In vitro and in vivo approaches to the characterization of the α2-adrenoceptor

1 In order to more fully understand the role of the α₂-adrenoceptor in brain function, a combination of in vitro and in vivo techniques were utilized including radioligand binding, autoradiography, brain microdialysis and antisense oligonucleotides.

2 Binding studies showed the tritiated form of the selective α₂-adrenoceptor antagonist, RX821002 (methoxy-idazoxan) labelled an apparent single population of sites in rat brain membranes with high affinity (1 nM), for which prazosin had low affinity (1107 nM). Similar studies in rabbit brain membranes found that prazosin and oxymetazoline were able to displace. [³H]-RX821002 in a biphasic manner indicating the presence of subtypes of α₂-adrenoceptors.

3 Receptor autoradiography revealed a distribution of [³H]-RX821002 binding in rat brain consistent with the labelling of all %aL₂-adrenoceptor subtypes, namely α_2A/D-, α_2B and α_2C.

4 In rat, in vivo brain dialysis experiments demonstrated peripherally administered RX821002 elevated basal noradrenaline in frontal cortex and also, although to a lesser extent, in ventral hippocampus. RX821002 was also able to elevate extracellular dopamine in the striatum.

5 A 7-day i.c.v. infusion of an antisense oligonucleotide targeting the α_2A/D-adrenoceptor, resulted in a significant reduction in the autoradiographic density of [³H]-RX821002 binding in specific brain areas, notably the lateral septal nuclei and anterior hypothalamic area.

6 Several years of research by our group has extended our knowledge of the pharmacology and function of the α₂-adrenoceptor and has provided evidence of the roles of this receptor in the control of monoamine turnover. The successful use of antisense technology to knockdown expression of the α_2A/D subtype provides future opportunities to explore the physiology of this receptor subtype.