatory projections to the rostral gustatory NST in rats aged postnatal day 1 (P1) to P34. Golgi impregnation studies were performed to analyze morphological development of dendrites in regions of the rostral NST that were identified as anterior tongue terminal fields. Results demonstrate that afferent fibers of the anterior tongue project to the rostral NST in rats as young as P1. The volume of NST terminal fields increased from P1 to approximately P16-P20, and was adult-like after approximately P20. Developmental increases in terminal field volume resulted from a preferential expansion in the rostrocaudal plane. Planar length of first-order dendrites associated with fusiform, multipolar, and ovoid neurons, and second-order dendrites of fusiform and ovoid neurons, increased approximately three-fold between P4 and P16-20. First-order dendritic length for all morphological types was adult-like after approximately 20-25 days of age, whereas second-order dendritic length of multipolar neurons increased significantly between P30 and P60-70. Histochemical studies confirmed that activity of the mitochondrial respiratory enzymes cytochrome c oxidase (EC 1.9.3.1), succinate dehydrogenase (EC 1.3.99.1), and NADH-dehydrogenase (EC 1.6.99.3) increased monotonically during the developmental period in which planar growth of first-order dendrites was observed. The present results, in combination with results from previous studies, indicate that morphological and metabolic development fo the NST occurs concomitantly with morphological development of taste receptors and peripheral gustatory nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of GABA and GABA-transaminase containing neurons in the gustatory zone of the nucleus of the solitary tract

Previous cytoarchitectural and electron micrographic studies have indicated that the gustatory zone of the nucleus of the solitary tract (NST) may contain local circuit neurons. It is known that neurons of the caudal "visceroceptive" NST contain GABA, glutamic acid decarboxylase (EC 4.1.1.15), and GABA-transaminase (GABA-T; 4-aminobutyrate: 2-oxoglutarate aminotransferase; EC 2.6.1.19). The present study was conducted to determine whether or not neurons in the gustatory zone of the NST of rat contain GABA and the principle degradative enzyme of GABA, GABA-T. Transganglionic transport of horseradish peroxidase (HRP) was used to identify chorda tympani (CT) nerve terminal fields. Immunohistochemical studies were combined with transport experiments to evaluate the organization of GABA immunoreactive neurons in CT terminal fields. Results show that GABA immunoreactive neurons and puncta are located within CT terminal fields. These neurons evince small ovoid morphologies resembling Golgi interneurons, and comprise an average of 18% of total neurons in CT terminal fields. Independent histochemical studies reveal that approximately 82% of GABA immunoreactive neurons within CT terminal fields exhibit GABA-T activity. Retrograde transport of HRP was used in additional studies to evaluate whether or not axons of putative GABAergic neurons project to the second-order central gustatory relay located in the caudal parabrachial nucleus (PBNc), to the caudal NST, or to regions surrounding the rostral or caudal NST. Combined studies indicate that GABA immunoreactive neurons in the gustatory NST do not project axons to the PBNc, to the caudal NST, or to regions adjacent to the rostral or caudal NST.(ABSTRACT TRUNCATED AT 250 WORDS)