Structure and function of gustatory neurons in the nucleus of the solitary tract. I. A classification of neurons based on morphological features

Classification by morphology of multipolar neurons of the human principal olivary nucleus

The principal olivary nucleus is the largest part of the inferior olivary complex and is involved in the spatial and temporal organization of movement and motor learning. Nearly all neurons in this nucleus is multipolar along with having a highly complex dendritic tree and significant asymmetry in shape. In this study, we updated the current classification scheme, examined morphological differences between the proposed groups, and investigated age-related morphological changes. Histological preparations were digitized by a light microscope and a sample of 259 images of neurons was analyzed by 17 computationally generated parameters of morphology. These were reduced to the four variables of principal component analysis and the sample was classified by k-means method of clustering into three clusters. The differences between clusters were documented and for medium-sized neurons the relationship between four morphological parameters and age were investigated. Finally, for two of the age groups the changes in the morphology were explored. This study includes a detailed and robust classification of the PON neurons and the findings improve upon past qualitative work.

Information processing in brainstem bitter taste-relaying neurons defined by genetic tracing

Bitter reception is mediated by taste receptor cells that coexpress multiple T2Rs, a family of G-protein-coupled receptors. However, it remains elusive how bitter taste information is translated in the brain into appropriate behavioral responses. Here we used a combination of genetic tracing and electrophysiological and immunohistochemical analyses in mice to functionally characterize the neurons in the solitary tract nuclei of the medulla, which receive input from mT2R5-expressing cells. The neurons defined by a transneuronal tracer originating from mT2R5-expressing cells receive glutamatergic synaptic input via the AMPA receptor. The satiety peptide cholecystokinin increases glutamatergic transmission, suggesting an interaction between information processing of taste and the homeostatic control of feeding. Nevertheless, the tracer-labeled neuron types are heterogeneous, and can be classified into catecholamine and pro-opiomelanocortin neurons. Our data reveal that the architectural solution in the first-order central relay that processes information from mT2R5-expressing cells uses unique ensembles of neurons with different neurotransmitters.

Physiological and anatomical properties of intramedullary projection neurons in rat rostral nucleus of the solitary tract

The rostral nucleus of the solitary tract (rNTS), the first-order relay of gustatory information, not only transmits sensory information to more rostral brain areas but also connects to various brain stem sites responsible for orofacial reflex activities. While much is known regarding ascending projections to the parabrachial nucleus, intramedullary projections to the reticular formation (which regulate oromotor reflexive behaviors) remain relatively unstudied. The present study examined the intrinsic firing properties of these neurons as well as their morphological properties and synaptic connectivity with primary sensory afferents. Using in vitro whole cell patch-clamp recording, we found that intramedullary projection neurons respond to depolarizing current injection with either tonic or bursting action potential trains and subsets of these groups of neurons express A-type potassium, H-like, and postinhibitory rebound currents. Approximately half of the intramedullary projection neurons tested received monosynaptic innervation from primary afferents, while the rest received polysynaptic innervation, indicating that at least a subpopulation of these neurons can be directly activated by incoming sensory information. Neuron morphological reconstructions revealed that many of these neurons possessed numerous dendritic spines and that neurons receiving monosynaptic primary afferent input have a greater spine density than those receiving polysynaptic primary afferent input. These results reveal that intramedullary projection neurons represent a heterogeneous class of rNTS neurons and, through both intrinsic voltage-gated ion channels and local circuit interactions, transform incoming gustatory information into signals governing oromotor reflexive behaviors.

A survey of oral cavity afferents to the rat nucleus tractus solitarii

Visualization of myelinated fiber arrangements, cytoarchitecture, and projection fields of afferent fibers in tandem revealed input target selectivity in identified subdivisions of the nucleus tractus solitarii (NTS). The central fibers of the chorda tympani (CT), greater superficial petrosal nerve (GSP), and glossopharyngeal nerve (IX), three nerves that innervate taste buds in the oral cavity, prominently occupy the gustatory-sensitive rostrocentral subdivision. In addition, CT and IX innervate and overlap in the rostrolateral subdivision, which is primarily targeted by the lingual branch of the trigeminal nerve (LV). In the rostrocentral subdivision, compared with the CT terminal field, GSP appeared more rostral and medial, and IX was more dorsal and caudal. Whereas IX and LV filled the rostrolateral subdivision diffusely, CT projected only to the dorsal and medial portions. The intermediate lateral subdivision received input from IX and LV but not CT or GSP. In the caudal NTS, the ventrolateral subdivision received notable innervation from CT, GSP, and LV, but not IX. No caudal subnuclei medial to the solitary tract contained labeled afferent fibers. The data indicate selectivity of fiber populations within each nerve for functionally distinct subdivisions of the NTS, highlighting the possibility of equally distinct functions for CT in the rostrolateral NTS, and CT and GSP in the caudal NTS. Further, this provides a useful anatomical template to study the role of oral cavity afferents in the taste-responsive subdivision of the NTS as well as in subdivisions that regulate ingestion and other oromotor behaviors.

Genetic tracing of the gustatory neural pathway originating from Pkd1l3-expressing type III taste cells in circumvallate and foliate papillae

Polycystic kidney disease 1-like 3 (Pkd1l3) is expressed specifically in sour-sensing type III taste cells that have synaptic contacts with afferent nerve fibers in circumvallate (CvP) and foliate papillae (FoP) located in the posterior region of the tongue, although not in fungiform papillae (FuP) or the palate. To visualize the gustatory neural pathways that originate from type III taste cells in CvP and FoP, we established transgenic mouse lines that express the transneuronal tracer wheat germ agglutinin (WGA) under the control of the mouse Pkd1l3 gene promoter/enhancer. The WGA transgene was accurately expressed in Pkd1l3-expressing type III taste cells in CvP and FoP. Punctate WGA protein signals appeared to be detected specifically in type III taste cells but not in other types of taste cells. WGA protein was transferred primarily to a subset of neurons located in close proximity to the glossopharyngeal (GL) nerve bundles in the nodose/petrosal ganglion (NPG). WGA signals were also observed in a small population of neurons in the geniculate ganglion (GG). This result demonstrates the anatomical connection between taste receptor cells (TRCs) in the FoP and the chorda tympani (CT) nerves. WGA protein was further conveyed to neurons in a rostro-central subdivision of the nucleus of the solitary tract (NST). These findings demonstrate that the approximately 10 kb 5'-flanking region of the mouse Pkd1l3 gene functions as a type III taste cell-specific promoter/enhancer. In addition, experiments using the pkd1l3-WGA transgenic mice reveal a sour gustatory pathway that originates from TRCs in the posterior region of the tongue.

Types of taste circuits synaptically linked to a few geniculate ganglion neurons

The present study evaluates the central circuits that are synaptically engaged by very small subsets of the total population of geniculate ganglion cells to test the hypothesis that taste ganglion cells are heterogeneous in terms of their central connections. We used transsynaptic anterograde pseudorabies virus labeling of fungiform taste papillae to infect single or small numbers of geniculate ganglion cells, together with the central neurons with which they connect, to define differential patterns of synaptically linked neurons in the taste pathway. Labeled brain cells were localized within known gustatory regions, including the rostral central subdivision (RC) of the nucleus of the solitary tract (NST), the principal site where geniculate axons synapse, and the site containing most of the cells that project to the parabrachial nucleus (PBN) of the pons. Cells were also located in the rostral lateral NST subdivision (RL), a site of trigeminal and sparse geniculate input, and the ventral NST (V) and medullary reticular formation (RF), a caudal brainstem pathway leading to reflexive oromotor functions. Comparisons among cases, each with a random, very small subset of labeled geniculate neurons, revealed "types" of central neural circuits consistent with a differential engagement of either the ascending or the local, intramedullary pathway by different classes of ganglion cells. We conclude that taste ganglion cells are heterogeneous in terms of their central connectivity, some engaging, predominantly, the ascending "lemniscal," taste pathway, a circuit associated with higher order discriminative and homeostatic functions, others engaging the "local," intramedullary "reflex" circuit that mediates ingestion and rejection oromotor behaviors.

Gustatory neural circuitry in the hamster brain stem

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second central relays for the taste pathway, respectively. Taste neurons in the NST project to the PbN, which further transmits taste information to the rostral taste centers. Nevertheless, details of the neural connections among the brain stem gustatory nuclei are obscure. Here, we investigated these relationships in the hamster brain stem. Three electrode assemblies were used to record the activity of taste neurons extracellularly and then to electrically stimulate these same areas in the order: left PbN, right PbN, and right NST. A fourth electrode, a glass micropipette, was used to record from gustatory cells in the left NST. Results showed extensive bilateral communication between brain stem nuclei at the same level: 1) 10% of 96 NST neurons projected to the contralateral NST and 58% received synaptic input from the contralateral NST; and 2) 12% of 43 PbN neurons projected to the contralateral PbN and 21% received synaptic input from the contralateral PbN. Results also showed extensive communication between levels: 1) as expected, the majority of 119 NST neurons, 82%, projected to the ipsilateral PbN, but 85% of the 20 NST neurons tested received synaptic input from the ipsilateral PbN, as did 59% of 22 NST neurons that did not project to the PbN; and 2) although few, 3%, of 119 NST cells projected to the contralateral PbN and 38% received synaptic input from the contralateral PbN. These results demonstrated that taste neurons in the NST not only project to, but also receive descending input from the bilateral PbN and that gustatory neurons in the NST and PbN also communicate with the corresponding nucleus on the contralateral side.

Genetic tracing of the gustatory and trigeminal neural pathways originating from T1R3-expressing taste receptor cells and solitary chemoreceptor cells

We established transgenic mouse lines expressing a transneuronal tracer, wheat germ agglutinin (WGA), under the control of mouse T1R3 gene promoter/enhancer. In the taste buds, WGA transgene was faithfully expressed in T1R3-positive sweet/umami taste receptor cells. WGA protein was transferred not laterally to the synapse-bearing, sour-responsive type III cells in the taste buds but directly to a subset of neurons in the geniculate and nodose/petrosal ganglia, and further conveyed to a rostro-central region of the nucleus of solitary tract. In addition, WGA was expressed in solitary chemoreceptor cells in the nasal epithelium and transferred along the trigeminal sensory pathway to the brainstem neurons. The solitary chemoreceptor cells endogenously expressed T1R3 together with bitter taste receptors T2Rs. This result shows an exceptional signature of receptor expression. Thus, the t1r3-WGA transgenic mice revealed the sweet/umami gustatory pathways from taste receptor cells and the trigeminal neural pathway from solitary chemoreceptor cells.

Cholinergic modulation of neurons in the gustatory region of the nucleus of the solitary tract

The rostral portion of the nucleus of the solitary tract (rNST) is an obligatory relay for gustatory afferent input on its way to the forebrain. Previous studies have demonstrated excitation of rNTS neurons by glutamate and substance P and inhibition by gamma-aminobutyric acid (GABA) and met-enkephalin (ENK). Despite the existence of cholinergic neurons and putative terminals within the rNTS, there are no data on the effects of acetylcholine (ACh) on rNTS processing. Here, we use patch-clamp recording of rNTS neurons in vitro to examine ACh-mediated responses and voltage-gated conductances in these cells. Results revealed (1) intrinsic voltage-gated inhibition via activation of voltage-gated potassium A-channels (I(A)), found almost exclusively in the medial rNTS, and hyperpolarization-activated potassium/sodium channels (I(h)), found more frequently in the lateral rNST; and (2) ligand-gated inhibition via activation of muscarinic m2 ACh receptors (mAChRs) linked to inward rectifier potassium channels (K(ir)) evenly distributed throughout the rNTS, a mechanism dependent on cholinergic inputs. Muscarinic responses were blocked by AFDX-116, a selective m2 mAChR antagonist, and by BaCl2, an antagonist of K(ir) channels. In addition, many rNTS neurons exhibited excitation via alpha7 and non-alpha7 nicotinic AChRs. Non-alpha7 nAChRs, blocked by 10 microM mecamylamine, occurred more frequently in the lateral rNTS. In contrast, alpha7 nAChRs, blocked by 20 nM methyllcaconitine, were evenly distributed across the nucleus. As previously reported for voltage-activated conductances, none of these currents was related to neuronal morphology. These voltage- and ligand-dependent inhibitory mechanisms would be expected to contribute to the modulation of gustatory processing through the NST.

In vivo recordings from rat geniculate ganglia: taste response properties of individual greater superficial petrosal and chorda tympani neurones

Coding of gustatory information is complex and unique among sensory systems; information is received by multiple receptor populations located throughout the oral cavity and carried to a single central relay by four separate nerves. The geniculate ganglion is the location of the somata of two of these nerves, the greater superficial petrosal (GSP) and the chorda tympani (CT). The GSP innervates taste buds on the palate and the CT innervates taste buds on the anterior tongue. To obtain requisite taste response profiles of GSP neurones, we recorded neurophysiological responses to taste stimuli of individual geniculate ganglion neurones in vivo in the rat and compared them to those from the CT. GSP neurones had a distinct pattern of responding compared to CT neurones. For example, a small subset of GSP neurones had high response frequencies to sucrose stimulation, whereas no CT neurones had high response frequencies to sucrose. In contrast, NaCl elicited high response frequencies in a small subset of CT neurones and elicited moderate response frequencies in a relatively large proportion of GSP neurones. The robust whole-nerve response to sucrose in the GSP may be attributable to relatively few, narrowly tuned neurones, whereas the response to NaCl in the GSP may relate to proportionately more, widely tuned neurones. These results demonstrate the diversity in the initial stages of sensory coding for two separate gustatory nerves involved in the ingestion or rejection of taste solutions, and may have implications for central coding of gustatory quality and concentration as well as coding of information used in controlling energy, fluid and electrolyte homeostasis.

Synaptic and morphologic properties in vitro of premotor rat nucleus tractus solitarius neurons labeled transneuronally from the stomach

Neurons in the rat nucleus tractus solitarius (NTS) possess morphologic characteristics that have been correlated with the type of synaptic information they receive. These features have been described for viscerosensory neurons but not for premotor NTS neurons. The morphologic and synaptic features of neurons in the rat caudal NTS were assessed using whole-cell patch-clamp recordings and biocytin labeling in brainstem slices. Gastric-related premotor NTS neurons were identified for recording after inoculation of the stomach wall with a transneuronal retrograde viral label that reports enhanced green fluorescent protein. Three morphologic groups of NTS neurons were identified based on quantitative aspects of soma area and proximal dendritic arborization, measures that were consistent across slice recordings. The most common type of cell (group I) had relatively small somata and one to three sparsely branching dendrites, whereas the other groups had larger somata and more than three dendrites, which branched predominantly close to (group II) or distant from (group III) the soma. Voltage-clamp recordings revealed spontaneous excitatory and inhibitory postsynaptic currents in all neurons, regardless of morphology. Gastric-related premotor NTS neurons composed two of the three morphologic types (i.e., groups I and II). Compared with unlabeled neurons, these cells were less likely to receive constant-latency synaptic input from the tractus solitarius. These results refute the hypothesis that general patterns of synaptic input to NTS neurons depend on morphology. Gastric premotor neurons comprise a subset of NTS morphologic types, the organization of the viscerosensory input to which has yet to be defined.

Organization of cortico-cortical associative projections in rats exposed to ethanol during early postnatal life

The fine organization of cortico-cortical associative projections was investigated in adult rats exposed to inhalation of ethanol during the first postnatal week. Ethanol-treated and control animals received cortical injections of biotinylated dextran amine combined with N-methyl-D-aspartic acid, in order to obtain a Golgi-like retrograde labeling of associative pyramidal neurons. The results obtained from the analysis of labeling can be summarized as follows: (a) there are fewer associative projection neurons in ethanol-treated than in normal animals; (b) the ratio between the number of supragranular and infragranular associative neurons is higher in ethanol-treated animals compared to controls; (c) the basal dendrites of pyramidal associative cells of layer 2/3 display a simplified dendritic branching in ethanol exposed cases as compared to controls; (d) the cluster analysis shows that normal dendrites can be clearly subdivided into different groups according to their geometric properties, whereas dendrites from animals exposed to ethanol follow less robust grouping criteria. These differences are discussed in consideration of the functional alterations that characterize the fetal alcohol syndrome.

Mechanisms involved in the water intake-related pressor response in the rat

OBJECTIVE

In this study we aimed to characterize and clarify the mechanisms involved in the acute blood pressure increase observed concomitantly with water intake in moderately dehydrated rats.

DESIGN

Short-term water deprivation was employed as a model to induce controlled water intake to study concomitant cardiovascular responses in the rat.

METHODS

Male Wistar rats were deprived of water for 18-24 h before the experiments and were allowed to drink for 20 s periods during the experimental session. During these periods water intake was accompanied by steady arterial pressure increases. This pressor response was unaffected by topical anesthesia of the oral cavity. Direct administration of water into the stomach did not cause pressor responses. The pressor response was not affected by bilateral adrenal demedullation or by pretreatment with diazepam, homatropine methyl bromide, d(CH2)5 Tyr(Me)AVP, losartan or RX821002. The pressor response was significantly reduced by ganglionic blockade with mecamylamine or pretreatment with the alpha1-adrenoceptor antagonist, prazosin.

CONCLUSIONS

Our results show that: (1) short-term dehydration can be used as a model to study cardiovascular responses associated with water intake in rats; and (2) the sympathetic nervous system and vascular smooth muscle alpha1-adrenoceptors are involved in the pressor response to water intake by dehydrated rats.

Subthalamic nucleus lesions reduce low frequency oscillatory firing of substantia nigra pars reticulata neurons in a rat model of Parkinson's disease

Single unit recordings performed in animal models of Parkinson's disease revealed that output nuclei neurons display modifications in firing pattern and firing rate, which are supposed to give rise to the clinical manifestations of the illness. We examined the activity pattern of single units from the substantia nigra pars reticulata, the main output nuclei of the rodent basal ganglia, in urethane-anesthetized control and 6-hydroxydopamine-lesioned rats (a widespread model of Parkinson's disease). We further studied the effect of a subthalamic nucleus lesion in both experimental groups. Subthalamic nucleus lesion produces behavioral improvement in animal models of Parkinson's disease, and was expected to reverse the changes induced by 6-hydroxydopamine lesions. A meticulous statistical investigation, which included a non-biased classification of the recorded units by means of cluster analysis, allowed us to identify a low frequency oscillation of firing rate ( approximately 0.9 Hz) occurring in approximately 35% of the units recorded from 6-hydroxydopamine-lesioned rats, as the main feature differentiating 6-hydroxydopamine-lesioned and control rats. Subthalamic nucleus lesions significantly reduced the proportion of oscillatory units in 6-hydroxydopamine-lesioned rats. However, the population of nigral units recorded from rats bearing both lesions still differed significantly from control units. These results suggest that oscillatory activity in the basal ganglia output nuclei may be related to some clinical features of parkinsonism, and suggest a putative mechanism through which therapeutic interventions aimed at modifying subthalamic nucleus function produce clinical benefit in Parkinson's disease.

Development of salt-responsive neurons in the nucleus of the solitary tract

The rodent gustatory system has become a popular and useful model for the study of brain development because of this system's protracted period of postnatal maturation and its sensitivity to subtle changes in the animal's sensory environment. The goal of this investigation was to improve our understanding of dendritic remodeling exhibited by second-order gustatory neurons by presenting a comprehensive and definitive description of the development of the dendritic architecture of taste-sensitive neurons in the rostral nucleus of the solitary tract. Extracellular and intracellular recording and intracellular labeling techniques were used to examine the structure and function of individual gustatory neurons in three groups of rats: (1) Postnatal day 13-21 (PND13-21), (2) Postnatal day 22-28 (PND22-28), and (3) Adult (postnatal day 60-90). We found that neurons that responded to all three of the salts in our taste array ("Salt Sensitive") exhibited a striking increase in the number of dendritic branch points, maximum branch order, swelling density, and spine density between the PND13-21 and PND22-28 periods. These increases were followed by a period of dendritic remodeling during which the values for all measures except spine density decreased significantly. The neurons that did not respond to all three salts exhibited no change in the number of dendritic branches, branch order, or spine density during development, but they did undergo a decrease in swelling density. We also found that there was a significant decrease in the total dendritic length and cell volume of Salt Sensitive neurons between the PND22-28 and Adult periods, whereas the cells that did not respond to all three salts exhibited an increase in dendritic length and cell volume between postnatal day 28 and adulthood. Finally, we found that the dendrites of the Adult Salt Sensitive neurons were more restricted in the rostrocaudal axis than either the PND13-21 or PND22-28 Salt Sensitive cells. In contrast, there were no significant changes in the rostrocaudal extent of the dendritic arbors of cells that did not respond to all three salts. When viewed in the context of the extant literature and our own preliminary studies that used modified salt diets, we propose that these results provide strong support for the hypothesis that there is a relationship between postnatal dendritic development (particularly remodeling) and the animal's sensitivity to salts.

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.

Structure and function of gustatory neurons in the nucleus of the solitary tract. IV. The morphology and synaptology of GABA-immunoreactive terminals

In the visual, auditory and somatosensory systems, insight into the synaptic arrangements of specific types of neurons has proven useful in understanding how sensory processing within that system occurs. The neurotransmitter GABA is present in the nucleus of the solitary tract and based on the fact that the vast majority of cells respond to GABA, its agonists and antagonists, and that over 45% of synaptic terminals in the rostral subdivision of the nucleus of the solitary tract are GABA-immunoreactive, GABA is thought to play an important role in gustatory processing. The following study was carried out to establish the distribution of GABA-immunoreactive terminals within the nucleus of the solitary tract. Specifically, the distribution on to physiologically-identified gustatory neurons was determined using post-embedding electron immuno-histochemistry. GABA-immunoreactive terminals synapse with gustatory neuronal somata and all portions of their dendrites, but non-GABAergic terminals synapse only with distal dendrites of the gustatory cells and on to correspondingly small unidentified dendritic profiles in the neuropil. There is a differential distribution of two subtypes of GABA-immunoreactive terminals on to proximal and distal portions of the gustatory neurons as well. Finally, a model for the synaptic arrangements involving gustatory and GABAergic neurons is proposed.

Developmental changes in the dendritic architecture of salt-sensitive neurons in the nucleus of the solitary tract

Recent studies have provided evidence that brainstem gustatory neurons undergo substantial dendritic growth during a period of postnatal development that coincides with the maturation of their response to salts, suggesting a relationship (perhaps causal) between the physiology and morphology of developing salt-sensitive neurons. In an initial effort to explore this issue, we used extracellular and intracellular recording and intracellular labeling techniques to examine the structure and function of individual gustatory neurons in the rostral nucleus of the solitary tract (rNST) of young (postnatal day [P] 22-28) and adult rats. We found that P22-28 cells that responded to all three of the salts in our taste array had a greater dendritic length, a greater cell volume, and more dendritic branches than the cells that responded to one salt. As a group, taste-sensitive neurons in P22-28 animals had a higher maximum dendritic branch order and a trend toward more dendritic branch points than gustatory neurons in adult animals. The dendritic arbors of P22-28 taste neurons that responded to all three salts were larger (greater surface area and volume), more extensive in the rostrocaudal axis and exhibited a higher maximum branch order, more branch points and higher swelling density than adult cells that responded to all three salts. These results demonstrate that the morphology of salt-sensitive gustatory neurons in developing animals is closely related to the number of salts that evoke a response. The data also support the postulate that gustatory neurons in the rat brainstem undergo substantial dendritic remodeling between the fourth week of life and adulthood. Dendritic remodeling may play an important role in the maturation of the rNST response to NaCl.

The use of cluster analysis for cell typing

In invertebrates, large neurons, identifiable in each animal, have proven to be useful models for investigating basic neurobiological phenomena. In vertebrates, the number of neurons and the complexity of nervous systems increase and 'identifiability' is lost. To compensate for this, other approaches must be adopted to study the vertebrate brain. One successful approach has been to identify cell types, recognizable in each individual. The identification of cell types in central nuclei has helped us understand the organization of these nuclei and has provided an important foundation for examining possible relationships between the structure and function of neurons. Unfortunately, not all nuclei are composed of neurons of readily identifiable types. Nuclei lacking distinct cell types are, in general, less well understood than nuclei with morphologically distinct cell types. This article describes a statistical approach known as cluster analysis that we used to define cell types in the nucleus of the solitary tract-a nucleus that had been suggested to contain identifiable cell types but within which cell typing had proven difficult. Similar techniques have been used to classify Golgi-impregnated cells in the ventrobasal complex of the dog, the subthalamic nucleus of the bushbaby and the human retina. Cluster analysis has also been used in the gustatory system, in general, and nucleus of the solitary tract, in specific, to classify electrophysiologically characterized cells. The method also includes a technique for verifying the utility of the resulting classification.

Ascending and descending projections from the rostral nucleus of the solitary tract originate from separate neuronal populations

Anterograde studies have shown that neurons within the rostral (gustatory) nucleus of the solitary tract project to the parabrachial nucleus, as well as to sites within the medulla including the reticular formation and caudal nucleus of the solitary tract. In order to determine the degree to which the same neurons contribute to both projections, injections of retrograde tracers were made simultaneously into both the parabrachial nuclei and medullary reticular formation of the rat. Only a small proportion of neurons were double labeled. Consistent with studies in hamster, labeled neurons projecting to the parabrachial nuclei in rat consisted of both stellate and elongate neurons, concentrated within the central subdivision of the rostral nucleus of the solitary tract. Injections into the medullary reticular formation also labeled both stellate and elongate neurons but these were concentrated in the ventral subdivision of the nucleus. The results of the present study demonstrate that different populations of neurons in the nucleus of the solitary tract contribute to ascending and descending pathways. This suggest a possible functional specialization within the nucleus of the solitary tract for those neurons whose output eventually reaches the forebrain compared to those neurons with local connections.

Structure and function of gustatory neurons in the nucleus of the solitary tract: II. Relationships between neuronal morphology and physiology

This study employed intracellular recording and labeling techniques to examine potential relationships between the physiology and morphology of brainstem gustatory neurons. When we considered the neuronal response to the four "prototypic" tastants, we were able to demonstrate a positive correlation between breadth of responsiveness and the number of dendritic branch points. An analysis of the response to eight tastants also revealed an association between dendritic spine density and the breadth of responsiveness, with more narrowly tuned neurons exhibiting more spines. Interestingly, a neuron's "best response" was a relatively poor predictor of neuronal morphology. When we focused on those neurons that responded to only one tastant, however, a number of potentially important relationships became apparent. We found that the cells that only responded to quinine were smaller than the neurons that only responded to NaCl, HCl, or sucrose. The HCl-only neurons, however, were more widespread in the rostrocaudal dimension that the neurons that only responded to NaCl. A number of additional structure-function relationships were identified when we examined the neuronal response to selected tastants. We found that neurons that responded to sucrose but not quinine, as well as neurons that responded to quinine but not sucrose, were more widespread in the mediolateral dimension than neurons that responded to both sucrose and quinine. We also discovered that the neurons that responded to NaCl, but not to NH4Cl or KCl, were larger than neurons that responded to all three salts. We believe that these results support the hypothesis that there are relationships between the structure and function of gustatory neurons in the nucleus of the solitary tract, with the data highlighting the importance of three themes: 1) the relationship between dendritic specializations and tuning, 2) the relationship between dendritic arbor orientation and response properties, and 3) the potential importance of stimulus-specific neurons.

Cell types in the rostral nucleus of the solitary tract

The rostral subdivision of the nucleus of the solitary tract (rNST) is not laminated or otherwise organized into clearly segregated cell types. Although a variety of experimental approaches have yielded a wealth of information, the definition of cell types in this nucleus has been difficult, as reflected in the sometimes contradictory literature on morphological cell typing. The present review discusses how rNST neurons have been classified in the past and adds to the evidence that distinct neuron types exist in this nucleus. Consistencies in the literature, as well as inconsistencies among studies, are discussed. Furthermore, we have included a summary of our own results that help provide additional data relevant to cell typing. The definition of cell types in other central nervous system nuclei has helped our understanding of the organization of these nuclei and our understanding of the relationships between the morphology and function of neurons. It is hoped that this synthesis of the extant literature will facilitate the many ongoing efforts to correlate neuronal morphology and physiology in the gustatory system.