MODULATION OF SYNAPTIC EVENTS BY HEAVY-METALS IN THE CENTRAL-NERVOUS-SYSTEM OF MOLLUSKS

1. The effects of heavy metals (Pb2+, Hg2+, and Zn2+) on synaptic tran smission in the identified neural network of Helix pomatia L. and Lymn aea stagnalis L. (Gastropoda, Mollusca) were studied, with investigati on of effects on inputs and outputs as wells as on interneuronal conne ctions. 2. The sensory input running from the cardiorenal system to th e central nervous system and the synaptic connections between central neurons were affected by heavy metals. 3. Lead and mercury (10(-5)-10( -3) M) eliminated first the inhibitory, then the excitatory inputs run ning from the heart to central neurons. At the onset of action lead in creased the amplitude of the excitatory postsynaptic potentials, but b lockade of sensory information transfer occurred after 10-20 min of tr eatment. 4. The monosynaptic connections between identified interneuro ns were inhibited by lead and mercury but not by zinc. Motoneurons wer e found to be less sensitive to heavy metal treatment than interneuron s or sensory pathways. 5. The treatment with Pb2+ and Hg2+ often elici ted pacemaker and bursting-type firing in central neurons, accompanied by disconnection of synaptic pathways, manifested by insensitivity to sensory synaptic influences. 6. Zn2+ treatment also sometimes induced pacemaker activity and burst firing but did not cause disconnection o f the synaptic transmission between interneurons. 7. A network analysi s of heavy metal effects can be a useful tool in understanding the con nection between their cellular and their behavioral modulatory influen ces.