Helfert R H, Sommer T J, Meeks J, Hofstetter P, Hughes L F
Department of Surgery, Southern Illinois University School of Medicine, Springfield 62794-9638, USA.
J Comp Neurol. 1999 Apr 12;406(3):285-98.
The central nucleus of the inferior colliculus (ICc) is a major processing center for the ascending auditory pathways. Gamma-aminobutyric acid (GABA) and excitant amino acids (EAAs) are essential for coding many auditory tasks in the IC. Recently, a number of neurochemical and immunocytochemical studies have suggested an age-related decline in GABAergic inhibition in the ICc, and possibly excitant-amino-acid-mediated excitation as well. The objective of this study was to compare quantitatively changes in the synaptic organization of the ICc among three age groups (3, 19, and 28 months) of Fischer-344 rats. Immunogold electron microscopic methods were used to determine if there were age-related changes in the density, distribution, or morphology of GABA-immunoreactive (+) and GABA-immunonegative (-) synapses in the ICc. The data suggest similar losses of excitatory and inhibitory synapses in the ICc. There were significant reductions in the densities of GABA+ and GABA- synaptic terminals (approximately 30% and approximately 24%, respectively) and synapses (approximately 33% and approximately 26%, respectively) in the ICc of 28-month-old rats relative to 3-month-olds. The numeric values, which were adjusted to consider changes in volume of the IC with age, depict similar effects, although the effect magnitude for the adjusted values was reduced by approximately 9%. For both types of synapses, the decreases did not differ significantly from each other. The reductions in synaptic numbers appeared, to be related to a similar numeric decline in dendrites, in particular those with calibers of between 0.5 and 1.5 microm. The number and distribution of synaptic terminals on the remaining dendrites of GABA- neurons appeared not to undergo major age-related changes. GABA+ neurons, on the other hand, may have evolved patterns of synaptic and dendritic change during aging in which the distribution of synaptic terminals shifts to dendrites of larger caliber. In the 19-month group, the synaptic areas were elevated in terminals apposed to dendrites with calibers of 1.5 microm or less. However, this increase in synaptic size did not persist in the aged animals. No neuronal losses were detectable among the three age groups. Thus, the decrease in GABA and EAAs identified in the IC by previous studies may be attributable to synaptic and dendritic declines, rather than cell loss.
下丘中央核（ICc）是听觉上行通路的主要处理中心。γ-氨基丁酸（GABA）和兴奋性氨基酸（EAA）对于IC中许多听觉任务的编码至关重要。最近，一些神经化学和免疫细胞化学研究表明，ICc中GABA能抑制存在与年龄相关的下降，兴奋性氨基酸介导的兴奋可能也会下降。本研究的目的是定量比较Fischer-344大鼠三个年龄组（3、19和28个月）ICc突触组织的变化。采用免疫金电子显微镜方法确定ICc中GABA免疫反应性（+）和GABA免疫阴性（-）突触的密度、分布或形态是否存在与年龄相关的变化。数据表明ICc中兴奋性和抑制性突触有类似的减少。相对于3个月大的大鼠，28个月大的大鼠ICc中GABA+和GABA-突触终末密度（分别约为30%和约24%）以及突触密度（分别约为33%和约26%）显著降低。考虑到IC体积随年龄的变化进行调整后的数值呈现出类似的效应，尽管调整后数值的效应幅度降低了约9%。对于这两种类型的突触，减少幅度彼此之间无显著差异。突触数量的减少似乎与树突数量的类似减少有关，特别是那些直径在0.5至1.5微米之间的树突。GABA-神经元其余树突上突触终末的数量和分布似乎未发生与年龄相关的重大变化。另一方面，GABA+神经元在衰老过程中可能已经形成了突触和树突变化的模式，其中突触终末的分布转移到了直径更大的树突上。在19个月大的组中，与直径为1.5微米或更小的树突相对的突触终末的突触面积增加。然而，这种突触大小的增加在老年动物中并未持续。在三个年龄组中未检测到神经元损失。因此，先前研究在IC中确定的GABA和EAA的减少可能归因于突触和树突的减少，而非细胞损失。
