Different modes of action of morphine and epibatidine in Straub tail reactions in mice

The Straub tail reaction in mice is an S-shaped dorsiflexion of the mouse tail, long known as a sensitive and specific bioassay for morphine (CAS 57-27-2). It is based on a contraction of the sacro-coccygeal dorsalis muscles. In the case of morphine the reaction is thought to be induced by a long-lasting stimulation of the muscle's motor innervation at the level of the lumbosacral spinal cord. A similar reaction caused by epibatidine differs pharmacologically, because in this case the reaction can be specifically inhibited by the ganglionic blocking agent mecamylamine. This points to a site of action of epibatidine (CAS 140111-52-0) at the neuromuscular junction of the sacro-coccygeal dorsalis muscles and requires the presence of muscle fibres which react to depolarization by sustained contraction. The present study provides evidence of the existence of such fibres using histochemical and immunohistochemical methods.