Functional maturation of developing interneurons in the molecular layer of mouse dentate gyrus.

The dentate gyrus is the main target for cortical inputs to the hippocampal formation and is particularly strongly controlled by synaptic inhibition. Many GABAergic interneurons migrate from the dentate molecular layer towards their final position in the hilus during the first two postnatal weeks. During this critical period of development we monitored the intrinsic and synaptic properties of developing interneurons in the molecular layer of mouse hippocampal slices. We focussed on multipolar cells in the middle portion of the molecular layer. With increasing age, input resistance decreased and action potential waveform changed to larger amplitude and shorter duration. Repetitive spiking was scarce at early stages, while trains of action potentials could be readily elicited after the first postnatal week. At all ages, we observed spontaneous postsynaptic currents which were almost exclusively GABA(A) receptor-mediated and increased in frequency with age. All developmental changes in intrinsic and synaptic properties occurred between p 6-8 and p 9-11, indicating a rapid functional maturation at the end of the first postnatal week. Parallel immunohistochemical experiments revealed that calretinin positive cells formed the major part of developing interneurons in the middle molecular layer. Together, the data shows a rapid functional maturation of intrinsic and synaptic properties of interneurons in the dentate molecular layer and an early integration into synaptic networks with clear prevalence of inhibitory synaptic inputs.