Cholinoceptive neurons in the retina of the chick: an immunohistochemical study of the nicotinic acetylcholine receptors

Time-Dependent Decline in Multifocal Electroretinogram Requires Faster Recording Procedures in Anesthetized Pigs

Purpose

The time-dependent effect of anesthetics on the retinal function is debated. We hypothesize that in anesthetized animals there is a time-dependent decline that requires optimized multifocal electroretinogram (mfERG) recording procedures.

Methods

Conventional and four-frame global-flash mfERG recordings were obtained approximately 15, 60, and 150 minutes after the induction of propofol anesthesia (20 pigs) and isoflurane anesthesia (nine pigs). In six of the propofol-anesthetized pigs, the mfERG recordings were split in 3-minute segments. Two to 4 weeks after initial recordings, an intraocular injection of tetrodotoxin (TTX) was given and the mfERG was rerecorded as described above. Data were analyzed using mixed models in SAS statistical software.

Results

Propofol significantly decreases the conventional and global-flash amplitudes over time. The only significant effect of isoflurane is a decrease in the global-flash amplitudes. At 15 minutes after TTX injection several of the mfERG amplitudes are significantly decreased. There is a linear correlation between the conventional P1 and the global-flash DR mfERG-amplitude (R² = 0.82, slope = 0.72, P < 0.0001). There is no significant difference between the 3-minute and the prolonged mfERG recordings for conventional amplitudes and the global-flash direct response. The global flash–induced component significantly decreases with prolonged mfERG recordings.

Conclusions

A 3-minute mfERG recording and a single stimulation protocol is sufficient in anesthetized pigs. Recordings should be obtained immediately after the induction of anesthesia. The effect of TTX is significant 15 minutes after injection, but is contaminated by the effect of anesthesia 90 minutes after injection. Therefore, the quality of mfERG recordings can be further improved by determining the necessary time-of-delay from intraocular injection of a drug to full effect.

Translational Relevance

General anesthesia is a possible source of error in mfERG recordings. Therefore, it is important to investigate the translational relevance of the results to mfERG recordings in children in general anesthesia.

Fixation strategies for retinal immunohistochemistry

Immunohistochemical and ex vivo anatomical studies have provided many glimpses of the variety, distribution, and signaling components of vertebrate retinal neurons. The beauty of numerous images published to date, and the qualitative and quantitative information they provide, indicate that these approaches are fundamentally useful. However, obtaining these images entailed tissue handling and exposure to chemical solutions that differ from normal extracellular fluid in composition, temperature, and osmolarity. Because the differences are large enough to alter intercellular and intracellular signaling in neurons, and because retinae are susceptible to crush, shear, and fray, it is natural to wonder if immunohistochemical and anatomical methods disturb or damage the cells they are designed to examine. Tissue fixation is typically incorporated to guard against this damage and is therefore critically important to the quality and significance of the harvested data. Here, we describe mechanisms of fixation; advantages and disadvantages of using formaldehyde and glutaraldehyde as fixatives during immunohistochemistry; and modifications of widely used protocols that have recently been found to improve cell shape preservation and immunostaining patterns, especially in proximal retinal neurons.

Retinal ganglion cell neuroprotection induced by activation of alpha7 nicotinic acetylcholine receptors

The α7nAChR agonist, PNU-282987, has previously been shown to have a neuroprotective effect against loss of retinal ganglion cells (RGCs) in an in vivo glaucoma model when the agent was injected into the vitreous chamber of adult Long Evans rat eyes. Here, we characterized the neuroprotective effect of PNU-282987 at the nerve fiber and retinal ganglion cell layer, determined that neuroprotection occurred when the agonist was applied as eye drops and verified detection of the agonist in the retina, using LC/MS/MS. To induce glaucoma-like conditions in adult Long Evans rats, hypertonic saline was injected into the episcleral veins to induce scar tissue and increase intraocular pressure. Within one month, this procedure produced significant loss of RGCs compared to untreated conditions. RGCs were quantified after immunostaining with an antibody against Thy 1.1 and imaged using a confocal microscope. In dose-response studies, concentrations of PNU-282987 were applied to the animal's right eye two times each day, while the left eye acted as an internal control. Eye drops of PNU-282987 resulted in neuroprotection against RGC loss in a dose-dependent manner using concentrations between 100 μM and 2 mM PNU-282987. LC/MS/MS results demonstrated that PNU-282987 was detected in the retina when applied as eye drops, relatively small amounts of PNU-282987 were measured in blood plasma and no PNU-282987 was detected in cardiac tissue. These results support the hypothesis that eye drop application of PNU-282987 can prevent loss of RGCs associated with glaucoma, which can lead to neuroprotective treatments for diseases that involve α7nAChRs.

Classification of nAChRβ2-immunoreactive retinal ganglion cells and their tectal projections in chicks

The relationship between the type of retinal ganglion cell (RGC) and the retinoreceptive layer of the tectum is investigated by the immunostaining of RGCs with nicotinic acetylcholine receptorβ2 (nAChRβ2) antibody and intracellular staining by DiI and also by anterograde degeneration and biotinylated dextran amine labeling of retinotectal fibers in chicks. The results strongly suggest that many of the RGCs that express immunoreactivity to nAChRβ2 send axons to tectal layer 7 and are mainly classified into the simple-type of Groups II and III, which contain the cells providing middle-sized to large dendritic fields with simple dendritic arborization. These nAChRβ2-immunoreactive RGCs receive visual information via the multiple sublayers of the inner plexiform layer.

The effects of nicotine on the human electroretinogram

PURPOSE

To examine the effects of nicotine on responses from the human retina measured electrophysiologically.

METHODS

Electroretinogram (ERG) responses were obtained from ten healthy, visually normal adults who were nonsmokers. Nicotine (2 and 4 mg) and a placebo were administered in the form of gum 30 minutes before testing in two separate experiments. ERG responses were collected and analyzed using a full-field ERG system. Responses were recorded from one eye of each subject using a bipolar contact-lens electrode. Intensity-response curves were obtained under both dark- and light-adapted conditions. In experiment 1, both dark- and light-adapted tests were completed sequentially. In experiment 2, only light-adapted testing was performed. Intensity-response functions were analyzed using the Naka-Rushton equation.

RESULTS

In experiment 1, compared with placebo, dark-adapted b-wave amplitude responses decreased significantly after chewing gum containing both 2 and 4 mg of nicotine. Under light-adapted conditions, the peak b-wave amplitude was significantly decreased after chewing gum containing 4 mg of nicotine. In experiment 2, light-adapted b-wave amplitudes were increased after 4 mg nicotine. Oscillatory potentials were measured but no significant effects under nicotine were observed.

CONCLUSIONS

To the knowledge of the authors, this is the first demonstration that nicotine by itself affects responses in the human retina. These data support reports of the expression of nicotinic acetylcholine receptors in rabbit and nonhuman primate retina.

Muscarinic signaling influences the patterning and phenotype of cholinergic amacrine cells in the developing chick retina

Background

Many studies in the vertebrate retina have characterized the differentiation of amacrine cells as a homogenous class of neurons, but little is known about the genes and factors that regulate the development of distinct types of amacrine cells. Accordingly, the purpose of this study was to characterize the development of the cholinergic amacrine cells and identify factors that influence their development. Cholinergic amacrine cells in the embryonic chick retina were identified by using antibodies to choline acetyltransferase (ChAT).

Results

We found that as ChAT-immunoreactive cells differentiate they expressed the homeodomain transcription factors Pax6 and Islet1, and the cell-cycle inhibitor p27^kip1. As differentiation proceeds, type-II cholinergic cells, displaced to the ganglion cell layer, transiently expressed high levels of cellular retinoic acid binding protein (CRABP) and neurofilament, while type-I cells in the inner nuclear layer did not. Although there is a 1:1 ratio of type-I to type-II cells in vivo, in dissociated cell cultures the type-I cells (ChAT-positive and CRABP-negative) out-numbered the type-II cells (ChAT and CRABP-positive cells) by 2:1. The relative abundance of type-I to type-II cells was not influenced by Sonic Hedgehog (Shh), but was affected by compounds that act at muscarinic acetylcholine receptors. In addition, the abundance and mosaic patterning of type-II cholinergic amacrine cells is disrupted by interfering with muscarinic signaling.

Conclusion

We conclude that: (1) during development type-I and type-II cholinergic amacrine cells are not homotypic, (2) the phenotypic differences between these subtypes of cells is controlled by the local microenvironment, and (3) appropriate levels of muscarinic signaling between the cholinergic amacrine cells are required for proper mosaic patterning.

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Labeled lines in the retinotectal system: markers for retinorecipient sublaminae and the retinal ganglion cell subsets that innervate them

Axons of retinal ganglion cells (RGCs) carry visual information to the brain. In most vertebrates, the major synaptic target of RGCs is the optic tectum. In the chick, RGC axons form synapses in just 4 of 16 histologically recognizable laminae (the retinorecipient laminae [RRLs]), and arbors of individual RGCs are confined to a single RRL. To analyze the development and function of these parallel pathways, markers are required that selectively label them. Here, we have identified molecular markers for individual RRLs and for RGCs that project to them. Some of the markers may mediate or modulate signaling through the separate pathways: neuropeptides (substance P, neuromedin B, somatostatin-I and -II) and their receptors (substance P receptor), neurotransmitter synthetic enzymes (choline acetyltransferase) and the corresponding receptors (acetylcholine receptor beta2) and calcium-binding proteins (parvalbumin and calbindin). Other markers are adhesive proteins that could mediate selective connectivity of RGC subsets within specific RRLs (cadherin-7, cadherin-11, reelin and neuropilin-1). We further show that RGC subsets whose axons project to specific RRLs are heterogeneous with respect to the retinal sublaminae within which their dendrites arborize. Our results define laminar-specified circuits from retina to brain and support a model in which RGCs transmit information from multiple sources to single central laminae, where it can be integrated.

The role of the retinal pigment epithelium in eye growth regulation and myopia: a review

Myopia is increasing in prevalence world-wide, nearing epidemic proportions in some populations. This has led to expanded research efforts to understand how ocular growth and refractive errors are regulated. Eye growth is sensitive to visual experience, and is altered by both form deprivation and optical defocus. In these cases, the primary targets of growth regulation are the choroidal and scleral layers of the eye that demarcate the boundary of the posterior vitreous chamber. Of significance to this review are observations of local growth modulation that imply that the neural retina itself must be the source of growth-regulating signals. Thus the retinal pigment epithelium (RPE), interposed between the retina and the choroid, is likely to play a critical role in relaying retinal growth signals to the choroid and sclera. This review describes the ion transporters and signal receptors found in the chick RPE and their possible roles in visually driven changes in eye growth. We focus on the effects of four signaling molecules, otherwise implicated in eye growth changes (dopamine, acetylcholine, vasoactive intestinal peptide (VIP), and glucagon), on RPE physiology, including fluid transport. A model for RPE-mediated growth regulation is proposed.

Choline acetyltransferase-immunoreactive neurons in the retina of adult and developing lampreys

The presence of choline acetyltransferase-immunoreactive (ChATir) amacrine cells is reported for the first time in the retinas of three species of lamprey (Lampetra fluviatilis, Ichthyomyzon unicuspis, and Petromyzon marinus). In the three species, the ChATir cells were mainly distributed in the inner plexiform layer (IPL), which in lampreys extends from the inner nuclear layer (INL) to the inner limiting membrane. These cells had a bipolar, triangular or stellate appearance, and gave rise to processes coursing in the inner plexiform layer. In transforming lampreys, ChATir processes formed two asymmetrical inner and outer subplexuses in the inner plexiform layer, which is reminiscent of the distribution of processes of ChATir cells in the On and Off sublaminae reported in jawed vertebrates. The larval retina lacked ChAT immunoreactivity, and ChATir cells and processes appeared at early metamorphosis throughout the retina, exhibiting in late transforming stages an organization similar to that of adults. This first report of ChATir cells in the lamprey retina indicates that the appearance of cholinergic circuits in the retina of vertebrates occurred before the divergence of the agnathan and gnathostome lines.

Localization of voltage-sensitive L-type calcium channels in the chicken retina

L-type calcium channels have been associated with synaptic transmission in the retina, and are a potential site for modulation of the release of neurotransmitters. The present study documents the immunohistochemical localization of neuronal alpha1 subunits of L-type calcium channels in chicken retina, using antibodies to the alpha1c, alpha1d and alpha1f subunits of L-type calcium channels. The alpha1c-like subunits were localized to Müller cells, with predominantly radial processes, and a prominent band of horizontal processes in the outer plexiform layer. The antibody to alpha1d subunits labelled most, if not all, cell bodies. The antibody to a human alpha1f subunit strongly labelled photoreceptor terminals. Fainter immunoreactivity was detected in the inner segments of the photoreceptors, a subset of amacrine cells, two bands of labelling in the inner plexiform layer and many ganglion cells. The differential cellular distributons of these alpha1-subunits suggests subtle functional differences in their roles at different cellular locations.

Development of retinal ganglion cell structure and function

In this review, we summarize the main stages of structural and functional development of retinal ganglion cells (RGCs). We first consider the various mechanisms that are involved in restructuring of dendritic trees. To date, many mechanisms have been implicated including target-dependent factors, interactions from neighboring RGCs, and afferent signaling. We also review recent evidence showing how rapidly such dendritic remodeling might occur, along with the intracellular signaling pathways underlying these rearrangements. Concurrent with such structural changes, the functional responses of RGCs also alter during maturation, from sub-threshold firing to reliable spiking patterns. Here we consider the development of intrinsic membrane properties and how they might contribute to the spontaneous firing patterns observed before the onset of vision. We then review the mechanisms by which this spontaneous activity becomes correlated across neighboring RGCs to form waves of activity. Finally, the relative importance of spontaneous versus light-evoked activity is discussed in relation to the emergence of mature receptive field properties.

Evidence of muscarinic acetylcholine receptors in the retinal centrifugal system of the chick

In this study we characterize the presence of muscarinic acetylcholine receptors (mAChR) in the isthmo-optic nucleus (ION) of chicks by immunohistochemistry with the M35 antibody. Some M35-immunoreactive fibers were observed emerging from the retinal optic nerve insertion, suggesting that they could be centrifugal fibers. Indeed, intraocular injections of cholera toxin B (CTb), a retrograde tracer, and double-labeling with M35 and CTb in the ION confirmed this hypothesis. The presence of M35-immunoreactive cells and the possible mAChR expression in ION and ectopic neuron cells in the chick brain strongly suggest the existence of such a cholinergic system in this nucleus and that acetylcholine release from amacrine cells may mediate interactions between retinal cells and ION terminals.

The transport rate of cholera toxin B subunit in the retinofugal pathways of the chick

This study investigated the transport rate of the tracer, cholera toxin B subunit, within the retinofugal pathway of the chick hatchlings. Following intraocular injections, the chicks were allowed to survive for various time-periods. The immunoreactivity of cholera toxin B subunit was then examined in the retinofugal pathways. Two hours post-injection, retinal ganglion cells began to take up the tracer and transport it to the most rostroventral portion of the optic tectum. After a 4 h survival period, the labeled retinal axons progressively innervated all retinofugal targets. Within the tectum, the labeling density varied from layer to layer with heavily labeled terminals in layer 5b, less label in layer 7 and the most diffuse label in layers 2-4. Scattered labeling was seen in the nucleus dorsolateralis anterior thalami, pars lateralis, the nucleus geniculatus lateralis, pars ventralis, the nucleus basal optic root, the nucleus lateralis anterior thalami, and the pretecal lentiformis nucleus of mesencephalon. After 6- and 8 h survival periods, increased labeling was seen in all retinofugal nuclei. There were increased numbers of retinal terminals in all retinorecipient layers of the tectum. It was noted that some of the retinal axons "overshot" into layers deeper than layer 7. In addition, retinal projections were found scattered throughout the ipsilateral nucleus basal optic root. Maximal labeling in all retinofugal targets was observed at a 10 h survival period. The present study suggests that cholera toxin B subunit can be used to trace retinal axons along their retinofugal paths up to the small terminal zones at a rate of 4.25 mm/h or 102 mm/day. Also, evidence of synchronous retinal terminations in layers 5b and 7 indicates that the transport of cholera toxin B subunit is independent of axon diameters of retinal ganglion cells. Finally, given the changing status of the embryo, the rapid transport of cholera toxin B subunit can be applied for tracing developing pathways.

Spontaneous activity in developing turtle retinal ganglion cells: pharmacological studies

Extracellular recordings were obtained from the ganglion cell (GC) layer during correlated spontaneous bursting activity (SBA) in the immature turtle retina. Pharmacological agents were bath-applied, and their effects on burst and correlation parameters were determined. SBA requires synaptic transmission. It was blocked in the presence of curare and mecamylamine, two cholinergic nicotinic antagonists, and enhanced with neostigmine, a cholinesterase inhibitor. SBA was profoundly inhibited during blockade of glutamatergic receptors with the broad spectrum antagonist kynurenate and it vanished with 6,7-dinitroquinoxaline-2-3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), two AMPA/kainate receptor antagonists. Blockade of NMDA receptors with D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) led only to a modest reduction in SBA. Blockade of GABAA receptors with bicuculline prolonged the duration of the bursts. Inhibition of GABA uptake with nipecotic acid led to a decrease in burst rate. Blockade of K+ channels with cesium (Cs+) and tetraethylammonium (TEA) led to a dramatic decrease in excitability. Burst propagation between neighboring GCs was reduced by K+ channel blockade. Gap junction blockade had no consistent effect on bursts or correlation parameters. None of these drugs had a strong effect on the refractory period between bursts. We conclude that correlated SBA in immature turtle GCs requires both cholinergic nicotinic and glutamatergic (mainly through AMPA/kainate receptors) synaptic transmission. GABAergic activity modulates the intensity and the duration of the bursts. Extracellular K+ is involved in lateral activity propagation and increases retinal excitability, which may be required for burst generation.

Specific expression of ezrin, a cytoskeletal-membrane linker protein, in a subset of chick retinotectal and sensory projections

Lamina-specific neuronal connections are a fundamental feature in many parts of the vertebrate central nervous system. In the chick, the optic tectum is the primary visual centre, and it has a multilaminated structure consisting of 15 laminae, of which only three or four receive retinal projections. Each of the retinorecipient laminae establishes synaptic connections selectively from one of a few subsets of retinal ganglion cells (RGCs). We have generated a series of monoclonal antibodies that appear to stain only one of the retinorecipient laminae. One of these, TB4, stained lamina F which receives inputs from a subpopulation of approximately 10-20% of RGCs which express the presynaptic acetylcholine receptor beta2-subunit. TB4 recognized a single 79-kDa protein on immunoblotting. cDNA cloning and immunochemical analysis revealed that the TB4 antigen molecule was ezrin, a cytoskeletal-membrane linker molecule belonging to the ezrin-radixin-moesin family. Unilateral enucleation of the eye, both prior to and after the establishment of retinotectal projections, attenuated the lamina-selective staining with TB4 in the contralateral tectum, suggesting that ezrin is anterogradely transported from RGCs to lamina F. Ezrin was thus expressed in a subset of RGCs that project to lamina F. Similar subset-selective expression and resultant lamina-selective distribution of ezrin were also observed in the lamina-specific central projections from the dorsal root ganglia. The staining pattern with TB4 in the dorsal root ganglia and spinal cord indicated that high expression of ezrin was restricted in cutaneous sensory neurons, but not in muscle sensory neurons. Since ezrin modulates cell morphology and cell adhesion profiles by linking membrane proteins with the cytoskeleton, it was suggested that ezrin is involved in the formation and/or maintenance of lamina-specific connections for neuronal subpopulations in the visual and somatosensory systems.

Developmentally regulated spontaneous activity in the embryonic chick retina

Even before birth and the onset of sensory experience, neural activity plays an important role in shaping the vertebrate nervous system. In the embryonic chick visual system, activity in the retina before vision has been implicated in the refinement of retinotopic maps, the elimination of transient projections, and the survival of a full complement of neurons. In this study, we report the detection of a physiological substrate for these phenomena: waves of spontaneous activity in the ganglion cell layer of the embryonic chick retina. The activity is robust and highly patterned, taking the form of large amplitude, rhythmic, and wide-ranging waves of excitation that propagate across the retina. Activity waves are most prominent and organized between embryonic days 13-18, coinciding with the developmental period during which retinal axons refine their connections in their targets. The spatial and temporal features of the patterns observed are consistent with the role of activity patterns in shaping eye-specific projections and retinotopic maps but inconsistent with the hypothesis that they specify lamina-specific projections in the tectum. Antagonists of glutamatergic and glycinergic transmission and of gap junctional communication suppress spontaneous activity, whereas antagonists to GABAergic transmission potentiate it. Based on these results, we propose that spontaneous activity in the ganglion cells is regulated by chemical inputs from both bipolar and amacrine cells and by gap junctional coupling involving ganglion cells.

Motion-sensitive neurons in the chick retina: a study using Fos immunohistochemistry

Fos immunohistochemistry was used to characterize neurons in the chick retina activated by optokinetic and stationary stimuli. Higher percentages of co-localization of Fos and the alpha5 subunit of the nicotinic acetylcholine receptor, and Fos and GABA were observed in retinal neurons after optokinetic compared to the stationary stimulation. These results indicate an involvement of the cholinergic and GABAergic circuitries in the motion detection by chick retinal cells.

Differential co-localization of nicotinic acetylcholine receptor subunits with calcium-binding proteins in retinal ganglion cells

Immunohistochemistry was used to examine the co-occurrence of nicotinic acetylcholine receptor subunits with calcium-binding proteins in ganglion cells of the chick retina. The alpha3 subunit was rarely observed in ganglion cells containing calbindin, calretinin, or parvalbumin. On the other hand, the alpha8 subunit was more often co-localized with all calcium-binding proteins studied. These results may be related to the high calcium permeability of nicotinic receptors that contain the alpha8 subunit.

The triggering of spreading depression in the chick retina by nicotinic receptor agonists

Spreading depression was evoked in vitro in retinas of 3-6 day old chickens by the nicotinic cholinergic agonists nicotine and cytisine. The response was reproducible and inhibited by the nicotinic cholinergic receptor antagonist mecamylamine and by the NMDA receptor antagonist -3-(2 carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP). The response to nicotinic agonists was not inhibited by alpha-bungarotoxin. The data show that spreading depression can be evoked in the chick retina by alpha-bungarotoxin insensitive nicotinic acetylcholine receptor subtypes and that the response is dependent upon NMDA receptor activation. This nicotine evoked spreading depression was inhibited by cadmium chloride indicating the involvement of voltage sensitive calcium channels. It is therefore argued that nicotine evokes spreading depression indirectly, as a result of calcium sensitive glutamate release. The glutamate released thus exerting its effects via NMDA receptors.

Presumptive presynaptic nicotinic acetylcholine receptors in the chick tectum: effects of lesions of the lateral spiriform nucleus

There are indications that nicotinic acetylcholine receptor subunits in the superficial layers of the chick tectum (Cajal's layers 1-7) may be transported from the retina. However, nicotinic receptor subunits are detectable by immunohistochemistry in all layers of the optic tectum. In this study, we performed unilateral electrolytic lesions of the lateral spiriform nucleus, which projects to the deep layers of the tectum and contains high amounts of nicotinic receptors in its perikarya. Following lesions of the lateral spiriform nucleus, both the alpha 5 and the beta 2 subunits were markedly depleted in the neuropil of the deep layers of the ipsilateral optic tectum (layers 8-13). No changes were observed in somata that contain either subunit. The present results suggest that most of the nicotinic acetylcholine receptor subunits in the chick optic tectum occur in axonal systems and could then constitute presynaptic receptors.

Optical recordings of the effects of cholinergic ligands on neurons in the ganglion cell layer of mammalian retina

Cholinergic regulation of the activity of rabbit retinal ganglion cells and displaced amacrine cells was investigated using optical recording of changes in intracellular free calcium ([Ca2+]i). Labeling of neurons in the mature retina was achieved by injecting calcium green-1 dextran (CaGD) into the isolated retina. Nicotine increased ganglion cell [Ca2+]i, affecting every loaded cell in some preparations; the pharmacology of nicotine was consistent with an action at neuronal nicotinic receptors, and specifically it was kappa-(neuronal-)bungarotoxin-sensitive but alpha-bungarotoxin-insensitive. Muscarine also raised [Ca2+]i, but it was less potent than nicotine, affecting only a subpopulation of ganglion cells, with an M1-like muscarinic receptor pharmacology. Neither the nicotine- nor muscarine-induced increases of ganglion cell [Ca2+]i were blocked by the glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and aminophosphonopentanoic acid. Therefore, the effects of cholinergic agonists on ganglion cell [Ca2+]i were not attributable to an indirect effect mediated by glutamatergic bipolar cells. The effects of nicotine and muscarine were abolished in calcium-free solution, indicating that the responses depend on calcium influx. Displaced (Cb) cholinergic amacrine cells were also loaded with CaGD and were identified by selective labeling with the nuclear dye 4',6-diamidino-2-phenyl-indole. Cb amacrine cells did not respond to either nicotine or muscarine, but responded vigorously to the glutamate receptor agonist kainic acid. There is anatomical evidence indicating that cholinergic amacrine cells make synaptic contact with each other, but the present results do not support the hypothesis that communication between these cells is cholinergic.

Neurogenesis of cholinoceptive neurons in the chick retina

Immunocytochemistry and [3H]thymidine autoradiography were combined in this study to determine the neurogenesis of cholinoceptive cells in the chick retina. After injections of [3H]thymidine between embryonic days 1 and 11, the time of birth of retinal neurons containing either the alpha 3 or the alpha 8 subunit of the nicotinic acetylcholine receptors was determined. The results indicate that the alpha 3-positive neurons in the ganglion cell layer leave the cell cycle from E2 through E7, and those in the inner nuclear layer (amacrine and displaced ganglion cells) from E2 through E9. The alpha 8-positive cells in the ganglion cell layer were born from E1 through E7, and those in the inner nuclear layer (amacrine and bipolar cells) from E2 through E11. These data suggest that the time of birth of cholinoceptive neurons in the chick retina follows the general pattern of cell generation in the chick retina, and that alpha 8-positive cells in the ganglion cell layer start to leave the cell cycle almost one day earlier than the alpha 3-positive cells in the same layer.

Target-independent diversification and target-specific projection of chemically defined retinal ganglion cell subsets

In diverse vertebrate species, defined subsets of retinal ganglion cells (RGCs, the neurons that project from retina to brain) are distinguishable on the basis of their dendritic morphology, physiological properties, neurotransmitter content and synaptic targets. Little is known about when this diversity arises, whether diversification requires target-derived signals, and how subtype-specific projection patterns are established. Here, we have used markers for two chemically defined RGC subsets in chick retina to address these issues. Antibodies to substance P (SP) and the nicotine acetylcholine receptor (AChR) beta 2 subunit label two small (&lt; 10%), mutually exclusive groups of RGCs in mature retina. SP and AChRs accumulate in distinct RGCs before retinotectal synapses have formed. Moreover, both populations of RGCs form in retinae that develop following tectal ablation or transplantation to the coelomic cavity. Thus, RGC subsets acquire distinct neurotransmitter phenotypes in the absence of extraretinal cues. In the mature optic tectum, SP- and AChR-positive RGC axonal arbors are confined to distinct retinorecipient (synaptic) laminae. In the developing tectum, SP- and AChR-positive axons are initially intermingled in a superficial fiber layer, but then enter and arborize in appropriate laminae soon after those laminae form. Importantly, SP-positive axons, which synapse in a superficial lamina, never extend into the deeper, AChR-positive lamina. Tectal interneurons rich in SP receptors are concentrated in the lamina to which SP-positive RGC axons project, and a set of cholinergic (choline acetyltransferase-positive) tectal projection neurons elaborate dendrites in the lamina to which AChR-positive RGC axons project. These populations of tectal neurons, which are likely targets of the RGC subsets, form in tecta that develop following enucleation. Thus, RGCs and their targets can diversify in each others absence. Accordingly, we propose that the lamina-selective connectivity we observe reflects the presence of complementary cues on RGC subsets and their laminar targets.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

Differential development of alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors in the chick retina

The development of cells containing neuronal nicotinic receptors (nAChRs) in the chick retina was investigated by means of immunohistochemical techniques with antibodies directed against the alpha 3 and alpha 8 nAChR subunits. The alpha 3 subunit is one of the major alpha-bungarotoxin-insensitive nicotinic receptor subunits in the chick retina, whereas alpha 8 appears to be the most common alpha-bungarotoxin-sensitive subunit in the same structure, alpha 3-like immunoreactivity (alpha 3-LI) was first detected in cells of the vitreal margin, on the embryonic day 4.5 (E4.5). alpha 8-LI was first detected in the same type of cell almost a day later. However, the processes of alpha 8-LI cells developed much faster than those of alpha 3-LI cells, generating visible stained laminae in the prospective inner plexiform layer as early as E7. alpha 3-LI was only clearly seen in laminae of the inner plexiform layer by E12. By this date, both alpha 3 and alpha 8-LI were seen in the same types of cells as in the adult retina, i.e., amacrines, displaced ganglion cells, and cells of the ganglion cell layer for alpha 3-LI; and amacrines, bipolar cells, and cells of the ganglion cell layer for alpha 8-LI. These results reveal different patterns of development of cells containing the alpha 3 and alpha 8 nAChR subunits in the chick retina and indicate that those nAChR subunits are expressed in the chick retina before choline acetyltransferase-positive cells can be detected and well before synaptogenesis. These data also suggest that nAChRs may have a developmental function in the retina.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Bipolar cells of the chick retina containing alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors

Two cDNA clones for nicotinic acetylcholine receptor (nAChR) subunits sensitive to alpha-bungarotoxin (alpha-Bgt) have been isolated, the so-called alpha-Bgt binding proteins alpha 1 (or alpha 7 nAChR subunit) and alpha 2 (or alpha 8 nAChR subunit). Immunohistochemical experiments have shown that both alpha 7 and alpha 8 subunits, as well as subunits insensitive to alpha-Bgt (beta 2 and alpha 3), are present in amacrine and ganglion cells of the chick retina. However, only the alpha 8 subunit was observed in presumptive bipolar cells. The present study investigated in detail the pattern of distribution of the bipolar cells containing the alpha 8 nAChR subunit and its relation to the pattern of distribution of bipolar cells immunoreactive to protein kinase C (PKC). Presumptive alpha 8- and PKC-like immunoreactive (alpha 8-LI and PKC-LI) bipolar cells were observed sending their dendrites to the outer plexiform layers and their axons to the inner plexiform layer. Whereas alpha 8-LI bipolar cells corresponded to 40-53% of the whole population of bipolar cells, PKC-LI bipolar cells represented only 6-8% of the same population. The soma sizes of the alpha 8-LI bipolar cells were slightly smaller (mean +/- S.D.; 4.9 +/- 0.8 microns) than the soma sizes of the PKC-LI bipolar cells (5.4 +/- 0.9 microns). Double-labeling experiments indicated that probably all PKC-LI bipolar cells also contain alpha 8-LI. This indicates that two distinct groups of cholinoceptive bipolar cells exist in the chick retina, one that contains PKC-LI, and another one that does not.

c-fos and NGFI-A mRNA of rat retina: evidence for light-induced augmentation and a role for cholinergic and glutamate receptors

When rats are exposed to room light from the dark, there is a transient increase of mRNA for the immediate-early genes c-fos and NGFI-A in the retina. Augmentation of c-fos and NGFI-A mRNA by light is apparently associated with activation of cholinergic nicotinic and muscarinic receptors as it can be suppressed by the nicotinic antagonist mecamylamine and the muscarinic antagonist atropine. Moreover, the light-induced increase of c-fos mRNA in retina appears to be associated with activation of glutamate receptors also as the noncompetitive inhibitor of N-methyl-D-aspartate receptors dizocilpine (MK-801) partially suppressed the increase of the c-fos message. Light-induced NGFI-A mRNA augmentation is apparently modulated by the same receptors. We were unable to detect light-induced changes of c-jun mRNA.

[Leu5]enkephalin-like immunoreactive amacrine cells are under nicotinic excitatory control during darkness in chicken retina

Based on the principle that retinal levels of [Leu5]enkephalin-like immunoreactivity (LELI) are set by the rate of release and thus reflect neural activity, we partially defined the dark-associated increase in excitatory control of LELI amacrine cells in chicken. Retinal levels of LELI were measured by radioimmunoassay (RIA). Intravitreal injection of cholinergic antagonists decreased the rate of depletion of LELI during the dark phase, suggesting the presence of cholinergic excitatory control of the LELI neurons. This cholinergic control involves nicotinic rather than muscarinic receptors, as tubocurarine appeared over 100 times more effective than atropine in inhibiting the decrease in retinal levels of LELI in the dark. (The ED50s were estimated at 3.2 and 450 nmol, respectively.) The lack of effect of the antagonists when applied during the light phase, suggest that there is little cholinergic input to the LELI amacrine cells in the light. Superfusing isolated retinas with buffer containing tubocurarine (10 microM) decreased the efflux of LELI by 35%, compared to the spontaneous release during the dark. Atropine (10 microM) had no effect on the release of LELI, and pilocarpine (100 microM) increased the release of LELI from retinas superfused in the light by 20%. We conclude that, in addition to previously reported glycinergic and dopaminergic inhibition, the LELI amacrine cells receive cholinergic excitatory input. A shift in balance between glycinergic and dopaminergic inhibitory, and cholinergic excitatory control may underly the light-driven variation in activity of the LELI neurons in chicken retina.

Different subsets of displaced ganglion cells in the pigeon retina exhibit cholecystokinin-like and enkephalin-like immunoreactivities

Immunohistochemical and retrograde tracing techniques were combined in order to identify chemically specific displaced ganglion cells in the pigeon retina. About 15% of the displaced ganglion cells that were retrogradely labeled following injections of different tracers into the accessory optic nucleus were shown to contain cholecystokinin8-like immunoreactivity. These cells were medium to large (15-30 microns) and located mostly in the peripheral retina. Another population of about 9% of the retrogradely labeled displaced ganglion cells was shown to contain leucine-enkephalin-like immunoreactivity. These cells were medium-sized (11-18 microns) and distributed almost evenly throughout the retina. These two types of displaced ganglion cells represent together only about 0.1% of the total number of ganglion cells in the pigeon retina. Taken together with previous results, these data indicate that the displaced ganglion cells of the avian retina may comprise several chemically specific cell types. The present results also contribute information on the chemical heterogeneity of retinal ganglion cells.

Neurons of the chick brain and retina expressing both alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive nicotinic acetylcholine receptors: an immunohistochemical analysis

Immunohistochemical methods were used to study the possible co-localization of two alpha-bungarotoxin-sensitive (alpha 7 and alpha 8) and two alpha-bungarotoxin-insensitive (beta 2 and alpha 3) subunits of the nicotinic acetylcholine receptors in neurons of the chick brain and retina. Several structures contained neurons that were doubly-labeled with antibodies against the alpha 7 subunit and the beta 2 subunit. These structures included, for example, the interpeduncular nucleus, nucleus spiriformis lateralis, optic tectum, pretectal visual nuclei, and the lateral hypothalamus. Double-labeling with antibodies against the alpha 7 and alpha 8 subunits was also seen in several regions, which included the interpeduncular nucleus, visual pretectum, lateral hypothalamus, dorsal thalamus, and the habenular complex. In the retina, many cells in the inner nuclear layer were observed to contain alpha 8 and alpha 3 subunits, whereas neurons in the ganglion cell layer were seen to contain alpha 7 and alpha 8 or, less frequently, alpha 7 and alpha 3 subunits. These results indicate that alpha-bungarotoxin-sensitive and alpha-bungarotoxin-insensitive subunits of the nicotinic receptors are co-expressed by neurons of the chick brain and retina.

Distribution of neuropeptide Y, substance P, and choline acetyltransferase in the developing visual system of the pigeon and effects of unilateral retina removal

The distribution of three neuroactive substances, neuropeptide Y, substance P, and choline acetyltransferase, was studied by immunocytochemical methods in central visual regions of adult, developing, and ablated pigeon brains. In normal adult brains, neuropeptide Y-positive cells and processes were present in the nucleus pretectalis, the nucleus of the basal optic root, the nucleus of the marginal optic tract, and the visual Wulst. Substance P-positive cells and processes were found in the optic tectum and in the visual Wulst. Stained fibers and terminal-like processes, but no cells, were also observed in several visual thalamic nuclei. Choline acetyltransferase-positive cells and processes were located in the optic tectum, visual Wulst, the nucleus isthmo opticus, nucleus isthmi and certain visual thalamic nuclei. Cholinergic fibers and processes, but no cells, were present in the nucleus principalis precommissuralis, the supraoptic decussation, and the nucleus lentiformis mesencephali, pars magnocellularis. In the course of development, the distribution of immunoreactivity for all three substances was found to vary. These changes often involved either progressive increases or decreases in the density of labeled cells, neuropil and/or terminal-like profiles. Experiments with retina ablated pigeons clearly demonstrated that changes in the normal pattern of immunoreactivity distribution only occurred if the retina was removed immediately after hatching, i.e., before retinofugal connections have been established. The adult pattern of immunoreactivity for all three substances appears to be reached at about the same time that the anatomical and functional maturation of the pigeon visual system is completed. The present results suggest that this temporal correlation reflects the important role that retinal afferents play in the development of these putative peptidergic and cholinergic systems.

Temporally correlated expression of nAChR genes during development of the mammalian retina

In situ hybridizations and RNase protection assays have been used to characterize nicotinic acetylcholine receptor (nAChR) gene expression in the developing and adult rat retina. At the earliest time examined (embryonic day 13) a low level of alpha-3 and beta-4 gene expression could be detected. During the next 48 hr there was a dramatic induction of the alpha-3, alpha-4, beta-2, beta-3 and beta-4 genes in the recently differentiated retinal ganglion cells. By post-natal day 4 we detected nAChR gene expression in the inner nuclear layer. In the adult retina, in situ hybridizations showed these genes are expressed by cells residing in the ganglion and inner nuclear layers. These results suggest a common regulatory mechanism for the induction of nAChR expression in retinal ganglion cells during development. In addition, the variety of nAChR genes expressed in the retina imply a relatively large number of different types of nAChRs can be expressed by these cells.

Immunohistochemical localization of nicotinic acetylcholine receptor subunits in the mesencephalon and diencephalon of the chick (Gallus gallus)

Monoclonal antibodies against two alpha-bungarotoxin-binding subunits (alpha 7 and alpha 8) of the nicotinic acetylcholine receptors (nAChRs) were used as immunohistochemical probes to map their distribution in the chick diencephalon and mesencephalon. The distribution of the alpha 7 and alpha 8 nAChR subunits was compared to the distribution of immunoreactivity produced by a monoclonal antibody against the beta 2 structural subunit of the nAChRs. Structures that contained high numbers of alpha 7-like immunoreactive (LI) somata included the intergeniculate leaflet, nucleus intercalatus thalami, nucleus ovoidalis, organum paraventricularis, nucleus rotundus, isthmic nuclei, nucleus trochlearis, oculomotor complex, nucleus interstitio-pretecto-subpretectalis, stratum griseum centrale of the optic tectum, and nucleus semilunaris. Neuropil staining for alpha 7-LI was intense in the nucleus dorsomedialis hypothalami, nucleus geniculatus lateralis ventralis, griseum tecti, isthmic nuclei, nucleus lentiformis mesencephali, nucleus of the basal optic root, and stratum griseum et fibrosum superficiale of the tectum. High numbers of alpha 8-LI somata were found in the stratum griseum et fibrosum superficiale of the tectum and the nucleus interstitio-pretecto-subpretectalis, and intense neuropil staining for alpha 8-LI was found in the dorsal thalamus, nucleus geniculatus lateralis ventralis, lateral hypothalamus, griseum et fibrosum superficiale of the tectum. High numbers of beta 2-LI somata were found only in the nucleus spiriformis lateralis, whereas neuropil staining for beta 2-LI was intense in the nucleus geniculatus lateralis ventralis, nucleus suprachiasmaticus, nucleus lateralis anterior, nucleus habenularis lateralis, area pretectalis, griseum tecti, nucleus lentiformis mesencephalis, nucleus externus, and nucleus interpeduncularis, and in the stratum griseum centrale, stratum griseum et fibrosum superficiale, and stratum opticum of the tectum. These results indicate that there are major disparities in the localization of the alpha-bungarotoxin-binding alpha 7 and alpha 8 nAChR subunits and the beta 2 structural nAChR subunit in the chick diencephalon and mesencephalon. These nAChR subunits appear, however, to coexist in several regions of the chick brain.

Directional asymmetry of the frog monocular optokinetic nystagmus: cholinergic modulation

The frog monocular optokinetic gaze nystagmus (OKN) was studied by coil recordings after intravitreal administration of cholinergic drugs into the closed eye. Before injection, the frog displayed OKN for stimulations in the temporo-nasal (T-N) direction only. The injection of muscarinic agonists, as well as that of nicotinic antagonists, provoked the appearance of a naso-temporal (N-T) component, the slow phase velocity gain then being strongly and significantly increased. The abolition of the OKN directional asymmetry indicates that acetylcholine seems to act in opposite ways through muscarinic and nicotinic binding sites. The GABAergic and cholinergic systems may interact to generate and modulate OKN in the frog.

GABA-like immunoreactive cells containing nicotinic acetylcholine receptors in the chick retina

The possibility that GABA-like immunoreactive cells of the chick retina also contain neuronal nicotinic acetylcholine receptors was investigated by means of immunohistochemical techniques. Double-labeled cell bodies containing GABA-like immunoreactivity and nicotinic receptor-like immunoreactivity were seen in the inner third of the inner nuclear layer and were presumably amacrine cells. Approximately 29-36% of the GABA-positive cells in the inner nuclear layer contained nicotinic receptor immunoreactivity. Their soma sizes ranged from 5-12 microns. Some double-labeled cells ranging from 7-21 microns were observed in the ganglion cell layer as well. Between 9-37% of the GABA-positive cells in this layer contained nicotinic receptor-like immunoreactivity. Following injection of a retrograde tracer into the optic tectum, some of the retrogradely labeled cells were also double labeled with antibodies against GABA and nicotinic receptors. This indicates that at least some of the GABA-positive cells containing nicotinic acetylcholine receptors in the ganglion cell layer are indeed ganglion cells. The present data appear to represent the first demonstration of the presence of acetylcholine receptors in GABA-containing cells in the retina, thus providing a basis for a possible influence of acetylcholine upon those presumptive GABAergic cells.

A subpopulation of displaced ganglion cells of the pigeon retina exhibits substance P-like immunoreactivity

Immunohistochemical and retrograde tracing techniques were combined to demonstrate the occurrence of displaced ganglion cells (DGCs) exhibiting substance P-like immunoreactivity (SP-LI) in the pigeon retina. Following injections of rhodamine-labeled latex microspheres into the nucleus of the basal optic root (accessory optic system), about 5200 DGCs were observed to contain rhodamine fluorescence in the contralateral retina. Approximately 26% of the retrogradely labeled DGCs also contained SP-LI. The soma sizes of the doubly labeled DGCs ranged from 12 to 24 microns, and their distribution mirrored the overall distribution of DGCs projecting to the nucleus of the basal optic root. The density of doubly labeled DGCs ranged from 2 to 15 cells/mm2, with density peaks occurring in the superior-nasal and inferior-temporal retinal quadrants. Larger DGCs projecting to the nBOR (25-32 microns) were never seen to contain SP-LI. Together with previous results of enucleation experiments, these data indicate the existence of a subpopulation of SP-LI DGCs which are connected with the accessory optic system in the pigeon. The present results also contribute information on the heterogeneity of retinal ganglion cells transmitters and modulators.

Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed alpha 2, alpha 3, alpha 4 ...) and structural (termed beta 2, beta 3, beta 4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the alpha 4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as beta 2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that alpha 3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while alpha 4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that alpha 3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina.

GABAA receptors in the retina of the cat: an immunohistochemical study of wholemounts, sections, and dissociated cells

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter used by many neurons of the mammalian retina. To identify the synaptic targets of these cells, we undertook an immunohistochemical study with a monoclonal antibody that recognizes the GABAA receptors (62-3G1, generously donated by A. de Blas). This antibody labels the somata of at least one group of amacrine cells in the inner nuclear layer. It also labels two groups of somata in the ganglion cell layer; one small and the other much larger. The small cells are likely to be displaced amacrine cells based on their size, although some could be gamma ganglion cells. The much larger receptor-positive cells are clearly ganglion cells, based both on their size and the antibody labeling of the initial portion of their axon. In the peripheral retina, the size of these large somata suggests that many are beta ganglion cells. However, at any point across the retina the density of these cells never exceeded 50% of the density of beta cells as a whole. The antibody also labels a dense plexus of processes that extends throughout the inner plexiform layer (IPL), with a marked concentration in the inner third of the layer. This is the portion of the IPL in which the rod bipolar cells terminate. It is difficult to recognize processes of individual cells in the IPL, so retinae were dissociated. The rod bipolar cells were identified by protein kinase C immunoreactivity (Negishi et al., 1988; Karschin & Wäsle, 1990). They were not labeled by the GABAA receptor antibody. This is surprising in light of tight-seal, whole cell voltage-clamp recordings that have shown that the rod bipolars express functional GABAA receptors. One possible explanation is that the antibody recognizes only a subset of the GABAA receptors.

A light- and electron-microscopic investigation of the optic tectum of the frog, Rana pipiens, I: The retinal axons

There are several different groups of ganglion cells in the retina of the frog. Although their axons are thought to terminate in different layers of the optic tectum, little is known about the morphology of their terminal arbors or their synaptic targets. The present paper reports the results of a layer-by-layer study of horseradish peroxidase labeled retinal axons in the optic tectum of Rana pipiens. Light and electron microscopy was used to study the axon's laminar distribution, patterns of arborization, and synaptic contacts. Labeled retinal axons were found in all of the superficial layers of the tectum (A-G). From layer to layer, the retinal axons differed markedly in the diameter of their parent axons (0.2-3.0 microns) and in the morphology and horizontal extent of their terminal arbors. Five classes of synaptic terminals could be distinguished in the tectum. The retinal terminals belonged to a class characterized by round, medium-sized synaptic vesicles. They made synaptic contact with dendrites and other axon terminals in each of the layers. They were always the presynaptic component. The postsynaptic dendrites were often the vertically oriented processes of cells located in the deeper layers. The postsynaptic terminals belonged to a class distinguished by their flat, medium-sized vesicles. These terminals in turn contacted what appeared to be dendrites. In layer eight, the retinal axons were often large, spoon-shaped boutons that ended in apposition with the somata of the layer.

Target dependence of chick retinal ganglion cells during embryogenesis: cell survival and dendritic development

The survival of retinal ganglion cells and the dendritic development were investigated a) in normal chick embryos, b) in embryos whose primordial optic lobes and adjacent areas were removed (target reduced embryos), and c) in embryos whose optic nerves were transected (target deprived embryos) in order to study the influences of central targets on developing ganglion cells. The ganglion cells were stained postmortem with the carbocyanine dye DiI. Cell body and dendritic field diameters were measured in whole-mounted retinae before and after the period of cell death at embryonic day 10 (E10) and E16. The cell densities within the ganglion cell layer were counted in cresyl violet/thionine stained retinae. The central retinal projection in target reduced embryos was studied with the anterogradely transported fluorescent marker rhodamine-B-isothiocyanate (RITC). In normal embryos, the earliest dendritic processes were observed at E6 in the central retina, whereas at E10 elaborate dendritic branching was found across the retina. Different morphological types of ganglion cells could be identified at E16. In both, target reduced embryos and target deprived embryos, the initial dendritic growth and pattern of ramification were indistinguishable from those of normal embryos up to E10. Cell body diameters, dendritic tree diameters, and cell densities were not significantly different. At the end of the naturally occurring cell death period (E16), the ganglion cell density was strongly reduced in both experimental groups compared to controls. In particular, when the optic nerve was transected, it resulted in the almost complete degeneration of ganglion cells. In target reduced embryos, a small population (about 5% of the normal number) of ganglion cells survived. The proportion of large cells was increased within the total population compared to normal retinae. Displaced ganglion cells were not affected by partial target removal but strongly affected by transection of the optic nerve. Anterograde labelling from the retina revealed that in target reduced embryos the remaining ganglion cells innervated non-tectal primary visual nuclei. The present results suggest the following: a) Before the onset of the cell death period, the growth and ramification of ganglion cell dendrites occur independently of central visual targets. b) In target reduced embryos, a small population of ganglion cells survives, namely, those cells that project to remaining central areas. Complete disconnection from central targets by transecting the optic nerve leads to the degeneration of almost all ganglion cells. c) The surviving ganglion cell population consists mainly of large ganglion cells.

Distributions of choline acetyltransferase and acetylcholinesterase activities in the retinal layers of pigeon red and yellow fields

The activities of choline acetyltransferase and acetylcholinesterase were assayed in submicrogram samples from layers of pigeon retina. Red and yellow fields were sampled separately to investigate quantitatively the relationship between these enzymes of acetylcholine metabolism and the gradient of inner plexiform layer complexity, increasing from the yellow field to the red. Choline acetyltransferase and acetylcholinesterase activities were concentrated in and near the inner plexiform layer, within which two peaks of activity for each enzyme were obtained. The distributions of enzyme activities indicate that populations of amacrine cells in the pigeon retina are cholinergic. The quantitative similarities between the enzyme activities in red and yellow fields suggest that the cholinergic system may not be specifically involved in the increase in inner plexiform layer complexity across the pigeon retina.

Differential effects of axotomy on substance P-containing and nicotinic acetylcholine receptor-containing retinal ganglion cells: time course of degeneration and effects of nerve growth factor

The time course of degeneration of chick retinal ganglion cells was examined with Nissl stains and immunohistochemical methods for detection of substance P-like immunoreactive and nicotinic acetylcholine receptor immunoreactive neurons. Small lesions were made in the retinae, adjacent to the optic nerve head, and were subsequently sectioned parallel to the vitreal surface, permitting direct comparison of normal and axotomized retinal ganglion cells distal to the site of axon damage. At four and six days after surgery, a large number of degenerating cells with clear cytoplasm and pyknotic nuclei were seen. After eight, 10 and 14 days, many retinal ganglion cells displayed a chromatolytic response with dispersed Nissl granules, eccentric nuclei and the cells appeared crenulated. The number of apparently normal neurons in the ganglion cell layer in the axotomized region was reduced by about 50% six days following surgery, by about 70% on the 10th day and by about 75% on the 17th day. The remaining neurons in the ganglion cell layer were identified as displaced amacrine cells. From day 2 onwards, increased numbers of glial cells were present in the optic fibre, ganglion cell and inner plexiform layers. Many glial cells were enlarged and displayed extensive cytoplasmic processes, while others showed mitotic activity. Somata and proximal dendrites of retinal ganglion cells were intensely stained for substance P-like immunoreactivity at two and four days following surgery. At six, eight and 10 days, staining intensity was markedly reduced though still evident and at 14 and 17 days, substance P-like immunoreactivity had virtually disappeared. The persistence of limited substance P-like immunoreactive ganglion cells 10 days after surgery indicates that these cells have a relatively protracted response to axotomy. Nicotinic acetylcholine receptor-like immunoreactivity in the ganglion cells at two and four days following axotomy was substantially reduced. The majority of faintly stained nicotinic acetylcholine receptor-like immunoreactive ganglion cells, as visualized in counterstained sections, did not exhibit pyknosis in the immediate period following axotomy. Double label studies demonstrated that substance P-like immunoreactive ganglion cells were distinct from the nicotinic acetylcholine receptor-like immunoreactive ganglion cells. In a second set of experiments, nerve growth factor was then placed into the vitreous humor following intra-retinal axotomy. The somata, dendrites and proximal axons of lesioned substance P-like immunoreactive ganglion cells in these retinae were more intensely stained for a longer period of time and appeared more robust than cells from untreated retinae.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotransmitters, receptors, and neuropeptides in the accessory optic system: an immunohistochemical survey in the pigeon (Columba livia)

Immunohistochemical techniques were used to survey the distribution of several conventional transmitters, receptors, and neuropeptides in the pigeon nucleus of the basal optic root (nBOR), a component of the accessory optic system. Amongst the conventional neurotransmitters/modulators, the most intense labeling of fibers/terminals within the nBOR was obtained with antisera directed against glutamic acid decarboxylase (GAD) and serotonin (5-HT). Moderately dense fiber plexuses were seen to label with antibodies directed against tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT). GAD-like immunoreactivity (GAD-LI) was found in many small and medium-sized perikarya within the nBOR. Some of the medium-sized cells were occasionally positive for ChAT-LI. Cell body and dendritic staining was also commonly seen with the two tested antisera against receptors-anti-GABA-A receptor and anti-nicotinic acetylcholine receptor. The antisera directed against various neuropeptides produced only fiber labeling within the nBOR. The densest fiber plexus staining was observed with antiserum against neuropeptide Y (NPY-LI), while intermediate fiber densities were seen for substance P (SP-LI) and cholecystokinin (CCK-LI). A few varicose fibers were labeled with antisera against neurotensin (NT), leucine-enkephalin (L-ENK), and the vasoactive intestinal polypeptide (VIP). Unilateral enucleation produced an almost complete elimination of TH-LI in the contralateral nBOR. SP-LI and CCK-LI were also decreased after enucleation. No apparent changes were seen for all other substances. These results indicate that a wide variety of chemically-specific systems arborize within the nBOR. Three of the immunohistochemically defined fiber systems (TH-LI, SP-LI, and CCK-LI fibers) were reduced after removal of the retina, which may indicate the presence of these substances in retinal ganglion cells. In contrast, the fibers exhibiting ChAT-LI, GAD-LI, 5-HT-LI, NPY-LI, NT-LI, L-ENK-LI, and VIP-LI appear to be of nonretinal origin. Two different populations of nBOR neurons exhibited GAD-LI and ChAT-LI. However, these two populations together constituted only about 20% of the nBOR neurons.