Basal Optic Nucleus Research Articles

A multivariate approach to structural heterogeneity of retinal ganglion cells

Knowing neuronal types is essential for understanding the structural and functional organization of the nervous system. It has long been recognized that neuronal types should be discovered and not defined. This can be done using cluster analysis (CA). Despite there being many studies using CA to classify neurons, only a few of them meet its formal prerequisites. In the present study, we provide an example of using CA in combination with other multivariate techniques for examining neuronal diversity. A special emphasis is put on formal prerequisites to the data and procedure. The data under scrutiny are a sample of ganglion cells projecting to the basal optic nucleus [accessory optic system-projecting ganglion cells (AOS GCs)] in the common frog. There is physiological evidence that these cells comprise at least two functional types but their structural heterogeneity has not been addressed. Cells were labeled with horseradish peroxidase in vivo and examined in whole-mounted retinae using light microscopy. A sample of well-stained cells was obtained and used to estimate 18 structural parameters. A variety of clustering algorithms were used to classify the cells. The joint polar distribution of dendrite mass was monomodal. CA did not reveal a statistically reliable cluster structure in the sample. The clusters were not cohesive and well isolated. ANOVA-on-Ranks revealed no significant between-cluster differences. Our formal conclusion is that functionally distinct frog AOS GCs do not differ in morphology or dendritic arbor orientation. The advantages and limitations of the adopted approach are discussed.

Modulation of visual inputs to accessory optic system by theophylline during hypoxia

Neural tissues from fresh water turtles have been electrophysiologically studied in vitro due to their significant resistance to hypoxia. Such neurons have resting membrane potentials that are similar to intact animals and receive similar synaptic inputs evoked by sensory stimuli. One mechanism to reduce the brain's metabolic requirement in the absence of oxygenated blood flow was investigated by blocking adenosine receptors before and during hypoxia. Extracellular and whole-cell patch recordings were made from the basal optic nucleus, whose neurons respond to visual stimuli in vitro. While the addition of the adenosine antagonist theophylline to oxygenated superfusate had minimal effect on the neural activity, theophylline in superfusate bubbled with nitrogen strongly increased activity compared to either oxygenated theophylline or control superfusate bubbled with nitrogen. The increase in spontaneous activity was due to increases to both amplitude and frequency of excitatory synaptic events. Even during these increases, the neurons continued to exhibit their direction-sensitive responses. These results indicate that adenosine may play a role in protecting the viability of the brainstem during hypoxia without reducing visually mediated brainstem reflex control.

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.

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.

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.

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.

Membrane properties and monosynaptic retinal excitation of neurons in the turtle accessory optic system.

Using an eye-attached isolated brain stem preparation of a turtle, Pseudemys scripta elegans, in conjunction with whole cell patch techniques, we recorded intracellular activity of accessory optic system neurons in the basal optic nucleus (BON). This technique offered long-lasting stable recordings of individual synaptic events. In the reduced preparation (most of the dorsal structures were removed), large spontaneous excitatory synaptic inputs [excitatory postsynaptic potentials (EPSPs)] were frequently recorded. Spontaneous inhibitory postsynaptic potentials were rarely observed except in few cases. Most EPSPs disappeared after injection of lidocaine into the retina. A few EPSPs of small size remained, suggesting that these EPSPs either were from intracranial sources or may have been miniature spontaneous synaptic potentials from retinal ganglion cell axon terminals. Population EPSPs were synchronously evoked by electrical stimulation of the contralateral optic nerve. Their constant onset latency and their ability to follow short-interval paired stimulation indicated that much of the population EPSP's response was monosynaptic. Visually evoked BON spikes and EPSP inputs to BON showed direction sensitivity when a moving pattern was projected onto the entire contralateral retina. With the use of smaller moving patterns, the receptive field of an individual BON cell was identified. A small spot of light, projected within the receptive field, guided the placement of a bipolar stimulation electrode to activate retinal ganglion cells that provided input to that BON cell. EPSPs evoked by this retinal microstimulation showed features of unitary EPSPs. Those EPSPs had distinct low current thresholds. Recruitment of other inputs was only evident when the stimulation level was increased substantially above threshold. The average size of evoked unitary EPSPs was 7.8 mV, confirming the large size of synaptic inputs of this system relative to nonsynaptic noise. EPSP shape was plotted (rise time vs. amplitude), with the use of either evoked unitary EPSPs or spontaneous EPSPs. Unlike samples of spontaneous EPSPs, data from many unitary EPSPs formed distinct clusters in these scatterplots, indicating that these EPSPs had a unique shape among the whole population of EPSPs. In most BON cells studied, hyperpolarization-activated channels caused a slow depolarization sag that reached a plateau within 0.5-1 s. This property suggests that BON cells may be more complicated than a simple site for convergence of direction-sensitive retinal ganglion cells to form a central retinal slip signal for control of oculomotor reflexes.

The accessory optic system in the frog, Rana pipiens: An electron microscopic study of the retinal afferents utilizing the anterograde tracer biocytin

The retinal afferents to the basal optic nucleus in the frog, Rana Pipiens, were labeled anterogradely with biocytin and subsequently studied at the electron microscopic level. Labeled synaptic terminals in the nucleus varied in size from 0.5 μm to 2.0 μm and made symmetric synaptic contacts with large and small dendrites, although very rare axospinous and axosomatic contacts were also demonstrated.

Visual responses and connectivity in the turtle pretectum.

1. Using an isolated turtle brain preparation, we made extracellular spike recordings in the dorsal midbrain during visual stimulation. Single units were isolated by their response to a slow-moving full-field visual pattern imaged on the contralateral retina. This stimulus elicits responses from the basal optic nucleus (BON) and the cerebellar cortex using a similar preparation. Direction and speed tuning were then analyzed, as well as the size and position of the receptive field. 2. In one brain stem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction sensitive (DS) to contralateral retinal stimulation. The location and properties of these cells indicate that they are in the mesencephalic lentiform nucleus (nLM). Anterograde transport of intravitreally injected horseradish peroxidase revealed that this pretectal nucleus receives direct input from the contralateral eye. 3. All but 2 of the 48 cells of the nLM were strongly DS. The most effective stimulus was a slowly moving complex visual pattern that drifted nasally in the contralateral visual field. Brief flashes of spots, patterns, or diffuse light were much less effective. Receptive fields were large and usually (9 of 13 cells) centered in the superior visual field near the horizon and nasal to the blind spot. 4. The visual responses of nLM cells were compared to those of cells in the superficial layers of the optic tectum. In contrast to nLM, the responses of tectal cells were heterogeneous and frequently not DS. Neither tectum or nLM cells had much spontaneous spike activity during darkness or stationary patterns. On the other hand, visual responses of nLM cells were very similar to those of the BON, where neurons also had low spontaneous activity, preferred slow-moving patterns, and were DS. However, nLM and BON exhibit different distributions of preferred directions. Most nLM cells preferred temporal-to-nasal motion, whereas BON cells preferred almost any direction, although few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brain stem including the BON. 5. The visual properties of nLM cells recorded in vitro were very similar to those that were recorded in intact turtles.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal projections in the cane toad, Bufo marinus.

The location and extent of retinorecipient areas in the cane toad, Bufo marinus, were established by anterograde transport of cobaltic-lysine complex from the cut optic nerve. Most of the labeled optic axons travelled in the marginal optic tract, while others were in the axial optic tract, and/or the basal optic tract. Retinal projections terminated in both contralateral and ipsilateral targets. In addition to the optic tectum, the main visual center, retinorecipient areas included the suprachiasmatic nucleus, rostral visual nucleus, neuropil of Bellonci, corpus geniculatum thalamicum, ventrolateral thalamic nucleus (dorsal part), posterior thalamic neuropil, uncinate neuropil, pretectal nucleus lentiformis mesencephali and basal optic nucleus. While all of these retinorecipient areas receive optic fibers from both eyes, the ipsilateral retinal projections were observed to be generally sparser than those from the contralateral retina. A sparse optic fiber projection covers the surface of the ipsilateral optic tectum and is most prominent rostromedially and caudolaterally. The position and the extent of each of the retinorecipient areas were determined in relation to a three-dimensional coordinate system. Morphometric analysis showed that 85.3% of the retinorecipient area is in the contralateral optic tectum, 10.4% in contralateral non-tectal areas, 1.6% in the ipsilateral optic tectum and 2.7% in ipsilateral non-tectal areas. The presence of an ipsilateral tectal projection and the well defined pretectal visual neuropil complex may be related to the highly developed visual behavior and visual acuity of Bufo marinus.

Cholinergic, Monoaminergic and Peptidergic Innervation of the Primary Visual Centers in the Brain of the Lizards &lt;i&gt;Gekko gecko&lt;/i&gt; and &lt;i&gt;Gallotia galloti&lt;/i&gt; (Part 2 of 2)

In order to study the relationship between retinal projections and immunohistochemically identified neurotransmitter systems in the primary visual centers of the brain in lizards, intraocular injections of horseradish peroxidase were combined with immunohistochemistry. Antibodies raised against six substances were applied: choline acetyltransferase (ChAT), serotonin (5-HT), tyrosine hydroxylase (TH), dopamine (DA), substance P (SP), and leu-enkephalin (LENK). In the primary visual centers of the lizards <i>Gekko gecko </i>and <i>Gallotia galloti, </i>notable overlap was observed between retinofugal fibers with: 1) ChAT-immunoreactive fibers in almost all primary visual centers; 2) 5-HT-immuno-reactive fibers in the ventral lateral geniculate body and the basal optic nucleus; 3) TH-immunoreactive fibers in the nucleus ovalis and the dorsal lateral geniculate body; 4) SP- and LENK-immunoreactive fibers in the perirotundal belt; and 5) TH- and SP-immunoreactive fibers in the pretectal posterodorsal nucleus. The latter nucleus also contains dopaminergic cell bodies that lie outside the retinal target area but have dendrites extending into it. Several differences were noted in the distribution of 5-HT-, TH-, DA-, and LENK-immunoreactive fibers in the tectum of the midbrain in the two species studied. Distinct laminae of 5-HT-immunoreactive fibers (layer 9) and TH- and DA-immunoreactive fibers (layers 9 and 11) are present in G. <i>gecko </i>but absent or, at least, less distinct in G. <i>galloti. </i>On the contrary, the optic layers in the tectum of G. <i>galloti </i>show a rather dense plexus of LENK immunoreactive fibers, whereas the corresponding layers in G. <i>gecko </i>are devoid of LENK-immunoreactivity. Since only a very few ChAT immunoreactive fibers were observed in the optic nerve of G. <i>galloti, </i>most of the observed immunoreactive fibers in the primary visual centers are considered to have an extraretinal origin. Putative sources of the cholinergic, the monoaminergic, and the peptidergic innervation of the primary visual centers in reptiles include the isthmic nucleus, the raphe nuclei, the substantia nigra and the nucleus of the posterior commissure, as reported in other amniotes.

Effects of synaptic drugs on turtle optokinetic nystagmus and the spike responses of the basal optic nucleus

Behavioral and electrophysiological measures were used to elucidate the retinal modulation of oculomotor control in the turtle. Eye movements were recorded following intravitreal applications of 2-amino-4-phosphonobutyrate (APB) and the GABA antagonists picrotoxin and bicuculline. Visual responses of single basal optic nucleus (BON) neurons of the accessory optic system were studied in parallel experiments. The effectiveness of APB, a glutamate analog thought to act selectively on the retinal ON pathway, was assessed independently by recording electroretinograms or ganglion cell activity. Injections of APB into the turtle's eye reduced or blocked the injected eye's ability to drive horizontal optokinetic nystagmus, as also observed in rabbit and cat (Knapp et al., 1988; Yucel et al., 1989). Single-unit recordings from the BON during APB superfusion (50-200 microM APB) of the contralateral retina demonstrated that these cells, which are direction-sensitive and respond to the offset of light flashes, have their responses to moving stimuli blocked by APB. During the APB effect, GABA antagonists were applied to the same eye. Although moderate doses of APB were sufficient to block optokinetic or BON light responses, the addition of GABA blockers still elicited a spontaneous temporal-to-nasal nystagmus (Ariel, 1989) or visually responsive yet direction-insensitive responses from BON cells (Schuerger et al., 1990). These results are discussed in terms of the retinal output to pathways involved in oculomotor control of optokinetic nystagmus.

Electrophysiological evidence for a direct projection of direction-sensitive retinal ganglion cells to the turtle's accessory optic system

1. The direct retinal input pathway to the basal optic nucleus (BON), the primary nucleus of the turtle accessory optic system, was characterized physiologically. We tested the hypothesis that directional information encoded in retinal ganglion cells can influence the BON via a direct pathway. Using an in vitro whole-brain, eyes-attached preparation, we demonstrated the directness of this pathway by 1) antidromic activation of retinal ganglion cells from the contralateral BON and 2) orthodromic activation of the BON from the contralateral optic nerve. 2. Of 72 physiologically classified retinal ganglion cells, 9 could be antidromically activated from the contralateral BON with low current (less than 200 micro A). Eight of these cells were direction-sensitive (DS). The ninth cell did not respond to visual stimulus movement. The antidromic latencies ranged from 2.2 to 6.1 ms with a mean of 3.8 ms. These latencies were quite consistent for each cell, having an average SD of 0.08 ms. Moreover, consistent responses could always be recorded at stimulation rates up to 100 Hz. 3. With current stimulation of the contralateral optic nerve, the orthodromic conduction latency of 17 BON single units ranged from 2.5 to 6.6 ms with a mean of 4.6 ms. These latencies were more variable for an individual cell, having an average SD of 0.3 ms. Responses to individual current pulses could never be consistently evoked at stimulation rates greater than 40 Hz. 4. DS responses were recorded in BON single units after the removal of the dorsal midbrain, including the optic tectum and pretectum as well as the telencephalon. Three of these cells were activated orthodromically by current stimulation delivered to the contralateral optic nerve. Thus directional information reaches the BON via a direct projection from the contralateral retina. 5. Visual response properties of DS retinal ganglion cells were compared with those of BON cells to examine the transformations that take place in the brain stem. Applying a limaçon model to the responses of both DS retinal ganglion cells and BON cells revealed that both types of cells have very similar direction tuning. However, the distribution of maximally responsive directions in the retina may differ from that of the BON. 6. Because DS retinal ganglion cells project directly to the BON, and because BON cells lose their direction sensitivity after retinal application of GABA antagonists, we conclude that the BON receives essential directional information directly from DS retinal ganglion cells. This directional information in the BON may represent a retinal slip error signal necessary for retinal image stabilization.