Repartition and drug sensitivity of dopamine and L-isoproterenol-sensitive adenylate cyclases in rat brain homogenates

Abstract

The characteristics of dopamine, 1-isoproterenol, and d-LSD stimulated adenylate cyclases were studied in homogenates of fresh or frozen tissues. In rat striatum, when the assay was done in the presence of 1 mM MgSO4, dopamine (10(-4) M) stimulated the enzyme activity by 3.5-fold. This effect was completely blocked by fluphenazine (10(-5) M; Ki = 9 X 10(-9) M) and by phentolamine (Ki = 3 X 10(-7) M). d-LSD stimulated the adenylate cyclase activity (Km = 1.4 X 10(-7) M) by interacting with the dopaminergic receptors. Maximal adenylate cyclase stimulation by d-LSD was 1.4-fold; as a matter of fact, this compound acted as a partial agonist on the dopaminergic receptors. l-Isoproterenol (Km = 10(-6) M) activated an adenylate cyclase present in rat striatum homogenates through a receptor distinct from the dopaminergic receptor; this stimulation was not affected by addition of fluphenazine or phentolamine but suppressed by dl-propranolol (10(-4) M). The topographical distributions of dopamine adenylate cyclase activity and endogeneous dopamine content were examined in homogenates prepared from discs punched out from serial frozen (-7 degrees C) slices of the striatum. A 4.8-fold progressive decrease in the amount of cyclic AMP produced in the presence of dopamine (10(-4) M) was observed from the rostral to the caudal part of the structure. The d-LSD-sensitive adenylate cyclase followed a similar distribution. It should be noted that the topographic distribution of endogeneous dopamine is quite comparable to the distribution of the dopamine-sensitive adenylate cyclase, suggesting that this enzyme is an integral part of the dopamine synapses. We also reported that the frontal cortex contains a dopamine-sensitive adenylate cyclase. In conclusion, we trust that the micromethod described for adenylate cyclase assay will be of some use in the study of the precise topographic distribution of catecholamine sensitive adenylate cyclases in different structures of brain.