Long-term chemical sympathectomy leads to an increase of neuropeptide Y immunoreactivity in cerebrovascular nerves and iris of the developing rat.

Short-term (surgical) and long-term (chemical) sympathectomy have revealed the presence of a population of neuropeptide Y-like immunoreactive nerve fibres which do not degenerate in parallel with noradrenaline-containing nerves supplying cerebral vessels and the iris of the rat. Two days after bilateral removal of the superior and middle cervical ganglia of 7-week-old rats, noradrenaline-containing nerves could not be detected along any of the arteries of the rat circle of Willis or of the iris, but 18-32% of neuropeptide Y-like immunoreactive nerves remained. Long-term treatment (6 weeks) with guanethidine commencing in developing 1-week-old rats caused degeneration of the sympathetic neurons in cervical ganglia and disappearance of 5-hydroxydopamine-labelled nerves (that showed dense-cored vesicles at the electron microscope level) from rat cerebral vessels, but did not significantly change the density of neuropeptide Y-like immunoreactive axons on the vessels. Furthermore, whilst in control rats neuropeptide Y-like immunoreactivity was localized largely within 5-hydroxydopamine-labelled cerebrovascular nerves, after long-term sympathectomy with guanethidine, neuropeptide Y-like immunoreactivity was seen only in nerves lacking small dense-cored vesicles. A small number of catecholamine-containing nerves appeared along the internal carotid and anterior cerebral arteries after long-term sympathectomy; these may arise from neurons of central origin. These results suggest that as a consequence of long-term sympathectomy with guanethidine, compensatory changes occur, involving an increase in the expression of neuropeptide Y-like immunoreactivity in non-sympathetic axons in cerebrovascular nerves and iris of the rat. In contrast, the neuropeptide Y-like immunoreactive nerves in the dura mater appear to be entirely sympathetic, since none were present after short-term sympathectomy and none appeared after long-term sympathectomy.