5HT1A serotonin receptor agonists inhibit Plasmodium falciparum by blocking a membrane channel

Abstract

To identify new leads for the treatment of Plasmodium falciparum malaria, we screened a panel of serotonin (5-hydroxytryptamine [5HT]) receptor agonists and antagonists and determined their effects on parasite growth. The 5HT1A receptor agonists 8-hydroxy-N-(di-n-propyl)-aminotetralin (8-OH-DPAT), 2,5-dimethoxy-4-iodoamphetamine, and 2,5-dimethoxy-4-bromophenylethylamine inhibited the growth of P. falciparum in vitro (50% inhibitory concentrations, 0.4, 0.7, and 1.5 microM, respectively). In further characterizing the antiparasitic effects of 8-OH-DPAT, we found that this serotonin receptor agonist did not affect the growth of Leishmania infantum, Trypanosoma cruzi, Trypanosoma brucei brucei, or Trichostrongylus colubriformis in vitro and did not demonstrate cytotoxicity against the human lung fibroblast cell line MRC-5. 8-OH-DPAT had similar levels of growth inhibition against several different P. falciparum isolates having distinct chemotherapeutic resistance phenotypes, and its antimalarial effect was additive when it was used in combination with chloroquine against a chloroquine-resistant isolate. In a patch clamp assay, 8-OH-DPAT blocked a P. falciparum surface membrane channel, suggesting that serotonin receptor agonists are a novel class of antimalarials that target a nutrient transport pathway. Since there may be neurological involvement with the use of 8-OH-DPAT and other serotonin receptor agonists in the treatment of falciparum malaria, new lead compounds derived from 8-OH-DPAT will need to be modified to prevent potential neurological side effects. Nevertheless, these results suggest that 8-OH-DPAT is a new lead compound with which to derive novel antimalarial agents and is a useful tool with which to characterize P. falciparum membrane channels.

FIG. 1.

Dose-dependent inhibition of P. falciparum growth by 8-OH-DPAT. FUP (chloroquine-sensitive), FVO, (chloroquine-resistant), Indochina (chloroquine-resistant), and Thailand (multidrug-resistant) isolates were used to demonstrate the growth-inhibitory effects of 8-OH-DPAT. Percent inhibition was determined with a [³H]hypoxanthine incorporation assay.

FIG. 2.

Blocking of an ion channel on the parasitophorous vacuole membrane of P. falciparum by a 5HT1A agonist, as shown by patch clamp recordings of parasitized erythrocytes. (A) Control baseline recordings showing noncontinuous sweeps of current recorded with the pipette potential clamped at −100 mV (transmembrane potential not known). Each line shows every third sweep from a series of 50 sweeps. The channel is open when the amplitude of the recording is increased and closed (blocked) when the recording is flat. (B) Recordings 2 min after the 5HT1A agonist 8-OH-DPAT (0.1 mg/ml) was added to the bath under the same conditions as the control (A).