Acetylcholinesterase localization in cat retina: a comparison with choline acetyltransferase

Differential co-localization with choline acetyltransferase in nervus terminalis suggests functional differences for GnRH isoforms in bonnethead sharks (Sphyrna tiburo)

The nervus terminalis (NT) is a vertebrate cranial nerve whose function in adults is unknown. In bonnethead sharks, the nerve is anatomically independent of the olfactory system, with two major cell populations within one or more ganglia along its exposed length. Most cells are immunoreactive for either gonadotropin-releasing hormone (GnRH) or RF-amide-like peptides. To define further the cell populations and connectivity, we used double-label immunocytochemistry with antisera to different isoforms of GnRH and to choline acetyltransferase (ChAT). The labeling patterns of two GnRH antisera revealed different populations of GnRH-immunoreactive (ir) cell profiles in the NT ganglion. One antiserum labeled a large group of cells and fibers, which likely contain mammalian GnRH (GnRH-I) as described in previous studies and which were ChAT immunoreactive. The other antiserum labeled large club-like structures, which were anuclear, and a sparse number of fibers, but with no clear labeling of cell bodies in the ganglion. These club structures were choline acetyltrasferase (ChAT)-negative, and preabsorption control tests suggest they may contain chicken-GnRH-II (GnRH-II) or dogfish GnRH. The second major NT ganglion cell-type was immunoreactive for RF-amides, which regulate GnRH release in other vertebrates, and may provide an intraganglionic influence on GnRH release. The immunocytochemical and anatomical differences between the two GnRH-immunoreactive profile types indicate possible functional differences for these isoforms in the NT. The club-like structures may be sites of GnRH release into the general circulation since these structures were observed near blood vessels and resembled structures seen in the median eminence of rats.

Dual substrate model for novel approach towards a kinetic study of acetylcholinesterase inhibition by diazinon

Limited reports as compared to other insecticides appear in the literature for acetylcholinesterase (AChE) inhibition by diazinon. In the current study, new kinetic parameters of AChE inhibition by diazinon have been investigated. The assay was done with bovine retinal AChE using two different substrate (ASCh) concentrations in the absence and presence of diazinon (0.08-1.28 mM). The optical density was monitored up to 25 min (reaction time) for the assay. New kinetic parameters k'(oms), K'(sms), k(oms), K(sms), K'(asms) and K(asms) ) were calculated from these experimental data.

Demonstration of cholinergic ganglion cells in rat retina: expression of an alternative splice variant of choline acetyltransferase

Acetylcholine acts as a neurotransmitter in the retina. Although previous physiological studies have indicated that some retinal ganglion cells may be cholinergic, several immunohistochemical studies using antibodies to choline acetyltransferase (ChAT) have stained only amacrine cells but not ganglion cells. Recently, we identified a splice variant of ChAT mRNA, lacking exons 6-9, in rat peripheral nervous system. The encoded protein was designated as ChAT of a peripheral type (pChAT), against which an antiserum was raised. In the present study, we examined expression of pChAT in rat retina, both at the protein level by immunohistochemistry using the antiserum and at the mRNA level by RT-PCR. Immunohistochemistry revealed that although no positive neurons were found in untreated intact retinas, many neurons became immunoreactive for pChAT after intravitreal injection of colchicine. Damage of the optic nerve was also effective in disclosing positive cells. Such positive neurons were shown to be ganglion cells by double labeling with a retrograde tracer that had been injected into the contralateral superior colliculus. Western blot analysis and RT-PCR revealed a corresponding band to the pChAT protein and to the amplified pChAT gene fragment, respectively, in retinal samples. In addition, ChAT activity was definitely detected in retinofugal fibers of the optic nerve. These results indicate the presence of cholinergic ganglion cells in rat retina.

Developmental relationship between cholinergic amacrine cell processes and ganglion cell dendrites of the mouse retina

Ganglion cells of the mammalian retina undergo structural remodeling before their dendrites are confined to functionally distinct laminas within the inner plexiform layer. It has been proposed that cholinergic amacrine cells provide laminar cues that remodel ganglion cell dendrites, because their processes stratify before those of the ganglion cells. To address this possibility, it is necessary to know whether cholinergic cells contact all or only some classes of ganglion cells during development. We, therefore, used two-photon microscopy to simultaneously reconstruct the dendritic arbors of different classes of ganglion cells and terminal processes of cholinergic cells in neonatal mouse retina. We determined that, after birth, cholinergic cells contacted only a subset of ganglion cells. Large bistratified cells (LBCs), resembling direction selective ganglion cells in other species, had dendrites that fasciculated with the cholinergic plexuses. The LBCs received numerous presynaptic cholinergic contacts shortly after birth. In contrast, large monostratified cells (LMCs), ramifying outside the cholinergic plexuses at maturity, received few, if any, cholinergic contacts even at early stages when their dendrites overlapped with the cholinergic processes. These observations suggest that cholinergic cells provide laminar cues for only subsets of ganglion cells. They also indicate that the synaptic organization between amacrine and ganglion cells may be specified early in development.

Kinetic constants for the inhibition of camel retinal acetylcholinesterase by the carbamate insecticide lannate

We have designed this study to determine various kinetic parameters of camel retinal membrane-bound acetylcholinesterase (AChE; EC 3.1.1.7) inhibition by carbamate insecticide lannate [methyl N-[[(methylamino)carbonyl]oxy] ethanimidothioate]. All these kinetic constants were derived by simple graphical methods. The value of kinetic parameters was estimated as follows: 0.061 microM)(-1), 1.14 (microM)(-1), 0.216 microM, 0.016 min(-1), 0.0741 (gammaM min)(-1) 0.746 microM, and 4.42 microM for velocity constant (Kv), new inhibition constant (Knic), dissociation constant (Kd), carbamylation rate constant (k2c), overall carbamylation rate constant (k'2), 50% inhibition constant (K150), and 99% inhibition constant (K199), respectively. These unique methods may be used to estimate such kinetic parameters for time-dependent inhibition of enzymes by variety of chemicals, insecticides, herbicides, and drugs.

Kinetic analysis of the toxicological effect of tacrine (Cognex) on human retinal acetylcholinesterase activity

For the first time, kinetic parameters of the effect of tacrine, an anti-cholinesterase inhibitor of therapeutic potential in Alzheimer's disease has been studied on human retinal acetyl-cholinesterase (AChE). Tacrine inhibited the AChE activity in a concentration dependent manner, the IC(50) being about 45 nM. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.120 mM and this value was increased by 4-52.8% in the presence of tacrine. V(max) was observed to be 2.23 micromol/h per mg protein for the control system, while it was decreased by 14.73-56.25% in the tacrine treated systems. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the mixed type, i. e. a combination of competitive and noncompetitive inhibition. The values of K(i) and K(I) were estimated to be as 37.76 and 64.36 nM, respectively.

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.

Kinetics of the inhibition of acetylcholinesterase in camel retina by cisplatin

The inhibitory effect of cisplatin (CDDP) on camel retina acetylcholinesterase (AChE) was characterized. The CDDP effect was independent of the time of incubation with AChE before the addition of substrate, indicative of reversible inhibition. Moreover, dilution data prove that CDDP is a reversible inhibitor of camel retina AChE. Cisplatin inhibited AChE activity of camel retina in a concentration- and time-dependent manner, the IC50 values being 5.32 and 0.196 mM at 5 min and 24 h incubation times, respectively. The IC50 has dual components, i.e. directly proportional and inversely proportional to 0-1.5 h and 1.5-24 h incubation periods, respectively. The Michaelis-Menten constant (Km) for the hydrolysis of acetylthiocholine iodide (ASCh) was found to be 0.0796 mM and Vmax was 0.668 micromol/min/mg protein. Kmapp and Vmaxapp both decreased as the CDDP concentration increased. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure uncompetitive type. The value of Ki was estimated as 0.811 mM by the primary and secondary replots of the Lineweaver-Burk and Dixon plots. Kiapp decreased while Vmaxiapp increased after increasing the ASCh concentration.

The DAPI-3 amacrine cells of the rabbit retina

In the rabbit retina, the nuclear dye, 4,6,diamidino-2-phenylindole (DAPI), selectively labels a third type of amacrine cell, in addition to the previously characterized type a and type b cholinergic amacrine cells. In this study, these "DAPI-3" amacrine cells have been characterized with respect to their somatic distribution, dendritic morphology, and neurotransmitter content by combining intracellular injection of biotinylated tracers with wholemount immunocytochemistry. There are about 100,000 DAPI-3 amacrine cells in total, accounting for 2% of all amacrine cells in the rabbit retina, and their cell density ranges from about 130 cells/mm2 in far-peripheral retina to 770 cells/mm2 in the visual streak. The thin varicose dendrites of the DAPI-3 amacrine cells form a convoluted dendritic tree that is symmetrically bistratified in S1/S2 and S4 of the inner plexiform layer. Tracer coupling shows that the DAPI-3 amacrine cells have a fivefold dendritic-field overlap in each sublamina, with the gaps in the arborization of each cell being occupied by dendrites from neighboring cells. The DAPI-3 amacrine cells consistently show the strongest glycine immunoreactivity in the rabbit retina and they also accumulate exogenous [3H]-glycine to a high level. By contrast, the AII amacrine cells, which are the best characterized glycinergic cells in the retina, are amongst the most weakly labelled of the glycine-immunopositive amacrine cells. The DAPI-3 amacrine cells costratify narrowly with the cholinergic amacrine cells and the On-Off direction-selective ganglion cells, suggesting that they may play an important role in movement detection.

The inhibitory effect of cyclophosphamide on camel retina acetylcholinesterase activity

Kinetic parameters for the effect of cyclophosphamide (CP) on the camel retina acetylcholinesterase (AChE) activity were investigated for the first time in the present study. It was found that 18 micrograms of retina protein and an incubation time of 4.0 min were suitable conditions for linear of AChE activity. The CP effect was independent of time of incubation with AChE before the addition of substrate, which shows it reversible action. Moreover, dilution data prove that CP is a reversible inhibitor of camel retina AChE. Cyclophosphamide (0.2-2.4 mM) inhibited activity of camel retina in a concentration-dependent fashion, the IC50 being about 1.17 mM. The Michaelis constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.106 mM and the Vmax was 0.765 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the pure noncompetitive type. The value of Ki was estimated as 0.763 mM by the primary Dixon and secondary replots of the Lineweaver-Burk plot.

Kinetics for camel (Camelus dromedarius) retina acetylcholinesterase inhibition by methotrexate in vitro

This work addresses the kinetic analysis of the interaction of methotrexate (MTX) with camel retina acetylcholinesterase (A ChE, EC 3.1.1.7). It was found that the MTX effect was reversible in nature. The IC50 was determined, by two methods, to be 1.362 mM. The Michaelis-Menten constant (Ks) for the hydrolysis of acetylthiocholine iodide (ASCh) by AChE was 0.123 mM in the control system, and the MTX-treated systems showed a 10-35% decrease in this value. The Vmax was 0.789 mumol/min/mg protein for the control system, while it was decreased by 23-76% in the MTX-treated systems. The Lineweaver-Burk plot, Dixon plot and their secondary replots indicated that the inhibition was a linear mixed type; i.e., uncompetitive and noncompetitive. The values of Ki and KI were estimated as 0.782 and 0.404 mM, respectively. The use of camel retina as a model for the study of human retina may open new avenues for studying various aspects of AChE.

The nature of the inhibition of camel retina acetylcholinesterase (EC 3.1.1.7) activity by tetrahydroaminoacridine

The nature of the inhibition of camel retina acetylcholinesterase (AChE) activity by tetrahydro-aminoacridine (THA, tacrine) has been investigated in the present study. The non-significant change of the percent inhibition of AChE by THA with respect to various lengths of the preincubation period showed the type of the reversible inhibition. THA reversibly inhibited AChE activity in a concentration dependent manner; IC50 was 0.23 microM while the IC100 was 14.22 microM. The K(m) for the hydrolysis of acetylthiocholine iodide was found to be 62.6 microM in the control system; a value increased in the THA treated systems. The Vmax was 0.472 mumole/min/mg protein for the control system, while it decreased in the THA treated systems. Dixon, as well as Lineweaver-Burk, plots and their secondary replots indicated that the nature of the inhibition is of the linear mixed type, which is considered to be a partial competitive and pure non-competitive mixture. The values of Ki(slope) and K'i(intercept) were estimated as 0.068 microM and 0.181 microM, respectively. The K'i was greater than Ki indicating that THA has a greater affinity of binding for the peripheral site than the active site of the camel retina AChE. The use of camel retina as a good experimental animal model may open new avenues for studying acetylcholine and AChE metabolism.

Acetylcholinesterase in the developing ferret retina

The function of acetylcholinesterase (AChE) is to terminate the action of acetylcholine at the cholinergic synapse. Recent evidence suggests additional roles for acetylcholinesterase as a peptidase and/or a protease which is expressed by growing neurites as part of their invasion of developing neural structures. We report the localization of acetylcholinesterase in developing ferret retina. AChE histochemical staining is seen in the developing inner plexiform layer (IPL) of ferret retina at birth (post-natal day zero, PO), the earliest developmental stage examined. Transient expression is seen at the border between the ganglion cell layer and the nerve fiber layer at P14 and P21. A small amount of transient expression is seen in the outer plexiform layer (OPL) at this age as well. By P28, the transient expression in the OPL is at its peak, and is found at photoreceptor terminals and associated with apparent horizontal cell axons. Labeling is also seen intracellularly in the inner nuclear layer (INL), at the OPL/INL border, suggesting that horizontal cells are the source of the transient AChE expression in the OPL. Overt synaptic profiles also appear in the inner plexiform layer (IPL) at P21 and P28. About 2 days layer, the eyes open and the photoreceptor outer segments are fully developed. By 2 weeks later, at P42, the AChE staining pattern in the retina has taken on its adult appearance: no reaction product in the outer retina; intracellular reaction product in the Golgi apparatus of a subset of amacrine and displaced amacrine cells which manufacture AChE; and extracellular reaction product at both synaptic and non-synaptic sites in the IPL. These data are consistent with a role for AChE as a peptidase early in development, and as an enzyme essential in the termination of synaptic action at mature synapses.

AChE-staining of type II ganglion cells, processes and terminals in the cochlea of the mustached bat

There have been a number of reports showing that ganglion cells of sensory neurons may be stained by traditional acetylcholinesterase (AChE) histochemical techniques commonly used to demonstrate efferent nerve fibers and terminals. AChE-staining has been described for cell bodies in the vestibular and spiral ganglia; staining of peripheral and central processes, however, is rare and the presence of reaction product in afferent terminals has not been reported. The outer hair cells of mustached bats, Pteronotus parnellii, differ from those of most mammals in that they typically have a single, large efferent terminal surrounded by 5-7 small, afferent terminals. In this animal an AChE-positive reaction was found not only in efferent fibers and terminals but also in type II ganglion cells, their peripheral and central processes and in outer hair cell terminals. The stained cell bodies were smaller than the unstained type I ganglion cells and they were much fewer in number. The processes of the stained cells could be followed from the soma. The central processes were dispersed throughout the VIIIth nerve trunk. Stained peripheral processes were evident in the osseous spiral lamina, floor of the tunnel of Corti and first space of Nuel and in the outer spiral plexus along the sides of the outer phalangeal (Deiters') cells. AChE-stained afferent terminals were easy to identify after transection of the crossed olivocochlear bundle (COCB) and subsequent degeneration of large efferent terminals. These results are of interest in that assessments of efferent nerve histochemistry after COCB transection need to recognize the potential contribution of AChE reaction product in afferent terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholinesterase and choline acetyltransferase localization patterns do correspond in cat and rat retinas

Is acetylcholinesterase (AChE) a reliable marker for cholinergic activity in the cat and rat retinas? To evaluate this question, radial sections, labeled for AChE, have been compared to sections labeled for choline acetyltransferase (ChAT). Within the inner plexiform layer (IPL) of each species, two lightly-stained AChE bands are revealed which correspond to the depths of ChAT immunoreactivity. Although retinal AChE is not limited exclusively to sites where ChAT is present, AChE and ChAT activity do occur in the same IPL sublaminae. Used with proper caution, AChE is a reliable secondary indicator of cholinergic activity.

Solubilization and biochemical characterization of the melatonin deacetylase from Xenopus laevis retina

Melatonin deacetylase, an enzyme activity recently discovered in the Xenopus laevis retina, regulates local melatonin levels. The deacetylase occurs in retina, retinal pigment epithelium, and skin, all sites of melatonin action, and is widely distributed among vertebrates. We have solubilized the enzyme from Xenopus retina and pigment epithelium using nonionic detergents, and have developed a specific enzyme assay. We have characterized the enzyme and now report that the deacetylase is relatively specific for melatonin and is inhibited by the melatonin precursor N-acetylserotonin and the product of the deacetylase, 5-methoxytryptamine. Inhibition of deacetylase activity by eserine (physostigmine) suggests a relationship between deacetylase and cholinesterase activities. However, among a variety of cholinesterase inhibitors tested, only eserine inhibits the deacetylase. Furthermore, eserine is much less potent as an inhibitor of the deacetylase than the cholinesterases, and purified cholinesterases failed to deacetylate melatonin. We also show that melatonin deacetylase and aryl acylamidase (an enzyme related to cholinesterases) activities are differentially extractable from Xenopus ocular tissues, and that they exhibit different pH optima and inhibition profiles. Our results provide an initial characterization of the Xenopus retinal melatonin deacetylase, and indicate that deacetylase activity is distinct from cholinesterase and aryl acylamidase activities.

Hydrolysis of substance P in the rabbit retina: I. Involvement of acetylcholine and acetylcholinesterase. An in vivo study

The laminar patterns of acetylcholinesterase (AChE) activity and substance P (SP) immunoreactivity within the inner plexiform layer (IPL) of the rabbit retina show striking similarities. Discrete bands of SP-immunoreactivity were seen at 1-7%, 40-48% and 85-95% depth of IPL. AChE activity was present throughout the entire thickness of the IPL with moderately stained bands in each sublamina (3-24% in sublamina a and 62-89% in sublamina b depth IPL). These bands were bordered on both sides by bands of even greater density (in sublamina a 0-3% and 24-34% and in sublamina b 55-62% and 89-100% depth IPL). Cell processes staining for choline acetyltransferase (ChAT) have previously been shown to ramify at 19-24% and 63-79% depth levels. Thus, SP- and ChAT-immunoreactive bands are located in both sublaminae, positioned within regions of moderate AChE activity and flanked by bands with greater AChE activity. This strong morphological correspondence and reported interactions between acetylcholine (ACh), AChE and SP in vitro provide the basis for the present study to determine whether such interactions can be demonstrated in vivo. Retinas infused with ACh showed a 60% average increase in SP-IR as compared with untreated retinas from the same animals. Treatment with diisopropylfluorophosphate (DFP) also resulted in a 56% increase in SP-IR. The ability of ACh to induce increased levels of SP was not inhibited by CoCl2, atropine or mecamylamine, ruling out the possibilities of polysynaptic transmission or involvement of muscarinic or nicotinic receptors. Infusion of ACh did not increase the levels of preprotachykinin-mRNA indicating that the increase in SP-IR is not due to de novo synthesis but rather to inhibition of the enzyme(s) responsible for SP degradation. Whether AChE functions alone or in concert with other enzymes to hydrolyze SP cannot be determined from these experiments but is addressed in a separate study.

Distribution in brain and retina of four enzymes of acetyl CoA synthesis in relation to choline acetyl transferase and acetylcholine esterase

Eleven regions of mouse brain and twelve layers of monkey retina were assayed for choline acetyl transferase (ChAT), acetylcholine esterase (AChE), and 4 enzymes that synthesize acetyl CoA. The purpose was to seek evidence concerning the source of acetyl CoA for acetylcholine generation. In brain ATP citrate lyase was strongly correlated with ChAT as well as AChE (r = 0.914 in both cases). Weak, but statistically significant correlation, was observed between ChAT and both cytoplasmic and mitochondrial thiolase, whereas there was a significant negative correlation between ChAT and acetyl thiokinase. In retina ChAT was essentially limited to the inner plexiform and ganglion cell layers, whereas substantial AChE activity extended as well into inner nuclear, outer plexiform and fiber layers, but no further. ATP citrate lyase activity was also highest in the inner four retinal layers, but was not strongly correlated with either ChAT or AChE (r = 0.724 and 0.761, respectively). Correlation between ChAT and acetyl thiokinase was at least as strong (r = 0.757), and in the six inner layers of retina, the correlation between ChAT and acetylthiokinase was very strong (r = 0.932).

The cholinergic system in retinal transplants in rats

Retinas from embryonic day 13 or 14 Sprague-Dawley albino rats were transplanted to the brainstem of newborn rats with unilateral eye enucleation at birth. Two months after the transplantation, the activity and distribution of acetylcholinesterase and choline acetyltransferase were studied using histochemical and immunocytochemical methods respectively. Results obtained showed that the staining patterns of these two cholinergic enzymes in the retinal transplants were essentially the same as those observed in the retinas of normal rats and in the control retinas of the recipient animals. The similarities in the distribution of these two cholinergic enzymes in these retinas suggest that the cholinergic system in the retinal transplants is likely to be functional.

Cholinergic amacrine cells in the developing cat retina

Antiserum directed against the ACh-synthesizing enzyme choline acetyltransferase (ChAT) was used to determine the development of cholinergic amacrine cell distributions in wholemounted kitten retinae. From birth (P0) two populations of cholinergic amacrine cells were immunolabelled; one population was located in the inner nuclear layer (INL) and the other was displaced to the ganglion cell layer (GCL). Dendrites from cholinergic amacrines in the INL stratified in the outer third of the inner plexiform layer (IPL), whilst those from displaced amacrines in the GCL stratified in the centre of the IPL. There was a centroperipheral gradient of development in both populations, and the total number of cholinergic amacrines in the GCL always exceeded that of the INL. The most dramatic increase in the number of amacrines expressing ChAT was between P0 and P5 when the numbers doubled. During the same period there was an increase in the soma size of both populations in the peripheral retina. In the GCL, cholinergic amacrines did not attain adult size until P21 whereas those of the INL reached adult proportions by P5. Overall, between P0 and P21 there was a fivefold increase in the number of immunoreactive cholinergic amacrines in the GCL and a 3.5-fold increase in those of the INL. In general, the distribution, soma size, percentage per layer, and total number of cholinergic amacrines in both populations resembled those of the adult by P21.

The morphology and topographic distribution of substance-P-like immunoreactive amacrine cells in the cat retina

In cat retinal wholemounts, substance-P-like immunoreactivity (SP-IR) was localized in a distinct population of amacrines whose cell bodies were normally placed in the ganglion cell layer. Although displaced amacrines accounted for 80-95% of the SP-IR amacrines in peripheral retina, this proportion decreased considerably within the area centralis, accounting for 50-80% of the labelled cells at maximum density. The SP-IR cells in both the inner nuclear and ganglion cell layers gave rise to well-defined varicose dendrites of uniform appearance that stratified around 60% depth (S3/S4) of the inner plexiform layer. In addition, sparse fine dendrites in stratum 1 (S1) could sometimes be traced to inner nuclear cells and occasionally to displaced amacrines. The combined SP-IR cell density ranged from less than 50 cells mm-2 in the far periphery to more than 500 cells mm-2 in the area centralis; the maximum density showed little individual variation despite wide differences in the proportion of displaced cells. The 39,000 SP-IR amacrines in a mapped retina had a triangular topographic distribution, with intermediate isodensity lines extending vertically in superior retina and horizontally along both arms of the visual streak. Colocalization experiments established that all SP-IR cells in cat retina showed GABA-like immunoreactivity, and that the SP-IR amacrines were quite distinct from the cholinergic amacrines identified by choline acetyltransferase immunohistochemistry.

Neurofibrillar long-range amacrine cells in mammalian retinae

A distinct population of wide-field, unistratified amacrine cells are shown to be selectively stained by using neurofibrillar methods in rabbit and cat retinae. Their cell bodies may be located in the inner nuclear, inner plexiform or ganglion cell layers and they branch predominantly in stratum 2 of the inner plexiform layer. Characteristically, each cell has two or more long-range distal processes which extend for 2-3 mm beyond a more symmetrical, proximal dendritic field of 0.6-0.8 mm diameter. Although the neurofibrillar long-range amacrines account for less than 1 amacrine in 500, they achieve effective coverage of the retina by both the proximal and distal dendrites.

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.

Substance P-like immunoreactive amacrine cells in the cat retina

Substance P-like immunoreactivity was localized by immunocytochemical techniques to two subpopulations of amacrine cells in the cat retina. One cell was a unistratified amacrine with processes ramifying within stratum 4 of the inner plexiform layer. The other cell type was a bistratified cell with processes in both stratum 1 (s1) and stratum 4 (s4). Both cell types were seen with their somas displaced to the ganglion cell layer as well as in the conventional amacrine location in the inner nuclear layer. Substance P cells were present in the greatest density within the area centralis and decreased in number toward the periphery. The ratio of amacrine to displaced amacrine cells also decreased peripherally. However, the coverage by immunoreactive fibers in s4 remained three times that seen in s1. Computer-assisted analysis confirmed the location of substance P-containing processes at 5-15% (s1) and 50-70% (s4) depth levels in the inner plexiform layer. A comparison of substance P-like immunoreactivity in light- and dark-adapted cat retinas showed no apparent differences in the distribution of immunoreactivity due to lighting conditions.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.