No abstract

Two types of amacrine cell immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine (CA)-synthetic pathway, are described in the rhesus monkey retina with the indirect-immunofluorescent method. These 2 types of neuron differ in soma size, plane of arborization in the inner plexiform layer, levels of the enzyme TH as quantified by microspectrofluorometry, and population density. Type 1 CA cells have comparatively large cell bodies almost exclusively in the innermost row of the inner nuclear layer; their processes arborize in the outermost stratum of the inner plexiform layer; they give rise to fine predominantly radially oriented fibers in the inner nuclear layer; and there are about 26 type 1 CA cells/mm2. Type 2 CA amacrine cells have relatively small cell bodies located in the inner nuclear layer (44.4%), the inner plexiform layer (35.6%) and the ganglion cell layer (20%), and their processes arborize in the center of the inner plexiform layer. Although type 2 CA amacrine cells are more numerous (35 cells/mm2) than type 1 CA cells, type 1 CA amacrine cells are 3.5 x brighter than type 2 CA cells and therefore likely to contain 3.5 X more TH. Thus the primate retina contains 2 distinct catecholaminergic neuronal pathways that could have different functional roles in vision.

In the present work, we have characterized the maturation of the GABAergic system in mammalian retina. Immunoreactivity for GABA, GAD (glutamic acid decarboxylase, EC 4.1.1.15) -65 and -67 in the adult rat retina was localized in cells in the inner nuclear and ganglion cell layers. This pattern was established around postnatal day 8 and included transient GABA and GAD-67 expression in horizontal cells. GAD activity was very low at P1 and P4, increasing after P8, reaching maximal activity by P21 and decreasing to attain adult values by P30. GABA content was approximately constant from P1 to P13, increasing thereafter to reach adult levels. GAD protein content increased progressively with postnatal development and the two isoforms could be distinguished at P8. The disparity between retinal GABA content vs. presence and activity of the synthesizing enzyme, led us to investigate the alternative pathway for GABA synthesis that utilizes putrescine as a substrate. Highest levels of ornithine decarboxylase activity (the limiting step for putrescine synthesis) were found between P1 and P4, decreasing to very low levels after P13. The same pattern was observed for putrescine content in the retina. Highest amounts were found at P1, that decreased and remained constant after P13. Additionally, approximately 40% of tritiated putrescine incorporated by P1, P4 and adult retinas was converted into GABA. Our results suggest the existence of two different sources of GABA in mammalian retina, one that uses glutamate as a precursor and predominates in the mature nervous system and another that utilizes putrescine and is present transiently at early developmental stages.

No abstract

The retinas of placental mammals appear to lack the large number and morphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the cone sheath and by labeling of cone outer segments with monoclonal anti-visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone subpopulations in placental mammals. Besides a dominant rod population (max. = 400,000/mm 2 ) four subtypes of cones (max. = 3000/mm 2 ) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A second group of single cones lacking oil droplets was labeled by OS-2 antibody. The topography of these cone subtypes showed striking anisotropies. The COS-1 labeled single cones without oil droplets were found all over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemisphere. The population of OS-2 labeled cones was also ubiquitous although slightly higher in the upper hemisphere (200/mm 2 ). The COS-1 labeled cones bearing an oil droplet, including the principal member of double cones, were concentrated (800/mm 2 ) in the inferior, non-tapetal half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The additional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence in the area centralis, and the correlation with the non-tapetal inferior hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.

The results show, for the first time, that the expression of S- and M-opsin is dependent on normal thyroid hormone levels during development.

DOPA decarboxylase (DDC; aromatic-L-amino acid decarboxylase; EC 4.1.1.28) is absent in retinas from 6-day-old chicken embryos (E6) but is expressed in retina of E8 embryos, in the presumptive outer plexiform layer. Thereafter, DDC appears in cell bodies of presumptive amacrine cells. The dopamine (DA) content of E9/10 and E15/16 retinas, preincubated with L-DOPA for 1 h, increased 250-and 600-fold, respectively, showing that DDC is active since early in development. Intercellular communication, measured by endogenous cyclic AMP accumulation, was observed when retinas from E9/10 to E15/16 were pre-incubated for 1 h with 1 mM L-DOPA, washed and followed by incubation in the presence of 0.5 mM 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor. Cyclic AMP accumulation was prevented when pre-incubation with L-DOPA was carried out in the presence of carbidopa. Moreover, the accumulation of cyclic AMP was inhibited by SCH 23390 (2 lM). The incubation of retinas in medium previously conditioned by retinapigmented epithelium (RPE) also increased its cyclic AMP content with the characteristics described for L-DOPA. Our results show that dopaminergic communication takes place in the embryonic retina, before tyrosine hydroxylase expression, provided L-DOPA is supplied to the tissue. It also shows that RPE is a potential source of L-DOPA early in development.

The number and topographical distribution of photoreceptors was studied in whole-mounted retinas of Cebus apella. It was estimated a total of 48 million rods and 3.8 million cones. The average peak foveal cone density and the Nyquist Limit at the foveola were estimated as 169, 127 cells/mm(2) and 46.77+/-7.98 cyc/deg, respectively. A cone-enriched rim was found near the ora serrata, more noticeable in the nasal retina. Rod distribution was asymmetrical along horizontal and vertical meridians with a higher density in the dorsal retina. The rod/cone ratio was variable and asymmetrical along both meridians.