Inhibitory neuronal networks in the cortex comprise various specialized GABAergic interneuron types. In hippocampus, excitatory synaptic innervation onto many interneurons is permanently potentiated or depressed by neuronal activity. The plasticity in glutamatergic synapses is akin to long-term potentiation (LTP) and –depression (LTD) originally described in hippocampal principal cells, except that in most interneurons it is independent of NMDA receptors. Plasticity in GABAergic neuron population is diverse and at least four distinct long-term plasticity forms are elicited in the interneurons of the hippocampal CA1 area. Anatomical analyses demonstrate that plasticity rules are specific to an anatomical interneuron type.

We investigated activity-induced LTP and LTD in excitatory glutamatergic synapses onto identified hippocampal CA1 area interneurons in vitro in acute slice preparation and in vivo in urethane-anesthetized rat. We used minimally invasive recording methods to measure alterations in the synaptic strength. In intracellular recordings from slices we utilized gramicidin-perforated current clamp recordings whereupon recorded neurons were filled with neurobiotin to mark cells for subsequent anatomical analysis. In anesthetized rats we recorded postsynaptic spike probability and spike delay to afferent glutamatergic pathway stimulation. In vivo recorded cells were extracellular recorded and juxtacellularly labeled with neurobiotin for post hoc anatomical analyses of their structure and molecular identity.

We saw both LTP and LTD in glutamatergic afferents onto CA1 area interneurons in vitro and in vivo. We suggest that the cell type-specific plasticity in interneurons plays a role in physiological hippocampal function.