TRACE EYEBLINK CONDITIONING INCREASES CA1 EXCITABILITY IN A TRANSIENTAND LEARNING-SPECIFIC MANNER

Time-dependent, learning-related changes in hippocampal excitability w ere evaluated by recording from rabbit CAI pyramidal neurons in slices prepared at various times after acquisition of trace eyeblink conditi oning. Increased excitability (reduced postburst after hyperpolarizati ons and reduced spike-frequency adaptation) was seen as early as 1 hr after acquisition to behavioral criterion, was maximal in neurons stud ied 24 hr later, and returned to baseline within 7 d, whereas behavior al performance remained asymptotic for months. Neurons were held at -6 7 mV to equate voltage-dependent effects. No learning-related effects were observed on input resistance, action-potential amplitude or durat ion, or resting membrane potential. The excitability changes were lear ning-specific, because they were not seen in neurons from very slow le arning (exhibited <30% conditioned responses after 15 training session s) or from pseudoconditioned control rabbits. Neurons from rabbits tha t displayed asymptotic behavioral performance after long-term retentio n testing (an additional training session 14 d after learning) were al so indistinguishable from control neurons. Thus, the increased excitab ility of CA1 neurons was not performance- or memory-dependent. Rather, the time course of increased excitability may represent a critical wi ndow during which learning-specific alterations in postsynaptic excita bility of hippocampal neurons are important for consolidation of the l earned association elsewhere in the brain.