Structural analysis of the microglia-interneuron interactions in the CA1 hippocampal area of the APP/PS1 mouse model of Alzheimer's disease

Microglia can interact with glutamatergic neurons and, through control of synaptic elements, regulate their physiological function. Much less is known about the partnership between microglia and GABAergic inhibitory interneurons. Here, we compared the interactions between microglia and parvalbumin (PV+) and somatostatin (SOM+) expressing interneurons in the CA1 hippocampal area of APP/PS1 transgenic mice that mimic certain aspects of the Alzheimer's disease (AD). We first uncovered a high level of interactions between microglia and two types of interneurons, with 98% of SOM+ and 90% of PV+ cells receiving different types of putative microglial contacts. The latter included the microglia soma to the interneuron soma (Soma(MG)-to-Soma(IN)), the microglia process to the interneuron soma (Process(MG)-to-Soma(IN)) and the microglia process to the interneuron dendrite (Process(MG)-to-Dend(IN)) interactions. Moreover, we found significantly larger areas of interaction for the Soma(MG)-to-Soma(IN) and the Process(MG)-to-Dend(IN) type of contacts between microglia and SOM+ cells. In contrast, PV+ cells exhibited larger areas for the Process(MG)-to-Soma(IN) interactions. Second, in APP/PS1 mice, although the overall microglia interactions with interneurons remained preserved, the fraction of interneurons receiving putative microglia contacts on their dendrites was reduced, and larger areas of interactions were observed for somatic contacts, suggesting a stronger modulation of the interneuron output by microglia in AD. In summary, these results reveal microglia as important partners of hippocampal PV+ and SOM+ GABAergic cells, with interneuron type-specific pattern of interactions. Thus, microglia may play an essential role in the operation of interneurons under normal conditions and their dysfunction in disease.

Automated summary

The study revealed a high level of interactions between microglia and parvalbumin and somatostatin-expressing interneurons, with different patterns of interactions observed.