Partial depletion of neuropeptide Y from noradrenergic perivascular and cardiac axons by 6-hydroxydopamine and reserpine.

The effects of 6-hydroxydopamine (6-OHDA) and reserpine on the storage of neuropeptide Y (NPY) in noradrenergic cardiovascular nerves were examined with both immunohistochemistry and radioimmunoassay (RIA). Immunohistochemical double-labelling techniques demonstrated that NPY was located only in noradrenergic axons in the guinea-pig carotid artery, mitral valve, thoracic inferior vena cava, thoracic aorta, superior mesenteric artery and small saphenous vein. Treatment with 6-OHDA in vivo eliminated noradrenergic, NPY-containing axon terminals from all tissues, but preterminal axons were still prominent in the superior mesenteric artery. The greatest depletion of NPY detected by RIA after 6-OHDA treatment was found in tissues with a predominance of terminal noradrenergic axons, such as the small saphenous vein, whereas NPY accumulating in preterminal axons masked the loss of NPY from terminal axons in the superior mesenteric artery. After treatment with doses of reserpine that led to a rapid depletion of noradrenaline (NA) from perivascular nerves, NPY was still detected histochemically at all times although levels sometimes appeared to be reduced. RIA demonstrated that the partial depletion of NPY after reserpine consisted of a rapid phase seen in the vena cava and saphenous vein after the highest doses, and a slower phase of NPY depletion from all tissues after all doses of reserpine. The greatest depletion of NPY from terminal axons by reserpine (in small saphenous vein) was 85-90%. These results demonstrate that some NPY can be stored in noradrenergic perivascular axons in the absence of noradrenaline, but that partial depletion of NPY from axon terminals results when NA stores are depleted by reserpine. The variation in extent of NPY depletion between tissues after drug treatments can be explained by variation in the ratio of preterminal to terminal axons.