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Biomedical subjects

D G Green

Publications and source records attributed to D G Green.

85 records · Page 5Linked to original sources
PubMed1966

Postsynaptic versus presynaptic inhibition in antagonistic stretch reflexes.

Motoneurons of the cat gastrocnemius-soleus muscle were studied intracellularly with conventional glass micropipettes. Each of these motoneurons was made to fire repetitively by stretch of its own muscle (gastrocnemius-soleus), and by current injected through the impaling microelectrode. By comparing the amount of inhibitory influence from antagonistic stretch of posterior biceps on the repetitive firing in these two different situations, an estimate could be obtained of the relative contribution of postsynaptic inhibition in this type of antagonistic stretch reflex. Even when the experimental conditions were such as to favor presynaptic inhibition, only strong postsynaptic inhibitory effects were seen; presynaptic inhibition was not found.

Animals↗
PubMedDate not supplied

Electrophysiological estimates of visual sensitivity in albino and pigmented mice.

We have estimated the absolute threshold of congenic albino and pigmented mice (C57) using ERG, pupillary light reflex, and VEP. We are unable to detect strain differences using ERGs or VEPs, but pupillary thresholds appear to be different. In addition, as we have previously reported for rats, VEP thresholds are considerably lower than the ERG b-wave thresholds. The VEP thresholds agree with behavioral data from pigmented mice made by others. The mouse VEP thresholds are close to the VEP thresholds in rats and the psychophysical thresholds of two human observers.

Albinism↗
PubMedDate not supplied

Behavioral estimates of absolute threshold in rat.

Dark-adapted thresholds of albino and pigmented rats were estimated using behavioral methods. Albino and pigmented rats who had been water deprived learned to bar press for water reinforcement when a light stimulus was presented. Absolute threshold was defined to be the light intensity at which bar pressing behavior was significantly modified by the presence of the light stimulus. Albino rats had an average threshold of -5.23 log cd/m2 and the pigmented rats had a threshold of -5.0 log cd/m2. These values are close to -5.3 log cd/m2, the psychophysical threshold of human observers in the same apparatus. Consistent with our earlier electrophysiology, these behavioral experiments provide no evidence for an albino/pigmented sensitivity difference. Comparisons are made between behavioral and electrophysiological determinations of absolute threshold in albino and pigmented rats. Thresholds determined behaviorally agree remarkably well with those derived from visual evoked potentials.

Albinism↗
PubMedDate not supplied

Spectral properties of turtle cones.

Microelectrodes were used to record from red and green cones of the turtle Pseudemys scripta elegans. The purpose of this study was to determine the action spectra of the red and green cone photopigments, and to look closely for direct interactions between the two cone classes. An isolated retina preparation was employed so that cones could be stimulated from the outer segment side, thereby avoiding the oil droplets that reside in the inner segments of many cones and normally filter incident light. In agreement with some previous electrophysiological studies, we found little evidence for significant direct connections between red and green cones. Exceptions to this rule are noted and discussed. Measurements indicate that this result does not appear to be due to a general loss of cone connectivity in the isolated retina preparation. Action spectra of the cone photopigments differed markedly from action spectra reported for cones in the eyecup preparation. In contrast to cones in the eyecup, cones in the isolated retina showed higher short-wavelength sensitivity and had action spectra that were adequately described by photopigment nomograms. A model of cone optical properties suggests that in the eyecup up to about 40% of the light that reaches a cone outer segment may do so without first passing through an oil droplet.

Animals↗
PubMedDate not supplied

Visual thresholds in albino and pigmented rats.

Albino rats have recently been reported to have increment thresholds against dim backgrounds that are two log units higher than those of pigmented rats. We, on the other hand, have failed to confirm these differences using electroretinogram b-waves and pupillary light reflexes. This paper reports on experiments using evoked potentials from cortex and colliculus and single-unit recordings from colliculus. We recorded visual-evoked potentials from cortex and superior colliculus in the strains of albino (CD) and pigmented (Long-Evans) rats used in the earlier studies. Thresholds were determined on eight fully dark-adapted animals by extrapolating intensity-response curves to the point at which there was zero evoked potential. The average dark-adapted threshold for the visual-evoked cortical potential was -5.6 log cd/m2 in pigmented and -5.80 log cd/m2 in albino animals. The average dark-adapted threshold for the superior colliculus evoked response was -5.54 log cd/m2 in pigmented and -5.84 log cd/m2 in albinos. The differences were not statistically significant. On the same apparatus, the average absolute threshold for three human observers was -5.3 log cd/m2, a value close to the rat dark-adapted thresholds. Thus, visual-evoked cortical potentials and superior collicular evoked potentials failed to confirm the report of higher dark-adapted thresholds for albinos. In addition, we find that single units in superior colliculus in the albino rat respond to very dim flashes.

Albinism↗
PubMedDate not supplied

Retinal image quality in the rodent eye.

Many rodents do not see well. For a target to be resolved by a rat or a mouse, it must subtend a visual angle of a degree or more. It is commonly assumed that this poor spatial resolving capacity is due to neural rather than optical limitations, but the quality of the retinal image has not been well characterized in these animals. We have modified a double-pass apparatus, initially designed for the human eye, so it could be used with rodents to measure the modulation transfer function (MTF) of the eye's optics. That is, the double-pass retinal image of a monochromatic (lambda = 632.8 nm) point source was digitized with a CCD camera. From these double-pass measurements, the single-pass MTF was computed under a variety of conditions of focus and with different pupil sizes. Even with the eye in best focus, the image quality in both rats and mice is exceedingly poor. With a 1-mm pupil, for example, the MTF in the rat had an upper limit of about 2.5 cycles/deg, rather than the 28 cycles/deg one would obtain if the eye were a diffraction-limited system. These images are about 10 times worse than the comparable retinal images in the human eye. Using our measurements of the optics and the published behavioral and electrophysiological contrast sensitivity functions (CSFs) of rats, we have calculated the CSF that the rat would have if it had perfect rather than poor optics. We find, interestingly, that diffraction-limited optics would produce only slight improvement overall. That is, in spite of retinal images which are of very low quality, the upper limit of visual resolution in rodents is neurally determined. Rats and mice seem to have eyes in which the optics and retina/brain are well matched.

Accommodation, Ocular↗
PubMedDate not supplied

Photoreceptor coupling in turtle retina.

Photoreceptors in the isolated turtle retina of two species of turtle, Chelydra serpentina and Pseudemus scripta elegans, were penetrated with double-barrel electrodes. Physiological responses were recorded through one barrel and Neurobiotin tracer was injected from the other. Intracellular injection of Neurobiotin revealed patterns of tracer-coupled photoreceptors. Both the patterns of tracer coupling and the electrophysiology suggest a high degree of specificity of connections. Rods seem to be coupled only to rods and green and red cones seem to be coupled to cones of the same spectral type. Receptive-field profiles, measured with a thin, sharply focused slit of light, often had well-defined peaks and troughs in sensitivity. We have taken advantage of this observation and used the position of a peak in sensitivity to locate the position on the retina of a coupled cell. In one rod, it was possible to correlate physiological and morphological data and to show that the peaks in the physiological receptive field occurred at positions on the retina where there were dye-coupled cells. This provides direct evidence that gap junctions produce the physiological coupling between rods.

Animals↗
PubMedDate not supplied

Effects of inhibiting glutamine synthetase and blocking glutamate uptake on b-wave generation in the isolated rat retina.

The purpose of the present experiments was to evaluate the contribution of the glutamate-glutamine cycle in retinal glial (Müller) cells to photoreceptor cell synaptic transmission. Dark-adapted isolated rat retinas were superfused with oxygenated bicarbonate-buffered media. Recordings were made of the b-wave of the electroretinogram as a measure of light-induced photoreceptor to ON-bipolar neuron transmission. L-methionine sulfoximine (1-10 mM) was added to superfusion media to inhibit glutamine synthetase, a Müller cell specific enzyme, by more than 99% within 5-10 min, thereby disrupting the conversion of glutamate to glutamine in the Müller cells. Threo-hydroxyaspartic acid and D-aspartate were used to block glutamate transporters. The amplitude of the b-wave was well maintained for 1-2 h provided 0.25 mM glutamate or 0.25 mM glutamine was included in the media. Without exogenous glutamate or glutamine the amplitude of the b-wave declined by about 70% within 1 h. Inhibition of glutamate transporters led to a rapid (2-5 min) reversible loss of the b-wave in the presence and absence of the amino acids. In contrast, inhibition of glutamine synthetase did not alter significantly either the amplitude of the b-wave in the presence of glutamate or glutamine or the rate of decline of the b-wave found in the absence of these amino acids. Excellent recovery of the b-wave was found when 0.25 mM glutamate was resupplied to L-methionine sulfoximine-treated retinas. The results suggest that in the isolated rat retina uptake of released glutamate into photoreceptors plays a more important role in transmitter recycling than does uptake of glutamate into Müller cells and its subsequent conversion to glutamine.

ATP-Binding Cassette Transporters↗
PubMedDate not supplied

A dissection of the electroretinogram from the isolated rat retina with microelectrodes and drugs.

The origins of the a- and b-wave of the ERG were studied using simultaneous recordings made across the receptor layer and the full thickness of a piece of isolated albino rat retina. An inwardly directed current flowing across the rod outer segments was eliminated from the recording when postsynaptic activity was blocked with cobalt or when current source density measurements were made along the length of the outer segments. Rod photovoltages were inferred by removing extraneous field potentials from the recordings made across the photoreceptor layer. The spatial properties of the photovoltage indicates the responses came from an area about 100 microm in diameter. The glutamate analog. APB, which blocks depolarizing bipolar cells, eliminated the b-wave but left the a-wave unaffected. The ERG component due to depolarizing bipolar cells was inferred by subtracting recordings obtained before and after APB. After treatment with APB a slow component remained. This component was completely blocked by barium (200 microM), which blocks potassium channels on Müller cells. Barium had virtually no effect on low-intensity photovoltages but did affect the amplitude and shape of the saturated responses. Barium increased the amplitude of the component of the ERG which underlies the b-wave. It was concluded that the depolarizing bipolar cells directly generate the b-wave of the ERG.

Aminobutyrates↗