Retinal Ganglion Cell Projections Research Articles

Temporal regulation of EphA4 in astroglia during murine retinal and optic nerve development

Eph receptors and their ephrin ligands play important roles in many aspects of visual system development. In this study, we characterized the spatial and temporal expression pattern of EphA4 in astrocyte precursor cell (APC) and astrocyte populations in the murine retina and optic nerve. EphA4 is expressed by immotile optic disc astrocyte precursor cells (ODAPS), but EphA4 is downregulated as these cells migrate into the retina. Surprisingly, mature astrocytes in the adult retina re-express EphA4. Within the optic nerve, EphA4 is expressed in specialized astrocytes that form a meshwork at the optic nerve head (ONH). Our in vitro and in vivo data indicate that EphA4 is dispensable for retinal ganglion cell (RGC) axon growth and projections through the chiasm. While optic stalk structure, APC proliferation and migration, retinal vascularization, and oligodendrocyte migration appear normal in EphA4 mutants, the expression of EphA4 in APCs and in the astrocyte meshwork at the ONH has implications for optic nerve pathologies.

Semaphorin 3d guides laterality of retinal ganglion cell projections in zebrafish

The optic chiasm is an important choice point at which retinal ganglion cell (RGC) axons either cross the midline to innervate the contralateral brain or turn back to innervate the ipsilateral brain. Guidance cues that regulate this decision, particularly those directing the midline crossing of contralateral axons, are still not well understood. Here we show that Sema3d, a secreted semaphorin expressed at the midline, guides the crossing of RGC axons in zebrafish. Both Sema3d knockdown and ubiquitous overexpression induced aberrant ipsilateral projections, suggesting that Sema3d normally guides axons into the contralateral optic tract. Live imaging in vivo showed that RGC growth cones responded to ubiquitous Sema3d overexpression by pausing for extended periods and increasing their exploratory behavior at the midline, suggesting that Sema3d overexpression causes the midline environment to become less favorable for RGC axon extension. Interestingly, Sema3d overexpression did not affect growth cone behaviors before the midline, suggesting that RGC axons normally respond to Sema3d only upon reaching the midline. After Sema3d knockdown, growth cones grew across the midline but then paused or repeatedly retracted, impairing their ability to leave the midline region. Our results indicate that a proper balance of Sema3d is needed at the midline for the progression of RGC axons from the chiasm midline into the contralateral optic tract.

Brn3a-Expressing Retinal Ganglion Cells Project Specifically to Thalamocortical and Collicular Visual Pathways

Retinal ganglion cells (RGCs) innervate several specific CNS targets serving cortical and subcortical visual pathways and the entrainment of circadian rhythms. Recent studies have shown that retinal ganglion cells express specific combinations of POU- and LIM-domain transcription factors, but how these factors relate to the subsequent development of the retinofugal pathways and the functional identity of RGCs is mostly unknown. Here, we use targeted expression of an genetic axonal tracer, tau/beta-galactosidase, to examine target innervation by retinal ganglion cells expressing the POU-domain factor Brn3a. Brn3a is expressed in RGCs innervating the principal retinothalamic/retinocollicular pathway mediating cortical vision but is not expressed in RGCs of the accessory optic, pretectal, and hypothalamic pathways serving subcortical visuomotor and circadian functions. In the thalamus, Brn3a ganglion cell fibers are primarily restricted to the outer shell of the dorsal lateral geniculate, providing new evidence for the regionalization of this nucleus in rodents. Brn3a RGC axons have a relative preference for the contralateral hemisphere, but known mediators of the laterality of RGC axons are not repatterned in the absence of Brn3a. Brn3a is coexpressed extensively with the closely related factor Brn3b in the embryonic retina, and the effects of the loss of Brn3a in retinal development are not severe, suggesting partial redundancy of function in this gene class.

Ephrin-As and neural activity are required for eye-specific patterning during retinogeniculate mapping

In mammals, retinal ganglion cell (RGC) projections initially intermingle and then segregate into a stereotyped pattern of eye-specific layers in the dorsal lateral geniculate nucleus (dLGN). Here we found that in mice deficient for ephrin-A2, ephrin-A3 and ephrin-A5, eye-specific inputs segregated but the shape and location of eye-specific layers were profoundly disrupted. In contrast, mice that lacked correlated retinal activity did not segregate eye-specific inputs. Inhibition of correlated neural activity in ephrin mutants led to overlapping retinal projections that were located in inappropriate regions of the dLGN. Thus, ephrin-As and neural activity act together to control patterning of eye-specific retinogeniculate layers.

Loss of Circadian Photoentrainment and Abnormal Retinal Electrophysiology inMath5Mutant Mice

To determine how the absence of retinal ganglion cells (RGCs) in Math5 (Atoh7) mutant mice affects circadian behavior and retinal function. The wheel-running behavior of wild-type and Math5 mutant mice was measured under various light-dark cycle conditions. To evaluate retinal input to the suprachiasmatic nuclei (SCN) anatomically, the retinohypothalamic tracts were labeled in vivo. To assess changes in retinal function, corneal flash electroretinograms (ERGs) from mutant and wild-type mice were compared under dark- and light-adapted conditions. Alterations in retinal neuron populations were evaluated quantitatively and with cell-type-specific markers. The Math5-null mice did not entrain to light and exhibited free-running circadian behavior with a mean period (23.6 +/- 0.15 hours) that was indistinguishable from that of wild-type mice (23.4 +/- 0.19 hours). The SCN showed no anterograde labeling with a horseradish peroxidase-conjugated cholera toxin B (CT-HRP) tracer. ERGs recorded from mutant mice had diminished scotopic a- and b-wave and photopic b-wave amplitudes. The scotopic b-wave was more severely affected than the a-wave. The oscillatory potentials (OPs) and scotopic threshold response (STR) were also reduced. Consistent with these ERG findings, a pan-specific reduction in the number of bipolar cells and a smaller relative decrease in the number of rods in mutant mice were observed. Math5-null mice are clock-blind and have no RGC projections to the SCN. RGCs are thus essential for photoentrainment in mice, but are not necessary for the development or intrinsic function of the SCN clock. RGCs are not required to generate any of the major ERG waveforms in mice, including the STR, which is produced by ganglion cells in some other species. The diminished amplitude of b-wave, OPs, and STR components in Math5 mutants is most likely caused by the decreased abundance of retinal interneurons.

The Rod Photoreceptor Pattern Is Set at the Optic Vesicle Stage and Requires Spatially RestrictedcVaxExpression

How and when positional identities in the neural retina are established have been addressed primarily with respect to the topographic projections of retinal ganglion cells onto their targets in the brain. Although retinotectal map formation is a prominent manifestation of retinal patterning, it is not the only one. Photoreceptor subtypes are arranged in distinct, species-specific patterns. The mechanisms used to establish photoreceptor patterns have been relatively unexplored at the mechanistic level. We performed ablations of the eye anlage in chickens and found that removal of the anterior or dorsal optic vesicle caused loss of the area centralis, which is a rod-free central area of the retina, and severely disorganized other aspects of the rod pattern. These observations indicate that the anteroposterior and dorsoventral distribution of rods is determined by the optic vesicle stage. To investigate the molecular mechanisms involved, the rod distribution was analyzed after viral misexpression of several patterning genes that were previously shown to be important in positional specification of retinal ganglion cells. Ectopic expression of FoxG1, SOHo1,or GH6 transcription factors expressed in the anterior optic vesicle and/or optic cup, respectively, did not affect the rod pattern. This pattern therefore appears to be specified by an activity acting before, or in parallel with, these factors. In contrast, misexpression of the ventrally restricted transcription factor, cVax, severely disturbed the rod pattern.

ERK signaling is required for eye-specific retino-geniculate segregation.

In the mammalian visual system, retinal ganglion cell (RGC) projections from each eye, initially intermixed within the dorsal-lateral geniculate nucleus (dLGN), become segregated during the early stages of development, occupying distinct eye-specific layers. Electrical activity has been suggested to play a role in this process; however, the cellular mechanisms underlying eye-specific segregation are not yet defined. It is known that electrical activity is among the strongest activators of the extracellular signal-regulated kinase (ERK) pathway. Moreover, the ERK pathway is involved in the plasticity of neural connections during development. We examine the role of ERK in the segregation of retinal afferents into eye-specific layers in the dLGN. The activation of this signaling cascade was selectively blocked along the retino-thalamic circuitry by specific inhibitors, and the distribution of RGC fibers in the dLGN was studied. Our results demonstrate that the blockade of ERK signaling prevents eye-specific segregation in the dLGN, providing evidence that ERK pathway is required for the proper development of retino-geniculate connections. Of particular interest is the finding that ERK mediates this process both at the retinal and geniculate level.

The winged helix transcription factor Foxg1 facilitates retinal ganglion cell axon crossing of the ventral midline in the mouse.

During normal development, retinal ganglion cells (RGCs) project axons along the optic nerve to the optic chiasm on the ventral surface of the hypothalamus. In rodents, most RGC growth cones then cross the ventral midline to join the contralateral optic tract; those that do not cross join the ipsilateral optic tract. Contralaterally projecting RGCs are distributed across the retina whereas ipsilaterally projecting RGCs are concentrated in temporal retina. The transcription factor Foxg1 (also known as BF1) is expressed at several key locations along this pathway. Analysis of Foxg1 expression using lacZ reporter transgenes shows that Foxg1 is normally expressed in most, if not all, nasal RGCs but not in most temporal RGCs, neither at the time they project nor earlier in their lineage. Foxg1 is also expressed at the optic chiasm. Mice that lack Foxg1 die at birth and, although the shape of their eyes is abnormal, their retinas still project axons to the brain via the optic chiasm. Using anterograde and retrograde tract tracing, we show that there is an eightfold increase in the ipsilateral projection in Foxg1-/- embryos. The distributions of cells expressing the transcription factors Foxg1 and Nkx2.2, and cell-surface molecules Ephb2, ephrin B2 and SSEA-1 (Fut4) have been correlated to the normally developing retinothalamic projection and we show they are not much altered in the developing Foxg1-/- retina and optic chiasm. As much of the increased ipsilateral projection in Foxg1-/- embryos arises from temporal RGCs that are unlikely to have an autonomous requirement for Foxg1, we propose that the phenotype reflects at least in part a requirement for Foxg1 outwith the RGCs themselves, most likely at the optic chiasm.

Mutations affecting retinotectal axonal pathfinding in Medaka, Oryzias latipes

We screened for mutations affecting retinotectal axonal projection in Medaka, Oryzias latipes. In wild-type Medaka embryos, all the axons of retinal ganglion cells (RGCs) project to the contralateral tectum, such that the topological relationship of the retinal field is maintained. We labeled RGC axons using DiI/DiO at the nasodorsal and temporoventral positions of the retina, and screened for mutations affecting the pattern of stereotypic projections to the tectum. By screening 184 mutagenized haploid genomes, seven mutations in five genes causing defects in axonal pathfinding were identified, whereas mutations affecting the topographic projection of RGC axons were not found. The mutants were grouped into two classes according to their phenotypes. In mutants of Class I, a subpopulation of the RGC axons branched out either immediately after leaving the eye or after reaching the midline, and this axonal subpopulation projected to the ipsilateral tectum. In mutants of Class II, subpopulations of RGC axons branched out after crossing the midline and projected aberrantly. These mutants will provide clues to understanding the functions of genes essential for axonal pathfinding, which may be conserved or partly divergent among vertebrates.

Crossed and uncrossed retinal projections to the hamster circadian system.

The hamster suprachiasmatic nucleus (SCN), site of the circadian clock, has been thought to be equally and completely innervated by each retina. This issue was studied in animals that had received an injection of the tracer cholera toxin subunit B (CTb) conjugated to Alexa 488 into the vitreous of one eye, with CTb-Alexa 594 injected into the other. Retinal projections to the SCN and other nuclei of the circadian system were simultaneously evaluated by using confocal laser microscopy. Each retina provides completely overlapping terminal fields throughout each SCN. Although SCN innervation by the contralateral retina is slightly denser than that from the ipsilateral retina, there are distinct SCN regions where input from one side is predominant, but not exclusive. A dense terminal field from the contralateral retina encompasses, and extends dorsally beyond, the central SCN subnucleus identified by calbindin-immunoreactive neurons. Surrounding the dense terminal field, innervation is largely derived from the ipsilateral retina. The densest terminal field in the intergeniculate leaflet is from the contralateral retina, which completely overlaps the ipsilateral projection. Most nuclei of the pretectum receive innervation largely, but not solely, from the contralateral retina, although the olivary pretectal nucleus has very dense patches of innervation derived exclusively from one retina or the other. Retina-dependent variation in terminal field density within the three closely examined nuclei may indicate areas of specialized function not previously appreciated. This issue is discussed in the context of the melanopsin-containing retinal ganglion cell projections to several nuclei in the circadian visual system.

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus, intergeniculate leaflet, and visual midbrain: bifurcation and melanopsin immunoreactivity.

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.

Adenylate Cyclase 1 as a Key Actor in the Refinement of Retinal Projection Maps

cAMP occupies a strategic position to control neuronal responses to a large variety of developmental cues. We have analyzed the role of calcium-stimulated adenylate cyclase 1 (AC1) in the development of retinal topographic maps. AC1 is expressed in retinal ganglion cells (RGCs) from embryonic day 15 to adulthood with a peak during the first postnatal week. At that time, the other calcium-stimulated AC, AC8, is expressed in the superior colliculus (SC) but not in the RGCs. In mice of the barrelless strain, which carry an inactivating mutation of the AC1 gene, calcium-stimulated AC activity is reduced by 40-60% in the SC and retina. RGC projection maps were analyzed with a variety of anterograde and retrograde tracers. After an initially normal development until postnatal day 3, retinal fibers from the ipsilateral and contralateral eye fail to segregate into eye-specific domains in the lateral geniculate nucleus and the SC. Topographic defects in the fine tuning of the retinotectal and retinogeniculate maps are also observed with abnormalities in the confinement of the retinal axon arbors in the anteroposterior and mediolateral dimensions. This is attributable to the lack of elimination of misplaced axon collaterals and to the maintenance of a transient ipsilateral projection. These results establish an essential role of AC1 in the fine patterning of the retinal map. Calcium-modulated cAMP production in the RGCs could constitute an important link between activity-dependent changes and the anatomical restructuring of the retinal terminal arbors within central targets.

Excitatory mechanisms in the suprachiasmatic nucleus: the role of AMPA/KA glutamate receptors.

A variety of evidence suggests that the effects of light on the mammalian circadian system are mediated by direct retinal ganglion cell projection to the suprachiasmatic nucleus (SCN). This synaptic connection is glutamatergic and the release of glutamate is detected by both N-methyl-D-asparate (NMDA) and amino-methyl proprionic acid/kainate (AMPA/KA) iontotropic glutamate receptors (GluRs). It is well established that NMDA GluRs play a critical role in mediating the effects of light on the circadian system; however, the role of AMPA/KA GluRs has received less attention. In the present study, we sought to better understand the contribution of AMPA/KA-mediated currents in the circadian system based in the SCN. First, whole cell patch-clamp electrophysiological techniques were utilized to measure spontaneous excitatory postsynaptic currents (sEPSCs) from SCN neurons. These currents were widespread in the SCN and not just restricted to the retino-recipient region. The sEPSC frequency and amplitude did not vary with the daily cycle. Similarly, currents evoked by the exogenous application of AMPA onto SCN neurons were widespread within the SCN and did not exhibit a diurnal rhythm in their magnitude. Fluorometric techniques were utilized to estimate AMPA-induced calcium (Ca(2+)) concentration changes in SCN neurons. The resulting data indicate that AMPA-evoked Ca(2+) transients were widespread in the SCN and that there was a daily rhythm in the magnitude of AMPA-induced Ca(2+) transients that peaked during the night. By itself, blocking AMPA/KA GluRs with a receptor blocker decreased the spontaneous firing of some SCN neurons as well as reduced resting Ca(2+) levels, suggesting tonic glutamatergic excitation. Finally, immunohistochemical techniques were used to describe expression of the AMPA-preferring GluR subunits GluR1 and GluR2/3s within the SCN. Overall, our data suggest that glutamatergic synaptic transmission mediated by AMPA/KA GluRs play an important role throughout the SCN synaptic circuitry.

Vax2 inactivation in mouse determines alteration of the eye dorsal-ventral axis, misrouting of the optic fibres and eye coloboma.

Vax2 is a homeobox gene whose expression is confined to the ventral region of the prospective neural retina. Overexpression of this gene at early stages of development in Xenopus and in chicken embryos determines a ventralisation of the retina, thus suggesting its role in the molecular pathway that underlies eye development. We describe the generation and characterisation of a mouse with a targeted null mutation of the Vax2 gene. Vax2 homozygous mutant mice display incomplete closure of the optic fissure that leads to eye coloboma. This phenotype is not fully penetrant, suggesting that additional factors contribute to its generation. Vax2 inactivation determines dorsalisation of the expression of mid-late (Ephb2 and Efnb2) but not early (Pax2 and Tbx5) markers of dorsal-ventral polarity in the developing retina. Finally, Vax2 mutant mice exhibit abnormal projections of ventral retinal ganglion cells. In particular, we observed the almost complete absence of ipsilaterally projecting retinal ganglion cells axons in the optic chiasm and alteration of the retinocollicular projections. All these findings indicate that Vax2 is required for the proper closure of the optic fissure, for the establishment of a physiological asymmetry on the dorsal-ventral axis of the eye and for the formation of appropriate retinocollicular connections.

Asymmetrical Activation of Human Visual Cortex Demonstrated by Functional MRI with Monocular Stimulation

We have demonstrated asymmetric activation patterns in the visual cortices of normal humans who have undergone functional MRI with monocular photic stimulation. The contralateral hemisphere is activated more strongly and to a greater spatial extent than the ipsilateral hemisphere when either eye is stimulated. This asymmetry can be explained by nasotemporal asymmetries which have been described in anatomical studies of the visual system in primates and humans. In part, the representation of the monocular crescent of the temporal hemifield of either eye, which exists only in the crossed projection, may explain this. In addition, within the binocular field, there is a biased crossed projection of nasal retinal ganglion cells which drive the contralateral ocular dominance columns in V1. Finally, the blind spot representation in the ipsilateral visual cortex may also contribute to the observed asymmetries. Our study may in effect provide a functional correlate of the anatomical asymmetries that have been observed in humans and animals.

Slit Inhibition of Retinal Axon Growth and Its Role in Retinal Axon Pathfinding and Innervation Patterns in the Diencephalon

We have analyzed the role of the Slit family of repellent axon guidance molecules in the patterning of the axonal projections of retinal ganglion cells (RGCs) within the embryonic rat diencephalon and whether the slits can account for a repellent activity for retinal axons released by hypothalamus and epithalamus. At the time RGC axons extend over the diencephalon, slit1 and slit2 are expressed in hypothalamus and epithalamus but not in the lateral part of dorsal thalamus, a retinal target. slit3 expression is low or undetectable. The Slit receptors robo2, and to a limited extent robo1, are expressed in the RGC layer, as are slit1 and slit2. In collagen gels, axon outgrowth from rat retinal explants is biased away from slit2-transfected 293T cells, and the number and length of axons are decreased on the explant side facing the cells. In addition, in the presence of Slit2, overall axon outgrowth is decreased, and bundles of retinal axons are more tightly fasciculated. This action of Slit2 as a growth inhibitor of retinal axons and the expression patterns of slit1 and slit2 correlate with the fasciculation and innervation patterns of RGC axons within the diencephalon and implicate the Slits as components of the axon repellent activity associated with the hypothalamus and epithalamus. Our findings suggest that in vivo the Slits control RGC axon pathfinding and targeting within the diencephalon by regulating their fasciculation, preventing them or their branches from invading nontarget tissues, and steering them toward their most distal target, the superior colliculus.

T Differential Effects of Acetylcholine and Glutamate Blockade on the Spatiotemporal Dynamics of Retinal Waves

In the immature vertebrate retina, neighboring ganglion cells express spontaneous bursting activity (SBA), resulting in propagating waves. Previous studies suggest that the spontaneous bursting activity, asynchronous between the two eyes, controls the refinement of retinal ganglion cell projections to central visual targets. To understand how the patterns encoded within the waves contribute to the refinement of connections in the visual system, it is necessary to understand how wave propagation is regulated. We have used video-rate calcium imaging of spontaneous bursting activity in chick embryonic retinal ganglion cells to show how glutamatergic and cholinergic connections, two major excitatory synaptic drives involved in spontaneous bursting activity, contribute differentially to the spatiotemporal patterning of the waves. During partial blockade of cholinergic connections, cellular recruitment declines, leading to spatially more restricted waves. The velocity of wave propagation decreases during partial blockade of glutamatergic connections, but cellular recruitment remains substantially higher than during cholinergic blockade, thereby altering correlations in the activity of neighboring and distant ganglion cells. These findings show that cholinergic and glutamatergic connections exert different influences on the spatial and temporal properties of the waves, raising the possibility that they may play distinct roles during visual development.

Extrinsic Modulation of Retinal Ganglion Cell Projections: Analysis of the Albino Mutation in Pigmentation Mosaic Mice

Tyrosinase is a key enzyme involved in the synthesis of melanin in the retinal pigment epithelium (RPE). Mice that are homozygous for the albino allele at the tyrosinase locus have fewer retinal ganglion cells with uncrossed projections at the optic chiasm. To determine the site of the albino gene action we studied the projections of retinal ganglion cells in two types of pigmentation mosaic mice. First, we generated mosaic mice that contain a translocated allele of the wild-type tyrosinase on one X chromosome but that also have the lacZ reporter transgene on the opposite X chromosome. In these lacZ/tyrosinase mice, which are homozygous for the albino allele on chromosome 7, X-inactivation ensures that tyrosinase cannot be functional within 50% of the retinal ganglion cells and that these individual cells can be identified by their expression of the lacZ reporter gene product, β-galactosidase. The proportion of uncrossed retinal ganglion cells expressing β-galactosidase was found to be identical to the proportion that did not express it, indicating that the albino mutation associated with axonal behavior at the optic chiasm must affect ganglion cells in a cell-extrinsic manner. Second, to determine whether the RPE is the source of the extrinsic signal, we generated aggregation chimeras between pigmented and albino mice. In these mosaic mice, the extent of the uncrossed projection corresponded with the amount of pigmented cells within the RPE, but did not correspond with the genotypes of neural retinal cells. These studies demonstrate that the albino mutation acts indirectly upon retinal ganglion cells, which in turn respond by making axonal guidance errors at the optic chiasm.

Dynamics of primate P retinal ganglion cells: responses to chromatic and achromatic stimuli.

1. The majority of primate retinal ganglion cells (RGCs) project to the parvocellular layers of the lateral geniculate nucleus (LGN). These P cells play a central role in early visual processing. 2. An improved method of systems analysis has allowed us to explore the dynamics of the colour-opponent subregions of P-cell receptive fields with a single chromatic stimulus. The data show that the centre and surround subregions of the P-cell receptive field have similar temporal responses, but the surround is slightly delayed. The centre and surround demonstrate a large degree of chromatic selectivity. 3. The responses of the centre and surround subregions were fitted with a linear model and the model was used to predict the responses of P cells to new chromatic and achromatic stimuli. Although linear models predict the chromatic responses well, simple linear combinations of centre and surround responses fail to predict P-cell responses to achromatic stimuli. 4. The temporal responses of the different subpopulations of P cells, such as ON/OFF or L-centre/M-centre were not significantly different.

Development and regulation of substance P expression in neurons of the tadpole optic tectum.

Activity-dependent synaptic plasticity is characteristic of developing visual systems. Using the frog retinotectal system, we investigated the extent to which afferent input affects neurotransmitter expression in a target structure. We have concentrated on a particular subpopulation of tectal cells that is immunoreactive to substance P (SP). Early in development, SP expression in tectal neurons was restricted to the anterior lateral region of the tectum. As tadpoles developed, this expression expanded into progressively more posterior and medial regions in a manner that closely followed the gradient of tectal maturation. At all times, however, anterior and lateral tectal regions had a greater percent of SP-like immunoreactive (SP-ir) cells than posterior and medial ones. Horseradish peroxidase (HRP) labeling of the retinal ganglion cell projection in conjunction with SP immunocytochemistry demonstrated that innervation by retinal ganglion cell terminals preceded the expression of SP by tectal cells. This suggested that the optic nerve may influence SP differentiation and/or expression. In support of this idea, transection of the optic nerve resulted in a decrease in SP expression in the deafferented tectal lobe of tadpoles. This result, opposite to that seen previously in the adult, also indicates that optic nerve-dependent regulation of SP expression in the developing and mature systems occurs through different pathways.