Biomedical subjects

A S Ramoa

Publications and source records attributed to A S Ramoa.

31 records · Page 2Linked to original sources

PubMed1988

Blockade of intracortical inhibition in kitten striate cortex: effects on receptive field properties and associated loss of ocular dominance plasticity.

We have investigated the importance of GABAergic inhibition for the receptive field properties and plasticity of cells in the visual cortex of kittens. Osmotic minipumps were used to continuously infuse the GABA-antagonist, bicuculline methiodide (BIC), into striate cortex. Extracellular recordings were made during BIC infusion to assess neuronal response properties during the blockade of inhibition. Recordings were also made from other kittens after concurrent monocular deprivation and BIC infusion to investigate the importance of response selectivity for ocular dominance plasticity. The minipump delivery technique was used to produce a large volume of cortex presumably free of GABA-ergic inhibition. Compared to recordings in saline-infused control hemispheres, about half of the cells in bicuculline-infused hemispheres had abnormally low orientation selectivity. The low selectivity was generally accompanied by marked anomalies in several other receptive field properties. Particularly striking was the large size of the receptive fields. At eccentricities less than 10 deg many receptive fields subtended from 10 to over 30 deg of arc. The less selective neurons also had abnormal responses to flashed stimuli, giving strong transient responses to the onset and offset of large stationary stimuli which filled their receptive fields. These results imply that intracortical inhibition normally suppresses responses to stimuli within a large excitatory zone beyond the classical receptive field. Inhibition is necessary for the normal orientation selectivity of many cells, although the selectivity may be partially established by the cell's excitatory input. Additionally, intracortical inhibition appears to be necessary for the antagonism and segregation of ON and OFF receptive field subregions. In our study of plasticity, we exploited the fact that BIC treatment greatly increases the range of stimuli that activate cortical neurons. Kittens were monocularly deprived for 7 days concurrently with cortical infusion of BIC. After cessation of the drug treatment, physiological recordings were made. Response properties had returned to normal but neurons in BIC-infused hemispheres had a significantly reduced ocular dominance shift compared to neurons in control hemispheres. This is probably related to the reduced selectivity of cells during BIC infusion. The suggestion here is that there is diminished ocular dominance plasticity in BIC-infused hemispheres because of an increased probability of correlated activity between spontaneous discharge from the closed eye and the cortical activity evoked by the open eye afferents.

Animals↗

PubMed1988

Dendritic growth and remodeling of cat retinal ganglion cells during fetal and postnatal development.

We have studied the development of retinal ganglion cell morphology in the cat's visual system from early fetal to postnatal times. In particular, we have examined the contribution of growth and remodeling to the establishment of mature retinal ganglion cell form. Ganglion cells were identified by retrograde labeling with rhodamine latex microspheres deposited in the superior colliculus and lateral geniculate nucleus between embryonic day 34 (E34; birth = E65) and adulthood. To reveal the fine morphological details of retrogradely labeled ganglion cells, 48 hr later Lucifer yellow was injected intracellularly in living retinae that had been dissected and maintained in vitro. Our results show that at E35-37 the majority of ganglion cells are very simple in morphology, with a few dendritic processes that are generally aligned in a radial direction towards or away from the optic disc. During the ensuing 2 week period, there is a progressive growth and elaboration of dendrites. By E50, some ganglion cells resembling the adult alpha, beta, and gamma classes can be identified based on comparisons of the appearance and dimensions of their dendritic trees and somata with neighboring filled cells. However, ganglion cell dendrites and axons at this age express several transient morphological features. The axons of ganglion cells give rise to delicate processes originating from the intraretinal portion of the axon, including side branches, present in about half of the cells, and occasionally bifurcations that give rise to axon collaterals. These transient axonal features are present throughout development, including the neonatal period; no axon collaterals were observed after postnatal day 15, while axonal side branches persisted even at P31 but were gone by adulthood. Ganglion cell dendrites exhibit excessive branches and exuberant somatic and dendritic spines. Quantitative analysis of these processes shows that after E45 dendritic trees increase dramatically in complexity, reaching the peak number of spines and branch points by the first week of postnatal life. The number of dendritic processes then falls abruptly to reach near-adult levels by the end of the first postnatal month. Even though dendritic morphology closely resembles that seen in the adult at this age, ganglion cell bodies and dendrites must continue to grow to reach their adult size.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Transient morphological features of identified ganglion cells in living fetal and neonatal retina.

The function and morphology of retinal ganglion cells in the adult mammalian visual system has been well studied, but little is known about how the adult state is achieved. To address this question, the morphological changes that retinal ganglion cells undergo during development were studied. Ganglion cells were first identified by retrograde labeling with rhodamine latex microspheres deposited in retinorecipient targets in fetal and early postnatal cats. The structure of ganglion cells was then revealed by intracellular injection of Lucifer yellow in living retinas removed and maintained in vitro. As early as 2 weeks before birth, a morphologically diverse assortment of ganglion cells is present, some of which resemble the alpha, beta, and gamma classes found in the adult. However, in contrast to the adult, developing ganglion cells exhibit several transient features, including excessive axonal and dendritic branching and exuberant somatic and dendritic spines. These morphological features indicate that there is a transient network of connectivity that could play an important role in the final determination of retinal ganglion cell form and function.

Animals↗

PubMed1987

Receptive field properties of cells in area 19 of the cat.

We have recorded extracellularly from single cells in area 19 of the cat for the purpose of providing a quantitative description of response characteristics. A prominent feature of this area is a high incidence of cells that are end-stopped. Drifting sinusoidal gratings were used to determine spatial and temporal characteristics of the discharge region. In addition, we have conducted independent tests to characterize end zones of receptive fields. When a grating patch was used to stimulate the discharge region alone, all of the cells showed a band-pass spatial frequency tuning characteristic. The optimal spatial frequency ranged from 0.1 to 1.13 cycles/deg, and the distribution had a peak at 0.4 cycles/deg. The bandwidth at half peak amplitude ranged widely from 0.7 to 3.3 octaves (mean 2.0 octaves). When gratings were also presented to the end zones, responses to stimulation of the central region were suppressed. The surround was phase-insensitive in that the relative phase between the grating in the two regions generally did not affect the strength of the suppression. To determine spatial characteristics of the end-zone inhibition, the spatial frequency of the end-zone grating was changed while that for the central pattern was fixed. All cells showed a band-pass characteristic for end-zone inhibition, but in each case, the tuning width was broader than that for excitation. The mean spatial frequency bandwidth of end-zone inhibition was 2.7 octaves. The peak of the inhibition generally coincided with the peak of the excitatory spatial frequency tuning of the discharge center. Considered together, these results show that neurons in area 19 share common properties with those in areas 17 and 18, but they exhibit phase-insensitive end-zone inhibition more frequently.

Animals↗

PubMed1987

Dark-reared cats: unresponsive cells become visually responsive with microiontophoresis of an excitatory amino acid.

The visual system of kittens reared in total darkness is grossly abnormal. Although estimates vary, substantial proportions of cells in the visual cortex of these animals are unresponsive to visual stimulation. Additional cells are weakly responsive or erratic. We have considered the possibility that these neurons receive subthreshold input which might be evident if an excitatory neurochemical agent is applied during extracellular recording with a microelectrode. To test this notion, we have recorded from cells in the striate cortex of dark-reared kittens during microiontophoretic application of an excitatory amino acid, DL-homocysteate (DLH). Using this technique, we find that virtually all cells in the visual cortex of dark-reared kittens are responsive to visual stimulation. Prior to application of DLH, 27% of the cells were unresponsive to visual stimuli. Following iontophoresis of DLH, half of these cells responded with excitatory discharge to visual stimuli and the other half exhibited an inhibitory response in that the elevated maintained activity was suppressed during presentation of a visual stimulus. Additional cells from these animals, which were initially visually responsive, were also studied. For some of these units, responses were weak prior to administration of DLH and we were able to obtain a more clear estimate of selectivity for stimulus orientation during microiontophoresis of the drug. In these cases, and for the few cells which were initially responsive and orientation selective, we observed no major differences in selectivity before and after DLH application.

Animals↗

PubMed1987

Dark-reared kittens: GABA sensitivity of cells in the visual cortex.

Most cells in the visual cortex of dark-reared kittens are unselective for stimulus orientation and we examined the notion that this might be due to insufficiently developed gamma-aminobutyric acid (GABA) receptors. We recorded from cortical neurons and examined their sensitivity to iontophoretically applied GABA. As expected, most units were non-selective for orientation, but application of GABA suppressed impulse activity of these cells just as for orientation selective neurons. This result suggests that the development or maintenance of GABA receptors is not critically dependent on visual experience.

Animals↗

PubMed1987

Binocular interaction in the dorsal lateral geniculate nucleus of the cat.

We have investigated binocular interaction in the dorsal lateral geniculate nucleus (LGN) of the cat. Neurons were recorded extracellularly during visual stimulation with sinusoidal gratings which were presented at different interocular phases (disparities). The large majority of cells (91%) exhibited some type of binocular interaction. For 75% and 16% of the total number of cells, the binocular interaction was inhibitory or facilitatory, respectively. For the remaining 9% of cells, no interaction was evident. In marked distinction from visual cortex, the facilitatory and inhibitory interactions in the LGN are independent of the relative interocular phase of the patterns. Neurons in the LGN are therefore insensitive to the stereoscopic depth cue, retinal disparity.

Animals↗

PubMed1986

Effect of contrast on spatial frequency tuning of neurones in area 17 of cat's visual cortex.

Previous investigations have revealed that perceived spatial frequency of gratings rises as contrast is lowered. In order to account for this finding it has been postulated that the spatial frequency which produces the maximum response from cortical neurones decreases with contrast. We have examined this hypothesis by determining optimal spatial frequency at 3-5 different contrast levels for 37 neurones in the cat striate cortex. For the complete sample no systematic changes in optimal spatial frequency was observed. However, a shift in the predicted direction was found for cells tuned to high spatial frequencies.

Animals↗

PubMed1985

Comparison of response properties of cells in the cat's visual cortex at high and low luminance levels.

Receptive-field organization of cells in the cat's striate cortex and lateral geniculate nucleus (LGN) was investigated by using bars of light as stimuli. The aim was to determine if differences occur between conditions of high and low luminance levels. Of 72 cortical cells studied, the receptive fields of 63 were clearly different at high compared with low luminances. Units that gave on-off responses to flashed bars, for example, typically displayed on-only responses at low luminance. By far the most frequent change was that off responses were reduced or absent at low luminance levels. Of 63 cells that showed clear changes, 54 were of this type. This altered receptive-field organization appears to remain for extended periods (we have monitored the steady-state case for up to 2 h). Additional tests allow us to rule out the possible influence of overall changes in response strength and scattered light. To see if similar changes in receptive-field organization are present at the level of the LGN, we recorded from a small number of cells in the LGN (n = 10) and from an additional five afferent fibers in the cortex. In each case, there was a change in center-surround organization between high and low luminance levels similar to that previously reported for retinal ganglion cells. The excitatory responses from the surround for both on-center and off-center cells were absent at low luminance. Taken together, the results suggest that surround responses that can be elicited from ganglion cells and LGN cells make an important contribution to the receptive-field organization of cortical neurons. Changes in receptive-field organization of cortical cells are apparently not accompanied by alterations of other basic response properties. Orientation (7 cells) and spatial frequency (53 cells) selectivity remain relatively unchanged when measured at different luminances. Although optimal spatial frequency is slightly lower at low luminance levels, the low spatial frequency attenuation remains unaltered. Since receptive-field changes between high and low luminance levels suggest that a unit's classification may also vary, we examined simple and complex cell characteristics using sinusoidal gratings (65 cells). Contrary to what we had anticipated, the degree of modulation of responses was relatively independent of luminance, indicating that cell classification does not vary with stimulus luminance.

Animals↗

PubMedDate not supplied

Dendritic development of retinal ganglion cells after prenatal intracranial infusion of tetrodotoxin.

The dendritic form of a cell may be established by many factors both intrinsic and environmental. Blockade of action potentials along the course of axons and in their postsynaptic targets dramatically alters the development of axonal morphology. The extent to which blockade of target cell activity retrogradely alters the dendritic morphology of the presynaptic cells is unknown. To determine whether the establishment of dendritic form by developing retinal ganglion cells depends on activity within their targets, the sodium channel blocker, tetrodotoxin (TTX), was administered via minipumps to the diencephalon of cat fetuses from embryonic day 43 (E43) to E57. At E57 retinae were removed and living retinal ganglion cells injected in vitro with Lucifer yellow to reveal their dendritic morphology. In the TTX-treated animals both alpha and beta types of retinal ganglion cells were present, as were putative gamma cells. Overall, the dendrites of retinal ganglion cells in TTX-treated animals appeared qualitatively and quantitatively similar to those of untreated animals. The only significant change in the TTX-treated cases was a small increase in the number of dendritic spines on the non-beta cells. These results indicate that the acquisition of basic dendritic form of developing ganglion cells is not influenced by the action potential activity within their targets, and that it is also independent of the terminal branching patterns of their axons.

Action Potentials↗

PubMedDate not supplied

The role of spontaneous retinal activity before eye opening in the maturation of form and function in the retinogeniculate pathway of the ferret.

During early mammalian development, inputs from the two retinas intermix within the lateral geniculate nucleus (LGN), then segregate during the first postnatal week into layers that receive input from a single retina. Functionally, the LGN also changes markedly during the first postnatal month; early geniculate responses to retinal input are mainly excitatory, then inhibitory circuits mature within the LGN. These remarkable changes in form and function of the retinogeniculate pathway occur at a time when patterned visual activity is not present, but retinal ganglion cells already manifest spontaneous action potential activity. To examine the role of early retinal activity in these critical developmental processes, we placed the slow release polymer Elvax embedded with tetrodotoxin (TTX) into the vitreous chamber of one or both eyes of neonatal ferrets. Animals receiving monocular injection of TTX had the other eye treated with Elvax containing control citrate buffer. Intraocular injection of horseradish peroxidase was made at the end of the period of TTX treatment to reveal the retinal terminals in the LGN. Chronic monocular or binocular blockade of retinal activity during the first postnatal week did not prevent eye-specific segregation, although it made the boundaries between layers less distinct. Retinal terminals ended preferentially in the appropriate layer, but a large number of terminals were also present in the inappropriate layer. Further segregation was achieved during the second postnatal week of activity blockade, when most retinal terminals ended preferentially in the appropriate geniculate layer and sharper layer boundaries were present. However, a small but significant number of terminals still extended into the inappropriate layer. Together, these findings indicate that monocular as well as binocular blockade of retinal activity resulted in some anomalous retinogeniculate projections and delayed eye-specific patterning, but segregation was largely intact at the end of the second postnatal week. We also report here that intraocular tetrodotoxin had a marked effect on the maturation of intrinsic geniculate circuits prior to eye opening. Whole-cell patch-clamp recordings in the LGN slice preparation revealed that activity blockade prevented the maturation of the slow, but not the fast, hyperpolarizing potential of LGN neurons during the first postnatal month and up to P38, the oldest age studied. In conclusion, these results indicate that spontaneous retinal activity modulates the time course of binocular segregation but does not alone account for the segregation of retinogeniculate terminals. However, early retinal activity plays an important role in developing the intrinsic circuitry of the LGN.

Action Potentials↗

PubMedDate not supplied

A comparison of inhibition in orientation and spatial frequency selectivity of cat visual cortex.

Neurones in the visual cortex are highly selective for orientation and spatial frequency of visual stimuli. There is strong neurophysiological evidence that orientation selectivity is enhanced by inhibitory interconnections between columns in the cortex which have different orientation sensitivities, an idea which is supported by experiments using neuropharmacological manipulation or complex visual stimuli. It has also been proposed that selectivity for spatial frequency is mediated in part by a similar mechanism to that for orientation, although evidence for this is based on special use of visual stimuli, which hampers interpretation of the findings. We have therefore examined selectivity for both orientation and spatial frequency using a technique which allows direct inferences about inhibitory processes. Our method uses microiontophoresis of an excitatory amino acid to elevate maintained discharge of single neurones in the visual cortex. We then present visual stimuli both within and outside the range of orientations and spatial frequencies which cause a cell to respond with increased discharge. Our results show that orientations presented on either side of the responsive range usually produce clear suppression of maintained discharge. In marked contrast, spatial frequencies shown to either side of the responsive range have little or no effect on maintained activity. We conclude that there is an intracortical organization of inhibitory connections between cells tuned to different orientations but not different spatial frequencies.

Animals↗

PubMedDate not supplied

Short-term synaptic plasticity in the visual cortex during development.

The maturation of short-term synaptic plasticity was studied in slices of the visual cortex obtained from rats during the first 47 days of postnatal life. Responses of cortical neurons to repetitive stimulation of the white matter at frequencies >5 Hz were examined by recording intracellularly at the resting membrane potential level. Paired-pulse facilitation, an increase in the excitatory intracellular response following an initial response, was present in approximately 40% of the neurons studied from postnatal day 5 (P5) to P10. Most of the remaining neurons studied at these ages did not reveal paired-pulse interactions. There was a progressive, age-related increase in the proportion of cells displaying paired-pulse depression, a decrease in the second excitatory response relative to the first, and a concomitant decrease in the proportion of cells displaying paired-pulse facilitation. Thus, at P31-P47 approximately half of the neurons revealed depression of synaptic transmission following an initial stimulus, while most of the other neurons displayed a lack of temporal interactions. At these later ages, inhibitory potentials also displayed paired-pulse interactions. Maturation of paired-pulse depression of the excitatory response is temporally correlated with the development of intracortical inhibitory mechanisms and may reflect subtractive or shunting inhibition in the postsynaptic neuron as well as presynaptic inhibitory mechanisms. Consistent with a role of GAGAergic inhibition, application of GABA receptor antagonists produced reversible blockade of paired-pulse depression. In conclusion, cortical neurons display substantial maturation in short-term synaptic plasticity during the first postnatal month. Temporal facilitation may be important in enhancing excitatory neurotransmission at a time when synapses are very immature. In the mature cortex, suppressive temporal interactions could provide an important substrate for neuronal processing of visual information.

Animals↗