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R F Miller

Publications and source records attributed to R F Miller.

At least 289 records · Page 16Linked to original sources
PubMed1988

Feasibility of continuous glutamate monitoring in perfused retinal tissue with a potentiometric biosensing probe.

A potentiometric biosensing probe for glutamate has been evaluated as a possible tool to measure the release of glutamate from the isolated retina of Bufo marinus. This probe is based on carbon dioxide detection, following enzymatic conversion of glutamate to gamma-aminobutyric acid (GABA) via glutamic acid decarboxylase (GAD). Probe response characteristics of dynamic range, limit of detection, pH dependency, and selectivity are described. Probe modifications were required for sensor operation in an upside down configuration which was demanded by the need to mount and perfuse the retinal tissue directly at the sensor tip. Overall, these results indicate that this particular potentiometric biosensor is not well suited for direct glutamate measurements in retinal tissue because of pH incompatibility between the sensor and the tissue, and because of high background carbon dioxide levels released from the retina at the pH optimum of the probe. Despite this drawback, the sensor could be utilized to provide a continuous "downstream" monitor of glutamate levels during the course of an experiment, after pH buffering of tissue perfusate. Alternative approaches to probes more compatible with direct tissue measurements are discussed.

Animals↗
PubMed1988

Glutamate receptors of ganglion cells in the rabbit retina: evidence for glutamate as a bipolar cell transmitter.

1. Intracellular and extracellular recordings were obtained from ganglion cells in the rabbit retina. The effects of glutamate analogues and antagonists were studied using a perfusion method for drug application. 2. Kainate (KA) excited all ganglion cells directly and caused a large increase in firing rate. N-Methyl-DL-aspartate (NMDLA) also excited ganglion cells but it was less potent and caused burst firing. 3. Quisqualate (QQ) and (RS)-2-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) excited many ganglion cells and were approximately as potent as KA. Less frequently, QQ and AMPA had inhibitory effects possibly due to polysynaptic action. 4. General glutamate antagonists such as cis-2,3-piperidine dicarboxylic acid (PDA) and kynurenic acid blocked the light input to all ganglion cells. PDA and kynurenic acid blocked the effects of KA and NMDLA, but not carbachol, indicating that they act as glutamate antagonists in the rabbit retina. Kynurenic acid did not block the excitatory action of QQ, even though light responses were abolished. 5. Amacrine cells were depolarized by KA or QQ and less potently by NMDLA. Their light-evoked responses were blocked by PDA. 6. We conclude that the light input to ganglion cells in the rabbit retina is predominantly mediated by KA receptors. This is consistent with the idea that 'on' and 'off' bipolar cells are excitatory and release glutamate.

Action Potentials↗
PubMed1988

Bipolar origin of synaptic inputs to sustained OFF-ganglion cells in the mudpuppy retina.

1. The synaptic inputs to sustained OFF-center ganglion cells of the mudpuppy retina were studied using a superfused retina-eye-cup preparation. Intra- and extracellular electrophysiological recording techniques were carried out during bath application of 2-amino-4-phosphonobutyrate (APB), a glutamate analog that selectively blocks the light responses of ON-bipolars but has minor effects on OFF-bipolar or horizontal cells. 2. The use of APB reduced ganglion cell inputs to those arising from the OFF-bipolar channel. In this way, the existence and polarity (depolarizing vs. hyperpolarizing) of direct or indirect bipolar connections to ganglion cells was determined. 3. Cobalt application was used to block synaptic transmission and demonstrate that APB does not have a direct excitatory action on ganglion cells. 4. Intracellular recording experiments included the use of pulsatile and sustained current injection to evaluate the input resistance changes associated with light, the action of APB, and the excitatory, inhibitory, or disafacilitory nature of the postsynaptic potentials. 5. Some intracellularly recorded cells were stained with horseradish peroxidase (HRP) to verify the ganglion cell origin of the recordings. 6. The OFF-ganglion cell population of the mudpuppy appears to be a heterogeneous group of cells. Sustained OFF-ganglion cells can receive dominant inputs through either the ON- or OFF-bipolar cell pathway or through a mixture of the two. 7. Based on the analysis of this study, we divided sustained OFF-ganglion cells into three subclasses. For one class, light causes the removal of a sustained excitatory input which originates from the OFF-bipolar channel (i.e., a light-evoked disfacilitation); a second class of cells is almost entirely driven by the ON-bipolar channel through a sustained light-evoked inhibitory input; and a third class receives both a light-evoked sustained disfacilitory input from the OFF-bipolar channel and a sustained inhibitory input through the ON-bipolar pathway. Thus the retina appears to use a variety of mechanisms that result in a common response to flashing light stimuli. 8. The results of this study show that APB can be a powerful tool for pharmacologically deciphering the functional connections that exist between outer and inner retinal neurons.

Aminobutyrates↗
PubMed1988

Synaptic inputs and morphology of sustained ON-ganglion cells in the mudpuppy retina.

1. Sustained ON-ganglion cells from the mudpuppy retina were studied with a combined approach, including intracellular and extracellular recording from the superfused retina-eyecup preparation, pharmacology with bath-applied 2-amino-4-phosphonobutyrate (APB), and retrograde and intracellular staining using horse radish peroxidase (HRP). 2. Bath application of micromolar levels of APB selectively blocks the light response of ON-bipolar cells; APB was used to separate synaptic inputs into those which originate from ON- vs. OFF-bipolar cells. This approach clearly demonstrates that the light response of the vast majority of sustained ON-ganglion cells is primarily the result of sustained excitatory inputs that arise (directly or indirectly) from ON-bipolar cells. 3. APB revealed the presence of transient excitatory OFF-inputs in many sustained ON-ganglion cells that are normally not evident. 4. Five sustained ON-ganglion cells were intracellularly stained with HRP and their morphology was analyzed with the aid of a computer-assisted neuron reconstruction system. The stained cells are anatomically similar, based on quantitative analysis of a number of morphological parameters. The dendritic trees of all five cells are primarily confined to sublamina b of the inner plexiform layer, although some cells have a small number of processes that ramify in sublamina a. These latter processes may relate to the transient excitatory OFF-inputs revealed with APB application. 5. Ganglion cells which are morphologically similar to the stained, intracellularly sustained ON-ganglion cells were found in a collection of Golgi-like cells that were labeled by retrograde HRP transport. This raises the possibility that sustained ON-ganglion cells in the mudpuppy may constitute a morphologically identifiable class of retinal ganglion cells in this species. There is also some suggestion that a morphologically similar class of OFF-cells may be present.

Aminobutyrates↗
PubMed1988

Do N-methyl-D-aspartate receptors mediate synaptic responses in the mudpuppy retina?

Whole-cell recordings of amacrine and ganglion cells in the superfused retina-eyecup preparation of the mudpuppy were obtained in order to determine which excitatory amino acid receptor (EAAR) subtype mediates the synaptic responses of these neurons. All third-order retinal neurons tested were depolarized by kainic acid (KA, N-methyl-D-aspartate (NMDA), and quisqualate (QQ). The responses evoked by NMDA were blocked by the addition of D-2-amino-5-phosphonovaleric acid (D-AP5) and D-2-amino-7 phosphonoheptonoic acid (D-AP7) to the perfusate. When the actions of exogenously applied NMDA were completely blocked by D-AP5 and D-AP7, the light-evoked responses of inner retinal neurons persisted without any apparent reduction or, alternatively, a slight enhancement of the response was observed. Light-evoked responses of bipolar, amacrine, and ganglion cells associated with the On pathway were attenuated by L-AP5 in a manner similar to its lower-order homolog L-2-amino-4-phosphonobutyrate (AP4); nevertheless, L-AP5 was not an effective NMDA antagonist. Although synaptic transmission between retinal second- and third-order neurons appears to be mediated by EAARs, the NMDA receptor does not appear to play a prominent role under our experimental conditions. Nevertheless, our results suggest that the racemic mixture of AP5 should not be used as an NMDA antagonist in retinal research, due to the AP4-like actions of its L-enantiomer.

2-Amino-5-phosphonovalerate↗
PubMed1987

Subtle actions of 2-amino-4-phosphonobutyrate (APB) on the Off pathway in the mudpuppy retina.

The principal action of 2-amino-4-phosphonobutyrate (APB) applied to the retina is the elimination of the On-bipolar cell light response. In this study, we report that APB also has more subtle effects; it sometimes enhances Off responses of Off bipolar cells and On, Off, and On-Off inner retina neurons. Possible mechanisms for this enhancement are considered.

Action Potentials↗
PubMed1987

Barium reverses the transretinal potassium gradient of the amphibian retina.

Barium chloride (Ba2+) was added to the bathing medium of the perfused retina-eyecup preparation of the tiger salamander. The electroretinogram (ERG) and intraretinal extracellular potassium activity ([K+]o) were analyzed using a double-barrel electrode, one of which was ion-selective for K+. The action of Ba2+ on the ERG was to attenuate the c-wave/slow PIII complex. In addition, Ba2+ dramatically decreased the [K+]o in the outer retina, enough to reverse the transretinal potassium gradient. These findings, together with the known properties of Müller cell K+ channels, form the basis of an explanation of why Ba2+ blocks slow PIII and not the b-wave of the ERG.

Animals↗
PubMed1987

Horizontal cells contribute to the receptive field surround of ganglion cells in the rabbit retina.

The influence of horizontal cells on ganglion cells in the rabbit retina was examined by injecting current intracellularly into horizontal cells while simultaneously monitoring the extracellular spike activity of nearby single-unit ganglion cells. Hyperpolarizing current injected into horizontal cells decreased the firing rate of on-center brisk ganglion cells and increased the firing rate of off-center brisk ganglion cells. Depolarizing current produced opposite effects on on-center and off-center brisk ganglion cells. These findings are consistent with the view that horizontal cells in the rabbit contribute to the antagonistic surround excitation of brisk ganglion cells.

Action Potentials↗
PubMed1987

Excitatory amino acid receptors of rod- and cone-driven horizontal cells in the rabbit retina.

Intracellular recordings were obtained from horizontal cells in the superfused retina-eyecup preparation of the rabbit. Rod- and cone-dominated horizontal cells were studied using bath-applied excitatory amino acid analogues. Cone-dominated horizontal cell somas were depolarized by kainate (KA) or quisqualate (QQ) and their light responses were reduced or abolished. They were not affected by N-methyl-DL-aspartate (NMDLA) at concentrations up to 2 mM or by 2-amino-4-phosphonobutyrate (APB), a selective agonist for the ON bipolar cell. When synaptic transmission was blocked with cobalt, horizontal cell somas were hyperpolarized. Under these conditions, KA and QQ caused large depolarizations suggesting that these agents have a direct action on horizontal cell somas. Excitatory amino acid antagonists such as cis-2,3-piperidine dicarboxylic acid (PDA) and kynurenic acid (Kyn) hyperpolarized horizontal cell somas to the level of the light-driven membrane potential. These antagonists blocked both the light-driven responses and the depolarizing action of KA. The specific NMDA antagonist 2-amino-7-phosphonoheptanoate (AP-7) had no effect on the membrane potential or light-driven responses of horizontal cell somas. In contrast to a previous report, we found no evidence that low concentrations of NMDLA could hyperpolarize horizontal cells or act as a KA antagonist in the rabbit retina. Rod-dominated axon terminals were identified by waveform, threshold, and the presence of a large rod after-potential evoked by high light intensity. These cells were depolarized by KA and their light responses were attenuated. NMDLA and APB had no effect on these cells. The general antagonists, PDA and Kyn, hyperpolarized axon terminals and blocked their light-evoked responses. The specific NMDA antagonist, AP-7, had no effect on these cells. These results suggest that the synaptic receptors that mediate light input to both rod- and cone-dominated horizontal cells are kainate or quisqualate receptors. This implies that the rod and cone transmitters of the rabbit retina are similar, with the characteristics of an excitatory amino acid, such as glutamate.

Animals↗
PubMed1986

Kynurenic acid distinguishes kainate and quisqualate receptors in the vertebrate retina.

Excitatory amino acid receptors (EAARs) underlie major synaptic pathways in the brain, retina and spinal cord. Several subclasses of EAARs have been proposed, based on pharmacological studies using a variety of agonists and antagonists. Kynurenic acid (Kyn), a metabolite of tryptophan, has been recently proposed as a potent EAAR antagonist. In this report, we show that Kyn can be used to separate two distinct classes of EAAR in the vertebrate retina: it blocks kainic acid (KA) responses but has minimal effects on responses mediated by quisqualate (QQ). At concentrations which block the KA responses, Kyn also blocks the light-evoked synaptic responses of all types of third-order neurons in the retina. These results suggest that KA receptors are the major receptor subtypes which underlie synaptic transmission and that QQ receptors are minimally utilized by light-activated pathways under the conditions of our experiments.

Animals↗
PubMed1986

Neurotensin actions in the retina: mechanisms and variability.

The effects of neurotensin on mudpuppy retinal cells were studied using extracellular and intracellular electrophysiological recording techniques and bath application of the peptide. Ganglion and amacrine cells (but not bipolar or horizontal cells) were reversibly depolarized by low micromolar concentrations of neurotensin. Depolarizations also occurred with neurotensin application during cobalt block of synaptic transmission and were accompanied by decreased input resistances. This suggests neurotensin may act directly on amacrine and ganglion cells as a conventional excitatory transmitter. However, in many retinas, cells responded to light stimuli and to other drugs but not to neurotensin. These negative results are important in considering the peptide's normal role in retinal function.

Action Potentials↗
PubMed1986

A perfused rabbit retina preparation suitable for pharmacological studies.

We present a method for maintaining the isolated retina-eyecup of the rabbit in a manner which permits the introduction of pharmacological agents at a controlled concentration. As judged by physiological criteria, the retina is well maintained under these conditions. The stability of the preparation is excellent; intracellular or extracellular recordings from single cells can be maintained through multiple solution changes. By using Co2+ to block synaptic transmission, we can distinguish direct from transsynaptic effects. Use of this preparation should facilitate the investigation of neurotransmission in the mammalian retina.

Animals↗
PubMed1986

Necrotizing enterocolitis: an unusual cause of rectal bleeding in a term infant.

A 5-day-old term infant presented to the emergency department with a seven-hour history of rectal bleeding. He had had an uncomplicated prenatal course with no significant risk factors during the third trimester or perinatal period. The infant presented in minimal distress but was admitted for further workup and evaluation. The workup revealed necrotizing enterocolitis, which ultimately required surgical management. The infant was discharged with a colostomy, and then was readmitted ten weeks later for reanastomosis.

Ampicillin↗
PubMed1986

Technetium-99m-glucoheptonate lung imaging; has it a role in the diagnosis of bronchial carcinoma?

It has been shown by Vorne et al. and Passamonte et al. that technetium-99m-glucoheptonate (Tc-GH) concentrates in bronchial carcinomas. The two studies reported similar sensitivities (88 and 91%, respectively) but discordant specificities (88 and 10%, respectively). In order to assess the uptake of Tc-GH in bronchial carcinomas we performed lung imaging on patients with an abnormal chest radiograph who were undergoing routine investigations for bronchial carcinoma. 108 patients (85 carcinoma, 23 non-carcinoma) were given 555 MBq (15 mCi) Tc-GH intravenously and scanned 5 h later. Images were assessed by one investigator (J.A.L.) without knowledge of the patient's clinical details or other results. Localized uptake of Tc-GH was seen in 71 out of 85 patients who proved to have bronchial carcinoma. Uptake was also seen in 16 out of 23 patients with a variety of non-malignant lung conditions. Sensitivity for bronchial carcinoma was 84%, and specificity 30%. Uptake did not vary significantly with histology. Eleven out of 12 patients with proven tuberculosis had increased uptake of Tc-GH; two of those patients also had inactive lesions which did not concentrate the tracer. In conclusion, Tc-GH is of limited value in the diagnosis of bronchial carcinoma, because of its low specificity. A possible role in the diagnosis of active tuberculosis may exist, following further assessment.

Adult↗
PubMed1986

A functional organization of ON and OFF pathways in the rabbit retina.

Intracellular electrophysiological recordings were obtained from amacrine and ganglion cells in an isolated, superfused retina-eyecup preparation of the rabbit. Cells were characterized physiologically, after which cell-staining was accomplished by intracellular iontophoresis of HRP. A computer-assisted image-processing system was used to study the dendritic stratification pattern of HRP-labeled neurons within the inner plexiform layer (IPL). Our results support the concept that the IPL is functionally divided into a distal OFF region and proximal ON layer. ON and OFF ganglion and amacrine cells show dendritic arborizations consistent with this division and ON-OFF ganglion cells have processes in both portions of the IPL. It appears that these functional subdivisions of the IPL reflect excitatory, but not necessarily inhibitory, inputs. Thus, the pattern of dendritic arborization of a cell appears to predict its physiological response polarity, regardless of the type of inhibition it receives.

Animals↗