Specialised sympathetic neuroeffector associations in immature rat iris arterioles

Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature ra ts occurs via the activation of a,,adrenoceptors alone, while in immature r at iris arterioles, vasoconstriction occurs via activation of both a,and al pha(2)-adrenoceptors. In mature rats the vast majority of sympathetic varic osities form close neuroeffector junctions. Serial section electron microsc opy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay withi n 4 mu m of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment wi th 5-hydroxydopamine. 47 % of varicosities formed close associations with m uscle cells and 88 % formed close associations with muscle cells or melanoc ytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distrib ution of synaptic vesicles in single varicosities did not show the asymmetr ic accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restric tion of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of post synaptic responses.