Tests for neuroplasticity in the anuran retinotectal system

Mapping of retinotectal projections in the tree frog Hyla regilla was carried out by both behavioral and electrophysiological recording techniques following tectal ablations, with and without optic nerve regeneration. Scotomata produced by unilateral and bilateral half tectum ablations and by unilateral rectangular midtectal excisions were found to remain essentially unaltered in all cases through recovery periods up to 334 days. Similarly, electrophysiological mapping of the rostral half tectum separated by Gelfilm implants from the caudal tectum for up to 191 days yielded a normal rostral visual field. The stability of the retinotectal projection in adult Hylidae observed in these experiments contrasts with the plastic readjustments obtained in young goldfish in which the retinotectal system is still probably growing and presumably capable of field regulation. The results are taken to support the original explanatory model for developmental patterning of retinotectal connections in terms of selective cytochemical affinities between retinal and tectal neurons.     

Topographic projections between the nucleus isthmi and the tectum of the frog Rana pipiens.

The connections between the nucleus isthmi and the tectum in the frog have been determined by several anatomical techniques: iontophoresis of horseradish peroxidase into the tectum, iontophoresis of 3H-porline into the nucleus isthmi and the tectum, and Fink-Heimer degeneration staining after lesions of the nucleus isthmi. The results show that the nucleus isthmi projects bilaterally to the tectal lobes. The ipsilateral isthmio-tectal fibers are distributed in the superficial layers of the tectum, coincident with the retionotectal terminals. The contralateral isthmio-tectal fibers travel anteriorly adjacent to the lateral optic tract and cross the midline in the supraoptic ventral decussation, where they turn dorsally and caudally; upon reaching the tectum, the fibers end in two discrete layers, layers 8 and A of Potter. The tectum projects to the ipsilateral nucleus isthmi and there is a reciprocal topographic relationship between the two structures. Thus, a retino-tecto-isthmio-tectal route exists which may contribute to the indirect ipsilateral retinotectal projection which is observed electrophysiologically. The connections between the nucleus isthmi and the tectum in the frog are strinkingly similar to the connections between the parabigeminal nucleus and the superior colliculus of mammals.     

Development of the retinotectal system in normal quail embryos: cytoarchitectonic development and optic fiber innervation

The development of the optic tectum and the establishment of retinotectal projections were investigated in the quail embryo from day E2 to hatching day (E16) with Cresyl violet-thionine, silver staining and anterograde axonal tracing methods. Both tectal cytodifferentiation and retinotectal innervation occur according to a rostroventral-caudodorsal gradient. Radial migration of postmitotic neurons starts on day E4. At E14, the tectum is fully laminated. Optic fibers reach the tectum on day E5 and cover its surface on day E10. 'Golgi-like' staining of optic fibers with HRP injected in vitro on the surface of the tectum reveals that: growing fronts are formed exclusively by axons extending over the tectal surface; fibers penetrating the outer tectal layers are always observed behind the growing fronts; the penetrating fibers are either the tip of the optic axons or collateral branches; as they penetrate the tectum, optic fibers give off branches which may extend for long distances within their terminal domains; the optic fiber terminal arbors acquire their mature morphology by day E14. The temporal sequence of retinotectal development in the quail was compared to that already established for the chick, thus providing a basis for further investigation of the development of the retinotectal system in chimeric avian embryos obtained after xenoplastic transplantation of quail tectal primordia into the chick neural tube.     

Development of the transient ipsilateral retinotectal projection in the chick embryo: A numerical flourescence-microscopic analysis

The ipsilateral retinotectal projection in the developing chick was examined by using rhodamine-B-isothiocyanate (RITC)as an anterograde and retrograde vital marker for the retinal ganglion cells and their axons. Staining of the entire retina following intravitreal RITC injection between incubation days 3 and 16 revealed a small number of anterogradely labeled fibers in the optic tract and the anterior half of the optic tectum ipsilateral to the injection site. The total number of ipsilaterally projecting fibers was estimated to be about 2,000 on developmental day 9. The ipsilateral projection totally disappeared after day 15. The arrangement of fibers within the ipsilateral projection was examined by local anterograde RITC staining of localized retinal regions between days 9 and 10. The projection was retinotopically organized along the dorsoventral axis such that fibers of dorsal retinal origin projected on the ventral tectal half, whereas fibers of ventral retinal orgin projected on the dorsal tectal half. The localization of ipsilaterally projecting ganglion cell bodies was examined by retrograde RITC staining during days 9 and 15. Ganglion cells of all four quadrants of the central retina contributed to the production of the ipsilateral projection. The ipsilaterally growing retinotectal fibers did not represent collaterals of contralaterally projecting retinotectal axons. We assume that the tendency of early growing retinotectal axons to grow straight, as well as the ability of axonal growth cones to “sample” the environment, lead to a crossing of axons to the contralateral side. Ipsilateral projections would therefore represent “pathfinding errors.” Explanations for the elimination of the ipsilateral retinotectal projection are discussed.     

Nucleus isthmi enhances calcium influx into optic nerve fiber terminals in Rana pipiens

We examined the role of nucleus isthmi in enhancing intracellular calcium concentrations in retinotectal fibers in the frog optic tectum in vitro. The intracellular calcium levels were measured using the fluorescent calcium-sensitive dye, Calcium Green-1 3000 mw dextran conjugate (CG-1), which was injected into one optic nerve. Electrical stimulation of the labeled optic nerve alone increased tectal CG-1 fluorescence whereas electrical stimulation of nucleus isthmi alone had no effect on CG-1 fluorescence. Electrical stimulation of the nucleus isthmi ipsilateral to the labeled tectum, followed by electrical stimulation to the optic nerve can enhance calcium uptake more than a double pulse stimulation of the optic nerve alone. Maximum enhancement of the calcium signal by nucleus isthmi occurs when optic nerve stimulation follows the ipsilateral nucleus isthmi stimulation by 10 ms. These results suggest that nucleus isthmi input can facilitate retinotectal neurotransmission, and the mechanism could be used to allow the frog to attend to a single prey stimulus in an environment of several prey stimuli.     

Morphological and functional changes in the retinotectal system of the pigeon during the early posthatching period

Anterograde transport of either HRP or wheat germ agglutinin-conjugated HRP was used to study the posthatching development of the retinotectal connection in the pigeon. The functional maturation of the retinotectal system was also investigated by recording electroretinographic (ERG) and tectal evoked (TEP) responses to either flash or pattern stimuli. Two main morphological changes occurred in the retinotectal system during the first 6 days after hatching: an ipsilateral retinofugal component that was present at hatching disappeared and the outer tectal layers were progressively invaded by the contralateral retinofugal axons, which at hatching were limited to the stratum griseum et fibrosum superficiale of the dorsolateral tectal quadrant. During the early posthatching period, at the same developmental stage at which an ERG to unpatterned or patterned stimulation could first be recorded, a visually evoked response could be elicited in the contralateral optic tectum. Therefore, the retina and optic tectum seem to start functioning simultaneously, the limiting factor being the late maturation of photosensitive lamellae in the outer segments of the developing photoreceptors. During the first 20 days posthatching, the retinotectal system undergoes extensive development as revealed by latency and amplitude changes of the visually evoked potentials. We suggest that the pigeon visual system serves as a useful model for studies concerning visual development and plasticity.     

Recovery of the ipsilateral oculotectal projection following nerve crush in the frog: evidence that retinal afferents make synapses at abnormal tectal locations

The ipsilateral oculotectal projection in the frog is a topographic mapping of the binocular part of the visual field of one eye on the ipsilateral tectal lobe. The underlying neuronal circuitry consists of the topographic, crossed retinotectal projection and an intertectal pathway which relays information from a given point in one tectal lobe to the visually corresponding point in the other. During optic nerve regeneration, there is a period when the terminals of retinotectal afferents are found at abnormal locations in the opposite tectal lobe. Whether they form functional synapses at this time is not known. If so, one would expect to observe correlated abnormalities in the ipsilateral oculotectal projection. To determine whether such abnormalities exist, we have made parallel electrophysiological studies of the recovery of the retinotectal and ipsilateral oculotectal projections following crush of one optic nerve. The earliest stage of recovery was characterized by a lack of significant topographic order in the retinotectal projection and by the absence of a physiologically observable ipsilateral projection. Within a short time, the retinotectal projection became topographically organized and a similarly organized ipsilateral projection appeared. While topographic, the retinotectal projection at intermediate times was abnormal in that the multiunit receptive fields recorded at individual tectal loci were greatly enlarged. Multiunit receptive fields were similarly enlarged in the ipsilateral projection. In addition, some ipsilateral fields included areas of visual space not normally represented in the projection. The abnormalities in both projections subsequently disappeared over the same time course. Throughout recovery there was a high correlation between multiunit receptive field sizes in the contralateral tectal lobe and those at visually corresponding points in the ipsilateral tectal lobe. Enlarged multiunit receptive fields in the contralateral tectal lobe could not be accounted for in terms of optical or retinal abnormalities since single unit receptive field sizes were normal. Nor could they be accounted for in terms of changes in recording characteristics since simultaneously recorded fields activated by the undisturbed eye were normally sized. We conclude that the enlarged fields in the contralateral tectal lobe indicate the presence at individual tectal loci of afferents from wider than normal retinal regions. Similar considerations ruled out optical, retinal, and recording abnormalities as the explanation for the enlarged multiunit receptive fields in the ipsilateral tectal lobe.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence for shifting connections during development of the chick retinotectal projection

The pattern in which optic axons invade the tectum and begin synaptogenesis was studied in the chick. The anterogradely transported marker, horseradish peroxidase, was injected into one eye of embryos between 5 and 16 days of development (E5 to E16). This labeled the optic axons in the brain. The first retinal axons arrived in the most superficial lamina of the tectum on E6. They entered the tectum at the rostroventral margin. During the next 6 days of development the axons grew over the tectal surface. First they filled the rostral tectum, the oldest portion of the tectum, and then they spread to the caudal pole. Shortly after the first axons entered the tectum on E6, labeled retinal axons were found penetrating from the surface into deeper tectal layers. In any given area of the tectum, optic axons were seen penetrating deeper layers shortly after arriving in that area. Electron microscopic examination showed that at least some of the labeled axons in rostral tectum formed synapses with tectal cells by E7. These results show two things which contrast with results from previous studies. First, there is no delay between the time the retinal axons enter the tectum and the time they penetrate into synaptic layers of the tectum. Second, the first retinotectal connections are formed in rostral tectum and not central tectum. Retrograde tracing showed the first optic axons that arrived in the tectum were from ganglion cells in central retina. Previous studies have shown that the ganglion cells of central retina project to the central tectum in the mature chick. This opens the possibility that the optic axons from central retina, which connect to rostral tectum in the young embryo, shift their connections to central tectum during subsequent development. As they enter the tectum the growth cones of retinal axons appear to be associated with the external limiting membrane. During the time that connections would begin to shift in the tectum a second population of axons appears at the bottom of stratum opticum, some with characteristics of growth cones. This late-appearing population may represent axons shifting their connections. These results have implications for theories on how the retinotopic pattern of retinotectal connections develops.     

Responses of the optic tectum to telencephalic stimulation in catfish

In order to test physiologically for cerebrotectal connections in a fish, averaged evoked potentials and unit responses were recorded from the optic tectum following electrical stimulation applied to the telencephalon in the siluroid teleost Ictalurus nebulosus. A single shock applied to the area dorsalis centralis (Dc) of the telencephalon, and only to this area, elicits a sequence of deflections in the ipsilateral optic tectum: an initial negative peak at about 8 ms, (= N8), a larger N25 and a slow P50-N95. The configurations, depth profiles, latencies and susceptibility to repetitive stimulation, together with the known tectal anatomy, suggest that the first wave is due to the afferent fibers from the telencephalon and that N25 is due to deep tectal neurons. Telencephalic input exerts a conditioning effect on the field potentials and unit responses evoked by direct optic nerve shock. Such a shock elicits, in the contralateral tectum, small negative, optic tract axon peaks followed by a large N6, believed to be postsynaptic, and a still later P12. As a first approximation it is argued that the telencephalic input and the retinal input are activating different sets of neuronal elements in the optic tectum, since the configuration and depth profile of the telencephalic and optic nerve shock-elicited potentials are different. A conditioning Dc stimulus has a long-lasting effect on the form of the optic nerve field potential, maximally when the pallial shock precedes the optic by about 90 ms. The effect, observed by subtracting the conditioned from the unconditioned tectal response to optic nerve shock, is a difference wave with N11 and P20. The unit activity from deep tectal laminae is either activated or accelerated following Dc stimulation, while superficially located neurons are not affected. In another group of tectal units, the optic nerve shock-induced response is depressed by a preceding pallial dorsalis centralis stimulus. The evidence is compatible with the assumption of direct projections from Dc to the deep layers of the tectum, but the timing could also permit indirect pathways. In any case, the influence is not simple or identical for different tectal cell classes.     

Retinal fibers alter tectal positional markers during the expansion of the retinal projection in goldfish

Although widely accepted, the theory, that neurones carry immutable cytochemical markers which specify their synaptic connections, is not consistent with plastic reorganizations. Half retinal fish were therefore tested for changed markers following expansion. Optic nerve crush at the time of the half retinal ablation resulted in regeneration of a normal, restricted projection; but nerve crush following expansion (many months later) resulted in reestablishment of the expanded projection, assessed both by electrophysiological mapping and by radioautography. Since this implied changed markers, the half retina and tectum were tested independently using the ipsilateral tectum and eye as controls. In normal fish, removal of one tectum and deflection of the corresponding optic tract toward the remaining tectum resulted in regeneration of a positionally normal but ipsilateral map. In experimental fish, after the half retina had expanded its projection to the contralateral tectum, its optic tract was deflected to the control tectum. After 40 days it had regenerated a normal, restricted map indicating that the retinal markers had not changed. Such restricted projections did not expand in the presence of the normal projection even after a year or more. Similarly, the optic tract from the normal eye was deflected to cause innervation of the tectum containing the expanded half retinal projection. After 40 days, the projection regenerated from the normal eye was similar to the expanded half retinal projection. Areas of the normal retina corresponding to the missing areas of the half retina were not represented. Tectal markers had been altered by the half retinal fibers. In a final group, tecta were denervated and tested at various intervals by innervation from ipsilateral half retinal eyes. After five months of denervation, the regenerating fibers were no longer restricted to the rostral tectum but formed an expanded projection initially. Apparently tectal markers are induced by the retinal fibers, changed during expansion, and disappear during long-term denervation.     

A comparison of the normal and regenerated retinotectal pathways of goldfish

This is a light and electron microscopic study of the retinotectal pathway: intact and after regeneration of the optic nerve. The spatiotemporal pattern of axonal outgrowth and termination was studied with the methods of proline autoradiography, horseradish peroxidase (HRP) labeling, and fiber degeneration. The spatial order of optic fibers in the normal and regenerated pathways was assessed by labeling small groups intraretinally with HRP and then tracing them to the tectum. The labeled fibers occupied a greater fraction of the cross section of the regenerated than the normal optic tract. At the brachial bifurcation, roughly 20% of the regenerated fibers chose the incorrect brachium vs. less than 1% of the normals. In tectum, the regenerated optic fibers reestablished fascicles in stratum opticum, but they were less orderly than in the normals. The retinal origins of the fibers in the fascicles were established by labeling individual fascicles with HRP and then, following retrograde transport, finding labeled ganglion cells in whole-mounted retinas. Labeled cells were more widely scattered over the previously axotomized retinas than over the normal ones. A similar result was obtained when HRP was applied in the tectal synaptic layer. All of these results indicate that the pathway of the regenerated optic fibers is less well ordered than the intact pathway. Both autoradiography and HRP showed that the regenerating optic fibers invaded the tectum from the rostral end, and advanced from rostral to caudal and from peripheral to central tectum, along a front roughly perpendicular to the tectal fascicles. Synapses of retinal origin were noted electron microscopically in the tectum at the same sites where autoradiography indicated that the fibers had arrived. No retinal terminals were seen where grain densities were at background levels. Fiber ingrowth and synaptogenesis apparently occurred simultaneously. The synapses were initially smaller and sparser than in normals, but were in the normal tectal strata and contacted the same classes of post synaptic elements as in normals.     

The central projections in the retina in Necturus maculosus

The projections of the retina in Necturus maculosus were studied by injecting radioactive proline into one eye. Labeling was seen in both the contralateral and ipsilateral diencephalon and tectum. The contralateral fibers are divided into three major tracts: the marginal, axial, and basal. The ipsilateral fibers separate into a marginal and an axial optic tract. The contralateral and ipsilateral axial optic tracts have a similar distribution. The contralateral and ipsilateral marginal optic tracts projecting to the diencephalon also have a similar distribution. However, in the tectum the ipsilateral marginal optic tract ends in the anterior third while the contralateral extends almost the entire length of the tectum. The retinotectal ipsilateral projection ends in clumps as has been described in other vertebrates. A direct ipsilateral retinotectal projection has not been described in any other amphibian.     

Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

Topographic projections between the nucleus isthmi and the optic tectum in a teleost,Navodon Modestus

Following horseradish peroxidase injections into the optic tectum of a teleost,Navodon modestus, reciprocal and topographic projections between the nucleus isthmi and the ipsilateral optic tectum were determined. The isthmo-tectal fibers diverge to the optic tectum while maintaining the spatial arrangements of the isthmic cells from which the fibers originate. The tecto-isthmic projections also keep the spatial arrangements in the optic tectum. The tectal fibers converge near the nucleus isthmi and terminate in the non-cellular portion of the nucleus. The reciprocal topography is apparent in the combined results of 9 experiments with one tectal injection in each region. No labeled cells and fibers were found in the contralateral nucleus isthmi.     

Topographic and morphometric effects of bilateral embryonic eye removal on the optic tectum and nucleus isthmus of the leopard frog

Rana pipiens were raised through metamorphosis after extirpation of both eye primordia at Shumway embryonic stage 17 (Shumway '40). The visual connections between the isthmic nuclei and the optic tectum were examined in these animals using horseradish peroxidase (HRP) histochemistry. Isthmo-tectal projections are normally aligned with the primary retinotectal map. We asked whether these connections would develop normal topographic organization in the absence of normal retinal input. HRP was formed into a solid pellet (? 200–500 μm diameter) and inserted into one tectal lobe on the tip of a fine metal probe. The procedure produced relatively restricted retrograde label in somas and dendrites in both isthmi nuclei. In the nucleus isthmus ipsilateral to the tectal lobe receiving the HRP pellet, processes of tecto-isthmi neurons were labeled by anterograde transport. The topography of the isthmo-tectal and tecto-isthmic projections were identical in the developmentally enucleated animals and in normal frogs, even though eye removal severely reduced the volume of the optic tecta and the isthmi nuclei. Thus our analyses indicate that retinal contacts do not play an active role in the development of the positional or polarity cues that are involved in “mapping” projections between central visual nuclei. These results are discussed in the context of peripheral specification of central connections and in terms of models that have recently been proposed to explain the development of the retinotectal system.     

Spatial arrangement of radial glia and ingrowing retinal axons in the chick optic tectum during development

Neuroanatomical tracing of retinal axons and axonal terminals with the fluorescent dye, DiI, was combined with immunohistochemical characterization of radial glial cells in the developing chick retinotectal system. Emphasis was placed on the mode of the tectal innervation by individual retinal axons and on the distribution and fate of the tectal radial glial cells and their spatial relation to retinal axons. It was obvious from fluorescent images obtained from anterogradely filled axons that these axons deserted the superficial stratum opticum (SO) to penetrate the stratum griseum et fibrosum superficiale (SGFS) by making right-angled turns within the SO. Frequently, axons which had invaded the SGFS were bifurcated and had a superficial branch which remained within the SO. Terminal axonal arborization occurred at various depths within the SGFS. Characterization of the tectal glial cells and their radial fibers by means of the anti-filament antibody, R5, and post-mortem staining with the fluorescent dye, DiI, revealed the following. (a) At least from day E8 to P1, tectal glial fibers traversed all tectal layers from the periventricular location of their somata to the superficial interface between SO and pia mater. In this interface they enlarged and formed characteristic endfeet. (b) Glial endfeet covered the whole tectal surface. They showed at early ages anterior-posterior differences having a higher density in the posterior tectum. These differences disappeared at embryonic day E13. (c) After innervation, glial endfeet of the anterior tectal third were arranged in rows parallel to the retinal fibers within the SO. This arrangement was not observed in eyeless embryos. (d) Radial glial fibers could be stained with R5 from day E8 to late embryonic stages throughout their entire length. (e) At the first posthatching days, only the segments of the radial glial fibers restricted to the thickness of the SO were R5-positive, although the fibers still traversed throughout the depth of the tectum. The results are discussed in context to the genesis of the retinotectal projection.     

Synaptic transmission of excitation from the retina to cells in the pigeon's optic tectum

Intracellular recordings were used to study the synaptic excitation of optic tectum neurons in the pigeon. Electrical stimulation of both contralateral optic nerve and ipsilateral optic tract evoked in the tectal neurons EPSPs which in most cases were followed by an IPSP. An extrapolation procedure based on response latency was used to reveal that the EPSPs were mediated by way of mono-, di- and polysynaptic connections with the retinal endings. The laminar location of the recorded cells was estimated according to the field potential and the recording depth with the exception of the cell which was intracellulary stained with HRP. Monosynaptic EPSPs were recorded from cells in the retinorecipient region (sublayers IIa-f) as well as in the non-retinorecipient region (sublayers IIg-j and layer III) of the tectum, while di- and polysynaptic EPSPs were never recorded from the input layers. Tectofugal projections arise largely from layer III neurons. Thus, these results indicate that retinal excitation is transmitted to the output tectal cells by way of mono-, di- or polysynaptic pathways. The conduction velocities of most retinal fibers mediating the EPSP ranged from 4 to 22 m/s (average 12 m/s). However, in a number of retinal fibers the conduction velocities were in a faster range, up to 36 m/s.     

青蛙前视盖与视顶盖间神经传导的细胞内电生理研究(英文)

目的已有许多研究报告了青蛙的前视盖对视顶盖起抑制作用,但关于此神经活动的特性尚不清楚。本研究探讨了这种复杂的神经活动的机理。方法用细胞内记录方法,通过电刺激前视盖的神经细胞核来记录视顶盖细胞的神经活动。结果前视盖的电刺激在同侧视顶盖主要唤起了两种神经反应:一种是兴奋性(excitatory postsynaptic potential,EPSP)和抑制性突触后电位(an inhibitory postsynaptic potential,IPSP)同时出现,另一种是单纯的IPSP,后者在本记录中占主导地位。另外我们也记录到了某些投射到前视盖的视盖投射细胞的神经电位。它揭示了视顶盖和前视盖之间存在着交叉性的相互作用。短潜时的EPSP可能是通过单突触进行传导的,而大多数的IPSP是通过多突触方式进行神经信息传递的。几乎98%被记录的视盖细胞对前视盖的刺激显示出了抑制性反应。结论前视盖的神经细胞对视顶盖的神经活动发挥了强烈的抑制性作用。     

Postsynaptic potentials of tectal neurons evoked by electrical stimulation of the pretectal nuclei in bullfrogs (Rana catesbeiana)

Postsynaptic responses of the tectal cells to electrical stimulation of pretectal (Lpd/P) nuclei were intracellularly recorded in the bullfrog (Rana catesbeiana). The pretectal stimulation elicited mainly two types of responses in the ipsilateral tectum: an EPSP followed by an IPSP and a pure IPSP. The latter predominates in the tectal cells responding to ipsilateral pretectal stimulation. In a few cells, biphasic hyperpolarization appeared under stronger stimulus intensities. Only one type of response was found in the contralateral tectum, a pure IPSP. The antidromically invaded tecto-pretectal projecting cells were recorded in both tecta, which revealed reciprocal connections between the tectum and particular pretectal nuclei. This paper demonstrates the synaptic nature underlying pretectotectal information transfer. EPSPs with short latencies were concluded to be monosynaptic. Most IPSPs were generated through polysynaptic paths, but monosynaptic IPSPs were also recorded in both optic tecta. Nearly 98% of impaled tectal cells (except for intra-axonally recorded and antidromically invaded cells) showed inhibitory responses to pretectal stimulation. The results provide strong evidence that pretectal cells broadly inhibit tectal neurons as suggested by behavioral and extracellular recording studies.