Hiroyuki Uchiyama

A retinopetal nucleus in the preoptic area in a teleost,Navodon modestus

Following horseradish peroxidase (HRP) applications to the optic nerve of a teleost (Navodon modestus), a retinopetal nucleus was identified in the contralateral preoptic area. The nucleus was composed of small (7-10 microns) round cells. Centrifugal fibers from the nucleus were traced to the inner nuclear layer of the ipsilateral retina by both orthograde HRP and Fink-Heimer methods. The cells in the nucleus showed no neurosecretion. The retinopetal nucleus or neurons could not be found in Carassius carassius. No retinopetal neurons were found in the optic tectum in both species.

Circadian Rhythms of Rod–Cone Dominance in the Japanese Quail Retina

When the Japanese quail is held in constant darkness, retinal responses (ERG b-waves) increase during the animal’s subjective night and decrease during its subjective day. Rod photoreceptors dominate the b-wave responses (λmax = 506 nm) to all stimulus intensities at night but only to those intensities below the cone threshold during the day. Above the cone threshold, cones dominate b-wave responses (λmax, ∼550–600 nm) during the day regardless of the state of retinal adaptation. Apparently a circadian oscillator enables cone signals to block rod signals during the day but not at night. The ERG b-wave reflects the activity of bipolar cells that are postsynaptic to rods and cones. The ERG a-wave reflects the activity of both rods and cones. The amplitude of the isolated a-wave (PIII) changes with time of day, as does that of the b-wave, but its spectral sensitivity does not. The PIII responses are maximal at ∼520 nm both day and night and may reflect multiple receptor mechanisms. The shift in rod–cone dominance detected with the ERG b-wave resembles the Purkinje shift of human vision but, unlike the Purkinje shift, does not require a change in ambient light intensity. The shift occurs in constant darkness, with a period of ∼24 hr indicative of a circadian rhythm in the functional organization of the retina.

Migration of GnRH-immunoreactive neurons from the olfactory placode to the brain: a study using avian embryonic chimeras.

Previous studies suggest that gonadotropin-releasing hormone (GnRH) neurons appear in the olfactory placode and subsequently migrate into the brain during embryonic development. The aim of the present study was to obtain direct evidence for migration of GnRH neurons from the olfactory placode into the brain. Olfactory placodes from quail embryos were transplanted isotopically and isochronically, to replace the unilaterally ablated olfactory placodes of chick embryos. The chimeric embryos were allowed to develop for several days until they reached the embryonic stages when GnRH neurons are seen in the brain in normal embryos. Quail olfactory epithelia were formed in the host chick embryos. Quail olfactory nerves were also formed and reached the olfactory bulb or primordial olfactory bulb. GnRH-immunoreactive cells of quail origin revealed by a triple staining method were observed in the quail olfactory epithelium, quail olfactory nerve, chick olfactory bulb, and septo-preoptic area. These results indicate that GnRH neurons originate in the olfactory placode and migrate into the telencephalon including the septo-preoptic area. A migratory route of GnRH neurons was well documented by the use of a quail neuron-specific antibody, QN. The migratory route in the brain is discussed with special reference to the terminal nerve. A GnRH-immunoreactive neuronal group of chick origin appeared in the diencephalon of chimeric embryos. These diencephalic neurons may be of non-placodal origin. FMRFamide-immunoreactive neurons of quail origin were also found in the quail olfactory nerve and the host olfactory bulb, suggesting that FMRFamide neurons also originate in the olfactory placode and migrate into the brain.

Fiber connections and synaptic organization of the preoptic retinopetal nucleus in the filefish (balistidae, teleostei)

Neuron cell bodies of the preoptic retinopetal nucleus (PRN), located in the diencephalon of the filefish , Navodon modestus, project axons to the retina. In the present study, the fiber connections and synaptic organization of the preoptic retinopetal nucleus (PRN) were investigated light- and electron-microscopically. The majority of neural cell bodies are located in the rostral half of this rostro-caudally elongated nucleus. Four types of synaptic terminals are distinguishable. The first (L) consists of large, irregularly shaped terminals that contain electron-dense mitochondria and numerous synaptic vesicles. These profiles make asymmetrical multi-synaptic contacts and gap junctions with somata and dendrites. The L terminals are also presynaptic to a second class of terminals (P), which have pleomorphic synaptic vesicles and form synapses onto dendrites. F terminals which have flat synaptic vesicles were also seen PRN. Very few S terminals were also seen in PRN. This type of terminal contains spherical synaptic vesicles of various sizes and a few pale mitochondria. S terminals form asymmetrical synapses with somata, dendrites and P terminals. Following unilateral tectal ablation, degenerating fibers from the lesion were traced into PRN bilaterally, although ipsilateral projections were far more numerous. L terminals exhibit degenerative changes after large tectal resection, whereas S terminals degenerate after contralateral eye enucleation. Therefore, a tecto-PRN-retinal circuit and a reciprocal connection between the retina and PRN have been documented. The similarity between PRN in the filefish and retinopetal nuclei in other classes of vertebrates, especially the isthmo-optic nucleus in birds, is discussed.

Cytoarchitecture, synaptic organization and Fiber Connections of the nucleus olfactoretinalis in a teleost (Navodon modestus)

Cytoarchitecture, synaptic organization and fiber connections of the nucleus olfactoretinalis (NOR) in a teleost, Navodon modestus, have been studied light- and electron-microscopically using an HRP or HRP-degeneration combined method. Following HRP injections into the optic nerve, most contralateral and a few ipsilateral neurons in the NOR were labeled. There are two types of neurons in NOR. Type I neurons have a medium-sized spindle-shaped soma with a round nucleus, and type II neurons have a large oval soma with an invaginated nucleus and contain cored vesicles (80-130 nm in diameter). Afferent terminals which form synaptic contacts with cell bodies of NOR neurons were classified into 3 types according to their morphological characteristics; S, F1 and F2 terminals. S terminals originated in ipsilateral area ventralis telencephali pars supracommissuralis (Vs). These terminals contain both spherical and cored vesicles, and make synaptic contacts with both type I and type II neurons. F1 terminals, which originated in ipsilateral area dorsalis telencephali pars posterior (Dp), are large in profile, and contain flat vesicles and mitochondria with irregularly arranged cristae. These terminals make synaptic contacts only with type I neurons. F2 terminals are small in profile, and contain flat vesicles, cored vesicles and small mitochondria with regularly arranged cristae. F2 terminals make synaptic contacts with both type I and type II neurons. The functional significance of NOR and the relationship between NOR and the ganglion of the nervus terminalis are discussed.

Retinal neurones specific for centrifugal modulation of vision

The avian retina receives centrifugal projections from a midbrain nucleus, the isthmo-optic nucleus. We labelled target cells for the isthmo-optic fibres by intracellular injections of Lucifer Yellow in fixed retinal slices. The isthmo-optic recipient (IOR) cells had no major dendrites extending into the inner plexiform layer, but had a thin axon-like process running horizontally in the junction between the inner nuclear layer and the inner plexiform layer. The IOR cells were morphologically similar to the association amacrine cells of Cajal. Immunohistochemical localization of aspartate and glutamate in the IOR cells suggested that they may use these excitatory amino acids as neurotransmitters.

Centrifugal inputs enhance responses of retinal ganglion cells in the japanese quail without changing their spatial coding properties

Centrifugal fibers originating in the midbrain innervate the avian retina. Stimulation of the centrifugal fibers enhances the responses of ganglion cells in the retinas of both chick and pigeon. The enhanced responses have been attributed to disinhibition, a reduction of the inhibitory surround component of the receptive fields of retinal ganglion cells. We found that stimulation of the centrifugal fibers in Japanese quail enhances the responses of retinal ganglion cells to drifting sine-wave gratings over a wide range of spatial frequencies. Our results do not support the idea that centrifugal inputs selectively influence receptive field surrounds. We also found that centrifugal inputs changed the temporal response properties of retinal ganglion cells by enhancing their responses to sine-wave gratings drifting across the retina at higher temporal frequencies (> 5 Hz). The result shows that centrifugal inputs from the midbrain can enhance responses of retinal ganglion cells without affecting the center-surround organization of their receptive fields. The centrifugal modulation of retinal responses may have a role in shifting visual attention.

Pretectum and Accessory Optic System in the Filefish Navodon modestus (Balistidae, Teleostei) with Special Reference to Visual Projections to the Cerebellum and Oculomotor Nuclei

The fiber connections of the accessory optic system (AOS) were investigated in a balistid fish, Navodon modestus (filefish), by means of horseradish peroxidase (HRP) and degeneration methods. Following injections of HRP into the corpus cerebelli, neurons in two retinal recipient nuclei, the area pretectalis pars dorsalis (APd) and area pretectalis pars ventralis (APv), were labeled retrogradely. In addition, a few neurons near the nucleus of the accessory optic tract (nAOT) were labeled. These neurons have dendrites extending into nAOT. Neurons in APv were also labeled by HRP injections into the oculomotor complex (nIII). However, no neurons were labeled in APd or nAOT. A few neurons in the lateral part of APv were labeled by HRP injections into the abducens nucleus (nVI). Three nuclei of the AOS, APd, APv and nAOT, were shown to receive tectal projections by the Fink-Heimer method. Thus, APv receives retinal and tectal projections, and in turn projects to corpus cerebelli, nIII and nVI. Specific efferent connections of the AOS in teleosts are discussed from phylogenetic aspects.

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 μm 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. J. Comp. Neurol. 476:146–153, 2004.

Computation of motion direction by quail retinal ganglion cells that have a nonconcentric receptive field

One type of retinal ganglion cells prefers object motion in a particular direction. Neuronal mechanisms for the computation of motion direction are still unknown. We quantitatively mapped excitatory and inhibitory regions of receptive fields for directionally selective retinal ganglion cells in the Japanese quail, and found that the inhibitory regions are displaced about 1-3 deg toward the side where the null sweep starts, relative to the excitatory regions. Directional selectivity thus results from delayed transient suppression exerted by the nonconcentrically arranged inhibitory regions, and not by local directional inhibition as hypothesized by Barlow and Levick (1965).