Pharmacological characterization of the inhibitory effects of neurotensin on the rabbit ileum myenteric plexus preparation.

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 there is co-secretion of noradrenaline, the evoked release of [³H]-amezinium is relatively small, yohimbine increases the release, and clonidine can cause only moderate inhibition. When there is no or very little co-secretion of noradrenaline, the evoked release of [³H]-amezinium is at least doubled, yohimbine causes no further increase and clonidine produces strong inhibition.