Norepinephrine attenuates serotonin inhibition of pial venous tone

Serotonin 5-HT(7) receptors mediate relaxation of porcine pial veins

Isolated porcine pial veins in the presence of active muscle tone have been shown to exhibit rhythmic contractions (RC) that are inhibited by serotonin (5-HT) in a concentration-dependent manner. The 5-HT inhibition of RC is mediated by an as yet unidentified 5-HT receptor subtype located on the vascular smooth muscle. 5-carboxamidotryptamine, which is a potent but nonselective agonist at 5-HT(7) receptors, has been shown to be the most potent inhibitor of RC in porcine pial veins. Therefore, the present study was designed to determine if the 5-HT-mediated inhibition of RC in pial veins is mediated by 5-HT(7) receptors and if 5-HT(7) receptor mRNA is expressed in endothelium-denuded pial veins; the study was done with the use of an in vitro tissue bath and RT-PCR techniques. Our findings indicated that 5-HT inhibition of RC in porcine pial veins was prevented by 5-HT(7)-receptor antagonists (clozapine, pimozide, and LY-215840) in a concentration-dependent manner. Furthermore, a strong PCR signal for the 5-HT(7) receptor was consistently detected in endothelium-denuded pial veins. Sequence analysis of the amplified products confirmed their high degree of homology with the porcine and/or human 5-HT(7)-receptor gene. Taken together, these data suggest that the 5-HT-induced inhibition of RC in porcine pial veins is at least in part mediated by 5-HT(7) receptors located on the venous smooth muscle.

Mechanism of prejunctional muscarinic receptor-mediated inhibition of neurogenic vasodilation in cerebral arteries

Nitric oxide (NO) is a major transmitter in mediating cerebral neurogenic vasodilation in several species. Recent findings have suggested that acetylcholine, which is costored with NO in cerebral perivascular nerves, plays a role in modulating NO release, presumably by acting on muscarinic (M) receptors on nitric oxidergic nerve terminals. The present study was designed using an in vitro tissue bath technique to pharmacologically characterize the presynaptic muscarinic-receptor subtype(s) that mediate modulation of NO release and therefore neurogenic vasodilation and to investigate further the possible mechanisms involved in this presynaptic modulation in porcine basilar arteries. Transmural nerve stimulation (TNS) elicited a frequency-dependent, tetrodotoxin-sensitive relaxation. The relaxation was abolished by nitro-L-arginine (30 microM) and was completely reversed by L-arginine and L-citrulline, but not by their D-enantiomers. Atropine (0.01-1 microM), pirenzepine (an M1-receptor antagonist, 0. 01-1 microM), and methoctramine (an M2-receptor antagonist, 0.01-1 microM), but not 4-DAMP (an M3-receptor antagonist) or tropicamide (an M4-receptor antagonist) at concentrations as high as 10 mM, significantly increased the TNS-elicited relaxation. This relaxation, on the other hand, was significantly attenuated by arecaidine but-2-ynyl ester tosylate (an M2-receptor agonist, 0.1 microM) but was not affected by McN-A-343 (an M1-receptor agonist, 1 microM). Double-labeling immunohistochemical study demonstrated that perivascular M2 receptor-immunoreactive fibers were completely coincident with NADPH diaphorase fibers. Furthermore, the muscarinic receptor-mediated modulation of TNS-elicited relaxation was completely prevented by omega-conotoxin GVIA (0.1 microM), a specific N-type Ca2+ channel inhibitor, but was still observed in the presence of tetraethylammonium (1 mM), 8-bromo-cAMP (0.5 mM), and pertussis toxin. It is concluded that the presynaptic M2 receptors on porcine cerebral perivascular nitric oxidergic nerves mediate inhibition of NO release. The inhibition is due primarily to a decreased Ca2+ influx through N-type Ca2+ channels.