Specific binding of 3H-guanfacine to alpha 2-adrenoceptors in rat brain. Comparison with 3H-clonidine.

1. 3H-clonidine and the new antihypertensive drug, 3H-guanfacine (N-amino-2-[2,6-dichlorophenyl]-acetamide hydrochloride), bind with a high affinity to alpha 2-adrenoceptors in the rat central nervous system. 2. Dissociation curves for 3H-clonidine and 3H-guanfacine binding indicate the presence of high and low affinity binding sites. 3. In various rat brain regions, guanfacine is 0.5 to 8 times more selective for alpha-2 adrenoceptors than clonidine. 4. In the corpus striatum, dopamine is 7 times more potent at 3H-clonidine sites than at 3H-guanfacine sites. 5. In conclusion, the radioligand 3H-guanfacine appears more selective for alpha 2-adrenoceptors than 3H-clonidine.