Autoradiography following 3H-glycine (Gly) uptake and immunocytochemistry with a Gly-specific antiserum were used to identify neurons in Macaca monkey retina that contain a high level of this neurotransmitter. High-affinity uptake of Gly was shown to be sodium dependent whereas release of both endogenous and accumulated Gly was calcium dependent. Neurons labeling for Gly included 40-46% of the amacrine cells and nearly 40% of the bipolars. Synaptic labeling was seen throughout the inner plexiform layer (IPL) but with a preferential distribution in the inner half. Bands of labeled puncta occurred in S2, S4, and S5. Both light and postembedding electron microscopic (EM) immunocytochemistry identified different types of amacrine and bipolar cell bodies and their synaptic terminals. The most heavily labeled Gly+ cell bodies typically were amacrine cells having a single, thick, basal dendrite extending deep into the IPL and, at the EM level, electron-dense cytoplasm and prominent nuclear infoldings. This cell type may be homologous with the Gly2 cell in human retina (Marc and Liu: J. Comp. Neurol. 232:241-260, '85) and the AII/Gly2 of cat retina (Famiglietti and Kolb: Brain Res. 84:293-300, '75; Pourcho and Goebel: J. Comp. Neurol. 233:473-480, '85a). Gly+ amacrines synapse most frequently onto Gly- amacrines and both Gly- and Gly+ bipolars. Gly+ bipolar cells appeared to be cone bipolars because their labeled dendrites could be traced only to cone pedicles. The pattern of these labeled dendritic trees indicated that both diffuse and midget types of biopolars were Gly+. The EM distribution of labeled synapses showed Gly+ amacrine synapses throughout the IPL, but these composed only 11-23% of the amacrine population. Most of the Gly+ bipolar terminals were in the inner IPL, where 70% of all bipolar terminals were labeled. These findings are consistent with previous data from cats and humans and suggest that both amacrine and bipolar cells contribute to glycine-mediated neurotransmission in the monkey retina.
The distribution patterns of G A B A immunoreactive ( + ) and immunonegative (-) amacrine cell synapses a n d profiles in the inner plexiform layer (IPL) were analyzed in three macaque monkey retinas using postembedding electron-microscopic (EM) immunogold cytochemistry. Synapses and profiles were counted at 5 % intervals throughout the I P L depth in three EM montages (total area = 6509 fim 2 ), with 0% depth at the inner nuclear l a y e r / I P L border. Nearly 7 0 % of all amacrine synapses were G A B A + , and they contacted all major classes of neurons that arborize in the I P L : bipolars (45%), ganglion cells (25%), and G A B A + (20%) a n d G A B A -(10%) amacrines. A major relationship was seen between G A B A + amacrine processes a n d bipolar terminals: 7 6 % of all amacrine-to-bipolar synapses were G A B A + , and 8 2 % of bipolar output dyads contained at least one G A B A + amacrine.G A B A + amacrine profiles (N = 2455) were concentrated in three wide bands at I P L depths of 0 -2 5 % , 4 0 -6 0 % , and 7 5 -1 0 0 % , corresponding to the dense bands seen with light-microscopic immunocytochemistry. In contrast, G A B A + amacrine synapses (N = 1081) were distributed evenly throughout the IPL depth, rather than being confined to the three dense bands. G A B Aamacrine synapses (N = 516) were concentrated at 4 0 % and 6 0 % depths.Each category of amacrine output synapses had a characteristic pattern of stratification in the I P L . G A B A + amacrine-to-bipolar synapses occurred throughout the I P L but were most frequent at 2 0 % and 8 0 % I P L depths, where the dendrites of midget cone bipolars arborize (Polyak, 1941). In contrast, G A B A + amacrine-to-ganglion cell synapses were concentrated at 3 0 % and 7 0 % I P L depths, near the dendritic arborizations of parasol ganglion cells (Watanabe & Rodieck, 1989). G A B A + synapses o n t o bipolars a n d amacrines were also concentrated at the level of rod bipolar terminals. G A B A + amacrines must play significant but different roles in O N and O F F midget and parasol pathways as well as the rod pathway.
Retinal ganglion cells were labeled retrogradely by localized injections of HRP into different regions of the pretectum, tectum, and optic tract in 26 cats. Retinal projection zones in the pretectum were labeled anterogradely in the same cats by intravitreal injections of 3H-proline. This allowed the HRP injection sites to be located with respect to the retinal termination zones. The form of the projection zones from retina to pretectum was determined from serial reconstructions of either coronal or horizontal sections. The zones are best distinguished in horizontal sections, where they are seen as four roughly parallel strips on either side of the brain. They are more-or-less parallel to the anterior border of the tectum, and appear to traverse the entire width of the retinal projection to the tectum. Each zone is similar in form for the ipsilateral and contralateral projections, although the contralateral projection is thicker and denser. Binocular injections of 3H-proline showed that the projections from the two eyes were in register and did not interdigitate. Cells labeled by HRP injections in the anteromedial end of the pretectum were concentrated in the lower nasal quadrant of the contralateral retina, and the lower temporal quadrant of the ipsilateral retina. Posterolateral injections labeled cells in the upper quadrants. There is thus a rough retinotopic mapping along the elongated axis of the pretectum. When the distributions of ganglion cells labeled by HRP injections to different parts of the pretectum are combined, they show a concentration in both the visual streak and area centralis, and thereby reflect, at least qualitatively, the relative spatial distribution of the entire ganglion-cell population. About 85% of the retinal projection to the pretectum is contralateral. For all of the HRP injections, the spatial density of labeled cells was always low, accounting for no more than 3% of the total spatial density of ganglion cells in any retinal region. Several types of ganglion cells were labeled following injections to most regions of the pretectum; these included alpha, beta, and epsilon cells, as well as small-bodied cells showing a variety of morphologic forms. Alpha cells were labeled mainly from the anterolateral end of the pretectum, but other cell types were labeled from all injected regions. In the peripheral retina, 2% of the labeled cells were alpha cells, 32% were beta cells, 19% were epsilon cells, and the remaining 47% were small cells whose dendrites only occasionally filled to any significant extent.(ABSTRACT TRUNCATED AT 400 WORDS)
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