The monosynaptic connection: modulating influence of opioid peptides on the plasticity of presynaptic neurons and identified synapses.

An investigation of the effect of opioid peptides (leucine-enkephalin and methionine-enkephalin) on the plastic properties of the system of monosynaptically connected LPa7-LPa3 and RPa3, and LPa8-LPa3 and RPa3, neurons in the brain of the edible snail was carried out. It was demonstrated that all three elements in the system under investigation (the presynaptic neuron, the postsynaptic neuron, and the synapse) display the same type of plasticity, i.e., habituation to rhythmic stimulation. The enkephalins exert a modulating effect on the plastic properties of the presynaptic neuron and synapse: they slow down the habituation of the presynaptic neuron in response to intracellular stimulation and the development of habituation at a level of the synapse as well. However, the changes in the character of the postsynaptic response in the presence of enkephalins are not the direct consequence of their influence on the plastic properties of the presynaptic neuron. In addition, the enkephalins decrease the efficiency of synaptic transmission in this system: they decrease the duration of the EPSP in the postsynaptic neuron.