THE TIME COURSE OF THE DEVELOPMENT OF THE SYMPATHETIC INNERVATION OF THE VASCULATURE OF THE RAT TAIL

The development of the sympathetic innervation of the tail vasculature in the rat has been examined using catecholamine fluorescence and immunohistochemical techniques to demonstrate tyrosine hydroxylase (TH) and neuropeptide Y (NPY). The tail was found to be largely devoid of noradrenergic terminals at birth. At the earliest ages, axons within nerve trunks and paravascular axon bundles showed high levels of catecholamine fluorescence, but this virtually disappeared as the innervation of the effectors was achieved. The perivascular plexus on the caudal artery was established over the first six postnatal weeks along a rostrocaudal gradient which was retained in the adult, i.e. proximal regions were more densely innervated than distal ones. The innervation of the rest of the vasculature developed relatively late during this period, with the exception of the arteriovenous anastomoses present in the distal half of the tail. These became innervated about 10 days earlier than the adjacent caudal artery at the same levels, and received a much denser innervation in the adult. At all developmental stages, distributions of TH- and NPY-immunoreactive nerve fibres were identical to those seen with catecholamine fluorescence. The sequence of development suggests that the different vascular targets are innervated by subsets of sympathetic neurones having the same neurochemistry but developing independently.