L-aspartate: evidence for a role in cone photoreceptor synaptic transmission in the carp retina

Light-dependent plasticity of the morphology of horizontal cell terminals in cone pedicles of fish retinas

Horizontal cell terminals lateral to the synaptic ribbons in goldfish cone pedicles give rise to 0.3 micron long, finger-like extensions in the light-adapted state. These structures, called spinules, disappear almost completely after dark adaptation. The ultrastructure of the horizontal cell terminals is characterized by the presence of occasional microtubules, microfilaments and sparse irregular vesicles; in the dark, large multivesicular bodies can also be found. Two types of membrane densities are described in the horizontal cell terminals, one of which is typically located at the tip of the spinules. Their positive reaction to E-PTA makes it probable that the spinules are synaptic structures. Reconstruction of serial sections shows about 12 spinules per terminal in the light but only two in the dark. Formation and disappearance of the spinules takes about 60 min and involves a transitional stage in the form of a spherical structure. Spinules can be found in five other teleost species, with a darkness-induced reduction in number, but not in the horizontal cell terminals of the mudpuppy, turtle and mammals.

The synthesis of neuroactive amino acids from radioactive glucose and glutamine in the rat retina: effects of light stimulation

The effect of light stimulation in vitro on the labelling of neuroactive amino acids derived from [14C]glucose or [14C]glutamine in the rat retina has been studied. [14C]Glutamine, at 700 microM, provided about 50% of the tissue pools of glutamate, aspartate, and GABA; and the labelling of these decreased on light stimulation, both in the photoreceptor cells (glu and asp) and in the inner retina (glu, asp, and GABA). In contrast, there were no significant changes in the entry of label derived from [14C]glucose, although similar trends were apparent in the data obtained for the photoreceptor cell layer. The pools may, therefore, be separate. Other results support the contention that glucose is the principal energy source for the retina, its entry into non-amino acid derivates being decreased on light stimulation.

Calcium spikes in toad rods

1. When the retina of the toad, Bufo marinus, was superfused with 6-12 mM-tetraethylammonium chloride (TEA), intracellular recordings from rods showed large, depolarizing regenerative potentials. For brief exposures to TEA, these potentials occurred during the recovery phase of the light responses; whereas, during longer exposures, they were spontaneous in darkness but suppressed during illumination. Similar regenerative potentials were observed during perfusion with 3-10 mM-4-aminopyridine and 1-2 mM-BaCl2. 2. The amplitude of the regenerative potentials depended upon the extracellular Ca concentration ([Ca2+]o). Lowering [Ca2+]o decreased their amplitude and in zero [Ca2+]o they were reversibly abolished. Increasing [Ca2+]o by 1.5-2 times produced a small hyperpolarization of membrane potential and a large augmentation in regenerative response amplitude. However, larger increases in [Ca2+]o produced large membrane hyperpolarizations and reversibly suppressed the regenerative responses. 3. High concentrations of Sr2+ in TEA also enhanced regenerative activity but did not affect the rod resting membrane potential. The amplitude of regenerative potentials increased continuously with increasing [Sr2+]o, and in 28 mM-Sr2+ the rods generated 60-70 mV action potentials, even in the absence of extracellular Na+. 4. The regenerative potentials were blocked by 25 microM-Cd2+, 50-100 microM-Co2+, 5mM-Mg2+, and 100 microM-D-600. They were unaffected by 2 microM-TTX or 2-5 mM-Na aspartate. 5. In Ringer containing 12 mM-TEA, large anode break responses could be recorded from rods at the termination of inward current pulses. These anode break responses were also suppressed by Co2+ and unaffected by TTX or Na aspartate. 6. We conclude that the membrane of toad rods contains a conductance normally selective for Ca2+, which is activated by depolarization. In normal Ringer, the inward current through this conductance produces little effect, since it is balanced by a large outward current, probably carried by K+. TEA and other agents appear to block this outward current, permitting the Ca2+ current to become regenerative.

Drug-induced changes in catecholaminergic cells of the fish retina

The changes in fluorescence intensity and number of visible catecholaminergic cells (CA-cells), as revealed by means of a histofluorescence technique, were used as indicators of the effects of various pharmacological agents upon CA-cells in the retina of fishes (Cyprinus carpio and Eugerres plumieri). The study includes in vivo and in vitro experiments. In the in vivo experiments, intravitreal injection, two or three hours before eye enucleation, of 10 microgram L-DOPA, dopamine, or noradrenaline accentuated CA-cell fluorescence and increased the number of visible cells, whereas 10 microgram of tyramine, octopamine, synephrine, or adrenaline reduced the endogenous fluorescence. Intramuscular injection of reserpine (3 mg/kg) abolished CA-cell fluorescence. In the in vitro experiments, pieces of isolated retinas were incubated for three or 30 minutes in media containing different drugs. Only minor changes in fluorescence were detected after three minutes of incubation, but after 30 minutes, dopamine (20 microM) markedly enhanced CA-cell fluorescence. Carbachol (20 mM), acetylcholine (10 mM) plus BW-anticholinesterase (1 mM) or substance P(1.6 x 10(-2) mM), all reduced CA-cell fluorescence. Kainic acid (20 mM) abolished fluorescence from CA-cell somata, while fluorescent fiber networks remain unchanged. L-aspartate (5 mM) and GABA (10 mM) in the incubation medium did not influence fluorescence intensity. The results are relevant to, and consistent with, electrophysiological observations of dopamine-mediated spatial effects on horizontal cell potentials.

A simple method for the preparation of D-alpha-aminoadipic acid

High quantity (1 g and more) of racemically and chromatographically pure D-alpha-aminoadipic acid was prepared by selective metabolism of the L-isomer of the commercially available DL-alpha-aminoadipate by Pseudomonas putida. The overall yield of this preparation averaged 40%. The final product has [a]25D value of -25 degrees. This procedure can be useful in the synthesis of high purity D-alpha-amino-adipate, a compound shown recently to be a useful tool in the study of neurotransmission mechanism mediating synaptic excitation.

New approaches to ophthalmic electrodiagnosis by retinal oscillatory potential, drug-induced responses from retinal pigment epithelium and cone potential

New clinical methods are proposed to assess (1) neuronal activities of the retinal inner layers including amacrine cells by means of the oscillatory potential, (2) photopic function through the rapid decay in the off-response and (3) activities of the retinal pigment epithelium through susceptibility of the standing potential of the eye to osmotic stress and to Diamox. These new methods are able to reveal otherwise undetectable retinal disorders.

Antagonism of excitatory amino acid-induced responses and of synaptic excitation in the isolated spinal cord of the frog

1. A range of compounds has been tested for excitatory amino acid agonist or antagonist activity and for effects on synaptic activity on isolated hemisected spinal cords of frogs. 2. L-Monoamino dicarboxylic acids of chain length up to 8 carbon atoms (L-alpha-aminosuberate) were all agonists. 3. Within a series of D-monoamino dicarboxylic acids, and with diamino dicarboxylic acids (mainly unresolved mixtures of diasteroisomers), there was a progression from agonist activity, for compounds of chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length equal to or shorter than glutamate, to antagonist activity, for compounds of longer chain length, D-alpha-Aminosuberate (D alpha SD) was the most potent antagonist. 4. The antagonist actions of these substances showed a Mg2+--like selectivity with respect to depolarizations produced by different excitants. N-methyl-D-aspartate (NMDA) was the most susceptible agonist and quisqualate and kainate the least susceptible. Responses to other excitatory amino acids, including L-glutamate and L-aspartate, showed intermediate sensitivity to the antagonists. 5. A parallelism was observed between the relative potencies of mono- and diamino dicarboxylic acids as NMDA antagonists and their relative potencies as depressants of synaptic responses. 6. The results support the concept of different types of excitatory amino acid receptors, with NMDA and its antagonists acting predominantly on one type. These NMDA receptors are probably transmitter receptors activated by an excitatory amino acid transmitter.

Effects of catecholamines and related compounds on horizontal cells in the fish retina

The effects of catecholamines (CA) and certain related compounds in the superfusate were examined on the intracellularly recorded potential from horizontal cells in the fish (Eugerres plumieri) retina. The stimulated retinal area consisted of a central spot 1.0 mm in diameter and an annulus 2.0 mm in inner diameter and 4.0 mm in outer diameter; both forms of monochromatic stimuli were centered relative to the recording microelectrode. Each of the CA (dopamine, noradrenaline, and adrenaline) produced an analogous effect on the hyperpolarizing response of all types of horizontal cells. The depolarizing response of the C (R/G)-type cells was found to change variably with the CA. The effect of dopamine (DA) among the CA was most pronounced when they were used at an equivalent amount. With 10--50 microM, the action of DA was variable but in general its effect was to increase slightly both center and surround responses. In some cases, however, DA initially augmented the surround and reduced the center response. Large amounts of the CA (100--200 microM) augmented the center response and attenuated the surround response considerably; these reciprocal changes usually were associated with moderate depolarization of the cells (5--10 mV). Recovery then occurred in 15--20 minutes. These results indicate that the CA, up to a certain amount, do not directly affect the transmission from photoreceptors to horizontal cells, since the center response became larger. At the same time, the lateral propagation of an S-potential appears to be selectively affected by the CA, suggesting that the adrenergic system participates in this phenomenon. When an excess of these compounds (200-500 microM) was given, the cells were rapidly depolarized to near 0 MV and eventually the light-induced responses were abolished. Large amounts (5--10 mM) of metabolic products of the CA (DOPAC and VMA) were found to reduce the center response slightly more than the surround. alpha-Methylnoradrenaline, 5-hydroxydopamine, and serotonin also caused the same but less effect on horizontal cells as did DA. Reserpine and clonidine mimicked the CA effect only if these compounds had been preceded by repeated applications of one of the CA or if the retina had been pretreated with Marplan. Propranolol, haloperidol, and apomorphine affected neither the horizontal cell membrane potential nor the CA effect. However, phentolamine in large amounts (500 microM) markedly diminished the DA action. Therefore, alpha-adrenergic receptors appear to be involved in the CA-induced changes observed in the horizontal cell response.

Similarities in effects of acetylcholine and dopamine on horizontal cells in the fish retina

The effect of acetylcholine (ACh) on cone- and rod-connected horizontal cells (generating photopic and scotopic L-type S-potentials, respectively) in the fish (Eugerres plumieri) retina was compared with that of dopamine (DA), and some similarities were found in the effects. About two-thirds of the L-type horizontal cells examined were sensitive to a test solution containing ACh (10 mM in concentration) and BW-anticholinesterase (BW; 1.0 mM). ACh with BW augmented a center response and attenuated a surround response as did DA (0.2 mM), although the latter effect was more pronounced and longer-lasting. ACh with BW frequently produced oscillations of the horizontal cell membrane potential, as did DA with clonidine (alpha-adrenergic stimulant). Phentolamine (alpha-adrenergic blocker) interrupted both the effects of DA and ACh on the center and surround responses, while hexamethonium (cholinergic antagonist) appeared to interfere selectively with the effect of ACh. Therefore, the ACh-induced changes observed in the horizontal cell membrane potential are assumed to be mediated by the adrenergic system in the fish retina.

Effects of some amino acids on horizontal cells in the fish retina

Effects of some amino acids on horizontal cells were examined in the fish (Eugerres plumieri) retina. L-aspartate (5 mM in concentration) and L-glutamate (10 mM) consistently depolarized all types of horizontal cells, although the effect of L-aspartate was more pronounced. gamma-Aminobutyric acid (GABA; 10 mM) consistently hyperpolarized the cone-connected L- and C-type horizontal cells, whereas the effect on the rod-connected cells was slight or nonexistent. The hyperpolarizing effect of GABA on the cone-horizontal cells was markedly diminished by picrotoxin (0.1 mM). Usually, glycine (10 mM) showed slight and variable effects. It merely depolarized some cone-horizontal cells but weakly hyperpolarized some rod-horizontal cells. In a few cases, however, glycine began to drastically hyperpolarize the cone-horizontal cells in retinas that had been repeatedly treated with certain agents used. Taurine and beta-alanine (10 mM each) had negligible effects, although the former slightly depolarized and the latter merely hyperpolarized both cone- and rod-horizontal cells.