Andrew P. Mariani

Dopamine-containing amacrine cells of rhesus monkey retina parallel rods in spatial distribution

Dopamine-containing amacrine cells of rhesus monkey were found everywhere outside of the foveola in whole, flat retinas by the formaldehyde-glutaraldehyde fluorescent method. There were about 7500 such cells in a single retina and their density, determined by cell counts and measured by a nearest neighbor method, was minimal in foveal and peripheral regions and maximal at 3 mm from the center of the fovea. Compared to density distributions of other retinal neuron types, dopamine-containing amacrine cells correlated only with rods, which also had a peak density at 3 mm eccentricity. Cones and ganglion cells peaked in the foveal pit, or within 1 mm of it, respectively. As the distribution of dopamine-containing cells followed that of rods, it is suggested that dopamine could be involved in the rod neuronal circuitry of primates.

GM1 ganglioside-induced recovery of nigrostriatal dopaminergic neurons after MPTP: an immunohistochemical study

The administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mice results in the loss of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) from the mouse striatum and a loss of cells containing tyrosine hydroxylase (TH)-immunoreactivity from the substantia nigra. The cells that remained in the nigra after MPTP treatment were smaller in diameter than normal cells. Treatment with GM1 ganglioside beginning 24 h after establishing the MPTP lesion resulted in partial restoration of DA and DOPAC content in the striatum and an increase in the diameter of the TH-immunoreactive nigra cells. It appears, therefore, that treatment of MPTP-intoxicated mice with GM1 ganglioside results in the partial restoration of both the biochemistry and morphology of dopaminergic neurons.

Immunohistochemical evidence for epinephrine-containing retinal amacrine cells

The enzyme for the synthesis of epinephrine, phenylethanolamine-N-methyltransferase, has been localized, by an indirect immunofluorescent staining method, to a subpopulation of amacrine cells in the rat retina. The immunoreactive cells are located primarily in the inner nuclear layer and send a single process to the inner plexiform layer. Most of the immunoreactivity is found in the center of the inner plexiform layer. A small percentage of immunoreactive cell bodies were found in the inner plexiform layer and occasionally cells were observed in the ganglion cell layer. These epinephrine-containing amacrine cells are morphologically distinct from the dopamine-containing amacrine cells previously described by formaldehyde fluorescence and we speculate from reports in the literature that epinephrine-containing amacrine cells may play a role in modulating the activity of dopamine-containing amacrine cells.

Neural circuitry of the cat retina: cone pathways to ganglion cells

Bipolar cells transfer signals transduced by photoreceptors in the distal retina to amacrine and ganglion cells of the proximal retina. Light microscopic studies of such cells in the cat retina indicate that there are at least nine different morphological varieties; one variety contacts only rods, while the other varieties are cone bipolars (Kolb et al., 1981). Electrical responses to light have been recorded from several varieties of cone bipolar cell in the cat retina and the stains of these cells show that the axon terminals of the center-depolarizing units branch in the inner two thirds of the cat’s inner plexiform layer (IPL) and that the axon terminals of the center-hyper-polarizing units can be found in the outer third of the IPL. The division of the visual system into onand off-center pathways appears to originate with spatially segregated interactions between specific bipolar and ganglion cell types in the IPL. and in the nature of the contacts the former make with cones in the outer plexiform layer (OPL). Cajal (1933) inferred that there were two classes of bipolar cells in mammalian retina: those devoted to rods, and those to cones. This recognition was based on the observation that the synaptic terminals of the rods and cones ended at different levels in the OPL, the rod spherules occupying a thick band distal to a thin line of cone pedicles. In consequence the dendrites of the rod and cone bipolars have characteristic appearances, reaching to different levels of the OPL. In Fig. I three bipolar cells of the cat retina are seen in close proximity. The rod bipolar is central. It is characterized by the high, irregular plane of termination of its dendrites, on the tips of which can be seen minute beads. To either side of this lie cone bipolars with lower, more planar dendritic terminals. The cell on the left has an extremely flat appearing dendritic top. In primate retina “flat top” was the term first coined by Polyak (1941) to describe such cells. On the right is another variety of cone bipolar whose dendrites each bear a distinct terminal cluster, each of which contacts a single cone. This is the “invaginating” variety of cone bipolar cell (Boycott and Kolb, 1973). When Golgi impregnated cells such as these have been subject to electron microscopy, earlier conjectures based on light microscopy have been extended and confirmed. Rod bipolars in mammalian species have never been observed to contact cones (Kolb, 1970; Boycott and Kolb, 1973). The “flat” cone bipolar has been found to make only a single unique and specialized sort of contact with cones, the “flat” contact or “basal junction”, while the invaginating cone bipolar cell makes, also exclusively, an entirely different sort of contact with cones, where dendritic fingers invaginate the proximal surface of the cone pedicle to approach a synaptic ribbon and become the central elements of triads at the ribbon synaptic complex (Kolb, 1970; Boycott and Kolb, 1973; Mariani, 1980). A flat contact (FC, Fig. 2) has a smoothly curved length of electron dense membranes on the preand postsynaptic sides. Occasional perpendicular cross bridges can be seen in the cleft, however no accumulation of vesicles occurs presynaptically at such contacts. An invaginating contact (IC, Fig. 2) is recognized at the tip of an invaginating cone bipolar dendrite where it approaches the synaptic ribbon. On both sides of the ribbon, and flanking the central element, lie dendritic terminals of horizontal cells, to complete the triad. A long process extends from the left of the cone pedicle (CP, Fig. 2) to contact a rod spherule (RS, Fig. 2) with a minute gap junction. Thus the anatomy suggests and the physiology confirms (Nelson, 1977) that interactions between rod and cone systems commence at the most distal level of retinal processing in the cat and other rod-dominated mammals. The axonal terminations of bipolar cells in the cat, like their dendritic terminals, are highly distinctive. Returning to Fig. 1, the rod bipolar (center) has a very simple club-shaped terminal with few or no bifurcations of the terminal arborization. Rod bipolar processes extend proximally to the very lowest stratum of the IPL approaching the tops of ganglion cell bodies. The axonal arborization of the flat cone bipolar (left) is much higher in the IPL. Its profuse planar branching lies just under the layer of amacrine cell bodies, the distal-most one third of the IPL. We have called this layer “sublamina a” (Famiglietti and Kolb, 1976). The invaginating cone bipolar cell (right) has a bushy axonal arborization whose central plane is intermediate between that of the flat cone bipolar cell and

GABAergic synapses and benzodiazepine receptors are not identically distributed in the primate retina

The distribution of benzodiazepine receptors (BZR) was compared to the distribution of gamma-aminobutyric acid (GABA)-ergic synapses in the rhesus monkey retina using monoclonal antibodies against the BZR and polyclonal antisera to glutamate decarboxylase (GAD), the GABA-synthesizing enzyme which labels the presynaptic terminals of the GABAergic synapses. Indirect immunofluorescence including dual fluorochroming for both BZR and GAD indicates that although both were localized to the inner plexiform layer and adjacent cell body layers, their distributions were largely non-overlapping. Thus, in the primate retina, BZRs are not exclusively associated with GABAergic synapses.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment decreases dopamine and increases lipofuscin in mouse retina

The compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a relatively selective neurotoxin that destroys dopamine (DA)-containing nigrostriatal neurons. We have now studied the effects of MPTP on retinal dopaminergic neurons. Acute treatment resulted in the accumulation of DA when evaluated by direct chemical analysis or histofluorescence. Chronic treatment resulted in a decrease of DA, an apparent loss of fluorescent cells, and a striking increase of lipofuscin in the retina. Thus, MPTP may be a useful drug for studying the dopaminergic neuronal system of retina and the possible link between neurons and the accumulation of lipofuscin.

Intracellular recordings from a biplexiform ganglion cell in macaque retina, stained with horseradish peroxidase

A biplexiform ganglion cell, which is characterized by dendritic contacts with rods, has been penetrated with an HRP-filled microelectrode in the retina of Macaca fascicularis. Its fine curvy axon could be traced to the optic disc. One of several dendritic processes ascended through all retinal layers and ended in the layer of rod spherules. Under all conditions of chromatic adaption, the cell produced depolarizing responses with a rapid onset and a slow decay. Besides a strong rod input the recordings indicate signals from at least two spectrally different cone mechanisms.

IV. Benzodiazepine pharmacology of cultured mammalian CNS neurons

Many neurons cultured from the embryonic mammalian central nervous system (CNS) express benzodiazepine receptors while some neurons differentiate specific transmitter phenotypes like glutamic acid decarboxylase (GAD), the synthetic enzyme for gamma-aminobutyric acid (GABA). The benzodiazepine receptors in these cultured neurons are often, if not always coupled to a practically ubiquitous GABA-mediated function, activation of Cl- ion conductance. The transmitter signal serves to inhibit neuronal excitability and is facilitated by clinically important benzodiazepines. Here we review some details regarding the pharmacological actions of benzodiazepines on membrane excitability.

Chemical synapses between turtle photoreceptors

Rod photoreceptors of the snapping turtle retina wer Golgi impregnated and studied in the electron microscope. Telodendria arising from the synaptic bases ended at rods and cones as lateral or central elements of the ribbon synaptic complex, thus providing clear evidence of chemical synapses between turtle photoreceptors.

Synaptic organization of type 2 catecholamine amacrine cells in the rhesus monkey retina.

SummaryTwo types of amacrine cell immunoreactive for tyrosine hydroxylase, the rate-limiting enzyme in the catecholamine synthetic pathway, are present in the retina of the rhesus monkey,Macaca mulatta. The well-known dopaminergic, or type 1 catecholamine amacrine cells have relatively large cell bodies almost exclusively in the inner nuclear layer with processes that densely arborize in the outermost stratum of the inner plexiform layer and fine, radially-oriented fibres in the inner nuclear layer. Type 2 catecholamine amacrine cells, in contrast, have smaller cell bodies in the inner nuclear layer, the inner plexiform layer and the ganglion cell layer, and have sparsely-branching processes ramifying in the centre of the inner plexiform layer. Although type 2 catecholamine cells are more numerous than type 1 catecholamine amacrines, type 2 cells contain less than one-third the amount of tyrosine hydrolase as the type 1 cells. Electron microscopy of retinal tissue immunoreacted for tyrosine hydrolase by the peroxidase-antiperoxidase method revealed synaptic input from amacrine cells at conventional synapses, and bipolar cells at ribbon synapses onto the type 2 catecholamine amacrine cells. Curiously, although the synaptic input is comparatively easily found, the output synapses, or synapses of the type 2 catecholamine amacrine cells onto other neuronal elements, are rarely found. Some synapses of the type 2 catecholamine cells onto non-immunoreactive amacrine cells have been identified, however. This unusual pattern of synaptic organization, with many identifiable input synapses but few morphologically characterizable output synapses, suggests a paracrine function for the dopamine released by the type 2 catecholamine amacrine cells in the primate retina.