The Vasocontractive Action of Norepinephrine and Serotonin in Deep Arterioles of Rat Cerebral Gray Matter

To examine the direct effects of norepinephrine (NE) and serotonin (5-HT) on the contractility of arterioles in the gray matter of the rat cerebrum, we micro-perfused arterioles in vitro and observed the changes in luminal diameter under the stop-flow condition with constant intraluminal pressure. While the average diameter of the lumen of arterioles was 39.9±9.7 μm (n=7) in Hepes-buffered saline, the average in 10−7 M NE in the extraluminal solution changed into smaller in saline by 21.1±5.4% (n=7). The contractile effect of NE shows a dose-dependent curve between the 10−7 and 10−5 M. The contractile response to 10−6 M NE was significantly reduced by yohinbin, an α2 blocker. 10−6 M NE applied to the lumen also caused contraction of arterioles by 12.4±5.3% in diameter (n=5). 5-HT at 10−7 M in the extraluminal solution caused contraction of arterioles by 10.9±4.4% in diameter (n=7). 5-HT in the extraluminal solution caused contraction of arterioles in a dose dependent manner between 10−10 and 10−6 M. The contractile effect of 5-HT at 10−6 M was strongly reduced by 10−6 M ketanserin, a 5-HT2 receptor antagonist. 5-HT applied to the lumen had no effect at all (n=6), however NE applied to the lumen caused contraction. These results strongly suggest that 5-HT plays a significant role in arteriolar contractility only from the cerebrospinal fluid (CSF) side, while NE is an important regulator of arteriolar contractility from both the CSF and blood circulation sides.