Search

The present study describes the major aspects of how monoaminergic (serotonin, dopamine) systems change in the course of regeneration of the brain in the earthworm (Eisenia fetida), investigated by immunocytochemistry, HPLC assay, and ligand binding. Following brain extirpation, the total regeneration time is about 80 days at 10°C. On the 3rd postoperative day serotonin, and on the 11th postoperative day tyrosine hydroxylase-immunoreactive neurons can be observed in the wound tissue. Thereafter the number of the immunoreactive cells increases gradually, and by the 76th–80th postoperative days all serotonin- and tyrosine hydroxylase-immunopositive neurons can be found in their final positions, similarly to those observed in the intact brain. Labeled neurons located in the dorsal part of the regenerated brain appear earlier than the cells in lateral and ventral positions. Both serotonin- and tyrosine hydroxylase-immunoreactive neurons of the newly formed brain seem to originate from undifferentiated neuroblasts situated within and around the ventral ganglia and the pleura. Dopaminergic (tyrosine hydroxylase-immunoreactive) elements may additionally derive from the proliferation of neurons localized in the subesophageal ganglion and the pharyngeal nerve plexus. Following brain extirpation, both serotonin and dopamine levels, assayed by HPLC, first increase in the subesophageal ganglion; by the 25th day of regeneration, the monoamine content decreases in it and increases in the brain. Hence it is suggested that monoamines are at least partly transported from this ganglion to the regenerating brain. At the same time, ³H-LSD binding can be detected in the regenerating brain from the 3rd postoperative day, showing a continuous increase until the 80th postoperative day, suggesting a guiding role of postsynaptic elements in the monoaminergic reinnervation of the newly formed brain. J. Exp. Zool. 296A:18–29, 2003. © 2003 Wiley-Liss, Inc.

Innervation of the γ straddler vibrissa and distribution of individual γ vibrissal sensory nerve fibers within infraorbital nerve branches were investigated in adult rats. In the close vicinity (2 mm) of the whisker-pad, one-fifth of the nerve fibers innervating the γ vibrissa were still located in nerves to neighboring (β, δ, C1 and D1) whiskers, while one-tenth of the innervating fibers were placed in nerves of more distant sinus hairs. This convergence of γ vibrissal sensory nerve fibers to single vibrissa from neighboring nerves is suggested to be a consequence of overlapping vibrissal representation in the Gasserian ganglion.

Reorganization of the cerebellar glomerulus, the main synaptic complex within the granule cell layer, was investigated using quantitative morphological techniques. All afferents to the cerebellar cortex, including mossy-fibers, were surgically destroyed by undercutting the cerebellar vermis. Fifteen days after the operation, which resulted in the removal of the main excitatory afferent to the glomerulus, a significant reorganization of the whole synaptic complex was observed, whereas the structural integrity of the glomerulus was remarkably well preserved. This was indicated by the observation that the number of granule cell dendrites (≈50 per glomerulus), as well as the number of dendritic digits (≈210 per glomerulus) bearing most of the ≈230 synaptic junctions per glomerulus, did not change significantly after mossy-fiber degeneration. The total number of synapses in the reorganized glomerulus did not change either, despite the disappearance of two-thirds of (excitatory) synaptic junctions caused by mossy-fiber degeneration. In the reorganized glomeruli, however, the inhibitory, GABA-containing Golgi axonal varicosities became the dominant synaptic type—about four-fifths (≈200) of all synapses within the glomerulus—whereas the dendritic synapses between the granule cells represented only one-fifth of all synaptic junctions. The quantitative data of the reorganized cerebellar glomerulus demonstrate both a remarkable constancy and a plasticity of he excitatory granule cells and inhibitory Golgi neurons building up this synaptic complex. constancy (the preservation of certain specific structural features) is represented by an eventually unchanged number of dendrites and synaptic junctions within the deafferented lomerulus. Such constancy was made possible, however, by the morphogenetic plasticity of both nerve-cell types to produce new, dendrodendritic and axo-dendritic synapses to compensate for the loss of mossy-fiber synapses.