The spatial pattern of the synaptic vesicular apparatus as a correlate of transmitter storage models

Essential functions of synapsins I and II in synaptic vesicle regulation

Synaptic vesicles are coated by synapsins, phosphoproteins that account for 9% of the vesicle protein. To analyse the functions of these proteins, we have studied knockout mice lacking either synapsin I, synapsin II, or both. Mice lacking synapsins are viable and fertile with no gross anatomical abnormalities, but experience seizures with a frequency proportional to the number of mutant alleles. Synapsin-II and double knockouts, but not synapsin-I knockouts, exhibit decreased post-tetanic potentiation and severe synaptic depression upon repetitive stimulation. Intrinsic synaptic-vesicle membrane proteins, but not peripheral membrane proteins or other synaptic proteins, are slightly decreased in individual knockouts and more severely reduced in double knockouts, as is the number of synaptic vesicles. Thus synapsins are not required for neurite outgrowth, synaptogenesis or the basic mechanics of synaptic vesicle traffic, but are essential for accelerating this traffic during repetitive stimulation. The phenotype of the synapsin knockouts could be explained either by deficient recruitment of synaptic vesicles to the active zone, or by impaired maturation of vesicles at the active zone, both of which could lead to a secondary destabilization of synaptic vesicles.

Population trends in the fine spatial re-organization of synaptic elements in forebrain regions of chicks 0.5 and 24 hours after passive avoidance training

Two regions in the forebrain of domestic chicks (Gallus domesticus), the intermediate and medial hyperstriatum ventrale and the lobus parolfactorius, have previously been shown to be important centres of biochemical, pharmacological and physiological change following one-trial passive avoidance training. The purpose of the present study was to examine, at the electron microscopic level, the fine spatial re-arrangement of synaptic structures in the intermediate and medial hyperstriatum ventrale (at 30 min), and in the lobus parolfactorius (at 24 h), post-training using comprehensive biometrical designs, image analysis and stochastic approaches. In intermediate and medial hyperstriatum ventrale, no significant differences in the numerical density of synapses either between control and trained chicks, or between hemispheres, were revealed using the disector method. However, after training, a nested-ANOVA demonstrated an increase in the thickness of pre- and post-synaptic electron densities (estimated via image analysis) only in the left intermediate and medial hyperstriatum ventrale, whereas synaptic apposition zone profiles increased in length bilaterally. In presynaptic terminals from the intermediate and medial hyperstriatum ventrale, stochastic analysis revealed that training resulted in the re-distribution of synaptic vesicles between two spatial pools relative to synaptic apposition zones, in both hemispheres producing a large number of synaptic vesicles closer to synaptic apposition zones; a nearest neighbour analysis of synaptic apposition zone profiles indicated that the lateral shape of the synaptic apposition zone after training is more complex in both hemispheres. In the lobus parolfactorius at 24 h post-training the main changes in synaptic fine structure involved a shift of synaptic vesicles away from synaptic apposition zones in the right hemisphere with the distance between synaptic apposition zones decreasing; in the left lobus parolfactorius, synaptic apposition zones became more regular/round in shape with a greater distance between them after training. These data suggest that the initial acquisition of memory involves population changes in the fine spatial organization of synaptic vesicles and synaptic apposition zones in synapses in the intermediate and medial hyperstriatum ventrale, which indicate a possible tendency towards greater synaptic efficacies. These changes are as dynamics as the molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.

Interactions between brain mitochondria and cytoskeleton: evidence for specialized outer membrane domains involved in the association of cytoskeleton-associated proteins to mitochondria in situ and in vitro

The surface distribution of several proteins (porin, hexokinase, and two proteins associated with microtubules or actin filaments) on the outer membrane of brain mitochondria was analyzed by immunogold labelling of purified mitochondria in vitro. The results suggest the existence of specialized domains for the distribution of porin in the outer mitochondrial membrane. Similarities between the distribution of porin and the distribution of microtubule-associated proteins bound in vitro to mitochondria suggested that mitochondria and microtubules interact by binding microtubule-associated proteins to porin-containing domains of the outer membrane. This hypothesis was supported by biochemical studies on outer mitochondrial proteins involved in in vitro binding of cytoskeleton elements. In vitro interactions between mitochondria and microtubules or neurofilaments were analyzed by electron microscopy. These studies revealed cross-bridging between the outer membrane of mitochondria and the two cytoskeleton elements. Cross-bridging was influenced by ATP hydrolysis and by several proteins associated with the surface of mitochondria or with microtubules. In addition, unidentified proteins which were recognized by antibodies to all intermediate filaments subunits were associated either with the mitochondrial surface or with microtubules. This data suggest the participation of additional cytoplasmic proteins in the interactions between cytoskeleton elements and mitochondria.

Spatial re-arrangement of the vesicle apparatus in forebrain synapses of chicks 30 min after passive avoidance training

A quantitative ultrastructural study of synapses was carried out in the forebrain IMHV (intermediate and medial hyperstriatum ventrale) of 1-day-old chicks 30 min after training to avoid pecking at a bead coated with methyl anthranilate. In 10 birds (5 control and 5 trained), the length, curvature and the number of the synaptic active zone profiles were measured, and the active zone profile length was observed to increase in trained chicks. The spatial arrangement of synaptic vesicles with respect to the active zone was examined using a statistical stereological approach. This showed that vesicles are not located uniformly but accumulate in two spatial pools which appear to rearrange following training, with a greater number of vesicles near the active zone. These data may reflect subtle changes in the functional efficacy of synapses in the IMHV in the initial phases of memory formation.

Cytoskeleton-mediated, age-dependent lateral topography of lectin-gold-labelled molecules on the plasma membrane of cultured neurons: a statistical view

In dissociated spinal cord neurons (12-day-old mouse embryo, monolayer culture), an electron microscopic study was carried out to examine quantitatively the rearrangement of wheat-germ agglutinin-gold-labelled molecules on the neuronal somatic surface at two developmental stages (on the fifth and 15th days in vitro), and after cytoskeletal interruptions. In tests, before labelling the cultures were incubated with colchicine or cytochalasin in order to affect microtubules or mostly actin filaments, respectively. Samples of electron micrographs that display soma membrane (profile) fragments were quantified. A set of stochastic geometry approaches was accomplished, which allowed statistical and stereological analysis of labelling. Images that illustrate the lateral (surface) patterns of label were simulated. On the fifth day in vitro, both colchicine and cytochalasin were found to cause an increase in the surface density and aggregation of wheat-germ agglutinin label relative to controls, the effect of cytochalasin being significantly more profound. By the 15th day in vitro, treatment with both drugs led to a similar tendency towards heavy aggregation of wheat-germ agglutinin labels. In contrast, neuron processes showed an opposite tendency of label rearrangement, which suggests lateral migration of labelled molecules, as a result of drug action. Possible molecular mechanisms involved in the phenomena are discussed.