PubMed Health⌕ Search

Biomedical subjects

D A Redburn

Publications and source records attributed to D A Redburn.

At least 19 recordsLinked to original sources
PubMed1995

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function.

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

Animals↗
PubMed1993

The role of GABA during development of the outer retina in the rabbit.

Horizontal cells are among the first to mature in the neonatal mammalian retina and they are the first to establish the position of the outer synaptic layer which is subsequently formed by invading terminals of both rod and cone photoreceptors. During the period of cone synaptogenesis, horizontal cells transiently express the full complement of GABAergic properties (uptake, release, synthesis and storage of GABA); later during development of rod terminals, these properties are down-regulated. Given the reports of GABA's role in other developing neuronal systems (for review: 10), we have examined the effect that GABA, produced from horizontal cells, might have on photoreceptor maturation in rabbit retina. Results from our previous studies show that lesioning the horizontal cell with kainic acid in vivo leads to a displacement of cone photoreceptor cells and a disappearance of their synaptic terminals, while rod cells maintain their normal position and produce an overabundance of terminals. Similar effects are seen with the GABA-A receptor antagonists, picrotoxin and bicuculline. New evidence from 3H-thymidine studies suggests that the effects of kainic acid are specific and that cell division, migration and differentiation in other cell types do not appear to be affected. This body of work is summarized and possible mechanisms of action are suggested which could account for the apparent ability of GABA to help maintain the normal position of cone cell bodies and regulate cone synaptogenesis.

Animals↗
PubMed1992

Development of the glutamate system in rabbit retina.

We have investigated two characteristics of the glutamate system in the developing rabbit retina. 1) Glutamate immunoreactivity was observed at birth within developing processes of four cell types; two of which, photoreceptors and ganglion cells, are known to be glutamatergic in the adult. Two other cell types, type A horizontal cells and amacrine cells, are immunoreactive to both glutamate and GABA at birth, suggesting that endogenous pools of glutamate in GABAergic neurons serve as precursor for GABA synthesis. Thus it appears that endogenous glutamate pools are present within neurons prior to synaptogenesis as part of the early expression of either the glutamate or GABA transmitter phenotype. 2) Analysis of 3H-glutamate metabolism during retinal development showed that rapid conversion of glutamate to glutamine does not occur until the second postnatal week, coincident with the expression of Muller (glial) cell activity. In the absence of glial metabolism in the neonate, extracellular concentrations of glutamate remain relatively high and are likely to have major effects on neuronal maturation.

Aging↗
PubMed1992

gamma-Aminobutyric acid immunoreactivity in multiple cell types of the developing rabbit retina.

We have previously demonstrated that the neonatal rabbit retina contains a larger complement of cells that accumulate [3H]-GABA than does the adult. In order for these neurons to be classified as GABAergic, they must also contain endogenous GABA. We now report that these same neonatal cell populations are also immunoreactive to GABA antisera. In frozen sections from rabbit retina, treated with GABA antisera, immunoreactive processes in both synaptic layers were observed at postnatal day 1. The appearance of immunofluorescent fibers precedes that of photoreceptor and bipolar cell terminals in the outer plexiform layer and is diminished by postnatal day 5. Also noted, was a 50% decrease in the density of GABA-immunoreactive cell bodies in the inner nuclear and ganglion cell layers, accompanied by an increase in cell volume and a shift toward a more spherical cell shape of the remaining cells. At postnatal day 1 and 3, we also observed immunoreactive cells having the characteristic morphology of interplexiform cells. This cell type sends branches to both the outer and inner plexiform layers, thus a morphological basis for communication between the two developing plexiform layers is present as early as postnatal day 1. Thus, retinas from neonatal rabbits have a larger complement of cells that stain for endogenous GABA than does the adult. These results coupled with our previous studies suggest that GABAergic properties are expressed by a larger number of cell types in the neonate than in the adult and are consistent with the general hypothesis that GABA functions as a trophic agent during development.

Animals↗
PubMed1991

Melatonin inhibits ACh release from rabbit retina.

Previous studies have suggested that melatonin, released from photoreceptors, may modulate retinal dark-adaptive responses by inhibition of dopamine release from retinal interneurons. We have broadened these studies to examine the effect of melatonin on release of another retinal neurotransmitter, acetylcholine (ACh). The ACh system in rabbit retina has been localized to starburst amacrine cells, which release ACh in response to a variety of experimental stimuli, including direct potassium depolarization, flashing light, and glutamatergic as well as GABAergic inputs. The effect of melatonin on release of endogenously synthesized [3H]-ACh was measured in perfusates from retinas or retinal synaptosomes preloaded with [3H]-choline chloride. Melatonin significantly inhibited ACh release stimulated by potassium in intact retina but not in synaptosomes. Stimulation of intact retina by flashing light or by the glutamate receptor agonist, kainic acid, was also inhibited by melatonin. In contrast, there was no significant effect of melatonin on picrotoxin-induced release. These findings suggest that melatonin does have an inhibitory effect on ACh release, either by direct interaction with the cholinergic amacrine cell, or indirectly via GABAergic but not glutamatergic neurons.

Acetylcholine↗
PubMed1991

Acetylcholine release from the rabbit retina mediated by kainate receptors.

The cholinergic amacrine cells of the rabbit retina may be labeled with 3H-choline (3H-Ch), and the activity of the cholinergic population may be monitored by following the release of 3H-ACh. Glutamate analogs caused massive ACh release, up to 50 times the basal efflux, with the following rank order of potency: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) greater than quisqualate (QQ) = kainate (KA) much greater than NMDA (in magnesium-free medium) much greater than glutamate greater than aspartate. In contrast, the release of 3H-Ch was unchanged. Submaximal doses of each agonist were used to establish the specifity of glutamate antagonists. Kynurenic acid was selective for KA much greater than QQ, and 6,7-dinitroquinoxaline-2,3-dione (DNQX) was selective for KA greater than QQ much greater than NMDA. At low doses, which selectively blocked the response to KA, both antagonists blocked the light-evoked release of ACh. These results suggest that ACh release may be produced via several glutamate receptors, but the physiological input to the cholinergic amacrine cells is mediated by KA receptors. Because these cells receive direct input from cone bipolar cells, this work supports previous evidence that the bipolar cell transmitter is glutamate.

Acetylcholine↗
PubMed1990

Kainic acid lesioning alters development of the outer plexiform layer in neonatal rabbit retina.

The first synaptic relay in the primary visual pathway occurs between terminals of photoreceptors and second-order neurons within the outer plexiform layer of the retina. During development, one of these types of second-order neurons, the type A horizontal cell, differentiates and assumes mature characteristics several days before any other cells ramifying in that synaptic layer. In neonates, horizontal cells appear to be GABAergic during the first 5 days of postnatal life and in addition they also are responsive to kainic acid. We have previously suggested that they may play a pioneering role in the postnatal development of the outer plexiform layer, perhaps providing structural guidance or trophic substances such as GABA, for synaptic development. To test this hypothesis, we first demonstrated that a single intraocular injection of kainic acid within 24 hr of birth results in a permanent and selective loss of type A horizontal cells in the outer retina. Retinas from animals maintained for 5 days postinjection were harvested for analysis of postnatal development of the outer plexiform layer in the absence of horizontal cells. One of the major findings was that kainic acid treatment caused a reversal of the normal complement of photoreceptor cell types, resulting in an abnormally high rod/cone ratio. The distribution of cell processes within the outer plexiform layer was also altered and normal synaptic connections were not made. In spite of these changes in the constituents of the outer plexiform layer, the normal position of the synaptic layer was not affected by the loss of horizontal cells. These results rule out the possibility that horizontal cells provide a structural barrier which is an absolute requirement for establishing the location of the outer plexiform layer. Rather, these cells may be more involved in cell differentiation and synaptogenesis.

Animals↗
PubMed1990

Evidence for functionally distinct subclasses of gamma-aminobutyric acid receptors in rabbit retina.

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian retina, where it serves many roles in establishing complex response characteristics of ganglion cells. We now provide biochemical and physiological evidence that at least three subclasses of GABA receptors (A1, A2, and B) contribute to different types of synaptic integration. Receptor binding studies indicate that approximately three-fourths of the total number of [3H]GABA binding sites in retina are displaced by the GABAA receptor antagonist, bicuculline, whereas one-fourth are displaced by the GABA-B receptor agonist, baclofen. GABAA receptors can be described by a three-site binding model with KD values of 19 nM, 122 nM, and 5.7 microM. Benzodiazepines and barbiturates potentiate binding to the GABAA site, which suggests that significant numbers of GABAA receptors are coupled to regulatory sites for these compounds and thus are classified as GABAA1 receptors. The response to pentobarbital appears to involve a conversion of low-affinity sites to higher-affinity sites, and is reflected in changes in the densities of sites at different affinities. Functional studies were used to establish which of the different receptor subclasses regulate release from cholinergic amacrine cells. Our results show that GABA suppresses light-evoked [3H]acetylcholine release via GABAA2 receptors not coupled to a benzodiazepine or barbiturate regulatory site, and enhances release via GABAB receptors. GABAA1 sites do not appear to control acetylcholine release in rabbit retina.

Acetylcholine↗
PubMed1989

Darkness stimulates rapid synthesis and release of melatonin in rat retina.

The presence of melatonin in retina has been widely reported for over two decades although studies of its functional importance within the retina have only recently been emphasized. We have analyzed the biochemical characteristics of melatonin synthesis and release, focusing on rapid changes in response to light/dark conditions. Our major findings are consistent with the following conclusions: (1) melatonin synthesis is stimulated within minutes after exposure to darkness, and may reflect an increase in N-acetyl transferase activity; (2) melatonin is not stored, but rather it diffuses freely throughout the retina immediately after it is synthesized; and (3) the dark-induced increase in retinal melatonin release is a synthesis-coupled response and does not involve separate secretion mechanisms. The characteristics of melatonin synthesis and release described herein would be consistent with the proposed role of melatonin as a local paracrine effector of dark-adaptive responses in retina.

Animals↗
PubMed1988

AP4 inhibits chloride-dependent binding and uptake of [3H]glutamate in rabbit retina.

Glutamate is one of the major neurotransmitters used by primary and secondary neurons of the visual pathway in retina. AP4(2-amino-4-phosphonobutyric acid) preferentially blocks the activity of one functional subclass of retinal neurons, ON bipolar cells, apparently by acting as an agonist at a hyperpolarizing glutamate receptor. We have used in vitro binding assays to examine different subclasses of presumptive glutamate receptors in retinal membrane fractions. One subclass consists of AP4-sensitive binding sites which require calcium and chloride for maximal binding and which are inhibited by freeze-thaw procedures. In addition, AP4 inhibits chloride-dependent [3H]glutamate uptake into retinal synaptosomes and intact retina. [3H]glutamate which is accumulated via the AP4-sensitive mechanism can be subsequently released by depolarizing levels of potassium. The pharmacological selectivity of AP4-sensitive glutamate receptors on ON bipolar cells measured electrophysiologically is very similar to that of AP4-sensitive, [3H]glutamate binding and uptake, measured biochemically in subcellular fractions. These results raise the possibility that AP4-sensitive glutamate recognition sites in retina may be linked to two separate effectors, one which gates ion channels and leads to hyperpolarization, and another which acts as a glutamate transporter.

Aminobutyrates↗
PubMed1988

Neurotransmitter systems in the outer plexiform layer of mammalian retina.

Melatonin represents a second type of chemical signal released from photoreceptors in response to increased darkness, one with characteristics which are significantly different from those of glutamate. Concise spatial and temporal aspects of the photoreceptor signal are conserved through discrete glutamatergic synapses. Different classes of post-synaptic neurons each have appropriate subclasses of glutamate receptors which transmit sign conserving or sign inverting images of the visual mosaic. In contrast, melatonin, because of its highly lipophilic nature is not released by stimulus-coupled secretion mechanisms, but rather by simple diffusion. Thus control of melatonin "release" may be less concise than glutamate. In addition, melatonin may diffuse beyond the confines of the synaptic area to target cells throughout the retina. Effects of melatonin in retina are not well understood; however, current hypotheses suggest that, perhaps via its control of dopamine systems in the inner retina, melatonin plays an important role in dark adaptation and in various retinal processes which exhibit a circadian rhythm. Melatonin and glutamate may represent "co-transmitters" which provide the visual pathway with two types of signals, with melatonin providing widespread modulatory influences on the discrete visual information conveyed via glutamatergic circuits.

Animals↗
PubMed1987

Synaptosomal neurotransmitter uptake systems in the retina and brain nuclei of light- and dark-adapted rabbits.

High affinity uptake rate for [14C]aspartate and [3H]dopamine by retinal homogenate (H), Pl (outer plexiform layer, OPL), and P2 (inner plexiform layer, IPL) retinal synaptosomal fractions were not significantly different between light- and dark-adapted rabbits. However, there were significant increases in the dark in [3H]gamma-aminobutyric acid high affinity uptake rate by retinal H and P2 but not that of Pl. There was a significantly higher [3H]choline uptake rate by retinal H, Pl and P2 in the dark-adapted compared to light-adapted rabbits, but there was no significant change in this rate for synaptosomal fractions from the lateral geniculate body, superior colliculus, visual cortex (VA I + II), caudate nucleus (CN) and hippocampus (HP). Data obtained in this study, along with reports of others, indicate that the change in retinal neurotransmission functions may not always be parallel with the change in high affinity uptake rates of neurotransmitters by retinal synaptosomal fractions. Data obtained indicate an increase in retinal cholinergic neuronal activities in the dark and indicate that optic nerves are not cholinergic and cholinergic neurons in brain nuclei, such as VA, CN and HP, are not significantly influenced by optic nerve inputs in light and dark conditions.

Acclimatization↗
PubMed1987

An indoleamine system in photoreceptor cell terminals of the Long-Evans rat retina.

Uptake of 3H-serotonin is localized to the outer plexiform layer in Long-Evans rat retinas. Autoradiographic accumulation is seen only after in vitro incubation in the light, with retinas isolated from the underlying sclera. Potassium stimulates the release of 3H-serotonin. In this species, amacrine cells do not accumulate these compounds; thus the outer plexiform layer appears to be the only site of uptake and release of this indoleamine. The age-related loss of 3H-serotonin accumulation in the outer plexiform layer of retinal dystrophic rats coincides temporally with the spontaneous degeneration of photoreceptor cells that occurs in this species. Electron-microscopic autoradiography of 3H-serotonin accumulation further confirms that uptake is localized to rod and cone terminals in the outer plexiform layer. The specific accumulation of indoleamines into rod and cone terminals that is observed in the light but is absent in darkness suggests that indoles have an important physiological role in photoreceptors.

5,7-Dihydroxytryptamine↗
PubMed1986

Glutamate receptor agonists release [3H]GABA preferentially from horizontal cells.

A total of 5-6 different cell types in vertebrate retinas accumulate [3H]gamma-aminobutyric acid (GABA). In frog retina, specific populations of cells in the horizontal, amacrine and ganglion cell layers are labeled autoradiographically after a 15-min in vitro incubation with [3H]GABA. Cells which may be bipolar or interplexiform cells are also labeled. Similar autoradiographic patterns are observed in chick retina except for the absence of labeled bipolar or interplexiform cells. In rat retinas, [3H]GABA uptake is limited primarily to Muller and amacrine cells. Depolarizing glutamate receptor agonists (glutamate, aspartate and kainic acid) applied in an in vitro perfusion system, stimulated massive release of [3H]GABA from frog and chick retina but not from rat retina. Under these conditions, autoradiographic labeling of horizontal cells was virtually depleted, while labeling of other cell types remained robust. In contrast, potassium caused release of the label from all 3 types of retina, and loss of autoradiographic labeling occurred uniformly in all cell types. We conclude that [3H]GABA-accumulating horizontal cells possess depolarizing glutamate receptors and that activation of these receptors leads to a release of GABA stores. On the other hand, Muller cells and the various subclasses of [3H]GABA-accumulating amacrine, bipolar and/or interplexiform cells, do not release GABA in response to glutamate receptor stimulation and thus appear to be relatively insensitive to excitatory amino acids.

Animals↗
PubMed1986

Postnatal development of 3H-GABA-accumulating cells in rabbit retina.

Light and electron microscopic autoradiography demonstrates that 3H-GABA is accumulated by horizontal cells in neonatal rabbit retina but not in the adult. A specific population of horizontal cells appears to be mature at birth and they avidly accumulate 3H-GABA during a 15-minute incubation period in vitro. Uptake into horizontal cells is not observed after the fifth postnatal day; 3H-GABA-accumulating horizontal cell bodies and their processes are the first identifiable components that clearly mark the future location of the outer plexiform layer at birth and as such, may be considered pioneering elements. Our observations raise the interesting possibility that the pioneering horizontal cell may provide structural and/or chemical factors necessary for the subsequent development of the outer plexiform layer of the retina. Labeling patterns of other retinal cells also show varying degrees of change during development. A population of amacrine cells accumulate 3H-GABA at birth. These cells show little change in their morphological or 3H-GABA uptake properties from birth to adulthood. Müller cells show weak accumulation of 3H-GABA at birth. Subsequent to this time, labeling of Müller cells is significantly more robust, resulting in Müller cell domination of retinal autoradiographic patterns in more mature retinas. Every cell body in the ganglion cell layer accumulates 3H-GABA at birth. The number of labeled cells declines during postnatal development, resulting in a very limited adult population. We conclude that the ability of retinal cells to accumulate 3H-GABA does not remain constant during postnatal development; rather each cell population displays a unique maturation sequence that results in a dramatic developmental shift in the number and types of GABA-accumulating cells present in the retina.

Age Factors↗
PubMed1985

Light evoked release of acetylcholine in response to a single flash: cholinergic amacrine cells receive ON and OFF input.

By pharmacologically blocking the inhibitory inputs to the cholinergic amacrine cells of the rabbit retina, we have been able to detect the light-evoked release of ACh in response to a single flash. Under these conditions ACh is released equally at light ON and light OFF. This implies that: the cholinergic amacrine cells receive ON and OFF input; they respond to light with depolarizing transients; and the inputs to this system have a basic symmetry.

Acetylcholine↗