Stability and variability of synapses in the adult molluskan CNS

Synaptic transmission was examined between identified neurons in the centra l nervous system (CNS) of the freshwater mollusk, Lymnaea stagnalis. Four i dentified neurons were used: Right Pedal Dorsal one (RPeD1; a dopaminergic respiratory interneuron), Visceral Dorsal two and three (VD2/3), and Viscer al Dorsal four (VD4; a cardiorespiratory interneuron), Neuron RPeD1 synapse s onto both VD2/3 and VD4, while VD4 makes a reciprocal synapse onto RPeD1, When compared from animal to animal, the connections were variable in sign . Previously, we demonstrated that, in a given animal, the RPeD1 --> VD4 sy napse could be either inhibitory, biphasic, or undetectable. The present st udy now expands this concept of variability by showing that the RPeD1 --> V D2/3 synapse was either excitatory or undetectable from animal to animal, w hile the synapse from VD4 to RPeD1 was observed as inhibitory, biphasic, de polarizing, excitatory, or undetectable. Next, we used 1-day organ culture to determine if the variability observed between animals is a product of on going change to the sign of these identified synapses and whether or not th e extent of change could be influenced by the culture conditions. Changes t o the sign of transmission occurred within minutes and, more commonly, afte r 24-h organ culture. All three synapses were investigated before and after 1-day organ culture, in either defined medium (DM) or brain-conditioned me dium (CM). Regardless of culture conditions, the RPeD1 --> VD2/3 synapse sh owed no change of sign, i.e., it was relatively stable. However, the synaps es between RPeD1 and VD4 did change sign, and when cultured in CIM, the VD4 --> RPeD1 synapse changed significantly more than in DM. These data indica te that variability of some synapses reflects changes at these synapses. Th is is the first report that specific synapses In an adult CNS can change si gn, and that the sign of transmission can be modulated by environmental ren ditions. (C) 2000 John Wiley & Sons, Inc.