Convergence of oral and extraoral information in the superior secondary gustatory nucleus of the channel catfish

Connections of the commissural nucleus of Cajal in the goldfish, with special reference to the topographic organization of ascending visceral sensory pathways

The primary general visceral nucleus of teleosts is called the commissural nucleus of Cajal (NCC). The NCC of goldfish has been divided into the medial (NCCm) and lateral (NCCl) subnuclei that receive inputs from subdiaphragmatic gastrointestinal tract and the posterior pharynx, respectively. Fiber connections of the NCC were examined by tract-tracing methods in the goldfish Carassius auratus. Tracer injections into the NCC suggested that the NCC projects directly not only to the secondary visceral sensory region in the rhombencephalic isthmus and other brain stem centers, but also to the forebrain, similar to the situations in mammals, birds, and the Nile tilapia. Although fiber connections of the NCCm and NCCl were basically similar, the NCCm was the more important source of ascending general visceral fibers to the forebrain. Topographic organization was recognized regarding projections to the isthmic secondary visceral sensory zone; input from the NCCm is represented in the secondary general visceral sensory nucleus, while input from the NCCl in the lateral edge of the secondary gustatory nucleus. Moreover, specific injections into different regions of the vagal lobe revealed that the dorsomedio-ventrolateral axis of the lobe is represented in the lateromedial axis of the secondary gustatory nucleus. These observations suggest fine topographic organization of ascending visceral sensory pathways to the isthmic secondary centers. It should also be noted that the reception of primary afferents from the posterior pharynx and projections to the secondary gustatory nucleus suggest that the NCCl may be regarded as a gustatory rather than a general visceral sensory structure.

The pallium and mind/behavior relationships in teleost fishes

Three interrelated pallial areas mediate behaviors reflective of the cognitive and emotional aspects of the teleost mind. The dorsocentral area (Dc) has specific associations with both of the other pallial areas and projects to major lower sensorimotor centers. While Dc generally functions as an output or modulatory component of the pallium, it probably also has integrative features important for certain behaviors. The dorsolateral region (Dl) has dorsal (Dld) and ventral (Dlv) divisions. In association with the dorsal part of Dc, Dld processes visual information via a 'tectal loop' which is hypertrophied in certain coral reef species. The region also receives afferents related to other modalities. Functionally, Dld resembles the tetrapod sensory neocortex. Anatomical and behavioral data (i.e. involvement in spatial and temporal learning) strongly suggest that Dlv is homologous to the tetrapod hippocampus. The dorsal part of the dorsomedial area (Dmd) processes acoustic, lateral line, gustatory, and multimodal information. It has reciprocal connections with Dld such that the Dmd and Dld together can be considered the teleost nonolfactory 'sensory pallium'. Behavioral studies indicate that Dmd creates the 'fear' necessary for defense/escape and avoidance behaviors and controls several components of species-typical sexual and aggressive behavior (responsiveness, behavioral sequencing, and aspects of social cognition). While the functional results generally support the anatomical evidence that Dmd is homologous to the tetrapod amygdala, a case can also be made that Dmd has 'sensory neocortex-like' features. Understanding the interrelationships of Dl, Dmd, and Dc seems a necessary 'next step' in the identification of the neural processes responsible for mental experiences such as those of a unified sensory experience (Umwelt) or of feelings of discomfort versus well-being.

Anterior and posterior oral cavity responsive neurons are differentially distributed among parabrachial subnuclei in rat

The responses of single parabrachial nucleus (PBN) neurons were recorded extracellularly to characterize their sensitivity to stimulation of individual gustatory receptor subpopulations (G neurons, n = 75) or mechanical stimulation of defined oral regions (M neurons, n = 54) then localized to morphologically defined PBN subdivisions. Convergence from separate oral regions onto single neurons occurred frequently for both G and M neurons, but converging influences were more potent when they arose from nearby locations confined to the anterior (AO) or posterior oral cavity (PO). A greater number of G neurons responded optimally to stimulation of AO than to PO receptor subpopulations, and these AO-best G neurons had higher spontaneous and evoked response rates but were less likely to receive convergent input than PO-best G neurons. In contrast, proportions, response rates, and convergence patterns of AO- and PO-best M neurons were more comparable. The differential sensitivity of taste receptor subpopulations was reflected in PBN responses. AO stimulation with NaCl elicited larger responses than PO stimulation; the converse was true for QHCl stimulation. Within the AO, NaCl elicited a larger response when applied to the anterior tongue than to the nasoincisor duct. Hierarchical cluster analysis of chemosensitive response profiles suggested two groups of PBN G neurons. One group was composed of neurons optimally responsive to NaCl (N cluster); the other to HCl (H cluster). Most N- and H-cluster neurons were AO-best. Although they were more heterogenous, all but one of the remaining G neurons were unique in responding best or second-best to quinine and so were designated as quinine sensitive (Q+). Twice as many Q+ neurons were PO- compared with AO-best. M neurons were scattered across PBN subdivisions, but G neurons were concentrated in two pairs of subdivisions. The central medial and ventral lateral subdivisions contained both G and M neurons but were dominated by AO-best N-cluster G neurons. The distribution of G neurons in these subdivisions appeared similar to distributions in most previous studies of PBN gustatory neurons. In contrast to earlier studies, however, the external medial and external lateral-inner subdivisions also contained G neurons, intermingled with a comparable population of M neurons. Unlike cells in the central medial and ventral lateral subnuclei, nearly every neuron in the external subnuclei was PO best, and only one was an N-cluster cell. In conclusion, the present study supports a functional distinction between sensory input from the AO and PO at the pontine level, which may represent an organizing principle throughout the gustatory neuraxis. Furthermore, two morphologically distinct pontine regions containing orosensory neurons are described.

Parallel medullary gustatospinal pathways in a catfish: possible neural substrates for taste-mediated food search

Taste and tactile fibers in the facial nerve of catfish innervate extraoral taste buds and terminate somatotopically in the facial lobe (FL)-a medullary structure crucial for gustatory-mediated food search. The present study was performed to determine the neural linkages between the gustatory input and the spinal motor output. Spinal injections of horseradish peroxidase (HRP) label spinopetal cells in the octaval nuclei, the nucleus of the medial longitudinal fasciculus, and reticulospinal neurons (Rsps) in the brainstem medial reticular formation (RF), including the Mauthner cell. A somatotopically organized, direct faciospinal system originating from superficial cells scattered in the lateral lobule of the facial lobe (ll) is also labeled. The brainstem reticulospinal cells are segmentally organized into 14 clusters within eight segments of the reticular formation and includes one cluster (RS5) directly ventral to the FL. Injections of HRP or fluorescent tracers into the medial lobule of the FL label a facioreticular projection terminating around the Rsps of RS5. DiI injections into this area of the RF retrogradely label deeply situated bipolar neurons, especially in the medial and intermediate lobules of the FL. Electrophysiological recordings in and around RS5 show units with large receptive fields and with responses to chemical and tactile stimulation. The FL projects to the spinal cord via two pathways: (1) a topographically organized direct faciospinal pathway, and (2) an indirect facioreticulospinal pathway in which reticular neurons process and integrate gustatory information before influencing spinal circuitry for motor control during food search.

Axonal projection patterns of neurons in the secondary gustatory nucleus of channel catfish

The second gustatory nucleus of teleost fishes receives ascending fibers from the primary gustatory center in the medulla and sends efferent fibers to several nuclei in the inferior lobe of the diencephalon. Similar to the corresponding parabrachial nucleus in birds and mammals, the secondary gustatory nucleus of catfish consists of several cytoarchitectonically distinct subnuclei which receive input from different portions of the primary gustatory nuclei. However, it is unclear how the subnuclear organization relates to the processing of gustatory information in the hindbrain and the subsequent transmission of that information to the forebrain. To determine whether cells within different subnuclei of the secondary gustatory nucleus of channel catfish project to different diencephalic targets, single cells were intracellularly labeled with biocytin. Three subnuclei have been identified in the secondary gustatory nucleus: a medial subnucleus spanning most of the rostrocaudal extent of the nucleus, a central subnucleus and a dorsal subnucleus, the latter two located in the rostrolateral portion of the complex. Cells throughout the secondary gustatory nucleus typically possessed similar collateral projections to several nuclei in the inferior lobe, although four of the six cells filled in the medial subnucleus projected only to nucleus centralis. The only apparent subnucleus-specific projection pattern involved cells at the rostral edge of the secondary gustatory nucleus and in the secondary visceral nucleus. Axons of these cells terminated only in restricted portions of nucleus lobobulbaris. These results suggest that efferents from different subnuclei of the secondary gustatory nucleus of catfish, like those of the parabrachial nucleus of birds and mammals, do not possess simple, topographical projections to target nuclei in the diencephalon.

Taste and tactile responsiveness of neurons in the posterior diencephalon of the channel catfish

Many teleosts possess an enlargement of the ventral diencephalon called the inferior lobe. In ostariophysine species (e.g., carps and catfishes), this region receives ascending fibers from the primary and secondary gustatory centers in the hindbrain. Extracellular unit activity was recorded from identified nuclei in the inferior lobe of the channel catfish to characterize taste and tactile responsiveness from the different nuclei associated with gustatory projections. Taste responses (to amino acids and nucleotides) were recorded from units in the nucleus centralis (nCLI), nucleus lobobulbaris (caudal portion--nLB, rostrolateral portion--rl nLB, and parvicellular portion--nLBp), and lateral thalamic nucleus (nLT), supporting the proposed gustatory role for these nuclei. Tactile responsiveness was distinct between different nuclei in the caudal inferior lobe. Units from the nCLI and nLB had lower spontaneous activity than those from other nuclei, and typically had receptive fields including the whole extraoral body surface, ipsilaterally. Units from the rl nLB and nLBp had receptive fields, often including both oral and extraoral surfaces, bilaterally, but rl nLB receptive fields typically included the whole body, while those from nLBp units were often restricted to the head and mouth. The apparent electrophysiological distinction between these nuclei, combined with their different connectivity patterns, suggest that the gustatory nuclei in the inferior lobe of channel catfish are involved in various different sensory processing mechanisms.

Diencephalic gustatory connections in the channel catfish

Vertebrate gustatory systems include a tertiary ascending pathway from a secondary gustatory nucleus in the hindbrain to several forebrain nuclei. This connection is prominent in catfish, corresponding to their highly developed sense of taste. Iontophoretic injections of horseradish peroxidase were used to identify the specific target nuclei of the tertiary gustatory pathway in channel catfish and to characterize those nuclei by their respective connections. Efferents from the secondary gustatory nucleus (nGS) ascend in the tertiary gustatory tract to the caudal inferior lobe, where they terminate caudally in the nucleus lobobulbaris (nLB) and nucleus centralis (nCLI), and rostrally in the nucleus diffusus (nDLI). Secondary projections from the facial lobe (FL) also terminate in the nLB and in the nucleus subglomerulus (nSG). The nLB forms three cell groups (caudal--nLB, rostrolateral--rl nLB, parvicellular--nLBp), which project to the facial lobe, vagal lobe, and telencephalon, respectively. Cells from the nCLI project throughout the caudal inferior lobe and to the acousticolateral torus semicircularis and telencephalon, while the nDLI and nSG have intrinsic connections within the inferior lobe. The lateral thalamic nucleus projects from this region back to the nGS. Through these identified connections several mechanisms for the processing of gustatory information can be proposed. The descending projections from the nLB and nLT could provide feedback to the primary and secondary gustatory nuclei, and could modulate feeding-related motor circuits in the medulla. The connections of nCLI and nLBp with the telencephalon allow for the involvement of gustation in learning processes and other complex behaviors.