Pharmacological or genetic targeting of Transient Receptor Potential (TRP) channels can disrupt the planarian escape response

Abstract

In response to noxious stimuli, planarians cease their typical ciliary gliding and exhibit an oscillatory type of locomotion called scrunching. We have previously characterized the biomechanics of scrunching and shown that it is induced by specific stimuli, such as amputation, noxious heat, and extreme pH. Because these specific inducers are known to activate Transient Receptor Potential (TRP) channels in other systems, we hypothesized that TRP channels control scrunching. We found that chemicals known to activate TRPA1 (allyl isothiocyanate (AITC) and hydrogen peroxide) and TRPV (capsaicin and anandamide) in other systems induce scrunching in the planarian species Dugesia japonica and, except for anandamide, in Schmidtea mediterranea. To confirm that these responses were specific to either TRPA1 or TRPV, respectively, we tried to block scrunching using selective TRPA1 or TRPV antagonists and RNA interference (RNAi) mediated knockdown. Unexpectedly, co-treatment with a mammalian TRPA1 antagonist, HC-030031, enhanced AITC-induced scrunching by decreasing the latency time, suggesting an agonistic relationship in planarians. We further confirmed that TRPA1 in both planarian species is necessary for AITC-induced scrunching using RNAi. Conversely, while co-treatment of a mammalian TRPV antagonist, SB-366791, also enhanced capsaicin-induced reactions in D. japonica, combined knockdown of two previously identified D. japonica TRPV genes (DjTRPVa and DjTRPVb) did not inhibit capsaicin-induced scrunching. RNAi of DjTRPVa/DjTRPVb attenuated scrunching induced by the endocannabinoid and TRPV agonist, anandamide. Overall, our results show that although scrunching induction can involve different initial pathways for sensing stimuli, this behavior’s signature dynamical features are independent of the inducer, implying that scrunching is a stereotypical planarian escape behavior in response to various noxious stimuli that converge on a single downstream pathway. Understanding which aspects of nociception are conserved or not across different organisms can provide insight into the underlying regulatory mechanisms to better understand pain sensation.

The Endocannabinoid System and Invertebrate Neurodevelopment and Regeneration

“…Additional actions of cannabinoids in planaria may be mediated through TRP-like receptors. Planarians display what is known as “scrunching” in response to noxious stimuli, such as low pH, elevated temperatures, or amputation [ 129 , 130 ]. This behavior is described as a cilia-independent alteration in oscillatory gait, which is thought to be mediated through the TRP channels [ 129 , 130 ].…”

“…Planarians display what is known as “scrunching” in response to noxious stimuli, such as low pH, elevated temperatures, or amputation [ 129 , 130 ]. This behavior is described as a cilia-independent alteration in oscillatory gait, which is thought to be mediated through the TRP channels [ 129 , 130 ]. As mentioned, TRPV channel activation has been observed in response to anandamide in mammalian models [ 131 , 132 ].…”

“…As mentioned, TRPV channel activation has been observed in response to anandamide in mammalian models [ 131 , 132 ]. A recent study demonstrated that two previously identified TRPV-encoding genes ( DjTRPVa and DjTRPVb ) partially mediate anandamide sensing in Dugesia japonica [ 130 ]. Exposure to 100 μM of anandamide resulted in the scrunching behavior associated with noxious stimuli, as well as an unexpected increase in head lifting or head wiggling [ 130 ].…”

“…A recent study demonstrated that two previously identified TRPV-encoding genes ( DjTRPVa and DjTRPVb ) partially mediate anandamide sensing in Dugesia japonica [ 130 ]. Exposure to 100 μM of anandamide resulted in the scrunching behavior associated with noxious stimuli, as well as an unexpected increase in head lifting or head wiggling [ 130 ]. When TRPV channels, DjTRPVa and DjTRPVb were knocked down via RNA interference, a decreased behavioral response associated with a delay in onset of scrunching was observed, highlighting the involvement of TRPV channels in anandamide-induced scrunching, but also supporting the involvement of other receptor(s) in anandamide sensing [ 130 ].…”

“…Exposure to 100 μM of anandamide resulted in the scrunching behavior associated with noxious stimuli, as well as an unexpected increase in head lifting or head wiggling [ 130 ]. When TRPV channels, DjTRPVa and DjTRPVb were knocked down via RNA interference, a decreased behavioral response associated with a delay in onset of scrunching was observed, highlighting the involvement of TRPV channels in anandamide-induced scrunching, but also supporting the involvement of other receptor(s) in anandamide sensing [ 130 ]. In addition to these behaviors, we have recently demonstrated that anterior segments of bisected Dugesia dorotocephala (formerly Dugesia japonica ) exhibited difficulty/complete inability to right when cultured with concentrations as low as 0.5 µM of the CB1 inverse agonist AM251 (1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide), suggesting that CB1 activation may contribute to coordination of muscle contractions [ 122 ].…”