A radioautographic study of retinal projections in Type I and Type II lizards

The retinofugal projections of 5 species (Acanthodactylus boskianus, Scincus scincus, Tarentola mauritanica, Uromastix acanthinurus and Zonosaurus ornatus) belonging to 5 different families of Type I and Type II lizards have been examined by means of the radioautographic method. In the 5 species the retinal ganglion cells project to the contralateral hypothalamus (nucleus suprachiasmaticus), thalamus (nucleus geniculatus lateralis pars ventralis, nucleus geniculatus lateralis pars dorsalis), pretectum (nuclei lentiformis mesencephali, geniculatus pretectalis, postero-dorsalis griseus tectalis), tectum opticum (layer 2 to layer 6 of the stratum griseum et fibrosum superficiale) and tegmentum mesencephali (nucleus opticus tegmenti). Ipsilateral optic fibers were never observed in Uromastix acanthinurus, whereas an uncrossed quota was visible in both nucleus geniculatus lateralis pars dorsalis and nucleus postero-dorsalis in the other species. An ipsilateral retinotectal projection was observed only in Tarentola mauritanica. With the exception of the nucleus griseus tectalis the contralateral optic centers identified in this material have to a large extent been observed in other reptiles belonging to the different orders. The presence in reptiles of a general pattern of contralateral visual projections indicates that these were established very clearly in the course of evolution. Similarities become apparent when this plan is compared with that observed in birds. In marked contrast the ipsilateral component in reptiles is unstable and mutable in nature. This ipsilateral retinotectal projections do not appear to be a feature restricted to Type I lizards. On the other hand, the presence of this optic component cannot be linked solely to nocturnal habits.     

Retinal recipient nuclei in the painted turtle,Chrysemys picta: An autoradiographic and HRP study

Retinofugal pathways in the painted turtle were examined with autoradiographic and HRP methods. The majority of the retinal fibers decussate at the optic chiasm and course caudally to terminate in 12 regions of the diencephalon and mesencephalon. The pars dorsalis of the lateral geniculate nucleus is the densest target in the thalamus. Two nuclei dorsal to pars dorsalis—the dorsal optic and dorsal central nuclei—receive optic input. Three nuclei ventral to pars dorsalis are retinal targets—the ventral geniculate nucleus, nucleus ventrolateralis pars dorsalis, and nucleus ventrolateralis pars ventralis. Contralateral fibers course through the pretectum where they terminate in nucleus geniculatis pretectalis, nucleus lentiformis mesencephali, nucleus posterodorsalis, and the external pretectal nucleus. Retinal fibers also terminate within the superficial zone of the optic tectum. HRP material demonstrates three optic fiber layers—laminae 9, 12, and 14. Optic fibers leave the main optic tract as a distinct accessory tegmental optic pathway and terminate in the basal optic nucleus. Ipsilateral retinal terminals occur in a pars dorsalis and a pars ventralis of the lateral geniculate nucleus, the dorsal optic nucleus, nucleus posterodorsalis, the basal optic nucleus, and in laminae 9 and 12 of the optic tectum. Rostrally, the ipsilateral tectal fibers occupy two zones along the medial and lateral tectal roof; these zones converge caudally and are continuous along the caudal wall of the tectum.     

Projections of the retinorecipient pretectal nuclei in the pigeon (Columba livia)

We have used anterograde autoradiographic and retrograde HRP techniques to investigate the efferent connections of the retinorecipient pretectal nuclei in the pigeon. In the accompanying paper we identified these nuclei in the pigeon as the nucleus lentiformis mesencephali--pars lateralis and pars medialis, the tectal gray, the area pretectalis, and pretectalis diffusus. Although there are reports of a few of the projections of these nuclei, they had not previously been the subject of a detailed study. We found that different cell types in the lentiformis mesencephali, pars medialis and the lentiformis mesencephali, pars lateralis have descending projections to different targets. These targets include the inferior olive, the cerebellum, the lateral pontine nucleus, the nucleus papillioformis, the nucleus of the basal optic root, the nucleus mesencephalicus profundus, pars ventralis, the nucleus principalis precommissuralis, and the stratum cellulare externum. We found that a few cells in the lentiformis mesencephali project to the medial pontine nucleus, but that a much heavier projection arises from the nucleus laminaris precommissuralis, which is medial to the nucleus lentiformis mesencephali, pars medialis. The tectal gray has predominantly ascending projections to the diencephalon. The nuclei that it projects to are the nucleus intercalatus thalami, the nucleus of the ventral supraoptic decussation, the nucleus posteroventralis, the ventral lateral geniculate nucleus, the nucleus dorsolateralis medialis, and the nucleus dorsolateralis anterior. The tectal gray also projects topographically to layers 4 and 8-13 of the optic tectum. Area pretectalis has both ascending and descending projections. It has ipsilateral ascending projections to the nucleus dorsolateralis anterior, pars magnocellularis, the nucleus lateralis anterior, and the nucleus ventrolateralis thalami. It has ipsilateral descending projections to the central gray, the nucleus of the basal optic root, pars dorsalis, the lateral pontine nucleus, and the deep layers of the optic tectum. It has contralateral projections to the area pretectalis, the nucleus Campi Foreli, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the cerebellum, and the Edinger-Westphal nucleus. The efferent projections of pretectalis diffusus are limited. It projects contralaterally to the pretectalis diffusus, and ipsilaterally to the nucleus of the ventral supraoptic decussation, the lateral pons, and the cerebellum.4     

Cytoarchitecture and fiber connections of the superficial pretectum in a teleost,Navodon modestus

Fiber connections of the so-called nucleus geniculatus lateralis (or the nucleus pretectalis superficialis pars parvocellularis) in a teleost, Navodon modestus, were examined by means of the horseradish peroxidase (HRP) tracing method. The nucleus receives fibers from the contralateral retina, ipsilateral optic tectum and nucleus isthmi, and projects bilaterally to the nucleus intermedius of Brickner and ipsilaterally to the optic tectum and raphe nuclei. The fiber connections suggest that the nucleus relays mainly visual information to the inferior lobe (hypothalamus) but not to the telencephalon. The nucleus is not a homologous structure to the lateral geniculate nucleus in other vertebrate classes.     

Pretectal connections in turtles with special reference to the visual thalamic centers: a hodological and gamma-aminobutyric acid-immunohistochemical study

Projections of the pretectal region to forebrain and midbrain structures were examined in two species of turtles (Testudo horsfieldi and Emys orbicularis) by axonal tracing and immunocytochemical methods. Two ascending gamma-aminobutyric acid (GABA)ergic pathways to thalamic visual centers were revealed: a weak projection from the retinorecipient nucleus lentiformis mesencephali to the ipsilateral nucleus geniculatus lateralis pars dorsalis and a considerably stronger projection from the nonretinorecipient nucleus pretectalis ventralis to the nucleus rotundus. The latter is primarily ipsilateral, with a weak contralateral component. The interstitial nucleus of the tectothalamic tract is also involved in reciprocal projections of the pretectum and nucleus rotundus. In addition, the pretectal nuclei project reciprocally to the optic tectum and possibly to the telencephalic isocortical homologues. Comparison of these findings with previous work on other species reveals striking similarities between the pretectorotundal pathway in turtles and birds and in the pretectogeniculate pathway in turtles, birds, and mammals.     

Diameters and terminal patterns of retinofugal axons in their target areas: an HRP study in two teleosts (Sebastiscus and Navodon)

Studies in various vertebrate classes, particularly amphibians and mammals, have revealed that retinal ganglion cells with different functional properties project by means of axons of correspondingly different diameters onto specific target regions. Whether a similar pattern exists in teleosts is partly investigated in the present study. HRP was injected into the optic nerve of Sebastiscus and Navodon. The calibers of intraretinal HRP-labeled axons were classed as fine (ca. 0.8 micron), medium (ca. 1.3 micron), and coarse (ca. 2.5 microns). The calibers of HRP-labeled retinofugal axons were then determined in their target areas, and these can be summarized as follows: Optic hypothalamus: fine, medium. Lateral geniculate nucleus: fine. Dorsolateral thalamic nucleus: fine, medium. Area pretectalis: fine. Nucleus of the posterior commissure: fine, medium. Area ventralis lateralis, contralateral: fine, medium, coarse; ipsilateral: coarse. Optic tectum, stratum opticum: fine, medium; stratum fibrosum et griseum superficiale: fine, medium, coarse, segregated in sublayers; stratum album centrale: fine, medium, coarse. Therefore, fine fibers were found to reach all target areas except the ipsilateral area ventralis lateralis, and these were the only fibers found in the lateral geniculate nucleus, area pretectalis, and stratum griseum centrale of the optic tectum. Coarse fibers, on the other hand, were found only in the area ventralis lateralis and the optic tectum (stratum fibrosum et griseum superficiale and stratum album centrale). Terminal patterns of these fibers were also studied. Most fine fibers take tortuous courses giving off a few branches and terminate with many varicosities, and medium and coarse fibers give off several finer branches and terminate with bulbous swellings. The physiological significance of these findings is discussed. In addition, retrogradely labeled (retinopetal) cells were found in the olfactory bulb and the area ventralis pars ventralis of the telencephalon, as well as in the preoptic area and the dorsolateral thalamic nucleus.     

Extratelencephalic projections of the avian visual Wulst. A quantitative autoradiographic study in the pigeon Columbia livia

The efferent projections of the pigeon visual Wulst upon the diencephalon and mesencephalon were investigated using the autoradiographic technique following the combined injection of [3H] proline and [3H] leucine into the rostral hyperstriatum accessorium. Repeated measures of silver grain densities were performed bilaterally in different brain structures using a computer-assisted system of image analysis. The density values were compared (Mann-Whitney U-Test) with those recorded in three homolateral control structures (tractus opticus, n. rotundus, n. pretectalis principalis) and in corresponding contralateral areas and nuclei. The data showed ipsilateral projections from the visual Wulst and via the tractus septomesencephalicus upon the dorsal thalamus (n.: dorsolateralis anterior superficialis parvocellularis), ventral thalamus (n.: intercalatus, ventrolateralis, geniculatus lateralis pars ventralis--GLv), pretectum (n.: superficialis synencephali, geniculatus pretectalis, griseus tectalis, pretectalis: diffusus, pars lateralis and pars medialis, area pretectalis) as well as to the nucleus of the basal optic root, n. spiriformis medialis and optic tectum (layer 2-4, 6, 7, 12 and 13). Crossed projections were observed to pass through the supraoptic decussation and the posterior commissure, however only the contralateral n. GLv was found to be significantly labeled. Interspecies variations in the organization of descending visual Wulst projections, related to the terminal distribution and relative size of the crossed components may be linked to differences in the degree of overlap of the binocular fields. Correspondingly, this may reflect the degree of bilateralization upon the Wulst of direct input from the visual thalamus.     

A reconsideration of the primary visual system of the turtle Emys orbicularis.

The retinocerebral projections of Emys orbicularis were investigated by means of [3H]-proline or HRP, administered by intraocular injection. Two newly-hatched, two juvenile and seven adult specimens were examined. The results reveal contralateral retinal projections to fifteen sites: two in the hypothalamus (the nuclei suprachiasmaticus and periventricularis), five in the thalamus (the nuclei ovalis, geniculatus lateralis ventralis, geniculatus laleralis dorsalis, dorsolateralis anterior and ventrolateralis), five in the pretectal region (the nuclei geniculatus pretectalis, opticus pretectalis ventrolateralis, lentiformis mesencephali, posterodorsalis and griseus tectalis), two in the optic tectum (the stratum opticum and the stratum fibrosum et griseum superficiale), and one in the tegmentum (the nucleus opticus tegmenti). Ipsilateral projections to nine of these sites at thalamic, pretectal, tectal and tegmental levels, while weak, could be clearly demonstrated. These results differ considerably from those obtained in a previous investigation using a Nauta-paraffin technique; it is suggested that the differences are due to limitations of the latter technique. A review of the existing literature on the Chelonian primary visual system reveals considerable terminological diversity, and a standard nomenclature for the primary visual centres of turtles is proposed.     

Ipsilateral retinal projections in Japanese quail, Coturnix coturnix japonica

Ipsilateral retinal projections were investigated in Japanese quails by means of the Fink-Heimer method after retinal extirpation, and by means of direct injection of horseradish peroxidase or cobalt iontophoresis into the optic nerve. Ipsilateral projections were found in the nucleus lateralis anterior thalami, nucleus dorsolateralis anterior thalami pars lateralis, nucleus geniculatus lateralis pars ventralis, nucleus lentiformis mesencephali pars magnocellularis and nucleus ectomamillaris. No ipsilateral retino-tectal projections were observed.     

An experimental study of the retinal projections of the European eel (Anguilla anguilla), carried out at the catadromic migratory silver stage

A radioautographic study of the European eel (Anguilla anguilla) was carried out in ten female specimens at the catadromic migratory silver stage. Terminal arborizations of contralaterally projecting visual fibres were identified in ten hypothalamic structures (area optica preoptica ventralis and the nuclei suprachiasmaticus, opticus hypothalamicus ventromedialis, preopticus magnocellularis lateralis, posterioris lateralis, posterioris dorsalis periventricularis posterioris dorsalis lateralis, posterioris dorsalis medialis, posterioris ventralis lateralis, and posterioris ventralis periventricularis), ten thalamo-pretectal structures (Areas C1 and C2, area optica tractus opticus ventrolateralis and the nuclei dorsolateralis thalami, ventrolateralis thalami pars ventralis, opticus ventralis thalami, geniculatus lateralis, opticus pretectalis partes dorsalis et ventralis, and opticus commissurae posterioris), and in the tectal strata opticum partes externa et interna, fibrosum et griseum superficiale, griseum centrale and album centrale. An accessory optic system was identified, and a contralateral retinal projection to the anterior region of the anterior semicircular torus (n. opticus dorsolateralis mesencephali) was identified. Ipsilateral projections to hypothalamic and thalamopretectal structures were also observed. Apart from the retinal projection to the preoptic area, which is exceptionally important in the silver eel, the general plan of organization of the primary visual centres of this form is comparable to that described in other species of teleost. However, the architecture of some primary visual centres shows characteristics similar to those described in more primitive Actinopterygians.     

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus).

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.     

Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

Retinal projections in the freshwater butterfly fish, Pantodon buchholzi (Osteoglossoidei). II. Differential projections of the dorsal and ventral hemiretinas.

Pantodon buchholzi, the freshwater butterfly fish, is a member of the Osteoglossomorpha, the most primitive of the four major teleost radiations. The projections of fibers originating in the dorsal and ventral hemiretinas in Pantodon, as determined with autoradiography, are reported here. Fibers originating in the ventral hemiretina reach their targets through the axial, medial and dorsal optic tracts. Fibers that originate in the dorsal hemiretina reach their points of termination by way of the axial, medial and ventral optic tracts. Projections of the various tracts to preoptic, thalamic, tubercular, pretectal and tectal regions, as described in the previous study of total retinal projections, were verified. The retinal projections to the preoptic, thalamic and tubercular nuclei do not map topographically. Ventral hemiretinal fibers are mapped, however, onto the dorsal part of the nucleus pretectalis superficialis pars parvocellularis, the rostral part of the dorsal accessory optic nucleus, the entire nucleus pretectalis periventricularis pars ventralis and the dorsomedial portion of the optic tectum. Ventral hemiretinal fibers also supply most if not all the retinal innervation to the central pretectal nucleus. In contrast, dorsal hemiretinal fibers are mapped onto the ventral part of nucleus pretectalis superficialis pars parvocellularis, the entire dorsal accessory optic nucleus and the ventrolateral portion of the optic tectum. The dorsal and ventral hemiretinal projections to the tectum about at a cytoarchitectonically recognizable point, indicating that no discontinuity is present in the retinal connectivity with the tectum. The pars parvocellularis of nucleus pretectalis superficialis is a simple, unfolded, and nonlaminar structure in Pantodon. This structure contrasts markedly with the more complex, folded structure of the nucleus in the majority of other examined teleosts. The orientation of the projections from the dorsal and ventral hemiretinas onto this nucleus in Pantodon is congruent with that seen in other fishes only after a schematic unfolding of the nucleus in these fishes.     

Autoradiographic localization of nicotinic acetylcholine receptors in the brain of the zebra finch (Poephila guttata)

We have localized nicotinic acetylcholine receptors in the zebra finch brain by using three 125I-labelled ligands: alpha bungarotoxin and two monoclonal antibodies to neuronal nicotinic receptors (MAb 35 of Tzardos et al., J. Biol. Chem., 250: 8635-8645, '81; and MAb 270 of Whiting and Lindstrom: J. Neurosci. 6: 3061-3069, '86). Unfixed brains from intact adult male and female zebra finches were prepared for in vitro autoradiography. Low-resolution film autoradiograms and high-resolution emulsion autoradiograms were prepared for each of the three ligands. The major brain structures that bind all three of the ligands are hippocampus; hyperstriatum dorsalis; hyperstriatum ventralis; nucleus lentiformis mesencephali; nucleus pretectalis, some layers of the optic tectum; nucleus mesencephalicus lateralis; pars dorsalis; locus ceruleus; and all cranial motor nuclei except nucleus nervi hypoglossi. The major structures labelled only by [125I]-alpha bungarotoxin binding included hyperstriatum accessorium and the nuclei: preopticus medialis, medialis hypothalami posterioris, semilunaris, olivarius inferior, and the periventricular organ. Of the song control nuclei, nucleus magnocellularis of the anterior neostriatum; hyperstriatum ventralis, pars caudalis; nucleus intercollicularis; and nucleus hypoglossus were labelled. The binding patterns of the two antibodies were similar to one another but not identical. Both labelled nucleus spiriformis lateralis and nucleus geniculatus lateralis, pars ventralis especially heavily and also labelled the nucleus habenula medialis; nucleus subpretectalis; nucleus isthmi, pars magnocellularis; nucleus reticularis gigantocellularis; nucleus reticularis lateralis; nucleus tractus solitarii; nucleus vestibularis dorsolateralis; nucleus vestibularis lateralis; nucleus descendens nervi trigemini; and the deep cerebellar nuclei. Lobus parolfactorius and nucleus vestibularis medialis were labelled by only MAb 270, whereas only MAb 35 labelled nucleus laminaris and the medial and lateral pontine nuclei. These data extend previous reports of cholinergic participation in the song system (Ryan and Arnold: J. Comp. Neurol. 202: 211-219, '81) to suggest that the zebra finch song system may contain several closely related nicotinic receptors. In several brain nuclei it appeared that certain anatomical portions of a nucleus or a certain class of neurons were specifically labelled. Furthermore, in certain cases, the labelling appeared to be clustered around Nissl-stained cell nuclei, thus suggesting that the receptors are concentrated on or in somata.     

Immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon

Choline acetyltransferase, a specific marker for cholinergic neurons, has been immunohistochemically localized in the mesencephalon and in the caudal diencephalon of the chicken. A complete series of transverse sections through the mesencephalon is presented. In the diencephalon, cholinergic fibers were found in the stria medullaris, the fasciculus retroflexus, and the ventral portion of the supraoptic decussation. The nucleus triangularis and the nucleus geniculatus lateralis, pars ventralis also contained cholinergic fibers. Small cholinergic cell bodies were found in the medial habenula. In the pretectum, cholinergic fibers innervated the nucleus lentiformis mesencephali and the tectal gray. The nucleus spiriformis lateralis also contained cholinergic fibers, while most of the cell bodies in the nucleus spiriformis medialis were cholinergic. In the mesencephalon, labelled fibers were found in the nucleus intercollicularis and in all layers of the optic tectum except the stratum opticum. The highest density of tectal cholinergic fibers was in the stratum griseum et fibrosum superficiale (SGFS), layer f. Radial cells located in SGFS, layer i were also cholinergic. In the isthmic nuclei, cholinergic fibers were found in the pars magnocellularis, while the pars parvicellularis and the nucleus semilunaris contained labelled cells. The oculomotor, Edinger-Westphal, trochlear, and trigeminal motor nuclei all had cholinergic cell bodies. Cholinergic axons were present in the oculomotor and trochlear nerves. In the tegmentum, cell bodies were labelled in the nucleus mesencephalicus profundus, pars ventralis, while the nucleus interpeduncularis had dense cholinergic innervation. Our localization of cholinergic cell bodies and fibers has been compared with earlier autoradiographic and anatomical studies to help define cholinergic systems in the avian brain. For example, the results indicate that the chicken may have a cholinergic habenulointerpeduncular system similar to that reported in the rat. Establishing the cholinergic systems within the avian midbrain is important for designing future neurophysiological and pharmacological studies of cholinergic transmission in this region.     

Fiber connections of the nucleus isthmi in the carp (Cyprinus carpio) and tilapia (Oreochromis niloticus)

The nucleus pretectalis (NP) is a prominent nucleus in the percomorph pretectum and has been shown to project to the nucleus isthmi in the filefish by an HRP tract-tracing method [Ito et al., 1981], but a homologous nucleus to the NP is apparently lacking in ostariophysans. The present study examined fiber connections of the nucleus isthmi in an ostariophysan teleost, the carp (Cyprinidae, Cyprinus carpio), to identify a nucleus homologous to the percomorph nucleus pretectalis. Identical studies in a percomorph tilapia (Cichlidae, Oreochromis niloticus) were also performed. Injections of biotinylated dextran amine (BDA) or biocytin to the carp nucleus isthmi labeled cells in the ipsilateral optic tectum and nucleus ruber of Goldstein [1905]. Labeled tectal neurons were located in the stratum periventriculare (SPV) and the stratum fibrosum et griseum superficiale (SFGS). The somata in the SPV were pyriform and those in the SFGS were fusiform. No labeled cells were found in the pretectum. Labeled terminals were seen in the ipsilateral nucleus pretectalis superficialis pars parvocellularis (PSp), optic tectum, and bilateral nucleus ruber. Terminals in the nucleus ruber appear to come from tectal neurons in the SFGS labeled by isthmic injections. Thus the nucleus isthmi has reciprocal fiber connections with the ipsilateral optic tectum, receives projections from the ipsilateral nucleus ruber, and projects to the ipsilateral PSp. The nucleus pretectalis homologue is apparently absent in the carp. Studies in tilapia showed that the nucleus isthmi receives bilateral projections from the NP and optic tectum. In addition, the present study revealed a previously unknown afferent from the nucleus ruber to the percomorph nucleus isthmi. The tilapia nucleus isthmi projects to the same targets as in the carp. Isthmic projection neurons in the tilapia optic tectum were located in the SPV and pyriform with a similar shape to the carp SPV neurons that project to the nucleus isthmi. No labeled cells were found in the SFGS of tilapia optic tectum. The fusiform neurons in the SFGS of the carp optic tectum possess various hodological similarities with the NP and may correspond to the NP neurons of percomorphs.     

Intensity and pattern discrimination after lesions of the pretectal complex,accessory optic nucleus and ventral geniculate in pigeons

Pigeons were trained to discriminate between pairs of visual stimuli that differed in intensity or pattern. After completion of traianing, bilateral, stereotaxic lesions were made in various cell groups in the mesencephalon and diencephalon that receive terminals of the optic tract. The target regions were nucleus ectomamillaris (accessory optic nucleus), nucleus lentiformis mesencephali and area pretectalis (pretectal complex) and the nucleus geniculatus lateralis, pars ventralis (ventral geniculate). In some cases, combined lesions of nucleus lentiformis mesencephali and area pretectalis were made. Lesions of nucleus ectomamillaris, nucleus lentiformis mesencephali, area pretectalis, or ventral geniculate did not produce major impairments of discrimination performance nor did combined lesions of nucleus lentiformis mesencephali and area pretectalis. A number of cases of intended destruction of the ventral geniculate also had extensive damage to the overlying nucleus rotundus. In several of these cases of combined destruction of nucleus rotundus and ventral geniculate, the previously reported discrimination deficits following nucleus rotundus lesions did not appear. In those cases in which the nucleus rotundus deficit was observed, the lesions were found to include the nucleus subpretectalis, which, like nucleus rotundus, receives tectofugal fibers via the brachium of the superior colliculus. The data of the ventral geniculate + rotundus cases and ventral geniculate + rotundus + subpretectalis cases suggest that sensory deficits following a lesion in a particular cell group may not necessarily indicate that the sensory information is processed in that cell group, but rather that the lesion had deprived other cell groups of the appropriate input necessary for their proper functioning.     

Evolution of reptilian visual systems: retinal projections in a nocturnal lizard, Gekko gecko (Linnaeus)

On the basis of the development of the dorsal ventricular ridge of the telencephalon, lizards can be divided into a type I group, to which Gekko and the majority of lizard families belong, and a type II group with more derived features, of which Iguana is representative. Most studies of retinal projections have utilized lizards of the type II group, which are adapted to a diurnal niche. Gekko gecko is differently adapted in that it is nocturnal. Study of the retinal projections was undertaken in Gekko gecko in order to insure that conclusions regarding the pattern of retinal pathways in saurians would be based on a sample which was more representative of the total range of variation. Unilateral removal of the retina by suction cannula was carried out on 12 adult specimens of Gekko gecko. After survival times of 10 to 74 days, brains were processed with various silver methods. The retina projects contralaterally to the pars dorsalis and pars ventralis of the lateral geniculate nucleus and the pars ventralis of the ventrolateral nucleus in the thalamus, nuclei geniculatus pretectalis, lentiformis mesencephali, and posterodorsalis in the pretectum, layers 8–14 of the optic tectum and nucleus opticus tegmenti. Additionally, the retina projects ipsilaterally to the dorsal and ventral lateral geniculate nuclei and to the pretectal nuclei, as well as to the optic tectum, particularly layers 8 and 9. The finding of ipsilateral retinothalamic projections in Gekko supports the idea that this pathway is generalized among saurians. However, presence of ipsilateral retinothalamic projections and the degree of binocular overlap cannot be correlated when lizards, snakes, crocodiles, and turtles are compared. The functional significance of this pathway therefore remains obscure. Ipsilateral retinotectal projections have not been previously described in land vertebrates other than mammals. Whether their presence is correlated with nocturnal visual habits or is generalized among type I lizards remains to be determined. The pattern of retinal projections has been studied in too few representatives of non-mammalian land vertebrates to presently permit conclusions regarding the origin of non-decussating pathways.     

Observations on the projections and intrinsic organization of the pigeon optic tectum: an autoradiographic study based on anterograde and retrograde, axonal and dendritic flow.

Three aspects of the labelling pattern seen after the injection of 13 different radioactive amino acids into the pigeon optic tectum have been described: The efferent projections of the optic tectum; the specific labelling of two pathways; and the dendritic organisation of tectal layer III neurons based on the retrograde and anterograde movement of label within these dendrites. Discrete injections of tritiated amino acid that involved all or only the superficial tectal layers suggested that layer III gave rise to the massive non-topographically organised and bilateral projections (fibers crossing within the decussato supraoptica ventrlis) upon the nuclei rotundus, subpraetectalis and interstitio-praetecto-subpraetectalis and to the ipsilaterally directed pathways terminating within the nuclei praetectalis, triangularis, subrotundus, dorsolateralis anterior thalami, posteroventralis and ventrolateralis thalami. Layer III neurons may also be the source of efferents to the posterior dorsolateral thalamus (the layer III pathway), the pontine grey and, bilaterally to the reticular formation and of the layer IV or tectal commisural pathway terminating within the contralateral tectal cortex. In contrast projections originating from layer II were generally topographically organised and terminated either within certain of the isthmic nuclei (n. isthmi pars parvocellularis, n. isthmo-opticus and n. semilunaris) or ran within layer I (layer I pathways) to end in the pretectum (griseum tectale) and ventral thalamus (n. ventrolateralis thalami, n. geniculatus, pars ventralis). A small projection from layer II upon the ipsilateral nucleus rotundus may also be present. Triated serine and tyrosine were found to be particularly effective in labeling perikarya as well as axons and terminals. The layer I pathway could be selectively labelled after tectal injections of 3H-GABA while the cell bodies of Ipc neurons were labelled in a retrograde fashion after tectal injections of 3H-glycine, serine or alanine. Intrinsic tectal labelling was found by correlation with Golgi material to reflect both anterograde and retrograde transport of label within dendrites of layer III cells. Anterograde movement of label indicated that the terminal portions of layer III cell dendrites ended in an orderly radial arrangement within sublayers IIb and IId, while the retrograde movement of label resulted in the labelin of layer III perikarya outside the injection field.