Ultrastructural study of the effects of 6-hydroxydopamine on the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis)

The present study examined the ultrastructural changes in the sinuatrial and atrioventricular nodes of the heart of the monkey (Macaca fascicularis) after 6-hydroxydopamine (6-OHDA) at survival times of 1, 3, 5, 7 and 14 days. Changes ranged from dissolution of the cytoplasm to amorphous appearance and darkening of the nodal cells. Initially, the ultrastructural changes were quite similar in both sinuatrial (SA) and atrioventricular (AV) nodal cells but in the later stage, especially at fourteen days, affected SA nodal cells showed empty-looking appearance while affected AV nodal cells displayed a darkened appearance. The cardiac neurons also showed ultrastructural changes such as diffuse accumulation of glycogen particles, distended cisternae of the rough endoplasmic reticulum and increased lipofuscin granules. Some of the vacuolated axonal profiles containing large dense-cored vesicles were in close association with the somata of the cardiac neurons. There were also changes in the non-neuronal cells such as darkening and vacuolation of the cells capping the neurons. Macrophages and Schwann cells were activated to engulf the degenerating nodal cells and axonal profiles. The ultrastructural changes in the nodal cells and the cardiac neurons reflect a disturbance in the cell metabolism presumably brought about by the impairment of the sympathetic nervous system.