Receptor protection experiments confirm the identity of presynaptic alpha 2-autoreceptors

Receptor protection experiments were carried out in cerebrocortical slices from rabbits in order to study the sites at which drugs with alpha-adrenoceptor affinity modulate the release of noradrenaline. The slices were preincubated with 3H-noradrenaline. They were then superfused with 3H-noradrenaline-free medium and stimulated electrically (3 Hz) twice for 2 min each, after 60 and 250 min of superfusion (S1, S2). Phenoxybenzamine was added from 85 to 95 min of superfusion. Potential protecting drugs were present for 5 min before and during the exposure to phenoxybenzamine and then washed out together with the latter. Phenoxybenzamine 0.1 and 1 mumol/l increased the evoked overflow of tritium by 77 and 287%, respectively, as indicated by the S2/S1 overflow ratio. When cocaine was present throughout superfusion, phenoxybenzamine 0.1 and 1 mumol/l increased the evoked overflow by 97 and 353%, respectively. Clonidine 0.1-100 mumol/l, when added before and during the contact with phenoxybenzamine, reduced or even abolished the increase caused by the latter. This interaction was not changed when cocaine was included in the superfusion fluid. The increase caused by phenoxybenzamine was also reduced or abolished by noradrenaline 1-100 mumol/l (tested in the presence of cocaine), yohimbine 0.01-1 mumol/l and phentolamine 0.1-10 mumol/l. Only high concentrations of clonidine, noradrenaline, yohimbine and phentolamine changed the evoked overflow when given alone (and subsequently washed out). The effect of phenoxybenzamine was not modified by prazosin 1 mumol/l, morphine 1 mumol/l and naloxone 10 mumol/l.(ABSTRACT TRUNCATED AT 250 WORDS)