Background: Invertebrates express opiate receptors and synthesize opiatealkaloids such as morphine and morphine-6beta-glucuronide. Most of this work has been demonstrated inimmune and neural tissues of various invertebrates. We hypothesized that morphinergic signaling may alsotake place in Mytilus edulis gill since they are innervated, in part, with dopamine nerves. Material/Methods:Ciliary activity from excised gills was evaluated via stroboscopic synchronization of metachronal waveformation before and after drug exposure. Nitric oxide was determined in real-time via an amperometricprobe following drug application. Real-time RT-PCR was performed on excised gill tissue to confirm thepresence of the micro opiate receptor transcript Results: Incubation of M. Edulis excised gill filamentsreveal spontaneously lateral cilia beating in a metachronal wave of about 600 beats per minute, whichwas significantly decreased by morphine in a concentration dependent and naloxone reversible manner.Exposure of the spontaneously beating cilia to SNAP, a nitric oxide donor, also diminished the beatingrate in a concentration dependent manner. Exposing the cilia to L-NAME blocked the morphine induced cilio-inhibition,demonstrating that morphine was working to inhibit the cilia via NO. Furthermore, the gill tissue containedmicro opiate receptor transcripts, which was micro3 in nature. Conclusions: As in mammals, opiate signalingis not confined to neural tissues. This report demonstrates the occurrence of opiate signaling for thefirst time in an invertebrate's respiratory tissue.

Keywords: Cilia - physiology, Gills - physiology, Morphine - pharmacology, Mytilus edulis - physiology, Nitric Oxide - metabolism, Receptors, Opioid, mu - genetics, S-Nitroso-N-Acetylpenicillamine - pharmacology, Transcription, Genetic