Morphological types of neurons located at taste-responsive sites in the solitary nucleus of the hamster

Inhibitory modulation of optogenetically identified neuron subtypes in the rostral solitary nucleus

Inhibition is presumed to play an important role in gustatory processing in the rostral nucleus of the solitary tract (rNST). One source of inhibition, GABA, is abundant within the nucleus and comes both from local, intrasolitary sources and from outside the nucleus. In addition to the receptor-mediated effects of GABA on rNST neurons, the hyperpolarization-sensitive currents, Ih and IA, have the potential to further modulate afferent signals. To elucidate the effects of GABAergic modulation on solitary tract (ST)-evoked responses in phenotypically defined rNST neurons and to define the presence of IA and Ih in the same cells, we combined in vitro recording and optogenetics in a transgenic mouse model. This mouse expresses channelrhodopsin 2 (ChR2) in GAD65-expressing GABAergic neurons throughout the rNST. GABA positive (GABA+) neurons differed from GABA negative (GABA-) neurons in their response to membrane depolarization and ST stimulation. GABA+ neurons had lower thresholds to direct membrane depolarization compared with GABA- neurons, but GABA- neurons responded more faithfully to ST stimulation. Both IA and Ih were present in subsets of GABA+ and GABA- neurons. Interestingly, GABA+ neurons with Ih were more responsive to afferent stimulation than inhibitory neurons devoid of these currents, whereas GABA- neurons with IA were more subject to inhibitory modulation. These results suggest that the voltage-gated channels underlying IA and Ih play an important role in modulating rNST output through a circuit of feedforward inhibition.

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.

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

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

Neural coding of taste by simultaneously recorded cells in the nucleus of the solitary tract of the rat

The nucleus of the solitary tract (NTS) receives input from taste buds on the rostral tongue from the chorda tympani (CT) nerve. How this input is processed by the NTS was the subject of the present investigation. Here we used tetrodes to record from pairs or small groups of NTS cells as they responded to taste stimuli or electrical stimulation of the CT nerve in urethane-anesthetized rats. Once a pair (or small group) of NTS cells were isolated and identified as showing an evoked response to CT nerve stimulation, taste stimuli were presented in separate trials. Tastants consisted of 0.1 M NaCl, 0.01 M HCl, 0.01 M quinine HCl, and 0.5 M sucrose. Responses to various patterns of CT stimulation were then recorded. Functional connections among simultaneously recorded NTS cells were implied from analysis of cross-correlation functions of spike trains. We identified four groups of cells, not all of which responded to taste, with staggered latencies of response to CT nerve stimulation, ranging from ∼3 to 35 ms in ∼8- to 12-ms increments. Analyses of putative functional connectivity along with latencies of CT-evoked responses suggested that CT input arrives at the NTS in pulses or waves, each of which activates recurrent excitatory connections among NTS cells. These actions may amplify the incoming signal and refine its temporal pattern.

Afferent connections of the parabrachial nucleus in C57BL/6J mice

Although the mouse is an experimental model with an increasing importance in various fields of neuroscience, the characteristics of its central gustatory pathways have not yet been well documented. Recent electrophysiological studies using the rat and hamster have revealed that taste processing in the brainstem gustatory relays is under the strong influence of inputs from forebrain gustatory structures. In the present study, we investigated the organization of afferent projections to the mouse parabrachial nucleus (PbN), which is located at a key site between the brainstem and gustatory, viscerosensory and autonomic centers in the forebrain. We made injections of the retrograde tracer fluorogold centered around the "waist" area of the PbN, whose neurons are known to be highly responsive to taste stimuli. Retrogradely labeled neurons were found in the infralimbic, dysgranular and agranular insular cortex as well as the claustrum; the bed nucleus of the stria terminalis and the substantia innominata; the central nucleus of the amygdala; the lateral and medial preoptic areas, the paraventricular, the dorsomedial, the ventromedial, the arcuate, and the lateral hypothalamic areas; the periaqueductal gray, the substantia nigra pars compacta, and the ventral tegmental area; the supratrigeminal nucleus, rostral and caudal nucleus of the solitary tract; the parvicellular intermediate and gigantocellular reticular nucleus; the caudal and interpolar divisions of the spinal trigeminal nucleus, dorsomedial spinal trigeminal nucleus, and the area postrema. Numbers of labeled neurons in the main components of the gustatory system including the insular cortex, bed nucleus of the stria terminalis, central nucleus of the amygdala, lateral hypothalamus, and rostral nucleus of the solitary tract were quantified. These results are basically consistent with those of the previous rat and hamster studies, but some species differences were found. Functional implications of these afferent inputs are discussed with an emphasis on their role in taste.

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.

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.

Relationship between structure and function of neurons in the rat rostral nucleus tractus solitarii

To investigate the relationship between the structure and function of neurons in the rostral (gustatory) nucleus tractus solitarii (rNTS), we analyzed the morphological and biophysical properties of rNTS neurons by performing whole-cell recordings in a brain slice preparation. Overall, neurons (n = 58) had a mean somal diameter of 16 microns, an average dendritic length of 598 microns, an average dendritic thickness of 0.91 microns, and a spine density of 0.037 spines/microns. Neurons were separated into three groups (elongate, multipolar, and ovoid) on the basis of previously established morphological criteria. The highest percentage (49%) of neurons were classified as ovoid, while 35% were multipolar and only 16% were elongate. The most frequently observed firing pattern, in all three cell types, elicited by a 1,200 ms, 100 pA depolarizing current pulse was a regularly firing spike train. However, the intrinsic firing properties of the remaining neurons were different. Thirty-one percent of the ovoid neurons responded with a short burst of action potentials and 44% of the elongate neurons showed a delay in the onset of the spike train following a hyperpolarizing prepulse. Less than 16% of the multipolar neurons demonstrated either of these firing characteristics. Therefore, rNTS neurons with similar morphology do not have unique biophysical properties. However, the data suggest that there may be subpopulations of the three morphological types, each of which displays a different firing pattern. Since the structure and function of the three morphological groups were not strictly correlated, these subpopulations may represent functional groups.

Distribution of synapses on identified cell types in a gustatory subdivision of the nucleus of the solitary tract

Two morphological types of neurons in the rostral nucleus of the solitary tract (NST) in the hamster send axons to the parabrachial nucleus (PBN). Elongate cells have oval cell bodies and 2 mediolaterally oriented primary dendrites. Large stellate cells have polygonal cell bodies and 3-5 radiating primary dendrites. Both cell types are located in the rostral central subdivision of the NST, surrounded by primary afferent axons from the oral cavity. This study uses electron microscopy to evaluate the synaptic inputs to horseradish peroxidase (HRP)-labelled elongate and stellate PBN projection cells. Three types of axon terminals provide most of the synapses on the labelled cells. Primary-like terminals contain large, clear, round vesicles and engage in asymmetrical synaptic junctions; they resemble gustatory (facial) afferent terminals identified previously (Whitehead, J. Comp. Neurol. 244:72, 1986). Axon terminals containing small, pleomorphic vesicles (SP terminals), form symmetrical junctions, and resemble Golgi II endings. Terminals containing medium-sized pleomorphic vesicles (MP terminals) form asymmetrical junctions. These types of axon terminals distribute differentially on the labelled cells. Primary-like inputs are largely restricted to distal dendrites and their spines. SP terminals provide more synaptic coverage than primary-like or MP terminals; for both cell types the SP inputs are concentrated proximally on dendrites and cell bodies. The data suggest that elongate and large stellate cells function as second-order projection neurons in the ascending taste system. The density, spatial distribution, and timing of activation of the various types of synapses could relate to the electrophysiological response properties of the projection neurons.