Tectal Neuropil Research Articles

Sources of electrical transients in tectal neuropil of the frog, Rana pipiens

We have studied the outer neuropil layers in frog tectum where the ummyelinated optic nerve fibers terminate. At any point the neuropil an extracellular microelectrode records several different visually evoked electrical transients, distinct by size and shape. When classified by shape alone, each transient falls into one of 3 distinct classes. Some of these transients are binocularly driven, as originally described by Finch and Collett. The aggregate of the receptive fields of all the elements recorded at a single point defines a multiunit receptive field (MURF). Each MURF is characteristically oval, and divided into 3 sections along its long axis. Each section represents the aggregate of the receptive fields associated with one class of transient; i.e. transients belonging to only one specific class can be evoked by stimulating that part of the visual field corresponding to the appropriate section of the MURF. All of the MURFs mapped by recording in a single tectum are radially arranged in visual space about a central point, or ‘visual pole’. Several conclusions are made. First, the two larger types of transient are generated postsynaptically by electrically active dendritic elements, specifically the beaded dendritic appendages of tectal neurons. The smallest type of transient is of presynaptic origin. Second, these tectal elements have a local and global anatomical order across the tectum, which accounts for both the tripartite structure of the MURFs and their radial arrangement about a visual pole. Third, since the large transients are of postsynaptic origin, genuine recordings of single retinal ganglion cell (RGC) activity can be made only in the optic nerve or retina itself. Fourth, information is conveyed over the unmyelinated optic nerve fibers at pulse rates as high as 80/s and is transsynaptically effective at such rates. Finally, the electrically active tectal dendritic elements, with their highly organized spatial arrangement, are an important component of the frog's visual processing apparatus, instead of being merely relays or repeaters.

Regeneration of retinal axons in grafts of peripheral and central nervous tissue in the adult rat

Peripheral nerves were autologously grafted to the retinae of adult rats. The distal end of the nerves was inserted into fetal tectal tissue placed within cortical lesion cavities. Two to 8 months later, anterograde pathway tracing using horseradish peroxidase demonstrated retinal axons regenerating along the length of the PNS nerve graft in 87% of animals with viable PNS and fetal tectal grafts. Of these animals, retinal axons grew into the tectal neuropil in 23% of cases but specific innervation of appropriate target regions was not detected.

Developing retinotectal projection in larval goldfish.

The retinotectal projection in larval goldfish was studied with the aid of anterograde filling of optic fibers with HRP applied to the retina. The results show that optic fibers have already reached the tectum and begun to form terminal arbors in newly hatched fish. The projection is topographic in that fibers from local regions of the retina project to discrete patches of tectum, with the smallest patch covering 3.5% of the total surface area of tectal neuropil. Many fibers in young larvae have numerous short side branches along their length and only some of them show evidence of terminal sprouting. The arbors are approximately elliptical in shape and average about 1,500 microns 2. Growth cones are seen frequently. In older larvae, terminal arbors are larger and more highly branched, and they have begun to resemble those in adult fish. Fibers terminate in two strata; those in the upper layer are smaller (1,800 microns 2 on average) than those in the deeper stratum (4,000 microns 2 on average). The fraction of tectal surface area covered by individual arbors (the "tectal coverage") ranges from 1.5% to 3% of the total surface area of the tectal neuropil. In contrast, the tectal coverage of individual arbors in young adult goldfish is much smaller, ranging from 0.02% to 0.42% of tectal surface area (Stuermer, '84, and unpublished). This apparent increase in precision of the map in older animals is not due to retraction of arbors, which are slightly larger in adults, but is accounted for by overall tectal growth: the tectal neuropil in goldfish increases in area by about 250-fold during this period (Raymond, '86).

Retinotopic organization of the developing retinotectal projection in the zebrafish embryo

Developing retinal axons in the zebrafish embryo were stained with HRP or with the fluorescent dyes dil and diO to study the formation of the retinotectal projection. Retinal axons leave the eye at 34-36 hr postfertilization (PF), invade the tectum at 46-48 hr PF, and innervate the tectal neuropil at 70-72 hr PF. Dorsal and ventral axons occupy separate aspects of the optic nerve and tract and pass into their retinotopically appropriate ventral and dorsal hemitectum, respectively. Nasal and temporal axons are segregated in the nerve, mixed in the tract, and are coextensive over the rostral half of tectum until 56 hr PF. They then segregate again, due to the progression of nasal axons into the open caudal tectum. Thus, at 70-72 hr PF, dorsal and ventral as well as temporal and nasal axons occupy their retinotopically appropriate tectal quadrants. After ablation of the temporal retina prior to the time of axonal outgrowth, the nasal axons bypass the vacant rostral tectum to terminate in the caudal tectal half. Temporal axons in the absence of nasal axons remain restricted to their appropriate rostral tectal half, suggesting that nasal and temporal axons possess a preference for their retinotopically appropriate tectal domains. Measurements of individual terminal arbors and the tectal areas in embryos and in adult zebrafish showed that individual arbors are large with respect to the embryonic tectum but are about 14-15 times smaller than in the adult. However, the proportion of tectum covered by embryonic arbors is about 7 times larger than in the adult, suggesting that a higher precision of the adult projection is achieved as a result of a greater enlargement of the tectum than of the arbors.

Regenerated optic fibers in goldfish reestablish a crude sectoral order in the visual pathway

The goldfish optic pathway is regenerated after an optic nerve crush. We have examined the axonal topography of the regenerated pathway by labeling, with horseradish peroxidase (HRP), axons originating from retinal sectors or annuli. The positions of the labeled axons in the cross section of the pathway were compared to the normal and related to the factors that may influence axonal pathfinding. The positions of retinal axons in the cross section of the normal pathway are predictable from the retinal addresses of the ganglion cells described by the polar coordinates r (the distance from the optic disc) and theta (the sectoral or clockface position). The two coordinates map orthogonally onto the cross section of the pathway; r varies monotonically along one axis; theta varies along a perpendicular axis. The normal r-order, present in the nonregenerated stump of the experimental nerve, was severely degraded and perhaps lost entirely in the regenerated optic nerve, tract, and brachia. Sectoral order was also lost as the axons passed the crush site, but it was reestablished, albeit crudely, in the regenerated tract and brachia where axons tended to occupy positions appropriate to their dorsal, ventral, nasal, and temporal retinal origins. The exit sequence of the regenerated axons from the stratum opticum into the tectal neuropil was normal: temporal first, nasal last. These results suggest that the regenerating fibers followed some theta-specific cue located in the nonaxonal environment. It seems likely that the original axons probably followed the same cue. In contrast, the absence of r-order suggests that there is no r-specific cue for the regenerates to follow. It seems likely that the original r-order was a consequence of nonspecific influences--the orderly spatiotemporal growth of the retina and the existence of a permissive region for axonal growth.

Anomalous retrograde labeling of brain cells from the eye in the goldfish: Evidence for long distance growth of sprouted neurites

We have used retrograde labeling with horseradish peroxidase (HRP) and a wheat germ agglutinin conjugate of HRP (WGA:HRP) to investigate the projections of the nucleus postglomerulosus (nPg) both in normal goldfish and in animals which had undergone retinal removal. In normal animals, our evidence indicates that nPg projects only to the optic tectum. Using small HRP and WGA:HRP application sites in the tectum, we have shown that nPg cells have broadly spread terminals in the tectal neuropil and that there is no obvious correspondence between the rostrocaudal axis of the nPg and the deployment of the terminal arbors of its cells along the rostrocaudal axis of the tectum. In addition, we found no evidence for an nPg projection to the eye in normal animals. After retinal removal we found that nPg cells were more readily backfilled from small tectal applications of HRP. However, our most interesting observation was that at 4–6 weeks and more after ocular surgery, we could retrogradely label the cells of the nPg with intraocular or retroocular injections of WGA: HRP. At the same postoperative times, we were also able to label neurites in the atrophied optic nerve by microinjecting WGA:HRP into the contralateral midbrain tegmentum. Finally, we found that the cells of the nPg undergo a hypertrophic response, similar to that seen in other neurons after axotomy, following retinal removal or section of the dorsomedial brachium of the optic tract. Thus, these cells respond to retinal denervation of the tectum with a response characteristic of axotomized cells although their axons have not been cut. Similar changes were also seen in the nucleus isthmi on both sides of the brain following retinal removal. We interpret our data to indicate that cells of the nPg can respond to optic (and thus heterotypic) denervation of their terminal field by sprouting processes which grow away from the terminal field, through denervated optic pathways, to the retinaless eye. This interpretation requires that the sprouted processes grow for several millimeters.

Contact cell polarization and tensile fields: their role in morphogenesis and cytodifferentiation : L.V. Boloussov, B.N. Belintzev, A.V. Lakirev, A.G. Zaraiskii. Dept of Embryology, Moscow State University

The plasticity of somatosensory afferents in the adult axolotl tectum was studied in two ways. First, normal adult axolotls were monocularly enucleated, and second, adult animals which had been monocular since embryogenesis had the tectum contralateral to the remaining eye excised. After a survival time of about one year the brains of these animals were studied electrophysiologically and histochemically. In the enucleated adults, the deprived tectum lacked visual activity, and the acetylcholinesterase staining, dense in the superficial neuropil of the normally innervated tectum, was much reduced. Somatosensory units remained deep in the tectal neuropil, as in normally innervated tecta. Serotonergic terminals associated with somatosensory responses were correspondingly largely restricted to the deeper tectal neuropil in these animals. This result is in contrast to that of embryonic enucleation where somatosensory activity and serotonergic terminals are found in the superficial tectal neuropil of the adult. Autoradiographic results from the tectal excision experiments showed that retinal axons in adults regrew to innervate the ipsilateral tectum when deprived of their normal targets. Associated with the ipsilateral innervation both visual units and acetylcholinesterase staining were found in the superficial tectum. In spite of this change in the visual innervation of the tectum, there was little difference in the distribution of somatosensory responses or serotonergic terminals; both remained relatively superficial and appeared to be unaffected by the late ingrowing retinal axons. This results is in contrast to the findings from embryonic transplants to genetically eyeless embryos which, as adults, show somatosensory responses and serotonergic endings restricted to the deeper tectal neuropil

Pre- and postsynaptic correlates of interocular competition and segregation in the frog.

Segregated zones of termination between converging inputs that arise from different presynaptic populations are a common property of topographically organized zones within the vertebrate central nervous system. Increasing evidence suggests that such segregation is at least in part established on the basis of competitive interactions that depend upon the activity patterns within each afferent population. However, the cellular mechanisms of these interactions are poorly understood. We have used a preparation in which a stereotyped interdigitating pattern of retina-specific termination stripes are produced in frog tecta innervated by two retinas as a result of embryonic implantation of a third eye primordia. In these animals it has been possible to examine the relationship between the number of retinal ganglion cells in each of the retinas innervating a striped tectum, the volumetric changes in the tectum as a result of this double innervation, and the pattern of eye-specific segregation that is produced. Counts of retinal ganglion cells in the retinas of the three-eyed frogs with one completely striped tectal lobe revealed no significant differences between cell numbers in the doubly innervating retinas and the normal retinas of the same animals. The average increase in retinal ganglion cell innervation to the striped tecta of these animals was 100%. However the tecta only increased in total volume by 26%. This later increase consisted of a 25% increase in the volume of the deep lying and predominantly cellular tectal laminae and a 37% increase in the superficial retinotectal synaptic zone. In many of these same animals HRP and 3H-proline were used to differentially label the set of stripes from each retina and measurements of the extent of each projection were performed. We found that the volume of tectal neuropil occupied by a striped projection is relatively unrelated to the number of ganglion cells making up that projection. Observations of the striping pattern after HRP processing to visualize stripes in whole unsectioned tecta indicate that the periodicities and rostrocaudal orientation of stripes are robust over a wide range of relative innervation densities. When one projection is much smaller than the other, stripes appear to break down into a series of "puffs" or islands of retina-specific termination zones. Nevertheless, these puffs still have a rostrocaudal alignment and the spacing of fully formed stripes. These observations suggest that the formation of exclusive termination zones may be a threshold phenomenon: so after a certain innervation density is reached one input can take over a unit of target neuropil in an all-or-none manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Substance P-containing ganglion cells become progressively less detectable during retinotectal development in the frog Rana pipiens.

Substance P-like immunoreactivity (SPLI) was immunohistochemically analyzed in the retinae and optic tecta of Rana pipiens embryos and tadpoles between stages 25 of Shumway (S25) and XXV of Taylor and Kollros (TKXXV). A population of retinal ganglion cell (RGC) somata display SPLI. The number of labeled cell bodies increases in proportion and staining intensity between S25 and TKX and progressively decreases toward the end of metamorphosis. At TKXXV, only occasional cells in the periphery of the retina displaying SPLI can be observed in the RGC layer, heralding the adult condition, in which SPLI can only be seen rarely in occasional RGCs. An increasing proportion of optic nerve axons display SPLI from S25 through TKXVI, decreasing progressively thereafter toward the end of the larval period. Concurrently, SPLI appears for the first time in the superficial tectal neuropil between TKIII and TKV, with progressively increasing staining intensity and in a discrete lamina previously shown to contain retinofugal terminals in the adult. These observations corroborate inferences from previous studies indicating the existence of populations of peptidergic RGCs that terminate within precisely restricted synaptic loci in the tectum and presumably perform different functional operations in the adult. Previous observations, however, necessitated various experimental manipulations involving injuries to the visual system in order to demonstrate neuroactive peptide-like immunoreactivity in RGCs, thus allowing the possibility of posttraumatic expression of anomalous peptide phenotypes that may not reflect normal features of RGCs. The present study eliminates this variable and provides further evidence of the existence of peptidergic RGCs.

Regrowth of retinal axons after lesions of the brachium and pretectal region in the rat

Suction lesions of the left rostral superior colliculus (SC) and pretectal area were made in young rats. One to 50 days later, the right eye of each animal was injected with horseradish peroxidase (HRP). Animals were subsequently perfused and the brains processed with tetramethylbenzidine. In many older animals, lesioned areas were covered with thin membranes composed of glial and connective tissue cells. HRP-labelled optic axons were traced running through these membranes for up to 3 mm. On occasion, regrowing retinal fibres reached the rostral border of the remaining SC but did not grow into the tectal neuropil.

Map formation in the developing Xenopus retinotectal system: an examination of ganglion cell terminal arborizations

Single axonal arbors of retinal ganglion cells have been stained by injecting cobalt extracellularly into the retinae of Xenopus embryos and tadpoles. The axonal endings of the earliest retinal axons to arrive in the midbrain were usually simple in appearance, often ended in growth cones, and terminated in tectal regions appropriate to their location in the eye. Thus, a topographic projection exists very early in the development (stages 37 to 39) of the projection, before the elaboration of complex axonal arbors. Retinal axons began acquiring more mature features, exemplified by the elaboration of terminal arbors, by stage 39. The arbors of most ganglion cells were elongated in the rostral-to-caudal dimension during early larval life (stages 40 to 45) and covered a large portion of tectal neuropil. During mid-larval stages (stages 46 to 50), arbors covered a relatively smaller proportion of the tectal neuropil. A quantitative analysis of this change suggests that the apparent decrease in size of the arbors, with respect to the tectum, is due to rapid growth of tectal neuropil and not due to retraction of an initially diffuse arbor. Thus, the refinement in targeting of axonal arbors during development is a phenomenon distinct from that seen during regeneration.

Growth cone-target interactions in the frog retinotectal pathway.

The growth cones of retinal ganglion cell axons were studied in the optic tract and tectum with horseradish peroxidase (HRP) histochemistry and electron microscopy. The ganglion cell growth cones has many morphological features similar to those described in vitro and in other in vivo systems. However, we found that some processes formed highly differentiated terminal arborizations, while retaining growth cones on many of their branches. In addition, ultrastructural examination of the tectal neuropil revealed that many ganglion cell axonal processes had characteristics of both growth cones and presynaptic endings. These findings are discussed in the context of the hypothesis of shifting connections and the evidence that retinotectal map formation involves several mechanisms, including a process that depends on the action potential activity in the optic fibers.

The relation between soma position and fibre trajectory of neurons in the mesencephalic trigeminal nucleus of Xenopus laevis.

In adult Xenopus laevis the mandibular and ophthalmic branches of the trigeminal nerve were backfilled with CoCl2 or cobaltous lysine and whole brains silver intensified to reveal neurons of the mesencephalic Vth nucleus (mes. V). The nucleus contains about 100 cells arranged in a band extending arch-like from the ventrolateral margin of the optic tectum to the midline. Many cells possess a small number of short dendritic processes that arborize in the tectal neuropil; in some cells one dendrite terminates within the ependyma or ventricle. A single axon arises from each cell and courses in layer 7 to the margin of the tectum. Axon collaterals arise close to the cell body to terminate principally within layer 6, but occasionally also in layers 8 and 9. Collaterals occurring more caudally terminate in layer 6. These findings suggest that mes. V cells acts as tectal interneurons as well as conveying somatosensory information to the tectum from the mouth region. In the dorsal roof of the tectum the trajectory of a fibre is related to the distance of the soma from the midline. Mes. V cells located at the lateral end of the nucleus possess axons that course initially in a mediolateral direction before turning along the ventrolateral margin of the tectum. Cells positioned close to the midline have axons that project rostrocaudally the entire length of the tectum. The axons of cells located at intermediate positions within the nucleus course at correspondingly oblique angles through the dorsal roof of the tectum. Thus in this area there is a more or less 90 degrees rang in the orientation of mes. V fibres to the longitudinal axis. It is proposed that this topographical relationship between soma position and axon trajectory arises through a developmental mechanism, in which mes. V fibres grow during larval life sequentially into the medial zone of tectal growth and become subsequently displaced rostrolaterally, owing to the further addition of tectal tissue medially, through an angle dependent upon the parent cell's date of birth.

Qualitative and quantitative measures of plasticity during the normal development of the Rana pipiens retinotectal projection

We have examined the following aspects of retinal development in the frog, Rana pipiens: (1) the overall pattern of cell addition to the retina; (2) the relative rate of retinal ganglion cell (RGC) accretion; and (3) the changes in RGC density during larval development. In addition, we have studied the development of the retinal projection onto the tectum by means of the anterograde transport of horseradish peroxidase (HRP) and measurements of the volume of tectal neuropil at several larval stages and in postmetamorphic frogs. We find that the addition of new cells to the retina of Rana pipiens larva is restricted to the ciliary margin and that this addition is concentric at all larval stages. Additionally, the morphometric measures of retinal and tectal growth, along with the HRP histochemistry, indicate that the retinal projection exhibits considerable plasticity during normal development. The plasticity we observe in normal development may explain why the retinotectal projection can compensate its area and volume in experimental paradigms that effect drastic changes in innervation density.

The relationship between retinal axon ingrowth, terminal morphology, and terminal patterning in the optic tectum of the frog.

AbstractThe mode of entry of retinal ganglion cell (RGC) axons and the detailed morphology of their arbors has been studied in the tectal lobes of the frog Ranapipiens. These tecta were subjected intact to HRP histochemistry and subsequently flal‐mounted for examination under a compound microscope. Subsets of RGC axons were labelled by inserting small pellets of solidified HRP into the optic chiasm region or into the tectal neuropil itself. Annular or semiannular patterns of retinal ganglion cell arbors were always observed after placement of HRP in the chiasm region. HRP pellets in the tectal neuropil labelled segments of annuli in caudal tectal regions. These tectal patterns corresponded to a similar annular distribution of HRP‐filled RGC bodies in the topographically appropriate regions of the contralateral retina.The morphologies and trajectories of RGC axons were found to vary systematically depending on the age of the animal and the region of the tectal lobe examined. In rostral tectum the tangential spread of large‐caliber RGC axon arbors appears to decrease with increasing animal age and in large adult frogs there is a general tendency for arbors in caudal tectum to be larger than arbors in rostral tectum. In addition, axons contributing to terminal annuli in the central region of young and old frog tecta travel in a strict rostrocaudal direction over the anterior regions of the lobe. A predominantly rostrocaudal trajectory is also established by axons terminating in lateral and medial regions of the tecta of young postmetamorphic frogs. In the lateral and medial regions of mature optic tecta, however, axons arborizing in peripheral terminal annuli turn rather abruptly off the medial and lateral optic tracts and establish radially oriented trajectories. The annular patterning of terminal arbors and the age‐ and position‐dependent differences in arbor morphology are discussed in the context of a developmental sequence where arbors from newly arrived RGC axons distribute themselves along the tectal borders.The question of whether such a high degree of fiber and terminal ordering actually dictates synaptic patterning in the tectal lobe has also been addressed in this study by using the stereotyped striped pattern found in the dually innervated tectal lobes of three‐eyed frogs (Constantine‐Paton and Law, 78;Law and Constantine‐Paton, '81). The presentresults indicate that eye‐specific segregation in the terminal zone is independent of bundling of axons in the optic tract. Stripes are also shown to be independent of the path taken by axons en route to the tectum and of the way in which axons enter the tectal plate. These observations and our earlier conclusions (Reh et al., '83) concerning order in the Rana optic nerve are discussed in terms of recent models of retino tectal development which postulate a role for passive “extra‐tectal” factors in the formation of this visual map.

Differences between embryos and adults in the plasticity of somatosensory afferents to the axolotl tectum

The plasticity of somatosensory afferents in the adult axolotl tectum was studied in two ways. First, normal adult axolotls were monocularly enucleated, and second, adult animals which had been monocular since embryogenesis had the tectum contralateral to the remaining eye excised. After a survival time of about one year the brains of these animals were studied electrophysiologically and histochemically. In the enucleated adults, the deprived tectum lacked visual activity, and the acetylcholinesterase staining, dense in the superficial neuropil of the normally innervated tectum, was much reduced. Somatosensory units remained deep in the tectal neuropil, as in normally innervated tecta. Serotonergic terminals associated with somatosensory responses were correspondingly largely restricted to the deeper tectal neuropil in these animals. This result is in contrast to that of embryonic enucleation where somatosensory activity and serotonergic terminals are found in the superficial tectal neuropil of the adult. Autoradiographic results from the tectal excision experiments showed that retinal axons in adults regrew to innervate the ipsilateral tectum when deprived of their normal targets. Associated with the ipsilateral innervation both visual units and acetylcholinesterase staining were found in the superficial tectum. In spite of this change in the visual innervation of the tectum, there was little difference in the distribution of somatosensory responses or serotonergic terminals; both remained relatively superficial and appeared to be unaffected by the late ingrowing retinal axons. This results is in contrast to the findings from embryonic transplants to genetically eyeless embryos which, as adults, show somatosensory responses and serotonergic endings restricted to the deeper tectal neuropil

Laminar organization of peptide-like immunoreactivity in the anuran optic tectum.

Peptide, 5-hydroxytryptamine (5-HT)-, tyrosine hydroxylase (TOH)-, and glial fibrillary acidic protein (GFAP)-like immunoreactivity was studied in the optic tectum of Rana pipiens. Peroxidase-antiperoxidase and indirect immunofluorescence single- and double-labeling methods were used to compare differential laminar distribution of each of these substances. Substance P (SP), leucine-enkephalin (LENK), cholecystokinin octapeptide (CCK8), bombesin (BOM), avian pancreatic polypeptide (APP), and possibly neurotensin display unique individual patterns of laminar distribution of processes and cell bodies throughout the tectum. A correlative analysis of the topographical distribution of SP, LENK, BOM, and APP on the basis of double-labeled sections shows a precise laminar segregation of these substances. Vasoactive intestinal peptide-, beta-endorphin-, and ranatensinlike immunoreactivity is consistently absent from our material. 5HT- and TOH-like immunoreactivity discloses a reticular array of fibers without clear evidence of laminar organization. This peptide-like laminar organization is particularly elaborate throughout the superficial neuropil of the optic tectum, the major retinorecipient zone. The pattern of lamination demonstrated in the present study differs in several important features from that previously described on the basis of several histological methods. The cells of origin of processes (axons and/or dendrites) in the superficial tectal neuropil may be either intrinsic or extrinsic to the tectum. Special reference is made to conflicting evidence regarding the possibility of a retinal contribution to peptide-like tectal lamination.

A banded distribution of retinal afferents within layer 9A of the normal frog optic tectum

A banded distribution of retinal ganglion cell axons within layer 9A of the superficial tectal neuropil in Rana pipiens was revealed through anterograde labeling with horseradish peroxidase. Layer 9A previously has been demonstrated to mediate binocular vision through a polysynaptic pathway by way of the nucleus isthmi 5,8,9. This nucleus interconnects analogous regions of the two tectal lobes such that isthmic axons retinotopically map the visual world of the ipsilateral eye within tectal layers 9A and 8 9,10. Thus, we have found that a pattern of retinal ganglion cell bands occurs in binocular regions of normal frogs. This pattern is similar, but not identical, to the experimentally produced stripes previously observed in the doubly innervated tecta of 3-eyed and single tecta frogs 2,12–14. Qualitative and quantitative comparisons of these two types of afferent segregation patterns have implicated several structural and functional parameters which might be involved in band formation.

The serotonergic somatosensory projection to the tectum of normal and eyeless salamanders.

The spinotectal somatosensory projection was compared in normal, genetically eyeless, and embryonically manipulated salamanders. In normal animals, serotonin fluorescence was restricted to the intermediate tectalneuropil. This same region showed both high levels of serotonin uptake and somatosensory single unit electrical activity. In mutant eyeless salamanders and in normal animals enucleated early in development, serotonin fluorescence, serotonin uptake, and somatosensory activity were present in the superficial tectal neuropil. One-eyed animals, either genetically normal axolotls with one eye enucleated embryonically or genetically eyeless animals in which a normal eye had been transplanted, showed normal intermediate serotonin fluroescence and somatosensory physiology in the visually innervated half-tectum. In the visually uninnervated half-tectum, they showed superficial serotonin fluorescence and somatosensory physiology. In normal animals, 5,7-dihydroxytryptamine (5,7-DHT), a specific poison for serotonergic fibers, eliminated physiological responses in the contralateral somatosensory tectal region. The 5,7-DHT poisoning also abolished U.V.-induced serotonin fluorescence in the intermediate tectal neuropil. These results are discussed in terms of (1) evidence for serotonin as a central neurotransmitter for somatosensory information in the tectum, (2) the effects of eyelessness on tectal organization, and (3) related results in other animals.

Anatomy and physiology of experimentally produced striped tecta

Transplantation of a third eye primordium to the forebrain region of a frog (Rana pipiens) embryo causes two retinal projections to converge on a single tectal lobe. These projections form stereotyped eye-specific termination bands (Constantine-Paton, M., and M. I. Law (1978) Science 202: 639-641) that are similar to the source-specific stripes found normally in many regions of the mammalian brain. In the present study, we use quantitative analyses of anatomical data and double labeling techniques to demonstrate that induced bands of three-eyed frogs are approximately 200 micrometers wide, that they invariably run in a rostrolateral to caudomedial direction, and that they represent interdigitating synaptic zones which fill the entire superficial neuropil of the dually innervated tectal lobe. This periodic segregation pattern is not seen in the superinnervated diencephalon or in the optic tracts. Morphometric measurements on animals with dually innervated tectal lobes revealed an approximate 30% hyperplasia that was restricted largely to the deeper predominantly cellular tectal layers. Each of the banded retinal projections occupied neuropil volumes that were roughly 50% smaller than the volume occupied by the noncompeting retina of the same animal. Eye-specific segregation was detected with extracellular recording techniques as an ability of one of the two eyes to elicit consistently more and larger action potentials throughout a radial penetration of the superficial tectal neuropil. In several preparations, electrolytic lesions confirmed that physiologically defined positions of eye dominance corresponded to a band from the same eye. Maps of the visual field projections within dually innervated tecta were relatively normal and their orientation was consistent with the initial embryonic orientation of the retinas. Eye laterality, time of arrival, or fasciculation during growth are not responsible for this induced banding. Instead, our results are attributed to two mechanisms that are probably fundamental to neural mapping. These are: differential affinities between retinal and tecta loci which normally align the projection by bringing together appropriate pre- and postsynaptic areas and interactions among retinal ganglion cell fibers. The latter causes axons from physically neighboring retinal ganglion cell bodies to terminate together within the tectum and consequently increases the internal order of map. We propose that these same two mapping mechanisms may be responsible for the afferent segregation found in diverse regions of many vertebrate brains.