Structural and functional consequences of neonatal deafferentation in the superficial layers of the hamster's superior colliculus

Synapse maturation is enhanced in the binocular region of the retinocollicular map prior to eye opening

In the developing visual system of mammals, retinal axons from the two eyes compete for postsynaptic partners. After eye opening, this process is regulated in part by homeostatically constrained competition for synaptic connectivity with target neurons. However, prior to eye opening, the functional and synaptic basis of binocular map development is unclear. To examine the role of binocular interactions during early stages of visual map development, we performed in vitro patch-clamp recordings from the superior colliculus (SC) of neonatal mice. Using newly designed slice preparations, we compared retinocollicular synapse development in the medial SC, which receives binocular input, and the lateral SC, which is predominantly monocular. Surprisingly, we found that at P6-7, when eye-specific segregation has just emerged, retinocollicular synapses were stronger and more mature and dendritic arbors were more elaborate in the medial than the lateral SC. Furthermore, monocular enucleation of the ipsilateral eye at P0 selectively reduced synaptic strength and dendritic branching in the medial SC and abolished the differences normally observed between the two slices at P6-7. This specifically implicates binocular interactions in the development of retinocollicular connectivity prior to eye opening. Our findings contrast with the predictions of a constrained-connectivity model of binocular map development and suggest instead that binocular competition prior to eye opening enhances retinocollicular synaptic strength and the morphological development of retino-recipient neurons.

Somatosensory cross-modal plasticity in the superior colliculus of visually deafferented rats

The effects of neonatal visual deafferentation on the final adult pattern of cortico-collicular connections from the rat primary somatosensory cortex barrel field were studied by injecting an anterograde tracer (BDA) into different locations of the barrel cortex. Collicular afferents originating in the barrel cortex normally end in the intermediate collicular strata (SGI and SAI). However, neonatal visual deafferentation caused an invasion of abundant somatosensory cortical afferents into the lateral portions of the superficial collicular strata (SGS and SO). Moreover, anterograde-labelled fibers in the intermediate strata were more densely packed in visually deafferented animals. In order to study the activity of the altered somatosensory cortico-collicular connection, the effects of two different types of whisker stimuli on c-fos expression in the SC were analyzed (apomorphine treatment and enriched environment exploration). In stimulated control animals, c-fos expression was clearly evident in neurons of the intermediate layers 2 h after whisker stimulation. Similar stimulation in adult animals that underwent neonatal visual deafferentation triggered higher levels of c-fos expression in the superficial collicular layers that were invaded by cortico-collicular axonal branches. In exploration experiments, increased levels of c-fos expression were also detected in lateral parts of the intermediate layers of visually deafferented animals. These results suggest that the ascending fibers of somatosensory cortical origin can recruit deafferented superficial collicular neurons that enabling them to participate in extravisual behavioural responses mediated by collicular circuits.

Deprivation-induced cortical reorganization in children with cochlear implants

A basic finding in developmental neurophysiology is that some areas of the cortex cortical areas will reorganize following a period of stimulus deprivation. In this review, we discuss mainly electroencephalography (EEG) studies of normal and deprivation-induced abnormal development of the central auditory pathways in children and in animal models. We describe age cut-off for sensitive periods for central auditory development in congenitally deaf children who are fitted with a cochlear implant. We speculate on mechanisms of decoupling and reorganization which may underlie the end of the sensitive period. Finally, we describe new magentoencephalography (MEG) evidence of somatosensory cross-modal plasticity following long-term auditory deprivation.

Functional Topography of Converging Visual and Auditory Inputs to Neurons in the Rat Superior Colliculus

We have used a slice preparation of the infant rat midbrain to examine converging inputs onto neurons in the deeper multisensory layers of the superior colliculus (dSC). Electrical stimulation of the superficial visual layers (sSC) and of the auditory nucleus of the brachium of the inferior colliculus (nBIC) evoked robust monosynaptic responses in dSC cells. Furthermore, the inputs from the sSC were found to be topographically organized as early as the second postnatal week and thus before opening of the eyes and ear canals. This precocious topography was found to be sculpted by GABAA-mediated inhibition of a more widespread set of connections. Tracer injections in the nBIC, both in coronal slices as well as in hemisected brains, confirmed a robust projection originating in the nBIC with distinct terminals in the proximity of the cell bodies of dSC neurons. Combined stimulation of the sSC and nBIC sites revealed that the presumptive visual and auditory inputs are summed linearly. Finally, whereas either input on its own could manifest a significant degree of paired-pulse facilitation, temporally offset stimulation of the two sites revealed no synaptic interactions, indicating again that the two inputs function independently. Taken together, these data provide the first detailed intracellular analysis of convergent sensory inputs onto dSC neurons and form the basis for further exploration of multisensory integration and developmental plasticity.

Connection from the dorsal column nuclei to the superior colliculus in the rat: Topographical organization and somatotopic specific plasticity in response to neonatal enucleation

Somatosensory stimuli from the body to deep and intermediate strata of the superior colliculus (SC) are relayed from the dorsal column nuclei (DCN), gracile (GrN) and cuneate (CuN). Electrophysiological studies have shown that the somatosensory representation in SC is arranged into a map-like pattern. However, there is a lack of studies confirming a morphological correlate of such an organization. On the other hand, after neonatal enucleation in rodents, somatosensory inputs ascend from their normal termination territory in intermediate and deep collicular strata to invade the more dorsally located visual strata. However, the origin of these reactive afferents has not been specified. By using anterograde (biotinylated dextran amine 10,000; BDA) and retrograde (Fluoro-Gold; FG) tracers, we studied separately the connection from GrN and CuN to the intact and neonatally deafferented SC. GrN-collicular afferents were found to terminate mainly within the periphery of the caudomedial SC quadrant, whereas CuN-collicular fibers innervated primarily the lateral part of the rostrolateral and caudolateral collicular quadrants, in a way consistent with previously described functional data. Retrograde tracing experiments using FG injected in SC confirmed this topographical arrangement. Injections of BDA in GrN or CuN of neonatally enucleated rats showed that reactive fibers reaching superficial strata are only those CuN-collicular fibers innervating the caudolateral SC quadrant, where the forelimb is represented. The present results provide an anatomical substrate for the known somatotopic organization of tactile representation in SC and further reinforce the previous proposal that the plastic reorganization of DCN-collicular afferents following neonatal enucleation constitutes a functional compensatory response.

Potential role of endogenous brain-derived neurotrophic factor in long-term neuronal reorganization of the superior colliculus after bilateral visual deprivation

The role of the brain-derived neurotrophic factor (BDNF), the BDNF receptor (TrkB), and the glutamic acid decarboxylase (GAD67) after neonatal, bilateral nerve deafferentiation during postnatal development was investigated in the rat superior colliculus (SC). BDNF and GAD67 mRNA expression were significantly increased in optic (Op) and intermediate gray (InG) layers at 5, 8, 15, and 21 days after birth, but not in adult animals. However, TrkB mRNA expression was not modified at any time tested. At 15 days, where changes in BDNF and GAD67 mRNAs were more evident, an upregulation of the NMDAR(1A) mRNA glutamate receptor in the Op and InG, a modification in the pattern of synaptic zinc in the superficial layers of SC, and an increased synaptophysin immunoreactivity in the Op was found. This indicates the existence of a synergic mechanism between BDNF and NMDA to determine refinement of connections after the loss of visual input in SC.

Reorganization of the corticotectal projections introduced by neonatal monocular enucleation in the Monodelphis opossum and the influence of serotoninergic depletion

The influence of neonatal serotoninergic lesion (performed with s.c. injection of 5,7-dihydroxytryptamine) on the plasticity of the developing corticotectal projection was studied in the gray short-tailed opossum (Monodelphis domestica). As a first step, the placement and density of neurons projecting from the visual cortical areas to the superior colliculus was established in the adult opossum. Injections of retrogradely transported fluorescent dyes into the superior colliculus of intact three-month-old animals labeled neurons of cortical layer V. In this species, there are three visual areas: the striate area and two secondary areas, the laterally placed peristriate area and the medial visual area. The population of the labeled neurons was denser in peristriate and medial visual areas than in the striate area. Secondly, the influence of neonatal monocular enucleation on the extent of this projection was investigated, alone or in combination with a serotoninergic lesion. Injection of dyes into the superior colliculi of three-month-old animals that were unilaterally enucleated on the second postnatal day also labeled neurons of cortical layer V. However, the density of the cortical neurons projecting to the superior colliculus contralateral to the remaining eye was much lower. This reduction was most profound in the striate visual area. No significant modifications of this projection were found on the side ipsilateral to the remaining eye. In another group of opossums, unilateral enucleation on the second postnatal day was combined with serotoninergic lesion. Brains of some of the treated pups were immunostained for serotonin on the fifth postnatal day. At this age, 70-80% of serotoninergic axons in the brain were missing. However, in about three weeks these axons had regrown, and their density in the neocortex was approximately the same as in the control animals. We conclude that severe reduction of the serotoninergic innervation during the early postnatal period did not influence the plastic changes induced in the corticotectal projection by unilateral enucleation.

The corticotectal projection of the rat in vitro: development, anatomy and physiological characteristics

In this study, the formation of the corticotectal projection of the rat in organotypic slice culture was investigated, using both anatomical and physiological approaches. The establishment of fibre connections from visual cortex to superior colliculus explants was monitored after 3, 6, 14, 20 and 30 days in vitro by cortical injections of Dil. As in cortical cultures without cocultured colliculus, fibres anterogradely labelled by this procedure spread radially from the injection site into the surroundings of the explant, without displaying any directional preference. Especially, layer V pyramidal cells could be seen to extend processes not only to the collicular target, but also in the opposite direction, suggesting that no axonal guidance was exerted by the projection target. The total number of fibres projecting in the direction of the colliculus was not higher than of those projecting in the opposite direction. However, fibres projecting into the colliculus were significantly longer. This was also the case when the colliculus was placed next to the pial side of the cortical explant, indicating that outgrowth direction was not related to this observation. We therefore assume a chemotrophic rather than a chemotactic influence of the projection target on cortical axons, which is based on direct contact of axons to the target tissue. It cannot be excluded, however, that the failure to detect chemotactic guidance was caused by the lack of diffusion gradients in our culture system. Innervation of the collicular slice exclusively originated from layer V pyramidal cells, irrespective of the position of the collicular target. Fibre courses suggested that discrimination of the projection target was achieved upon encounter with the collicular surface by direct membrane contact. Inside the collicular tissue, fibre arborizations occurred preferredly in up to three layers perpendicular to the surface. Even after the smallest tracer injections, termination fields were diffusely distributed over the collicular slice. Also, the spatial distribution of retrogradely stained projection neurons did not differ statistically from an equal distribution. Thus, a high degree of convergence and divergence was observed anatomically in the corticotectal projection formed in vitro, corresponding to the immature state in vivo. The functionality of the corticotectal projection was assessed by intracellular recordings from collicular neurons. Electrophysiological properties, such as membrane potential (-68 +/- 11 mV), membrane resistance (35.4 +/- 27.7 M omega) and the time constant (3.0 +/- 2.1 ms) were comparable to reference values, confirming the viability of our culture preparation. The functionality of corticotectal transmission was revealed by intracellularly recorded responses of collicular cells to extracellular cortical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of eye enucleation on NADPH-diaphorase positive neurons in the superficial layers of the rat superior colliculus

Dihydronicotinamide adenine-dinucleotide phosphate diaphorase (NADPH-d) positive neurons in the superficial layers of superior colliculus (SC) were studied in the adult rat after eye enucleation at postnatal day 5 (P5). Bilaterally, NADPH-d histochemistry revealed either weakly or intensely labeled neurons. In the SC contralateral to the enucleation, the volume of superficial layers decreased significantly, whereas the total number of NADPH-d positive neurons was only slightly reduced, thus resulting in an increased cell density. Bilaterally, the number of NADPH-d positive neurons was around 20% of Nissl-stained neurons. While the number of neurons which were weakly positive for NADPH-d was unchanged contralateral to the enucleation (thus resulting in a significant increase in their percentage on the overall NADPH-d population), the number of intensely labeled neurons decreased by 30%. Intensely labeled neurons were classified with respect to cell size and dendritic distribution. Some (126) were reconstructed and analyzed on the computer, in order to quantitate morphological differences in dendritic distribution in the denervated and control SC. The percent of neurons which could be assigned to some classes (marginal, stellate, narrow field vertical and wide field vertical) was reduced contralateral to the enucleation. In addition, vertically-oriented neurons (narrow field vertical, wide field vertical and pyriform) showed a significant decrease in soma size, dendritic length and number of branch points. And finally, the overall orientation of dendrites on narrow and wide field vertical neurons was more dispersed, when compared to the control colliculus. Thus, P5 eye enucleation affects the adult morphology of NADPH-d positive neurons in the superficial layers of the rat SC, resulting in increased cell density, changed relative number of cells in each morphological type, and altered soma size, dendritic length and orientation in specific neurons.

Development of the hamster serotoninergic system: cell groups and diencephalic projections

Nuclei of the circadian visual system are extensively innervated by serotoninergic neurons and rhythmicity is modulated by the serotoninergic system. This study investigated the temporal relationships between prenatal origins of serotoninergic cell groups and perinatal innervation of structures in the hamster circadian visual system as well as in the remaining diencephalon. Serotonin-immunoreactive (5-HT-IR) neurons of the B4-B9 complex were first seen on embryonic day 8 (E8). The number of neurons increases sharply by E10 when the first 5-HT-IR cells are evident in the medulla (B1-B3 complex). The distribution of serotoninergic neurons in the hamster brainstem is generally adult-like by E14. Thick 5-HT-IR fibers arch around the mesencephalic flexure at E10 and reach more rostral mesencephalic areas at E11. A branch of the medial forebrain bundle (MFB) projects ventrally toward the retrochiasmatic area; a second branch ascends along the fasciculus retroflexus. Fibers cross the midline in the supraoptic commissure by E12, other arrive in the lateral geniculate region, and a branch of the MFB extends toward the mammillary area. At E13, a periventricular medial thalamic branch of the MFB is seen, axons appear in the supramammillary commissure, and a fine fasciculus between the medial thalamus and intergeniculate leaflet is visible. Lateral, paraventricular, and retrochiasmatic hypothalamic areas and centro- and dorsomedial thalamus are densely innervated at E14. The mammillary area and lateral geniculate body are moderately innervated, and the first fibers appear in the deep laminae of the superior colliculus. The innervation of the suprachiasmatic nuclei, periventricular hypothalamus, and superficial layers of the superior colliculus occurs postnatally. The results are consistent with serotoninergic system development in other species.

Immunochemical heterogeneity in the tecto-LP pathway of the rat

The projection from the rat's superior colliculus (SC) to the lateral posterior nucleus of the thalamus (LP) has previously been described as arising from a morphologically homogeneous population of neurons in the stratum opticum (SO). The present study combined immunocytochemistry with retrograde tracing and lesion techniques to determine whether or not the SC-->LP projection arose from neurons that were also neurochemically homogeneous. The combination of retrograde tracing and immunocytochemistry with an antibody directed against calbindin-D 28K (CBD) showed that 64.4% of the neurons that project from SC to LP contain this calcium-binding protein. Retrograde tracing and immunocytochemistry for adenosine deaminase (ADA) showed that a smaller number of tecto-LP cells (15.7%) were immunoreactive (IR) for this enzyme. Moreover, nearly all (93.0%) of the ADA-IR tecto-LP cells also contained CBD-IR. Adenosine deaminase-IR axons in LP were restricted to the dorsomedial portion of the nucleus and their density was substantially reduced after ablation of the ipsilateral superficial SC laminae. The lateral posterior nucleus contained numerous CBD-IR cells and fibers throughout its extent and it was thus difficult to determine the extent to which the extra-perikaryal CBD-IR in this nucleus was dependent upon the tecto-LP pathway. Nevertheless, destruction of the ipsilateral SC did reduce the density of CBD-IR in LP. These results suggest that the SC-->LP projection in rat arises from at least four neurochemically distinct cell groups: 1) those that contain CBD, 2) those that contain both CBD and ADA, 3) a very small population that contains only ADA, and 4) a group that is not recognized by either of these markers. Our results further suggest that ADA containing fibers may have a more restricted terminal distribution in LP than axons that contain only CBD.

Computer methods in neuroanatomy: determining mutual orientation of whole neuronal arbors

In most neurons orientation can be recognized because their arbors are more or less polarized and/or flattened. These are morphological characteristics of great functional importance. This paper deals with three-dimensional display and mathematical definition of orientation planes and vectors in whole arbors. An orientation plane can be derived from the flattest rectangular prism with which it is possible to enclose the arbor, or may be found by best-fit least square determination (based on all digitized points of the arbor). Both approaches allow description and comparison, in quantitative terms, of the orientation of neurons under various normal, pathological or experimental conditions.

Normal development and effects of deafferentation on the morphology of superior collicular neurons projecting to the lateral posterior nucleus in hamster

Visually responsive neurons in the superficial layers of the hamster's superior colliculus (SC) can be divided into distinct morphological and functional classes. In the preceding paper (Mooney et al., '91), we showed that neonatal enucleation has only slight and insignificant effects upon the structural characteristics of cells within a given class, but results in a significant reduction of neurons (narrow and widefield vertical cells) with dorsally directed dendritic arbors. In an effort to determine whether this change reflected differential transneuronal degeneration of these cell types or alterations in the dendritic arbors of surviving cells, this study re-examined this issue by restricting the analysis to a specific and relatively homogeneous subpopulation of superficial layer neurons, those that project to the lateral posterior nucleus (LP). Physiological recordings demonstrated that most (64.7%) tecto-LP cells in neonatally enucleated hamsters develop somatosensory receptive fields. The combination of retrograde tracing and injection of cells with Lucifer yellow in a fixed slice preparation demonstrated that nearly 75% of tecto-LP cells in normal adult hamsters are widefield vertical cells while less than 25% of the neurons filled in neonatally enucleated adults are in this class. Most of the tecto-LP cells in the neonatally enucleated adult hamsters were either horizontal cells (19.5%), giant stellate cells (24.6%), or had dendrites that were directed only toward the deep SC laminae (10.3%). Differential enucleation-induced cell death could not account for all of these changes. Tecto-LP neurons were retrogradely labelled with the carbocyanine dye, Di-I, in hamsters on postnatal day (P-) 0 (the day of birth) through P-10. As early as P-0, most retrogradely labelled neurons could be identified as either widefield (44.6%) or narrowfield (18.9%) vertical cells. These results, when considered together with those from the normal adult and neonatally enucleated adult hamsters, support the conclusion that neonatal eye removal results in a reorganizaton of the dendritic arbors of some collicular neurons that have already undergone considerable development at the time of the lesion.