All three α₂-adrenoceptor types serve as autoreceptors in postganglionic sympathetic neurons

Abstract

Postganglionic sympathetic neurons and brain noradrenergic neurons use α_2A- and α_2C-adrenoceptors as presynaptic autoreceptors. The present experiments were carried out in order to see whether they possess presynaptic α_2B-autoreceptors as well. Pieces of atria, vasa deferentia, the occipito-parietal cortex and the hippocampus were prepared from either wildtype (WT) mice or mice in which both the α_2A- and the α_2C-adrenoceptor gene had been disrupted (α_2ACKO). The pieces were incubated with ³H-noradrenaline and then superfused and stimulated electrically. In a first series of experiments, single pulses or brief, autoinhibition-poor pulse trains were used for stimulation. The α₂-adrenoceptor agonist UK 14,304 (brimonidine) reduced the evoked overflow of tritium in all four tissues from WT mice but did not change it in any tissue from α_2ACKO mice. A different pattern was obtained with medetomidine as α₂ agonist. Like UK 14,304, medetomidine reduced the evoked overflow of tritium in all four tissues from WT mice and did not affect overflow in brain slices from α_2ACKO mice; however, in contrast to UK 14,304, medetomidine reduced evoked overflow also in atrial and vas deferens pieces from α_2ACKO mice, although with a lower maximum and potency than in WT preparations. The α-adrenoceptor antagonists rauwolscine, phentolamine, prazosin, spiroxatrine and WB 4101 shifted the concentration-response curve of medetomidine in α_2ACKO atria and vasa deferentia to the right. The pK_d values of the five antagonists against medetomidine in α_2ACKO atria and vasa deferentia correlated with pK_d values at prototypical α_2B radioligand binding sites but not at α_2A or α_2C binding sites. In a second series of experiments, autoinhibition-rich pulse trains were used for stimulation. Under these conditions, rauwolscine and phentolamine increased the evoked overflow of tritium from α_2ACKO atrial and vas deferens pieces but not from α_2ACKO brain slices. The increase was smaller (by 40% in atria and by 70% in the vas deferens) than previously observed in WT preparations (by 200–400%). In a last series of experiments, mRNA for the α_2B-adrenoceptor was demonstrated by RT-PCR in thoracolumbar sympathetic ganglia from WT, α_2AKO, α_2CKO and α_2ACKO mice but not from α_2BKO mice. The results show that brain noradrenergic neurons express only α_2A- and α_2C-adrenoceptors as autoreceptors. Postganglionic sympathetic neurons, however, can express α_2B-adrenoceptors as presynaptic autoreceptors as well. The α_2B-autoreceptors are activated by medetomidine but not by UK 14,304. They are also activated by previously released noradrenaline. The two-α₂-autoreceptor hypothesis has to be replaced by a three-autoreceptor hypothesis for postganglionic sympathetic neurons.