Release of [3H]‐amezinium from cortical noradrenergic axons: a model for the study of the α‐autoreceptor hypothesis

1 [3H]‐amezinium is taken up selectively into noradrenergic axons and their transmitter‐storing vesicles and is released from these axons by action potentials. We used it as a non‐α‐adrenergic marker in order to study the α‐adrenergic autoinhibition of noradrenaline release. 2 Rat occipitocortical slices were preincubated with [3H]‐amezinium 0.03 μm and then superfused and stimulated electrically (3 Hz for 3 min). The stimulation‐evoked overflow of tritium was measured in six groups of slices: from saline‐pretreated rats; from saline‐pretreated rats, the slices being exposed to exogenous noradrenaline before preincubation with [3H]‐amezinium; from saline‐treated rats, slices from which were exposed simultaneously to noradrenaline and cocaine before preincubation with [3H]‐amezinium; from rats in which noradrenaline stores had been depleted by pretreatment with α‐methyltyrosine (α‐MT); from α‐MT‐treated rats, the slices being exposed to noradrenaline before preincubation with [3H]‐amezinium; and from α‐MT‐treated rats, slices from which were exposed to noradrenaline plus cocaine before preincubation with [3H]‐amezinium. 3 The stimulation‐evoked overflow of tritium, expressed as a percentage of the tritium content of the tissue, was 1.15% in slices from saline‐pretreated rats, and was similar in slices from saline‐pretreated rats after exposure to noradrenaline or noradrenaline plus cocaine. It was 2.56% in slices from α‐MT‐treated rats, 1.20% from α‐MT‐treated rats after exposure to noradrenaline, and 2.88% from α‐MT‐treated rats after exposure to noradrenaline plus cocaine. 4 Yohimbine 0.1 and 1 μm increased the stimulation‐evoked overflow of tritium in slices from all groups of saline‐pretreated rats and in those slices from α‐MT rats that had been in contact with exogenous noradrenaline. Yohimbine did not change the evoked overflow in slices from α‐MT rats that had not been exposed to noradrenaline, or had been exposed to noradrenaline plus cocaine. 5 Clonidine 0.01 — 1 μm decreased the stimulation‐evoked overflow of tritium moderately in slices from saline‐pretreated rats, markedly in slices from α‐MT‐treated rats, and moderately again when the latter slices had been exposed to noradrenaline. 6 It is concluded that the action potential‐evoked release of [3H]‐amezinium as well as the modulation of this release by yohimbine and clonidine depend on the presence or absence of α‐adrenergic autoinhibition caused by the co‐secretion of noradrenaline. When ther