Differential susceptibility to ageing of rat preganglionic neurones projecting to the major pelvic ganglion and of their afferent inputs.

We have analysed age-related changes in the morphology of preganglionic neurones in the lumbosacral spinal cord, labelled following injection of retrograde tracers into the major pelvic ganglion of young adult and aged male rats. We have also examined changes in neurotransmitter-characterised spinal afferent inputs to these neurones, or to the nuclei in which they lie, using light and electron microscope immunohistochemistry. In previous investigations of the major pelvic ganglion, the sympathetic, but not parasympathetic, postganglionic neurones were seen to exhibit age-related changes and the same pattern is seen in the preganglionic neurones. This included an apparent reduction in the numbers of sympathetic preganglionic neurones, and a reduction in the length of their dendrites and the complexity of their branches. Ultrastructural immunohistochemical studies described here reveal significant reductions in the area of synaptic contact made by glutamate-immunoreactive boutons onto the dendrites of sympathetic (but not parasympathetic) preganglionic neurones, while contacts from boutons immunoreactive for glycine or gamma-aminobutyric acid (GABA) were unchanged. There is also a reduction in synaptic contacts received by sympathetic somata from boutons immunoreactive for none of these amino acids. Serotonin-immunoreactive terminals are closely associated with preganglionic autonomic neurones, and these are reduced in number in sympathetic, but not parasympathetic, spinal nuclei of aged rats. However, serial section electron microscopy has so far failed to demonstrate conventional synaptic contacts between serotonergic terminals and the dendrites or somata of the preganglionic autonomic neurones. In young animals, axon terminals immunoreactive for thyrotropin-releasing hormone (TRH) are abundant in all spinal laminae including area X, but in aged animals, such terminals are significantly reduced in number in regions containing preganglionic sympathetic, but not parasympathetic, neurones. These results indicate that the sympathetic preganglionic neuron populations that project to the major pelvic ganglion, and the spinal inputs they receive, show a number of degenerative changes in aged rats which are not seen parasympathetic preganglionic neuronal populations.