Opioid-dopamine interaction in planaria: A behavioral study

The behavioral response of planaria to the exposure to selective opioid agonists was studied. The μ agonist [d-ala², N-methyl-Phe⁴,Gly⁵-ol]enkephalin (DAMGO) and the δ agonist [D-Pen², D-Pen⁵]enkephalin (DPDPE) failed to alter motor activity at all doses tested. Low doses of the selective κ agonist (±)-trans-U-50-trans-3,4-dichloro-N-methyl-N[2-(1-pyrrodinyl)-cyclohexyl]benzene acetamide methasulphonate (U50, 488) and bremazocine-HCl increased motor activity leading to C-like position (CLP) and screw-like hyperkinesia (SLH). These changes were identical to those seen previously with the exposure to D2 or D1 dopamine receptor agonists, respectively. Higher doses of κ agonists produced the enhancement of CLP and SLH together with robust snake-like movements (SLM). This latter response, that was typical of stimulation of κ opioid receptors, was blocked by co-exposure to naloxone or the selective κ antagonist Nor-binaltorphimine (Nor-BNI). Finally, co-exposure to sulpiride or SH-23390 respectively blocked the CLP or SLH response produced by U50,488 or bremazocine. Our data indicate the presence of κ opioid receptors in planaria and suggest the functional interaction between the opioid and dopamine system in this simple animal model. Copyright (C) 1999 Elsevier Science Inc.