Comparison of nerves to cerebral and extracerebral blood vessels: A differential effect of alpha methyl tyrosine on norepinephrine content

Alpha methyl tryosine (AMT), an inhibitor of norepinephrine (NOR) synthesis, was injected in traperi toneall y (200 mg/kg) in Sprague Dawley rats, kept in a cold room, or at room temperature for 16 hours. Using formaldehyde induced NOR fluorescence, nerve counts were made on whole mounts of cerebral and femoral arterioles 14-300 m in diameter, utilizing a grid superimposed on the vessels. Cold had no effect on the number of visible (i.e. fluorescing) nerves. AMT had an appreciable effect but only on nerves to femoral arterioles, where a significant reduction in nerve count was observed in both cold stressed and non stressed rats, when compared with animals not given AMT. Since the counting technique is sensitive only to large depletions of NOR, we cannot conclude that AMT failed to affect NOR content in cerebrovascular nerves. However, if such an effect was present, it was much less than the effect of AMT on nerves to femoral vessels. We suggest that the differential effect of AMT on these 2 vascular beds may indicate a lower basal level of NOR release from cerebrovascular nerves, which would correlate with the difficulty of demonstrating basal sympathetic tone in this vascular bed.