Stimultaion of cyclic AMP formation by epinephrine and norepinephrine has been studied in discrete areas of rat brain that include the epinephrine-containing brain stem nuclei C-1 and C-2. In the C-1 area, epinephrine-stimulated cyclic AMP formation was partially reversed by 100 microM phentolamine and by 10--100 microM propranolol or alprenolol and hence appeared to involve activation of a mixture of both alpha- and beta-adrenergic receptors as has been reported for other rat brain areas such as the cerebral cortex. However, in the C-2-area, the epinephrine and norepinephrine stimulated cyclic AMP formation involved the activation of a single receptor type which was alpha-like in character. Stimulation of cyclic AMP formation by epinephrine in the C-2 area was antagonized by nanomolar concentrations of both phentolamine and yohimbine. The epinephrine-stimulated formation of cyclic AMP in the C-2 but not in the C-1 area was augmented in a strains of rats which exhibit spontaneous genetic hypertension (SHR) vs. Wistar-Kyoto controls. It is suggested that the enhanced epinephrine-stimulated cyclic AMP formation in the C-2 area of SHR rats could be a physiological compensatory response to some other hypertension-causing lesion which, for example, results in chronically reduced epinephrine release or in ruduced availability of epinephrine at its postsynaptic receptor thereby leading to receptor supersensitivity. Supporting this possibility was the finding that treatment of SHRs and control animals and reserpine resulted in enhancement of epinephrine-stimulated cyclic AMP formation in the C-2 area of control rats, essentially obliterating the difference between control and SHR. The findings are also interepreted as supporting the involvement of epinephrine neurons in central vaso-depressor mechanisms.
Adrenergic alpha-Antagonists Adrenergic beta-Antagonists