Electron microscopic demonstration of tyrosine hydroxylase-immunoreactive interplexiform cells in the lamprey retina

Retinal amino acid neurochemistry of the southern hemisphere lamprey, Geotria australis

Lampreys are one of the two surviving groups of the agnathan (jawless) stages in vertebrate evolution and are thus ideal candidates for elucidating the evolution of visual systems. This study investigated the retinal amino acid neurochemistry of the southern hemisphere lamprey Geotria australis during the downstream migration of the young, recently-metamorphosed juveniles to the sea and during the upstream migration of the fully-grown and sexually-maturing adults to their spawning areas. Glutamate and taurine were distributed throughout the retina, whilst GABA and glycine were confined to neurons of the inner retina matching patterns seen in most other vertebrates. Glutamine and aspartate immunoreactivity was closely matched to Müller cell morphology. Between the migratory phases, few differences were observed in the distribution of major neurotransmitters i.e. glutamate, GABA and glycine, but changes in amino acids associated with retinal metabolism i.e. glutamine and aspartate, were evident. Taurine immunoreactivity was mostly conserved between migrant stages, consistent with its role in primary cell functions such as osmoregulation. Further investigation of glutamate signalling using the probe agmatine (AGB) to map cation channel permeability revealed entry of AGB into photoreceptors and horizontal cells followed by accumulation in inner retinal neurons. Similarities in AGB profiles between upstream and downstream migrant of G. australis confirmed the conservation of glutamate neurotransmission. Finally, calcium binding proteins, calbindin and calretinin were localized to the inner retina whilst recoverin was localized to photoreceptors. Overall, conservation of major amino acid neurotransmitters and calcium-associated proteins in the lamprey retina confirms these elements as essential features of the vertebrate visual system. On the other hand, metabolic elements of the retina such as neurotransmitter precursor amino acids and Müller cells are more sensitive to environmental changes associated with migration.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Gamma-aminobutyric acid-synthesizing cells in the retina of the chameleon Chamaeleo chameleon

Antibodies directed against gamma-aminobutyric acid (GABA) and L-glutamic acid decarboxylases 65 and 67 kDa (GAD65 and -67) were used to study the GABAergic cell populations of the chameleon retina. GABA immunoreactivity was found in the two main types of retinal interneurons, amacrine and horizontal cells. Amacrine, displaced amacrine, and intra- and interplexiform cells displayed the strongest GABA immunoreactivity of all the retinal cell types. Horizontal cells formed a continuous GABA-immunoreactive cell layer lying against the outermost portion of the inner nuclear layer. In contrast to previous studies (Quesada et al. [1996] Cell Biol. Int. 20:395-400; [1999] Eur. J. Anat. 3:13-25), the present results demonstrate that the horizontal cells of the chameleon retina are GABA immunoreactive and that a subpopulation of these is immunolabelled by an antibody against GAD65. These results indicate that GABAergic synaptic transmission plays a key role in the outer plexiform layer of the vertebrate retina.

Review of larval and postlarval eye ultrastructure in the lamprey (Cyclostomata) with special emphasis on Geotria australis (Gray)

The literature dealing with the lateral eye in lampreys is briefly reviewed here. While there appears to be no longer much doubt that the short and long photoreceptor cells in the lamprey eye correspond to rods and cones, questions of dark/light adaptational changes, the nature of visual pigments, and the roles of retinal serotonin and melatonin need to be re-addressed. Eyes of the larval and postlarval ("macrophthalmia") stages of the lamprey Geotria australis were examined by electron microscopy and it was found that the larval retina is largely undifferentiated except for a small central zone surrounding the optic nerve head. The retina of the postlarval stage is fully differentiated and the photoreceptor outer segments undergo renewal, which appears to involve the retinal pigment epithelium (RPE). The distribution of larval RPE and choroidal pigments, postlarval ganglion cells, and the orientation of scleral collagen are unusual for vertebrates. No obvious positional or size differences of any retinal cell type were apparent between day- and night-fixed specimens.