The uptake of gamma-aminobutyric acid and glutamate by synaptosomes from the visual cortex of albino and pigmented rabbits.

The synaptosomal uptake of glutamate and gamma aminobutyric acid [GABA] in the visual cortex of albino and pigmented rabbits was compared. GABA uptake was similar in both pigmented and albino rabbits, but glutamate uptake was greater in the pigmented rabbit. The kinetics of glutamate uptake in albino and pigmented rabbits suggested that the number of functioning glutamate synapses may be lower in the albino. The significance of this with respect to the differences in visual processing in the two types of rabbit is discussed.

Heterogeneity of synaptic density in the adult pigmented rabbit's visual cortex.

In the adult pigmented rabbit, synaptic density in the lateral and medial part of the visual cortex was estimated along the projection area of the visual streak. A higher synaptic density distribution was observed in the lateral cortex (projection area of the nasal visual field) than in the medial cortex (projection area of the temporal visual field). This shows that there is a higher synaptic density in the region of the visual cortex receiving input from a retinal area with a high ganglion cell concentration than the area of the cortex receiving input from the retina with a low concentration of such cells. The regions of the visual cortex with higher and lower synaptic densities are the areas having a higher and lower magnification factor respectively.

Distribution of neurons in the pigmented rabbit's visual cortex.

Past observations revealed a relationship between mammalian retinal ganglion cell distribution and cortical and subcortical projection of the rabbit's visual field. High ganglion cell density sectors of the retina claimed larger areas in their cortical and subcortical projections (with a high magnification factor) than retinal areas with low ganglion cell density. In this study, it was observed that distribution of nonpyramidal neurons in laminae IV and VI of the pigmented rabbit's visual cortex was not uniform. The nonuniformity in layer IV roughly followed the variation in the magnification factor in the cortical projection of the animal's visual field. The highest density of nonpyramidal neurons in layer IV was observed in the cortical area with the highest magnification factor.

Visual cortex in the albino rabbit.

Cortical projection of the visual field on the albino rabbit's cortex has been investigated and compared with that in the pigmented rabbit. A lower magnification of the projection of the temporal visual streak (nasal visual field) in the albino rabbit is attributed to the absence of an area centralis-like organization of the retinal ganglion cells in the temporal visual streak in this animal. Cortical single units in the albino rabbit show a lesser variety in their receptive field organization than those in the pigmented rabbit.

Visual field representation in the newborn rabbit's cortex.

The representation of the visual field in the newborn rabbit's visual cortex appears on about the 10th day after the animal's birth coinciding with the time of its eye opening. Initially the visual response is obtained from a small anterolateral segment of the visual area of the cortex representing a few degrees of the extreme nasal vision of the animal. Even at this early stage, the visually responsive area of the rabbit's cortex seems to organize into V1 and V2 with the binocular response confined to V1 only. By the 15th day after the animal's birth, the visually responsive area in the newborn rabbit extends medially and posteriorly over the cortex, representing more of the animal's temporal and superior visual fields. By this time the binocular response is established over a narrow zone of the animal's visual cortex, representing a few degrees of the nasal visual field, on either side of the boundary between V1 and V2. Between the 16th and 17th day after the rabbit's birth, the cortical representation of the animal's visual field extends medially to the margin of the fissura sagittalis lateralis and caudally to the posterior pole of the hemisphere. In this visually responsive area of the cortex, the band-shaped visual field of the newborn rabbit is asymmetrically represented.

Retinotopic organization of the guinea pig's visual cortex.

Under anaesthesia the right visual cortex of the guinea pig was investigated with 1.5 M potassium citrate-filled glass microelectrodes. Single unit and small unit cluster response showed an organized three-part representation of the contralateral visual field on the animal's visual cortex. The major central division (V1) had the representation of a nasotemporally elongated visual field. This visual field extended for about 110 degrees horizontally and for about 70 degrees vertically. The nasal visual field was represented laterally on the cortex and the lower visual field anteriorly. The visual field representation was not uniformly magnified in V1; the magnification of the nasal field was higher than the rest of the visual field. On either side of V1, the guinea pig's cortex had two additional visually responsive strips. The strip V2L, situated lateral to V1, had a condensed representation of about 40 degrees of the animal's nasal visual field and mirror-imaged the visual field representation of the adjacent V1. The strip V2M, situated medial to V1, had a similar representation of about 40 degrees of the animal's temporal visual field and mirror-imaged the visual field representation of the area V1 adjacent to it. A binocularly responsive zone of cortex was observed on either side of the boundary between the areas V1 and V2L, representing between 10 degrees and 15 degrees of the nasal visual field on each side of the boundary line. Investigation of the retinal ganglion cell distribution of the animal showed a nasotemporally oriented 'visual streak' with a high ganglion cell density per unit area of the retina, near the optic disc. Away from the 'visual streak' the density of the ganglion cells of the retina diminished progressively. The configuration of the 'visual streak' and the general pattern of the ganglion cell distribution in the rest of the retina coincided with the asymmetry in the magnification of representation of the animal's visual field on the cortex.