MODIFICATION OF APLYSIA FEEDING NETWORK BY L-DOPA AND DOPAMINE-DEPENDENT LEARNING

Dopamine (DA) is a ubiquitous neuromodulator of neuronal networks in the animal kingdom and has a well-established role in modulating motor behavior and encoding reward information. Although the effects of DA and DA-dependent learning at the behavioral and molecular levels are well-understood, many questions still remain concerning the effects of DA at the network level. DA-dependent learning effects on a neuronal circuit were examined by measuring the biophysical properties before and after in vitro operant conditioning (OC) of the feeding circuit of Aplysia. OC reduced the excitability of B4 and the B4-to-B51 synaptic connection, with a trend towards an enhancement of the B4-to-B8 synaptic connection. These data indicate that OC can decrease the excitability of neurons and modify synaptic connections. The effects of L-DOPA were examined by applying either low or high concentrations of L-DOPA (40 or 250 mM) and then monitoring activity of up to 130 neurons simultaneously using a voltage-sensitive dye (RH-155). L-DOPA selected one of two distinct buccal motor patterns (BMPs): intermediate (low L-DOPA) or bite patterns (high L-DOPA). The selection of intermediate BMPs was associated with shortening of the second phase of the BMP (retraction), whereas the selection of bite BMPs was associated with shortening of both phases of the BMP (protraction and retraction). Selection of intermediate BMPs was also associated with truncation of individual neuron spike activity (decreased burst duration but no change in spike frequency or burst latency) in neurons active during retraction and in neurons projecting an axon through nerves Rn or n3. In contrast, selection of bite BMPs was associated with compression of spike activity (decreased burst latency and duration, and increased spike frequency) in neurons projecting through nerves Rn, n2, and n3, as well as increased spike frequency of protraction neurons. The large-scale voltage-sensitive dye (VSD) recordings delineated the spatial distribution of neurons active during BMPs and the modification of that distribution by the two concentrations of L-DOPA. Finally, a system was established for identifying neurons in VSD recordings which will enable a detailed analysis of the effects of L-DOPA and OC