Facilitation of inhibitory post-synaptic potentials in central neurones of Helix aspersa

Abstract

1. 1. Evoked potentials were recorded intracellularly in two adjacent nerve cells, E16 and E17, in the brain of Helix aspersa. The potentials were biphasic or inhibitory post-synaptic responses which followed stimulation of the anal nerve. 2. 2. The properties of the responses were compared with the action of transmitter compounds. The E16 response was similar to the action of dopamine, and the E17 response, to that of glutamate. The inhibitory responses could be selectively blocked by separate antagonists. The inhibitory post-synaptic potential in E16 was mediated by K + ions, and that in E17, by both K + and Cl − ions. 3. 3. When responses were paired, the second inhibitory potential was facilitated. Facilitation outlasted the first response by 0.5 sec in E16, and 10 sec in E17. During trains of responses repeated at 1/sec, facilitation increased slowly and declined over a longer time course than following a single stimulus. No change in post-synaptic transmitter efficacy or membrane resistance was observed during facilitation. 4. 4. Facilitation following a single response was reduced by high calcium and increased by high magnesium concentrations. The maximum amplitude of potentials during train-dependent facilitation was unaffected by calcium elevation. 5. 5. The possibility of two separate mechanisms underlying synaptic facilitation is discussed, and compared with similar findings at motor nerve endings in the frog and molluscan excitatory synapses. 6. 6. The existence of two cells with different facilitation times of 0.5 sec (E16) and 10 sec (E17) indicates a way in which priority pathways could be set up within the Central Nervous System.