Abstract

Compared with cloned, human (h)D₂ receptors (pK_i = 6.9), the antiparkinsonian agent piribedil showed comparable affinity for hα_2A- (7.1) and hα_2C- (7.2) adrenoceptors (ARs), whereas its affinity for hα_2B-ARs was less marked (6.5). At hα_2A- and hα_2C-ARs, piribedil antagonized induction of [³⁵S]guanosine-5′-O-(3-thio)triphosphate (GTPγS) binding by norepinephrine (NE) with pK_b values of 6.5 and 6.9, respectively. Furthermore, Schild analysis of the actions of piribedil at hα_2A-ARs indicated competitive antagonism, yielding a pA₂ of 6.5. At a porcine α_2A-AR-Gi1α-Cys351C (wild-type) fusion protein, piribedil competitively abolished (pA₂ = 6.5) GTPase activity induced by epinephrine. However, at a α_2A-AR-Gi1α-Cys351I (mutant) fusion protein of amplified sensitivity, although still acting as a competitive antagonist (pA₂ = 6.2) of epinephrine, piribedil itself manifested weak partial agonist properties. Similarly, piribedil weakly induced mitogen-activated protein kinase phosphorylation via wild-type hα_2A-ARs, although attenuating its phosphorylation by NE. As demonstrated by functional [³⁵S]GTPγS autoradiography in rats, piribedil antagonized activation by NE of α₂-ARs in cortex, amygdala, and septum. Antagonist properties were also expressed in a dose-dependent enhancement of the firing rate of adrenergic neurons in locus ceruleus (0.125–4.0 mg/kg i.v.). Furthermore, piribedil (2.5–4.0 mg/kg s.c.) accelerated hippocampal NE synthesis, elevated dialysis levels of NE in hippocampus and frontal cortex, and blocked hypnotic-sedative properties of the α₂-AR agonist xylazine. Finally, piribedil showed only modest affinity for rat α₁-ARs (5.9) and weakly antagonized NE-induced activation of phospholipase C via hα_1A-ARs (pK_b = 5.6). In conclusion, piribedil displays essentially antagonist properties at cloned, human and cerebral, rat α₂-ARs. Blockade of α₂-ARs may, thus, contribute to its clinical antiparkinsonian profile.