Learning: Neural analysis in the isolated brain of a previously trained mollusc, Pleurobranchaea californica

The neural manifestations of food avoidance learning in the mollusc, Pleurobranchaea, survive the surgical reduction of the preparation to the nearly isolated brain. These manifestations include increased synaptic inhibition and reduced synaptic excitation of the phasic paracerebral feeding command interneurons (PC_ps) in the brain in response to food stimulation of chemosensory structures left attached to the brain. The same changes are not evident, however, in brains removed from naive, control or satiated specimens. Therefore the nearly isolated brain preparation permits analysis of the cellular substrates of learning in relative isolation from non-associative motivational variables. The isolated brain preparation is here used to show that the increased synaptic inhibition consequent to associative training is distributed not only to the PC_ps but also to their identified central presynaptic inputs, including other identified feeding command interneurons (PSEs and ET_IIs; ref. 21). The decrease in PC_p excitation is explained in part by a training-induced inhibition of excitatory inputs to the PC_ps, and in part by a training-induced reduction in the efficacy of an identified polysynaptic excitatory pathway presynaptic to the PC_ps.