Opposite effects of glucocorticoid receptor activation on hippocampal CA1 dendritic complexity in chronically stressed and handled animals.

Remodeling of synaptic networks is believed to contribute to synaptic plasticity and long-term memory performance, both of which are modulated by chronic stress. We here examined whether chronic stress modulates dendritic complexity of hippocampal CA1 pyramidal cells, under conditions of basal as well as elevated corticosteroid hormone levels. Slices were prepared from naïve, handled or chronically stressed animals and briefly treated with vehicle or corticosterone (100 nM); neurons were visualized with a fluorescent dye injected into individual CA1 pyramidal cells. We observed that 21 days of unpredictable stress did not affect hippocampal CA1 apical or basal dendritic morphology compared with naïve animals when corticosteroid levels were low. Only when slices from stressed animals were also exposed to elevated corticosteroid levels, a significant reduction in apical (but not basal) dendritic length became apparent. Unexpectedly, animals that were handled or 3 weeks showed a reduction in both apical dendritic length and number of apical branch points when compared with naïve animals. Apical dendritic length and number of branch points were restored to levels found in naïve animals several hours after in vitro treatment with 100 nM corticosterone. All effects of acute corticosterone administration could be prevented by the glucocorticoid receptor antagonist RU38486 given during the last 4 days of the stress or handling protocol. We conclude that brief exposure to high concentrations of corticosterone can differently affect apical dendritic structure, depending on the earlier history of the animal, a process that critically depends on involvement of the glucocorticoid receptor.

Changing synaptic connections on cell bodies of growing identified spinal motoneurons of the eel, Anguilla.

As the target musculature they innervate grows throughout life, certain segmental motoneurons from the spinal cord of Anguilla, readily identified on the basis of their form and position, also increase in size. In doing so, they present a steadily increasing target to the spinal and supraspinal neurons that innervate them. How the afferent neurons respond was assessed by measuring features of their synaptic boutons contacting the motoneuronal perikarya, as seen with electron microscopy. About 60% of the perimeter of the perikaryal profile of each motoneuron was found to be covered with synaptic bouton profiles, a value that is independent of the size of the motoneuron. Furthermore, the distances between synaptic profiles, their contact sizes (measured as apposition length) and the number and size of the vesicles each profile contains were all found to be relatively constant and also independent of motoneuronal size. In contrast, the number of synaptic profiles contacting a motoneuron correlated well with its perikaryal size. Our findings indicate that the challenge of a growing neuronal target is met by a steady increase in the number of contacting boutons, the form and spacing of which are held relatively constant; this strategy will require continual synaptic realignment at the target.

Changes in size and number of spinal motoneurons in relation to growth of the musculature in the eel, Anguilla.

Estimates of the numbers of spinal motoneurones in relation to growth in the eel, Anguilla, were made by counting the axons contained within the ventral roots. Motoneuronal size was determined as the surface area of cell somata labelled retrogradely from applications of tracer (horseradish peroxidase, cobalt lysine) to the musculature or spinal nerves. The sizes and numbers of muscle fibres from red and white portions of the myotomal musculature were determined from frozen sections. Measurements of motoneurone size and number in relation to the size and number of the muscle fibres were obtained from the same body region for eels of body lengths ranging from 65 to 930 mm and for 3 different stages of the eel's life history (glass, yellow and silver eels). The mean sizes and the numbers of red and white muscle fibres, and motoneuron size, increase substantially in relation to body length; the numbers of axons in the ventral roots of fish of different body lengths, however, are very similar (approx. 130 per half segment). Silver eels are indistinguishable from yellow eels of a similar size with respect to the size and number of motoneurone somata and axons and to the number of muscle fibres. In silver eels red muscle fibres are similar in size to those of yellow eels, but the white fibres are larger.

Fusion of myocytes with larval muscle remnants during flight muscle development in the Colorado beetle.

The transformation of the slow contracting larval m. obliquus lateralis caudalis II during metamorphosis into the asynchronous indirect flight muscle, m. obliquus lateralis dorsalis, in the Colorado beetle, Leptinotarsa decemlineata, was examined by electron microscopy. Particular attention was paid to the fate of the larval muscle fibres, the origin and behaviour of the myoblasts for flight muscle development and the change of the myofibrillar filament lattice of the larva into that of the adult. In the pre-pupal period, the larval muscles dedifferentiate and fragment. At pupation, the muscle fibres consist of cell fragments containing very few myofibrils. The sarcoplasmic reticulum and the transverse tubular system are greatly reduced. The number of myoblasts developed from satellite cells by mitosis increases considerably. They penetrate the muscle fibre and surround the cell fragments. The new fibres of the flight muscle develop from myocytes fused with the larval fragments. The larval basal lamina, surrounding the cell fragments and myoblasts, is present in pupae up to 1 day old. In pupae about 2.5 days old new myofibrils appear that have the adult filament lattice. The insect muscle transformation and the repair of vertebrate muscle after injury show striking resemblances.