Synaptic remodelling of perforated synapses during development and maturation in a visual projection area in birds.

Perforated synapses are characterized by the appearance of one or more discontinuities in the postsynaptic density (PSD). These synapses are thought to represent intermediate stages in synaptic remodelling and turnover. To determine the dynamic level of these processes, the development of perforated synapses at different ages was examined in the neuropil of the ectostriatum, a visual projection area in birds. The overall number of perforated synapses per unit volume increases during development. Single and multiple perforated synapses, however, show a different developmental trend. Between 20 and 100 days, when the number of multiple perforated synapses decreases by 46%, single perforated synapses increase by 83% in number per unit volume. No significant changes in single or multiple perforated synapses can be observed at younger ages (i.e., 5 or 10 days). Various parameters increase between 10 and 20 days, followed by a reduction at 100 days: The relative frequency of perforations per synaptic contact, the length of the postsynaptic density including the size of the perforation, the length of the synaptic contact zone in perforated synapses excluding the size of the perforation, and the size of the perforation follow the same trend.

Monocular deprivation alters the development of synaptic structure in the ectostriatum of the zebra finch.

The effects of monocular deprivation, beginning at hatching, were examined in the neuropil of ectostriatum, a visual telencephalic projection area in birds. The volume of ectostriatum, the number of synapses and subsynaptic features like the presynaptic terminal size and the length of the postsynaptic contact zone were quantified in juvenile (20 d) and adult (100 d) zebra finches. Monocular deprivation affects almost all of the parameters mentioned above in juvenile birds, but only one (i.e., the size of presynaptic terminals) is permanently altered in adulthood. Both hemispheres are affected in juvenile birds with respect to the volume of ectostriatum, the length of synaptic contact zones and the presynaptic terminal size when compared to normal values. In normal birds the number of synaptic contacts is established at 20 days and remains fairly constant at 100 days. In experimental birds there is an increase in synapse number during this time period. However, no interhemispheric differences or differences compared to normal animals could be identified. The presynaptic terminals in experimental birds are smaller compared to normal values in young (25% for the deprived side; 19% for the non-deprived side) and adult (13% for the deprived side; 11% for the non-deprived side) animals. The only permanent effect caused by monocular deprivation in the ectostriatum is characterized by smaller presynaptic terminals. It is surprising that the tectofugal pathway that is believed to be mostly ipsilateral is not very vulnerable to monocular deprivation in adult animals. It is even more surprising that the deprivation effects are seen on both sides of the brain. The implications of this plasticity will be discussed in this paper.

Ultrastructural analysis of the development and maturation of synapses and subsynaptic structures in the ectostriatum of the zebra finch.

The development of synapses and subsynaptic features in the neuropil of the ectostriatum, a visual projection area in birds, was examined ultrastructurally at 5, 10, 20, and 100 days posthatching. The maturation of the synaptic complex is accompanied by a variety of different dynamic processes. The number of synapses in ectostriatum and the number of specific synaptic types vary with age as does the constellation of subsynaptic structures. At day 5, before eye opening, the total number of synapses is 16% of the adult value. These synapses, unlike synapses seen at maturity, have indistinct synaptic contact zones and generally are associated with few synaptic vesicles. Synapse number increases continuously until 20 days of age, paralleled by a steady increase of the observed brain volume. The largest increase in synapse number takes place during the time of eye opening (i.e., between 5 and 10 days). This increase is mainly due to an increase of asymmetric synapses, the most common type in the neuropil of ectostriatum (90% of the synapse population). At day 20 the number of synapses has reached its maximum and remains high in adulthood. Synapses on spines are more prominent in younger animals than in adults. The percentage of presynaptic terminals involved in synaptic contact with more than one postsynaptic element (multiple synapses) shows a significant reduction from 12% to 4% early in development (between days 10 and 20). Presynaptic terminal size and postsynaptic density (PSD) length increase until 20 days of age. From day 20 to adulthood the PSD shows a 10% reduction in contact length, and the presynaptic terminal further increases in size by 27%. Therefore, the pre- and postsynaptic structures described above continue to develop after the number of synapses remains constant.

Synaptic plasticity in the hypoglossal nucleus of female canaries: structural correlates of season, hemisphere, and testosterone treatment.

The caudal portion of the hypoglossal nucleus (nXIIts) contains the motor neurons that control the syrinx in songbirds. In canaries, song occurs seasonally, is principally produced by males, and appears to be produced predominantly by muscles on the left side of the syrinx. The present study measures the effect of seasonal change and manipulation of testosterone levels on synapse number and morphology in nXIIts in adult female canaries. We find that synapse density is lower in testosterone-treated birds than in control birds and lower in fall than in spring. The number of vesicles per presynaptic profile increases in the spring as a result of a general increase in this measure in all synapses. The number of vesicles per presynaptic profile also increases with testosterone treatment, primarily due to an increase in the proportion of synapses associated with unusually high vesicle counts. Together, these changes suggest that large reserves of neurotransmitter may be necessary to sustain singing. Several ultrastructural differences between hemispheres are found. Postsynaptic thickenings are longer, and postsynaptic processes are larger on the left side than on the right side. In the spring, there are more vesicles per synapse on the left than on the right, but this lateralization is reversed in the fall. Thus, lateralization of song production is associated with lateral asymmetries in synapse morphology. These hemispheric differences are relatively small, like those seen at the light microscope level, encouraging further consideration of peripheral as well as CNS sources of functional lateralization. The seasonal and testosterone-induced changes in synapse number and morphology may be components of the periodic reorganization of canary vocalization.

Morphology of Golgi-impregnated neurons in hyperstriatum ventralis, pars caudalis in adult male and female canaries.

Golgi-impregnated neurons in the song control nucleus hyperstriatum ventralis, pars caudalis (HVc) in male and female canaries (Serinus canarius) have been divided into classes, primarily on the basis of interneuronal variability in spine density and dendritic branching pattern. At least four neuronal classes are found in HVc: aspinous neurons and three classes of spiny neurons. The "furry" dendrite (FD) cell class consists of neurons with long dendrites that are densely packed with spines. Their cell bodies are between 10 and 15 microns in diameter. Neurons of the thick dendrite (TD) cell class also have long dendrites but only about half as many spines along their dendritic branches. Their cell bodies are between 12 and 18 microns in diameter. Neurons of the short dendrite (SD) cell class are characterized by a low spin density, very thin dendrites, and a small dendritic field. Their cell bodies are between 9 and 13 microns in diameter. The TD class can be divided into two subclasses on the basis of the shape of the dendritic field. Principal component factor analysis and cluster analysis provide objective support for this classification scheme. Neurons of subclass TD2 are sexually dimorphic. Neurons from males have dendritic trees that are about 70% larger and have 40% more dendritic endings than do neurons from females. There may also be small sex differences in dendritic morphology in the SD class and in the remainder of the TD class. There are clearly no sex differences in the dendritic morphology of neurons from the FD class. The direct pathway which is believed responsible for dimorphic song production in canaries is from HVc to nucleus robustus archistriatalis (RA) and then to the motor neurons which control the avian vocal organ. It is surprising that the most striking dimorphism in the present data occurs in neurons which, on morphological grounds, are unlikely to project to RA.

Afferent connections of the ectostriatum and visual wulst in the zebra finch (Taeniopygia guttata castanotis Gould)--an HRP study.

Afferent connections of the two main areas in the telencephalon, the visual wulst and the ectostriatum, were traced in the zebra finch by injection of horseradish peroxidase and staining with tetramethylbenzidine (TMB). Nuclei projecting to the hyperstriatum accessorium (HA) or the HIS region (lamina hyperstriatica intercalatus superior) were: (1) ipsilaterally the n. dorsalis anterior pars lateralis (DLL) with its two subdivisions DLLd and DLLv, the n. dorsolateralis anterior pars magnocellularis (DLAmc), and the area pretectalis (AP); (2) bilaterally the nucleus of the septomesencephalic tract (SPC) with the ipsilateral component coming from the medial, the contralateral component from the lateral part of the nucleus. As in the pigeon or the owl the ectostriatum of the zebra finch receives massive input, which is topographically ordered, from the n. rotundus. In addition to this pathway the ectostriatum receives additional visual input from the ipsilateral area pretectalis, the n. subrotundus and eventually a bilateral projection from the n. tegmenti pedunculopontinus pars compacta (TPC).