Learning in the Single-Cell Organism Physarum polycephalum: Effect of Propofol

Abstract

Propofol belongs to a class of molecules that are known to block learning and memory in mammals, including rodents and humans. Interestingly, learning and memory are not tied to the presence of a nervous system. There are several lines of evidence indicating that single-celled organisms also have the capacity for learning and memory which may be considered as basal intelligence. Here, we introduce a new experimental model for testing the learning ability of Physarum polycephalum, a model organism frequently used to study single-celled "intelligence". In this study, the impact of propofol on Physarum's "intelligence" was tested. The model consists of a labyrinth of subsequent bifurcations in which food (oat flakes soaked with coconut oil-derived medium chain triglycerides [MCT] and soybean oil-derived long chain triglycerides [LCT]) or propofol in MCT/LCT) is placed in one of each Y-branch. In this setting, it was tested whether Physarum memorized the rewarding branch. We saw that Physarum was a quick learner when capturing the first bifurcations of the maze; thereafter, the effect decreased, perhaps due to reaching a state of satiety. In contrast, when oat flakes were soaked with propofol, Physarum's preference for oat flakes declined significantly. Several possible actions, including the blocking of gamma-aminobutyric acid (GABA) receptor signaling, are suggested to account for this behavior, many of which can be tested in our new model.

Keywords: Physarum polycephalum; anesthetics; learning; memory; propofol; single-celled organism.

Figure 1

The 5-level Y-maze: (A) Exemplary course of an experiment with Physarum in a 5-level Y-maze. Starting with a rectangular chamber, Physarum spreads out along successive bifurcations. At each of the first 4 bifurcations, Physarum’s favorite food, an oat flake, is placed at the tip of the left arm (see white arrows). The oat flakes are either soaked with MCT/LCT or Propofol-^®Lipuro. The fifth bifurcation is devoid of an oat flake. (B) Probability representation of Physarum´s directional decisions at each bifurcation in the 5-level Y-maze with oat flakes soaked with MCT/LCT or (C) Propofol-^®Lipuro. The red lines represent the decision to the left (oat) and the blue lines to the right (no oat). The dashed black lines indicate the mean, and the grey dotted lines indicate the confidence intervals of two times the standard deviation. Values greater or less than this limit indicate significance (* = p < 0.05). The graphs represent 50 experiments for each condition (MCT/LCT or Propofol-^®Lipuro).

Figure 2

The 1-level Y-maze: (A) Exemplary experiment with Physarum in a dual 1-level Y-maze. Starting with a rectangular chamber, Physarum spreads out into the arms of the Y. A depot consisting of 3 µL of MCT/LCT was placed at the tip of the left arm; correspondingly, a depot consisting of 3 µL of Propofol-^®Lipuro was placed at the tip of the right arm. (B) Percentage representation of Physarum´s directional decisions (total 30 experiments).

Figure 3

Transcript alignment to GABA receptor. Alignment section of the first hit between Physarum transcript 03383 and the protein sequence O88871 coding for the GABA type B receptor subunit 2 in Rattus norwegicu.

Figure 4

Experimental set-up. Acrylic glass container providing a humidified and semisterile atmosphere equipped with a heightened base carrying the experimental maze. Above this, attached to a stand controlled by an automated remote control, a camera took photographs.