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Background: Dorsal root ganglion (DRG) is a structure that contains ~90% of somatic afferent neurons and only 5-10% of visceral afferent neurons. Within visceral afferent neurons is a subpopulation responsible for the heart innervations, known as cardiac spinal afferent neurons. Like many other cell types, the neurons constituting DRG are susceptible to changes related to ageing. Previous studies in rat have shown that the process of ageing does not affect the whole population of DRG neurons equally. In addition, there was no change in proportion of isolectin B4 (IB4)-positive (likely unmyelinated) and neurofilament 200 (N52)-positive (likely myelinated) neurons, while the total number of neurons is slightly decreased or remains unchanged. However, to the best of our knowledge, there are no studies about age-related changes of cardiac spinal afferent neurons. Objectives: The aim of this study was to investigate the effect of ageing on cardiac spinal afferent neurons in the rat. Materials and Methods: A patch loaded with retrograde tracer Fast Blue (FB) was applied to all chambers of the rat heart. Morphological and neurochemical characteristics of labeled cardiac spinal afferent neurons were assessed in young (2 months) and old (2 years) rats using IB4 and N52. Results: Our study shows that the number of cardiac spinal afferent neurons decreased with age and it was reduced to 15% of the total number of neurons found in young rats (1604 vs. 248). In addition, we have observed a difference in myelinated and unmyelinated neurons rate. The number of IB4- positive neurons increased significantly, whereas the proportion of N52-positive ones decreased significantly during the ageing process. The size of neuronal soma of IB4- positive neurons increased, while the size of N52-positive neurons remained unchanged. Conclusion: Unlike somatic spinal afferents, cardiac spinal afferent neurons underwent morphological and neurochemical changes during ageing process. Although there is a major decrease in total number of cardiac spinal afferent neurons, the innervation density and nociceptive capacity are preserved due to both, an increased proportion and compensatory growth of IB4-positive neurons. |
