PD-168077 maleate is a selective dopamine D4 receptor agonist, with a Ki of 9 nM. PD 168077 (0.2-25.0 mg/kg) induced nonstereotyped shuffling locomotion with uncoordinated movements, jerking, and yawning, which were insensitive to antagonism by CP-293,019, L-745,870, or haloperidol[1]. PD-168077 is one of the first agents to be identified as putative selective D4 agonists. It shows >100-fold selectivity over other members of the D2-like receptor family and over their D1-like counterparts; PD-168077 shows a 20-fold selectivity over α1, and α2, a 45-fold selectivity over 5-HT1A, and a 460-fold selectivity over 5-HT2A receptors; PD-168077 evidences intrinsic activity at the D4 receptor in terms of quinpirole-like inhibition of forskolin-stimulated cAMP accumulation or stimulation of [3H]thymidine uptake in CHO cells expressing the human D4 receptor[1]. In the PD-168077-treated cell, p-CaMKII exhibits a significantly increased clustering at synaptic sites, as indicated by the enhanced colocalization with PSD-95[2]. PD-168077 (1-200 ng) induced penile erection in a dose-dependent manner. The minimal effective dose was 50 ng, while the maximal response was found with 200 ng of the compound, which increased penile erection episodes from 0.3+/-0.03 to 1.7+/-0.21[3]. Apomorphine (0.1microg) and PD-168077 (0.1microg) induced penile erection episodes, which occurred concomitantly to an increase in extra-cellular dopamine and DOPAC (3,4-dihydroxyphenylacetic acid) concentration in the dialysate from the shell of the nucleus accumbens, as measured by intracerebral microdialysis. Stimulation of dopamine receptors (mainly of the D2 to D4 subtype) in the paraventricular nucleus induces the release of oxytocin in brain areas that influence the activity of mesolimbic dopaminergic neurons mediating the appetitive and reinforcing effects of sexual activity[4].
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