Two alpha(2)-adrenergic receptor subtypes, alpha(2A) and alpha(2C), inhibit transmitter release in the brain of gene-targeted mice

Abstract

alpha(2)-Adrenergic receptors play an essential role in regulating neurotransmitter release from sympathetic nerves and from adrenergic neurons in the CNS. However, the role of each of the three highly homologous alpha(2)-adrenergic receptor subtypes (alpha(2A), alpha(2B), alpha(2C)) in this process has not been determined unequivocally. To address this question, the regulation of norepinephrine and dopamine release was studied in mice carrying deletions in the genes encoding the three alpha(2)-adrenergic receptor subtypes. Autoradiography and radioligand binding studies showed that alpha(2)-receptor density in alpha(2A)-deficient brains was decreased to 9 +/- 1% of the respective wild-type value, whereas alpha(2)-receptor levels were reduced to 83 +/- 4% in alpha(2C)-deficient mice. These results indicate that approximately 90% of mouse brain alpha(2)-receptors belong to the alpha(2A) subtype and 10% are alpha(2C)-receptors. In isolated brain cortex slices from wild-type mice a non-subtype-selective alpha(2)-receptor agonist inhibited release of [(3)H]norepinephrine by maximally 96%. Similarly, release of [(3)H]dopamine from isolated basal ganglion slices was inhibited by 76% by an alpha(2)-receptor agonist. In alpha(2A)-receptor-deficient mice, the inhibitory effect of the alpha(2)-receptor agonist on norepinephrine and dopamine release was significantly reduced but not abolished. Only in tissues from mice lacking both alpha(2A)- and alpha(2C)-receptors was no alpha(2)-receptor agonist effect on transmitter release observed. The time course of onset of presynaptic inhibition of norepinephrine release was much faster for the alpha(2A)-receptor than for the alpha(2C)-subtype. After prolonged stimulation with norepinephrine, presynaptic alpha(2C)-adrenergic receptors were desensitized. From these data we suggest that two functionally distinct alpha(2)-adrenergic receptor subtypes, alpha(2A) and alpha(2C), operate as presynaptic inhibitory receptors regulating neurotransmitter release in the mouse CNS.