RT Journal Article
SR Electronic
T1 Studies of the Biogenic Amine Transporters. 14. Identification of Low-Efficacy “Partial” Substrates for the Biogenic Amine Transporters
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 251
OP 262
DO 10.1124/jpet.111.188946
VO 341
IS 1
A1 Richard B. Rothman
A1 John S. Partilla
A1 Michael H. Baumann
A1 Catrissa Lightfoot-Siordia
A1 Bruce E. Blough
YR 2012
UL http://jpet.aspetjournals.org/content/341/1/251.abstract
AB Several compounds have been identified that display low-efficacy, “partial substrate” activity. Here, we tested the hypothesis that the mechanism of this effect is a slower rate of induced neurotransmitter efflux than that produced by full substrates. Biogenic amine transporter release assays were carried out in rat brain synaptosomes and followed published procedures. [3H]1-methyl-4-phenylpyridinium (MPP+) was used to assess release from dopamine (DA) and norepinephrine nerve terminals, whereas [3H]5-hydroxytryptamine (5-HT) was used to assess release from 5-HT nerve terminals. A detailed time-course evaluation of DA transporter (DAT)-mediated efflux was conducted by measuring the efflux of [3H]MPP+ after the addition of various test compounds. In vivo microdialysis experiments compared the effects of the full substrates [(±)-1-(2-naphthyl)propan-2-amine (PAL-287) and (S)-N-methyl-1-(2-naphthyl)propan-2-amine (PAL-1046)], to that of a partial DAT/5-HT transporter substrate [(S)-N-ethyl-1-(2-naphthyl)propan-2-amine (PAL-1045)] on extracellular DA and 5-HT in the nucleus accumbens of the rat. The in vitro release assays demonstrated that partial substrate activity occurs at all three transporters. In the DAT efflux experiments, d-amphetamine (full substrate) promoted a fast efflux (K1 = 0.24 min−1) and a slow efflux (K2 = 0.008 min−1). For the partial DAT substrates, K1 = ∼0.04 min−1, and K2 approximated zero. The in vivo microdialysis experiments showed that the partial substrate (PAL-1045) was much less effective in elevating extracellular DA and 5-HT than the comparator full substrates. We conclude that low-efficacy partial DAT substrates promote efflux at a slower rate than full substrates, and “partiality” reflects the ultra-slow K2 constant, which functionally limits the ability of these compounds to increase extracellular DA. We speculate that partial biogenic amine transporter substrates bind to the transporter but are less effective in inducing conformational changes required for reverse transport activity.