| Hippocampal and piriform cortex pyramidal neurons in vitro exhibit transient learning-dependent reductions in the amplitude and duration of calcium-dependent post-burst afterhyperpolarizations (AHPs), accompanied by an increase in excitability (i.e., reduced accommodation) after multi-trial trace eyeblink conditioning, spatial learning, or olfactory discrimination learning, with a time-course appropriate to support consolidation of the learned tasks. It is not known if AHP plasticity is unique to either these pyramidal neuron populations or to tasks requiring multiple trials, or if learning-dependent AHP plasticity is a more conserved learning mechanism used in other regions and/or for more rapidly acquired single-trial tasks. Intracellular current-clamp recordings were made from pyramidal neurons in the lateral (LA) and basolateral (BLA) nuclei of the amygdala and from hippocampal pyramidal neurons in the ventral and dorsal CA1 (CA1v and CA1d, respectively) and CA3 at intervals from 1 hr to >72 hr following single-trial inhibitory avoidance (IA) learning. Significant learning-dependent reductions in AHPs and in accommodation were seen in pyramidal neurons of BLA 1 hr following training, but extinguished 24 hrs post-training. No learning-dependent AHP plasticity was observed in the LA following single-trial IA learning, or in BLA or LA following acquisition of a multi-trial spatial memory task. Following IA learning, reduced AHPs and reduced accommodation were seen in CA1v pyramidal neurons within 1 hr post-trial, plasticity which persisted 24 hr but was extinguished >72 hr post-trial. There was also a reduction in CA1v AHPs and accommodation 1 hr following simple exposure to the IA apparatus (novel context) but this plasticity was extinguished by 24 hr post-exposure. Reductions in AHPs and accommodation were also seen in CA1d pyramidal neurons, but this plasticity was delayed until 24 hr post-trial and extinguished >72 hr post-trial. Transient inactivation of the BLA either immediately before or immediately post-trial blocked learning-dependent changes in excitability in the hippocampus assessed 24 hr post-trial. These findings demonstrate conservation of AHP plasticity in BLA and CA1 as a mechanism for learning a single-trial task. This conserved learning-dependent plasticity appears to be evoked in specific neuronal populations, in particular tasks, with regional variations in time-course regulated by the demands of the task acquired. |
