The cerebellar projections to the superior colliculus and pretectum in the cat: An autoradiographic and horseradish peroxidase study

Efferent projections from the cerebellar nuclei to the superior colliculus and the pretectum have been studied using both retrograde and orthograde labeling techniques in the cat. In order to identify what parts of the cerebellar nuclei project to the superior colliculus and the pretectum, the retrograde horseradish labeling technique was employed. In another set of experiments, tritiated amino acids were injected into each of the cerebellar regions from which the cerebello-tectal and cerebellopretectal projections arise, and the laminar and spatial distributions of orthograde labeling in the superior colliculus and the pretectum were compared.The results showed that the cerebello-tectal projections arise from two different regions of the cerebellar nuclei: the caudal half of the medial nucleus and the ventrolateral part of the posterior interposed nucleus. Fibers arising from the medial nucleus distribute bilaterally in the superficial zone of the intermediate gray layer in the superior colliculus, while those originating from the posterior interposed nucleus terminate contralaterally in the deeper aspect of the intermediate gray layer and in the deep gray and white layers. Although the lateral nucleus does not contribute to the cerebello-tectal projection, it projects profusely to the pretectum contralaterally. The origin of the cerebello-pretectal projection lies in the parvicellular part of the lateral nucleus. Among several pretectal nuclei, the posterior pretectal, the medial pretectal nucleus and the reticular part of the anterior pretectal nucleus receive the cerebellar afferents.The findings of the differential projections from the cerebellum to the superior colliculus and the pretectum suggest that the cerebellum exerts a regulatory influence on visuo-motor and somato-motor transfer in these midbrain structures by differential circuits.     

The projection of the fovea to the superior colliculus in rhesus monkeys

Horseradish peroxidase was injected into the superior colliculus of seven Rhesus monkeys to determine whether retinal ganglion cells concerned with the fovea project to the superior colliculus or whether, as results of several experiments using other methods suggest, only the parafoveal retina projects directly to this region. The retinal distribution of ganglion cells labelled by retrograde transport showed unequivocally that there is a direct projection from the fovea to the superior colliculus. However, there was no convincing evidence that the nasotemporal overlap that exists in the retinogeniculate projections also occurs in the projection to the superior colliculus. The results cast serious doubt on schemes that attribute little or no role to the superior colliculi in foveal vision and which emphasize the projection of the fovea to the visual cortex and the peripheral retina to the superior colliculi.     

Ontogeny of enkephalin-like immunoreactivity in the rat hippocampus

The postnatal development of leucine5-enkephalin-like immunoreactivity within the hippocampal formation of the rat has been analyzed using immunocytochemical techniques. From the day of birth to postnatal day three, no intrinsic hippocampal elements exhibit immunoreactivity although labeled axons are found within the fimbria, within the alveus, and in the vicinity of the angular bundle. On postnatal day 4, a few immunoreactive hippocampal neurons can be seen in stratum radiatum of the region CA3 and by postnatal day 8, within the hilus, strata pyramidale and oriens of regio superior, and the subiculum. There is a dramatic increase in the incidence of immunoreactive perikarya between postnatal days 8 and 10 in all fields as well as the appearance of labeled neurons in CA1 stratum pyramidale and stratum granulosum of the dentate gyrus. Two days after the first appearance of immunoreactive perikarya, intensely immunoreactive neurons, labeled much more extensively than is ever seen in the adult, are encountered in each subfield of the hippocampus. The spatio-temporal order in both the emergence of perikaryal immunoreactivity and the transient appearance of intensely immunoreactive neurons follows that of neurogenesis, with immunoreactivity developing 12-14 days after the peak period of last cell division for a given hippocampal region. The incidence of immunoreactive perikarya in the dentate gyrus was quantified in rat pups ranging from postnatal days 8 to 19. The appearance of labeled neurons followed the spatio-temporal gradients that have been described for neurogenesis in this region as well. Immunoreactive perikarya emerged in the suprapyramidal stratum granulosum prior to their emergence in the infrapyramidal zone and in the temporal pole of the dentate earlier than in the mid-dorsoventral dentate. The lateral perforant path and mossy fiber axons, seen to exhibit enkephalin-like immunoreactivity in the adult hippocampal formation, differ in their relative maturity at the age immunoreactivity first appears. Immunoreactivity appears as early as postnatal day 4 in the lateral perforant path, an age at which these axons are just growing into their target field while it is not found within the mossy fibers until after postnatal day 10, an age at which mossy fiber bouton elaboration is well advanced and physiologically competent mossy fiber synapses with the regio inferior pyramidal cells have been established. The latter observation indicates that enkephalin is not necessary for synaptic transmission at the mossy fiber synapse.     

A sensitive period for ganglion cell degeneration and the formation of aberrant retino-fugal connections following tectal lesions in rats

Rats of 0, 3, 5, 10 and 30 days of age received unilateral tectal lesions. After surviving for 150 days the retinal ganglion cell layer of the contralateral eye was examined for evidence of a ganglion cell loss. The retino-fugal projections of the eye contralateral to the lesion were studied in autoradiographs. In the animals operated at 0 days of age, 33% of the ganglion cells had degenerated but in animals operated at 5 days of age, 67% of the ganglion cells had degenerated. The animals operated at 30 days of age show no significant cell loss. An aberrant retinal projection to the lateral posterior nucleus of the thalamus was found only in animals operated at 0 and 3 days of age.The retinal projection to the thalamus was investigated in normal rats of 0, 3, 5 and 10 days of age using the anterograde transport of horseradish peroxidase. There was a conspicuous projection to the lateral posterior nucleus in animals of 0 and 3 days of age, but in the 5-day-old rat the retinal projection to the lateral posterior nucleus was very small and similar to the adult pattern.We conclude that transecting the tectal terminals of retinal ganglion cells causes the ganglion cells to degenerate, unless they are old enough to have formed sustaining collaterals. In addition, the tectal lesion removes a major tectal input to the lateral posterior nucleus and, if carried out within the first few days, leads to the preservation of the normally transient retinal projection to the lateral posterior nucleus, presumably by reducing competition between axon terminals.     

Morphology of retino-geniculate axons in the cat

When horseradish peroxidase is injected into the optic tract, retino-geniculate axons and their terminals become filled throughout their length and resemble axons impregnated by the Golgi methods. In the optic tract, labeled axons and collateral branches ranging from 0.5 to 5.0 μm in diameter can be traced to the dorsal lateral geniculate nucleus. Larger diameter axons branch frequently and terminate in laminae A, Al, C and in the medial interlaminar nucleus. These axons have terminal swellings that range from small (1–2μm) and spherical to large (7–l0μm) and crenulated. The swellings may be arranged in clusters, in chains connected by fine axons, or singly at the ends of axons. Fine diameter axons project to laminae Cl and C2, and intermingle with larger fibers in C and in the medial interlaminar nucleus. These fine axons branch infrequently and usually give rise to single terminals or chains of swellings, but rarely bear clusters of terminals.The distribution of these groups of axons suggests that they may correspond to classes of retino-geniculate axons that have been characterized elect rophysiologically. The larger diameter axons show the same distribution as X- and Y-cell axons, while the finer axons appear to correspond to W-cell axons. Within each of these structural groups, however, individual axons display remarkable diversity in their overall patterns of branching as well as in the types and arrangements of their terminals.     

Growth of dendrites in the optic tectum of the chick embryo following destruction of the eye primordium.

The consequences of depriving the optic tectum of axons from the contralateral eye have been studied in Golgi-impregnated brains from a staged series of chick embryos. Following enucleation at 2–5 days of age, measurements of dendritic length and the numbers of branches at all orders for three cell types were performed with an automated three-dimensional tracking system at various survival times. Dendritic lengths and the number of middle order branches of neurons from control animals, aged 12–14 days (stages 38–40), are greater than those from non-innervated embryos of the same ages. However, by Day 18 (stage 44), no significant differences in length or branching are seen between neurons from control and experimental embryos. Observations of these neurons revealed qualitative differences between experimental and control embryos. Growth cones, varicosities and filopodia, indicators of dendritic differentiation, are more commonly associated with neurons from control Day 12 and 14 embryos, than operated embryos of the same stages. However, at Days 16 and 18 these growth characteristics are more usually seen on neurons from deafferented embryos than from controls.The deleterious effects observed in experimental animals between Days 12 and 14 are presumably caused by the absence of optic fibers. The eventual growth during late embryogenesis, of the cells deprived of optic input, may reflect a trophic influence not acting in the earlier period.     

Distribution of enkephalin-like immunoreactivity in the rat main olfactory bulb

Enkephalin-like immunoreactivity was localized within the main olfactory bulb of the rat using immunohistochemical techniques. These studies utilized well characterized antisera directed to either leu⁵- or met⁵-enkephalin. Specificity was established by absorption of the antisera with either 10 μM synthetic leu⁵- or met⁵-enkephalin.Specific enkephalin-like immunoreactivity was observed within several different cell populations including (1) periglomerular cells, (2) granule cells and their processes within the external plexiform layer and (3) occasional short-axon (horizontal) cells within the granule and external plaxiform layers. The granule cell layer contained the greatest number of immunoreactive cells. Only a limited number of immunoreactive cells were found in both the periglomerular and granule cell layers, suggesting the enkephalin-containing neurons represent a sub-population within each layer.The absence of immunoreactive processes in the periventribular white matter, as well as the morphologies of immunoreactive bulbar neurons, indicates that enkephalin is found exclusively within intrinsic olfactory bulb neurons.     

A demonstration of the dentato-reticulospinal projection in the cat

Experiments were performed to anatomically and electrophysiologically demonstrate the existence of a dentato-reticulospinal pathway in the cat. Reticulospinal neurons projecting to the lumbar region of the spinal cord were shown to respond to stimulation in the dentate nucleus at latencies as short as 0.8 ms. The latency of these responses could be varied by changing either stimulus strength or stimulus frequency. Furthermore, intracellular recordings revealed that these responses were associated with a graded depolarization with latencies as short as 0.8 ms. Collision experiments confirmed that the responses recorded in reticular neurons following spinal cord stimulation were antidromically evoked and that the orthodromically evoked responses to dentate stimulation were conducted to the spinal cord. To ensure that the short latency responses evoked in these cells by dentate stimulation were not the result of activating a cerebellar projection to the brainstem through the inferior cerebellar peduncle, an experiment was performed demonstrating that these responses could be blocked by lesions of the brachium conjunctivum. In the anatomical experiments, small injections of horseradish peroxidase limited to the rostromedial region of the medullary reticular formation resulted in the retrograde labeling of neurons in the contralateral dentate nucleus.On the basis of these electrophysiological and neuroanatomical findings, it was concluded that a dentatoreticulospinal system is present in the cat. a system by which the dentate nucleus may affect neuronal integration occurring in the spinal cord.     

Widespread cortical projections of the hippocampal formation in the cat

Efferent projections from the hippocampal formation to the cat's cortex were traced with the retrograde horseradish peroxidase technique. Different areas of the cortex of 31 cats were injected with small amounts of horseradish peroxidase. All subregions of the hippocampal formation were screened for labeled cells. It was found that, with the exception of the entorhinal injections, only subicular areas of the hippocampal formation contain labeled neurons. When HRP was injected into the entorhinal cortex, labeled cells are also found in the hippocampus proper. The most dense projection from the subicular cortex is directed to the medial part of the cortical hemisphere. Here, cingulate, retrosplenial and medial prefrontal fields receive a substantial number of subicular efferents. Furthermore, the entorhinal cortex is reached by a number of axons originating in the subicular area. Scarce projections from the subicular cortex terminate in the dorsal prefrontal, temporal, parietal and prepiriform cortex. It is suggested that the projection from the subicular cortex to the neocortical areas of the frontal pole (medial prefrontal cortex) is of special importance as it may constitute a link between the association areas of the neocortex and those regions of the limbic system thought to play a role in memory (subicular cortex, mamillary bodies, anterior thalamus, cingulate gyrus).     

On the extent of the lateral prefrontal cortex of the cat

It is known that the largest portion of the prefrontal cortex of the cat—defined as the projection area of the thalamic mediodorsal nucleus—lies around both sides of the frontal pole. Furthermore, a small area, situated in the anterior sylvian gyrus, is known to receive afferents from the mediodorsal nucleus. The purpose of the present experiment was to investigate whether the area in between these two separate regions, i.e. the orbital gyrus, receives afferents from the mediodorsal nucleus as well.Of nine injections of horseradish peroxidase into cortical loci within the orbital gyrus, only four resulted in retrogradely labelled neurons within the mediodorsal nucleus. Two of the latter loci lay on the border of the lateral part of the frontal pole region while the other two were situated next to the anterior sylvian gyrus. On the other hand, even large injections, when restricted to the orbital gyrus, failed to label cells within the mediodorsal nucleus.From these findings we draw the following conclusions. First, the prefrontal cortex of the cat consists indeed of two separate fields. Second, due to a lack of corresponding investigations in the dog, and due to possible discrepancies in the comparability of the orbital gyri of cat and dog, we do not know whether the lateral prefrontal cortices of dog and cat are organized similarly. Third, it is emphasized that differences in the organization of the prefrontal cortex of different mammalian species have been observed repeatedly and that further dissimilarities might be expected.     

Localization of enkephalin-like immunoreactivity in acetylcholinesterase-positive cells in the guinea-pig lateral superior olivary complex that project to the cochlea

Olivocochlear fibers have been demonstrated to have acetylcholinesterase-positive staining both in brainstem and cochlea. Olivocochlear fibres in the cochlea have also been determined to contain enkephalin-like immunoreactivity. In this study, we first determined the source of olivocochlear fibers in the guinea-pig using horseradish peroxidase and wheat germ agglutinin in retrograde transport studies. These cells were then examined for enkephalin-like immunoreactivity followed by acetylcholinesterase staining on the same sections to determine which cells and fibers showed staining for both. It was found that cells in the guinea-pig lateral superior olive that project to the cochlea have both enkephalin-like immunoreactivity staining and acetylcholinesterase-positive staining. Cells in other areas giving rise to olivocochlear fibers showed only acetylcholinesterase staining. These results suggest that there is co-localization of enkephalin and acetylcholine in a population of olivocochlear cells and fibers.     

Distribution of substance P-like immunoreactivity in the spinal cord and dorsal root ganglia of the human foetus and infant

Using the indirect immunofluorescence method, the distribution of substance P-like-immunoreactivity was studied in spinal cord and dorsal root ganglia of 25 human foetuses ranging from 12 to 29 weeks of gestational age. The spinal cord and dorsal root ganglia of three infants (1 day-, 2 and 4 month-old) were also investigated as a post-natal reference. On the whole, the substance P distribution patterns seen in infants were already visible throughout most of foetal life. The highest density of substance P-like-immunoreactive fibres was localized over the superficial layers of the dorsal grey horn. Punctiform immunofluorescence was often found over the white matter especially in the funiculi dorsalis et lateralis. In the ventral horn, substance P immunoreactive fibres were few and far between in the grey matter and were only detected from foetal stage 16 weeks. In addition, longitudino-frontal sections through the dorsal regions revealed repetitive arrangements of substance P-like-immunoreactive fibres along the whole spinal cord. In dorsal root ganglia only a few immunoreactive cells were observed. These findings demonstrate the wide and early occurrence of substance P-like-immunoreactivity in the human foetus spinal cord and dorsal root ganglia. They suggest that the development of the substance P neuronal system begins early in ontogenesis and is regionally differentiated.     

Morphological properties of physiologically characterized lamina III neurones in the cat spinal cord

Six lamina III interneurones of the cat spinal cord were impaled and stained with intracellular injections of horseradish peroxidase. The responses of these neurones varied considerably: 1 neurone responded to light brushing of its receptive field, whilst 4 cells were excited by strong pressure. Morphologically, they were also a heterogeneous population. Two neurones had rostro-caudally orientated dendritic arbors that were confined to the lamina, while four of the cells were vertically orientated and possessed dendrites that crossed lamina boundaries. There was no correlation between the physiological characteristics of a neurone and its morphology. Three of the vertically orientated neurones were examined ultrastructurally. The first of these cells received several types of synaptic terminal which were distributed in an organised pattern over the entire dendritic tree. This neurone possessed recurrent dendrites which participated in a variety of complex synaptic arrangements. The second neurone also participated in a variety of synaptic arrangements, including glomeruli in lamina II, and received contacts from vesicle-containing dendrites. It gave rise to collateral axons which arborized in lamina II and formed boutons which formed synapses with dendrites. The third cell possessed varicose dendrites which were probably artifactual. It is concluded that lamina III interneurones are a heterogeneous population by electrophysiological, morphological and ultrastructural criteria. They differ in many respects from lamina II neurones and from the cells of origin of ascending systems. The diversity of their inputs and their variation in morphology suggests that they receive input from a variety of primary afferent fibres and dorsal horn neurones and hence may integrate information from these sources.     

Cortical afferents of the nucleus accumbens in the cat,studied with anterograde and retrograde transport techniques

The cortical afferentation of the nucleus accumbens in the cat was studied with the aid of retrograde tracing techniques. Retrograde experiments were carried out with horseradish peroxidase or one of the fluorescent tracers Bisbenzimid, Nuclear Yellow and Fast Blue. In the anterograde experiments [³H]leucine and [³⁵S]methionine were used as tracers.Following injections in the nucleus accumbens, retrogradely-labelled cells were found in the medial frontal cortex, the anterior olfactory nucleus, the posterior part of the insular cortex, the endopiriform nucleus, the amygdalo-hippocampal area, the entorhinal and perirhinal cortices and the subiculum of the hippocampal formation. In the medial frontal cortex most of the labelled cells were found in layers III and V of the prelimbic area (area 32 of Brodmann), but retrogradely-filled neurons were also present in the infralimbic area and in the caudoventral part of the lateral bank of the proreal gyrus. Retrogradely-labelled cells in the entorhinal and perirhinal cortices were located in the deep cellular layers. Following large injections in the nucleus accumbens, retrograde labelling in the subiculum extended from the most dorsal, septal pole to the most ventral, temporal pole.Injections of anterograde tracers were placed in the frontal cortex, the entorhinal and perirhinal cortices and the hippocampal formation. The prelimbic area was found to project via the internal capsule to mainly the rostral half of the nucleus accumbens, whereas in the caudal half of the nucleus only a lateral region receives frontal cortical fibres. Following injections in the infralimbic area only fibres passing through the nucleus accumbens were labelled. Afferents from the entorhinal and perirhinal cortices reach the nucleus accumbens by way of the external capsule and terminate mainly in a ventral zone of the nucleus accumbens.Afferents from the entorhinal area are distributed to the entire accumbens, whereas the termination field of the perirhinal afferents is largely restricted to the lateral part of the nucleus accumbens. Both the frontal cortex and the entorhinal and perirhinal cortices appear to project also to the nucleus caudatus and the tuberculum olfactorium. These cortical areas also project to the contralateral striatum.Both anterograde and retrograde tracing experiments demonstrated a topographical relationship between the subiculum and the nucleus accumbens. The ventral pole of the subiculum projects via the fornix to the medial part of the caudal half of the nucleus accumbens and to a small dorsomedial area in its rostral half. Successively more dorsal portions in the subiculum project to successively more ventrolateral parts in the rostral nucleus accumbens. The projection from the hippocampus was found to extend also to the tuberculum olfactorium. The results of the present study do not provide unambiguous criteria for the delimitation of the nucleus accumbens in the cat.     

The synaptic organization of terminals traced from individual labeled retino-geniculate axons in the cat

Individual retino-geniculate axons in the dorsal lateral geniculate nucleus of the cat were filled with horseradish peroxidase and studied with both the light and electron microscope. A procedure was followed which allowed us to identify the size, shape and arrangement of particular terminal swellings by light microscopy and then to study their patterns of synaptic contacts with the electron microscope. Many of the labeled terminals in laminae A and Al have the same fine structural features as retino-geniculate terminals that have been described previously. They are large, contain round synaptic vesicles and pale mitochondria, and are the central processes in glomeruli where they form asymmetric contacts with dendrites and terminals containing pleomorphic synaptic vesicles. Other terminals have the same cytological features but are quite small and are not the central processes in glomeruli. Some of these small terminals form simple axo-dendritec contacts while others participate in very large glomeruli containing several terminals from a single retino-geniculate axon. These different patterns of synaptic contacts made by different terminals can be found on branches of a single axon and correspond to the variations in terminal arbors described in the preceding paper (MASON & ROBSON, 1978).     

Density of optic terminals in half tecta of goldfish with compressed retinotectal projections

If the caudal half of the tectum in goldfish is removed, the projection from the entire retina is compressed onto the remaining half tectum. The percentage increase in density of optic terminals and synaptic contacts in such half tecta compared to that in tecta with normal projections ranged from 20–112% and 19–109% respectively. Both measures had similar percentage increases in each individual fish, with the proportion of terminals with more than one synaptic contact remaining constant.Tectal sites available for optic termination are flexible so that a higher density is allowed in response to increased optic innervation.     

Evidence for intrinsic expression of enkephalin-like immunoreactivity and opioid binding sites in cat superior colliculus

We have investigated the cellular localization of opioid peptides and binding sites in the cat's superior colliculus by testing the effects of retinal deafferentation and intracollicular excitotoxin lesions on patterns of enkephalin-like immunostaining and opiate receptor ligand binding. In normal cats, enkephalin-like immunoreactivity marks a thin tier in the most dorsal stratum griseum superficiale, small neurons of the stratum griseum superficiale, and patches of fibers in the intermediate and deeper gray layers. Eliminating crossed retinotectal afferents by contralateral eye enucleation had little immediate effect on this pattern, although chronic eye enucleation from birth did reduce immunoreactivity in the superficial layers. By contrast, fiber-sparing destruction of collicular neurons by the excitotoxins N-methyl-D-aspartate and ibotenic acid virtually eliminated enkephalin-like immunoreactivity in the neuropil of the upper stratum griseum superficiale, presumably by killing enkephalinergic cells of the superficial layers. Such lesions did not eliminate the patches of enkephalin-like immunoreactivity in the deeper layers. In normal cats, opiate receptor ligand binding is dense in the stratum griseum superficiale, particularly in its upper tier, and moderately dense in the intermediate gray layer. Contralateral eye removal had no detectable effect on the binding pattern, but excitotoxin lesions of the colliculus dramatically reduced binding in both superficial and deep layers. Some ligand binding, including part of that in the upper stratum griseum superficiale, apparently survived such lesions. Similar effects were observed in the lateral geniculate nucleus: enucleation produced no change in binding, whereas excitotoxin lesions greatly reduced specific opiate binding. We conclude that in the superficial collicular layers, both enkephalin-like opioid peptides and their membrane receptors are largely expressed by neurons of intrinsic collicular origin. The close correspondence between the location of these intrinsic opioid elements and the tier of retinal afferents terminating in the upper stratum griseum superficiale further suggests that opiatergic interneurons may modulate retinotectal transmission postsynaptically.     

Cells of origin of brainstem afferents to lobules I and II of the cerebellar anterior lobe in the cat

Cells of origin of the brainstem afferents to lobules I and II of the cerebellar anterior lobe were identified by means of the retrograde horseradish peroxidase technique. In order to avoid diffusion into other lobules, injections were made under direct visual control through the fourth ventricle, after having removed ventral parts of the posterior lobe.With clearcut localization, the major projections originated from neurons of the following nuclei; the pontine nuclei (dorsal to the lateral nucleus, and the lateral and dorsal part of the peduncular nucleus) and nucleus corporis pontobulbaris; vestibular nuclear complex (the superior, medial and descending vestibular nuclei and group x), nucleus of Martin and interstitial nucleus of the vestibular nerve; the ventrolateral part of the external cuneate nucleus; lateral reticular nucleus (mainly the parvocellular portion); the inferior olivary complex (the caudal and central parts of the medial accessory nucleus and the lateral part of the dorsal accessory nucleus at middle levels). Small projections originated from the paramedian reticular nucleus, prepositus hypoglossi nucleus, nuclei raphe obscurus and pallidus, and the gracile and main cuneate nuclei.It was suggested that lobules I and II function not only as representations of the hindlimb-tail regions but also of the neck region by receiving afferents from the central cervical nucleus and the ventrolateral part of the external cuneate nucleus that receives dorsal root afferents C1 to C4.     

Patterns of synaptic contact upon individually labeled large cells of the dorsal lateral geniculate nucleus of the cat

Four large geniculate cells that were densely filled with horseradish peroxidase have been studied in detail, at first light microscopically, to define the shape and distribution of the dendritic arbors, and then electron microscopically, to define the pattern of synaptic contacts upon their surfaces. Three of the cells had the morphological characteristics of a class 1 cell³, and one cell was intermediate between class 1 and 2. All of these cells had few of the characteristic grape-like appendages, thought to receive retinal input within synaptic glomeruli. Retinal afferents were mainly distributed around proximal juxtasomatic dendritic segments and dendritic branch points. Some retinal afferents made simple axodendritic contacts, while others formed a part of a synaptic glomerulus. None of the retinal afferents forming synapses near the perikaryon were involved in glomeruli. Within the glomeruli involving class 1 cells, retinal terminals can relate both to dendritic segments, particularly near branch points, as well as to dendritic appendages. The few singly placed grape-like appendages were always contacted by retinal afferents, but not necessarily in a glomerular arrangement. Terminals interpreted as cortico-geniculate were seen most commonly upon peripheral dendritic segments, while those containing pleomorphic vesicles (F) were distributed more or less evenly over all parts of the dendritic and perikaryal surface. The perikaryon itself was contacted by F-type terminals but was not contacted by retino-geniculate or cortico-geniculate terminals. A class of slender axonal terminal, containing round synaptic vesicles and few or no mitochondria, was found contacting two of the four perikarya, but the origin of these slender axons is unknown.It is concluded that the surfaces of geniculate class 1 cells can be separated into several functionally distinct zones on the basis of the synaptic contacts they receive.     

Retinotectal reorganization in goldfish—III. Effect of thyroxine

Reorganization of the goldfish retinotectal projection after ablation of half the tectum is dependent on lighting conditions and season. It occurs in summer but not in winter in normal lighting conditions. Constant light prevents reorganization in summer. Treatment with thyroxine at times when it does not normally occur results in some reorganization of the projection in a majority of cases. This consists of reduplicated receptive fields and partial compression. The ability of thyroxine to induce reorganization is not related to the amount of optic fibre sprouting it produces, suggesting that it may be acting on synapse formation.A hormonal influence on reorganization has been demonstrated and this may underlie the effects of season and lighting conditions, which are both known to be associated with changes in hormone levels in fish.