The human endocannabinoid system (ECS) modulates many vital physiological and neuromodulatory functions. It comprises lipid endocannabinoids, e.g. Anandamide (AEA), which are synthesized on demand and which bind to receptors (e.g. CB1, CB2) to elicit a response. Single-celled protists can respond to endocannabinoids despite not possessing any of the known cannabinoid receptors suggesting they possess a rudimentary ECS with an unknown target for the endocannabinoids.
The ciliate Tetrahymena thermophila possesses a suite of endocannabinoids (including AEA), with N-palmitoylethanolamide (PEA) and N-oleoylethanolamide (OEA) being two of the dominant compounds. In humans, the natural receptor for the latter is the peroxisome proliferator-activated receptor alpha (PPARα) but this receptor has not been identified in the T. thermophila genome. It may be that Tetrahymena spp. possess a ‘PPAR-like’ receptor or they possess a different target, which mediate a reaction to these endocannabinoids.

This study examined the potential role of ‘PPARs’ in the ECS of Tetrahymena pyriformis. Cell death was examined in the presence of agonists to each of the three known PPAR types, i.e. OEA/PEA/AEA/Cannabidiol (CBD) (to PPARα), GW0742 (to PPARβ/δ), and Rosiglitazone (PPARγ). All agonists induced ciliate cell death and all, except PEA and Rosiglitazone, gave IC50 values similar to those recorded for human cells (GW0742, 12 μM; OEA, 45 μM; AEA and CBD, 4 μM). The negative action of some of these agonists could be alleviated by blocking PPAR isoforms with their respective antagonists (GW6471 for PPARα, GSK3787 for PPARβ/δ, and T0070907 for PPARγ). The action of AEA and GW0742 were alleviated with all antagonists, CBD was only alleviated with GW6471 (for PPARα), and OEA was alleviated by none. Results suggest that T. pyriformis possesses a PPAR-like protein/target for AEA, CBD, and GW0742, but not for OEA.

Since the target of OEA was not a PPAR, this study explored other potential targets by using antagonists against the Dopamine receptor (Haloperidol), Gαi/o (Pertussis toxin [PTX]), and Protein kinase A (H89). Only PTX slightly alleviated the effect of OEA, suggesting a Gαi/o receptor mechanism.

Although the results are preliminary, further study may lead to the mapping and deeper understanding of the nature of the ECS in Tetrahymena and these non-CB pathways may be a possible therapeutic target in key diseases such as cancer, obesity and diabetes.