Two α2-adrenergic receptor subtypes, α2A and α2C, 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.

Alpha2A-Adrenoceptors are Important Modulators of the Effects of D-Amphetamine on Startle Reactivity and Brain Monoamines

“…In the absence of a 2A -AR, or during pharmacological blockade of all a 2 -AR subtypes with an antagonist, the NE stores in noradrenergic neurons are susceptible to depletion by D-amph. This corroborates the evidence from a 2 -AR subtype-deficient mice revealing the principal role of the a 2A -AR in regulating the stimulation-induced NE release in cortical and hippocampal slices in vitro Trendelenburg et al, 1999Trendelenburg et al, , 2001Scheibner et al, 2001;Bücheler et al, 2002) and in the prefrontal cortex in vivo . Furthermore, the observed contribution of the a 2 -ARnoradrenergic system to modulation of the effects of D-amph is in good agreement with two earlier extensive studies on genetically modified mice, reporting increased sensitivity to psychostimulants when the brain NE homeostasis is disturbed, that is, in mice lacking the NE transporter (Xu et al, 2000) or incapable of synthesizing NE (Weinshenker et al, 2002).…”

“…The decreased DOPAC formation, reflecting reduced intraneuronal metabolism of DA resulting from reuptake blockade of DA by Damph, thus, is not affected by a 2A -AR. On the contrary, the formation of HVA occurs mainly extracellularly, and the elevated HVA concentrations of a 2A -KO mice may therefore represent increased DA release in the absence of a 2A -AR heteroreceptor-mediated inhibition (Bücheler et al, 2002). Supporting this view, the HVA levels of WT mice after combined Ati-D-amph treatment were elevated to the level observed in a 2A -KO mice after D-amph alone, except in the striatum, where the a 2C -AR subtype predominates.…”

Activation of α2 Adrenergic Receptors Suppresses Fear Conditioning: Expression of c-Fos and Phosphorylated CREB in Mouse Amygdala

“…a-2 adrenergic agonists and fear conditioning M Frances Davies et al fear encoding (Bucheler et al, 2002). Indeed, we have found that a 2A AR-deficient mice exhibit a higher level of freezing than controls in the discrete test (Davies et al, 2003).…”

“…Noradrenergic neurons within the nucleus tractus solitarius (NTS) project to the central nucleus (Asan, 1998), and the activation of the NTS (Zardetto-Smith and Gray, 1990) contributes to an increased norepinephrine release in the amygdala and enhances memory (Williams et al, 2000). Norepinephrine release from the ascending noradrenergic tracts is also under presynaptic control of both a 2C and a 2A ARs (Bucheler et al, 2002), where they act as an autoreceptors on noradrenergic terminals. The absence of a 2A AR-mediated presynaptic inhibition in a 2A ARÀdefi-deficient mice would likely promote a stronger release of norepinephrine evoked by the US to promote discrete cue On day 1 wild-type mice were either injected with saline or dexmedetomidine (20 mg/kg i.p.)…”

JP‐1302: a new tool to shed light on the roles of α2C‐adrenoceptors in brain

“…In contrast, mice that are homozygous for a 2C gene deletion or overexpression are viable and have been reasonably well-characterized. Overall, both strains of mouse confirm that a 2C is not responsible for the classical effects of non-selective a 2 -adrenoceptor agonists (see Kable et al, 2000), although the subtype does have some influence on neurotransmitter release (Bucheler et al, 2002;Zhang and Ordway, 2003), even if less so than does a 2A (Bucheler et al, 2002). However, the transgenic phenotypes make definition of the potential clinical applications of a 2C -selective ligands somewhat problematic.…”