Injury to Cone Synapses by Retinal Detachment: Differences from Rod Synapses and Protection by ROCK Inhibition

Attachment of a detached retina does not always restore vision to pre-injury levels, even if the attachment is anatomically successful. The problem is due in part to long-term damage to photoreceptor synapses. Previously, we reported on damage to rod synapses and synaptic protection using a Rho kinase (ROCK) inhibitor (AR13503) after retinal detachment (RD). This report documents the effects of detachment, reattachment, and protection by ROCK inhibition on cone synapses. Conventional confocal and stimulated emission depletion (STED) microscopy were used for morphological assessment and electroretinograms for functional analysis of an adult pig model of RD. RDs were examined 2 and 4 h after injury or two days later when spontaneous reattachment had occurred. Cone pedicles respond differently than rod spherules. They lose their synaptic ribbons, reduce invaginations, and change their shape. ROCK inhibition protects against these structural abnormalities whether the inhibitor is applied immediately or 2 h after the RD. Functional restoration of the photopic b-wave, indicating cone-bipolar neurotransmission, is also improved with ROCK inhibition. Successful protection of both rod and cone synapses with AR13503 suggests this drug will (1) be a useful adjunct to subretinal administration of gene or stem cell therapies and (2) improve recovery of the injured retina when treatment is delayed.

Coming of Age for the Photoreceptor Synapse

Purpose

To discuss the potential contribution of rod and cone synapses to the loss of visual function in retinal injury and disease.

Methods

The published literature and the authors' own work were reviewed.

Results

Retinal detachment is used as a case study of rod spherule and cone pedicle plasticity after injury. Both rod and cone photoreceptors terminals are damaged after detachment although the structural changes observed are only partially overlapping. For second-order neurons, only those associated with rod spherules respond consistently to injury by remodeling. Examination of signaling pathways involved in plasticity of conventional synapses and in neural development has been and may continue to be productive in discovering novel therapeutic targets. Rho kinase (ROCK) inhibition is an example of therapy that may reduce synaptic damage by preserving normal synaptic structure of rod and cone cells.

Conclusions

We hypothesize that synaptic damage contributes to poor visual restoration after otherwise successful anatomical repair of retinal detachment. A similar situation may exist for patients with degenerative retinal disease. Thus, synaptic structure and function should be routinely studied, as this information may disclose therapeutic strategies to mitigate visual loss.

Actin Dynamics, Regulated by RhoA-LIMK-Cofilin Signaling, Mediates Rod Photoreceptor Axonal Retraction After Retinal Injury

Purpose

Retraction of the axon terminals of rod photoreceptors after retinal detachment breaks the first synapse in the visual pathway, resulting in visual impairment. Previous work showed that the mechanism of axonal retraction involves RhoA signaling and its downstream effector LIM Kinase (LIMK) activation. We examined the response of the downstream component cofilin, a direct binding protein of actin filaments, as well as the regulation by RhoA-LIMK-Cofilin signaling of actin assembly/disassembly, in the presynaptic ribbon terminal of injured rod cells.

Methods

Injury was produced by retinal detachment or rod cell isolation. Detached porcine retina was probed for levels and localization of phosphorylated cofilin with Western blots and confocal microscopy, whereas rod cell cultures of dissociated salamander retina were examined for filamentous actin assembly/disassembly with a barbed end assay and phalloidin staining.

Results

A detachment increased phosphorylation of cofilin in retinal explants; phosphorylation occurred in rod terminals in sections of detached retina. Isolation of rod cells resulted in axon retraction accompanied by an increase in actin barbed ends and a decrease in net filament labeling. All changes were significantly reduced by either Rho kinase (ROCK) or LIMK inhibition, using Y27632 or BMS-5, respectively. Cytochalasin D also reduced retraction and stabilized filaments in isolated rod cells.

Conclusions

These results indicate that actin depolymerization via activation of RhoA downstream kinases and cofilin contributes to axon retraction. Preventing depolymerization, in addition to actomyosin contraction, may stabilize ribbon synapses after trauma.

Sema3A Reduces Sprouting of Adult Rod Photoreceptors In Vitro

Purpose

Rod photoreceptor terminals respond to retinal injury with retraction and sprouting. Since the guidance cue Semaphorin3A (Sema3A) is observed in the retina after injury, we asked whether Sema3A contributes to structural plasticity in rod photoreceptors.

Methods

We used Western blots and alkaline phosphatase (AP)-tagged neuropilin-1 (NPN-1) to detect the expression of Sema3A in an organotypic model of porcine retinal detachment. We then examined Sema3A binding to cultured salamander rod photoreceptors using AP-tagged Sema3A. For functional analysis, we used a microspritzer to apply a gradient of Sema3A-Fc to isolated salamander rod photoreceptors over 24 hours.

Results

Sema3A protein was biochemically detected in porcine retinal explants in the retina 7, 24, and 72 hours after detachment. In sections, NPN-1 receptor was bound to the inner and outer retina. For isolated rod photoreceptors, Sema3A localized to synaptic terminals and to neuritic processes after 1 week in vitro. In microspritzed rod photoreceptors, process initiation occurred away from high concentrations of Sema3A. Sema3A significantly decreased the number of processes formed by rod photoreceptors although the average length of processes was not affected. The cellular orientation of rod photoreceptors relative to the microspritzer also significantly changed over time; this effect was reduced with the Sema3A inhibitor, xanthofulvin.

Conclusion

Sema3A is expressed in the retina after detachment, binds to rod photoreceptors, affects cell orientation, and reduces photoreceptor process initiation in vitro. Our results suggest that Sema3A contributes to axonal retraction in retinal injury, whereas rod neuritic sprouting and regenerative synaptogenesis may require a reduction in semaphorin signaling.

Fasudil, a Clinically Used ROCK Inhibitor, Stabilizes Rod Photoreceptor Synapses after Retinal Detachment

Purpose

Retinal detachment disrupts the rod-bipolar synapse in the outer plexiform layer by retraction of rod axons. We showed that breakage is due to RhoA activation whereas inhibition of Rho kinase (ROCK), using Y27632, reduces synaptic damage. We test whether the ROCK inhibitor fasudil, used for other clinical applications, can prevent synaptic injury after detachment.

Methods

Detachments were made in pigs by subretinal injection of balanced salt solution (BSS) or fasudil (1, 10 mM). In some animals, fasudil was injected intravitreally after BSS-induced detachment. After 2 to 4 hours, retinae were fixed for immunocytochemistry and confocal microscopy. Axon retraction was quantified by imaging synaptic vesicle label in the outer nuclear layer. Apoptosis was analyzed using propidium iodide staining. For biochemical analysis by Western blotting, retinal explants, detached from retinal pigmented epithelium, were cultured for 2 hours.

Results

Subretinal injection of fasudil (10 mM) reduced retraction of rod spherules by 51.3% compared to control detachments (n = 3 pigs, P = 0.002). Intravitreal injection of 10 mM fasudil, a more clinically feasible route of administration, also reduced retraction (28.7%, n = 5, P < 0.05). Controls had no photoreceptor degeneration at 2 hours, but by 4 hours apoptosis was evident. Fasudil 10 mM reduced pyknotic nuclei by 55.7% (n = 4, P < 0.001). Phosphorylation of cofilin and myosin light chain, downstream effectors of ROCK, was decreased with 30 μM fasudil (n = 8–10 explants, P < 0.05).

Conclusions

Inhibition of ROCK signaling with fasudil reduced photoreceptor degeneration and preserved the rod-bipolar synapse after retinal detachment.

Translational Relevance

These results support the possibility, previously tested with Y27632, that ROCK inhibition may attenuate synaptic damage in iatrogenic detachments.

RhoA Signaling and Synaptic Damage Occur Within Hours in a Live Pig Model of CNS Injury, Retinal Detachment

PURPOSE

The RhoA pathway is activated after retinal injury. However, the time of onset and consequences of activation are unknown in vivo. Based on in vitro studies we focused on a period 2 hours after retinal detachment, in pig, an animal whose retina is holangiotic and contains cones.

METHODS

Under anesthesia, retinal detachments were created by subretinal injection of a balanced salt solution. Two hours later, animals were sacrificed and enucleated for GTPase activity assays and quantitative Western blot and confocal microscopy analyses.

RESULTS

RhoA activity with detachment was increased 1.5-fold compared to that in normal eyes or in eyes that had undergone vitrectomy only. Increased phosphorylation of myosin light chain, a RhoA effector, also occurred. By 2 hours, rod cells had retracted their terminals toward their cell bodies, disrupting the photoreceptor-to-bipolar synapse and producing significant numbers of spherules with SV2 immunolabel in the outer nuclear layer of the retina. In eyes with detachment, distant retina that remained attached also showed significant increases in RhoA activity and synaptic disjunction. Increases in RAC1 activity and glial fibrillary acidic protein (GFAP) were not specific for detachment, and sprouting of bipolar dendrites, reported for longer detachments, was not seen. The RhoA kinase inhibitor Y27632 significantly reduced axonal retraction by rod cells.

CONCLUSIONS

Activation of the RhoA pathway occurs quickly after injury and promotes synaptic damage that can be controlled by RhoA kinase inhibition. We suggest that retinal detachment joins the list of central nervous system injuries, such as stroke and spinal cord injury, that should be considered for rapid therapeutic intervention.

A Versatile Method of Patterning Proteins and Cells

Substrate and cell patterning techniques are widely used in cell biology to study cell-to-cell and cell-to-substrate interactions. Conventional patterning techniques work well only with simple shapes, small areas and selected bio-materials. This article describes a method to distribute cell suspensions as well as substrate solutions into complex, long, closed (dead-end) polydimethylsiloxane (PDMS) microchannels using negative pressure. This method enables researchers to pattern multiple substrates including fibronectin, collagen, antibodies (Sal-1), poly-D-lysine (PDL), and laminin. Patterning of substrates allows one to indirectly pattern a variety of cells. We have tested C2C12 myoblasts, the PC12 neuronal cell line, embryonic rat cortical neurons, and amphibian retinal neurons. In addition, we demonstrate that this technique can directly pattern fibroblasts in microfluidic channels via brief application of a low vacuum on cell suspensions. The low vacuum does not significantly decrease cell viability as shown by cell viability assays. Modifications are discussed for application of the method to different cell and substrate types. This technique allows researchers to pattern cells and proteins in specific patterns without the need for exotic materials or equipment and can be done in any laboratory with a vacuum.

ROCK inhibitors in ocular disease

<p class="ADMETabstracttext">Rho kinases (ROCKs) have a crucial role in actin-cytoskeletal reorganization and thus are involved in broad aspects of cell motility, from smooth muscle contraction to neurite outgrowth. The first marketed ROCK inhibitor, called fasudil, has been used safely for treatment of cerebral vasospasm since 1995 in Japan. During the succeeding decades ROCK inhibitors have been applied in many pathological conditions from central nervous system disorders to cardiovascular disease as potential therapeutic agents or experimental tools to help understand the underlying (patho)mechanisms. In 2014, a fasudil derivate named ripasudil was accepted for clinical use in glaucoma and ocular hypertension. Since ROCK kinases are widely expressed in ocular tissues, they have been implicated in the pathology of many ocular conditions such as corneal dysfunction, glaucoma, cataract, diabetic retinopathy, age-related macular degeneration, and retinal detachment. This paper aims to provide an overview of the most recent status/application of ROCK inhibitors in the field of eye disease.</p>

Lim kinase, a bi-functional effector in injury-induced structural plasticity of synapses

The structural plasticity of synaptic terminals contributes to normal nervous system function but also to neural degeneration, in the form of terminal retraction, and regeneration, due to process growth. Synaptic morphological change is mediated through the actin cytoskeleton, which is enriched in axonal and dendritic terminals. Whereas the three RhoGTPases, RhoA, Cdc42 and Rac, function as upstream signaling nodes sensitive to extracellular stimuli, LIMK-cofilin activity serves as a common downstream effector to up-regulate actin turnover, which is necessary for both polymerization and depolymerization. The dual effects of LIMK activity make LIMK a potential target of therapeutic intervention for injury-induced synaptic plasticity, as LIMK inhibition can stabilize actin cytoskeleton and preserve existing structure. This therapeutic benefit of LIMK inhibition has been demonstrated in animal models of injury-induced axon retraction and neuritic sprouting by rod photoreceptors. A better understanding of the regulation of LIMK-cofilin activity and the interaction with the microtubular cytoskeleton may open new ways to promote synaptic regeneration that can benefit neuronal degenerative disease.

LIM Kinase, a Newly Identified Regulator of Presynaptic Remodeling by Rod Photoreceptors After Injury

PURPOSE

Rod photoreceptors retract their axon terminals and develop neuritic sprouts in response to retinal detachment and reattachment, respectively. This study examines the role of LIM kinase (LIMK), a component of RhoA and Rac pathways, in the presynaptic structural remodeling of rod photoreceptors.

METHODS

Phosphorylated LIMK (p-LIMK), the active form of LIMK, was examined in salamander retina with Western blot and confocal microscopy. Axon length within the first 7 hours and process growth after 3 days of culture were assessed in isolated rod photoreceptors treated with inhibitors of upstream regulators ROCK and p21-activated kinase (Pak) (Y27632 and IPA-3) and a direct LIMK inhibitor (BMS-5). Porcine retinal explants were also treated with BMS-5 and analyzed 24 hours after detachment. Because Ca2+ influx contributes to axonal retraction, L-type channels were blocked in some experiments with nicardipine.

RESULTS

Phosphorylated LIMK is present in rod terminals during retraction and in newly formed processes. Axonal retraction over 7 hours was significantly reduced by inhibition of LIMK or its regulators, ROCK and Pak. Process growth was reduced by LIMK or Pak inhibition especially at the basal (axon-bearing) region of the rod cells. Combining Ca2+ channel and LIMK inhibition had no additional effect on retraction but did further inhibit sprouting after 3 days. In detached porcine retina, LIMK inhibition reduced rod axonal retraction and improved retinal morphology.

CONCLUSIONS

Thus structural remodeling, in the form of either axonal retraction or neuritic growth, requires LIMK activity. LIM kinase inhibition may have therapeutic potential for reducing pathologic rod terminal plasticity after retinal injury.

Position along the nasal/temporal plane affects synaptic development by adult photoreceptors, revealed by micropatterning

In retinal degeneration, death of photoreceptors causes blindness. Repair of the retina by transplanting photoreceptors has resulted in limited functional connectivity between transplanted and host neurons. We hypothesize that absence of appropriate biological cues, specifically positional (retinotopographic) cues, reduces synaptogenesis. Here we use micropatterning to test whether regional origin affects the early synaptic development of photoreceptors. Right and left retinas from salamanders were first labelled with dextran tetramethyl-rhodamine and fluorescein, respectively, bisected into nasal (N)/temporal (T) or dorsal (D)/ventral (V) halves, individually dissociated, mixed, and cultured for 1 week. Origin of cells was identified by the fluorescent label. Interactions between photoreceptors and neighboring (target) cells were assessed by the number of neuritic contacts with a presynaptic swelling (varicosity). Randomly-plated photoreceptors showed no preference for cellular origin, likely due to multiple potential interactions available to each cell. To reduce cell-cell interactions, culture substrate was patterned using a microfluidic device with 10 μm-wide channels separated by 200 μm, thus allowing only 1-2 targets per photoreceptor. In patterned cultures, 36.89% of N rod cells contacted T targets but only 27.42% of N rod cells contacted N targets; similarly 35.05% of T rod cells contacted N cells but only 17.08% contacted T cells. Thus, opposite regions were more permissive of contact. However, neither cone nor D/V rod cells showed preferences for positional origin of targets. In conclusion, micropatterning demonstrated that neuritic differentiation by rod cells depends on retinotopographic cues along the nasal/temporal plane, suggesting that transplanting rod cells of known positional origin will increase transplant success.

Vacuum-assisted fluid flow in microchannels to pattern substrates and cells

Substrate and cell patterning are widely used techniques in cell biology to study cell-to-cell and cell-substrate interactions. Conventional patterning techniques work well only with simple shapes, small areas and selected bio-materials. This paper describes a method to distribute cell suspensions as well as substrate solutions into complex, long, closed (dead-end) polydimethylsiloxane (PDMS) microchannels using negative pressure. Our method builds upon a previous vacuum-assisted method used for micromolding (Jeon et al 1999 Adv. Mater 11 946) and successfully patterned collagen-I, fibronectin and Sal-1 substrates on glass and polystyrene surfaces, filling microchannels with lengths up to 120 mm and covering areas up to 13 × 10 mm(2). Vacuum-patterned substrates were subsequently used to culture mammalian PC12 and fibroblast cells and amphibian neurons. Cells were also patterned directly by injecting cell suspensions into microchannels using vacuum. Fibroblast and neuronal cells patterned using vacuum showed normal growth and minimal cell death indicating no adverse effects of vacuum on cells. Our method fills reversibly sealed PDMS microchannels. This enables the user to remove the PDMS microchannel cast and access the patterned biomaterial or cells for further experimental purposes. Overall, this is a straightforward technique that has broad applicability for cell biology.

Mislocalized opsin and cAMP signaling: a mechanism for sprouting by rod cells in retinal degeneration

PURPOSE

In human retinal degeneration, rod photoreceptors reactively sprout neurites. The mechanism is unknown in part because of the paucity of animal models displaying this feature of human pathology. We tested the role of cAMP and opsin in sprouting by tiger salamander rod cells, photoreceptors that can produce reactive growth.

METHODS

In vitro systems of isolated photoreceptor cells and intact neural retina were used. cAMP signaling was manipulated with nucleotide analogues, enzyme stimulators, agonists for adenosine and dopamine receptors, and the opsin agonist, β-ionone. Levels of cAMP were determined by radioimmunoassay, and protein levels by Western blot and quantitative immunocytochemistry. Neuritic growth was assayed by image analysis and conventional and confocal microscopy.

RESULTS

cAMP analogues and stimulation of adenylyl cyclase (AC) directly or through G-protein-coupled receptors resulted in significant increases in neuritic growth of isolated rod, but not cone, cells. The signaling pathway included protein kinase A (PKA) and phosphorylation of the transcription factor cAMP response element-binding protein (pCREB). Opsin, a G-linked receptor, is present throughout the plasmalemma of isolated cells; its activation also induced sprouting. In neural retina, rod sprouting was significantly increased by β-ionone with concomitant increases in cAMP, pCREB, and synaptic proteins. Notably, opsin stimulated sprouting only when mislocalized to the plasmalemma of the rod cell body.

CONCLUSIONS

cAMP causes neuritic sprouting in rod, but not cone, cells through the AC-PKA-CREB pathway known to be associated with synaptic plasticity. We propose that in retinal disease, mislocalized rod opsin gains access to cAMP signaling, which leads to neuritic sprouting.

Organotypic culture of full-thickness adult porcine retina

There is a recognized demand for in vitro models that can replace or reduce animal experiments. Porcine retina has a similar neuronal structure to human retina and is therefore a valuable species for studying mechanisms of human retinal injury and degenerative disease. Here we describe a cost-effective technique for organotypic culture of adult porcine retina isolated from eyes obtained from an abattoir. After removing the anterior segment, a trephine blade was used to create multiple neural retina-Bruch's membrane-RPE-choroid-sclera explants from the posterior segment of adult porcine eyes. A piece of sterile filter paper was used to lift the neural retina off from each explant. The filter paper-retina complex was cultured (photoreceptor side up) atop an insert, which was held away from the bottom of the culture dish by a custom-made stand. The stand allows for good circulation of the culture medium to both sides of the retina. Overall, this procedure is simple, reproducible, and permits preservation of native retinal structure for at least seven days, making it a useful model for a variety of morphological, pharmacological, and biochemical studies on mammalian retina.

RhoA inactivation prevents photoreceptor axon retraction in an in vitro model of acute retinal detachment

PURPOSE

An early injury response to retinal detachment is disruption of synaptic connectivity between photoreceptors and second-order neurons. Most dramatic is the retraction of rod cell axons and their terminals away from the outer synaptic layer and toward their cell bodies. This study tested whether axonal retraction in detached retina was due to the activation of the small GTPase RhoA and was preventable using RhoA antagonists.

METHODS

Retinal detachments were created in in vitro preparations of porcine eyecups. RhoA activation was determined with a Rhotekin binding assay. To block axon retraction, drugs were applied to neural retinal explants either before or after detachment from the retinal pigment epithelium. Presynaptic movement was quantified by image analysis of double-labeled retinas examined with confocal microscopy.

RESULTS

Active RhoA increases transiently after detachment followed by morphologic evidence of axonal retraction over the next 24 hours. Pretreating the retina with a RhoA antagonist, CT-04, or a Rho kinase inhibitor, Y27632, at multiple concentrations significantly inhibited axonal retraction. Reducing calcium influx through L-type calcium channels with nicardipine also blocked retraction. To create a more plausible therapeutic scenario, drug treatments were delayed and applied after retinal detachment. The Rho kinase inhibitor, but not nicardipine, significantly blocked rod axonal retraction when applied up to 6 hours after detachment.

CONCLUSIONS

Thus, RhoA and downstream Rho kinase activity constitute part of the mechanism that produces rod axonal retraction in retinal explants. Treatments that manipulate RhoA signaling may promote synaptic stability after retinal detachment.

Using Laser Tweezers For Manipulating Isolated Neurons In Vitro

In this paper and video, we describe the protocols used in our laboratory to study the targeting preferences of regenerating cell processes of adult retinal neurons in vitro. Procedures for preparing retinal cell cultures start with the dissection, digestion and trituration of the retina, and end with the plating of isolated retinal cells on dishes made especially for use with laser tweezers. These dishes are divided into a cell adhesive half and a cell repellant half. The cell adhesive side is coated with a layer of Sal-1 antibodies, which provide a substrate upon which our cells grow. Other adhesive substrates could be used for other cell types. The cell repellant side is coated with a thin layer of poly-HEMA. The cells plated on the poly-HEMA side of the dish are trapped with the laser tweezers, transported and then placed adjacent to a cell on the Sal-1 side to create a pair. Formation of cell groups of any size should be possible with this technique. "Laser-tweezers-controlled micromanipulation" means that the investigator can choose which cells to move, and the desired distance between the cells can be standardized. Because the laser beam goes through transparent surfaces of the culture dish, cell selection and placement are done in an enclosed, sterile environment. Cells can be monitored by video time-lapse and used with any cell biological technique required. This technique may help investigations of cell-cell interactions.

RhoA and its role in synaptic structural plasticity of isolated salamander photoreceptors

PURPOSE

Adult salamander photoreceptors retract existing axons and extend new neuritic processes in vitro. In mammalian retina, similar forms of structural plasticity occur in injury and disease. The authors asked whether RhoA is present in photoreceptor axon terminals and whether activity in the RhoA-ROCK pathway contributes to the structural plasticity observed in rod and cone cells.

METHODS

Antibodies against RhoA were used to immunolabel Western blots sections and isolated neurons from salamander retina. Isolated photoreceptors were treated with lysophosphatidic acid (LPA; a RhoA activator) or Y27632 (an inhibitor of RhoA effector ROCK) for the first 24 hours, the first 3 days, or the last 24 hours of culture. Growth and retraction were assessed with time-lapse and image analyses.

RESULTS

RhoA protein was found throughout the retina, including in rod and cone synaptic terminals. When treated with LPA, photoreceptors significantly reduced the growth of new neuritic processes and presynaptic varicosities and retracted growth at the highest LPA concentrations. When treated with Y27632, rod cells significantly increased the number of varicosities, whereas cone cells increased process growth. Treatment with Y27632 also dramatically reduced retraction of the existing axon, which occurs spontaneously in rod cells during the first 24 hours of culture.

CONCLUSIONS

Thus, RhoA-ROCK activity reduces and retracts neuritic growth, but inhibition of activity increases neuritic development and blocks retraction. The results suggest that RhoA activation contributes to axon retraction by rod cells after retinal detachment, whereas inhibition of RhoA contributes to the neuritic sprouting seen in reattached and degenerating retina.

Cone and rod cells have different target preferences in vitro as revealed by optical tweezers

PURPOSE

When neural circuits are damaged in adulthood, regenerating and sprouting processes must distinguish appropriate targets to recreate the normal circuitry. We tested the ability of adult nerve cells to target specific cells in culture using the retina as a model system.

METHODS

Under sterile culture conditions, retinal cells, isolated from tiger salamander retina, were micromanipulated with optical tweezers to create pairs of first-order photoreceptor cells with second- or third-order retinal neurons. The development of cell contact and presynaptic varicosities, the direction and amount of neuritic growth, and nerve cell polarity were assessed after seven days in vitro. Cultures were labeled for rod opsin to distinguish rod from cone cells and for the alpha subunit of the trimeric G protein Go (Go alpha) to identify cone-dominated and mixed rod-cone ON bipolar cells.

RESULTS

Quantitative analysis of growth demonstrated that target preferences were cell-specific: Cone cells preferred second-order bipolar cells, whereas rod cells grew toward third-order neurons, which include amacrine and ganglion cells. In addition, when rod cells grew toward bipolar cells, they chose an abnormally high number of Go alpha-positive bipolar cells. These growth patterns were not affected by tweezers manipulation or the amount of growth. Cell orientation of the photoreceptor also did not affect preferences: Cells oriented away from dendritic processes could reorient their axonal pole toward the target cell.

CONCLUSIONS

Cone cells preferred normal partners, and rod cells preferred novel partners. These intrinsic preferences indicate that adult nerve cells can have differing capacities for targeting even if they come from the same cell class. Further,these differences may help explain the patterns of photoreceptor sprouting seen in retinal degeneration in which rod, but not cone, cells invade the inner retinal layers where third-order neurons are located.

The nitric oxide-cGMP signaling pathway differentially regulates presynaptic structural plasticity in cone and rod cells

Although abundant structural plasticity in the form of axonal retraction, neurite extension, and formation of presynaptic varicosities is displayed by photoreceptors after retinal detachment and during genetic and age-related retinal degeneration, the mechanisms involved are mostly unknown. We demonstrated recently that Ca(2+) influx through cGMP-gated channels in cones and voltage-gated L-type channels in rods is required for neurite extension in vitro (Zhang and Townes-Anderson, 2002). Here, we report that the nitric oxide (NO)-cGMP signaling pathway is active in photoreceptors and that its manipulation differentially regulates the structural plasticity of cone and rod cells. The NO receptor soluble guanylyl cyclase (sGC) was detected immunocytochemically in both cone and rod cells. Stimulation of sGC increased cGMP production in retinal cultures. In cone cells, quantitative analysis showed that NO or cGMP stimulated neuritic sprouting; this stimulatory effect was dependent on both Ca2+ influx through cGMP-gated channels and phosphorylation by protein kinase G (PKG). At the highest levels of cGMP, however, cone outgrowth was no longer increased. In rod photoreceptors, NO or cGMP consistently inhibited neuritic growth in a dose-dependent manner; this inhibitory effect required PKG. When NO-cGMP signaling was inhibited, changes in the neuritic development of cone and rod cells were also observed but in the opposite direction. These results expand the role of cGMP in axonal activity to adult neuritogenesis and suggest an explanation for the neurite sprouting observed in an autosomal recessive form of retinitis pigmentosa that is characterized by high cGMP levels in photoreceptor layers.

Cyclic AMP Prevents Retraction of Axon Terminals in Photoreceptors Prepared for Transplantation: An In Vitro Study

PURPOSE

Cell transplantation has emerged as a possible remedy for degeneration and injury in the central nervous system (CNS). In the retina, photoreceptor transplantation is a potential treatment for retinal degenerative disease. Graft survival has been well documented, but evidence of functional recovery is lacking. A major obstacle to recovery of vision is lack of synapse formation between grafted photoreceptors and host bipolar and horizontal cells. A prior study demonstrated that photoreceptors prepared for transplantation undergo rapid morphologic changes, including retraction of axon terminals toward their cell bodies, away from potential synaptic partners, a phenomenon that may interfere with graft-host synaptic interaction after transplantation. In this study, prevention of retraction of photoreceptor axon terminals was possible by pharmacological intervention.

METHODS

Photoreceptor sheets, prepared by vibratome sectioning, and full-thickness retinas, harvested from adult porcine eyes, were maintained in culture and treated with either the cyclic adenosine monophosphate analogue 8-(4-chlorophenylthio)-cyclic 3',5'-adenosine monophosphate (CPT-cAMP), or forskolin, an adenylyl cyclase stimulant, for up to 48 hours.

RESULTS

Both CPT-cAMP and forskolin treatments successfully blocked retraction of photoreceptor axon terminals. This effect was not due to cell toxicity and was reversed after removal of treatment, indicating its specificity.

CONCLUSIONS

Pharmacological manipulation of photoreceptor axonal plasticity may improve graft-host synaptic interaction after subretinal photoreceptor cell transplantation.

A highly Ca2+-sensitive pool of vesicles contributes to linearity at the rod photoreceptor ribbon synapse

Studies of the properties of synaptic transmission have been carried out at only a few synapses. We analyzed exocytosis from rod photoreceptors with a combination of physiological and ultrastructural techniques. As at other ribbon synapses, we found that rods exhibited rapid kinetics of release, and the number of vesicles in the releasable pool is comparable to the number of vesicles tethered at ribbon-style active zones. However, unlike other previously studied neurons, we identified a highly Ca(2+)-sensitive pool of releasable vesicles with a relatively shallow relationship between the rate of exocytosis and [Ca(2+)](i) that is nearly linear over a presumed physiological range of intraterminal [Ca(2+)]. The low-order [Ca(2+)] dependence of release promotes a linear relationship between Ca(2+) entry and exocytosis that permits rods to relay information about small changes in illumination with high fidelity at the first synapse in vision.

Synaptic Plasticity in Mammalian Photoreceptors Prepared as Sheets for Retinal Transplantation

PURPOSE

To investigate systematically the early morphologic changes in axon terminals of adult mammalian rod and cone photoreceptors prepared as a sheet for subretinal transplantation.

METHODS

An in vitro system was designed to maintain adult porcine retinas for up to 48 hours. Photoreceptor sheets, prepared by vibratome sectioning, and full-thickness retinas were cultured at temperatures similar to those in pretransplantation storage (4 degrees C) and after transplantation (37 degrees C). Changes in the outer nuclear and outer plexiform layers were analyzed, using immunohistochemistry, laser scanning confocal microscopy, and image analysis.

RESULTS

Morphologic changes were observed in photoreceptor sheets as early as 10 minutes after incubation. The most significant change was the retraction of photoreceptor axons and terminals toward their cell bodies. Retraction was temperature dependent, being exacerbated at 37 degrees C compared with 4 degrees C, at its maximum by 24 hours of culture, and present in sheets obtained from both superior and inferior retina. The cause of this movement was not preparation techniques associated with vibratome sectioning or gelatin removal. Retraction was also present in full-thickness neural retina incubated at 37 degrees C. Reduction in outer nuclear layer cell counts and thickness were also evident in these preparations, primarily in photoreceptor sheets.

CONCLUSIONS

Adult photoreceptor sheets, a potential graft preparation for retinal transplantation, show a rapid retraction of axon terminals toward the cell bodies during culture. Although retraction may impede synaptic integration after transplantation, this intrinsic plasticity could be redirected to stimulate graft-host interaction.

Adenosine A(2a) receptor-mediated inhibition of rod opsin mRNA expression in tiger salamander

The neuromodulator adenosine mediates dark-adaptive changes in retinal photoreceptors through A(2a) receptors. In cold-blooded vertebrates, opsin mRNA expression is lower at night than during the day. In the present study, we tested whether adenosine could inhibit opsin mRNA expression in cultured rod cells and if endogenous adenosine acts to suppress opsin mRNA in the intact retina at night. Semi-quantitative in situ hybridization showed that treatment with 100 nm of the A(2a)/A(2b) agonist N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) reduced opsin mRNA 41% in cultured rod cells. The effect of DPMA was blocked by 10 microm of the A(2a) antagonist 8-(3-chlorostyryl)caffeine (CSC) but not by 10 microm of the A(2b) antagonist alloxazine. One micromolar adenosine alone had no effect on opsin mRNA. However, in the presence of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA), 1 microm adenosine reduced opsin mRNA 61%. EHNA alone reduced opsin mRNA by 26%. Consistent with an A(2a) receptor mechanism, 100 nm forskolin (adenylate cyclase agonist) decreased opsin mRNA 34%. Finally, northern blots showed that intravitreal injection of 10 microm CSC at night increased opsin I mRNA 38%. Thus, endogenous adenosine suppresses rod opsin I mRNA expression at night; in vitro results indicate this reduction occurs through A(2a)-like receptor binding and stimulation of adenylate cyclase activity.

Activation of mislocalized opsin kills rod cells: a novel mechanism for rod cell death in retinal disease

Rod photoreceptors are highly compartmentalized sensory neurons that maintain strict ultrastructural and molecular polarity. Structural subdivisions include the outer segment, inner segment, cell body, and synaptic terminal. The visual pigment rhodopsin is found predominantly in membranes of the rod cell outer segment but becomes mislocalized, appearing throughout the plasma membrane of the cell in many retinal diseases and injuries. Currently, there is no known link between rhodopsin redistribution and rod cell death. We propose that activation of mislocalized rhodopsin kills rod cells by stimulating normally inaccessible signaling pathways. This hypothesis was tested in primary retinal cell cultures, which contain photoreceptors. In rod photoreceptors, opsin immunofluorescence occurred throughout the rod cell plasma membrane. Activation of this mislocalized opsin by photostimulation after formation of isorhodopsin or by incubation with beta-ionone (opsin agonist) killed 19-30% of rod cells. Rod cell death was apoptotic, as indicated by marked chromatin condensation and the requirement for caspase-3 activation. Rod cell death could be induced by forskolin (adenylate cyclase agonist), and conversely, beta-ionone-induced cell death could be blocked by cotreatment with SQ22536 (an adenylate cyclase inhibitor). Pertussis toxin (a G protein inhibitor) also blocked beta-ionone-induced cell death. The data support a mechanism by which activation of mislocalized opsin initiates apoptotic rod cell death through G protein stimulation of adenylate cyclase.

Dopamine D4 receptor-mediated regulation of rod opsin mRNA expression in tiger salamander

Light stimulates dopamine release in the retina and has been shown to rapidly up-regulate rod opsin mRNA. In the present study, we tested the effect of dopamine on rod opsin mRNA expression and examined the hypothesis that dopamine can mediate a light-evoked increase in opsin gene expression. Northern blots showed that a 30-min light-exposure increased rod opsin mRNA expression 27%. In situ hybridization on isolated rods showed that 500 nM dopamine and 1 microM quinpirole (dopamine D2/D3/D4 agonist) increased opsin mRNA 45% and 26%, respectively. The effect of quinpirole was selectively blocked by the D4 antagonist, L750,667 (20 microM). In very low density cultures, quinpirole increased opsin expression 46%, suggesting a direct effect on rod photoreceptors. Consistent with a dopamine D4 receptor mechanism, 1 microM H-89 (protein kinase A inhibitor) increased opsin mRNA 39%. Finally, intravitreal injection of quinpirole increased opsin mRNA 21% whereas injection of L750,667 (10 microM) blocked the light-evoked increase in opsin expression. These data show that rod opsin mRNA is up-regulated by dopamine binding a D4-like receptor on rods, possibly through inhibition of protein kinase A, and that endogenous dopamine can mediate the light-evoked increase in opsin mRNA expression.

Rapid glutamatergic alterations in the neural retina induced by retinal detachment

PURPOSE

Retinal detachment induces neurochemical changes in the neural retina over a span of days to weeks. However, little information is available on the acute response in the retina to detachment.

METHODS

Distribution of the neurotransmitters glutamate, glycine, and gamma-aminobutyric acid (GABA) and the metabolic amino acids aspartate and glutamine was examined immunocytochemically from 5 to 30 minutes and at 3 hours after retinal detachment in a salamander eyecup preparation.

RESULTS

Glutamate showed a rapid depletion from neuronal cell bodies in detached retina, whereas Müller cells, which normally sequester and metabolize glutamate, showed increased immunolabeling for glutamine. Changes occurred exclusively in detached retinal regions of the eyecup. Aspartate, a precursor for glutamate synthesis, also showed decreased labeling in neuronal cell bodies in detached retinal regions, although these changes were not as striking as those observed for glutamate. In contrast, the distributions of the inhibitory amino acid neurotransmitters glycine and GABA were not affected appreciably by acute retinal detachment.

CONCLUSIONS

These results indicate that retinal detachment induces rapid, localized alterations in the glutamatergic system of the neural retina that are consistent with a massive efflux of neuronal glutamate and concomitant alterations in glutamate metabolism. An acute efflux of neuronal glutamate in detached retina could contribute to excitotoxicity and to the initiation of structural alterations and changes in gene expression; it is also consistent with reported neurochemical changes associated with longer term retinal detachment.

L-type calcium channels in the photoreceptor ribbon synapse: localization and role in plasticity

Calcium (Ca(2+)) influx through voltage-gated Ca(2+)channels stimulates a variety of neural activities, including process outgrowth, neurotransmission, and synaptic plasticity. In general, L-type channels control Ca(2+) influx into the soma and dendrites, whereas other Ca(2+) channel types control presynaptic activities. Neurons that make ribbon synapses, however, are among a select group of nerve cells whose presynaptic Ca(2+)-dependent secretion is linked to L-type channels. Recently, photoreceptor ribbon synapses have been shown to be capable of dramatic structural remodeling and neuritic outgrowth. Here, we have examined 1) the distribution of dihydropyridine (DHP)-sensitive (L-type) Ca(2+) channels in photoreceptor presynaptic structures and 2) the role of these channels in axonal plasticity and process outgrowth in culture. Using anti-alpha(1C) and the fluorescent dihydropyridine, (-)-DM-BODIPY DHP, L-type channels were localized in the outer plexiform layer of retinal sections and in presynaptic terminals of freshly isolated photoreceptors. In the rod terminal, dense patches of label were present; their distribution and number matched that of synaptic ribbons. After 1-7 days in vitro, punctate alpha(1C) staining occurred along newly formed neurites and presynaptic varicosities. Functional channels were present throughout the culture period, as determined by fura-2 imaging. Channel blockage by nicardipine, a DHP antagonist, inhibited axonal remodeling. Specifically, it prevented axon retraction and lamellipodium formation, reduced neurite growth, and produced long, thin processes on some, primarily cone, photoreceptors. L-type Ca(2+) channel activity, therefore, not only stimulates neurotransmission but contributes to presynaptic structural plasticity at the ribbon synapse.

Micromanipulation of retinal neurons by optical tweezers

Micromanipulation by optical tweezers has been tested in cultures of mature isolated retinal cells to determine its potential for use in creating synaptic circuits in vitro. Rod and cone photoreceptors as well as other retinal nerve cell types could be optically trapped with a 980 nm diode laser mounted on an inverted light microscope using a 40x oil immersion objective numerical aperture of 1.3. Manipulation was done under sterile conditions using transparent culture dishes. To form cell groups, one half of a culture dish was made less adhesive by application of a thin layer of silicone elastomer. Unattached cells were trapped and relocated next to cells lying on an adhesive culture substrate. Optical trapping did not affect the ability of neurons to subsequently attach to the culture substrate. Up to 60% of trapped cells survived for 2 or more days. The pattern and rate of process outgrowth for manipulated cells was comparable to unmanipulated cells and by 2 days, cell-cell contacts were observed. Cultures were fixed at 2 and 5 days for electron microscopy. Organelle, nuclear and cytoplasmic structure of manipulated cells was completely normal and in photoreceptors, synaptic vesicles and ribbons were intact. Optical tweezers, therefore, provide a benign technique with which to micromanipulate whole neurons. The procedures also bestow increased precision to the study of cell-cell interactions by allowing the selection of potentially interacting cell types at a single cell level.

Age-related changes in the tiger salamander retina

Tiger salamanders have been used in visual science because of the large size of their cells and the ease of preparation and maintenance of in vitro retinal preparations. We have found that salamanders over 27 cm in length show a variety of visual abnormalities. Compared to smaller animals (15-23 cm), large animals exhibited a decrease in visual responses determined by tests of the optomotor reflex. Small animals responded correctly an average of 84.5% of the time in visual testing at three light levels compared to an average of 68.4% for the large animals with the poorest visual performance at the lowest level of illumination. In addition, large animals contained (i) histological degeneration of the outer retina, in particular, loss and disruption of outer segments and abnormalities of the retinal pigmented epithelium, (ii) loss of cells, including photoreceptors, by apoptosis as evaluated with the TUNEL technique, and (iii) an increase in the number of macrophages and lymphocytes within the retina as determined by morphological examination. These histological changes were present in all large animals and all quadrants of their retinas. In contrast, small animals showed virtually no retinal degeneration, no TUNEL-positive cells, and few immune-like cells in the retina. Since large animals are also older animals. the visual changes are age-related. Loss of visual function and histological degeneration in the outer retina also typify aged human eyes. Thus, we propose that large salamanders serve as an animal model for age-related retinal degeneration. In addition to providing a source of aging retina that is readily accessible to experimental manipulation, the salamander provides a pigmented retina with a mixed (2:1, rod:cone) population of photoreceptors, similar to the degeneration-prone parafoveal region of the human eye.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Injury-induced remodelling and regeneration of the ribbon presynaptic terminal in vitro

The neuronal response to axonal injury may relate to the type of insult incurred. Recently, neuritic and presynaptic varicosity regeneration by isolated adult salamander photoreceptors was demonstrated. We have used this system to compare the rod photoreceptor response to two types of injury: denervation/detargeting, the removal of pre- and postsynaptic partners from the axon terminal, and axotomy, the removal of the axon terminal itself. Cells were followed with time-lapse video microscopy for 24-48 h in culture and immunolabelled for SV2 or synaptophysin to identify synaptic vesicle-containing varicosities. Although all injured cells responded with regenerative growth, denervated/detargeted photoreceptors (i.e. neurons which retain their axon terminal) grew 80% more processes and fourfold more presynaptic varicosities than axotomized neurons. In cells which retained their original axon and terminal, varicosity formation generally began with axon retraction. Retraction was followed by elaboration of a lamellipodium and, by 48 h, development of varicosity-bearing neurites from the lamellipodium. Synaptic vesicle protein localization in denervated/detargeted cells paralleled axon terminal reorganization. Axotomized cells, in contrast, lacked synaptic vesicle protein immunoreactivity during this period. To detect synaptic protein synthesis, photoreceptors were examined for colocalization of synaptic vesicle protein with rab6, a Golgi marker, by confocal microscopy. As expected, synaptic vesicle protein staining was present in the Golgi complex during regeneration; however, in cells with an axon, new synaptic vesicle protein-labelled varicosities were found at early stages, prior to the appearance of immunolabel in the Golgi complex. The data demonstrate remarkable plasticity in the ribbon synapse, and suggest that in adult rod cells with an intact axon terminal, synaptic vesicle protein synthesis is not a prerequisite for the formation of new presynaptic-like terminals. We propose that preexisting axonal components are reutilized to expedite presynaptic renewal as an early response to denervation/detargeting.

Intersegmental fusion in vertebrate rod photoreceptors. Rod cell structure revisited

PURPOSE

To determine the cause of intersegmental fusion--a fusion between the plasma membranes of the inner and outer segments--in vertebrate rod cells.

METHODS

Rod cells from adult salamander, frog, and rat retinas were examined with electron microscopy. The fine structure of fusion and the frequency of occurrence were assessed in three types of preparations (intact eyeballs, eyecup preparations, and neural retinas detached from the retinal pigmented epithelium). The effects of temperature, depolarization, and calcium were examined in isolated neural retinas by maintaining retinas in culture under defined conditions. Retinas from anesthetized animals also were examined.

RESULTS

Intersegmental fusion in rod cells was consistently observed in all species after retinal detachment and maintenance in a standard Ringer's solution for 20 to 30 minutes. The amount of fusion seen in isolated amphibian retinas was sensitive to a number of factors: Elevated temperature increased the number of fused rod cells, whereas high KCl, low CaCl2, and Tricaine, which has calcium-related effects, reduced or prevented fusion. In attached retinas, fusion was seen infrequently; the incidence of fused cells in amphibian eyecup preparations, however, could be increased by incubation in the cold. The extensive continuity between the inner and outer segments created by their fusion did not disrupt intracellular structure. Moreover, the fusion seemed to be reversible with time.

CONCLUSIONS

Intersegmental fusion is a feature of vertebrate rod cells. Although seen predominantly after retinal detachment, it is influenced by factors that affect other well-known exoplasmic membrane fusions. Thus, it may be one of the many regulated membrane fusion-fission events within the rod photoreceptor.

Process outgrowth and synaptic varicosity formation by adult photoreceptors in vitro

To assess the regenerative capability of the photoreceptor synapse, we have isolated and cultured photoreceptors from the mature salamander retina. Both rod and cone photoreceptors were able to regenerate processes within 3 d of plating. Cells extended numerous actin-containing filopodia as well as a few neuritic processes. The neurites contained microtubules and formed synaptic vesicle-filled varicosities, as shown by immunostaining for tubulin and synaptic vesicle proteins and by electron microscopy. Furthermore, regenerated varicosities were capable of depolarization-induced vesicle labeling, suggesting that they can recycle synaptic vesicles and release neurotransmitter by synaptic vesicle exocytosis. Differences were observed between rod and cone cell synaptic regeneration in vitro, which resembled structural differences between their synaptic terminals in situ: rod cells formed multiple synaptic vesicle-filled varicosities along neurites at a distance from the soma, whereas cone cells tended to accumulate synaptic vesicles within the soma. The regeneration of neurites and synaptic vesicle-filled varicosities was abolished by microtubule depolymerizing agents, suggesting a role for microtubule-based vesicle transport in the formation of varicosities. Finally, process outgrowth and varicosity formation were independent of cell-cell contact and, indeed, proceeded in the complete absence of other cells. These findings suggest not only that differentiated photoreceptors are capable of synaptic renewal but that the regeneration of presynaptic-like terminals is an intrinsic ability of rod and cone cells.

Differential expression of synapsins I and II among rat retinal synapses

The synapsins are a family of synaptic vesicle-associated phosphoproteins thought to regulate the availability of vesicles for neurotransmitter release. In order to assess variability of synapsin isoform expression, we compared the localization of synapsins Ia, Ib, IIa, and IIb in the inner plexiform layer of the rat retina. Double labeling in conjunction with confocal fluorescence and electron microscopy allowed imaging of synapsin I and II immunoreactivity within single presynaptic terminals. No qualitative differences were observed between expression of the a and b isoforms of synapsin I in individual terminals; likewise, the a and b isoforms of synapsin II were identically distributed. In contrast, marked differences were seen upon comparison of synapsin I and synapsin II expression in single terminals. Our results indicate the existence of three classes of presumed amacrine cell synaptic terminals: synapsin I+/synapsin II-, synapsin I-/synapsin II+, and synapsin I+/synapsin II+. Each class of synapse has a different distribution among five IPL sublayers, suggesting that they represent different subpopulations of amacrine cells. Double labeling with an antibody to choline acetyltransferase indicates that synapsin I-/II+ terminals may be those of cholinergic amacrine cells. Furthermore, all synapsin II+ terminals appear to be distinct from those expressing the GABA synthetic enzyme glutamic acid decarboxylase. The observed variations in synapsin content suggest the existence of presynaptic terminal heterogeneity that is not apparent from conventional morphological studies.

Synapsins in the vertebrate retina: absence from ribbon synapses and heterogeneous distribution among conventional synapses

The vertebrate retina contains two ultrastructurally distinct types of vesicle-containing synapses: conventional synapses, made predominantly by amacrine cells, and ribbon synapses, formed by photoreceptor and bipolar cells. To identify molecular differences between these synapse types, we have compared the distribution of the synapsins, a family of nerve terminal phosphoproteins, with that of synaptophysin (p38) and SV2, two intrinsic membrane proteins of synaptic vesicles. We report an absence of synapsin I and II immunoreactivity from all ribbon-containing nerve terminals. These include terminals of rod cells in developing and adult rat retina, rod and cone cells in monkey and salamander retinas, and rat bipolar cells. Furthermore, we show that synapsins I and II are differentially distributed among conventional synapses of amacrine cells. The absence of the synapsins from ribbon synapses suggests that vesicle clustering and mobilization in these terminals differ from that in conventional synapses.

Neuroblast mitosis in dissociated culture: regulation and relationship to differentiation

Although neuron generation is precisely regulated during ontogeny, little is known about underlying mechanisms. In addition, relationships between precursor proliferation and the apparent sequence of developmental processes, including cell migration, neurite elaboration, transmitter expression and synaptogenesis remain unknown. To address these issues, we used a fully defined neuronal cell culture system derived from embryonic rat sympathetic ganglia (DiCicco-Bloom, E., and I. B. Black. 1988. Proc. Natl. Acad. Sci. USA. 85:4066-4070) in which precursors enter the mitotic cycle. We now find that, in addition to synthesizing DNA, neuroblasts also underwent division in culture, allowing analysis of developmental relationships and mitotic regulation. Our observations indicate that mitotic neuroblasts expressed a wide array of neuron-specific characteristics including extension of neuritic processes with growth cones, elaboration of neurotransmitter enzyme, synthesis and transport of transmitter vesicles and organization of transmitter release sites. These data suggest that neuroblasts in the cell cycle may simultaneously differentiate. Consequently, the apparent sequence of ontogenetic processes is not an immutable, intrinsic neuronal program. How, then, are diverse developmental events coordinated? Our observations indicate that neuroblast mitosis is regulated by a small number of epigenetic factors, including insulin and EGF. Since these signals also influence other processes in developing neurons, epigenetic regulation normally may synchronize diverse ontogenetic events.

Distribution of muscarinic acetylcholine receptors on processes of isolated retinal cells

Binding of propylbenzilylcholine mustard, a muscarinic acetylcholine receptor antagonist, to isolated retinal cells was examined with light microscopic autoradiography. Dissociation of the adult tiger salamander retina yielded identifiable rod, cone, horizontal, bipolar, amacrine/ganglion, and Müller cells. Preservation of fine structure was assessed with conventional electron microscopy. For all cell types, the plasmalemma was intact and free of adhering debris; in addition, presynaptic ribbon complexes were present in photoreceptor and bipolar axon terminals indicating that synaptic structures were retained. Specific binding to cell bodies and processes was analyzed separately by using morphometric and statistical techniques. The highest grain densities occurred on processes of amacrine/ganglion cells and axons and 2 degrees and 3 degrees dendrites of bipolar neurons. Bipolar cells, however, seemed to be a heterogeneous population because there was great variation in the density of binding sites on both their axons and distal dendrites. Intermediate levels of binding were found on bipolar 1 degree dendrites and horizontal cells. No specific binding was detected on Müller cells and most parts of photoreceptors. Comparisons between cells showed that grain densities were similar for bipolar axons and amacrine/ganglion cell processes but bipolar dendrites were richer in binding sites than horizontal cell dendrites. Thus, muscarinic receptors in the salamander retina are located on amacrine/ganglion, bipolar, and horizontal cells and primarily confined to the processes which compose the two synaptic layers. In the inner plexiform layer, muscarinic receptors reside on processes from all three inner retinal neurons: in the outer synaptic layer, receptors are only on second-order cells and are more numerous along bipolar than horizontal cell dendrites.

Growth and synapse formation among major classes of adult salamander retinal neurons in vitro

Adult neurons, isolated from the salamander retina, were maintained in low-density cell culture and examined for synapse formation by electrophysiological and electron microscopic techniques. Morphologically identifiable rod, cone, horizontal, bipolar, and amacrine/ganglion cells survived for many months, grew processes, and formed numerous cell contacts. Intracellular recordings showed the presence of a variety of voltage- and time-dependent conductances and both electrical and chemical transmission among these cells. At the ultrastructural level, gap junctions, monad ribbon synapses, and conventional synapses, like those present in the intact retina, were observed in sibling cultures. Thus, all major classes of adult retinal neurons, in addition to ganglion cells, are able to regenerate processes and reform synapses. The regenerated synaptic contacts are functional and structurally diverse.

Rod photoreceptors dissociated from the adult rabbit retina

Rod photoreceptors have been isolated from the adult rabbit retina using enzymatic and mechanical dissociation procedures; their fine structure, synaptic activity, and long-term viability were examined using conventional electron-microscopic, quick-freezing, and cell culture techniques. Freshly dissociated photoreceptors were well-preserved compared to their counterparts in the intact retina. About half of the cells, however, exhibited broad continuity between inner and outer segments. Quick-frozen, freeze-substituted rods differed from chemically fixed cells in 3 respects: (1) there was an increased amount of granular matrix in the cytoplasm, mitochondria, and rough endoplasmic reticulum; (2) branching and anastomosing profiles of smooth endoplasmic reticulum had disappeared from the inner segment; and (3) the number of synaptic vesicles within the spherule was highly variable, in some cases leaving synaptic ribbons completely denuded of their halo of vesicles. Light-adapted, solitary rod cells continued to be synaptically active: their endings were capable of endocytosis when placed in the dark in the presence of extracellular ferritin and tracer was incorporated into vesicles and vacuoles; this uptake was much reduced when the cells were incubated with the tracer in the light. Thus, synaptic vesicle regeneration was stimulated in the dark, suggesting that vesicles underwent exocytosis in the dark. Isolated rod cells adhered poorly to most standard substrates; without proper adhesion, cells deteriorated in 2-4 hr. However, photoreceptors did adhere to glutaraldehyde-fixed Vitrogen gels and could be maintained for over 48 hr on this substrate if kept in a complete medium at 22 degrees C. In contrast, Müller cells adhered quickly to a laminin substrate with their endfoot processes. The differential adhesion properties of Müller and photoreceptor cells may be useful in obtaining pure populations of glial cells or neurons from the adult mammalian retina.

Dissociated cingulate cortical neurons: morphology and muscarinic acetylcholine receptor binding properties

Identifiable cortical neurons were obtained from area 29c of rat cingulate cortex using enzymatic and mechanical dissociation techniques. Dissociated neurons were either analyzed morphologically with the electron microscope or processed autoradiographically to evaluate the distribution of specific 3H-propylbenzilylcholine mustard (PrBCM) binding. Ultrastructurally, neurons appeared healthy and contained a full complement of cytoplasmic organelles. Membranes were intact and no presynaptic endings adhered to cell bodies or dendrites. Dendritic spines were not observed in these dissociations and serial sections of identified neurons indicated that all dendritic processes were smooth. Receptor binding studies were conducted on small and medium-to-large pyramidal neurons and multipolar cells. Specific binding of PrBCM was determined by calculating the mean number of grains/10 micron somal perimeter or dendritic length and subtracting mean values from a matched series of neurons that were coincubated in atropine. Specific binding was to somata and dendrites of all neurons. Nonspecific binding was an average of 33% of total binding. A 2 X 2 factorial analysis of variance comparing total and nonspecific binding for pairs of processes indicated that there were no regional differences in dendritic binding, either by cell type or by order of dendritic branching. Both somatic and dendritic PrBCM binding was antagonized by pirenzepine (PZ); however, PZ appeared to be more effective at secondary dendritic, rather than at somatic and primary apical dendritic sites. Thus, the IC50 values for somata and primary apical dendrites of small pyramids were 6 X 10(-7) and 9 X 10(-7) M PZ, respectively, while that for secondary basal dendrites of the same neurons was 5.8 X 10(-8) M. Morphological and pharmacological results together suggest that (1) muscarinic receptors are present on the smooth surfaces of all pyramidal and multipolar neurons; (2) many of the binding sites are high affinity, PZ-sensitive, M1 receptors; and (3) this binding is associated with the postsynaptic specialization of symmetric, cholinergic synapses.

Rod cells dissociated from mature salamander retina: ultrastructure and uptake of horseradish peroxidase

To test the effects of isolation on adult neurons, we investigated the fine structure and synaptic activity of rod cells dissociated from the mature salamander retina and maintained in vitro. First, freshly isolated rod cells appeared remarkably similar to their counterparts in the intact retina: the outer segment retained its stack of membranous disks and the inner segment contained its normal complements of organelles. Some reorganization of the cell surface, however, was observed: (a) radial fins, present at the level of the cell body, were lost; and (b) the apical and distal surfaces of the inner and outer segments, respectively became broadly fused. Second, the synaptic endings or pedicles retained their presynaptic active zones: reconstruction of serially sectioned pedicles by using three-dimensional computer graphics revealed that 73% of the synaptic ribbons remained attached to the plasmalemma either at the cell surface or along its invaginations. Finally, tracer experiments that used horseradish peroxidase demonstrated that dissociated rod cells recycled synaptic vesicle membrane in the dark and thus probably released transmitter by exocytosis. Under optimal conditions, a maximum of 40% of the synaptic vesicles within the pedicle were labeled. As in the intact retina, uptake of horseradish peroxidase was suppressed by light. Thus, freshly dissociated receptor neurons retained many of their adult morphological and physiological characteristics. In long-term culture, the photoreceptors tended to round up; however, active zones were present even 2 wk after removal of the postsynaptic processes.

Use of a monoclonal antibody as a substrate for mature neurons in vitro

A mouse monoclonal antibody was produced against salamander retinal membranes. It binds to the retina as well as to a wide variety of other salamander tissues and is called Sal-1. Because retinal neurons dissociated from the mature salamander retina adhere poorly to standard substrates, cells were plated onto coverslips previously treated with Sal-1. This previous treatment resulted in a dramatic increase in cell-substrate adhesion. At low concentrations, the antibody had no detectable effect on the light response and fine structure of freshly dissociated rod cells. After several weeks in culture, retinal neurons continued to be attached to Sal-1-treated coverslips and appeared healthy. Furthermore, many cells had extended elaborate cell processes and achieved morphologies characteristic of mature neurons. These results show that antibodies can be used as substrates for the culture of mature neurons. This technique may also prove useful for in vitro studies of a variety of cell systems.

Rod-shaped bodies and crystalloid inclusions in ocular vascular endothelia of adult and developing Macaca mulatta

In addition to the usual organelles, endothelial cells of ocular blood vessels and Schlemm's canal in adult and fetal Macaca mulatta, contain two types of inclusions: rod-shaped bodies and crystalloids. The rod-shaped bodies are similar to those first described in arterial endothelia of the rat. They are between 0.2 to 0.35 micrometer in diameter, up to 2.5 micrometers in length, and are membrane bounded. The crystalloid inclusions are up to 1.2 micrometers in diameter and are associated with the granular endoplasmic reticulum. Their crystalline lattice consists of subunits, 28 nm in diameter, which are either arranged in rows separated by amorphous matrix or tightly packed in a honeycomb-like lattice. The rod-shaped bodies are much more numerous than the crystalloids; however, both kinds of inclusions are consistently found in both adult and fetal vessels. The represent normal components of ocular vascular endothelia which appear very early in the development of the eye.

Degeneration and regeneration of autonomic nerve endings in the anterior part of rhesus monkey ciliary muscle

The autonomic nerve plexus of the ciliary muscle was examined with the electron microscope in normal rhesus monkeys of different ages. In the anterior region of the muscle, at the boundary with the poorly innervated scleral spur and trabecular meshwork, 3.8-7.1% of the axons exhibit either degenerative or regenerative features. The cytoplasm of degenerating axons contains lamellated, dense and multivesicular bodies, vesicles, whorls of filaments, and membranous debris. The plasma membrane is often discontinuous and, on occasion, axonal debris and degenerative organelles are freely dispersed in the connective tissue spaces of the muscle. Degenerating axons contain a granular reaction product when stained for acid phosphatase activity. Regenerating axons are characterized by tightly packed mitochondria, glycogen particles, and aggregates of synaptic vesicles; they synapse with muscle cells and are negative to the acid phosphatase reaction. A quantitative analysis showed that in the anterior region of the ciliary muscle degenerating and regenerating axons increase in number with age, although the total number of axonal profiles remains constant. In the age groups examined, degenerating axons occurred with the same frequency as regenerating axons, thus, the age-dependent increase in axonal degeneration is accompanied by a parallel increase in axonal regeneration. We conclude that autonomic nerve endings in the anterior part of the ciliary muscle undergo a continuous process of renewal that is more prominent in old age.

Giant nerve fibers in the ciliary muscle and iris sphincter of Macaca mulatta

Unusually large nerve processes, containing numerous mitochondria, glycogen particles, and synaptic vesicles are described in both the ciliary muscle and the iris sphincter muscle of the rhesus monkey. The striking similarity of these axonal profiles to the dendritic enlargements observed by Sotelo and Palay (1968) is noted and the possibility that they represent growing ends of peripheral nerve fibers is suggested.