N-ethylmaleimide (NEM) diminishes alpha 2-adrenoceptor mediated effects on noradrenaline release

The effect of N-ethylmaleimide (NEM), which has been shown to abolish rather selectively inhibition of adenylate cyclase, on the alpha 2-adrenoceptor-mediated modulation of noradrenaline release was studied. Slices of the rabbit hippocampus were loaded with 3H-noradrenaline, superfused continuously and stimulated twice electrically. NEM (30 mumol/l) applied for 30 min enhanced both basal and stimulation-evoked tritium overflow significantly. Occupation of the receptor by the alpha 2-adrenoceptor agonist clonidine prior to and during NEM treatment did not protect the alpha 2-adrenoceptor-mediated autoinhibitory feedback system from being affected by NEM. Preincubation of the hippocampal slices with NEM was without any influence on 3H-noradrenaline uptake. The inhibitory effect of clonidine on 3H-noradrenaline release was attenuated in a non-competitive manner. In addition, the facilitatory effect of the alpha 2-adrenoceptor antagonist yohimbine on the stimulus-evoked tritium overflow was reduced. The facilitation of the evoked noradrenaline release by yohimbine or yohimbine or yohimbine and NEM converged with increasing concentrations of yohimbine, suggesting that yohimbine and NEM were acting at the same signal-transduction system. These results are compatible with the idea that NEM, by alkylating the Ni-unit of a presynaptically located adenylate cyclase, prevents the alpha 2-adrenoceptor-mediated modulation of noradrenaline release.