Functional anatomy of the tectum mesencephali of the goldfish. An explorative analysis of the functional implications of the laminar structural organization of the tectum

Putative cholinergic interneurons in the optic tectum of goldfish

We have demonstrated that a significant fraction of cells in layer 1 of goldfish optic tectum are immunoreactive for choline acetyltransferase. These cells constitute 4-7% of type XIV cells, which are thought to be intrinsic neurons and represent 95% of all tectal cells. The number of immunoreactive type XIV cells is unchanged following short-term enucleation. The possible integrative role of these putative cholinergic neurons is discussed in relation to sensory functions of the optic tectum.

Afferent and efferent connections of cerebellar lobe C3 of the mormyrid fish Gnathonemus petersi: an HRP study

The present paper is devoted to the extrinsic connections of lobe C3 of the highly differentiated corpus cerebelli of the electric fish Gnathonemus petersi. For this purpose, HRP injections or gels were placed in distinct parts of lobe C3 or its peduncle, in the pretectal region, and in the eye. Moreover, the presence of serotonin and tyrosine-hydroxylase was studied with immunohistochemical methods. The afferent connections of the rostral and caudal part of lobe C3 appear to differ considerably. Although both parts receive comparable projections from two pretectal nuclei (termed nucleus geniculatus and dorsal anterior pretectal nucleus) and the inferior olive, they receive projections from different parts of the nucleus lateralis valvulae, a large cell mass in the midbrain tegmentum, composed of small, tightly packed neurons. The caudal part of lobe C3 receives a projection from the most rostromedial cap of cells of this nucleus, whereas the rostral cap of lobe C3 receives efferents from the neighboring, more caudolateral, zone of cells of the nucleus lateralis valvulae. The caudal part of lobe C3, but not its rostral part, receives an additional projection from a nucleus in the isthmus region, termed nucleus Q. This nucleus sends a collateral projection to the torus longitudinalis. The efferents of both parts of lobe C3 project to slightly different parts of the midbrain tegmentum and the nucleus reticularis superior, and originate at least partly from eurydendroid cells. None of the nuclei and fiber tracts labeled could be shown to contain serotonin or catecholamines. The connections of lobe C3, as revealed by the present study, are compared with those of other parts of the mormyrid cerebellum and with those of the corpus cerebelli of other teleosts, with emphasis on the homology and functional significance of pretectocerebellar connections, the topical order in the cerebellar projections of the nucleus lateralis valvulae, and the relations between the cerebellum and torus longitudinalis. Comparison of the cerebellar connections in different teleostean species suggests that the strong development and the considerable differentiation of the cerebellum of mormyrids are related to at least two types of changes in the extrinsic connections, i.e.: a redistribution or parcelling of connections and the development of connections specific for mormyrids.

Topography of regenerating optic fibers in goldfish traced with local wheat germ injections into retina: evidence for discontinuous microtopography in the retinotectal projection

A small injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into dorsal or ventral peripheral retina in normal goldfish and in goldfish with prior optic nerve crush. Serial sections of tectum were subsequently taken for horseradish peroxidase (HRP) histochemistry 18 hours after injection and studied with light microscopy and densitometric reconstructions. In normals, a small, sharply delineated patch of product 200-300 microns wide was observed at the appropriate medial or lateral periphery of tectum. This product filled the entire SFGS, the main optic termination layer, and fell off abruptly at its edges. No labelling was detected in the optic pathways. In regenerates at about 20 days after nerve crush, these retinal injections yielded product that was dispersed across 1,000 microns or more of tectum but not in a uniform fashion. The densest product was biased toward the appropriate tectal position while product of intermediate density was mainly distributed along a path from the anterior end of tectum to this region. Product in the inappropriate half of tectum was much lighter and typically fiberlike in appearance. By about 40 days, product had condensed considerably at roughly the correct region of tectum but it was not as sharply delimited as in normals. Dense label occupied a single area about twice that of normals and exhibited flanking regions of light label extending for several hundred micrometers. At 59-148 days, a further condensation was observed but into more than one patch of product. The patches were of variable size and consisted of sharply delimited dense product which filled the entire SFGS at each position. Morphologically, these patches bore a remarkable resemblance to the ocular dominance columns previously seen in this system.

The organization of the diencephalon and the pretectum in the cichlid fish, Haplochromis burtoni

Although teleost fish comprise the largest vertebrate radiation, surprisingly little is known about the structure and development of their central nervous systems. Since teleosts are being used much more frequently as model systems in neurobiological research, particularly in understanding visual function, detailed information is needed about central nervous system structures and interconnections. By using the Bodian method with Nissl counterstaining we have analyzed the major nuclei in the diencephalon and pretectum of a cichlid fish, Haplochromis burtoni, which is dependent on vision for its survival. Although our results are broadly comparable with those from other teleost species, there are clear differences, particularly among the visual nuclei. By using animals of a range of sizes to confirm our descriptions we were able to identify possible developmental relationships among several nuclei.

Telencephalic terminals in the major retinal synaptic lamina of the goldfish optic tectum

Light and electron microscopic degeneration studies were used to examine the telencephalotectal pathway in goldfish. Both techniques showed that each telencephalic lobe sent bilateral projections to several tectal laminae. Degenerating synaptic terminals and fibers were observed in the major retinal projection lamina as well as in other tectal laminae. The terminals contained round to oval synaptic vesicles, asymmetric synapses and contacted relatively small postsynaptic profiles.

A Golgi study of the cell types of the dorsal torus semicircularis of the electric fish Eigenmannia: functional and morphological diversity in the midbrain

The dorsal torus semicircularis (torus) of the gymnotiform fish Eigenmannia was examined in Golgi-impregnated material. These results were correlated with those of a previous HRP study which used retrograde labelling techniques to identify the efferent cell types of the torus (Carr et al., '81, J. Comp Neurol. 203:649-670). The torus is a laminated midbrain nucleus of the electrosensory system. It receives somatotopically ordered electrosensory input from the medulla and caudal lobe of the cerebellum, proprioceptive input from the descending nucleus of the trigeminal nerve, and input from the optic tectum. The torus projects to the nucleus praeeminentialis, the optic tectum, nucleus electro-sensorius, parts of the central posterior thalamus, the pretectum, the lateral mesencephalic reticular formation (LMRA), the reticular formation, and the inferior olive. The torus has 12 laminae and 48 cell types by Golgi criteria. There are three major orientations to the dendritic fields of the toral neurons: purely horizontal neurons with dendrites confined to a single lamina, multipolar neurons whose dendrites often do not respect laminar boundaries, and vertical cells with dendrites that travel in the vertical bundles of dendrites and axons which pierce the torus at regular intervals. There are four major groups of vertically oriented neurons. The first has a predominantly horizontal dendritic tree with one or two vertical dendrites which connect the cell to a distant lamina. The second consists of "U"-shaped neurons with a horizontal arbor and two major dendrites which ascend in adjacent vertical bundles. The third group is made up of bilaminar neurons which receive input from two vertically separated dendritic arbors, and the fourth group is purely vertical in orientation. A group of four tegmental cell types in the LMRA also send their dendrites into the efferent tracts of the torus, and into lamina IX. The torus is similar in complexity and number of cell types to the mammalian inferior colliculus. The large number of cell types in these midbrain sensory nuclei, compared to the number of afferent inputs (seven or more for the torus) is notable and may reflect the parcellation of function associated with the parallel processing of these inputs.