This paper shows that Paramecium has GABAB receptors that can influence its behavior. The presence of those receptors is shown by immunochemistry. Using agonists, it is found that their activation inhibits the avoiding reaction; specifically, it reduces the duration of backward swimming in a depolarizing solution. This effect is partially suppressed by blockers of L-type calcium channels, therefore the mechanism seems to be that the activation of GABAB receptors inhibits the ciliary calcium channels responsible for the avoiding reaction.

Additionally, the authors show that paramecia release GABAB upon stimulation (using again a depolarizing solution of 40 mM KCl; which is probably not a very ecological stimulus). This is apparently similar to the physiology of GABAB in mammals (neurotransmitter release upon stimulation, and modulation of calcium channels by activated receptors).

What was not demonstrated, but would be very interesting to find out, is whether this GABAB release is sufficient to induce behavioral or physiological changes in neighboring paramecia, or even on itself.

A natural question is: what is it for? To understand this, one would probably need to characterize in more detail the conditions for GABAB release. One important thing to keep in mind is that paramecia swim very fast, much faster than the diffusion of any molecule. This means that once a paramecium releases something in the environment, that thing will essentially stay there while the paramecium moves on. Therefore, that thing will be a sort of marker placed in the environment. Suppose now that GABAB is released when the paramecium does an avoiding reaction. Then when it comes back to that location, the avoiding reaction will be inhibited, and therefore it will swim forward instead of changing direction. This may require the concentration to build-up over multiple passes of the same or different paramecia. Thus, this would confer some exploratory behavior, which could be helpful to exit a maze for example. The behavior would depend on the timescale of the effect, which would need to be measured.