O. L. Vaccarezza

Degeneration in the parvocellular portion of the lateral geniculate nucleus of the cebus monkey

SummaryThe synaptology of the Cebus lateral geniculate nucleus (LGN) was studied after varying (3–15 days) periods of survival following unilateral and bilateral eye enucleations. Part of the material was processed with the Glees and Nauta techniques for light microscopy while the rest was processed for electron microscope observation. The study revealed a variety of degenerated terminals in the parvocellular portion of the LGN and allowed the differentiation of the retinal from the extraretinal terminals. The most frequent synaptic type of retinal origin is a glomerular large central terminal (up to 20 μ long) which makes axodendritic and axoaxonic synaptic contacts with geniculate dendrites and peripheral small terminals. Simple axodendritic and axosomatic terminals of retinal and extraretinal origin were also found. The early changes affecting the geniculate neurons and astrocytes during the degenerative process are described.These results are discussed in relation to: 1) previous work on the LGN synaptology of cats and macaques; 2) the physiology of the LGN; 3) the phagocytic role of astrocytes; 4) the general problem of degeneration in the central nervous system. In addition, a correlation between the light and electron microscope observations is attempted.

Ultrastructure of cells and synapses in the parvocellular portion of the Cebus monkey lateral geniculate nucleus

SummaryThe ultrastructural study of the Cebus lateral geniculate nucleus shows the existence of a complex synaptic organization in which several synaptic types are recognized: a) axosomatic; b) simple axodendritic; c) glomeruli.Three main synaptic contacts were found in the glomeruli: 1. axodendritic 1, between central terminals and primary and secondary dendrites; 2. axodendritic 2, between peripheral terminals and dendrites and 3. axoaxonic, between central and peripheral terminals. At the axoaxonic junctions, the central are presynaptic to the peripheral terminals. Elliptical vesicles fill the peripheral terminals.These findings are discussed in relation with a) previous results of other authors in primates and cats; b) previous neurophysiological and pharmacological evidences for the existence of chemical synapses in the lateral geniculate nucleus; c) the possible functions of axoaxonic synapses.

Synaptic transmission in the degenerating lateral geniculate nucleus an ultrastructural and electrophysiological study

Abstract The functional and anatomical changes taking place at the main (optic) geniculate synapses after bilateral eye enucleation were studied using neurophysiological and ultrastructural techniques. Cessation of synaptic transmission in the lateral geniculate nucleus (LGN) took place between 48 and 72 hours after enucleation in two cats, and between 72 and 96 hours in five cats. During the period in which synaptic transmission was still preserved, the study of the discharges to orthodromic stimulation in the optic radiation and the analysis of the recovery cycle of the main geniculate neurons both indicated that a progressive deterioration of synaptic efficiency was taking place. The ultrastructural study of the main synapses of the LGN revealed, immediately after cessation of transmission, that the synaptic terminals were in the filamentous stage of degeneration. The gradual decrease in number of synaptic vesicles in the optic terminals of the LGN correlates well with the progressive deterioration of synaptic efficiency. It is suggested that these phenomena were due to a decrease in the amount of available transmitter for release at the arrival of each presynaptic spike. Possibly, the rate of transmitter synthesis was also altered.

Granulated vesicles in mitral cells and synaptic terminals of the monkey olfactory bulb

SummaryThe ultrastructural study of the Cebus monkey olfactory bulb showed the existence of granulated vesicles in mitral cells cytoplasm and in synaptic terminals of both plexiform layers. A close spatial relationship between granulated vesicles and Golgi complexes and the presence of dense granules inside the cisternae were observed. Elliptical vesicles in association with the granulated ones were found in the terminals synapsing with the mitral cell dendrites and somata. Granulated vesicles were more abundant in the inner plexiform layer.These findings are correlated with a) the origin and formation of granulated vesicles; b) previous pharmacological, physiological and histochemical evidences for the existence of adrenergic synaptic mechanisms in the olfactory bulb and c) the origin of the granulated vesicles containing terminals.

Retinal alterations induced by intravitreous colchicine.

SummaryA single intravitreal injection of 20 μg of colchicine induces remarkable alterations in several retinal structures, which are accompanied by lost of pupilar reflex, deficit in visual behaviour and finally temporary blindness. The pigment cells revealed, after colchicine administration the presence of dense bodies, vacuolae and mitochondrial damage. Outer segments of photoreceptors presented fragmentation, lysis and final dissapearance of the laminar structure. Inner segments showed vacuolization and mitochondrial alteration. Some bipolar and ganglion neurons showed vacuolization and accumulation of filaments in the soma. The effects on the visual cells are related to a possible action on the formation and removal of the outer segments. The effects on the neurons are discussed in relation with the action of colchicine on other CNS neurons and with the blocking action of this drug on axoplasmic flow.

Ultrastructural and electrophysiological aspects of denervated synapses in the lateral geniculate nucleus

Abstract The ultrastructural and electrophysiological alterations taking place in the lateral geniculate mucleus (LGN) were studied in two groups of cats: (I) after bilateral eye enucleation; (II) after electrolytic lesions of the optic tract (OT), or surgical section of the optic chiasm or OT. The study of the fine structure of the optic terminals in the LGN revealed, in both conditions, a progressive decrease in the number of synaptic vesicles together with an increase in the amount of neurofilaments (neurofibrillar hypertrophy). This stage was followed by the appearance of typical images of dark degeneration. The temporal course of this sequence of changes differed in the two cases. In (I) the neurofibrillar hypertrophy was completed in about 72 hr while in (II) it only took 48 hr. The dark degeneration stage appeared at 120 hr in (I) and at 72 hr in (II). The electrophysiological study showed that synaptic transmission ceased at 72–96 hr in (I) while in (II) it only took 48–54 hr. The study of the recovery cycle of the main geniculate synapses during the period of preserved transmission showed a progressive deterioration of synaptic efficiency. The results are interpreted as due to a decrease in the amount of available transmitter for release and to possible alterations in transmitter synthesis. The different temporal course of the alterations in conditions (I) and (II) is related to the length of the axon stump (in (I) ∼- 35 mm; in (II) ∼- 7 mm).

Lamination of the Cebus Lateral Geniculate Nucleus

SummaryThe mode of termination and the somatotopy of optic fibers in the Cebus lateral geniculate nucleus were studied with silver impregnation techniques. — The study of alternate sections impregnated with the Nauta-Laidlaw and Glees methods in unilaterally eye enucleated monkeys with variable survival periods showed the existence of 4 laminae in the parvocellular portion of the lateral geniculate nucleus. In the nucleus ipsilateral to the enucleation the degeneration was confined to the magnocellular 2 and parvocellular 3 and 5 laminae, while in the contralateral side the degeneration was limited to magnocellular 1 and parvocellular 4 and 6 laminae.The significance of the somatotopic arrangement is discussed in relation to previous anatomical and physiological findings.