Neuronal Analogues of Conditioning Paradigms

It has been suggested that there is a neuronal basis of conditioning that occurs at the synapse between two cells and that the firing of the postsynaptic cell is required for conditioning to occur. Neurophysiological investigations of learning have relied heavily on studies of the marine mollusk Aplysia californica because of its large, hardy, identifiable neurons. Results from these studies have not clearly determined whether electrophysiological procedures analogous to conditioning paradigms cause changes which could constitute the basis of in vivo conditioning. This study examined the conditionability of cells in the abdominal ganglion of the Aplysia californica in an attempt to replicate, clarify and extend previous results. It also investigated the role of postsynaptic cell firing in producing the observedconditioning. Individual cells within the ganglion were recorded from using intracellular recording techniques. Mild electrical stimulation of the inputs to the cell produced a postsynaptic potential. The postsynaptic potential was repeatedly paired with one of four forms of stimulation. In the conditioning procedure, the postsynaptic potential was followed in 300 milliseconds by repeated firing of the cell induced by intense stimulation of additional inputs to the cell. In the pseudoconditioning procedure, the stimulation induced firing followed the postsynaptic potential by 10 seconds. In the clamp procedure, the cell was hyperpolarized by current injection throughout the procedure. This prevented cell firing in response to the intense stimulation which was presented as in the conditioning procedure. In the current injection procedure, the postsynaptic potential was followed in 300 milliseconds by repeated cell firings induced byinjection of depolarizing current into the cell. The four procedures were given in counterbalanced orders. Following the conditioning and clamp procedures, thepostsynaptic potential was significantly larger.