Biosynthesis of acetylcholine in turtle photoreceptors

Processing and cryopreservation of human ureter tissues for single-cell and spatial transcriptomics assays

Characterizing the cellular heterogeneity of human ureter tissues using single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics provides a detailed atlas of cell types, signaling networks, and potential cell-cell cross talk underlying developmental and regenerative pathways. We describe an optimized protocol for generating, cryopreserving, and thawing single-cell suspensions from ureter tissues isolated post-cystectomy for scRNA-seq. In addition, we describe an optimized protocol for cryopreserving human ureter tissues for 10x Genomics Visium spatial gene expression platform. For complete details on the use and execution of this protocol, please refer to Fink et al. (2022).¹.

Isolation of mitochondria-derived mitovesicles and subpopulations of microvesicles and exosomes from brain tissues

Extracellular vesicles (EVs) are nanoscale vesicles secreted into the extracellular space by all cell types, including neurons and astrocytes in the brain. EVs play pivotal roles in physiological and pathophysiological processes such as waste removal, cell-to-cell communication and transport of either protective or pathogenic material into the extracellular space. Here we describe a detailed protocol for the reliable and consistent isolation of EVs from both murine and human brains, intended for anyone with basic laboratory experience and performed in a total time of 27 h. The method includes a mild extracellular matrix digestion of the brain tissue, a series of filtration and centrifugation steps to purify EVs and an iodixanol-based high-resolution density step gradient that fractionates different EV populations, including mitovesicles, a newly identified type of EV of mitochondrial origin. We also report detailed downstream protocols for the characterization and analysis of brain EV preparations using nanotrack analysis, electron microscopy and western blotting, as well as for measuring mitovesicular ATP kinetics. Furthermore, we compared this novel iodixanol-based high-resolution density step gradient to the previously described sucrose-based gradient. Although the yield of total EVs recovered was similar, the iodixanol-based gradient better separated distinct EV species as compared with the sucrose-based gradient, including subpopulations of microvesicles, exosomes and mitovesicles. This technique allows quantitative, highly reproducible analyses of brain EV subtypes under normal physiological processes and pathological brain conditions, including neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

In vitro Model Systems for Studies Into Retinal Neuroprotection

Therapy development for neurodegenerative diseases of the retina constitutes a major unmet medical need, and this may be particularly relevant for inherited diseases of the retina, which are largely untreatable to this day. Therapy development necessitates appropriate models to improve the understanding of the underlying degenerative mechanisms, as well as for the testing and evaluation of novel treatment approaches. This review provides an overview of various in vitro model systems used to study retinal neuroprotection. The in vitro methods and technologies discussed range from primary retinal cell cultures and cell lines, to retinal organoids and organotypic retinal explants, to the cultivation of whole eyeballs. The advantages and disadvantages of these methods are compared and evaluated, also in view of the 3R principles (i.e., the refinement, reduction, and replacement of live animal testing), to identify suitable in vitro alternatives for in vivo experimentation. The article further expands on the use of in vitro models to test and evaluate neuroprotective treatments and to aid the development of retinal drug delivery systems. Among the pharmacological agents tested and characterized in vitro are such that interfere with aberrant cyclic guanosine monophosphate (cGMP) -signaling or such that inhibit the activities of poly (ADP-ribose) polymerase (PARP), histone deacetylases (HDAC), calpain-type proteases, as well as unfolded protein response-related stress. We then introduce nanoparticle-based drug delivery systems and discuss how different in vitro systems may be used to assess their efficacy in the treatment of retinal diseases. The summary provides a brief comparison of available in vitro models and relates their advantages and limitations to the various experimental requirements, for instance, for studies into disease mechanisms, novel treatments, or retinal toxicity. In many cases, combinations of different in vitro models may be required to obtain a comprehensive view of the efficacy of a given retinal neuroprotection approach.

Preparing a Single Cell Suspension from Zebrafish Retinal Tissue for Flow Cytometric Cell Sorting of Müller Glia

Preparation of a single cell suspension from solid tissue is vital for a successful flow cytometry experiment. We report a detailed and reproducible method to produce a quality cell suspension from the zebrafish retina. Zebrafish retinas, especially their Müller glia cells, are of particular interest for their inherent regenerative capacity, making them a useful model for regenerative medicine and cell therapy research. Here, we detail a papain-based dissociation that is gentle enough to keep cells intact, but strong enough to disrupt cell-cell and cell-matrix interactions to yield a cell suspension that produces clean and reliable flow cytometric cell sorting results. This procedure consistently results in over 90% viability and three populations of cells based on GFP expression. The dissociation procedure described herein has been optimized for the collection of Müller glia from Tg(apoe:gfp) zebrafish retinas; however, the overall process may be applicable to other cell types in the fish retina, additional flow cytometric techniques, or preparing cell suspensions from similar tissues. © 2019 International Society for Advancement of Cytometry.

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

Ion channel compartments in photoreceptors: evidence from salamander rods with intact and ablated terminals

Vertebrate photoreceptors are highly polarized sensory cells in which several different ionic currents have been characterized. In the present study we used whole cell voltage-clamp and optical imaging techniques, the former combined with microsurgical manipulations, and simultaneous recording of membrane current and intracellular calcium signals to investigate the spatial distribution of ion channels within isolated salamander rods. In recordings from intact rods with visible terminals, evidence for five previously identified ionic currents was obtained. These include two Ca(2+)-dependent, i.e., a Ca(2+)-dependent chloride current [I(Cl(Ca))] and a large-conductance Ca(2+)- and voltage-dependent K(+) or BK current [I(K(Ca))], and three voltage-dependent currents, i.e., a delayed-rectifier type current [I(K(V))], a hyperpolarization-activated cation current (I(h)), and a dihydropyridine-sensitive L-type calcium current (I(Ca)). Of these, I(Cl(Ca)) was highly correlated with the presence of a terminal; rods with visible terminals expressed I(Cl(Ca)) without exception (n = 125), whereas approximately 71% of rods (40/56) without visible terminals lacked I(Cl(Ca)). More significantly, I(Cl(Ca)) was absent from all rods (n = 33) that had their terminals ablated, and recordings from the same cell before and after terminal ablation led, in all cases (n =10), to the loss of I(Cl(Ca)). In contrast, I(K(Ca)), I(K(V)), and I(h) remained largely intact after terminal ablation, suggesting that they arose principally from ion channels located in the soma and/or inner segment. The outward I(K)((Ca)) in terminal-ablated rods was reversibly suppressed on "puffing" a Ca(2+)-free extracellular solution over the soma and was appreciably enhanced by the L-type Ca(2+) channel agonist, Bay K 8644 (0.1-2 microM). These data indicate that rod photoreceptors possess discrete targeting mechanisms that preferentially sort ion channels mediating I(Cl(Ca)) to the terminal.

Papain: An Effective Permeation Enhancer for Orally Administered Low Molecular Weight Heparin

PURPOSE

The purpose of this study was to evaluate an effect of the proteolytic enzyme papain on permeation of low molecular weight heparin (LMWH) in vitro and in vivo.

MATERIALS AND METHODS

In vitro permeation studies were performed using rat small intestine as permeation barrier. In order to determine the ratio of papain to heparin resulting in the highest heparin permeation rate, molar ratios 1:1, 1:2 and 2:1 of papain to heparin were tested. Interactions of heparin with papain were investigated spectro-photometrically. For in vivo studies, 15 mg tablets containing heparin (13%), papain (64%) and hydroxyethylcellulose (22%) were orally administered to rats.

RESULTS

Since molar ratio papain to heparin 1:1 resulted in the highest permeation rate, it was used for in vivo studies. The results of binding studies of papain with heparin indicated a strong interaction between papain and heparin. Oral administration of tablets containing LMWH/papain/HEC resulted in sevenfold improvement of plasma anti-Xa activity in comparison to control. For tablets based on heparin/papain/HEC, a relative bioavailability of 9.1% vs. subcutaneous injection was obtained, whereas the relative bioavailability of control was 2.4%.

CONCLUSION

The co-administration of papain with heparin represents a new approach in improvement of absorption and bioavailability of orally administered heparin.

Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells

A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

Muscarinic acetylcholine receptors in the normal, developing and regenerating newt retinas

Immunoreactivity for m2 and m4 muscarinic acetylcholine receptors (mAChRs) was demonstrated in the adult newt retina. The m2 mAChR was localized to somata on either side of the inner plexiform layer (IPL), especially ganglion cells, and also distributed into two bands within the IPL. The distal band at a depth of 0-15% IPL co-localized with one of two choline acetyltransferase (ChAT) immunoreactive bands, while the proximal band at 85-100% depth did not overlap with either of the ChAT-ir bands. The m4 mAChR was localized to somata closely apposed to either side of the IPL, probably amacrine cell somata, and no immunoreactivity was detectable throughout the IPL. The time course of appearance of the m2 and m4 mAChRs was examined in both developing and regenerating retinas. Like acetylcholinesterase (AChE), the m2 was first detected in somata located at the most proximal level of the retina well before ChAT-ir cholinergic neurons appeared, while the m4 was detected at the time of appearance of ChAT, in both developing and regenerating retinas. When the outer plexiform layer (OPL) began to form, somata in the horizontal cell layer became transiently immunoreactive to the m2. The discrepancy in distribution of the m2 and ChAT in the IPL suggests that mAChR may play a role other than cholinergic neurotransmission. Furthermore, the similarity in time course of appearance of the m2 and m4, as well as other cholinergic system components [4], in both developing and regenerating retinas would suggest that the mechanisms that control neuronal differentiation during retinal development and regeneration are similar.

Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label immunocytochemistry was performed on sectioned and wholemount retina using antisera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), calretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline acetyltransferase (ChAT). H1 cells and their axon terminals label with CR, CB and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR, nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. The localization of nNOS in the H2 and H3 cells is a novel finding. None of these antibodies labels H4 cells. The photoreceptor subtypes have been differentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal member and rods and blue cones do not appear to label at all. ChAT-IR is located in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GABA is present in H1 horizontal cells and may be used in neurotransmission between horizontal cells and possibly for feedback pathways to photoreceptors. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, combined with available physiological evidence, suggests that NO may be involved in electrical coupling and/or modulation of synaptic input to these types of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photoreceptors in the outer plexiform layer of the turtle retina.

Confronting complexity: strategies for understanding the microcircuitry of the retina

The mammalian retina contains upward of 50 distinct functional elements, each carrying out a specific task. Such diversity is not rare in the central nervous system, but the retina is privileged because its physical location, the distinctive morphology of its neurons, the regularity of its architecture, and the accessibility of its inputs and outputs permit a unique variety of experiments. Recent strategies for confronting the retina's complexity attempt to marry genetic approaches to new kinds of anatomical and electrophysiological techniques.

Localization of choline acetyltransferase in the developing and adult turtle retinas

Acetylcholine has important epigenetic roles in the developing retina. In this study, cells that expressed choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine, were investigated in embryonic, postnatal, and adult turtle retinas by using immunofluorescence histochemistry. ChAT was present at stage 15 (S15) in cells near the vitreal surface. With the formation of the inner plexiform layer (IPL) at S18, ChAT-immunoreactive (-IR) cells were located in the inner nuclear layer (INL) and the ganglion cell layer (GCL). In the INL, presumed starburst amacrine cells were homogenous in appearance and formed a single row next to the IPL: This pattern was conserved until adulthood. In the GCL, however, there were multiple rows of ChAT-IR cells early in development, and this high density of labeled cells continued during the embryonic stages, until around birth. The high density of ChAT-IR cells in the GCL was due in part to a population of cells that expressed ChAT transiently. In postnatal stages and adult retinas, the presumed starburst amacrine ChAT-IR cells formed two mirror-like rows of homogenous cells on both borders of the IPL. Two cholinergic dendritic strata that were continuous with these cells were observed as early as S18, and their depths in the IPL were relatively stable throughout development. A third population of ChAT-IR cells was observed toward the middle of the INL around S25 and persisted into adulthood. Finally, cells in the outer nuclear layer (ONL) were ChAT-IR during the embryonic stages, were less immunoreactive during the postnatal stages, and were not immunoreactive in the adult retinas.

Choline acetyltransferase and acetylcholinesterase in the normal, developing and regenerating newt retinas

The presence of the choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) was demonstrated in the adult newt retina using immunocytochemical and histochemical techniques. Within the inner plexiform layer (IPL), two ChAT-positive bands were detected at relative depths of 0-15% and 45-60% of the total thickness (100%) of the IPL. AChE-positive band occupied approximately 0-60% of the IPL width with an intensive AChE-positive band at a depth of 20-40% within the IPL. Localizations of maximum ChAT and AChE activity were not exactly the same in the IPL of the mature retina. To elucidate whether retinal regeneration follows the same sequence of cellular differentiation steps that occur in retinal development, we examined the time course of appearance of the cholinergic neurons and AChE activity in both developing and regenerating retinas. The ChAT-positive cells were first detected in the retina just before or at the beginning of the morphological development of the IPL in both developing and regenerating retinas. AChE activity first became detectable in somata located at the most proximal layer of the retina before the ChAT-positive cells could be detected and well before the IPL developed in both developing and regenerating retinas. During subsequent development and regeneration, the outer plexiform layer, the IPL, and somata close to either side of the IPL became AChE-positive. The fact that the time course of the appearance of ChAT and AChE molecules during regeneration was similar to that observed during development suggests that common mechanisms may control both the development and the regeneration of the newt retina.

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.

Rapid charge movements and photosensitivity of visual pigments in salamander rods and cones

1. Photosensitivities of visual pigments were determined by measuring early receptor currents (ERCs) in voltage-clamped photoreceptors from larval salamanders. 2. As expected from previous work of others, the ERC elicited by a brief flash consisted of a rapid inward component followed by a larger and slower outward component. The magnitude of the outward component corresponded to the movement of about 0.18 electronic charge across the membrane per photoisomerization. 3. The time course of the ERC was independent of the flash intensity, the flash wavelength and the magnitude of the response. The outward component of the cone ERC declined about twice as rapidly as the outward component of the rod ERC.. 4. The amplitude of the ERC decreased as successive flashes bleached the cell's pigment. Using the proportional relation between the size of the ERC and the number of pigment molecules photoisomerized, photosensitivities of the native A2 pigments in rods, red-sensitive cones, blue-sensitive cones and UV-sensitive cones were determined. Calculated solution photosensitivities for rhodopsin, red-sensitive and blue-sensitive cone pigments were not significantly different and the average value for all three pigments at their respective absorption maxima was (7.3 +/- 1.6) x 10(-9) micron 2 molecule-1. A value of 44.0 x 10(-9) micron 2 molecule-1 was obtained in a single UV-sensitive cone. 5. Substitution of the native dehydroretinal chromophore in the red-sensitive cone pigment with 11-cis-retinal increased the solution photosensitivity to (9.6 +/- 0.62) x 10(-9) micron 2 molecule-1. 6. We conclude that cone pigments have large molecular absorption cross-sections and high quantum efficiencies of photoisomerization. These properties seem well suited for the receptive molecules of a highly sensitive, miniaturized transducer.

Enhanced in vitro uptake and retention of 3H-tetraphenylphosphonium by nervous system tumor cells

Photodynamic therapy is a promising treatment for human brain tumors because of the selective retention of certain compounds by tumor cells. Certain lipophilic cationic compounds, such as tetraphenylphosphonium (TPP), are selectively taken up by a variety of carcinomas. Although preferential retention of TPP has been demonstrated for the breast carcinoma cell line MCF-7, this compound had not been tested previously on cells derived from nervous system tumors. In the present study, tritiated-TPP (3H-TPP) uptake and retention for eight different cell cultures of three histologically different types of nervous system tumors was measured and the data were compared to a positive control (MCF-7) and negative controls (normal African Green monkey kidney epithelium (CV-1) and the normal human fibroblast (WI-38) cell lines). Uptake and retention characteristics could be grouped by specific pathological tumor types, but individual tumor variability was notable. Malignant astrocytoma (grade III/III glioblastoma) and malignant neurofibrosarcoma cells showed preferential uptake and retention of 3H-TPP relative to meningioma cells and normal controls. A clonogenic assay utilizing the cytotoxic lipophilic cationic compound dequalinium showed strong retainers of 3H-TPP to be more susceptible to the effects of dequalinium than weak retainers. These data demonstrate that certain human and experimental animal nervous system tumor cell lines retain lipophilic compounds possessing a delocalized positive charge. Lipophilic cationic compounds may be useful in the intraoperative delineation of tumor margins and in the photodynamic therapy of certain nervous system tumors.

Effects of modified chromophores on the spectral sensitivity of salamander, squirrel and macaque cones

1. Chemically modified retinal chromophores were used to investigate the mechanisms that produce the characteristic spectral absorptions of cone pigments. Spectral sensitivities of single cones from the salamander, squirrel and macaque retina were determined by electrical recording. The chromophore was then replaced by bleaching the pigment and regenerating it with a retinal analogue. 2. Exposing a bleached cone to 9-cis-retinal for a brief period (less than 20 min) caused its flash sensitivity to recover to about 0.2 of the pre-bleach value. Similar exposure to a locked 6-s-cis, 9-cis analogue gave a recovery to about 0.03 of the pre-bleach value. 3. Unlike the flash sensitivity, the saturating photocurrent amplitude often recovered completely after bleaching and regenerating the pigment. 4. When the 3-dehydroretinal chromophore in the salamander long-wavelength-sensitive (red) cone was replaced with 11-cis-retinal, shortening the conjugated chain in the chromophore, the spectral sensitivity underwent a blue shift of 67 nm. 5. Pigments containing the planar-locked 6-s-cis.9-cis-retinal analogue absorbed at substantially longer wavelength than those containing unmodified 9-cis-retinal. The opsin shift, a measure of the protein's ability to modify the chromophore's absorption was larger for the locked analogue than for 9-cis-retinal. This suggests that the native chromophore assumes a twisted 6-s-cis conformation in these pigments. 6. The spectral sensitivities of red and green macaque cones containing 9-cis-retinal or planar-locked 6-s-cis.9-cis-retinal retained the 30 nm separation characteristic of the native pigments. This suggests that the different absorptions of of the 6-7 carbon bond in the retinal chromophore.

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.

Release of endogenous excitatory amino acids from turtle photoreceptors

Responses to light are transmitted from photoreceptors to second-order retinal neurons by chemical synapses that may use an excitatory amino acid (EAA) as the neurotransmitter. This hypothesis is based primarily on the pharmacological actions of EAA agonists and antagonists on the membrane potentials and light responses of second-order neurons. But the release of endogenous EAAs, which is a critical criterion for the identification of EAAs as transmitters, has not been demonstrated. Here we report the use of outside-out membrane patches excised from rat hippocampal neurons to detect the release of EAAs from synaptic terminals of isolated turtle photoreceptors. Electrical stimulation of or application of lanthanum chloride to photoreceptors induced an increase in the frequency of opening of 50-pS channels in the patches. These channels were identified as the class of glutamate-activated channels that are also gated by aspartate and NMDA (N-methyl-D-aspartate). In several photoreceptor-patch pairs, spontaneous channel activity was observed near the synaptic terminals. These results provide strong evidence to support the hypothesis that both rods and cones of the turtle use an EAA as their neurotransmitter.

Binding pattern of alpha-bungarotoxin on horizontal cells of a marine teleost retina

A conjugate of alpha-bungarotoxin and a fluorescent marker (fluorescein isothiocyanate) has been used to localize "nicotinic" acetylcholine receptors on neurons in the outer plexiform layer of marine teleost retina. Toxin binding was confined to bipolar cell dendrites and to intermediate horizontal cells. The arrangement of labeled horizontal cells appears irregular in the whole retina, with a peak density in the ventral and dorsal quandrants. Alpha-bungarotoxin receptors on horizontal cells differ from those on bipolar cells and from those on dendrites in the inner plexiform layer in their sensitivity to agonists and antagonists such as d-tubocurarine and nicotine. They constitute a different type of "nicotinic" receptor that probably has a different function.

Neural organization of the retina of the turtle Mauremys caspica: a light microscope and Golgi study

The organization of the retina of the turtle species Mauremys caspica, found in fresh water ponds of Israel, has been examined by light microscopical techniques including examination of fresh wholemount retina, one micron blue-stained vertical sections and Golgi-stained material. The anatomical findings on Mauremys retina have been compared with those of the Pseudemys retina (Kolb, 1982) which is more commonly used for electrophysiological and neurochemical studies in the USA. The photoreceptors of Mauremys are similar in type and oil droplet content to Pseudemys photoreceptors except for the double cone in Mauremys. This cone type appears more abundant than in Pseudemys and the principal member contains a yellow oil droplet instead of an orange oil droplet. Golgi staining reveals that the cell types that have been seen in Pseudemys are found in Mauremys with identical morphology. In addition, two amacrine cell types that were not before described for Pseudemys have been added to the classification. One of these is the tristratified dopaminergic amacrine cell described in immunocytochemical studies (Witkovsky et al., 1984; Nguyen-Legros et al., 1985; Kolb et al., 1987). We have used these anatomical studies on Pseudemys and Mauremys retina to form a catalogue of neural types for the turtle retina in general. We conclude with an attempt to combine findings from anatomy, electrophysiology, and neurochemistry to form an overview of the organization of this reptilian retina.

Human retinas synthesize and release acetylcholine

Human retinas have the capacity to synthesize and release [3H]acetylcholine ([3H]ACh) after an incubation in [3H]choline ([3H]Ch). Synthesis of [3H]Ch by retinal homogenates was determined using either high-voltage paper electrophoresis (HVPE) or a two-step enzymatic/extraction assay for separating [3H]ACh from [3H]Ch. The enzymatic/extraction assay is shown to be accurate over a wide range of concentrations (10(-6)-10(-12) M). Homogenates of human retina synthesize [3H]ACh from [3H]Ch. We find an approximate Km of 50 microM and a Vmax of about 20 nmol/mg protein/h (at 37 degrees C) for the synthesis of labeled ACh by retinal homogenates. Human retinas also release [3H]ACh after a pulse of [3H]Ch. Release of labeled transmitter is stimulated by potassium depolarization. The potassium-stimulated release is partially blocked by magnesium or cobalt ions. Release data were analyzed by both the enzymatic/extraction assay and HVPE; the results are qualitatively identical in both cases. The data reported here provide additional evidence for cholinergic neurotransmission in the human retina.

Control of the light-regulated current in rod photoreceptors by cyclic GMP, calcium, and l-cis-diltiazem

The effect of calcium ions on the cGMP-activated current of outer segment membrane was examined by the excised-patch technique. Changes in the extracellular calcium concentration had marked effects on the cGMP-activated current, while changes in intracellular calcium concentration were ineffective. Changes in calcium concentration in the absence of cGMP had little, if any, effect on membrane conductance. These results suggest that both intracellular cGMP and extracellular calcium can directly affect the conductance underlying the light response in rod cells. The pharmacological agent l-cis-diltiazem reversibly inhibited the cGMP-activated current when applied to the intracellular side of an excised patch. When superfused over intact rod cells, l-cis-diltiazem reversibly blocked much of the normal light response. The isomer, d-cis-diltiazem, did not significantly affect either patches or intact rod cells. Thus, the light-regulated conductance has binding sites for both calcium and cGMP that may interact during the normal light response in rod cells and a site specific for l-cis-diltiazem that can be used to identify and further study the conductance mechanism.

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.

Studies of ionic currents in the isolated vestibular hair cell of the chick

The ionic currents in enzymatically isolated vestibular hair cells of the chick were studied by a whole-cell-clamp variation of the patch voltage clamp, and by single channel recording. At membrane potentials more negative than -80 mV the hair cell showed anomalous rectification, and at potentials more positive than -40 mV large outward K currents were observed in normal saline. The outward K current decreased at large positive potentials, showing an N-shaped I-V relation. The outward K current was carried mostly through the Ca-activated K channel. K currents through the anomalous rectifier channel showed a decay in normal saline. This decay was eliminated reversibly in Na-free saline when the isotonic KCl-EGTA solution was used as the internal medium. However, a fast decay was still observed in Na-free high-K external solution when isotonic CaCl-EGTA was used as the internal medium. An increase in [K]o decreased the decay rate of the inward K current. The single-channel conductance of the anomalous rectifier channel was 50 pS in 160 mM-K saline and 23 pS in 40 mM-K saline. In 100 mM-Ca, -Sr and -Ba salines a large inward current was observed. At positive potentials the inward current carried by Ca and Sr ions showed significant decay; the current became outward at large positive potentials. Since the decay of the inward current was eliminated when 100 microM-quinine was added to the bathing medium, it was probably due to the activation of some Ca-activated K conductance which remained even with isotonic CsCl-EGTA internal medium. The activation kinetics of the Ca channel were studied in 100 mM-Ba solution at low temperatures (9-13 degrees C). From a comparison of the time constants of activation with the time constants of the tail currents, it was concluded that the Ca channel follows Hodgkin-Huxley-type m2 kinetics. A slow component that deviated from m2 kinetics was frequently observed at relatively large positive potentials. The steady-state fluctuations of Ba current showed a power density spectrum reasonably well fitted by a sum of two Lorentzian functions. The spectrum has a low-frequency component which indicates kinetics close to the macroscopic activation process of the Ca channel and a high-frequency component that indicates very fast flickering kinetics operating in the Ca channel.

Mechanoelectrical transducer has discrete conductances in the chick vestibular hair cell

Properties of mechanoelectrical transduction were studied at the single-cell level by applying a whole-cell recording variation of the patch-clamp technique to dissociated vestibular hair cells of chicks. The hair bundle was directly stimulated by a glass rod, and transduction currents were recorded from the cell body. After a triangular movement of the stimulating probe, the transduction current was generated stepwise between discrete levels of amplitude. The minimum step amplitude was -1.8 pA at -27 mV in Na-containing normal saline.

Ionic currents of solitary horizontal cells isolated from goldfish retina

Solitary horizontal cells, dissociated from papain-treated goldfish retinas, produce action potentials and show a non-linear current-voltage relationship. Underlying ion-conductance mechanisms were analysed by a single-micro-electrode voltage-clamp technique. Pharmacological and ion-substitution experiments revealed that ionic currents could be separated into at least four voltage-dependent currents: a Ca current and three types of K currents. The Ca current was activated by membrane depolarization beyond -45 mV, reached a maximal value near 0 mV, and became smaller at more positive potentials. By extrapolation, the reversal potential was estimated to be approximately +50 mV. The Ca current was inactivated by accumulation of intracellular Ca ions but not by membrane depolarization. Co ions (4mM) blocked this current. The first type of K current showed anomalous (inward-going) rectification near the resting potential (congruent to -60 mV). Hyperpolarization from the resting level produced a large, almost steady inward current, while depolarization evoked only a small, steady outward current. The current-voltage relationship revealed a shallow negative resistance region at membrane potentials beyond -50 mV. The current was blocked by Cs (10 mM) or Ba (1 mM) ions. The second type of K current (the transient outward current) was activated by membrane depolarization beyond -25 mV. The peak amplitude increased almost exponentially as the membrane was depolarized. During steady depolarization this current decayed exponentially (time constant congruent to 500 ms at +20 mV). The current was inactivated by conditioning depolarization (greater than 10 s) beyond -30 mV and blocked by 4-aminopyridine (10 mM). The third type of K current was the maintained outward current which was activated by membrane depolarization beyond -20 mV, increased to a steady level in a few hundred milliseconds, and showed little inactivation. The amplitude increased as the membrane was depolarized. The current was blocked by tetraethylammonium ions (20 mM). A Ca-mediated K current was not detected. Action potentials and the non-linear current-voltage relationship of solitary horizontal cells can be explained qualitatively by the combination of the four ionic currents.

Adenosine-elicited accumulation of adenosine 3', 5'-cyclic monophosphate in the chick embryo retina

The cyclic AMP level of 17-day-old chick embryo retina increased from 20 to 331 pmol/mg protein when the tissue was incubated for 20 min in the presence of 4-(3-butoxy-4-methoxybenzyl-2-imidozolinone) (RO 20-1724). The addition of 0.5 mM-3-isobutyl-1-methylxanthine (IBMX) or 0.5 units/ml of adenosine deaminase (EC 3.5.4.4) to the medium reduced the increase of cyclic AMP content from 20 to 100 pmol/mg protein. Dipyridamole did not interfere with the rise of the retinal cyclic AMP level observed with RO 20-1724. The EC50 of 6-amino-2-chloropurine riboside (2-chloroadenosine)-elicited accumulation of cyclic AMP of retinas incubated in the presence of RO 20-1724 plus adenosine deaminase was approximately 1 microM. When retina incubation was carried out in the presence of 0.5 mM-IBMX, the 2-chloroadenosine dose-response curve was shifted to the right two orders of magnitude. Maximal stimulation of the cyclic AMP level of 17-day-old chick embryo retina incubated in the presence of 0.5 mM-IBMX was observed at 1 mM-adenosine concentration. This effect was not blocked by dopamine antagonists. Guanosine and adenine did not affect the retinal cyclic AMP level. AMP and ATP had a slight stimulatory effect. Adenosine response of embryonic retina increased sharply from the 14th to the 17th embryonic day. A similar, but not identical adenosine effect was observed in cultured retina cells.

Reactivation of contraction in detergent-lysed teleost retinal cones

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 micrometers (midnight dark-adapted) to 6 micrometers in fully light-adapted state. When dark-adapted fish are exposed to light (1.4 lux), cone myoids contract with a linear rate of 1.5 +/- 0.1 micrometers/min. We report here that detergent-lysed motile models of teleost retinal cones exhibit calcium- and ATP-dependent reactivated contraction, with morphology and rate comparable to that observed in vivo. For reactivation studies isolated dark-adapted retinas were lysed with nonionic detergent Brij-58 (0.1-1.0%). In reactivation medium containing 10(-5) M free calcium and 4 mM ATP, the lysed cones contracted with normal morphology at in vivo rates (1.4 +/- 1 micrometer/min). Little contraction was observed if ATP or detergent was deleted from the medium or if free calcium levels were less than 10(-8) M. Ultrastructural examination of cone models lysed with 1% Brij-58 revealed that, in spite of extensive extraction of the cytoplasmic matrix, cytoskeletal components (thin filaments, intermediate filaments, microtubules) were still present. Thus we have produced extensively extracted motile models of teleost retinal cones which undergo calcium- and ATP-dependent reactivated contraction with normal morphology at physiological rate.

Membrane properties of solitary horizontal cells isolated from goldfish retina

1. Solitary horizontal cells were obtained by dissociating the adult goldfish retina using the enzyme papain. The cells were identified on morphological grounds and could be kept in culture for over a week. 2. Solitary horizontal cells, penetrated with micro-electrodes, had resting potentials of about -75 mV in normal solution. When external K+ concentration was changed, the membrane potential varied from EK calculated from the Nernst equation. 3. All solitary horizontal cells tested showed an action potential in response to superthreshold depolarizing current pulses. The action potential had an overshoot of about +20 mV and a plateau potential lasting for several seconds. 4. The action potential appeared to be Ca-dependent for the following reasons: (a) TTX or low [Na+] did not affect the action potential, (b) Sr2+, Ba2+ or high [Ca2+] enhanced the action potential, while (c) Co2+ or high [Mg2+] blocked it. No regenerative activity has been observed in horizontal cells in the retina but it is possible that the regenerative mechanism is suppressed normally. 5. A role for K+ was indicated by an increase in the duration and amplitude of the action potential on the application of tetraethylammonium. 6. The steady-state current--voltage (I--V) curve, measured by applying constant current pulses, was S-shaped (current on the abscissa) and composed of inward- and outward-going rectifying regions and a transitional region between them. A similar non-linear I--V relationship has been reported in vivo. 7. The transitional region was characterized by a sudden potential jump and hysteresis, suggesting the presence of a 'negative resistance'. This potential jump appeared not to be produced by the Ca-conductance mechanism mentioned above, since similar jumps were observed in the presence of Co2+.

Lectin receptors on cells isolated from the turtle retina

The presence of specific oligosaccharides on the surface of retinal cells was examined by incubating FITC-labeled lectins with cells dissociated from papain-treated turtle retinas. The pattern and intensity of binding was found to vary among the cells examined. With Con A, there was strong surface staining of both rods and cones, with an intense ring of fluorescence above the nucleus. The bipolar and ganglion cells also showed strong surface labeling. In Müller (glial) cells there was intense fluorescence in the apical, microvillous region. In contrast, the horizontal cells and their axons showed weak staining. When RCA-60, RCA-120, and WGA were incubated with photoreceptors, bipolar cells, or horizontal cells, little fluorescence was visible. However, all three lectins bound strongly to the Müller cells. In contrast, the Lotus lectin did not bind to any of the cells examined. In all the cases, lectin binding was inhibited by the appropriate haptene sugar. Further, prior treatment cells with neuraminidase did not alter lectin binding to any cell type. These results suggest differences in the distribution of lectin receptors among specific cell types, and particularly between neurons and glial cells in the vertebrate retina.

Horizontal cell processes in teleost retina

Contacts between horizontal and bipolar cells are described in the retina of the teleost Eugerres plumieri. A single, long expansion observed in the external cone horizontal cells makes contact by means of a terminal button with the cell body of a bipolar. It represents the only contact between this class of horizontal cell and the bipolar soma. On the other hand, the medial and internal cone horizontal cells and the rod horizontal cells, which lack such a single, long expansion, display instead numerous short and fine expansions that terminate by means of a terminal knob on a bipolar cell body. The bipolar-destined, short expansions of the rod horizontal cell make contact with large bipolar cell bodies, whereas corresponding short expansions of cone horizontal cells contact small bipolar cell bodies. It is suggested that the ascending horizontal cell process forms presynaptic terminals in the photoreceptor triad complex, and that the single, long and the multiple, short bipolar-destined expansions are postsynaptic to the bipolar cell body.

Autoradiographic identification of acetylcholine in the rabbit retina

Rabbit retinas were studied in vitro under conditions known to maintain their physiological function. Retinas incubated in the presence of [3H]choline synthesized substantial amounts of both [3H]phosphorylcholine and [3H]acetylcholine. With time, [3H]phosphorylcholine proceeded into phospholipids, primarily phosphatidylcholine. Retinas pulse-labeled by a 15-min exposure to 0.3 microM [3H]choline were incubated for a subsequent hour under chase conditions designed either to retain newly synthesized acetylcholine within synapses or to promote its release. At the end of this time the two groups of retinas were found to contain equal amounts of radioactivity in the phospholipid pathway, but only the retinas incubated under the acetylcholine-protecting conditions contained [3H]acetylcholine. Freeze-dried, vacuum-embedded tissue from each retina was autoradiographed on dry emulsion. All retinas showed silver grains over the photoreceptor cells and faint labeling of all ganglion cells. In the retinas that contained [3H]acetylcholine, silver grains also accumulated densely over a few cells with the position of amacrine cells, over a subset of the cells of the ganglion cell layer, and in two bands over the inner plexiform layer. Fixation of the retina with aqueous osmium tetroxide retained only the radioactive compounds located in the photoreceptor and ganglion cells. Sections from freeze-dried tissue lost their water-soluble choline metabolites when exposed to water, and autoradiography of such sections again revealed radioactivity primarily in the photoreceptor and ganglion cells. Radioactive compounds extracted from the sections were found to faithfully reflect those present in the tissue before processing; analysis of the compounds eluted from sections microdissected along the outer plexiform layer showed [3H]acetylcholine to have been synthesized only by cells of the inner retina. Taken together, these results indicate that the photoreceptor and ganglion cells are distinguished by a rapid synthesis of choline-containing phospholipids, while acetylcholine synthesis is restricted to a few cells at both margins of the inner plexiform layer. They imply that the only neurons to release acetylcholine within the rabbit retina are a small group of probable amacrine cells.

Electron microscopic localization of [125I]alpha-bungarotoxin binding sites in the outer plexiform layer of the goldfish retina

Light and electron microscope autoradiography were performed on goldfish (Carassius auratus) retinas incubated in [125I]labelled alpha-bungarotoxin. The toxin was bound preferentially to membrane receptors in the inner and outer plexiform layers. Binding was suppressed by 10(-5) M nicotine or 10(-5) M native alpha-bungarotoxin. Electron microscopic analysis of the outer plexiform layer (OPL) strongly suggested that alpha-bungarotoxin binding sites were located on small bipolar cell dendritic processes that invaginated rod and cone synaptic terminals, and on large bipolar cell dendritic processes more proximally situated in the OPL. Large horizontal cell processes in the OPL and horizontal cell processes that invaginated rod and cone synaptic terminals did not appear to be labelled.

Similarities in effects of acetylcholine and dopamine on horizontal cells in the fish retina

The effect of acetylcholine (ACh) on cone- and rod-connected horizontal cells (generating photopic and scotopic L-type S-potentials, respectively) in the fish (Eugerres plumieri) retina was compared with that of dopamine (DA), and some similarities were found in the effects. About two-thirds of the L-type horizontal cells examined were sensitive to a test solution containing ACh (10 mM in concentration) and BW-anticholinesterase (BW; 1.0 mM). ACh with BW augmented a center response and attenuated a surround response as did DA (0.2 mM), although the latter effect was more pronounced and longer-lasting. ACh with BW frequently produced oscillations of the horizontal cell membrane potential, as did DA with clonidine (alpha-adrenergic stimulant). Phentolamine (alpha-adrenergic blocker) interrupted both the effects of DA and ACh on the center and surround responses, while hexamethonium (cholinergic antagonist) appeared to interfere selectively with the effect of ACh. Therefore, the ACh-induced changes observed in the horizontal cell membrane potential are assumed to be mediated by the adrenergic system in the fish retina.

Biochemical studies of isolated glial (Müller) cells from the turtle retina

A method has been developed for the preparation of large numbers of glial (Muller) cells from the turtle retina. After proteolytic dissociation of the retina, Muller cells were separated from retinal neurons by velocity sedimentation at unit gravity. Fractions containing >90 percent morphologically identifiable Muller cells were prepared by this procedure. Fractions containing only Muller cells were obtained by drawing selected cells individually into a micropipette under visual observation. Biochemical analyses of isolated Muller cells showed that (a) these cells did not synthesize and accumulate acetylcholine, gamma-aminobutyric acid, or catecholamines when incubated with appropriate radioactive precursors; (b) the specific activities of choline acetyltransferase (EC 2.3.1.6), glutamate decarboxylase (EC 4.1.1.15), and tyrosine hydroxylase (EC 1.14.16.2) in these cells were less than 2 percent of those found in the retina; (c) Muller cells, however, contained high activities of transmitter degrading enzymes-acetylcholinesterase (EC 3.1.1.7) and gamma-aminobutyrate- transamine (EC 2.6.1.19); and (d) the cells also possessed high levels of two presumably glial-specific-enzymes-glutamine synthetase (EC 6.3.1.2) and carbonic anhydrase (EC 4.2.1.1). These results, together with other findings, suggest that Muller cells are not capable of neurotransmitter syntheses but possess the enzymes necessary for two important roles in the retina: (a) the inactivation of certain transmitters after synaptic transmission by uptake and degradation, and (b) the maintenance of acid-base balance and the provision of a stable microenvironment in the retina by the removal of metabolic products such as carbon dioxide and ammonia.

Responses to light of solitary rod photoreceptors isolated from tiger salamander retina

Single, isolated rod photoreceptors were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. These solitary cells retained the morphological features of rods of the intact retina and could be maintained in culture for several days. When impaled with micropipettes for electrophysiological recording, dark-adapted solitary rods had during darkness a resting potential of approximately -45 mV and a steady-state slope resistance of 500 Momega at rest. The current-voltage relationship showed both inward- and outward-going rectification. The responses to light of solitary rods were similar to those recorded from rods in the intact retina stimulated with large-diameter spots of light. The reversal potential of the light response of solitary rods was near 0 mV when measured in either the inner or outer segment.

Synaptic transmission to the horizontal cells in the retina of the larval tiger salamander

1. The receptive field diameter for most horizontal cells far exceeds the lateral spread of processes for any cell. Therefore horizontal cells probably receive synaptic input from neighbours as well as from the photoreceptors. The electrical effects of these two synaptic inputs were studied. 2. We have characterized the electrical properties of the horizontal cell inputs by determining the current-voltage curves in dark and light. These curves were compared with those obtained in the presence of Co2+ or Mg2 was nearly identical to the curve in the light. 4. The putative transmitter substances glutamate, aspartate and GABA depolarized the cells by increasing conductance. Current-voltage curves measured in the presence of these substances intersected the dark and light curves at +50 mV, the same level at which the dark and light curves intersect. 5. The light response of cells uith broad receptive fields, between 1.0 and 2.0 mm, showed little or no change in conductance associated with the light response. The input resistance was near 20 Momega, and the current-voltage curves intersected at an extrapolated potential level near 200 mV. 6. In the presence of ACh, electrical properties of the broad field cells reverted to those of the narrow field cells: the receptive field was reduced to 0.5 mm, the imput resistance increased, and the current-voltage curves intersected near +50 mV. Thus ACh appeared to interrupt synaptic input from neighbouring horizontal cells. 7. The results confirm the suggestion that horizontal cells receive a tonic excitatory input from the photoreceptors which is decreased by light. They show that horizontal cells receive an additional input from their neighbours, not associated with a measurable conductance change. The input from neighbours is selectively interrupted by ACh, but the nature of this synapse and of the cholinergic action is not known.

Enrichment of spinal cord cell cultures with motoneurons

Spinal cord cell cultures contain several types of neurons. Two methods are described for enriching such cultures with motoneurons (defined here simply as cholinergic cells that are capable of innervating muscle). In the first method, 7-day embryonic chick spinal cord neurons were separated according to size by 1 g velocity sedimentation. It is assumed that cholinergic motoneurons are among the largest cells present at this stage. The spinal cords were dissociated vigorously so that 95-98% of the cells in the initial suspension were isolated from one another. Cells in leading fractions (large cell fractions: LCFs) contain about seven times as much choline acetyltransferase (CAT) activity per unit cytoplasm as do cells in trailing fractions (small cell fractions: SCFs). Muscle cultures seeded with LCFs develop 10-70 times as much CAT as cultures seeded with SCFs and six times as much CAT as cultures seeded with control (unfractionated) spinal cord cells. More than 20% of the large neurons in LCF-muscle cultures innervate nearby myotubes. In the second method, neurons were gently dissociated from 4-day embryonic spinal cords and maintained in vitro. This approach is based on earlier observations that cholinergic neurons are among the first cells to withdraw form the mitotic cycle in the developing chick embryo (Hamburger, V. 1948. J. Comp. Neurol. 88:221-283; and Levi-Montalcini, R. 1950. J. Morphol. 86:253-283). 4-Day spinal cord-muscle cultures develop three times as much CAT as do 7-day spinal cord-muscle plates, prepared in the same (gentle) manner. More than 50% of the relatively large 4-day neurons innervate nearby myotubes. Thus, both methods are useful first steps toward the complete isolation of motoneurons. Both methods should facilitate study of the development of cholinergic neurons and of nerve-muscle synapse formation.

Differential sensitivity of rods and cones in Xenopus retina to hemicholinium-3

Hemicholinium-3 (HC-3), a drug which prevents synthesis of acetylcholine in neurons, when injected intraperitoneally in doses as low as 2 X 5 mg/kg produces marked ultrastructural changes and damage in rod but not in cone photoreceptors. In rods it causes a reduction in cytoplasmic background density, swelling of the endoplasmic reticulum, ballooning of the outer membrane of the nucleus, leaching of the nucleoplasm and clumping of the nuclear chromatin. In dark-adapted rods HC-3 produces some loss of cytoplasmic synaptic vesicles but no reduction in numbers of those vesicles which lie adjacent to the synaptic ribbons. In light-adapted rods the drug does not cause such an apparent reduction of synaptic vesicles but does induce a considerable reduction in numbers of vesicles associated with the ribbons. These structural changes are discussed in the light of what is known about the pharmacology of HC-3 and neurotransmitter release from vertebrate photoreceptors.

Effects of light and dark upon photoreceptor synapses in the retina of Xenopus laevis

Photoreceptor synapses in Xenopus retina were studied after exposure to day/night cycles and continuous light or dark. In the rods, dense-core vesicles appear alongside the synaptic ribbons in animals exposed to light. In dark-adapted rods, electron-dense material is present in the synaptic clefts, but no dense-core vesicles are found associated with the synaptic ribbons. Cone photoreceptors do not show these ultrastructural changes in response to light and dark. Prolonged exposure to light (21 days) causes flattening of the synaptic vesicles associated with the synaptic ribbons in both rods and cones. The results are discussed in the light of what is known about transmitter release from photoreceptors.

Localization of acetylcholine receptors during synaptogenesis in retina

Nicotinic acetylcholine receptors are synthesized in chick embryo retina before synapses appear. Most of the receptors are found associated with neurites in the inner synaptic layer of the retina; later in development the receptors appear in the outer synaptic layer. Cells dissociated from retina and cultured in vitro form aggregates and also sort out into regions comprised either of neurites or cell bodies. Most of the nicotinic acetylcholine receptors of aggregates are found associated with neurite regions. The receptor distribution of cultured retina cells thus resembles that of intact retina.