Influence of muscarinic ligands on the amplitudes of the evoked surface potential's late components in the optic tectum of the urodele Plethodon jordani.

Recordings of field potentials from the tectal surface of an urodele amphibian were obtained in an in vitro preparation under influence of various muscarinic drugs. Bath applied acetylcholine (ACh) led to no change in the amplitudes or the shape of the evoked potentials. If the ACh-esterase blocker (-)-physostigmine was applied synchronously, the late components of the surface potential increased in amplitude. The non-selective cholinergic agonist carbachol showed a similar effect which was partially diminished by the nicotinic antagonist d-tubocurarine chloride (d-TC) and the muscarinic antagonist atropine sulfate. The application of the non-selective muscarinic agonist (+)-pilocarpine hydrochloride led to an increase of the late oligo- and polysynaptic events. This effect was reduced by the M(1)-antagonist pirenzepine dihydrochloride. The presented findings suggest that muscarinic receptors play a more important role in tectal processing than assumed in previous studies which emphasized the role of nicotinic receptors.

Projections of single retinal ganglion cells to the visual centers: an intracellular staining study in a plethodontid salamander.

The projection specificity of retinal ganglion cells and the morphology of their terminals were studied in the plethodontid salamander Plethodon jordani. In an in vitro approach, ganglion cells were stained with biocytin and reconstructed by means of light microscopy. Single retinal ganglion cells often have multiple terminal structures in the thalamus, pretectum, and tectum. The projection pattern in the diencephalic neuropils is related to the depth of the terminal arbor within the tectal fiber layer. Terminal arbors in the tectum differ in location, size, and branching pattern. The following types could be distinguished: The most superficial of the optic terminals in layer 1 are relatively small with a diameter of about 100 microm. With the exception of a few varicosities (beads) in the pretectal neuropils, their stem axons have no further collaterals or terminal arbors in the diencephalic neuropils. Intermediate terminals in layer 2 fan out to form a dense plexus with a medio-lateral extent of 180 microm on average. Some terminals in this layer show obvious antenna-like fibers reaching toward the surface of the tectum. The axons of layer 2 projecting neurons have additional collaterals and terminal arbors in the thalamus and pretectum. The deep layer 3 terminals spread out over a diameter of 400 microm on average and their degree of branching is moderate. The axons of layer 3 projecting ganglion cells have dense additional terminal arbors in the thalamus and pretectum. The deepest retinal terminals in the tectum are found within the predominantly efferent fiber layers. This type consists of an unbranched, but beaded axon which runs rostro-caudally with several bends and loops. The stem axon has an additional very dense terminal arborization in the neuropil of the nucleus Bellonci pars medialis and additional sparse collaterals in the pretectal area.

Connectivity of the salamander pretectum: an in-vitro (whole-brain) intracellular tracing study.

The amphibian optic tectum and pretectum have been analyzed in detail anatomically and physiologically, and a specific model for tecto-pretectal interaction in the context of the visual guidance of behavior has been proposed. However, anatomical evidence for this model, particularly the precise pattern of pretectotectal connectivity, is lacking. Therefore, we stained pretectal neurons intracellularly in an in-vitro preparation of the salamanders Plethodon jordani and Hydromantes genei. Our results demonstrate that the projections of neurons of the nucleus praetectalis profundus are divergent and widespread. Individual neurons may project divergently to telencephalic (ipsilateral amygdala and striatum), diencephalic (ipsi-and contralateral thalamus, contralateral pretectum), and mesencephalic (ipsi- and contralateral tectum and tegmentum) centers, and to the ipsi- and contralateral medulla oblongata and rostral spinal cord. The projection of pretectal cells to the optic tectum is bilateral; axonal structures do not show discernible patterns and are present in all layers of the superficial white matter. A classification of pretectal neurons on the basis of axonal termination pattern or dendritic arborization has not been possible. Our results do not support the hypothesis that a distinct class of pretectal neurons projects to a particular subset of tectal cells. Rather, the pretectum appears to influence the tectum indirectly, acting either on retinal afferents or modulating inhibitory interneurons.

Isthmotectal connections in plethodontid salamanders.

In the plethodontid salamander species Plethodon jordani and Hydromantes italicus, the morphology and connectivity of isthmic cells were investigated by means of intracellular staining with biocytin. Dendritic arborization, axonal pathways, and size and morphology of telodendritic structures in the fiber layers of the optic tectum were determined. The latter were studied electron microscopically. The majority of isthmic neurons project to both tectal hemispheres, each cell forming telodendritic structures of different extent in the ipsilateral and contralateral hemisphere. These structures are column-like with diameters of about 60 microm. The ipsilateral terminals extend through layers 1-3 of the tectal white matter and intermingle with terminals of contralateral retinal afferents in layers 1-3, as well as with ipsilateral retinal afferents in layer 3. The corresponding contralateral telodendritic structures are confined to layer 1 and are not in direct contact with ipsilateral retinal afferents in layer 3. Both telodendritic structures are located in the rostral two-thirds of the optic tectum, which are binocularly innervated. The rostrocaudal and mediolateral sites of isthmic telodendra are in register with the direct retinal map in each tectal hemisphere. Few isthmic cells project to the caudal one-third of the optic tectum, which is monocularly driven. These cells form only one axonal terminal in the ipsilateral tectal hemisphere. Size and structure of these ipsilateral terminals are similar to the ipsilateral telodendra of bilaterally projecting isthmic neurons.

Depth perception in salamanders: the wiring of visual maps.

Most members of the salamander family Plethodontidae exhibit fast and precise prey localization and are likely to use stereopsis for this task. Tectal and isthmic neurons were stained iontophoretically with biocytin after intracellular recording in order to investigate the fine structure and arborization pattern of dendritic and telodendritic trees. The back-projection of isthmic neurons to both tectal hemispheres exhibits a conspicuous and spatially restricted connection with tectal neurons and the afferent fiber layers, which probably plays an important role in the context of stereoscopic depth perception and detection of retinal disparities.