II. Pharmacological studies with derivatives of 2-aminotetralin, benzhydro[f]quinoline, benzhydro[g]quinoline, apomorphine and clonidine suggest a pharmacological dissimilarity between peripheral presynaptic dopamine receptors and alpha-2 adrenoceptors.

This study demonstrates that presynaptic dopamine receptors and alpha-2 adrenoceptors are pharmacologically different. A series of 2-aminotetralins, benzhydro[f]quinolines, benzhydro[g]quinolines, apomorphine and clonidine were studied to determine if they could stimulate presynaptic alpha-2 adrenoceptor and dopamine receptors. Presynaptic dopamine receptor activity was observed in di- and monohydroxy derivatives of 2-aminotetralins, dihydroxy derivatives of benzohydro[f]quinolines and benzohydro[g]quinolines and apomorphine. The greatest presynaptic dopamine receptor activity was observed with agents which maintained the dopamine moiety in the trans coplanar conformation. In contrast to these observations 1) monohydroxy derivatives of 2-aminotetralines were devoid of presynaptic alpha-2 adrenoceptor activity and 2) both cis and trans isomers of dihydroxy derivatives of benzohydro[f]quinolines and benzohydro[g]quinolines exhibited significant presynaptic alpha-2 adrenoceptors activity. These data suggest that presynaptic alpha-2 adrenoceptors and dopamine receptors represent separate functional entities. A discussion on the structure activity relationship associated with presynaptic alpha-2 adrenoceptor and dopamine receptor is provided.