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

Abstract

α 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 α 2-adrenergic receptor subtypes (α 2A, α 2B, α 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 α 2-adrenergic receptor subtypes. Autoradiography and radioligand binding studies showed that α 2-receptor density in α 2A-deficient brains was decreased to 9±1% of the respective wild-type value, whereas α 2-receptor levels were reduced to 83±4% in α 2C-deficient mice. These results indicate that approximately 90% of mouse brain α 2-receptors belong to the α 2A subtype and 10% are α 2C-receptors. In isolated brain cortex slices from wild-type mice a non-subtype-selective α 2-receptor agonist inhibited release of [ 3H]norepinephrine by maximally 96%. Similarly, release of [ 3H]dopamine from isolated basal ganglion slices was inhibited by 76% by an α 2-receptor agonist. In α 2A-receptor-deficient mice, the inhibitory effect of the α 2-receptor agonist on norepinephrine and dopamine release was significantly reduced but not abolished. Only in tissues from mice lacking both α 2A- and α 2C-receptors was no α 2-receptor agonist effect on transmitter release observed. The time course of onset of presynaptic inhibition of norepinephrine release was much faster for the α 2A-receptor than for the α 2C-subtype. After prolonged stimulation with norepinephrine, presynaptic α 2C-adrenergic receptors were desensitized. From these data we suggest that two functionally distinct α 2-adrenergic receptor subtypes, α 2A and α 2C, operate as presynaptic inhibitory receptors regulating neurotransmitter release in the mouse CNS.