[Effects of clonidine on the levels of cyclic nucleotides in rat brain regions: the change in cyclic nucleotides after clonidine alone or in combination with alpha-antagonists].

The following 4 Wistar rat groups were sacrificed by microwave irradiation after measuring blood pressure and heart rate: 1) saline control (1 ml/kg, i.v.), 2) clonidine alone (50 micrograms/kg, i.v.). 3) prazosin pretreated (0.1 mg/kg, i.v.), 4) yohimbine pretreated (1 mg/kg, i.v.). Cyclic nucleotides were analyzed in seven brain regions. Clonidine decreased blood pressure and heart rate 20 min after administration. These effects of clonidine were inhibited by yohimbine. Clonidine increased the levels of cyclic AMP in the medulla oblongata (including pons) and hypothalamus. Prazosin attenuated the cyclic AMP-increasing action of clonidine in the cerebellum. Yohimbine inhibited this action of clonidine in the cerebellum, striatum and hippocampus. Clonidine reduced cyclic GMP levels in the hypothalamus and striatum. Prazosin potentiated the cyclic GMP-reducing action of clonidine in the medulla oblongata. Yohimbine attenuated this action of clonidine in the medulla oblongata and hypothalamus. From these results, it is concluded that clonidine changes the levels of cyclic nucleotides in brain regions, especially in the part of the autonomic nervous center. In addition, it is indicated that alpha 1- or alpha 2-adrenoceptor plays a different role in the regulation of brain cyclic nucleotides, region by region.