?-Adrenoceptor-mediated inhibition of noradrenaline release in rabbit brain cortex slices

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Abstract
Brain cortex slices from rabbits were preincubated with [3H]noradrenaline and then superfused and stimulated electrically at 3Hz. In the presence of cocaine 30 μM, unlabelled noradrenaline, α-methylnoradrenaline, clonidine, oxymetazoline, xylazine and guanabenz decreased, whereas yohimbine, corynanthine, phentolamine, tolazoline and azapetine increased the stimulation-evoked overflow of tritium. Phenylephrine and prazosin had no effect on the evoked overflow except at concentrations that greatly accelerated the basal outflow of tritium. The results indicate that the noradrenergic axons of rabbit brain cortex are endowed with presynaptic α-adrenoceptors which are exclusively of the α2-type. Addition of various concentrations of cocaine, addition of pargyline, or stimulation at different current strengths was used to obtain either a high or a low stimulation-evoked overflow of tritium. Independently of the method used, a low evoked overflow coincided with a large percentage inhibition produced by 0.1μM clonidine, whereas a high evoked overflow coincided with a smaller percentage inhibition produced by clonidine. The results indicate that drugs which block the re-uptake of noradrenaline diminish the presynaptic inhibitory effect of α-adrenergic agonists by increasing the biophase concentration of released noradrenaline.