Tecto-isthmo-optic transmission in pigeons is mediated by glutamate and nitric oxide

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.

Association amacrine cells of Ramón y Cajal: Rediscovery and reinterpretation

In 1895, by means of the Golgi method, Santiago Ramón y Cajal discovered a cell having a unique morphology in the avian retina. This cell had its cell body in the amacrine cell level of the inner nuclear layer, only a few rudimentary dendrites at the outermost level of the inner plexiform layer (IPL), and a long axon coursing horizontally and terminating in the IPL. Despite having defined amacrine cells as cells without axons, Cajal named this cell type “association amacrine cell” (AAC). This discovery was not confirmed by other investigators for nearly a century. Very recently, however, isthmo-optic target cells (IOTCs), which receive the terminals of centrifugal fibers emanating from the isthmo-optic nucleus, have been identified as one type of AAC. As summarized and discussed in this review, the morphology of the AACs as described by Cajal has been completely confirmed. However, since these cells appear to be classical polarized, monoaxonal neurons and lack the dendritic interactions that are typical of amacrine cells, they should be regarded as a distinct type of retinal interneuron and not as amacrine cells.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Retinal target cells of the centrifugal projection from the isthmo-optic nucleus

Although the avian retina has long been known to receive projection from a midbrain nucleus, the isthmo-optic nucleus (ION), the output of its target cells has remained obscure. We labeled the isthmo-optic (IO) terminals in the Japanese quail retina, by using anterograde transport of fluorescent tracer injected into the ION, and then labeled target cells for these terminals by means of intracellular tracer injection under direct microscopic observation. Somata of the IO target cells (IOTCs) lie in the innermost zone of the inner nuclear layer of the ventral half of the retina and have no dendrites but an axon. The axons run in the inner plexiform layer (IPL) for up to 6 mm and terminate densely in a round or elliptical terminal field, about 90-290 microm in diameter, of the outermost zone of the IPL. Longer axons (> 2 mm) extend dorsally, but shorter ones (< 1 mm) project ventrally or horizontally, so the terminals are distributed widely in both dorsal and ventral halves of the retina. The IOTCs cannot be classified into any of the five conventional major classes of retinal cells, including amacrine cells, and are thought to be "slave" neurons whose output is controlled by the neurons in the brain. Topographic separation between input to and output from the IOTCs by the axons might be essential for the overall topographic organization of the centrifugal visual system in birds.

Cytoarchitecture of the avian optic tectum: neuronal substrate for cellular computation

To analyse cellular computation in the vertebrate brain, a thorough knowledge of the underlying anatomy, physiology and connectivity of the neuronal substrate is essential. This review compiles data on one of the best known structures of the vertebrate brain, the optic tectum of birds. The functions of this structure are multifold, but can be attributed largely to orientation and the basic analysis of sensory data in a spatial context. In the tectum, a wealth of data on physiology and anatomy has been gathered over more than a century and provides an excellent background for computational studies. The analysis of the optic tectum is facilitated by several principles of organisation, including the retinotopic input and the highly laminated layout with separated input and output layers. Moreover, the molecular mechanisms guiding the development and connectivity have been analysed in detail. As the avian tectum and the mammalian superior colliculus are partly homologous, the cellular mechanisms unraveled in the tectum can also be transferred to the colliculus and thus contribute to the understanding of the vertebrate visual system in general.