Robust Correlations Between Action Potential Duration and the Properties of Synaptic Connections in Layer 4 Interneurones in Neocortical Slices from Juvenile Rats and Adult Rat and Cat

Many studies of cortical interneurones use immature rodent tissue, while many recordings in vivo are made in adult cats. To determine the extent to which interneuronal circuitry studied with one approach can transfer to another, we compared layer 4 interneurones and their local connections across two age groups and two species and with similar connections in layers 3 and 5, using two common recording techniques: dual whole cell recordings at 20 degrees C and dual sharp electrode recordings at 35 degrees C. In each group, a range of morphological and electrophysiological characteristics was observed. In all groups, however, positive correlations were found between the width of the action potential and rise times and widths at half-amplitude of EPSPs and IPSPs and the EPSP paired pulse ratio. Multipolar interneurones with narrow spikes generated the fastest IPSPs in pyramidal cells and received the briefest, most strongly depressing EPSPs, while bitufted interneurones with broader spikes and adapting and burst firing patterns activated the broadest IPSPs and received the slowest, most strongly facilitating/augmenting EPSPs. Correlations were similar in all groups, with no significant differences between adult rat and cat, or between layers, but events were four times slower in juveniles at 20 degrees C. Comparisons with previous studies indicate that this is due in part to age, but in large part to temperature. Studies in adults were extended with detailed analysis of synaptic dynamics, which appeared to decay more rapidly than at juvenile connections. EPSPs exhibited the complexity in time course of facilitation, augmentation and depression previously described in other adult neocortical connections. That is, the time course of recovery from facilitation or depression rarely followed a simple smooth exponential decay. Facilitation and depression were not always maximal at the shortest interspike intervals, and recovery was often interrupted by peaks and troughs in mean EPSP amplitude with a periodicity around 80 Hz.