James S. Albert

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.

Distribution and innervation of lateral line organs in the channel catfish

The lateral line system of the channel catfish is formed by mechanoreceptive neuromasts located within five pairs of cephalic and one pair of trunk canals, as well as superficial lines of neuromasts, termed accessory and/or pit lines. Five pairs of pit lines occur on the head, and three pairs of superficial lines occur on the trunk. In addition to these mechanoreceptors, which are found in most teleost fishes, catfish also possess a total of over 4000 electroreceptive ampullary organs scattered over the entire body. The lateral line receptors are innervated by five pairs of lateral line nerves whose rami are secondarily associated with facial and trigeminal fibers that innervate taste buds and the dermis of the skin, respectively. The neuromasts of the trunk canal and the ramules of the posterior lateral line nerve that innervate them seem to be organized in a segmental pattern. The same is true for the intervertebral ramules of the recurrent facial ramus, which innervate the external taste buds on the trunk. The fibers of the gustatory and lateral line systems may use the neural crest, the developing spinal nerves, or both, to establish this segmental pattern. In this context, it may not be surprising that there is an intimate relationship among each of the sensory systems in the trunk. J. Comp. Neurol. 421:570–592, 2000.

TESTING HYPOTHESES OF NEURAL EVOLUTION IN GYMNOTIFORM ELECTRIC FISHES USING PHYLOGENETIC CHARACTER DATA

In this paper, we propose a method to test alternative hypotheses of phenotypic evolution. The method compares patterns observed in phylogenetic character data with patterns expected by explicit models of evolutionary process. Observed patterns of character‐state diversity are assessed from four properties of character‐state change derived from a phylogenetic analysis: the sequence and correlation of transformations on a cladogram and the spatial and functional localization of these transformations to parts of an organism. Patterns expressed in terms of the localization of transformations are compared with the expectations of null models that the number of transformations is proportional to measures of size or complexity. Deviations from the values expected by the null models are then compared with qualitative expectations of the models.

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.

Gonadotropin-releasing hormone neurons in the gourami midbrain: a double labeling study by immunocytochemistry and tracer injection

There are three groups of gonadotropin-releasing hormone (GnRH) neurons in the teleost brain. Midbrain GnRH neurons in the dwarf gourami send axons to various areas of the central nervous system. However, it is not clear whether midbrain GnRH neurons form a cell cluster separate from the nucleus of the medial longitudinal fasciculus (nMLF), which has been reported to project to the spinal cord. Thus, we performed a double labeling study. GnRH neurons were immunostained but were very faintly labeled with biocytin injected into the spinal cord. In contrast, nMLF neurons were strongly labeled with biocytin but were GnRH-immunonegative. GnRH neurons are distributed at almost the same rostrocaudal levels as nMLF neurons, but they constitute a separate cell group dorsocaudal to nMLF neurons.

Visual Thalamotelencephalic Pathways in the Sturgeon Acipenser, a Non-Teleost Actinopterygian Fish

Terrestrial vertebrates (amphibians, reptiles, birds, and mammals) possess two visual systems, the geniculate and extrageniculate pathways to the telencephalon. In cartilaginous fishes (e.g. sharks) both retinal and tectal neurons project to neurons in the thalamus, which themselves project to a single area in the telencephalon. The condition in ray-finned fishes (Actinopterygii) is ambiguous. In many teleosts there is a well developed extrageniculate pathway but no obvious geniculate system. This study reports on the thalamotelencephalic projections of a sturgeon, a non-teleost ray-finned fish. Several tract tracing methods (e.g., HRP, WGA-HRP, biocytin, BDA, DiI) were employed in conjunction with normal techniques for identifying neural structures (e.g., Nissl, Golgi). After injections of tracer into retinal and tectal recipient areas of the thalamus, labeled terminals were observed in the ventrolateral region of the caudal telencephalon, an area referred to as the thalamic projection area. After injections of tracer into the telencephalon, populations of retrogradely filled neurons were located in both the dorsal and ventral thalamus. These data demonstrate that thalamic neurons in both retinal and tectal pathways project directly to the telencephalon. These results support the view that two visual pathways are a primitive feature of vertebrate brain organization. These results are also consistent with the hypothesis that the ancestor of Acipenser and Teleostei (Actinopteri) acquired a novel visual pathway to the telencephalon through the ventral portion of the thalamus.

Terminal morphology of two branches arising from a single stem-axon of pretectal (PSm) neurons in the common carp

The induction of postsynaptic structures by presynaptic terminals is suggested in a teleost brain. Neurons in the nucleus pretectalis superficialis pars magnocellularis (PSm) in the common carp are known to send fibers to the corpus mamillare (CM) and the nucleus lateralis valvulae (NLV). Individual axons of PSm neurons bifurcate (or give off an axon collateral), both of which reach the target areas in the CM and NLV. The morphology of horseradish peroxidase‐labeled terminals in the CM and NLV appears quite different in light microscopy. Terminals in the CM appear as a fine network of beaded (2–4 μm in diameter) fibers, while those in the NLV are larger (8–12 μm in transverse diameter) and cup‐shaped, partially enveloping the soma of individual NLV neurons. In electron microscopy, however, these synapses in the CM and NLV share several ultrastructural similarities. Small (0.2 to 0.4‐μm thick, 0.4 to 0.7‐μm long) spine‐like protrusions arising from dendrites in the CM, and from cell bodies in the NLV, invaginate into the axon terminals, and the synaptic junctions are always formed at the base of the protrusion in both areas. Development of this unusual morphology is inferred to be directed from the presynaptic side. The morphological similarity of the spine‐like protrusions to the “spinule,” which is thought to be formed in response to synaptic activation, is discussed. J. Comp. Neurol. 378:379–388, 1997.

2315 Origin of GnRH innervation of pituitary in a teleost, dwarf gourami: a double labeling study using whole-brain in vitro tract tracing and immunocytoch

Naoyuki Yamamotol , Nobuhiko Sawai ‘*, Masami Yoshimoto’ , James S. Albert’, Hironobu Ito’ In most vertebrates, multiple gonadotropin-releasing hormone (GnRH) neuronal groups have been reported. Which of the GnRH cell groups regulates the pituitary is not well documented. We studied this question by a tracer st,udy combined with immunocytochemistry in a teleost, dwarf gourami. We applied a small cristal of biocytin to the pituitary connected to the excised brain and incubated in physiological salt solution for 6 to 24 hours. After fixation of the brain, a double labeling for biocytin and immunocytochemistry was performed. Retrogradely labeled neurons by biocytin were found in the preoptic area, the ventromedial hypothalamus, and occasionally in the ventral telencephalon. Double labeled neurons were found only in the nucleus preopticus’ pars parvicellularis. GnRH neurons of the terminal nerve ganglion and those in the midbrain were labeled by immunocytochemistry but not by biocytin. The present study suggests that the preoptic GnRH neuronal group regulates the pituitary.

1919 Telencephalic fiber connections and immunohistochemical studies in a teleost fish, tilapia

Masami Yoshimoto’, Yamamoto Naoyuki’, James S. Albert’, Sawai Nobuhiko2, ,Ito Hironobu’ Homology of telencephalic divisions between teleost fishes and other vertebrates is poorly known. This work focuses on the telencephalic fiber connections of olfactory and gustatory systems and immunolocalization of tyrosine hydroxylase (TH). The fiber connections of the telencephalon in Oreochromis (Tilapia) niloticus ‘were studied by means of horseradish peroxidase and biocytin methods. The area ventralis pars intermedia (Vi) and medial part of area dorsalis pars posterior (Dp) receive direct projection from the secondary gustatory nucleus (SGN). Futhermore, the Vi and Dp receive bilaterally projecting fibers from the olfactory bulb. The immunolocalization of TH was also examined in the telencephalon in tilapia. Fibers positive for TH were observed densely in the Vi and Dp. On the basis of the fiber connections of the olfactory bulb and SGN and the immunolocaliiation of TH, the Vi and Dp in the telencephalon of tilapia may be regarded as homologous with the amygdala of mammals.