Synaptic connexions and related postsynaptic pharmacology studied in the cerebral ganglion of Aplysia

Interneuronal connexions have been studied within the cerebral ganglion of Aplysia californica. Excitatory monosynaptic connexions between two groups of cells located on the dorsal surface of the ganglion near the cerebropleural connectives were analyzed in detail. The monosynapticity of these connexions was established not only by the strict one-to-one correlation between presynaptic action potential and excitatory postsynaptic response (EPSP) and the constant latency for any given cell pair, but also by the following criteria: (a) gradual change in the EPSP following tetraethylammonium injection into the presynaptic neurone, (b) sustained EPSP in the presence of a high external calcium ion concentration, (c) sensitivity of the EPSP amplitude to presynaptic polarization. Iontophoretic application of acetylcholine, 5-hydroxytryptamine and glutamate on the postsynaptic cells elicited excitatory responses in many cases. Inhibitory responses were obtained by local iontophoresis of dopamine, gamma-aminobutyric acid and occasionally also by acetylcholine. The only agent found to block the EPSP was bufotenine, which also readily blocked the 5-hydroxytryptamine response. Bufotenine was completely ineffective on the acetylcholine or glutamate excitatory responses. Of the various cholinolytics tested, none had an effect on the EPSP. Our data all point to 5-hydroxytryptamine as a transmitter in the studied synaptic connexions. However, it must be emphasized that in the absence of biochemical and histological evidence the role of the 5-hydroxytryptamine cannot be regarded as conclusive.