Synaptic organisation and behaviour-dependent activity of mGluR8a-innervated GABAergic trilaminar cells projecting from the hippocampus to the subiculum

• Published in: Brain Structure and Function Vol. 225; no. 2; pp. 705 - 734
• Main Authors: Katona, Linda, Hartwich, Katja, Tomioka, Ryohei, Somogyi, Jozsef, Roberts, J. David B., Wagner, Kristina, Joshi, Abhilasha, Klausberger, Thomas, Rockland, Kathleen S., Somogyi, Peter
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.03.2020; Springer Nature B.V
• Subjects: Acetylcholine receptors (muscarinic); Animals; Biomedical and Life Sciences; Biomedicine; Cell Biology; Female; GABAergic Neurons - metabolism; GABAergic Neurons - ultrastructure; Glutamatergic transmission; Glutamic acid receptors (metabotropic); Hippocampus; Hippocampus - metabolism; Hippocampus - ultrastructure; Interneurons; Male; Mice, Inbred C57BL; Neural Pathways - metabolism; Neural Pathways - ultrastructure; Neurology; Neuromodulation; Neurosciences; Neurotransmitter release; Original; Original Article; Parvalbumin; Rats, Sprague-Dawley; Receptor, Muscarinic M2 - metabolism; Receptors, Metabotropic Glutamate - metabolism; Septum; Sleep; Somatostatin; Subiculum; Synapses - metabolism; Synapses - ultrastructure; γ-Aminobutyric acid; Muscarinic; Movement; Sleep; Trilaminar cell; Inhibition; Burst firing; Medial septum; Theta; Hippocampus; mGluR8a
• ISSN: 1863-2653; 1863-2661; 0340-2061
• DOI: 10.1007/s00429-020-02029-2

In the hippocampal CA1 area, the GABAergic trilaminar cells have their axon distributed locally in three layers and also innervate the subiculum. Trilaminar cells have a high level of somato-dendritic muscarinic M2 acetylcholine receptor, lack somatostatin expression and their presynaptic inputs are enriched in mGluR8a. But the origin of their inputs and their behaviour-dependent activity remain to be characterised. Here we demonstrate that (1) GABAergic neurons with the molecular features of trilaminar cells are present in CA1 and CA3 in both rats and mice. (2) Trilaminar cells receive mGluR8a-enriched GABAergic inputs, e.g. from the medial septum, which are probably susceptible to hetero-synaptic modulation of neurotransmitter release by group III mGluRs. (3) An electron microscopic analysis identifies trilaminar cell output synapses with specialised postsynaptic densities and a strong bias towards interneurons as targets, including parvalbumin-expressing cells in the CA1 area. (4) Recordings in freely moving rats revealed the network state-dependent segregation of trilaminar cell activity, with reduced firing during movement, but substantial increase in activity with prolonged burst firing (> 200 Hz) during slow wave sleep. We predict that the behaviour-dependent temporal dynamics of trilaminar cell firing are regulated by their specialised inhibitory inputs. Trilaminar cells might support glutamatergic principal cells by disinhibition and mediate the binding of neuronal assemblies between the hippocampus and the subiculum via the transient inhibition of local interneurons.