RESOLUTION OF DOPAMINE AND BETA-1-ADRENERGIC-SENSITIVE AND BETA-2-ADRENERGIC-SENSITIVE ADENYLATE-CYCLASE ACTIVITIES IN HOMOGENATES OF CAT CEREBELLUM, HIPPOCAMPUS AND CEREBRAL-CORTEX

The stimulation of adenylate cyclase by dopamine and various β-adrenergic agonists has been investigated in homogenates from 3 areas of cat brain: the cerebral cortex, cerebellum and hippocampus. The purpose of the study was to determine whether the β-adrenergic receptors coupled to adenylate cyclase could be classified as either β1 and β2 subtypes in the different regions studied. The stimulation of adenylate cyclase by the β-adrenergic agonist, (-)isoproterenol (5 × 10-6 M), was completely blocked by the specific β-adrenergic antagonist, (-)alprenolol (10-5 M), but not by the dopaminergic antagonist, fluphenazine (10-5 M), whereas the stimulation of adenylate cyclase by (-)epinephrine (10-4 M) was blocked to varying extents by these two drugs in each of the 3 regions studied. The (-)epinephrine effect was always blocked in the combined presence of (-)alprenolol and fluphenazine. The adenylate cyclase stimulation by (-)epinephrine which is not blocked by (-)alprenolol was due to interaction of (-)epinephrine with a dopaminergic-sensitive adenylate cyclase which has been characterized in cerebral cortex, hippocampus and cerebellum. Regional differences in the affinity of β-adrenergic-sensitive adenylate cyclase for various agonists were investigated in the presence of fluphenazine (10-5 M). In the cerebellum the potency order was (±)protokylol > (±)hydroxybenzylisoproterenol > (±)isoproterenol > (-)epinephrine > (±)salbutamol > (-)norepinephrine, indicating the presence of a β2-adrenergic receptor. In the cerebral cortex the potency order was (-)isoproterenol > (±)protokylol > (±)hydroxybenzylisoproterenol > (-)epinephrine = (-)norepinephrine ((±)salbutamol being inactive). A similar pattern was found in the hippocampus indicating the presence of a β1-adrenergic receptor in these two regions. (±)Salbutamol was a partial agonist in the cerebellum and a competitive antagonist in the cerebral cortex. The ratio of the antagonist potencies of (±)practolol and (±)butoxamine preferential β1- and β2-adrenergic antagonists respectively, to block the stimulation of adenylate cyclase was 25 in the cerebellum, compared to 0.5 in the cerebral cortex and 1.6 in the hippocampus. These results confirm the presence of a β2 subtype of receptor coupled to adenylate cyclase in the former and β1 subtypes in the latter two regions. The comparison between the affinities of a series of β-adrenergic agonists and antagonists for the β-adrenergic receptors coupled with an adenylate cyclase in cerebral cortex and cerebellum with their affinities for well characterized β2-adrenergic receptors in lung and β1-adrenergic receptor in heart substantiated this conclusion. © 1979.