Optic Nucleus Research Articles

Distribution of hyaluronan in the central nervous system of the frog

The qualitative and quantitative distribution pattern of hyaluronan (HA), a component of the extracellular matrix (ECM), was studied in the frog central nervous system by using a highly specific HA probe and digital image analysis. HA reaction was observed in both the white and the gray matter, showing a very intense staining around the perikarya and dendrites in the perineuronal net (PN). In the telencephalon, strong reaction was found in different parts of the olfactory system, in the pallium, and in the amygdala. In the diencephalon, intensive staining was found in the nucleus of Bellonci, the dorsal habenula, the lateral and central thalamic nuclei, and the subependymal zone of the third ventricle. In the mesencephalon, layers of optic tectum displayed different intensities, with the strongest reaction in layers B, D, F, 3, and 5. Other structures of the mesencephalon showed regional differences. The PN was especially intensively stained around the perikarya of the toral nuclei, the oculomotor and trochlear nuclei, and the basal optic nucleus. In the rhombencephalon, the granular layer of cerebellum, the vestibulocochlear nuclei, the superior olive, the spinal tract of the trigeminal nerve, and parts of the reticular formation showed the most intense reaction in the PN. In the spinal cord, considerable HA staining was found in the white matter and around the perikarya of motoneurons. The present study is the first description of the HA-positive areas of frog brain and spinal cord demonstrating the heterogeneity of HA distribution in the frog central nervous system.

Localization of GABA (γ-aminobutyric acid) markers in the turtle's basal optic nucleus

Recent physiological data have demonstrated that retinal slip, the sensory code of global visual pattern motion, results from complex interactions of excitatory and inhibitory visual inputs to neurons in the turtle's accessory optic system (the basal optic nucleus, BON) [M. Ariel, N. Kogo, Direction tuning of inhibitory inputs to the turtle accessory optic system, J. Neurophysiol. 86 (2001) 2919–2930. [6], N. Kogo, T.X. Fan, M. Ariel, Synaptic pharmacology in the turtle accessory optic system, Exp. Brain Res. 147 (2002) 464–472. [23]]. In the present study, the inhibitory neurotransmitter γ-aminobutyric acid (GABA), its synthetic enzyme, glutamic acid decarboxylase (GAD-67) and its receptor subtypes GABA A and GABA B receptors were localized within the BON. GABA antibodies revealed cell bodies and processes, whereas antibodies against GAD revealed a moderate density of immunoreactive puncta throughout the BON. GAD in situ hybridization labeled BON cell bodies, indicating a possible source of inhibition intrinsic to the nucleus. Ultrastructural analysis revealed terminals positive for GAD that exhibit symmetric synaptic specializations, mainly at neuronal processes having small diameters. Neurons exhibiting immunoreactivity for GABA A receptors were diffusely labeled throughout the BON, with neuronal processes exhibiting more labeling than cell bodies. In contrast, GABA B-receptor-immunoreactive neurons exhibited strong labeling at the cell body and proximal neuronal processes. Both these receptor subtypes are functional, as evidenced by changes of visual responses of BON neurons during application to the brainstem of selective receptor agonists and antagonists. Therefore, GABA may be synthesized by BON neurons, released by terminals within its neuropil and stimulate both receptor subtypes, supporting its role in mediating visually evoked inhibition contributing to modulation of the retinal slip signals in the turtle accessory optic system.

Central Connection of the Optic, Oculomotor, Trochlear and Abducens Nerves in Medaka, Oryzias latipes

Medaka (Oryzias latipes) is one of the few vertebrate experimental animals in which inbred lines have been established. It is also a species that has advanced in genetic studies in a manner comparable to zebrafish. This fish is therefore a good model for studying functional organization of the nervous system, but anatomical analysis of its nervous system has been limited to embryonic stages. In the present study, we investigated anatomy of cranial nerves in adult fish focusing on the visual function, using an inbred strain of medaka. Cranial nerves of medaka were labeled using biocytin, revealing a central distribution of retinofugal terminals, retinopetal neurons, and oculomotor, trochlear and abducens motor neurons. The optic nerve of the adult medaka was of a complete decussation type. Retinofugal terminals were located in 8 brain nuclei, the suprachiasmatic nucleus, nucleus pretectalis superficialis, nucleus dorsolateralis thalami, area pretectalis pars dorsalis (APd), area pretectalis pars ventralis (APv), nucleus of the posterior commissure (NPC), accessory optic nucleus, and the tectum opticum. Retinopetal neurons were identified in 6 brain nuclei, the ganglion of the terminal nerve, preoptic retinopetal nucleus, nucleus dorsolateralis thalami, APd, APv, and NPC. The oculomotor neurons were mostly labeled ipsilaterally and were located dorsomedially, abutting the fasciculus longitudinalis medialis in the mesencephalon. The trochlear nucleus was located contralaterally and dorsolaterally adjacent to the fasciculus longitudinalis medialis in the mesencephalon. The abducens nucleus was located ipsilaterally in a ventrolateral part of the rhombencephalic reticular formation. These results, generally similar to those in other teleosts, provide the basis for future behavioral and genetic studies in medaka.

Shunting Inhibition in Accessory Optic System Neurons

The interaction of excitatory and inhibitory inputs to the accessory optic system was studied with whole cell recordings in the turtle basal optic nucleus. Previous studies have shown that visual patterns, drifting in the same preferred direction, evoke excitatory and inhibitory postsynaptic events simultaneously. Analysis of the reversal potentials for these events and their pharmacological profile suggest that they are mediated by AMPA and GABA(A) receptors, respectively. Here, neurons were recorded to study nonlinear interaction between excitatory and inhibitory responses evoked by electrical microstimulation of the retina and pretectum, respectively. The responses to coincident activation of excitatory and inhibitory inputs exhibited membrane shunting in that the excitatory response amplitude, adjusted for changes in driving force, was attenuated during the onset of the inhibitory response. This nonlinear interaction was seen in many but not all stimulus pairings. In some cases, attenuation was followed by an augmentation of the excitatory response. For comparison, the size of the excitatory response was evaluated during a hyperpolarizing current pulse that directly modulated voltage-sensitive channels of a slow rectifying I(h) current. Injection of hyperpolarizing current did not cause the attenuation of the excitatory synaptic responses. We conclude that there is a nonlinear interaction between these excitatory and inhibitory synaptic currents that is not due to hyperpolarization itself, but probably is a result of their own synaptic conductance changes, i.e., shunting. Since these events are evoked by identical visual stimuli, this interaction may play a role in visual processing.

Morphology of the turtle accessory optic system.

Neural signals of the moving visual world are detected by a subclass of retinal ganglion cells that project to the accessory optic system in the vertebrate brainstem. We studied the dendritic morphologies and direction tuning of these brainstem neurons in turtle (Pseudemys scripta elegans) to understand their role in visual processing. Full-field checkerboard patterns were drifted on the contralateral retina while whole-cell recordings were made in the basal optic nucleus in an intact brainstem preparation in vitro. Neurobiotin diffused into the neurons during the recording and was subsequently localized in brain sections. Neuronal morphologies were traced using appropriate computer software to analyze their position in the brainstem. Most labeled neurons were fusiform in shape and had numerous varicosities along their processes. The majority of dendritic trees spread out in a transverse plane perpendicular to the rostrocaudal axis of the nucleus. Neurons near the brainstem surface were often oriented tangential to that surface, whereas more cells at the dorsal side of the nucleus were oriented radial to the brainstem surface. Further analysis of Nissl-stained neurons revealed the largest neurons are located in the rostral and medial portions of the nucleus although neurons are most densely packed in the middle of the nucleus. The preferred directions of the visual responses of the neurons in this sample did not correlate with their morphology and position in the nucleus. Therefore, the morphology of the cells in the turtle accessory optic system appears dependent on its position within the nucleus while its visual responses may depend on the synaptic inputs that contact each cell.

Connectivity of the turtle accessory optic system

Recent whole-cell recordings show that there are multiple synaptic inputs to the accessory optic system of the pond turtle Pseudemys scripta elegans (the basal optic nucleus, BON), suggesting a complex role in visual processing. The BON outputs have now been investigated using transport of diI, rhodamine-conjugated and biotinylated dextrans. Although transport was primarily anterograde, contralateral retinal ganglion cells were labeled retrogradely, confirming that the injection site was a retinal target. Other retrogradely labeled neurons were found ipsilateral to the injection site, in the pretectum, the ventral tegmentum, the dorsal nucleus of the posterior commissure and the lateral habenular nucleus. However, other data indicate that the habenular cells were labeled by spread of the tracer from the BON to the adjacent fasciculus retroflexus and interpeduncular nucleus. Anterogradely labeled fibers projected from BON following three paths, a lateral bundle to the ipsilateral dorsal midbrain, an intermediate bundle to the ipsilateral pretectal area or the posterior commissure and a ventral fiber bundle to the tegmentum bilaterally. Some of these fibers projected caudally through the tegmentum and cerebellar peduncle to terminate just below the Purkinje cell layer of the cerebellar cortex. Fibers that coursed via the intermediate bundle to the posterior commissure were also seen reaching the contralateral pretectal area and the contralateral BON. Injections of the retrograde tracer Fluorogold were also made in the BON to confirm the reciprocal connectivity of both basal optic nuclei. The pathways revealed by these experiments indicate the existence of multiple afferent and efferent connections of the BON, supporting the view that the accessory optic system is more than a simple relay of retinal signals into the brainstem for optokinetic reflexes.

Synaptic pharmacology in the turtle accessory optic system.

The accessory optic system of the turtle (the basal optic nucleus, BON) receives both excitatory and inhibitory inputs that are direction-sensitive. When the dorsal midbrain is ablated, only the monosynaptic direction-sensitive input from the retina to the BON remains. To better understand the central visual processing performed by the accessory optic system, this study identifies the neurotransmitters and their receptors that mediate the synaptic excitation and inhibition of BON cells. We used a reduced in vitro turtle brainstem preparation in which the two eyes and brain were isolated pharmacologically. Patch recordings were made on BON neurons while drugs were applied to the brain, with the eyes bathed in control media and either exposed to visual pattern motion or subjected to electrical stimulation. An antagonist of the AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) subtype of glutamate receptor applied within the brain chamber blocked the visual responses. In response to electrical stimulation both excitatory and inhibitory synaptic events were blocked in BON cells, presumably by blocking direct excitation by retinal ganglion cell axons in the BON and indirect excitation of inhibitory interneurons elsewhere in the brainstem. An NMDA receptor antagonist was ineffective, even when the response was measured in a BON cell depolarized in Mg(2+)-free media. A GABA(A) receptor on the BON cell mediates the inhibitory responses to retinal stimulation. Injection of lidocaine into the contralateral eye caused an increase in spontaneous inhibitory post-synaptic potentials (IPSPs), suggesting that a tonic retinal output exists that reduces brainstem inhibition of BON cells. Also, there may be tonic inhibition of an excitatory path to BON neurons from within the brainstem, because bicuculline increased spontaneous excitatory post-synaptic potentials (EPSPs) observed in a BON cell without retinal input. These results indicate that the BON is a site of complex visual processing of competing visual signals and provide insight into how an interaction of excitation and inhibition creates a retinal slip signal in the accessory optic system.

Discharge patterns evoked by depolarizing current injection in basal optic nucleus neurons of the pigeon

The nucleus of the basal optic root of the accessory optic system in birds is involved in optokinetic nystagmus, which stabilizes images on the retina by compensatory movements of the eyes. The present paper studies the physiological and morphological properties of basal optic neurons in the pigeon by using a brain slice preparation and intracellular recordings. Sixty-one cells examined could be categorized into six types based on their firing patterns in response to depolarizing current injection. Type I cells (54%) fire spontaneously and more spikes as current intensity is increased. Type II cells (15%) discharge regular spikes with similar interspike intervals. Type III cells (5%) show an early burst followed by tonic firing. Type IV cells (5%) fire regular bursts with similar interburst intervals. Type V cells (16%) fire a few spikes in a cluster only at onset of current application. Type VI cells (5%) produce a hump-like depolarization or a single spike depending on current intensities. Seventeen cells stained with Lucifer yellow have multipolar or piriform perikarya (15–28 μm) with two to eight primary dendrites. In some cases, an axon is observed to originate from the cell body, traveling dorsolaterally or dorsally. The physiological significance of these findings is discussed.

Parcellation of the human pretectal complex: a chemoarchitectonic reappraisal

The pretectum is composed of numerous small nuclei that control various oculomotor functions. In all the non-human mammals investigated, the different pretectal nuclei have been named uniformly according to their structural and functional homology. However, the human pretectal nuclei still bear their traditional, in most cases misleading, nomenclature. In order to reveal the presumed chemoarchitectonic similarities between human and non-human pretectal nuclei, neuropeptide Y (NPY)- and vasoactive intestinal polypeptide (VIP)-immunohistochemistry was performed in the human pretectum, after being utilised successfully for the identification of different pretectal nuclei in the cat. No VIP neurones were observed in the human pretectal area, but numerous NPY cells were found in the ‘nucleus lentiformis’, and in the anterior bulge of the pretectum, while the ‘nucleus sublentiformis’ contained an abundant NPY fibre network. Some NPY neurones were present in the ‘principal pretectal nucleus’ as well. The olivary pretectal nucleus possessed NPY fibres, too. In the accessory optic system, the lateral terminal nucleus contained both NPY and VIP neurones, while in the dorsal terminal nucleus only NPY neurones were found. Our chemoanatomical findings were compared to the standard cytoarchitecture as well. Based on the homotopies in the spatial distribution pattern of NPY neurones in the cat and human pretectum, the current, widely accepted non-human anatomical nomenclature was applied to the morphologically homologous nuclei of the human pretectum. Accordingly, the ‘nucleus lentiformis’ (which contains numerous NPY cells) corresponds to the nucleus of the optic tract, the ‘nucleus sublentiformis’ (containing a dense network of NPY fibres) to the posterior pretectal nucleus, and the ‘nucleus of the pretectal area’ corresponds to the medial pretectal nucleus. We identified the anterior part of the pretectum as the human equivalent of the anterior pretectal nucleus in non-humans, including its two compact and reticular subdivisions. In addition, two accessory optic nuclei were verified chemoarchitectonically in the human brain.

Direction tuning of inhibitory inputs to the turtle accessory optic system.

Neurons in turtle accessory optic system (basal optic nucleus, BON) were studied to compare excitatory and inhibitory visual inputs. Using a reduced in vitro brain stem preparation with the eyes attached, previous studies only showed a monosynaptic retinal input to the BON from direction-sensitive retinal ganglion cells that share a common preferred direction. Now using an intact brain stem preparation, not only did BON neurons display inhibitory postsynaptic potentials [IPSP(C)s] spontaneously, but IPSP(C)s were also evoked by visual pattern motion, they had their polarity reversed near the chloride equilibrium potential and they were blocked by the GABA(A) antagonist bicuculline. Because excitatory postsynaptic currents had reversal potentials >0 mV, BON cells were recorded using patch electrodes filled with QX-314 or Cs+ to measure the cell's direction tuning also at that higher reversal potential. For most of the BON neurons studied, their visual excitation and inhibition had a very similar preferred direction, indicating that both synaptic inputs were maximally active onto the same cell under the same stimulus conditions. These competing inputs may result from connections between the pretectum and accessory optic nuclei. Such synaptic interactions may serve a functional role in the visual processing necessary to create retinal slip signals for oculomotor control.

Directional modulation of visual responses of pretectal neurons by accessory optic neurons in pigeons.

The nucleus lentiformis mesencephali and the nucleus of the basal optic root in birds, homologous to the nucleus of the optic tract and the terminal nuclei of the accessory optic tract in mammals, are involved in optokinetic nystagmus. The present study provides the first electrophysiological evidence that reversible blockade of the pigeon nucleus of the basal optic root by lidocaine can change visual responsiveness of pretectal neurons in a direction-dependent manner. Thirty pretectal cells examined were classified as unidirectional (80%), bidirectional (10%) and omnidirectional (10%) cells according to their directional selectivity. Among the unidirectional cells, seven cells changed firing rates in all directions of motion, 11 changed visual responses only in the preferred directions and six others did not change their responsiveness during lidocaine. Most of the bidirectional cells changed firing rates in the temporonasal direction, and two-thirds of the omnidirectional cells showed these changes in all directions. Thirteen lidocaine administration sites were marked within the nucleus of the basal optic root and 19 recording sites were marked within the nucleus lentiformis mesencephali. This histological verification indicates that the effects of lidocaine blockade in the accessory optic nucleus on the directional selectivity and visual responsiveness of pretectal cells appear to be related, to some extent, to the location of drug injections in the nucleus of the basal optic root. This study has found that visual neurons in the nucleus of the basal optic root, which predominantly prefer vertical and backward motion, could modulate the directional selectivity and visual responsiveness of neurons in the nucleus lentiformis mesencephali, which mainly prefer horizontal motion. It is conceivable that both nuclei work together in coordination and in competition during optokinetic nystagmus.

Metabolic changes in the nucleus of the optic tract after monocular enucleation as revealed by cytochrome oxidase histochemistry.

The histochemistry for the mitochondrial enzyme cytochrome oxidase (CO) was used to evaluate the levels of metabolic activity in neurons of the nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN) in the opossum (Didelphis aurita). The observations were performed in four groups: normal juveniles (4 months old), monocularly enucleated juveniles analysed when adults, normal adults (8 to 18 months old) and monocularly enucleated adults. CO labeled cells were observed to have a similar distribution along the NOT-DTN anteroposterior axis in both juvenile and adult normal animals. Monocular enucleation performed in adults produced a significant reduction of the reactive neuropil but not of the number of CO labeled cells in the deafferented NOT-DTN: the number of labeled neurons per section in the deafferented side matched those of the ipsilateral complex. In juveniles, however, this procedure caused a systematic reduction of the number of CO labeled cells in the contralateral NOT-DTN in comparison to the spared complex. The lack of reduction in the number of neurons found on the deafferented side of the NOT-DTN of monocularly enucleated adult opossums compared with the ipsilateral side might result from the presence of compensatory inputs to maintain their metabolic equivalence. However, when the monocular enucleation was performed in juvenile opossums, a statistically significant asymmetry of CO neurons in the NOT-DTN was observed. In other words, the compensatory mechanisms proposed for the adults were either absent or insufficient to achieve symmetry in juveniles, suggesting a more heavily reliance in the retinal input.

Cytoarchitectonic study of the brain of a perciform species, the sea bass (Dicentrarchus labrax). II. The diencephalon.

The cytoarchitecture of nuclei in the preoptic area, ventral thalamus, dorsal thalamus, epithalamus, hypothalamus, posterior tuberculum, synencephalon, and pretectum and the accessory optic nuclei was analyzed in a perciform teleost, the sea bass Dicentrarchus labrax, by using serial sections stained with cresyl-violet. In general, the cytoarchitecture of the preoptic area, ventral and dorsal thalamus, epithalamus, and synencephalon resembles the histological pattern of other teleosts. However, the parvocellular preoptic nucleus of sea bass has been subdivided into parvocellular and anteroventral parts for morphological and functional reasons. The hypothalamus of the sea bass seems to differ slightly from that of other teleosts. An elaborated lateral tuberal nucleus, with five subdivisions, and three different nuclei around the lateral recesses were recognized. A medial nucleus of the inferior lobe, which has been reported previously in the perciform Sparus aurata, is also present in the hypothalamus of sea bass but has not been described before in another advanced teleost. The organization of the pretectum and the accessory optic system is essentially similar in sea bass to that described in other perciforms with highly developed vision. The migrated portion of the posterior tuberculum of sea bass appears to differ from this region of the diencephalon in other teleosts. In sea bass, three cell masses that have been described previously only in the perciform Sparus aurata have been assigned to the migrated area of the posterior tuberculum. This study will provide the neuroanatomical basis for future morpho-functional studies to be done in the sea bass brain.

Immunocytochemical mapping of NPY and VIP neuronal elements in the cat subcortical visual nuclei, with special reference to the pretectum and accessory optic system

The aim of this study was to describe the distribution patterns of neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP)-immunoreactive (ir) neuronal elements in subcortical visual centers of the cat. Numerous NPY-ir neurons were present in the feline nucleus of the optic tract and in the anterior pretectal nucleus. Only a few NPY-ir neurons were found in the posterior, medial and olivary pretectal nuclei and in the accessory optic nuclei. Diffuse and heavily beaded NPY-ir fiber plexuses were observed throughout the superior colliculus, pretectum, and accessory optic system. Extensively arborising NPY-ir fibers were present in the mesencephalon and ventral lateral geniculate nucleus, while the dorsal visual thalamic nuclei contained only a few NPY-ir fibers. VIP-ir cells were present mainly in the accessory optic nuclei, and they were absent in the dorsal visual thalamus. Both NPY- and VIP-ir neurons were multipolar and fusiform in shape in the regions studied. Enucleation did not alter the appearance of NPY- and VIP-containing neuronal elements in the superior colliculus and pretectum while in the thalamus a subset of NPY-ir fiber population disappeared, indicating their retinal origin. Although there is a partial overlap in the topographical localization of the NPY- and VIP-ergic neurons in the pretectum, the colocalization of the two peptides could not be demonstrated. The present observations demonstrate the existence of two different and separate peptidergic (NPY and VIP) neuronal populations in the pretectum.

Synaptic and neurochemical features of calcitonin gene-related peptide containing neurons in the rat accessory optic nuclei

Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), including the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neurons are involved in visual circuitry, electron microscopic preparations were analyzed from normal rats and rats with optic nerve transections. A co-localization analysis was also made because CGRP-labeled neurons had features of GABAergic neurons. Thus, sections were prepared for light microscopy to determine whether CGRP-containing neurons also had glutamate decarboxylase (GAD) and other markers for GABAergic neurons, such as calcium binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron microscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that were postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating that retinal ganglion cells form synapses with CGRP-labeled neurons in the AON. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric synapses with unlabeled somata as well as dendritic shafts and spines. Consistent with this type of synapse being GABAergic were the co-localization data showing that about 90% of the CGRP-labeled neurons co-localized GAD in the AON. Many CGRP-labeled neurons showed immunostaining for CR (40%) whereas only a few had labeling for CB (5%). No CGRP-labeled neurons had PV. These data show that CGRP-containing neurons receive direct retinal input and represent a subpopulation of GABAergic neurons which differentially co-express calcium-binding proteins.

Analysis of direction-tuning curves of neurons in the turtle's accessory optic system.

Visual-movement sensitivity of neurons in the turtle's accessory optic system was investigated. Neuronal responses to stimulus direction and speed were analyzed to determine whether they reflect processing by a one-dimensional encoder of visual motion or whether they indicate directional integration of presynaptic direction-sensitive responses whose maximal-response directions are distributed. Both of these mechanisms make predictions about the functional relationship between stimulus direction and response. The responses of single units in the basal optic nucleus to visual stimulation in different directions were described by both cosine and wrapped normal fitting functions. The wrapped normal function (a Gaussian curve mapped onto a circle) performed at least as well as the cosine function and described directional tuning curves of varying widths. Unlike cosines, the addition of two wrapped normals could describe multi-lobed directional data. Next, it was demonstrated that these neurons did not encode visual motion projected onto a single, spatial axis. Responses to the projected speed along the maximal-response direction were systematically lower than responses to the actual speed along that direction. Thus, for speeds above 1 degrees/s, neuronal response varies with respect to direction but not speed. Summation of presynaptic direction-sensitive responses with distributed maximal-response directions (referred to as directional integration) is discussed as a means of accounting for these results.

Morphology of basal optic tract terminals in the turtle, Pseudemys scripta elegans.

The morphologies of axon terminals of retinal ganglion cells projecting to the basal optic nucleus (BON) via the basal optic tract (BOT) were studied in the red-eared turtle. The BOT was visualized on the ventral surface of the brainstem in vitro, and either biotinylated dextran amine was injected extracellularly or neurobiotin was injected into physiologically identified axons during intracellular recordings. Up to 16 hours after tracer injection, the brains were fixed, sectioned parasagittally, and stained for biotin and Nissl substance. The diameters and depths of extracellularly filled axons were measured at three BON sites. Fourteen axons were reconstructed from serial sections with the aid of appropriate computer software. Analysis of extracellularly filled retinal axons revealed that about three times more axons were present just inside the rostral border of the BON compared with its caudal border. Thick (2-4 microm) axons were located within 100 microm from the ventral border, whereas thin (<2 microm) axons were found throughout the nucleus. Only the thinnest axons (<1 microm) extended caudally from the nucleus, indicating that some extracellularly labelled fibers passed through the BON. The intracellularly filled axons were more similar to the thick axons filled extracellularly and arborized entirely within the BON. All of the intracellularly filled axons had thick ventral trunks from which many thin branches extended dorsally and obliquely within the BON. The thin branches bifurcated repeatedly to form bead-like varicosities or boutons that often formed clusters within regions of 150 microm3 or less. These clusters may reflect areas of focused synaptic contact on BON cells with specific direction preferences.

Direction Tuning of Individual Retinal Inputs to the Turtle Accessory Optic System

Neurons in turtle accessory optic system [basal optic nucleus (BON)] were recorded to study convergence of retinal afferents, using whole-cell patch electrodes in a reduced in vitro brainstem preparation with the eyes attached. BON cells primarily exhibit EPSPs from a contralateral retinal ganglion cell input and generate an output of action potentials. Visual responses were evoked by different directions of either full-field or local moving patterns. Direction tuning of action potentials was compared with that of EPSPs detected by passing the membrane voltage through an AC amplifier and window discriminator. This rough measure of retinal input indicated that the direction tuning of the full-field excitatory input from the retina matched that of the spike output for the same BON cell. Using local patterns within the receptive fields of the BON cells, it was estimated that one to four adjacent retinal inputs were being stimulated. The direction tuning of these inputs had preferred directions that were similar to that of the full-field spike output of the cell, irrespective of where the small window was placed within the receptive field. Because more than one retinal input may have been stimulated by the small stimulus window, subsets of those EPSPs that may represent responses of a single retinal afferent were identified based on their amplitude and rise time. Again, the preferred direction of those putative single retinal afferents matched the direction tuning of the spike output of the BON cell. These findings are discussed in terms of the formation of the retinal slip signal by the BON.

Connections between the nucleus of the optic tract and the accessory optic nuclei in the cat: WGA-HRP and GABA immunohistochemical study

Connections between the nucleus of the optic tract and the accessory optic nuclei in the cat: WGA-HRP and GABA immunohistochemical study

Calcitonin gene-related peptide immunoreactivity selectively labels accessory optic nuclei and pathways of the rat visual system.

The present study shows the distribution of calcitonin gene-related peptide (CGRP)-immunolabeled neuronal somata and fibers in the accessory optic system of adult rats. CGRP-immunoreactive cell bodies were small to medium-sized and mostly fusiform or oval-shaped. Both immunolabeled somata and fibers were found in the dorsal and lateral terminal nuclei as well as in the interstitial nucleus of the superior fasciculus (posterior fibers); whereas only immunoreactive fibers were found in the ventral division of the medial terminal nucleus, particularly its rostral portion. These results indicate that CGRP-containing neurons are present in all nuclear components of the accessory optic system and suggest that this neuropeptide may play a neuromodulative role in eye movements.