Eye-specific segregation of optic afferents in mammals,fish, and frogs: The role of activity

Eye-specific patches or stripes normally develop in the visual cortex and superior colliculus of many (but not all) mammals and are also formed, after surgically produced binocular innervation, in the optic tectum of fish and frogs. The segregation of ocular dominance patches or columns has been studied using a variety of anatomical pathway-tracing techniques, by electrophysiological recording of postsynaptic units or field potentials, and by the 2-deoxyglucose method following visual stimulation of only one eye. In the tectum of both fish and frogs and in the cortex and colliculus of mammals, eye-specific patches develop from initially diffuse, overlapping projections. Of the various mechanisms that might cause such segregation, the evidence favors an activity-dependent process that stabilizes synapses from the same eye because of their correlated activity. First, several environmental manipulations affect the segregation of afferents in visual cortex: strabismus and alternate monocular exposure apparently enhance segregation, whereas dark rearing slows the segregation process, and monocular deprivation causes the experienced eye to form larger patches at the expense of those of the deprived eye. Second, blocking activity in both eyes is effective in preventing the segregation both in the tectum of fish and frog and in the visual cortex of cat. With the eyes blocked, alternate stimulation of the optic nerves permits the segregation of ocular dominance, at least onto single cells in the cat visual cortex. These findings are discussed in terms of an activity-dependent stabilization of those synapses having correlated activity (those from neighboring ganglion cells within one eye) but not of those lacking correlated activity (those from left and right eyes). We suggest that the eye-specific patches represent a compromise between total segregation of the projections from the two eyes and the formation of a single continuous retinotopic map across the surface of the cortex or tectum.     

GABA-immunohistological observations, at the electron-microscopical level, of the neurons of isthmic nuclei in chicken, Gallus domesticus

Following a demonstration of Golgi-impregnated neurons and their terminal axon arborization in the optic tectum, the neurons of the nucleus parvocellularis and magnocellularis isthmi were studied by means of postembedded electron-microscopical (EM) γ-aminobutyric acid (GABA)-immunogold staining. In the parvocellular nucleus, none of the neuronal cell bodies or dendrites displayed GABA-like immunoreactivity in EM preparations stained by postembedded GABA-immunogold. However, numerous GABA-like immunoreactive and also unlabeled terminals established synapses with GABA-negative neurons. GABA-like immunoreactive terminals were usually found at the dendritic origin. Around the dendritic profiles, isolated synapses of both GABA-like immunoreactive and immunonegative terminals established glomerulus-like structures enclosed by glial processes. All giant and large neurons of the magnocellular nucleus of the isthmi displayed GABA-like immunoreactivity. Their cell surface was completely covered by GABA-like immunoreactive and unlabeled terminals that established synapses with the neurons. These neurons are thought to send axon collaterals to the parvocellular nucleus; their axons enter the tectum opticum. The morphological characteristics of neurons of both isthmic nuclei are like those of interneurons, because of their numerous axosomatic synapses with both asymmetrical and symmetrical features. These neurons are not located among their target neurons and exert their modulatory effect on optic transmission in the optic tectum at a distance.     

Activity-dependent tuning and the NMDA receptor

The refinement of the topographic map of visual space within the optic tectum of the frog is activity-dependent. The use of the three-eyed frog preparation to assay the operation of this fine-tuning mechanism indicates that this process is mediated by the NMDA receptor: Chronic in vivo treatment with APV, an NMDA antagonist, disrupts the segregation of retinal afferents into eye-specific zones while NMDA treatment sharpens this pattern. This latter effect is accompanied by a decreased sensitivity of the system to applied NMDA. Activation of the NMDA receptor may mediate the fine-tuning mechanism by initiating the stabilization of appropriate synapses. The requirements for NMDA receptor activation necessitate the convergence of terminals carrying correlated activity patterns. Such patterns of activity are provided by ganglion cells whose cell bodies lie near one another in the retina, and who should therefore, in an accurate visual map, terminate near one another in the tectum. Synapses from ganglion cells who do not neighbor one another in the retina have uncorrelated firing patterns and therefore do not activate the NMDA receptor. These synapses then would not be stabilized relative to one another. In addition to organizing the retinal projection, NMDA receptor activation may also modulate retinal ganglion cell arbor morphology, since chronic in vivo APV or NMDA treatments decrease arbor density. These results are discussed in terms of the effect of NMDA receptor activation on branch initiation and the rate of branch retraction.     

Interactions between tectal radial cells in the red-eared turtle, Pseudemys scripta elegans: an analysis of tectal modules.

The optic tectum is a major subdivision of the visual system in reptiles. Previous studies have characterized the laminar pattern, the neuronal populations, and the afferent and efferent connections of the optic tectum in a variety of reptiles. However, little is known about the interactions that occur between neurons within the tectum. This study describes two kinds of interactions that occur between one major class of neurons, the radial cells, in the optic tectum of Pseudemys using Nissl, Golgi and electron microscopic preparations. Radial cells have somata which bear long, radially oriented apical dendrites from their upper poles and short, basal dendrites from their lower poles. They are divided into two populations on the basis of the distribution of their somata in the tectum. Deep radial cells have somata densely packed in the stratum griseum periventriculare. Their plasma membranes form casual appositions. Middle radial cells have somata scattered throughout the stratum griseum centrale and stratum fibrosum et griseum superficiale and do not contact each other. The apical dendrites of both populations of radial cells participate in vertically oriented, dendritic bundles. The plasma membranes of the dendrites in these bundles form casual appositions in the deeper tectal layers and chemical, dendrodenritic synapses within the stratum fibrosum et griseum superficiale. The synapses have clear, round synaptic vesicles and slightly asymmetric membrane densities. Thus, radial cells interact via both casual appositions and chemical synapses. These interactions suggest that radial cells may form a basic framework in the tectum. Because both populations of radial cells extend into the stratum fibrosum et griseum superficiale and stratum opticum, they may receive input from some of the same tectal afferent systems. Because the deep radial cells alone have somata and dendrites in the deep tectal layers, they may receive additional inputs that the middle radial cells do not. Neurons in the two populations interact via chemical dendrodentritic synapses, thereby forming vertically oriented modules in the tectum.     

Spike-triggered dendritic calcium transients depend on synaptic activity in the cricket giant interneurons.

The relationship between electrical activity and spike-induced Ca2+ increases in dendrites was investigated in the identified wind-sensitive giant interneurons in the cricket. We applied a high-speed Ca2+ imaging technique to the giant interneurons, and succeeded in recording the transient Ca2+ increases (Ca2+ transients) induced by a single action potential, which was evoked by presynaptic stimulus to the sensory neurons. The dendritic Ca2+ transients evoked by a pair of action potentials accumulated when spike intervals were shorter than 100 ms. The amplitude of the Ca2+ transients induced by a train of spikes depended on the number of action potentials. When stimulation pulses evoking the same numbers of action potentials were separately applied to the ipsi- or contra-lateral cercal sensory nerves, the dendritic Ca2+ transients induced by these presynaptic stimuli were different in their amplitude. Furthermore, the side of presynaptic stimulation that evoked larger Ca2+ transients depended on the location of the recorded dendritic regions. This result means that the spike-triggered Ca2+ transients in dendrites depend on postsynaptic activity. It is proposed that Ca2+ entry through voltage-dependent Ca2+ channels activated by the action potentials will be enhanced by excitatory synaptic inputs at the dendrites in the cricket giant interneurons.     

Glutamate receptor binding in juvenile and adult Rana pipiens CNS

Autoradiographic methods were used to map NMDA and quisqualate-sensitive glutamate binding sites in the brain of mature and juvenile Rana pipiens frogs. NMDA and quisqualate-sensitive sites were consistently co-localized in the CNS. The highest glutamate binding occurred in the telencephalon, hypothalamus, and cerebellum. Glutamate binding sites were also specifical neuropil of the optic tectum, consistent with glutamate being the retinal ganglion cell neurotransmitter. The distribution of glutamate binding sites in the brain of juvenile postmate morphic frogs was similar to that in adults. In general, Quis binding increased about twofold in adults compared to juveniles, whereas NMDA binding did not show a comparable developmental increase. To test whether glutamate binding sites are located on retinal axon terminals or on tectal cell dendrites in the optic tectum, juvenile postmetamorphic frogs were enucleated unilaterally, and receptor binding was performed following 1, 3, 7, and 14 days survival. The denervated tectal neuropil showed a delayed decrease in NMDA-and quiequalate-sensitive binding, consistent with the receptors being located on postsynaptic tectal cell dendrites. © 1994 John Wiley & Sons, Inc.     

Axo-axonal synapses in the frog tectum opticum

A quantitative electron-microscopic investigation of synaptic endings in large sections showed that about 50% of all axo-axonal synapses are located in the outer zone of the neuropil (layer 9) of the tectum opticum ofRana temporaria L. These synapses are more numerous in the rostral part of the tectum than the caudal. Hardly any axo-axonal synapses lie deeper than 50–60 µ Most axo-axonal synapses are located on axon endings of retinal ganglionic cells, for after degeneration of the optic nerve the number of these synapses is reduced by two-thirds. During ontogenetic differentiation and regeneration of the optic nerve axo-axonal synapses develop before axo-dendritic and their presynaptic processes have the normal structure and differ sharply from the bulbs of growth of the optic fibers. On this basis the central origin of most presynaptic processes forming these synapses is postulated. The results point to the possibility of presynaptic control over the effectiveness of action of the efferent axons, primarily optic, terminating in the outer zone of the frog tectum opticum.     

Expression patterns of Ephs and ephrins throughout retinotectal development in Xenopus laevis

The Eph family of receptor tyrosine kinases and their ligands the ephrins play an essential role in the targeting of retinal ganglion cell axons to topographically correct locations in the optic tectum during visual system development. The African claw-toed frog Xenopus laevis is a popular animal model for the study of retinotectal development because of its amenability to live imaging and electrophysiology. Its visual system undergoes protracted growth continuing beyond metamorphosis, yet little is known about ephrin and Eph expression patterns beyond stage 39 when retinal axons first arrive in the tectum. We used alkaline phosphatase fusion proteins of EphA3, ephrin-A5, EphB2, and ephrin-B1 as affinity probes to reveal the expression patterns of ephrin-As, EphAs, ephrin-Bs, and EphBs, respectively. Analysis of brains from stage 40 to adult frog revealed that ephrins and Eph receptors are expressed throughout development. As observed in other species, staining for ephrin-As displayed a high caudal to low rostral expression pattern across the tectum, roughly complementary to the expression of EphAs. In contrast with the prevailing model, EphBs were found to be expressed in the tectum in a high dorsal to low ventral gradient in young animals. In animals with induced binocular tectal innervation, ocular dominance bands of alternating input from the two eyes formed in the tectum; however, ephrin-A and EphA expression patterns were unmodulated and similar to those in normal frogs, confirming that the segregation of axons into eye-specific stripes is not the consequence of a respecification of molecular guidance cues in the tectum.     

BDNF increases synapse density in dendrites of developing tectal neurons in vivo

Neuronal connections are established through a series of developmental events that involve close communication between pre- and postsynaptic neurons. In the visual system, BDNF modulates the development of neuronal connectivity by influencing presynaptic retinal ganglion cell (RGC) axons. Increasing BDNF levels in the optic tectum of Xenopus tadpoles significantly increases both axon arborization and synapse density per axon terminal within a few hours of treatment. Here, we have further explored the mechanisms by which BDNF shapes synaptic connectivity by imaging tectal neurons, the postsynaptic partners of RGCs. Individual neurons were co-labeled with DsRed2 and a GFP-tagged postsynaptic density protein (PSD95-GFP) to visualize dendritic morphology and postsynaptic specializations simultaneously in vivo. Immunoelectron microscopy confirmed that PSD95-GFP predominantly localized to ultrastructurally identified synapses. Time-lapse confocal microscopy of individual, double-labeled neurons revealed a coincident, activity-dependent mechanism of synaptogenesis and axon and dendritic arbor growth, which is differentially modulated by BDNF. Microinjection of BDNF into the optic tectum significantly increased synapse number in tectal neuron dendritic arbors within 24 hours, without significantly influencing arbor morphology. BDNF function-blocking antibodies had opposite effects. The BDNF-elicited increase in synapse number complements the previously observed increase in presynaptic sites on RGC axons. These results, together with the timescale of the response by tectal neurons, suggest that the effects of BDNF on dendritic synaptic connectivity are secondary to its effects on presynaptic RGCs. Thus, BDNF influences synaptic connectivity in multiple ways: it enhances axon arbor complexity expanding the synaptic territory of the axon, while simultaneously coordinating synapse formation and stabilization with individual postsynaptic cells.     

Target-Dependent Regulation of Retinal Nicotinic Acetylcholine Receptor and Tubulin RNAs During Optic Nerve Regeneration in Goldfish

A fundamental issue in central nervous system development regards the effect of target tissue on the differentiation of innervating neurons. We address this issue by characterizing the role the retinal ganglion cell target, i.e., the optic tectum, plays in regulating expression of tubulin and nicotinic acetylcholine receptor genes in regenerating retinal ganglion cells. Tubulins are involved in axonal growth, whereas nicotinic acetylcholine receptors mediate communication across synapses. Retinal ganglion cell axons were induced to regenerate by crushing the optic nerve. Following crush, there was a rapid increase in alpha-tubulin RNAs (3 days), which preceded the increase in nicotinic acetylcholine receptor RNAs (10-15 days). Both classes of RNAs approached control levels by the time retinotectal synapses and functional recovery were restored (4-6 weeks). If the optic nerve was repeatedly crushed or its target ablated, tubulin RNAs remained elevated, and the increase in receptor RNAs that would otherwise be seen 2 weeks after a single nerve crush did not occur. The interaction of retinal ganglion cell axons with their targets in the optic tectum appears, then, to exert a suppressive effect on the RNA encoding a cytoskeletal protein, tubulin, and an inductive effect on RNAs encoding nicotinic acetylcholine receptors involved in synaptic communication.     

Functional imaging reveals rapid development of visual response properties in the zebrafish tectum

The visual pathway from the retina to the optic tectum in fish and frogs has long been studied as a model for neural circuit formation. Although morphological aspects, such as axonal and dendritic arborization, have been well characterized, less is known about how this translates into functional properties of tectal neurons during development. We developed a system to provide controlled visual stimuli to larval zebrafish, while performing two-photon imaging of tectal neurons loaded with a fluorescent calcium indicator, allowing us to determine visual response properties in intact fish. In relatively mature larvae, we describe receptive field sizes, visual topography, and direction and size selectivity. We also characterize the onset and development of visual responses, beginning when retinal axons first arborize in the tectum. Surprisingly, most of these properties are established soon after dendrite growth and synaptogenesis begin and do not require patterned visual experience or a protracted period of refinement.     

A synaptic strategy for consolidation of convergent visuotopic maps

The mechanisms by which experience guides refinement of converging afferent pathways are poorly understood. We describe a vision-driven refinement of corticocollicular inputs that determines the consolidation of retinal and visual cortical (VC) synapses on individual neurons in the superficial superior colliculus (sSC). Highly refined corticocollicular terminals form 1-2 days after eye-opening (EO), accompanied by VC-dependent filopodia sprouting on proximal dendrites, and PSD-95 and VC-dependent quadrupling of functional synapses. Delayed EO eliminates synapses, corticocollicular terminals, and spines on VC-recipient dendrites. Awake recordings after EO show that VC and retina cooperate to activate sSC neurons, and VC light responses precede sSC responses within intervals promoting potentiation. Eyelid closure is associated with more protracted cortical visual responses, causing the majority of VC spikes to follow those of the colliculus. These data implicate spike-timing plasticity as a mechanism for cortical input survival, and support a cooperative strategy for retinal and cortical coinnervation of the sSC.     

Electrophysiological properties of isthmic neurons in frogs revealed by in vitro and in vivo studies

The frog nucleus isthmi (parabigeminal nucleus in mammals) is a visually responsive, cholinergic and anatomically well-defined group of neurons in the midbrain. It shares reciprocal topographic projections with the ipsilateral optic tectum (superior colliculus in mammals) and strongly influences visual processing. Anatomical and biochemical information indicates the existence of distinct neural populations within the frog nucleus isthmi, which raises the question: are there electrophysiological distinctions between neurons that are putatively classified by their anatomical and biochemical properties? To address this question, we measured frog nucleus isthmi neuron cellular properties in vitro and visual response properties in vivo. No evidence for distinct electrophysiological classes of neurons was found. We thus conclude that, despite the anatomical and biochemical differences, the cells of the frog nucleus isthmi respond homogeneously to both current injections and simple visual stimuli.     

Retinal neural progenitors express topographic map markers

Transplantation of neural stem cells for replacing neurons after neurodegeneration requires that the transplanted stem cells accurately reestablish the lost neural circuits in order to restore function. Retinal ganglion cell axons project to visual centers of the brain forming circuits in precise topographic order. In chick, dorsal retinal neurons project to ventral optic tectum, ventral neurons to dorsal tectum, anterior neurons to posterior tectum and posterior neurons to anterior tectum; forming a continuous point-to-point map of retinal cell position in the tectal projection. We found that when stem cells derived from ventral retina were implanted in dorsal host retina, the stem cells that became ganglion cells projected to dorsal tectum, appropriate for their site of origin in retina but not appropriate for their site of implant in retina. This led us to ask if retinal progenitors exhibit topographic markers of cell position in retina. Indeed, retinal neural progenitors express topographic markers: dorsal stem cells expressed more Ephrin B2 than ventral stem cells and, conversely, ventral stem cells expressed more Pax-2 and Ventroptin than dorsal stem cells. The fact that neural progenitors express topographic markers has pertinent implications in using neural stem cells in cell replacement therapy for replacing projecting neurons that express topographic order, e.g., analogous neurons of the visual, auditory, somatosensory and motor systems.     

Spike‐triggered dendritic calcium transients depend on synaptic activity in the cricket giant interneurons

The relationship between electrical activity and spike‐induced Ca²⁺ increases in dendrites was investigated in the identified wind‐sensitive giant interneurons in the cricket. We applied a high‐speed Ca²⁺ imaging technique to the giant interneurons, and succeeded in recording the transient Ca²⁺ increases (Ca²⁺ transients) induced by a single action potential, which was evoked by presynaptic stimulus to the sensory neurons. The dendritic Ca²⁺ transients evoked by a pair of action potentials accumulated when spike intervals were shorter than 100 ms. The amplitude of the Ca²⁺ transients induced by a train of spikes depended on the number of action potentials. When stimulation pulses evoking the same numbers of action potentials were separately applied to the ipsi‐ or contra‐lateral cercal sensory nerves, the dendritic Ca²⁺ transients induced by these presynaptic stimuli were different in their amplitude. Furthermore, the side of presynaptic stimulation that evoked larger Ca²⁺ transients depended on the location of the recorded dendritic regions. This result means that the spike‐triggered Ca²⁺ transients in dendrites depend on postsynaptic activity. It is proposed that Ca²⁺ entry through voltage‐dependent Ca²⁺ channels activated by the action potentials will be enhanced by excitatory synaptic inputs at the dendrites in the cricket giant interneurons. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 234–244, 2002; DOI 10.1002/neu.10032     

Alterations in the morphology of ganglion cell dendrites in the adult rat retina after optic nerve transection and grafting of peripheral nerve segments

Summary Transected ganglion cell axons from the adult retina are capable of reinnervating their central targets by growing into transplanted peripheral nerve (PN) segments. Injury of the optic nerve causes various metabolic and morphological changes in the retinal ganglion cell (RGC) perikarya and in the dendrites. The present work examined the dendritic trees of those ganglion cells surviving axotomy and of those whose severed axons re-elongated in PN grafts to reach either the superior colliculus (SC), transplanted SC, or transplanted autologous thigh muscle. The elaboration of the dendritic trees was visualized by means of the strongly fluorescent carbocyanine dye DiI, which is taken up by axons and transported to the cell bodies and from there to the dendritic branches. Alternatively, retinofugal axons regrowing through PN grafts were anterogradely filled from the eye cup with rhodamine B-isothiocyanate. The transection of the optic nerve resulted in characteristic changes in the ganglion cell dendrites, particularly in the degeneration of most of the terminal and preterminal dendritic branches. This occurred within the first 1 to 2 weeks following axotomy. The different types of ganglion cells appear to vary in their sensitivity to axotomy, as reflected by a rapid degeneration of certain cell dendrites after severance of the optic nerve. The most vulnerable cells were those with small perikarya and small dendritic fields (type II), whereas larger cells with larger dendritic fields (type I and III) were slower to respond and less dramatically affected. Regrowth of the lesioned axons in peripheral nerve grafts and reconnection of the retina with various tissues did not result in a significant immediate recovery of ganglion cell dendrites, although it did prevent some axotomized cells from further progression toward posttraumatic cell death.     

Active dendrites,potassium channels and synaptic plasticity

The dendrites of CA1 pyramidal neurons in the hippocampus express numerous types of voltage-gated ion channel, but the distributions or densities of many of these channels are very non-uniform. Sodium channels in the dendrites are responsible for action potential (AP) propagation from the axon into the dendrites (back-propagation); calcium channels are responsible for local changes in dendritic calcium concentrations following back-propagating APs and synaptic potentials; and potassium channels help regulate overall dendritic excitability. Several lines of evidence are presented here to suggest that back-propagating APs, when coincident with excitatory synaptic input, can lead to the induction of either long-term depression (LTD) or long-term potentiation (LTP). The induction of LTD or LTP is correlated with the magnitude of the rise in intracellular calcium. When brief bursts of synaptic potentials are paired with postsynaptic APs in a theta-burst pairing paradigm, the induction of LTP is dependent on the invasion of the AP into the dendritic tree. The amplitude of the AP in the dendrites is dependent, in part, on the activity of a transient, A-type potassium channel that is expressed at high density in the dendrites and correlates with the induction of the LTP. Furthermore, during the expression phase of the LTP, there are local changes in dendritic excitability that may result from modulation of the functioning of this transient potassium channel. The results support the view that the active properties of dendrites play important roles in synaptic integration and synaptic plasticity of these neurons.     

Identification and differential subcellular localization of the neuronal class C and class D L-type calcium channel alpha 1 subunits

To identify and localize the protein products of genes encoding distinct L-type calcium channels in central neurons, anti-peptide antibodies specific for the class C and class D alpha 1 subunits were produced. Anti-CNC1 directed against class C immunoprecipitated 75% of the L-type channels solubilized from rat cerebral cortex and hippocampus. Anti-CND1 directed against class D immunoprecipitated only 20% of the L-type calcium channels. Immunoblotting revealed two size forms of the class C L-type alpha 1 subunit, LC1 and LC2, and two size forms of the class D L-type alpha 1 subunit, LD1 and LD2. The larger isoforms had apparent molecular masses of approximately 200-210 kD while the smaller isoforms were 180-190 kD, as estimated from electrophoresis in gels polymerized from 5% acrylamide. Immunocytochemical studies using CNC1 and CND1 antibodies revealed that the alpha 1 subunits of both L-type calcium channel subtypes are localized mainly in neuronal cell bodies and proximal dendrites. Relatively dense labeling was observed at the base of major dendrites in many neurons. Staining in more distal dendritic regions was faint or undetectable with CND1, while a more significant level of staining of distal dendrites was observed with CNC1, particularly in the dentate gyrus and the CA2 and CA3 areas of the hippocampus. Class C calcium channels were concentrated in clusters, while class D calcium channels were generally distributed in the cell surface membrane of cell bodies and proximal dendrites. Our results demonstrate multiple size forms and differential localization of two subtypes of L-type calcium channels in the cell bodies and proximal dendrites of central neurons. The differential localization and multiple size forms may allow these two channel subtypes to participate in distinct aspects of electrical signal integration and intracellular calcium signaling in neuronal cell bodies. The preferential localization of these calcium channels in cell bodies and proximal dendrites implies their involvement in regulation of calcium-dependent functions occurring in those cellular compartments such as protein phosphorylation, enzyme activity, and gene expression.     

Control of Na+ spike backpropagation by intracellular signaling in the pyramidal neuron dendrites

The integrative function of neurons depends on the somato-dendritic distribution and properties of voltage-gated ion channels. Sodium, potassium, calcium, and hyperpolarization-activated cyclic nucleotide-gated K+ (HCN) channels expressed in the dendrites can be modulated by a number of neurotransmitters and second-messenger systems. For example, activation of protein kinases leads to an increase in dendritic excitability by removing a slow inactivation of Na+ channels and decreasing the activity of transient K+ channels in the apical dendrites of hippocampal pyramidal neurons. Consequently, action potentials propagating along the dendrites can be modified significantly by a variety of neuromodulatory synaptic inputs.     

Spatial Distribution of Excitatory Synapses on the Dendrites of Ganglion Cells in the Mouse Retina

Excitatory glutamatergic inputs from bipolar cells affect the physiological properties of ganglion cells in the mammalian retina. The spatial distribution of these excitatory synapses on the dendrites of retinal ganglion cells thus may shape their distinct functions. To visualize the spatial pattern of excitatory glutamatergic input into the ganglion cells in the mouse retina, particle-mediated gene transfer of plasmids expressing postsynaptic density 95-green fluorescent fusion protein (PSD95-GFP) was used to label the excitatory synapses. Despite wide variation in the size and morphology of the retinal ganglion cells, the expression of PSD95 puncta was found to follow two general rules. Firstly, the PSD95 puncta are regularly spaced, at 1–2 µm intervals, along the dendrites, whereby the presence of an excitatory synapse creates an exclusion zone that rules out the presence of other glutamatergic synaptic inputs. Secondly, the spatial distribution of PSD95 puncta on the dendrites of diverse retinal ganglion cells are similar in that the number of excitatory synapses appears to be less on primary dendrites and to increase to a plateau on higher branch order dendrites. These observations suggest that synaptogenesis is spatially regulated along the dendritic segments and that the number of synaptic contacts is relatively constant beyond the primary dendrites. Interestingly, we also found that the linear puncta density is slightly higher in large cells than in small cells. This may suggest that retinal ganglion cells with a large dendritic field tend to show an increased connectivity of excitatory synapses that makes up for their reduced dendrite density. Mapping the spatial distribution pattern of the excitatory synapses on retinal ganglion cells thus provides explicit structural information that is essential for our understanding of how excitatory glutamatergic inputs shape neuronal responses.