Nobuhiko Sawai

A New Interpretation on the Homology of the Teleostean Telencephalon Based on Hodology and a New Eversion Model

Various hypotheses regarding the homology of the teleostean telencephalon with that of other vertebrates have been proposed to date. However, a firm conclusion on this issue has yet to be drawn. We propose here a new hypothesis with a new eversion model. Our hodological data and the analysis of dorsal telencephalic organization in adult cyprinids suggest that: (1) the area dorsalis pars posterior corresponds to the lateral pallium; (2) ventral region of area dorsalis pars lateralis to the medial pallium; (3) pars medialis, dorsal region of pars lateralis, pars dorsalis, and pars centralis of the area dorsalis to the dorsal pallium, and (4) nucleus taenia to the ventral pallium. We propose in a three dimensional model that the eversion process occurs not only dorsolaterally but also caudolaterally. We consider that the caudally directed component dominates for ventral zones of the pallium, or the lateral and ventral pallia; and the periventricular surface of these zones shift caudally, laterally, and then rostrally in teleosts with pronounced telencephalic eversion. This new model fits well with the putative homology based on hodology and the organization of telencephalic divisions in the adult brain.

Preoptic gonadotropin-releasing hormone(GnRH) neurons innervate the pituitary in teleosts

In most teleosts, there are three groups of gonadotropin-releasing hormone (GnRH) neurons. In this study we addressed the question of GnRH neuronal innervation of the pituitary in the dwarf gourami and the tilapia using immunocytochemistry combined with biocytin tract tracing. Biocytin was applied to the pituitary attached to the brain in vitro. Similar results were obtained in both species. GnRH neurons retrogradely labeled with biocytin were observed only in the preoptic area. These results indicate that preoptic GnRH neurons innervate the pituitary. Negative labeling of biocytin in the terminal-nerve and midbrain GnRH neurons suggests that these two GnRH neuronal populations do not project to the pituitary. Biocytin-positive but GnRH-negative neurons were also observed in the preoptic area and the ventromedial parts of the hypothalamus, suggesting neuropeptidergic and aminergic innervation of the pituitary besides GnRH.

TELENCEPHALIC ASCENDING GUSTATORY SYSTEM IN A CICHLID FISH, OREOCHROMIS (TILAPIA) NILOTICUS

Central fiber connections of the gustatory system were examined in a percomorph fish Oreochromis (Tilapia) niloticus by means of the horseradish peroxidase (HRP), biocytin, and carbocyanine dye tracing methods. The primary gustatory areas in tilapia are the facial, glossopharyngeal, and vagal lobes of the medulla. The secondary gustatory nucleus (SGN) is a dumb‐bell‐shaped structure located in the isthmic region. In the SGN, there are two or three layers of neurons lining the ventromedial periphery of the nucleus and a molecular layer constituting of the major part of the nucleus. The SGN receives bilateral projections from the facial lobes and ipsilateral projections from the glossopharyngeal and vagal lobes. Ascending fibers originating from the SGN form the ipsilateral tertiary gustatory tract. A major part of the tract courses rostrally and terminates ipsilaterally in several diencephalic nuclei: the preglomerular tertiary gustatory nucleus (pTGN), the posterior thalamic nucleus, the nucleus diffusus lobi inferioris, the nucleus centralis of inferior lobe, and the nucleus recessus lateralis. The remaining small fiber bundle enters the medial and lateral forebrain bundles and terminates directly in two telencephalic regions; the area ventralis pars intermedia (Vi) and the area dorsalis pars posterior (Dp). Ascending fibers from the pTGN pass through the lateral forebrain bundle and terminate ipsilaterally in the dorsal region of area dorsalis pars medialis (dDm) of the telencephalon. Following biocytin injections into the dDm, small, round cells were labeled in the pTGN. After biocytin injections into the Vi and Dp of the telencephalon, retrogradely labeled cells were found in the ipsilateral SGN.

Developmental and aging change of orexin-A and -B immunoreactive neurons in the male rat hypothalamus

Orexin/hypocretin is indicated to affect various physiological functions and behaviors, such as energy balance, feeding, wake-sleep cycle, stress response, and reproduction. This study investigated postnatal development and aging changes of the orexin neuron in the male rat hypothalamus. The brain tissue of rats from 1 week to 24 months old was analyzed by immunohistochemistry for two forms of orexin peptides, orexin-A and -B. The number of immunoreactive cells for each age group was counted and the immunoreactive intensity was also analyzed in order to reveal the changes in the number of expressing cells and the relative amount of the peptides. The number of orexin immunoreactive cells increased from postnatal 2 weeks to maturation, then slightly decreased and stabilized until the age of 8 months old, but it was significantly decreased by 24 months old. The intensity of the immunoreaction followed almost the same pattern. Our findings demonstrate that orexin neurons are increased during maturation and then are significantly decreased during the period from 8 to 24 months old, indicating an involvement of orexin in the physiological changes in rat aging such as energy balance, sleep, stress response, and reproduction.

Fiber connections of the corpus mamillare in a percomorph teleost, tilapia Oreochromis niloticus.

The hypothalamus and perhaps its function appear to be similar among vertebrates. Thus, studying the teleostean hypothalamus could be a good model for understanding common neural circuits and mechanisms retained through the vertebrates. However, connections of the inferior lobe, which is considered the hypothalamus in teleosts, is poorly known. The corpus mamillare (CM) is a nucleus of the inferior lobe named after the mammalian mamillary body based on similarities in external morphology. Afferent connections of the CM have been reported only in cypriniform teleosts. These include projections from the nucleus pretectalis superficialis pars magnocellularis, a nucleus lacking in percomorph teleosts, and projections from the secondary gustatory nucleus. Efferent connections of the CM have not been reported in teleosts. In the present study, the CM and its subdivisions and the connections of these subnuclei were identified in isolated and maintained brains of tilapia Oreochromis niloticus by local DiI and biocytin injection. Afferent connections confirmed by reciprocal injections were from the nucleus diffusus lobi inferioris (NDLI) and the nucleus diffusus tori lateralis (NDTL). Efferent connections of each CM subnuclei were also reciprocally confirmed. These connections were to the area dorsalis pars medialis of the telencephalon, the nucleus ventromedialis (NVM) of the thalamus, the tectum opticum (TO), and the nucleus posterioris periventricularis. Because the NDLI is known to receive gustatory information in tilapia, the CM could relay gustatory inputs to multisensory areas, the TO and NVM, for which there are no current reports regarding gustatory inputs.

Fiber connections of the lateral valvular nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

Fiber connections of the lateral valvular nucleus were investigated in a percomorph teleost, the tilapia (Oreochromis niloticus), by tract‐tracing methods. Following tracer injections into the lateral valvular nucleus, neurons were labeled in the ipsilateral dorsal part of dorsal telencephalic area, corpus glomerulosum pars anterior, dorsomedial thalamic nucleus, central nucleus of the inferior lobe, mammillary body, semicircular torus, valvular and cerebellar corpus, in the bilateral rostral regions of the central part of dorsal telencephalic area, dorsal region of the medial part of dorsal telencephalic area, habenula, anterior tuberal nucleus, posterior tuberal nucleus, and spinal cord, and in the contralateral lateral funicular nucleus. Labeled fibers and terminals were found in the ipsilateral cerebellar corpus and bilateral valvula of the cerebellum. Tracers were injected into portions of the telencephalon, pretectum, inferior lobe, and cerebellum to confirm reciprocally connections with the lateral valvular nucleus and to determine afferent terminal morphology in the lateral valvular nucleus. Telencephalic fibers terminated mainly in a dorsolateral portion of the lateral valvular nucleus. Terminals from the corpus glomerulosum pars anterior, central nucleus of the inferior lobe, and mammillary body showed more diffuse distributions and were not confined to particular portions of the lateral valvular nucleus. Labeled terminals in the lateral valvular nucleus were cup‐shaped or of beaded morphology. These results indicate that the lateral valvular nucleus receives projections from various sources including the telencephalon, pretectum, and inferior lobe to relay information to the valvular and cerebellar corpus. In addition, the corpus glomerulosum pars anterior in tilapia is considered to be homologous to the magnocellular part of superficial pretectal nucleus in cyprinids. J. Comp. Neurol. 474:209–226, 2004.

An Immunohistochemical Study on the Expressional Dynamics of Kisspeptin Neurons Relevant to GnRH Neurons Using a Newly Developed Anti-kisspeptin Antibody

To investigate the reported discrepancy regarding the immunohistochemical expression of kisspeptin neurons, we produced a new antibody against synthetic peptide containing the same amino acid residual sequence as rat kisspeptin10. Although the antibody showed cross-reactivities against neurons other than kisspeptin neurons, these cross-reactivities were excluded by preabsorption with neuropeptide FF (NPFF). Immunohistochemistry using the antibody preabsorbed with NPFF showed specific kisspeptin immunoreactivities (IRs) in the arcuate nucleus (Arc), the inner layer of the median eminence, and the infundibulum in the rat hypothalamus. IRs were more intense in the adult female rats than in the males. This sexual dimorphism became observable at the 7th day after birth. These IRs intensified with age. Ovariectomy enhanced the IRs in the Arc in the female rats. In contrast, regarding IRs in the anteroventral periventricular nucleus (AVPV), only a few immunoreactive fibers were detected in the adult rats. We applied this antibody for the investigation of the interaction between kisspeptin fibers and gonadotropin-releasing hormone (GnRH) neurons. No direct morphological interaction between the cell bodies of GnRH neurons and kisspeptin fibers was observed in the medial preoptic area. Many projections of kisspeptin fibers were found close to the GnRH fibers in the lateral part of the median eminence. However, we did not observe any direct contact between kisspeptin fibers and the GnRH fibers. These results suggest that kisspeptin neurons regulate GnRH neurons not via the synaptic contact but via another information transmission process that is not synapse-dependent, such as volume transmission.

Topographical organization of an indirect telencephalo-cerebellar pathway through the nucleus paracommissuralis in a teleost, Oreochromis niloticus.

The nucleus paracommissuralis (NPC) of teleosts is a relay nucleus of an indirect telencephalo-cerebellar pathway. However, cells of origin in telencephalic subdivisions and terminal patterns of the NPC fibers in the cerebellum remain unclear. We studied these issues by means of tract-tracing methods in a cichlid, tilapia (Oreochromis niloticus). After tracer injections into the NPC, retrogradely labeled cells were found bilaterally in dorsal and ventral regions of the area dorsalis telencephali pars centralis (dDc and vDc) and area dorsalis telencephali pars dorsalis (Dd). Anterogradely labeled terminals were found in a caudal part of the bilateral corpus cerebelli (CC). The labeled terminals were restricted in the granular layer, which can be divided into dorsal and ventral regions based on cytoarchitecture. We injected tracers separately into the three telencephalic portions (dDc, vDc, and Dd) and into the dorsal or ventral regions of granular layer in the caudal CC. The results revealed a topographical organization of the indirect telencephalo-cerebellar pathway. A medial portion of the NPC received fibers from the vDc and projected to the ventral region of the caudal CC. An intermediate portion of the NPC received fibers from the dDc and Dd, and in turn projected to the dorsal region of the caudal CC. A lateral portion of the NPC received fibers from the Dd and in turn projected to the dorsal region of the caudal CC. The Dc is known to receive visual input via the area dorsalis telencephali pars lateralis, and the Dd is presumably a multimodal telencephalic portion. The present study suggests that the indirect telencephalo-cerebellar pathway through the NPC might convey descending visual and multimodal information to the CC in a topographical manner. We also demonstrated other indirect telencephalo-cerebellar pathways through the nucleus lateralis valvulae and the area pretectalis.

Retinal projections and retinal ganglion cell distribution patterns in a sturgeon (Acipenser transmontanus), a non-teleost actinopterygian fish.

Retinal projections in a sturgeon were studied by injecting biocytin or HRP into the optic nerve. The target areas are the preoptic area, thalamus, area pretectalis, nucleus of posterior commissure, optic tectum, and nuclei of the accessory optic tract. Furthermore, a few labeled fibers and terminals were found in a ventrolateral area of the caudal telencephalon. All retinal projections are bilateral, although contralateral projections were more heavily labeled. Retrogradely labeled neurons were found in the ventral thalamus bilaterally. Retinal projections in sturgeons are similar to those of other non-teleost actinopterygians and chondrichthyans (sharks), in terms of the targets and extent of bilateral projections. Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal whole mount preparations. The highest density areas were found in the temporal and nasal retinas, and a dense band of ganglion cells was observed along the horizontal axis between the nasal and temporal areas of highest density. The density of ganglion cells in the dorsal retina is the lowest. The total number of ganglion cells was estimated to be about 5 × 104 in a retina. The existence of a low density area in the dorsal retina suggests reduced visual acuity in the ventral visual field.

Tectal fiber connections in a non-teleost actinopterygian fish, the sturgeon Acipenser.

Tectal fiber connections were studied in members of an early branch of the actinopterygian lineage, the sturgeons Acipenser transmontanus and A. schrenkii, by means of biocytin, HRP, biotinylated dextran amine, and DiI tract tracing methods. The aim of this study is to elucidate the visual pathway via the optic tectum to the thalamus as a part of a series of studies on the visual pathways in sturgeons. After biocytin or biotinylated dextran amine injections to the optic tectum terminals are found bilaterally in the medial and lateral portions of both the dorsal thalamus and ventral thalamus. Ipsilateral projections are much more abundant. Tectal recipient areas in the thalamus overlap in part with the retinal recipient areas. After HRP or DiI injections to the dorsal or ventral thalamus, tectal neurons projecting to the thalamus were labeled in the ipsilateral or bilateral stratum periventriculare. Dendritic morphology of tectothalamic neurons suggests that they receive direct retinal input. These results suggest that visual information passes through the tectum to the thalamic areas which also receive direct retinal projections. In this regard, the visual system of Acipenser resembles that of chondrichthyans (sharks). Other fiber connections of the tectum are also described, which have not previously been studied by tracer methods in a sturgeon.