tokens: In brain membranes from spontaneously hypertensive rats clonidine , 10 (- 8 ) to 10 (- 5 ) M , did not influence stereoselective binding of [3H]-naloxone ( 8 nM ), and naloxone , 10 (- 8 ) to 10 (- 4 ) M , did not influence clonidine - suppressible binding of [3H]-dihydroergocryptine ( 1 nM ). These findings indicate that in spontaneously hypertensive rats the effects of central alpha - adrenoceptor stimulation involve activation of opiate receptors .