Abstract
Previous studies in our laboratory /3,17/ have demonstrated that serotonin (5-HT) appears to have a trophic-like effect in enhancing synapse formation and maintenance in both the developing and the adult central nervous system. In the present study, we focused on age-related changes in the density of the axosomatic and axodendritic synapses and the number of 5-HT-positive fibers in the chicken spinal cord, with special reference to differences between the ventral (laminae VII and IX) and the dorsal (lamina I) horn. At 1 week posthatching (PIW), a transient overproduction of synapses and 5-HT-immunoreactive fibers occurred in lamina IX; all parameters had returned to their initial levels by 1 month post-hatching (PIM). The density of synapses further decreased by about 40% between P6M and P2Y (2 years posthatching). Although the magnitude of the transient increase in lamina VII was less than that in lamina IX, the changing pattern of the synapses and the 5-HT-positive fibers was similar in both regions. In the ventral horn, thin 5-HT-positive fibers were most prominent at P1W and then decreased with development; thin 5-HT-positive fibers were still found at P6M but had almost disappeared by P2Y. By contrast, at P2Y the density of the synapses and the 5-HT-positive fibers in the dorsal horn was even higher than that of younger animals.Reduction of 5-HT levels in P2Y-old chickens by p-chlorophenylalanine (pCPA) administration decreased the synaptic density in lamina I but not in lamina IX. The results of this study demonstrate that 5-HT-mediated synaptic plasticity is markedly different in the ventral and dorsal horns of the aged chicken. In the ventral horn, synaptic plasticity reached a maximum at about P1W, remained stable in the young-adult period, and then finally disappeared in the aged chicken. Conversely, the results suggest that in the dorsal horn, 5-HT fibers continue to mediate the trophic influence on synaptic plasticity even in the old chicken.
Highlights
Chubakov and colleagues/4/first demonstrated using in vitro preparations that serotonin (5-HT)has a trophic-like role in facilitating the formation of chemical synapses
The results of this study demonstrate that 5-HT-mediated synaptic plasticity is markedly different in the ventral and dorsal horns of the aged chicken
The synaptic density decreases in a dose-dependent fashion in the presence of 5-HT depletors with different pharmacokinetics; at the maximum dose, almost 70% of the synapses disappear within one week after the decrease in the 5-HT level
Summary
Chubakov and colleagues/4/first demonstrated using in vitro preparations that serotonin (5-HT). Has a trophic-like role in facilitating the formation of chemical synapses. Subsequent in vivo studies in our laboratory /17,3/ have shown that 5-HT enhances synapse formation and maintenance in both the developing and the mature central nervous system (CNS). The synaptic density decreases in a dose-dependent fashion in the presence of 5-HT depletors with different pharmacokinetics; at the maximum dose, almost 70% of the synapses disappear within one week after the decrease in the 5-HT level. To exclude the possibility that synaptic loss occurs by nonspecific side effects of 5-HT depletors, Niitsu et al /16/ identified the 5-HT L. Receptor subtype mediating the facilitator role of 5-HT. The number of synapses in the ventral horn of the spinal cord could be artificially manipulated by administering a 5-HT2A antagonist or agonist or both
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