The Size, Number, and Fluorescence Intensity of 5HT-Immunoreactive Axon Terminals in the Aged Rat Lumbar Spinal Cord Motor Nucleus as Revealed by Confocal Fluorescence Microscopy and Computerized 3D Image Analysis

In this paper we describe changes in the morphology and distribution of serotoninergic (5HT)-like immunoreactive (IR) axon terminals In the aged rat spinal motor nucleus by use of immunofluorescence labeling and a computerized 3D analysis of image stacks recorded with a beam-scanning confocal microscope. The results are compared with corresponding data from young adult rats. 5HT-IR axonal profiles In the aged rat often had an altered morphology, with large, irregularly outlined profiles. Some of the 5HT-IR terminals in the aged rat had a complex appearance and extended numerous small processes; this may suggest that axonal proliferation also occurs. In several tissue samples the morphology of the axon profiles was altered to such an extent that they did not lend themselves to quantitative analysis. Quantitative analysis was performed on tissue samples showing moderate changes in the 5HT-IR axon plexus in the spinal cord of the aged rat. The results showed that the number of axon terminal-like objects was smaller by about 20% in the aged rat. The diameter of the terminals was, on average, 20% larger in the aged rat and the summed volume of axon terminal-like objects in a tissue stack was almost twice as large as that in young adult rats. The aggregated fluorescent signal of the axon terminals In a tissue stack, however, was similar in young adult and aged rats. Detailed analysis revealed that for most axon terminal diameter sizes, the fluorescent signal recorded in young adult rats was stronger than that recorded in aged rats.