Abstrakt: |
Effects of l-threo-3,4-dihydroxyphenylserine (l-threo-DOPS) on the spontaneous release and the stimulus(2 Hz)-evoked release of endogenous noradrenaline were studied in rat hypothalamic slices with functioning l-aromatic amino acid decarboxylase (AADC) and with AADC inhibition. In non-inhibited slices, spontaneous release was not modified by l-threo-DOPS at 1 Pm-100Nm, tended to increase at 1–10 μmand increased at 100 μm. Noradrenaline tissue content slightly increased at 100 μm. Stimulated release was concentration-dependently facilitated at 1–1000 Pmand tended to decrease gradually from a maximum at 10 Nm-10 μm. Under AADC inhibition, spontaneous release concentration-dependently increased at 10–100 μmby 60% of the increase seen in slices without AADC inhibition. Increase in noradrenaline tissue content was abolished. l-threo-DOPS produced a triphasic pattern on stimulated release; concentration-dependent facilitation at 1–1000 Pmsimilar to that seen in slices with functional AADC, no facilitation at 10–1000 Nm, and a concentration-dependent increment at 10–100 μm. The facilitation at 1 Nmwas stereoselective and was antagonized by (–)-propranolol 10 Nm, and no facilitation at 100 Nmwas restored to the maximum by yohimbine 10 Nm, DG-5128 10 Nmor S-sulpiride 1 Nm. Furthermore, l-threo-DOPS (1–1000 Pm)-induced facilitation was competitively antagonized by l-dopa methyl ester, a competitive antagonist for l-dopa, with a pA2value of 13·6, whereas it was noncompetitively antagonized by (–)-propranolol. |