Analysis of pre- and postsynaptic factors of the serotonin system in rabbit retina

[3H]Serotonin is accumulated by a specific set of amacrine cells in the rabbit retina. These cells also accumulate the neurotoxin, 5,7-dihydroxytryptamine, and show signs of necrosis within 4 h of in vivo exposure to the drug. Biochemical analysis of [3H]serotonin uptake reveal a sodium- and temperature-dependent, high affinity uptake system with a Km of 0.94 microM and Vmax of 1.08 pmol/mg protein/min. [3H]Tryptophan is also accumulated in rabbit retinal homogenates by a high affinity process. Accumulated [3H]serotonin is released in response to potassium-induced depolarization of intact, isolated retinas. In vitro binding studies of rabbit retinal homogenate membranes demonstrate specific sets of binding sites with characteristics of the postsynaptic serotonin receptor. These data strongly suggest that rabbit retina contains virtually all of the molecular components required for a functional serotonergic neurotransmitter system. The only significant difference between the serotonin system in rabbit retina and that in the well-established serotonin transmitter systems in nonmammalin retinas and in brains of most species is the relatively low concentration of endogenous serotonin in rabbit retinas, as demonstrated by high-performance liquid chromatography, histofluorescence, or immunocytochemistry.

Serotonin systems in the inner and outer plexiform layers of the vertebrate retina

In retinas of certain nonmammalian vertebrate species such as frog, pigeon, and chick, serotonin appears to function as the neurotransmitter of a specific population of amacrine cells. Neurochemical and morphological studies have demonstrated high endogenous levels of 5-hydroxytryptamine (5-HT) as well as uptake, release, and receptor-binding activity restricted to the inner plexiform layer. In retinas from most mammalian species, uptake, release, and receptor-binding activity have also been localized to amacrine cell terminals in the inner plexiform layer. However, serotonin content in mammalian retinas is low, and attempts to localize the endogenous store of 5-HT have failed. Thus the status of serotonin as a candidate in mammalian retina is still open to question. Our more recent studies have revealed a light-sensitive serotonin system associated with photoreceptor terminals in retinas of Long-Evans rats. Uptake, synthesis, and release of [3H]serotonin have been demonstrated. Endogenous levels of 5-HT decrease in the dark and increase in the light. Electrophysiological studies are needed to illucidate the functional role(s) of serotonin within retinas of different species.

Kainic acid-induced denervation supersensitivity of nicotinic, cholinergic receptors in ganglion cells of the rat retina

The rat retina contains both nicotinic and muscarinic cholinergic receptor sites as demonstrated by specific, high affinity binding of the nicotinic ligand, [3H]-alpha-bungarotoxin, and the muscarinic ligand, [3H]-quinuclidinyl benzylate. Seven days after an intraocular injection of 5 nmol of kainic acid, nicotinic binding was increased three-fold. We suggest that nicotinic sites may be located on ganglion cells because previous studies have shown that many ganglion cells are spared after kainic acid treatment and in fact, have an increased physiological response to ACh under these conditions. The increase in nicotinic sites may reflect a supersensitivity response to the loss of acetylcholine input after the kainic acid lesion. In contrast, muscarinic binding was decreased by 70% after kainic acid treatment. These data suggest that muscarinic sites are located on amacrine cells since these cells are destroyed by kainic acid treatment and some are known to be cholino-receptive. Some of the retinal muscarinic sites may function as inhibitory autoreceptors which regulate acetylcholine release from cholinergic amacrine cells.

Permanent alterations in muscarinic receptors and pupil size produced by chronic atropinization in kittens

Chronic mydriasis was induced in six kittens (four monocular, two binocular) and two adult cats (both monocular) by the daily topical application of atropine. Both the kittens and the adult cats were atropinized for a 13-week period with the treatment regimen beginning at the time of eye opening for the kittens. Pupil size measurements, obtained 1 year after the atropinization were discontinued, revealed that, although the pupils of the adult cats were normal, the pupils of the kittens' treated eyes were consistently smaller than pupils in control eyes. The status of the muscarinic receptors in the kittens' irides was investigated using 3H-QNB binding assays. In comparison with iris muscle homogenates from the control eyes, those from the treated eyes demonstrated an eightfold increase in the number of receptor binding sites. The results indicate that pupil size can be altered permanently by chronic mydriasis initiated early in the life of a kitten and that the permanent change in pupil size may result, in part, from a type of permanent supersensitivity response in the muscle following chronic blockade of muscarinic transmission by atropine.

GABA as a trophic factor during development

The process of synaptogenesis has been studied by many investigators to determine the factors which regulate synapse formation. We have used neonatal rabbit retina to investigate the role of the gamma-aminobutyric acid (GABA) neurotransmitter system during development. By utilizing an in vitro incubation treatment of isolated eyecups we found that treatment with nipecotic acid, a GABA uptake blocker, resulted in a 4-fold increase in the amount of specific 3H-muscimol binding. In addition, incubation of the tissue in the presence of the GABA agonists muscimol, 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridine-3-ol (THIP), or GABA itself led to similar increases in specific 3H-muscimol binding. The findings support the conclusion from previous studies that the induction of GABA receptors observed after in vivo treatment of 1-day-old rabbits with nipecotic acid resulted from an increase in the extracellular concentration of GABA. A possible role for GABA in the regulation of GABAergic synapse formation is presented in this report.

Intraocular injections of nipecotic acid produce a preferential block of neuronal 3H-GABA accumulation in adult rabbit retina

A procedure by which the activity of the retinal GABA uptake system can be manipulated in vivo has been developed. Intraocular injections of nipecotic acid, a proported GABA uptake blocker, were administered to adult rabbits every 48 hours for a two-week period. No behavioral or systemic changes were observed. Injections were well-tolerated with less than 10% loss of the tissue caused by physical damage or injection. Biochemical analyses demonstrated a dose-dependent inhibition of 14C-GABA uptake into retinal tissue. No effect on uptake was observed for saline-treated tissue. Autoradiographic analyses showed that in vivo treatment with nipecotic acid preferentially blocked accumulation of 3H-GABA into the amacrine cell bodies and processes in the inner plexiform layer. This treatment may be especially useful in assessing the functional significance of GABA transport in vivo.

Synaptic interactions in the GABA system during postnatal development in retina

Using biochemical analyses, we have demonstrated the presence of a high-affinity, sodium- and temperature-dependent uptake system for GABA in the retinas of newborn rabbits. The activity of this system two days after birth is approximately 70 percent of adult values, slowly increasing to adult level by postnatal day 6-8. An intraocular injection nipecotic acid (final concentration = 10 mM) into one-day-old rabbit pups resulted in a 60 percent inhibition in uptake activity. In order to study the possible role of the GABA uptake system in retinal development, we have determined the consequences of blocking GABA uptake with nipecotic acid on the postnatal development of post-synaptic GABA receptors, as measured by 3H-muscimol binding. Nipecotic acid treatment caused a significant increase in receptor binding in retinas prior to eye opening, with the maximal stimulation being one day after the intraocular injection. Our data indicate that the development of GABA receptor sites is influenced by the activity of the GABA uptake system and suggest that GABA may function as a trophic factor in the developing rabbit retina.

Autoradiographic analysis of 3H-glutamate, 3H-dopamine, and 3H-GABA accumulation in rabbit retina after kainic acid treatment

We have previously reported that exposure of isolated rabbit retina to 10(-3) M kainic acid produces profound morphological changes in specific retinal neurons (Hampton et al, 1981). We noted specific swelling of horizontal cell bodies and neurites, necrosis of cell bodies in the amacrine and ganglion cell layers, and swelling of elements in the inner plexiform layer. We now report a differential sensitivity to kainic acid of specific subclasses of amacrine cells autoradiographically labeled with 3H-glutamate, 3H-GABA, or 3H-dopamine. Three different effects were observed: (1) Labeling of neurons after incubation in 3H-glutamate was uniformly reduced while labeling of glia was much less affected. (2) The accumulation of 3H-dopamine was also decreased by kainic acid in two of the three labeled bands of the inner plexiform layer. The outermost labeled band was insensitive to kainic acid at the highest concentration tested (10(-2) M). These findings provide a basis for the subclassification of dopaminergic amacrine cells into at least two subclasses based on their sensitivity to kainic acid. (3) Kainic acid caused a dramatic increase in the labeling of GABAergic amacrine cell bodies and their terminals. This increased intensity may reflect a compensatory increase in uptake activity in response to kainic acid-induced depletion of endogenous GABA stores. These results confirm the highly toxic nature of kainic acid and demonstrate a high degree of specificity and complexity in its action in the retina.

The effects of 2-amino-4-phosphonobutyric acid (APB) on the ERG and ganglion cell discharge of rabbit retina

Perfusion of 100 microM 2-amino-4-phosphonobutyric acid (APB) into the in vivo rabbit eye-cup selectively and reversibly blocked the b-wave of the ERG and all On responses from retinal ganglion cells. In contrast, Off responses were occasionally enhanced, sometimes dramatically. The antagonistic surround inputs to Off ganglion cells, identified by their latency to light stimulation and magnesium sensitivity, were unchanged by APB. These observations suggest that APB selectively blocks depolarizing bipolar cells in rabbit retina in close agreement with the results of Slaughter and Miller (1981) from mudpuppy retina. We conclude that APB may be useful as a pharmacological tool to differentiate On and Off pathways in the rabbit visual system.

The cholinergic amacrine cells of rabbit retina receive on and off input: an analysis of [3H]-ACh release using 2-amino-4-phosphonobutyric acid (APB) and chloride free medium

Using the in vitro rabbit eye-cup we have examined the light-evoked release of ACh from cholinergic amacrine cells under conditions known to eliminate On responses in the retina. APB (100 microM), which blocks the photoreceptor/depolarizing bipolar cell synapse, reduced the light-evoked release of ACh by 80% but a small light-evoked response remained, which was potentiated by bicuculline. Depolarizing bipolar cells are also Cl- dependent. Cl- free medium caused a tenfold Ca2+-dependent increase in the release of ACh but some small light evoked release remained. These results indicate that the cholinergic amacrine cells receive On and Off input. Our findings are consistent with anatomical and electrophysiological evidence which suggests that the displaced cholinergic amacrine cells are On cells and the conventionally placed cholinergic amacrine cells are Off cells.

A tonic gamma-aminobutyric acid-mediated inhibition of cholinergic amacrine cells in rabbit retina

Using the in vivo rabbit eyecup, we have studied the light-evoked release of acetylcholine (ACh) which is presumed to indicate the activity of cholinergic amacrine cells. Gamma-Aminobutyric acid (GABA) inhibited the light-evoked release of ACh (IC50 congruent to 1 mM), but the GABA antagonists bicuculline (5 micro M) and picrotoxin (20 micro M) potentiated the light-evoked release and markedly increased the resting release of ACh. This bicuculline/picrotoxin-evoked release was calcium dependent and the effects of bicuculline, but not picrotoxin, were blocked by muscimol, a potent GABA agonist. Muscimol also inhibited the light-evoked release of ACh (IC50 less than 1 micro M) and was at least 1000 times more potent than GABA. Nipecotic acid (1 mM), a GABA transport blocker, also inhibited the light-evoked release of ACh, but the effect was slow in onset and recovery was prompt. We conclude that the cholinergic amacrine cells of rabbit retina are inhibited by GABA. The relatively weak action of GABA, compared to muscimol, may be due to the presence of avid GABA transport systems. We ascribe the excitatory effects of bicuculline and picrotoxin to the antagonism of endogenous GABA, suggesting that the cholinergic cells are influenced by a tonic release of GABA. This is consistent with the effects of nipecotic acid. Although we are unable to specify the synaptic arrangements involved, we suggest that the most likely interaction is directly between GABA amacrine cells and the cholinergic amacrine cells and/or their presumed bipolar cell inputs.

[3H]GABA binding in developing rabbit retina

We have studied the developmental sequence of the GABA system in the rabbit retina using an in vitro binding assay to monitor developmental changes in the post-synaptic receptor. A variety of tissue treatments including perchlorate and Triton X-100 were employed to optimize binding and remove endogenous factors which inhibit binding. Pre-treatment of the tissue with 0.05% Triton X-100 revealed high affinity binding for [3H]GABA which increased in a sigmoidal fashion with the post-natal age of the animal. A constant level of binding, at about 16% of adult levels, was noted until day 8, at which time a rapid increase occurred. At 16 days post-natal, the amount of specific binding reached a plateau near adult levels. Kinetic analysis of the GABA receptor showed an increase in the number of receptors (Bmax) with little or no change in the apparent affinity (KD). Our results suggest that the onset of post-synaptic receptor activity is delayed approximately 1 to 2 days, relative to the pre-synaptic components, and the period of rapid increase in GABA receptor binding coincides with the period of maximum increase in retinal synaptic density.

2-amino-4-phosphonobutyric acid and N-methyl-D-aspartate differentiate between "3H]glutamate and [3H] aspartate binding sites in bovine retina

Three pharmacologically and kinetically distinct binding sites for acidic amino acids were observed in synaptic membranes from bovine retina. One site preferentially binds aspartate with an apparent Kd of 6.3 nM and Bmax of 0.033 pmol/mg protein. [3H] Glutamate binds preferentially to two sites which have KdS of approximately 10 and 800 nM; and BmaxS of 0.148 and 0.417 pmol/mg protein, respectively. Only one of these sites, the 800 nM glutamate site, displayed a pharmacological specificity which was in extensive agreement with that reported for the putative glutamate receptor previously localized to a specific set of retinal neurons, the on-center, depolarizing bipolar cells.

3H-muscimol binding in synaptosomal fractions from bovine and developing rabbit retinas

Bovine and rabbit retina synaptosomal fractions bind the potent and specific GABA agonist, [3H]-muscimol with high affinity and limited capacity. The high degree of pharmacological specificity and the subcellular distribution of these binding sites are similar to those reported for [3H]-GABA binding sites. These observations suggest that these sites represent the recognition sites for GABA receptors. The specific binding of [3H]-muscimol in retinal homogenates from different aged rabbits reveal a distinct developmental profile with a fivefold to six fold increase in binding between days 5 and 13. Thus, it appears that GABA receptor development continues after eye opening in rabbits (day 9-10) and that receptor maturation is delayed by at least two to three days with respect to published of GABA uptake and evoked release.

Localization of kainic acid-sensitive cells in mammalian retina

Short-term (15 minutes) in vitro exposure to kainic acid (KA), a rigid structural analog of L-glutamic acid (Glu), caused two morphologically distinct neuronal lesions in retinas of several species. In rabbit retina, one type of lesion was characterized by rapid swelling after exposure to low concentrations of KA (10(-4)M). This lesion was observed in elements of both plexiform layers and, more specifically, in cell bodies and neurites of horizontal cells that contact cones. A few cell bodies from the amacrine cell layer showed some limited swelling. The swelling was completely blocked when sodium was removed from the incubation medium. The second type of lesion was generally seen after longer exposures of after exposure to higher concentrations of KA and was evidenced by degeneration of neurons in the amacrine and ganglion cell layers. One exception was noted in that a few cells from the ganglion cell layer degenerated even under low exposure conditions. The second type of lesion was not blocked by removal of sodium ions. Photoreceptor cells appeared resistant to all effects of KA. The results suggest that a correlation may exist between certain KA-induced lesions of the retina and putative glutamoreceptive neurons. At the same time, the two types of retinal lesions produced by KA are morphologically and chemically differentiable and may be useful in elucidating the differences between specific, Glu-related toxicity and nonspecific toxicity of KA.

Bacterial collagenase. Proposed adjunct to vitrectomy with membranectomy

Clostridiopeptidase A digested preretinal cicatricial tissue without causing morphological alteration of normal retina during a 30-minute incubation in the rabbit. Light and transmission electron microscopy were used to determine effects on the inner limiting membrane and retinal ganglion and Müller's cells and to evaluate enzyme digestion of preretinal scarring. Removal of the injected collangenase by vitrectomy resulted in normal electroretinograms and retinal morphology 48 hours postoperatively. If the enzyme was left in the eye for 24 hours, lens opacities, partial erosion of the inner limiting membrane, and extensive hemorrhage resulted. The specificity of action of the collagenase is due to the high degree of purity of the enzyme used and a substantial biochemical difference between scar collagen and basement membrane collagen. The injection of purified collagenase capable of digesting vitreal scar tissue while leaving the retina undamaged could represent a new approach to vitrectomy, specifically to facilitate certain cases of membranectomy.

Isolation of synaptosomal fractions from rabbit retina

Homogenization of rabbit retina in isotonic sucrose and differential rate centrifugation yielded two morphologically distinct synaptosomal fractions. One fraction was enriched in photoreceptor cell synaptosomes; the second fraction contained small synaptosomes derived from conventional sized synapses most abundant in the inner plexiform layer. Attempts to further purify these fractions using a variety of density gradients proved unsuccessful due to poor viability of photoreceptor cell synaptosomes. The synaptosomes prepared by our method are functionally stable as they demonstrate high affinity uptake for putative retinal neurotransmitters, neurotransmitter-sensitive adenylate cyclase activity, and calcium-dependent, potassium-stimulated release of [14C]GABA and [3H]dopamine.

The effects of intravitreal irrigation during vitrectomy on the electroretinogram

Intravitreal irrigation with Balanced Salt Solution Plus (BSS Plus) produced less decrease in b-wave amplitude than either normal saline solution or Balanced Salt Solution. BSS Plus was more suitable for intravitreal irrigation because it contained the appropriate bicarbonate, pH, and ionic composition necessary for maintenance of normal retinal electrical activity, and it contained glutathione, which is necessary for maintenance of endothelial cell adenosine triphosphatase and for protection against free radical damage and oxidative stress.