Biomedical subjects

G W Maguire

Publications and source records attributed to G W Maguire.

9 recordsLinked to original sources

PubMed1991

Modulation of the receptive field center response of cat retinal ganglion cells by the shift response signal through amacrine cells.

The effect of a stationary and an oscillating grating situated greater than 30 degrees from the RFC on the receptive field center response was examined. The transient firing rate of the center response was suppressed by the oscillating grating for all types of cells. The stationary grating also suppressed the transient firing but the degree of suppression was significantly less than that with the oscillating grating. There was also a significant elevation of the sustained firing rate in Y-cells. The on-going discharges were elevated in all types of units except on-center X-cells when the grating was oscillated. An increase in the area of a grating annulus did not increase the degree of suppression of the center response. This lack of spatial summation of the shift response was related to the properties of the on-off transient amacrine cells.

Animals↗

PubMed1990

Alterations of the cat's electroretinogram induced by the lesioning of the indoleamine-accumulating amacrine cells.

The neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) was used to destroy the indoleamine-accumulating amacrine cells located in the cat's retina. With 100 micrograms of 5,7-DHT, the alterations in the electroretinogram (ERG) were present in all of the treated eyes and the morphological changes were confined to some of the processes in the inner plexiform layer. The ERGs recorded from treated eyes consisted of negative waves at low intensities and depressed b wave amplitude at higher intensities. The duration of the b wave was not altered but the oscillatory potentials were strongly depressed. The changes were probably permanent. The differences in the ERG changes in cats and rabbits were suggested to arise from the differences in signal processing in the inner retina of rabbits and cats.

5,6-Dihydroxytryptamine↗

PubMed1985

A neural pathway for the shift response in the cat.

The shift response (McIlwain effect) was elicited by moving a large grating situated greater than 15 degrees from the conventional receptive field center (RFC). We examined the change in the amplitude of the shift response induced by placing a steady target on the RFC or the RF surround. We found that appropriate stimulation of the RFC or the RF surround will increase and inappropriate stimulation will decrease the amplitude of the shift response in a graded manner. The amplitude of the shift response was not correlated with the maintained activity but was correlated with the transient peak firing rate which is evoked by flashing the enhancing stimulus. A shift stimulus which elicits a strong shift response can be blocked by inappropriate stimulation. The results suggest that the shift signal is modulating a tonic signal which is present in the RF. Because the shift response is a transient excitation, we suggest that the shift response results from a disinhibitory process. A possible neural pathway for the shift response is presented.

Animals↗

PubMed1985

Physiological development of the kitten's retina: an ERG study.

The physiological development of the retina was followed by recording ERGs and OPs from kittens of different ages. We found that different properties of the retina attain adult values at different ages: the b-wave elicited by higher stimulus intensities became adultlike by five to seven weeks while the b-waves elicited by lower stimulus intensities required another 3-5 weeks; the implicit time for the b-waves elicited by the full intensity stimulus attained adult values by 10 weeks of age; and the oscillatory potentials did not become adultlike until 18 weeks of age. The physiological development of the ERG was related to the development of the photoreceptors and the ganglion cells as well as to the morphological development of the second order neurons. It was concluded that the physiological development, as the morphological development, proceeds in three stages: an initial slow phase during which the late receptor potential and the b-waves are first recorded; a second rapid phase, during which the amplitude of the b-waves and OPs increase rapidly; and a third slow differentiation phase during which the final development of the properties of the retina are attained.

Action Potentials↗

PubMed1985

Cat retinal ganglion cell receptive-field alterations after 6-hydroxydopamine induced dopaminergic amacrine cell lesions.

Optic tract single-unit recordings were used to study ganglion cell response functions of the intact cat eye after 6-hydroxydopamine (6-OHDA) lesioning of the dopaminergic amacrine cell (AC) population of the inner retina. The impairment of the dopaminergic AC was verified by high pressure-liquid chromatography (HPLC) with electrochemical detection of endogenous dopamine content and by [3H]dopamine high-affinity uptake; the dopaminergic ACs of the treated eyes demonstrated reduced endogenous dopamine content and reduced [3H]dopamine uptake compared with that of their matched controls. Normal appearing [3H]GABA and [3H]-glycine uptake in the treated retinas suggests the absence of any nonspecific action of the 6-OHDA on the neural retina. The impairment of the dopaminergic AC population was found to alter a number of response properties in off-center ganglion cells, but this impairment had only a modest effect on the on-center cells. An abnormally high proportion of the off-center ganglion cells in the 6-OHDA treated eyes possessed nonlinear, Y-type receptive fields. These cells also possessed shift-responses of greater than normal amplitude, altered intensity-response functions, reduced maintained activities, and more transient center responses. Of the on-center type cells, only the Y-type on-center cells were affected by 6-OHDA, possessing higher than normal maintained activities and altered intensity-response functions. The on-center X-cells were unaffected by 6-OHDA treatment. The dopaminergic AC of the photopically adapted cat retina therefore modulates a number of ganglion cell response properties and within the limits of this study is most prominent in off-center ganglion cell circuitry. When functioning normally, the dopaminergic AC of the cat's retina appears to make the receptive field of the off-center cell more sustained and may make its spatial summation characteristics more linear while adjusting the intensitive properties of neurons in both the on- and off-center pathways.

Animals↗

PubMed1984

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.

Age Factors↗

PubMed1983

Effects of chronic atropinization on visual acuity in kittens.

Atropine was instilled daily from the time of eye-opening until 14 weeks of age into either one or both eyes of developing kittens. Following a 4 week recovery period, grating visual acuity was determined using behavioral techniques under both monocular and binocular viewing conditions. The monocularly treated kittens behaviorally demonstrated an amblyopia in the treated eye and an absence of binocular summation. In contrast, the binocularly treated kittens appeared to show normal visual acuity in both eyes and their performance improved during binocular viewing conditions. Single-unit recordings in striate cortex indicated that binocular atropinization did not alter cortical binocularity, however, monocular atropinization resulted in a decrease in the proportion of binocularly excited neurons and a shift in ocular dominance to the untreated eye.

Amblyopia↗

PubMed1982

Optically induced anisometropia in kittens.

Anisometropia was simulated in 11 kittens during the critical period of visual system development by securing a high-powered minus lens in front of one eye. Behavioral determinations of monocular grating acuity indicated that the induced refractive error resulted in severe amblyopia in the defocused eye. Microelectrode recordings in striate cortex revealed that the deficits in visual acuity were paralleled by a decrease in binocularly innervated neurons and a marked shift in ocular dominance toward the nondeprived eye. "Position of paralysis" estimates revealed that all the anisometropic kittens exhibited anomalous interocular alignments that were indicative of estropia. There was also a significant reduction in the cross-sectional areas of dorsal lateral geniculate nucleus (LGNd) neurons in laminae innervated by the deprived eye. A 6 week period of binocular recovery initiated when three of the anisometropic kittens were approximately 16 weeks of age resulted in a partial recovery of visual acuity; however, there was little or no evidence for recovery of cortical binocularity, cortical ocular dominance, LGNd neuron cell size, or interocular alignment. In general the visual system alterations produced by the induced anisometropia appear to be qualitatively similar to the anomalies associated with prolonged unilateral lid suture.

Animals↗

PubMed1980

Axial lengths and refractive errors in kittens reared with an optically induced anisometropia.

An anisometropia was simulated in kittens during the critical period of development by securing a high-powered negative lens in front of one eye. Refractive error measurements obtained with an objective infrared optometer indicated that the deprived eyes of the anisometropic kittens were significantly more myopic than the normal eyes. A-scan ultrasonography showed that these differences in refractive error were correlated with an increase in the axial dimensions of the deprived eyes. The results of this experiment demonstrate that form deprivation associated with a habitually defocused retinal image produces an experimental myopia which is similar in nature to the refractive error changes produced by lid fusion and corneal opacification.

Age Factors↗