Larval Tiger Salamander Retina Research Articles

Chromophore switch from 11‐cis‐dehydroretinal (A2) to 11‐cis‐retinal (A1) decreases dark noise in salamander red rods

Dark noise, light-induced noise and responses to brief flashes of light were recorded in the membrane current of isolated rods from larval tiger salamander retina before and after bleaching most of the native visual pigment, which mainly has the 11-cis-3,4-dehydroretinal (A2) chromophore, and regenerating with the 11-cis-retinal (A1) chromophore in the same isolated rods. The purpose was to test the hypothesis that blue-shifting the pigment by switching from A2 to A1 will decrease the rate of spontaneous thermal activations and thus intrinsic light-like noise in the rod. Complete recordings were obtained in five cells (21 degrees C). Based on the wavelength of maximum absorbance, lambda max,A1 = 502 nm and lambda max,A2 = 528 nm, the average A2 : A1 ratio determined from rod spectral sensitivities and absorbances was approximately 0.74 : 0.26 in the native state and approximately 0.09 : 0.91 in the final state. In the native (A2) state, the single-quantum response (SQR) had an amplitude of 0.41 +/- 0.03 pA and an integration time of 3.16 +/- 0.15 s (mean +/- s.e.m.). The low-frequency branch of the dark noise power spectrum was consistent with discrete SQR-like events occurring at a rate of 0.238 +/- 0.026 rod(-1) s(-1). The corresponding values in the final state were 0.57 +/- 0.07 pA (SQR amplitude), 3.47 +/- 0.26 s (SQR integration time), and 0.030 +/- 0.006 rod(-1) s(-1) (rate of dark events). Thus the rate of dark events per rod and the fraction of A2 pigment both changed by ca 8-fold between the native and final states, indicating that the dark events originated mainly in A2 molecules even in the final state. By extrapolating the linear relation between event rates and A2 fraction to 0% A2 (100% A1) and 100% A2 (0% A1), we estimated that the A1 pigment is at least 36 times more stable than the A2 pigment. The noise component attributed to discrete dark events accounted for 73% of the total dark current variance in the native (A2) state and 46% in the final state. The power spectrum of the remaining 'continuous' noise component did not differ between the two states. The smaller and faster SQR in the native (A2) state is consistent with the idea that the rod behaves as if light-adapted by dark events that occur at a rate of nearly one per integration time. Both the decreased level of dark noise and the increased SQR amplitude must significantly improve the reliability of photon detection in dim light in the presence of the A1 chromophore compared to the native (A2) state in salamander rods.

The Metabotropic Receptor mGluR6 May Signal Through Go, But Not Phosphodiesterase, in Retinal Bipolar Cells

Bipolar cells are retinal interneurons that receive synaptic input from photoreceptors. Glutamate, the photoreceptor transmitter, hyperpolarizes On bipolar cells by closing nonselective cation channels, an effect mediated by the metabotropic receptor mGluR6. Previous studies of mGluR6 transduction have suggested that the receptor couples to a phosphodiesterase (PDE) that preferentially hydrolyzes cGMP, and that cGMP directly gates the nonselective cation channel. This hypothesis was tested by dialyzing On bipolar cells with nonhydrolyzable analogs of cGMP. Whole-cell recordings were obtained from On bipolar cells in slices of larval tiger salamander retina. Surprisingly, On bipolar cells dialyzed with 8-(4-chlorophenylthio)-cyclic GMP (8-pCPT-cGMP), or 8-bromo-cyclic GMP (8-Br-cGMP) responded normally to glutamate or L-2-amino-4-phosphonobutyrate (L-APB). Response amplitudes and kinetics were not significantly altered compared with cells dialyzed with cGMP alone. Comparable results were obtained with the PDE inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) or with 8-pCPT-cGMP and IBMX together, indicating that PDE is not required for mGluR6 signal transduction. Addition of the G-protein subunit G(o)alpha to the pipette solution suppressed the cation current and occluded the glutamate response, whereas dialysis with G(i)alpha or with transducin Gbetagamma had no significant effect on either the cation current or the response. Dialysis of an antibody directed against G(o)alpha also reduced the glutamate response, indicating a functional role for endogenous G(o)alpha. These results indicate that mGluR6 may signal through G(o), rather than a transducin-like G-protein.

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.

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.

Double-label analyses of the coexistence of somatostatin with GABA and glycine in amacrine cells of the larval tiger salamander retina

To investigate the possible GABAergic nature of somatostatin-immunoreactive neurons of the larval tiger salamander retina, somatostatin immunocytochemistry was combined with either γ-aminobutyric acid (GABA) immunocytochemistry or autoradiography of GABA high-affinity uptake. A total of 1,062 somatostatin cells were visualized in these studies. Double-label immunocytochemistry revealed that 96.3% of somatostatin-immunoreactive cells expressed GABA immunoreactivity. Double-label studies combining somatostatin immunocytochemistry with autoradiography of GABA high-affinity uptake revealed a slightly lower percentage (93%) of colocalization. Double-labelled cells were identified as Type 1, Type 2 and displaced amacrine cells. The small percentage of somatostatin-immunoreactive cells that did not co-label for GABA were identified as Type 1 amacrine cells. An analysis of retinal sections processed for double-label immunocytochemistry revealed that approximately 5% of GABA-immunoreactive cells in the amacrine and ganglion cell layers co-label for somatostatin. Somatostatin immunocytochemistry was combined with autoradiography of glycine high-affinity uptake to examine whether tiger salamander somatostatin-amacrine cells express this glycine marker. A total of 100 somatostatin-immunoreactive amacrine cells were visualized in double-label preparations. None of these cells were observed to exhibit glycine high-affinity uptake.

Synaptic organization of dopaminergic amacrine cells in the larval tiger salamander retina

The ultrastructural features and synaptic interactions of tyrosine hydroxylase-like-immunoreactive amacrine cells in the larval tiger salamander retina were examined using routine immunoelectron microscopy. The somas of tyrosine hydroxylase-like-immunoreactive amacrine cells were immunostained evenly throughout their cytoplasm. Their nuclei were generally unstained and possessed indented nuclear membranes. The processes of tyrosine hydroxylase-like-immunoreactive amacrine cells were homogeneously stained with the exception of their mitochondria, whose morphology was often distrupted by the staining procedure. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were characterized by an occasional dense-cored vesicle(s), in addition to a generally homogeneous population of small, round, agranular synaptic vesicles. They formed conventional synaptic junctions that were characterized by symmetrical synaptic membrane densities. A total of 168 synapses were observed that involved tyrosine hydroxylase-like-immunoreactive amacrine cell processes. A large percentage (79.8%) of these synaptic arrangements were found in sublayer 1 of the inner plexiform layer, while substantially lower percentages were observed in sublayers 3 (9.5%) and 5 (10.7%). They served as pre- and postsynaptic elements 63.1 and 36.9% of the time, respectively. Tyrosine hydroxylase-like-immunoreactive amacrine cell processes were presynaptic to amacrine cell processes (36.9% of total synaptic involvement) and processes that lack synaptic vesicles and whose origin remains uncertain (26.2%). They received synaptic input primarily from amacrine cell processes (31.0%). Tyrosine hydroxylase-like-immunoreactive amacrine cell processes also received a few ribbon synapses from bipolar cells (5.9%). Each of these synaptic relationships were observed in each of sublayers 1, 3 and 5 of the inner plexiform layer, with the majority of each arrangement being found in sublayer 1.

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 transmitter 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.

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

Previous studies have localized tyrosine hydroxylase, the rate-limiting enzyme for the production of dopamine, and γ-aminobutyric acid (GABA) to amacrine cell populations in the larval tiger salamander retina. Double-label immunocytochemistry was used to examine if tyrosine hydroxylase-like and GABA-like immunoreactivities colocalize in tiger salamander amacrine cells. A total of 2,162 tyrosine hydroxylase-like immunoreactive amacrine cells were observed in double-labelled sections. None of these cells were observed to express GABA-like immunoreactivity. Therefore, the present study demonstrates that dopamine and GABA are localized to distinct neuronal populations in the larval tiger salamander retina.

A double-label study demonstrating that all serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina express GABA-like immunoreactivity

A previous study localized serotonin-like immunoreactivity to amacrine cell populations in the larval tiger salamander retina. The present double-label immunocytochemical analysis of the tiger salamander retina was performed to determine if gamma-aminobutyric acid (GABA)-like immunoreactivity is expressed by serotonin-immunoreactive amacrine cells. More than 3,000 serotonin-amacrine cells were observed in double-label preparations, and all were found to express GABA-like immunoreactivity. This finding extends previous studies of serotonin-GABA coexistence in the retina by providing the first report of the co-localization of endogenous serotonin and GABA-like compounds in a retinal neuron.

A double-label study demonstrating that enkephalin and somatostatin are localized in separate populations of amacrine cells in the larval tiger salamander retina

Previous studies have localized enkephalin and somatostatin to amacrine cell populations in the larval tiger salamander retina. Double-label immunocytochemistry was utilized to examine if enkephalin- and somatostatin-like immunoreactivities are colocalized to tiger salamander amacrine cells. Of the more than 2000 labelled cells observed in double-labelled preparations, none were found to express both enkephalin and somatostatin immunoreactivity. Therefore, these studies demonstrate that in the larval tiger salamander retina, enkephalin and somatostatin are localized to separate populations of amacrine cells.

Double-label analyses demonstrating the non-coexistence of enkephalin and glycine in amacrine cells of the larval tiger salamander retina

Enkephalin immunocytochemistry was combined with either glycine immunocytochemistry or autoradiography of high-affinity glycine uptake to examine for colocalization of enkephalin and glycine in amacrine cells of the larval tiger salamander retina. A total of 995 enkephalin-immunoreactive amacrine cells were visualized in double-label preparations. None of the enkephalin-labelled cells was observed to co-label for markers of glycinergic activity.

Coexistence and function of glutamate receptor subtypes in the horizontal cells of the tiger salamander retina.

Effects of the major glutamate receptor agonists, kainate (KA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), quisqualate (QA), N-methyl-D-aspartate (NMDA), L-alpha-amino-4-phosphonobutyrate (L-AP4), and trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on horizontal cells (HCs) were studied in superfused larval tiger salamander retina. 20 microM of KA, AMPA, and QA mimicked the action of 3 mM glutamate in the absence and presence of 1 mM Co2+. 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the actions of KA and AMPA, but not those of QA and glutamate, indicative of the existence of CNQX-resistant QA receptors in the tiger salamander HCs. Prolonged application of ACPD hyperpolarized the HCs and enhanced the light responses, probably by shifting the resting HC voltage (Er) to a more hyperpolarized position. It is possible that the KA, AMPA, and CNQX-resistant QA receptors are involved in mediating the postsynaptic light responses in HCs, and ACPD receptors are involved in sensitivity adjustment of the HC responses.

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 γ-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.

Synaptic organization of serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina

Immunoelectron microscopy was used to investigate the ultrastructural features and synaptic relationships of serotonin-like immunoreactive amacrine cells in the larval tiger salamander retina. Serotonin-positive somas exhibited an evenly distributed peroxidase reaction product throughout their cytoplasm. Their nuclei were unstained and possessed indented nuclear membranes. Serotonin-immuno-reactive processes were generally stained throughout with the exception of their mitochondria, whose morphology was often disrupted by the staining reaction. They were further characterized by an occasional dense-cored vesicle/s in addition to a generally homogeneous population of small, round, clear synaptic vesicles. Serotonin-immunoreactive amacrine cell processes formed conventional synapses that were characterized by symmetrical synaptic membrane densities. A total of 222 synaptic arrangements were observed that involved the immunostained processes of serotonin-amacrine cells. As presynaptic elements, they primarily contacted amacrine cells processes (37.8%). They also provided substantial synaptic input to processes that lacked synaptic vesicles (16.2%) and whose origin was unidentified. Serotonin-processes provided a far fewer number of synaptic contacts onto the processes of bipolar cells (1.4%) and the somas of cells in the amacrine cell layer (0.5%). As postsynaptic elements, they received synaptic inputs from amacrine cells (27.9%) and bipolar cells (16.2%). With the exception of their synapses onto bipolar cells and the somas of cells in the amacrine cell layer, each of the synaptic relationships of serotonin-amacrine cells was observed in each of sublayers 1–5 of the inner plexiform layer.

Localization of serotoninlike‐immunoreactive amacrine cells in the larval tiger salamander retina

Light microscopic immunocytochemistry was used to study the populations of serotoninlike-immunoreactive cells in the larval tiger salamander retina. Of 1,135 serotonin-immunostained cells observed in transverse cryosections, 87% were identified as amacrine cells, whereas 13% were tentatively designated as displaced amacrine cells. The somas of the vast majority of serotonin-amacrine cells were situated in the innermost cell row of the inner nuclear layer. Only a few serotonin-immunostained amacrine cell somas were observed in the second row of cells from the inner nuclear layer. Serotonin-immunoreactive processes generally appeared as a diffuse plexus distributed evenly throughout all levels of the inner plexiform layer. As determined in whole-mount preparations, serotonin-amacrine cells were divisible into two populations on the basis of the diameters of their somas. Large cells (45%) ranged from 16 to 19 microns in diameter with the vast majority measuring 17-18 microns. Smaller and sometimes less intensely stained cells ranged from 14 to 16 microns in diameter with the large majority measuring 15 microns. The diameters of serotonin-displaced amacrine cells ranged from 19 to 22 microns with the large majority measuring 20 microns in diameter. An examination of whole-mount retinas revealed that serotonin-immunoreactive amacrine and displaced amacrine cells were distributed throughout the center and the periphery of the retina. The density of serotonin-amacrine cells (large and small combined) was calculated to be 173 +/- 4.5 (mean +/- standard error) cells per mm2.

Effects of CNQX, APB, PDA, and kynurenate on horizontal cells of the tiger salamander retina.

Effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 2-amino-4-phosphonobutyrate (APB), cis-2,3-piperidine dicarboxylic acid (PDA), and kynurenate (KYN) on the depolarizing actions of glutamate and kainate on horizontal cells (HCs) were studied in the larval tiger salamander retina. APB, PDA, and KYN hyperpolarized the HCs, but they failed to block either the actions of glutamate and kainate, or the HC light responses. APB and PDA did not cause membrane polarizations in either rods or cones, suggesting that the HC hyperpolarizations were not mediated by presynaptic actions of these compounds. CNQX, the newly synthesized non-NMDA (N-Methyl-D-Aspartate) receptor antagonist, blocked the HC light responses and the action of kainate, but not that of glutamate. These results suggest that the synaptic receptors in the tiger salamander HCs are probably non-NMDA although extra-synaptic NMDA receptors may exist in these cells.

Localization of putative GABAergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographic methods.

Putative GABAergic neurons in the larval tiger salamander retina were localized by a comparative analysis of glutamate decarboxylase immunoreactivity (GAD-IR), GABA-like immunoreactivity (GABA-IR), and high-affinity 3H-GABA uptake at the light microscopical level. Preliminary data showed that all GAD-IR neurons were double labeled for GABA-IR. However, because the weak somatic labeling with GAD-IR, we could not determine if the converse were true. Neurons commonly labeled with GABA-IR and 3H-GABA uptake include horizontal cells, type I (outer) and type II (inner) bipolar cells, type I (inner) and type II (outer) amacrine cells, and cell bodies in the ganglion cell layer (GCL). In addition, interplexiform cells were identified with GABA-IR. The presence of GABA-IR ganglion cells was indicated by GABA-IR fibers in the optic fiber layer and optic nerve as well as by a GABA-IR cell in the GCL that included a labeled axon. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30% in INL 1 (outer layer of somas), 15% in INL 2 (middle layer), 43-52% in INL 3 (inner layer), and about 21-26% in the GCL. Labeled processes were found in three bands in the inner plexiform layer, with the densest band located in the most proximal part. Postembedding labeling of 1-micron Durcupan resin sections for GABA-IR showed the same general pattern as obtained with 10-microns cryostat sections, with additional staining, however, of type II (inner) bipolar cell Landolt's clubs. Extensive colocalization of labeling was indicated, and we conclude that GABA-IR can serve as a valid and reliable marker for GABA-containing neurons in this retina and suggest that GABA serves as a transmitter for horizontal cells, several types of amacrine cell, a type of interplexiform cell, and perhaps a small percentage of type I and type II bipolar cells and ganglion cells.

Light microscopic localization of putative glycinergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographical methods.

Putative glycinergic neurons in the larval tiger salamander retina were localized by a comparative analysis of high affinity 3H-glycine uptake and glycine-like immunoreactivity (Gly-IR) at the light microscopic level. Commonly labeled neurons include at least three types of amacrine cell (Type IAd, Type IAb, Type IIAd; distinguished by soma location and dendritic ramification), cell bodies in the ganglion cell layer (GCL), and rarely observed Type II (inner) bipolar cells. With the increased resolution provided by Gly-IR, we identified a Type IAa amacrine cell, two types of Type IIAd amacrine cells, and Gly-IR interplexiform cells. Gly-IR axons in longitudinal sections of the optic nerve indicate the presence of Gly-IR ganglion cells. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30-40% in INL 2 (middle layer of somas), 30-40% in INL 3 (inner layer of somas), and about 5% in the GCL. Labeled processes were found throughout the full thickness of the inner plexiform layer (IPL), but with a much denser band in the proximal half (sublamina b). The only major difference between the two methods (3H-glycine uptake vs. Gly-IR) was that Type I (outer) bipolar cells were labeled only by 3H-glycine uptake; these cells were more lightly labeled with silver grains than cell bodies in either INL 2 or INL 3. Postembed labeling of 1 micron Durcupan plastic sections for Gly-IR showed the same pattern, but with much higher resolution, as obtained with 10 micron cryostat sections. This study indicates extensive colocalization of labeling by both probes in INL 2, INL 3, the IPL, and the GCL. We conclude that Gly-IR can serve as a valid and reliable marker for glycine-containing neurons in this retina and suggest that glycine serves as a transmitter for several morphologically distinct types of amacrine cell, an interplexiform cell, and perhaps a small percentage of Type II bipolar cells and ganglion cells.

Localization of tyrosine-hydroxylase-like-immunoreactive amacrine cells in the larval tiger salamander retina.

Immunocytochemistry was used to localize the populations of tyrosine-hydroxylase-like (TH)-immunoreactive cells in the tiger salamander retina. Ninety percent of these cells possessed somas that were situated in the innermost cell row of the inner nuclear layer and were classified as amacrine cells. Ten percent of TH-immunoreactive somas were located in the ganglion cell layer and were tentatively designated as those of displaced amacrine cells. The processes of TH-immunoreactive cells ramified most heavily in sublayer 1 of the inner plexiform layer, while a relatively small number of TH-labelled processes distributed in sublayers 3 and 5. Less than 1% of TH-immunoreactive cells in the amacrine cell layer exhibited a short process of somal origin that extended distally toward the outer plexiform layer. However, these processes did not cross the whole of the inner nuclear layer, and no immunolabelling was observed in the outer plexiform layer. An examination of retinal whole-mounts revealed that TH-immunoreactive amacrine and displaced amacrine cells were distributed throughout the center and periphery of the retina. The density of TH-immunolabelled amacrine cells was calculated to be 49 +/- 13 (mean +/- standard error) cells per mm2. The vast majority of TH-immunoreactive amacrine and displaced amacrine cells exhibited a stellate appearance and gave rise to three or more primary dendrites. A few TH-amacrine and displaced amacrine cells possessed two primary dendrites that emerged from opposite sides of their somas. The processes of TH-immunoreactive cells were generally poorly branched and varicose with terminal branches sometimes appearing thin and beaded. Because some TH-immunolabelled processes were very long, there was considerable overlap between the dendritic fields of neighboring TH-cells. Lastly, individual TH-immunoreactive amacrine and displaced amacrine cells were often observed in whole-mounts to provide processes that ramified at more than one level of the inner plexiform layer.

Localization of neurotensin-like immunoreactive amacrine cells in the larval tiger salamander retina.

Light microscopic immunocytochemistry was used to localize the populations of NT-like immunoreactive amacrine cells in the larval tiger salamander retina. Seventy-nine percent of NT-immunostained cells observed in transverse cryo-prepared sections were classified as Type 1 amacrine cells. Another 6% were classified as Type 2 amacrine cells, while 15% of the NT-cells had their cell bodies situated in the ganglion cell layer and were tentatively designated as displaced amacrine cells. Each type of NT-like immunoreactive cell was observed in the central and peripheral retina. NT-immunostained processes were observed to ramify in sublayers 3 and 5 of the inner plexiform layer. An examination of retinal whole mounts revealed that NT-amacrine cells were distributed throughout the center and periphery of the retina at a density of 82 +/- 24 cells/mm2. The dendritic fields of NT-immunostained amacrine and displayed amacrine cells were observed to be either symmetrically or asymmetrically distributed about their somas. Symmetrical dendritic fields were generally oval-shaped and ranged in diameter from 250 to 500 micron (major axis) by 150 to 250 micron (minor axis). Asymmetrical dendritic fields were observed to encompass one-half or less of an imaginary circle surrounding their soma of origin and were orientated in all directions. The processes forming asymmetrical dendritic fields ranged from 75 to 260 micron in length. Furthermore, partial overlap was often observed between the dendritic fields of adjacent NT-amacrine cells.