Effects of (+)-4-propyl-9-hydroxynaphthoxazine on midbrain dopamine neurons: an electrophysiological study

(+)-4-Propyl-9-hydroxynaphthoxazine (PHNO) is a highly potent, D2-selective dopamine (DA) receptor agonist. In the present study, we have examined the electrophysiological effects of PHNO on identified nigrostriatal DA (NSDA), mesoaccumbens DA (MADA) and Type I caudate neurons. Intravenous PHNO dose-dependently inhibited the firing rate of NSDA and MADA neurons in both chloral hydrate (CH)-anesthetized rats (ED50 values = 21.2 +/- 1.2 and 26.5 +/- 1.4 ng/kg, respectively) and locally anesthetized, paralyzed rats (ED50 values = 105.0 +/- 1.4 and 109.1 +/- 1.4 ng/kg, respectively). PHNO was significantly more potent in the CH-anesthetized rats. There was a significant, positive correlation between basal firing rate and log ED50 for NSDA neurons in both preparations and for MADA cells only in CH-anesthetized animals. Neither pretreatment with the D1 receptor agonist SKF 38393 nor hemitransection of the forebrain altered the rate-dependent nature of PHNO-induced inhibition of NSDA neurons. Likewise, depletions of serotonin with either p-chlorophenylalanine or 5,7-dihydroxytryptamine failed to alter the rate-dependent PHNO-induced inhibition of NSDA neurons. Iontophoretically applied PHNO inhibited both NSDA and MADA neurons to a similar degree as either DA or the D2 agonist quinpirole. In contrast, the similar effects of PHNO and quinpirole on Type I caudate neurons were significantly different than those of DA. These results suggest that PHNO inhibits midbrain DA neurons via stimulation of somatodendritic autoreceptors and that PHNO exhibits an electrophysiological profile characteristic of D2 agonists. In contrast to quinpirole, however, the rate-dependent nature of PHNO-induced inhibition of these cells was resistant to modulation.