Proliferation of arteriovenous anastomoses in the developing rabbit ear is enhanced after denervation

The innervation of the rabbit ear vasculature by noradrenergic and substance P-immunoreactive (SP-IR) nerves was investigated in both young and adult animals. All vascular segments were supplied by both noradrenergic and SP-IR nerves. In the ear margins, the arteriovenous anastomoses (AVAs) were more densely innervated than other vessels. In general, the density of both noradrenergic and SP-IR nerves increased with age until 10-12 weeks postnatum. The ear vasculature was denervated in young rabbits to test whether the normal proliferation of AVAs in the growing ear was altered in the absence of nerves. Surgical resection of all auricular nerve trunks except the auricular branch of the auriculo-temporal nerve, and removal of the ipsilateral superior cervical ganglion in 10- to 12-day-old rabbits left the ear devoid of both noradrenergic and SP-IR nerves for at least four weeks. Repeated administration of 6-hydroxydopamine (6-OHDA) to young rabbits produced degeneration of noradrenergic terminal axons, but preterminal nerve trunks survived. SP-IR nerves did not appear to be affected by 6-OHDA. These denervation regimes resulted in a 1.5- to 2-fold increase in the number of AVAs formed in the growing ear. Surgical denervation and 6-OHDA treatment also led to retarded growth of the media of the central ear artery, but this effect of 6-OHDA was probably not due to a specific action on vascular smooth muscle. Surgical resection of most of the dorsal auricular nerves in adult rabbits did not affect AVA density. Large differences in AVA density were apparent between groups of control animals from different parts of the country, or groups examined at different times of the year. These results demonstrate that the labile nature of AVAs in the rabbit ear can result in considerable variability in the absolute number of AVAs and suggest that both intrinsic and extrinsic factors may influence development of the microvasculature.