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.

Afferent and efferent fiber connections of the lobus inferior (LI) were studied in a percomorph teleost, Thamnaconus (Navodon) modestus. The LI of Thamnaconus is composed of the nucleus diffusus lobi inferioris (NDLI), the nucleus recessus lateralis pars lateralis et medialis (NRLl and NRLm), and the nucleus centralis lobi inferioris pars anterior et posterior (NCa and NCp). The NDLI receives projections from the secondary gustatory nucleus, preglomerular tertiary gustatory nucleus, corpus glomerulosum, dorsal region of the area dorsalis telencephali pars medialis (dDm), and area dorsalis telencephali pars lateralis. Different subdivisions of the dorsal telencephalon project to discrete regions of the NDLI. The NDLI projects to the corpus mamillare, NRLl, NCa, and NCp. Thus the NDLI could be regarded as an intrahypothalamic relay nucleus. The NCa receives projections from the NDLI and projects to the preglomerular tertiary gustatory nucleus, secondary gustatory nucleus, nucleus lateralis valvulae, and NRLl. The NCa appears to be primarily an extrahypothalamic projection nucleus. The NCp receives projections from the NDLI. Efferent connections of the NCp remain to be studied. The NRLl receives projections from the NDLI, and projects to the nucleus ruber (NR) of Goldstein [1905] and the preglomerular tertiary gustatory nucleus. Dense projections of the NR to the stratum opticum and stratum fibrosum et griseum superficiale of the optic tectum are demonstrated. The NRLm receives projections from the medial part of the dDm. Efferent connections of the NRLm remain unclear. The LI as a whole receives projections from the locus coeruleus and nucleus raphe superior. These results suggest that the LI receives gustatory and/or general visceral information from the secondary and tertiary gustatory nuclei, visual and somatosensory inputs from the corpus glomerulosum. Inputs from the dorsal telencephalic subdivisions could be of various modalities (e.g. visual, acousticolateral, gustatory and/or general visceral). The present study also suggests that information processed in the LI is transmitted to the optic tectum via the NR, to the corpus mamillare, to the secondary and tertiary gustatory nuclei, and to the cerebellum via the nucleus lateralis valvulae.

The cytoarchitecture and fiber connections of the nucleus lateralis valvulae of the carp (Cyprinus carpio) were studied by Nissl, Bodian, Golgi, and horseradish peroxidase methods. Chief cells composing the nucleus lateralis valvulae (NLV) are small and granular, and their axons terminate in the cerebellum. These neurons have no dendrite, and the cell body is enveloped in a single terminal of afferent fibers to the NLV. In order to observed local cell clustering, the NLV was three-dimensionally reconstructed with the aid of a computer image analysis system. Afferent sources to the NLV were the nucleus pretectalis superficialis pars magnocellularis (Northcutt and Braford, '84: Brain Res. 296:181-184), nucleus ventromedialis thalami (Ito et al., '86: J. Comp. Neurol 250:215-227), and the inferior lobe. The NLV projects to the inferior lobe and the cerebellum. In particular, the cerebellar projections were strong and topographically arranged. Some larger neurons lying just beneath the NLV, some of which were intermingled with the NLV neurons, projected to the torus longitudinalis. On the basis of the local cell clustering as well as NLV-cerebellar connections, three subdivisions of the NLV could be recognized, i.e., anterior, central, and posterior portions. The posterior portion was further subdivided into lateral and medial parts.

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. The results show that the ascending fiber connections of the central gustatory system in the percomorph teleost tilapia are essentially similar to those of mammals. That is, the pathway from the primary gustatory areas (facial, glossopharyngeal, and vagal lobes) through the SGN and pTGN to the dDm in tilapia corresponds with the mammalian gustatory pathway from the solitary nucleus through the pontine taste areas (nucleus parabrachialis) and the thalamic relay nucleus (ventral posteromedial nucleus) to gustatory neocortices. In addition, the pathway from the primary gustatory areas through the SGN to the Vi and Dp in tilapia corresponds with the pathway from the solitary nucleus through the pontine taste areas to the amygdala in mammals.

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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.

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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.