A double-label analysis demonstrating the non-coexistence of tyrosine hydroxylase-like and GABA-like immunoreactivities in amacrine cells of the larval tiger salamander retina

Synaptic organization of GABAergic amacrine cells in the salamander retina

The synaptic organization of GABA-immunoreactive (GABA-IR) amacrine cells in the inner plexiform layer (IPL) of salamander retina was studied with the use of postembedding immuno-electron microscopy. A total of 457 GABA-IR amacrine synapses, with identified postsynaptic elements, were analyzed on photomontages of electron micrographs covering 3,618 microm2 of the IPL. GABA-IR amacrine synapses were distributed throughout the IPL, with a small peak at the proximal margin of sublamina a. The majority of the output targets (81%) were GABA(-) neurons. Most of the contacts were simple synapses with one postsynaptic element identified as a process of an amacrine cell (55%), bipolar cell (19%) or ganglion cell (26%), and serial synapses were very rare. Of the 89 postsynaptic bipolar terminals, 63% participated in a reciprocal feedback synapse with the same presynaptic GABA-IR amacrine profile. There appeared to be no preference between GABA-IR amacrine contacts with rod- or cone-dominated bipolar cells (9.1% vs. 8.9%) or in the total number of amacrine synapses in sublaminas a and b (52% vs. 47%). The preponderance of amacrine cell input to bipolar cells in the OFF layer was derived from GABA-IR cells. These findings provide ultrastructural support to the existing physiological studies regarding the functional roles of the GABAergic amacrine cells in this species. Our results have added to the data base demonstrating that, in contrast to mammals, GABA-IR amacrine cells in amphibians and other nonmammals contact other amacrine cells more frequently, suggesting greater involvement of GABAergic amacrine cells in modulating lateral inhibitory pathways.

Immuocytochemical analysis of spatial organization of photoreceptors and amacrine and ganglion cells in the tiger salamander retina

In the present study, using double- or triple-label immunocytochemistry in conjunction with confocal microscopy, we aimed to examine the population and distribution of photoreceptors, GABAergic and glycinergic amacrine cells, and ganglion cells, which are basic but important parameters for studying the structure–function relationship of the salamander retina. We found that the outer nuclear layer (ONL) contained 82,019 ± 3203 photoreceptors, of which 52% were rods and 48% were cones. The density of photoreceptors peaked at ∼8000 cells/mm2 in the ventral and dropped to ∼4000 cells/mm2 in the dorsal retina. In addition, the rod/cone ratio was less than 1 in the central retina but larger than 1 in the periphery. Moreover, in the proximal region of the inner nuclear layer (INL3), the total number of cells was 50,576 ± 8400. GABAergic and glycinergic amacrine cells made up approximately 78% of all cells in this layer, including 43% GABAergic, 32% glycinergic, and 3% GABA/glycine colocalized amacrine cells. The density of these amacrine cells was ∼6500 cells/mm2 in the ventral and ∼3200 cells/mm2 in the dorsal area. The ratio of GABAergic to glycinergic amacrine cells was larger than 1. Furthermore, in the ganglion cell layer (GCL), among a total of 36,007 ± 2010 cells, ganglion cells accounted for 65.7 ± 1.5% of the total cells, whereas displaced GABAergic and glycinergic amacrine cells comprised about 4% of the cells in this layer. The ganglion cell density was ∼1800 cells/mm2 in the ventral and ∼600 cells/mm2 in the dorsal retina. Our data demonstrate that all three major cell types are not uniformly distributed across the salamander retina. Instead, they exhibit a higher density in the ventral than in the dorsal retina and their spatial arrangement is associated with the retinal topography. These findings provide a basic anatomical reference for the electrophysiological study of this species.

Asymmetrical blastomere origin and spatial domains of dopamine and neuropeptide Y amacrine subtypes in Xenopus tadpole retina

Amacrine cells are located almost exclusively in the inner nuclear layer (INL) of the retina, but they express a variety of neurotransmitters. To begin to elucidate the relative roles of the local environment and cell lineage in determining the different neurotransmitter subtypes of amacrine cells, we combined lineage tracing and immunocytochemical techniques to map the spatial distribution and clonal origin of dopamine (DA) and neuropeptide Y (NPY) amacrine cells in Xenopus tadpole retina. At the earliest period of neurotransmitter expression, both DA and NPY amacrine cells were distributed preferentially in center and intermediate annular regions, and in anterior and dorsal quadrants. Most of the DA and NPY cells first emerged as scattered cells and later as clusters (of 2 or more cells) that increased in number and size up to premetamorphic stages. These results suggest that DA and NPY amacrine subtypes may be influenced by environmental cues localized to specific regions of the retina. Lineage analysis showed that the percentage of DA or NPY amacrine cells produced by most blastomere progenitors is significantly different from that predicted by the number of cells in the retina produced by those blastomeres. Only two blastomeres produced over 90% of the DA amacrine cells and only four produced 97% of the NPY amacrine cells. Some retinal progenitors did not contribute at all to these two amacrine subtypes. There also is a marked asymmetry in the blastomere origin of DA and NPY amacrine cells. Two retinal progenitors produced significant numbers of NPY but very few DA amacrine cells. This analysis provides evidence that blastomere origin restricts the developmental choices of retinal progenitors.

Localization of substance P and GABA in retinotectal ganglion cells of the larval tiger salamander

The present study was performed as part of a systematic examination of the transmitter specificity of neuronal populations in the larval tiger salamander retina. Backfill-labeling of ganglion cells from the optic tectum was combined with double-label immunofluorescence histochemistry to determine if substance P and GABA are localized to ganglion cell populations in the tiger salamander retina. The triple-label analysis revealed the presence of substance P- and GABA-ganglion cells in both central and peripheral regions of the retina. Substance P-immunoreactive ganglion cells comprised 2% of the total population of backfill-labeled ganglion cells, while less than 1% of backfill-labeled ganglion cells expressed GABA immunoreactivity. Ganglion cells were not found to co-label for both substance P and GABA. Backfill-labeled displaced ganglion cells, which comprised 1.4% of the ganglion cell population, were not observed to be immunoreactive for either substance P or GABA. Forty-six point nine percent of substance P-cells in the ganglion cell layer were backfill-labeled and were identified as ganglion cells. GABA ganglion cells comprised less than 1% of GABA-immunoreactive cells in the ganglion cell layer. Therefore, the present study provides evidence for the presence of small populations of substance P- and GABA-ganglion cells in the larval tiger salamander retina. These observations suggest a functional diversity in the population of tiger salamander ganglion cells relative to their unique transmitter specificities.

A triple-label analysis demonstrating that enkephalin-, somatostatin- and neurotensin-like immunoreactivities are expressed by a single population of amacrine cells in the chicken retina

The combined results of previous double-label analyses provide evidence suggesting that the neuroactive peptides, enkephalin, somatostatin and neurotensin are expressed by a single population of amacrine cells in the chicken retina. In the present study, triple-label immunofluorescence histochemistry was used to confirm this relationship. An examination of more than fifteen thousand cells in sections collected from throughout the retina revealed that all labelled cells are immunopositive for endogenous enkephalin-, somatostatin- and neurotensin-like immunoreactivity. Therefore, these results reveal the presence of a single population of chicken amacrine cells, each member of which is characterized by its expression and presumed utilization of all three of these neuroactive peptides. However, the functional implications of the possibility of multiple signalling through these cells remain to be elucidated.

Colocalization of glycine in substance P-amacrine cells of the larval tiger salamander retina

The present study was performed as part of a systematic examination of glycine's coexistence with other classical transmitters and neuropeptides in neuronal populations of the larval tiger salamander retina. Substance P immunocytochemistry was combined with either glycine immunocytochemistry or autoradiography of glycine high-affinity uptake to examine whether tiger salamander substance P-amacrine cells express these glycine markers. Double-label analyses revealed two populations of substance P-amacrine cells that express glycine immunoreactivity and glycine high-affinity uptake. The large majority of double-labeled cells were situated in the innermost cell row of the inner nuclear layer, while a smaller number were located in the inner nuclear layer in the second cell row distal to the inner plexiform layer. Double-label immunocytochemistry revealed that these double-labeled cells accounted for 91.7% of substance P-immunoreactive amacrine cells. A slightly lower percentage (90.1%) of substance P-amacrine cells were found to exhibit a glycine high-affinity uptake mechanism. Substance P-amacrine cells that did not co-label for markers of glycine activity were situated in the innermost cell row of the inner nuclear layer. Substance P-immunoreactive displaced amacrine cells were not observed to co-label for either glycine immunoreactivity or glycine high-affinity uptake. The present study reveals that the large majority of substance P-amacrine cells in the larval tiger salamander retina co-express markers of glycine activity. This finding suggests a functional diversity in the population of tiger salamander substance P-amacrine cells relative to their coexisting relationship with a major inhibitory neurotransmitter.