Interactions between tectal radial cells in the red-eared turtle, Pseudemys scripta elegans: an analysis of tectal modules

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Response properties of visual neurons in the turtle nucleus isthmi

The optic tectum holds a central position in the tectofugal pathway of non-mammalian species and is reciprocally connected with the nucleus isthmi. Here, we recorded from individual nucleus isthmi pars parvocellularis (Ipc) neurons in the turtle eye-attached whole-brain preparation in response to a range of computer-generated visual stimuli. Ipc neurons responded to a variety of moving or flashing stimuli as long as those stimuli were small. When mapped with a moving spot, the excitatory receptive field was of circular Gaussian shape with an average half-width of less than 3°. We found no evidence for directional sensitivity. For moving spots of varying sizes, the measured Ipc response-size profile was reproduced by the linear Difference-of-Gaussian model, which is consistent with the superposition of a narrow excitatory center and an inhibitory surround. Intracellular Ipc recordings revealed a strong inhibitory connection from the nucleus isthmi pars magnocellularis (Imc), which has the anatomical feature to provide a broad inhibitory projection. The recorded Ipc response properties, together with the modulatory role of the Ipc in tectal visual processing, suggest that the columns of Ipc axon terminals in turtle optic tectum bias tectal visual responses to small dark changing features in visual scenes.

Regular mosaics of large displaced and non-displaced ganglion cells in the retina of a cichlid fish

Large retinal ganglion cells in the tilapid cichlid fish Oreochromis spilurus (standard length 15-54 mm) were filled with horseradish peroxidase and studied in flatmounts. Three types, with distinct patterns of dendritic stratification, formed spatially independent, nonrandom mosaics. One type (about 0.3% of all ganglion cells) resembled the outer (off) alpha cells of mammals. They were very large, with thick primary dendrites and large, sparsely branched planar trees in the outer part of the inner plexiform layer (IPL). About 300 were arrayed regularly across each retina, their exact number and spacing depending on its size. Their somata were often displaced into the IPL, even where neighbours in the mosaic were orthotopic. Another type (0.8%) resembled the inner (on) alpha cells of mammals. These had slightly smaller somata that were never displaced and smaller trees in the middle layers of the IPL. About 800 were arrayed uniformly and regularly across each retina. A rarer type (0.06-0.08%) had two planar trees: one forming a coarse mosaic in the outer part of the inner plexiform layer (co-planar with the trees of outer alpha-like cells) and another in the outer plexiform layer. These "biplexiform" cells were smaller and rounder than alpha-like cells and always displaced. The dendrites were finer and less tapered. Cells in which we could identify an outer plexiform tree failed to cover the retina completely, but were nonrandomly distributed. We draw three main conclusions: (1) some nonmammalian vertebrates have separate inner and outer mosaics of large ganglion cells like those of mammals, (2) the vertical displacement of ganglion cell somata can vary widely within a single mosaic and may thus be functionally irrelevant, and (3) biplexiform ganglion cells exist in fish but differ in morphology from the biplexiform types described in some other vertebrates.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. III. The tectorotundal projection

Nucleus rotundus is the primary thalamic recipient of projections from the optic tectum in pond turtles. Although the projection of the retina to the optic tectum is known to be topographically organized, earlier studies suggest that the tectorotundal projection is not topographically organized. Three types of analyses are used in this paper to characterize the organization of the projection of the optic tectum to nucleus rotundus. First, large iontophoretic injections of horseradish peroxidase into the optic tectum anterogradely fill axons with reaction product after the use of a cobalt-enhanced diaminobenzidine procedure. These preparations show that shafts of axons in the tectothalamic tract give rise to thinner, primary collaterals that enter nucleus rotundus from its caudolateral aspect and form sparsely branching arbors within the nucleus. Very thin secondary collaterals branch from these collateral bear terminal collaterals with frequent varicosities. Although the total size of such arbors is unknown, the evidence suggests that each arbor is large in relation to the size of nucleus rotundus. Thus, injection sites restricted to central tectum label axons throughout nucleus rotundus. Second, subtotal lesions of the tectum produce degeneration throughout nucleus rotundus in silver degeneration preparations. Finally, analysis of electron microscopic degeneration material indicates that tectal boutons are distributed along the full lengths of the dendrites of rotundal neurons, but not on their somata. These boutons form asymmetric synaptic junctions and contain round synaptic vesicles. In view of the relatively large size of the dendritic fields of rotundal neurons, these data suggest that the tectorotundal projection is both strongly convergent on individual neurons and strongly divergent from single tectorotundal axons. This type of organization is consistent with physiological evidence that rotundal neurons have receptive fields that cover at least one-half of the contralateral visual field and often include the entire hemifield. It seems unlikely that nucleus rotundus can be involved in neuronal transactions that preserve detailed spatial information, but it may be involved in processing information on other visual parameters such as stimulus velocity or color.

Regional specialization in retinal ganglion cell projection to optic tectum of Dipsosaurus dorsalis (Iguanidae)

Regional variation in the composition of the ganglion cell layer is a characteristic feature of vertebrate retinas. This suggests that corresponding patterns of regional variation might be expected in CNS structures which receive a direct, topographic projection from the retina. To investigate this possibility, I studied the retinotectal projection in an iguanid lizard (Dipsosaurus dorsalis), which shows a marked degree of regional variation in its retinal ganglion cell layer. Results of these experiments indicate that in D. dorsalis the optic tectum is differentiated horizontally into five regions which can be distinguished on the basis of cytoarchitecture and the patterning of retinal afferents, as visualized with silver degeneration and autoradiographic tracing techniques. These five areas are named according to their location: 1) Ventral Rim, 2) Lateral Wall, 3)Dorsolateral Area, 4) Dorsal Wall and 5) Medial Rim. Following enucleation, the Medial and Ventral Rim are characterized by fascicles of coarse degenerated fibers in layers 13, 12, and 9, by a relative absence of fine particulate debris in reduced silver material, and by the absence of silver grains in autoradiographs following intraocular injections of tritiated proline. They differ in the density of large calibre fibers and in the temporal course of degeneration. In the Lateral Wall the cellular layers are best differentiated, and nine retino-recipient layers can be distinguished below the stratum opticum. Degeneration is particularly heavy over the plexiform layers 9, 11, and 13. There is a superficial to deep gradient of degeneration density and of silver grains within layers 13 and 11. In the lower half of layer 11, argyrophilic debris and silver grains are aligned in vertical columns. The Dorsolateral Area is a circular region which covers the dorsolateral convexity of the tectum just caudal to mid tectal levels. It is characterized by a relatively low density of degenerated fibers in layers 13, 12, and especially in layer 9. Finally, the Dorsal Wall is similar to the Lateral Wall in the laminar distribution of degeneration and silver grains, except that there is an additional band of fine debris over layers 12 and 13 which is not visible in the Lateral Wall. Rostrally, the Dorsal Wall is greatly expanded (the Dorsal Expansion) and in this region a well developed plate of neurons lies immediately subjacent to the stratum opticum. This is the only tectal area which shows substantial degeneration at short survival times. These data suggest that in D. dorsalis there is horizontal variation in the morphology and patterning of retinal ganglion cell axons to the superficial tectal layers. Since Dipsosaurus retina exhibits a substantial degree of regional variation in the ganglion cell layer, and since the retina projects topographically onto the tectum, it is suggested that regional variation in the optic tectum may reflect regional variation in the ganglion cell layer of the retina.