[The endogenous opioid system tonically activates the locomotor neurons in the mollusk Lymnaea stagnalis].

Keeping specimens of the pond snail Lymnaea stagnalis in the aqueous solution of the opiate antagonist naloxone (0.1 mM) or naltrexone (0.1 mM) resulted in a suppression of motor activity, and, particularly, in a decrease in the rate of ciliary locomotion. The activity of ciliomotor serotonergic neurons of the Pedal A cluster was intracellularly recorded in preparations of isolated ganglia. Naloxone (0.1 mM) suppressed, whereas the opiate agonists morphine (0.1 mM) and DAGO (0.05 mM) accelerated the synaptically driven firing of the Pedal A neurons both in preparations of the entire CNS and isolated pedal ganglia. The obtained results testify to the occurrence of tonic activatory influence of the endogenous opioids on the ciliomotor pedal neurons of snails and suggest that this influence is, at least partially, mediated by their synaptic input.