Enkephalin in the goldfish retina

Vanilloid receptor 1 (TRPV1/VR1) co-localizes with fatty acid amide hydrolase (FAAH) in retinal amacrine cells

Fatty acid amide hydrolase (FAAH) is the degradative enzyme for anandamide (AEA), an endogenous ligand for the vanilloid receptor (TRPV1) and cannabinoid receptor 1. As FAAH and TRPV1 are integral membrane proteins, FAAH activity could modulate the availability of AEA for TRPV1 activation. Previous studies in this laboratory reported an extensive endocannabinoid system in goldfish retina. Immunocytochemistry was used to determine the relative distributions of FAAH-immunoreactivity (IR) and TRPV1-IR in goldfish retina. Here, we show the first example in an intact neural system in which TRPV1-IR co-localizes in subpopulations of FAAH-immunoreactive neurons, in this case amacrine cells. These cells are rare and consist of three subtypes: 1. ovoid cell with granular-type dendrites restricted to sublamina a, 2. pyriform cell with smooth processes in sublamina b, and 3. fusiform cell with smooth processes that project to sublaminae a and b. The varied appearances of reaction product in the dendrites suggest different subcellular localization of TRPV1, and hence function of FAAH activity regarding TRPV1 stimulation among the cell types. Ovoid and pyriform amacrine cells, but not fusiform cells, labeled with GAD-IR and constituted subsets of GABAergic amacrine cells. TRPV1 amacrine cells, though rare, are represented in the ON, OFF and ON/OFF pathways of the retina. As TRPV1 stimulation increases intracellular calcium with numerous downstream effects, co-localization of TRPV1 and FAAH suggests an autoregulatory function for anandamide. Due to the rarity of these cells, the three vanilloid amacrine cell types may be involved in global effects rather than feature extraction, for example: sampling of ambient light or maintaining homeostasis.

Retinal dopamine depletion in young quail mimics some of the effects of ageing on visual function

The hypothesis that retinal dopaminergic (DA) neurones are involved in the visual functions of interest was tested. The retinal DA in young quail was partially depleted by intravitreal injection of 6-hydroxydopamine (6-OHDA). It was found that the refractive state of 6-OHDA-treated birds became more myopic than normal (untreated) young, whereas the pupil diameter was not affected. The contrast sensitivity of 6-OHDA treated quail was significantly lowered (two to three times) at all spatial frequencies studied (0.25-5 c/d), and the peak latency of pattern electro-retinogram (PERG) response was prolonged by 3-4 msec (9%). Furthermore, the visual acuity and maximal amplitude of PERG response of the 6-OHDA-treated young quail were lower than those of normals. From histochemical studies, it was revealed that the morphology of the DA cells of 6-OHDA-treated young appeared similar to those of the old quail; the DA cells of 6-OHDA-treated retinae were less fluorescent and 2.5-5 times less numerous than respective controls. Combining the PERG and the morphological results, it would seem that the retinal DA plays an important role in the visual functions studied, and that loss of retinal DA could underlie some of the visual changes which occur during ageing.

Interaction between enkephalin and gamma-aminobutyric acid in the chicken retina: a double-label immunoelectron microscopic analysis

In the present study, double-label immunoelectron microscopy was used to examine the synaptic relationships between amacrine cell populations in the chicken retina that contain either enkephalin or gamma-aminobutyric acid (GABA) or both enkephalin and GABA. The objectives of the present study were twofold. First, the ultrastructural features and synaptic organization of enkephalin and enkephalin/GABA amacrine cells were compared. Second, the synaptic interactions between these populations and the population of GABA amacrine cells were examined. A total of 475 synaptic arrangements were observed to involved enkephalin or enkephalin/GABA amacrine cell processes. The synaptic relationships of enkephalin and enkephalin/GABA amacrine cells were quite similar. Each population was pre- and postsynaptic to amacrine cells, postsynaptic to bipolar cells, and presynaptic to processes possibly originating from ganglion cells. A substantial percentage of each population's pre- and postsynaptic relationships were with the processes of GABAergic amacrine cells. Moreover, when enkephalin and enkephalin/GABA amacrine cell processes were postsynaptic to bipolar cells, their dyadic partner was observed frequently to be a GABA amacrine cell process. The present study suggests a diversity in the population of chicken enkephalin amacrine cells with respect to their expression of the classical inhibitory transmitter GABA. Moreover, a functional relationship between enkephalinergic and GABAergic pathways is indicated by studies showing that both enkephalin and enkephalin/GABA amacrine cells exhibit substantial synaptic interaction with GABA amacrine cells.

Interaction between enkephalin and GABA in the chicken retina: further analyses of coexisting relationships

Previous studies have indicated an interactive relationship between enkephalin and gamma-aminobutyric acid (GABA) in the vertebrate retina. Among these studies are those that have demonstrated the colocalization of enkephalin and GABA in retinal amacrine cells. In the present study, enkephalin immunocytochemistry was combined with either autoradiography of tritiated GABA high-affinity uptake or GABA immunocytochemistry to further investigate the coexistence of GABA in enkephalin-amacrine cells of the chicken retina. A regional analysis revealed that the percentage colocalization of GABA high-affinity uptake in enkephalin-amacrine cells did not vary appreciably throughout the retina. Overall, 15.2% of enkephalin-amacrine cells exhibited high-affinity GABA uptake. Double-label immunofluorescence histochemistry revealed that 15.1% of enkephalin-amacrine cells express endogenous GABA-like immunoreactivity. These double-labelled cells were observed throughout central and peripheral regions of the retina. In each of the double-label analyses, only less intensely labelled enkephalin-amacrine cells expressed markers of GABA activity. The two double-label analyses reveal almost identical percentages of coexistence of GABA markers in chicken enkephalin-amacrine cells and therefore, provide supportive evidence for the GABAergic nature of these cells. These results suggest a functional diversity in the population of chicken enkephalin-amacrine cells and imply the possibility of multiple signalling through amacrine cells which contain enkephalin and GABA.

Quantitative analyses of the coexistence of gamma-aminobutyric acid in substance P-amacrine cells of the larval tiger salamander retina

The present study was performed as part of a systematic examination of gamma-aminobutyric acid's (GABA) coexistence with other classical transmitters and neuropeptides in neuronal populations of the larval tiger salamander retina. Substance P immunocytochemistry was combined with either GABA immunocytochemistry or autoradiography of high-affinity GABA uptake to examine for the presence of GABA in substance P-amacrine cells of the larval tiger salamander retina. Double-label analyses revealed two populations of substance P-amacrine cells that express both markers of GABA activity. One population was situated in the innermost cell row of the inner nuclear layer, while the other population was located in the ganglion cell layer. In both cases, these double-labelled cells accounted for approximately 10% of substance P-amacrine cells in their respective layers. The present study demonstrates, therefore, that substance P-amacrine cells in the larval tiger salamander retina can be categorized on the basis of their coexisting/non-coexisting relationships with GABA and suggests a possible functional diversity in the population of substance P-amacrine cells.

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

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

Colocalization of enkephalin-, glucagon-, and corticotropin-releasing factor-like immunoreactivity in GABAergic amacrine cells in turtle retina

The large number of amacrine cells which contain gamma-aminobutyric acid (GABA) in the turtle retina makes it difficult to examine specific GABAergic cell types. In order to selectively label subpopulations of GABAergic neurons, we have used fluorescent double-labeling immunocytochemical techniques to examine the localization of GABA-like immunoreactivity (LI) in amacrine cells which contain antigens resembling the neuropeptides glucagon (GLUC), corticotropin-releasing factor (CRF) or enkephalin (ENK). GABA-LI was found in 41% of the cells with GLUC-, 100% of the cells with CRF-, and 69% of the cells with ENK-LI. There were regional differences in the presence of GABA-LI in amacrine cell populations with ENK-LI. GABA-LI was present in about 80% of the cells with ENK-LI outside of the visual streak, while only 37% of the cells within the streak had GABA-LI. Based on the distinct morphologies and regional distributions of these peptidergic amacrine cells, we conclude that they represent different subpopulations of GABAergic amacrine cells in the turtle retina. Future studies can now utilize existing information regarding the synaptic connectivity of these peptidergic amacrine cells to help delineate the functions of GABAergic amacrine cells in the turtle retina.

Enkephalin-immunoreactive ganglion cells in the pigeon retina

A small number of enkephalin-like immunoreactive cells were observed in the ganglion cell layer of the pigeon retina. Many of these neurons were identified as ganglion cells, since they were retrogradely labeled after injections of fluorescent latex microspheres in the contralateral optic tectum. These ganglion cells were mainly distributed in the inferior retina, and their soma sizes ranged from 12-26 microns in the largest axis. The enkephalin-containing ganglion cells appear to represent only a very small percentage of the ganglion cells projecting to the optic tectum (less than 0.1%). Two to 7 weeks after removal of the neural retina, there was an almost complete elimination of an enkephalin-like immunoreactive plexus in layer 3 of the contralateral, rostrodorsal optic tectum. These data provide evidence for the existence of a population of enkephalinergic retinal ganglion cells with projections to the optic tectum.

Peptidergic neurons of teleost retinas

In retinas of teleost fish, neuropeptides typically have subtle, modulatory actions. The peptide effects typically have long latencies and durations, and, in some instances, they are known to be mediated by second messengers. Peptidergic neurons in teleost retinas have certain morphological features in common that are consistent with their function. Most peptidergic neurons are stratified amacrine cells with long, varicose processes; the processes of peptidergic centrifugal axons are also narrowly stratified and ramify extensively in the retina. The peptidergic amacrine cells are relatively infrequent, and, likewise, the centrifugal axons originate from a small number of perikarya in the brain. Cells that are so sparsely distributed and whose processes overlap so extensively are better-suited for modulation than for conveying detailed representations of visual space.

A double-label analysis demonstrating that all enkephalin-immunoreactive amacrine cells in the chicken retina express neurotensin immunoreactivity

A previous study demonstrated less than 50% co-existence between the populations of enkephalin- and neurotensin-like immunoreactive amacrine cells in the chicken retina. The present study was undertaken with the intent of re-examining this relationship using a more sensitive double-label paradigm. An examination of retinal cryosections collected throughout the retina revealed that all labelled cells express both enkephalin and neurotensin-like immunoreactivity. Therefore, these results indicate the presence of a single population of chicken amacrine cells the members of which express both these neuropeptides.

A re-examination of enkephalin's coexistence with gamma-aminobutyric acid in amacrine cells of the larval tiger salamander retina

Double-label immunocytochemistry was utilized to re-examine the colocalization of enkephalin and gamma-aminobutyric acid (GABA) in amacrine cells of the larval tiger salamander retina. A total of 465 enkephalin-immunoreactive amacrine cells were identified and in all cases these cells were GABA-immunoreactive. This finding corroborates a previous study that showed greater than 96% of enkephalin-amacrine cells in the tiger salamander retina to specifically accumulate [3H]GABA and provides additional evidence for the GABAergic nature of these enkephalin-amacrine cells.

Naloxone effects on the visual evoked potentials recorded from the main and accessory visual pathways of the cat

1. The effects produced by repetitive i.v. administration of naloxone (1, 2 or 4 mg/kg) on the visual evoked potentials (VEPs) recorded along the main and accessory visual pathways were investigated in a modified "encéphale isolé" cat preparation. 2. Naloxone provoked a progressive amplitude enhancement and latency reduction of some components, depending on the structure analyzed, the dose used and the number of administrations applied. Electroretinogram (ERG) and N1-P1 VEP components of optic chiasm (OCh), lateral geniculate body (LGB) and visual cortex (VC) did not present significant changes. 3. Late-latency components (more than 200 msec) appeared in the VEPs of LGB and VC, mainly when 4 mg/kg were used. 4. Our results suggest that endogenous opioids have a modulatory role in the processing of sensory information at different levels of the visual system.

Double-label analyses of somatostatin's coexistence with enkephalin and gamma-aminobutyric acid in amacrine cells of the chicken retina

Double-label analyses were performed to investigate somatostatin's coexistence with either enkephalin or gamma-aminobutyric acid (GABA) in amacrine cells of the chicken retina. Double-label immunocytochemistry revealed that although some amacrine cells labelled only for somatostatin or enkephalin, approx. 81% and 85% of somatostatin-immunopositive cells in the center and periphery of the retina, respectively, were also enkephalin-immunoreactive. Somatostatin-immunocytochemistry combined with autoradiography of high-affinity [3H]GABA uptake revealed that approx. 18% of somatostatin-immunoreactive amacrine cells exhibit high-affinity uptake of [3H]GABA.

The release of Leu5-enkephalin-like immunoreactivity from chicken retina is reduced by light in vitro

A superfusion system was established to examine the efflux of endogenous Leu5-enkephalin-like immunoreactivity (LE-LI) from isolated chicken retinas. Superfusion with buffer containing high concentration of K+ (60 mM KCl) increased the efflux of LE-LI by 96%. This effect was not observed when Co2+ (4 mM CoCl2) was present. Exposing the retinas to light decreased the efflux of LE-LI by 59% compared to that observed during superfusion in the dark. No effect of ambient light could be detected in the presence of Co2+. Upon reverse-phase high-performance liquid chromatography the material released by the retina comigrated with synthetic Leu5-enkephalin. These results demonstrate that the release of LE-LI from retinal neurons is increased during the dark, and it is concluded that the lighting conditions exert their effects by modifying the state of polarization of the LE-LI amacrine cells and hence the release of LE-LI from these neurons.

Synaptic organization of enkephalinlike-immunoreactive amacrine cells in the goldfish retina

Immunoelectron microscopy was used to examine the synaptic organization of enkephalinlike-immunoreactive amacrine cells in the goldfish retina. Enkephalin-immunostained processes sometimes contained dense-cored vesicles (115-145 nm) in addition to a generally homogeneous population of small, round, clear synaptic vesicles. A total of 194 synaptic relationships were observed that involved the immunostained processes of enkephalin-amacrine cells. The large majority of these were observed in sublayer 5 of the inner plexiform layer. In greater than 95% of the synaptic relationships, the enkephalin-immunostained profile served as the presynaptic element. In 58.8% of these relationships, enkephalin processes synapsed onto amacrine cell processes, while 30.4% of their synapses were onto processes that lacked synaptic vesicles. They also occasionally formed synaptic contacts (6.7%) onto the somas of cells located either in the inner nuclear or in the ganglion cell layers. Enkephalin profiles received synaptic input only from amacrine cells (4.1%), while no direct synaptic interaction was observed between enkephalin processes and bipolar cells. However, in sublayer 1, enkephalin profiles were found to synapse onto amacrine cell processes that were presynaptic to bipolar cell terminals. In the proximal inner plexiform layer, enkephalin processes were presynaptic to amacrine cell processes that as a group surrounded and sometimes provided synaptic input to extremely large and round bipolar cell endings.

Quantitative studies of enkephalin's coexistence with gamma-aminobutyric acid, glycine and neurotensin in amacrine cells of the chicken retina

Previous double-label studies demonstrate that enkephalin coexists with gamma-aminobutyric acid, glycine or neurotensin in amacrine cells of the chicken retina. The present study utilizes double- and triple-label paradigms to quantitatively analyze these coexisting relationships. Twenty-eight percent of enkephalin-like immunoreactive amacrine cells were found to exhibit high-affinity uptake of [3H]GABA, while 53% of enkephalin-amacrine cells specifically accumulate [3H]glycine. Moreover, the present study predicts that at least 26% of enkephalin-amacrine cells which accumulate [3H]glycine should also be immunoreactive for neurotensin.

Interactions between enkephalin and gamma-aminobutyric acid in the larval tiger salamander retina

Both double-label and intracellular electrophysiological recording techniques were utilized to investigate the interactions between enkephalin and gamma-aminobutyric acid in the larval tiger salamander retina. Double-label studies revealed that the vast majority (greater than 96%) of enkephalin-immunostained amacrine cells also exhibit high affinity uptake of [3H]gamma-aminobutyric acid. Electrophysiological evidence demonstrated that morphine and gamma-aminobutyric acid exert opposite effects on a population of On-Off ganglion cells. gamma-Aminobutyric acid decreased the activity of these cells, while enkephalin increased their activity. These findings support the idea that opiate-mediated pathways inhibit GABAergic pathways in the vertebrate retina.