Tectal graft modulation of audiogenic seizures in Long-Evans rat.

Audiogenic seizure (AGS) activity can be induced in the seizure-resistant Long-Evans rat by postnatal priming. This study examined the effects of unilateral lesions of the inferior colliculus (IC) and implantation of tectal grafts on AGS components. Animals were primed with a 10-kHz tone burst at 120 dB on postnatal day 14 and tested for AGS susceptibility on day 28, and then two groups were unilaterally lesioned including animals receiving embryonic day 16-17 grafts of caudal tectum. Subsequently, animals were repeatedly tested for wild running and clonic-tonic convulsion components of AGS. The results demonstrate that unilaterally grafted animals with partial IC lesions showed significant reduction in the incidence of clonus expression with greater terminal uniphasic wild running behavior. These effects were stronger than in animals with comparable unilateral lesions alone. Many neurons in graft cases were in direct contact with host tissues to provide a substrate for tissue interactions previously demonstrated to promote neuron survival and remediate IC functions.

Effects of tectal grafts on sound detection deficits induced by inferior colliculus lesions in hooded rats.

The midbrain inferior colliculus (IC), a major integrating center for auditory processing, provides a model for structural, functional, and behavioral recovery. The present study examined the role of IC in spatial sound detection, and the effects of neural transplantation in sparing of behavioral performance. Hooded rats were presented noise bursts at ambient noise levels and 15 dB above this level randomly at one of eight locations in the horizontal plane, and rats were required to suppress licking upon detecting signal presentations. Following training, rats received bilateral IC lesions, bilateral lesions followed in 1 wk by bilateral tectal grafts (embryonic Day 18), or were sham operated. During repeated testing 15 to 30 days and 40 to 45 days following surgical procedures, rats with lesions showed impaired detection task performance when compared to grafted or sham animals. Detection ratios were statistically higher for grafted rats than for rats with lesions at repeated testing times. Performance of grafted animals was not statistically separable from the sham control group; however, grafted rats never achieved preoperative detection rates nor rates as high as sham rats. Postmortem implantation of diI crystals unilaterally into grafts demonstrated fiber labeling in the ipsilateral IC and lateral lemniscus, and retrograde labeling of neurons in the remaining host IC and dorsal nucleus of lateral lemniscus. Combined with results of previous studies, the results of this study suggest that sparing of detection performance for sounds occurring at random spatial locations may be attributable to partial integration of host-graft pathways.

Tangential neuronal migration in the avian tectum: cell type identification and mapping of regional differences with quail/chick homotopic transplants.

This paper is a sequel to a previous report, using quail/chick chimeras with partial tectal transplants, in which a tangential invasion of host (chick) tectal territories by cells originating in the quail graft was demonstrated. The cells displaying this secondary tangential migration appeared restricted to two strata (stratum griseum centrale (SGC) and stratum griseum et fibrosum superficiale (SGFS)). Here we describe the morphology of the tangentially displaced neurons, as well as their overall distribution in the host tectal lobe, by means of an antibody that specifically recognizes quail cells, staining them in a Golgi-like manner. Neurons that migrated into the SGC are identified as multipolar projection neurons, typical of this stratum. The majority of cells that migrated into the SGFS correspond to horizontal neurons, as was also corroborated by observations in Golgi-impregnated material. These horizontal cells are concentrated in laminae b, d and f, where their processes form well delimited axonal plexuses. In confirmation of previous results, SGC neurons have a limited range of migration, whereas SGFS cells translocate across much longer distances. In reconstructions of appropriate cases, a remarkable polarity was noted. Significant invasion of chick tectum by quail cells mostly occurred in the rostral half of the host tectum. The long-range migration of superficial horizontal cells frequently reached, but did not cross, the rostral tectal boundary. Conversely, tangential migration in the caudal half of the host tectum was scarce and coincided with a typical arrangement of quail-derived radial columns interdigited with chick-derived columns. These findings are discussed in relation to existing data on immature neuronal populations, molecular marker distribution and polarity of the avian optic tectum.

Organization of visual cortical projections to fetal tectal transplants in rats: a study using multiple retrograde tracers.

Retrograde tracing techniques have been used to study the host visual cortical projection to fetal tectal tissue grafted to the midbrain of newborn host rats. To determine whether there is any topographic order in these cortical afferents, different parts of the grafts were injected with 3 different tracers: Fast blue (FB), Diamidino yellow dihydrochloride (DY), and either horseradish peroxidase (HRP) or rhodamine-labelled microspheres (Rh). The comparative visual cortical distribution of cells retrogradely labelled with the different dyes was then examined. Tectal tissue from 15-day-old pigmented rat embryos was injected via a glass micropipette onto the dorsal midbrain of anaesthetised newborn rats of the same strain. In adulthood, host rats were examined for the presence of grafts; 21 grafts were injected with retrograde tracers and the cortices of 12 of these animals were mapped to show the relative location of FB-, DY-, HRP- or Rh-labelled cells. Qualitative inspection of area 17 did not reveal consistent evidence of point-to-point visuotopic mapping in the cortico-transplant projection. However, within area 17 statistical analysis (chi 2 tests) revealed significant differences in most brains in the relative distribution of FB-, DY-, HRP- or Rh-labelled neurons. Areas 18 and 18a contained greater numbers of retrogradely labelled cells. In these extrastriate regions, statistical analysis also indicated significant differences in the relative distribution of neurons labelled with different tracers. These data thus provide evidence for a non-random pattern of cortical innervation of tectal grafts. Possible reasons for the absence of coherent, topographically organized cortico-transplant maps typical of the normal corticotectal projection are considered.

[Morphological changes of embryonic retina and tectum transplanted into the anterior eye chamber of adult rats].

Primordia of the retina and the tectum of different ages (13 to 20 embryonic days) were transplanted as single or double grafts into the anterior chamber of the eye of normal syngeneic adult rats (Fischer 344). After 2-37 weeks (mostly 4-16 weeks), host rats were sacrificed and their eyeballs were dissected, frozen and sliced to 10 microns thickness and examined histologically by using Nissl and acetylcholinesterase (AChE) stains. Survival of the retinal single grafts, the tectal single grafts and the double grafts of both tissues were 21.7% (5/23), 41.2% (14/34) and 73.9% (17/23), respectively. The single grafts of the retina surviving on the iris of the host anterior chamber were observed through the transparent cornea as an irregular, translucent and whitish masses, inside of which small blood vessels were seen to enter. The grafted tissues contained many darkly stained cells and formed tubular structures of varying sizes and in part exhibited rosette-like structures. The surviving single grafts of the tectum grew forming round, developing tissue on the host iris. The surface of the graft was smooth and vascularized. In Nissl sections, grafts contained large cells densely distributed, in particular in the center of the solid tissues. Areas of strong AChE-positive reaction in the graft were located close to the host iris. Double grafts of the retina and the tectum were observed on the host iris to fuse about 2 weeks after transplantation. On Nissl sections, the retinal part of the grafts of about 2 weeks (short term) of survival demonstrated rosette formation of two (i.e., inner and outer) layers composed of light and dark cells, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic activity in rat tectal grafts is influenced by host sensory innervation.

It has been shown previously that fetal tectal tissue grafted to the midbrain of newborn host rats grows, differentiates, and receives input from the host brain. In the present study, 4 neuroanatomical techniques have been combined to examine how metabolic activity in tectal transplants is influenced by an identified host sensory pathway. Tectal tissue from E15 pigmented rat embryos was transplanted to the midbrain region of anesthetized newborn rats of the same strain. Six to 22 weeks later, the functional relationship between tectal transplants and the visual system of the host animal was examined by mapping metabolic activity in the grafts and relating this activity to the presence or absence of host retinal innervation. Metabolic activity in tectal grafts was assessed using the radioactive 2-deoxy-glucose (2-DG) method and cytochrome oxidase (CO) histochemistry. Graft regions receiving input from host retinal axons were demonstrated by anterograde labeling after bilateral intraocular injections of HRP or WGA-HRP; all areas in grafts that were homologous to the superficial layers of normal superior colliculus (SC) were identified using AChE histochemistry. The levels of metabolic activity demonstrated with 2-DG and CO varied between animals and within individual grafts. Grafts that did not connect with the host showed only low metabolic activity. In grafts that received host input, localized areas of high metabolic activity were seen with both 2-DG and CO. Highest levels of activity were consistently found in area containing both intense AChE activity and a high density of host retinal innervation.

Fetal tectal or cortical tissue transplanted into brachial lesion cavities in rats: influence on the regrowth of host retinal axons.

Fetal neural tissue was transplanted into suction lesions of the left brachium and pretectal region in young rats. Tectal tissue was grafted into 6-18-day-old rats and cortical tissue was transplanted into 17-20-day-old animals. The aim was to determine whether grafts could potentiate the regrowth of damaged retinal axons and, as a consequence, stimulate the axons to reenter their host target, the superior colliculus (SC). Fifteen to 581 days after transplantation, host retinal projections were traced by injecting the right eye with horseradish peroxidase (HRP). Parallel series of frozen brain sections were stained for HRP histochemistry, acetylcholinesterase, Nissl, or neurofibrils. At all ages studied, grafts survived and grew within the wound cavity; survival was better in the older animals. Most cortical grafts and a small number of tectal grafts filled the wound cavity and formed complete tissue bridges across the lesion. The majority of tectal grafts were attached to one or the other side of the lesion and were connected to the opposite lesion face by glial and connective tissue membranes that formed over the lesion site. In many animals that received tectal transplants, host retinal axons were traced growing into the grafts. Regenerating axons innervated specific, localized areas within the grafts, and it appeared that the axons retained the ability to recognize their appropriate target cells within the graft neuropil. Comparable ingrowth into cortical grafts was not observed. Optic axons were occasionally seen reentering the superficial layers of the host SC; however, compared to fetal tectal grafts, the density of host SC innervation was sparse. The implications of these data are discussed with regard to the possible use of fetal neural tissue grafts as reconstructive tissue bridges in the mammalian central nervous system.

Tectal transplants into the occipital cortex of the newborn rat.

Embryonic day 15 rostral tectum (presumptive superior colliculus) was transplanted into the occipital cortex of newborn rats. One to two months later, the transplants were visualized and injected with either horseradish peroxidase (HRP) or wheat germ agglutinin conjugated with HRP (WGA/HRP). After the appropriate survival time and processing with either diaminobenzidine (DAB) or tetramethylbenzidine tetrahydrochloride (TMB), HRP-labelled pyramidal cells were found in layer V of the host ipsilateral occipital cortex. Thus, the occipitotectal connections are formed between host and graft despite the fact that the fibers must grow in a direction opposite to their normal course to reach the aberrantly positioned tectal graft.

Transplantation of various regions of embryonic brain tissue into the brain of adult rats.

Brain tissue from the cortex, midbrain (corpora quadrigemina ) or cerebellum was transplanted into the lateral ventricle or parenchyma of the right brain hemisphere of adult Wistar rats. Brain tissue transplants consisted of undifferentiated matrix cells and few neuroblasts. 30 and 110 days after operation transplants showed good development both in the lateral ventricle and inside parenchyma. They differentiated into organotypical and histotypical structures and cells similar to those formed in the normal development. Cortical structures developed from the cortex tissue, cerebellar structures from the cerebellum tissue, and nuclei white substance of corpora quadrigemina where formed from the midbrain tissue. Nerve and glial cells of transplants are well differentiated, tightly connected with the surrounding nervous tissue of the recipient, and remain viable by the end of the experiment. The immune response of the host to the transplant is not expressed. The behaviour of animals remains normal. The present experiments are the beginning of studies on grafting brain embryonal tissue to mammals with some nonhereditary and hereditary changes of the central nervous system (CNS) for our further investigations.

Transplantation of tectal tissue in rats. IV. Maturation of transplants and development of host retinal projection.

We have examined the time course of maturation of embryonic tectal tissue transplanted to the midbrain region of newborn rats and studied the development of the host retinal projection to the grafts. Transplants were examined 2-19 days after transplantation. The morphology of developing transplants was studied using Holmes silver and neutral red stained material. Tectal transplants attained their mature morphology about 17 days after transplantation. The time course of tectal transplant maturation appeared to be similar to that of normal superior colliculus in situ. The development of the host retinal projection into the transplants was examined by injecting the host eyes with horseradish peroxidase (HRP) at various times after transplantation. Retinal fibers anterogradely labeled with HRP were first seen growing into the transplants 3-4 days after transplantation. Ingrowing fibers were always located close to the surface of the transplants. The rate of growth of optic axons into the grafts was estimated to be about 250 to 300 micron per day. Patch-like arborizations of retinal afferents were formed soon after innervation and the mature pattern of optic innervation was established by about two weeks. There was no evidence for an initial transitory phase in which the axons invaded the whole transplant. The development of the host retinal projection preceded morphological maturation of the transplants. The mode of ingrowth of retinal axons into tectal grafts was in many respects similar to the way optic fibers grow into the superior colliculus during normal in situ development. The transplant technique thus provides an opportunity to manipulate and analyze the factors which guide optic fiber growth in intact brains.

Transplantation of tectal tissue in rats. I. Organization of transplants and pattern of distribution of host afferents within them.

We have examined the maturation of tectal tissue transplanted from fetal rats to the midbrain of newborns and have characterized the distribution of host retinal and cortical afferents within the transplants. The transplants develop characteristic internal order and connections which distinguish them from either embryonic cortex or retina placed in the same region. Host retinal afferents project to clearly circumscribed regions, where they synapse mainly on small dendrites or dendritic spines, and only rarely on vesicle-containing profiles. The retinorecipient areas contain few stained axons in neurofibrillar preparations and are almost always located at the surface of the transplant. There is very little overlap in the input from the two eyes into a single transplant even though the projections from each eye may lie adjacent to one another. Cortical afferents spread more broadly in the transplants, but are largely absent from areas of optic termination and from other more deeply located regions with sparse fiber staining properties. The observations suggest that when placed close to its normal location, tectal tissue can develop a number of features characteristic of normal superior colliculus. Appreciation of the internal order of the transplants makes it possible to investigate the cortical and retinal afferent pathways using physiological techniques.