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Background: Mossy cells comprise a large fraction of excitatory neurons in the hippocampal dentate gyrus, and their loss is one of the major hallmarks of temporal lobe epilepsy (TLE). The vulnerability of mossy cells in TLE is well known in animal models as well as in patients; however, the mechanisms leading to cellular death is unclear., Results: Transient receptor potential melastatin 4 (TRPM4) is a Ca 2+ -activated non-selective cation channel regulating diverse physiological functions of excitable cells. Here, we identified that TRPM4 is present in hilar mossy cells and regulates their intrinsic electrophysiological properties including spontaneous activity and action potential dynamics. Furthermore, we showed that TRPM4 contributes to mossy cells death following status epilepticus and therefore modulates seizure susceptibility and epilepsy-related memory deficits., Conclusions: Our results provide evidence for the role of TRPM4 in MC excitability both in physiological and pathological conditions., (© 2023. The Author(s).)

Layer I of the medial entorhinal cortex (MEC) contains converging axons from several brain areas and dendritic tufts originating from principal cells located in multiple layers. Moreover, specific GABAergic interneurons are also located in the area, but their inputs, outputs, and effect on local network events remain elusive. Neurogliaform cells are the most frequent and critically positioned inhibitory neurons in layer I. They are considered to conduct feed-forward inhibition via GABAA and GABAB receptors on pyramidal cells located in several cortical areas. Using optogenetic experiments, we showed that layer I neurogliaform cells receive excitatory inputs from layer II pyramidal cells, thereby playing a critical role in local feedback inhibition in the MEC. We also found that neurogliaform cells are evenly distributed in layer I and do not correlate with the previously described compartmentalization ("cell islands") of layer II. We concluded that the activity of neurogliaform cells in layer I is largely set by layer II pyramidal cells through excitatory synapses, potentially inhibiting the apical dendrites of all types of principal cells in the MEC., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Szocs, Henn-Mike, Agocs-Laboda, Szabo-Meleg and Varga.)

GABA released from heterogeneous types of interneurons acts in a complex spatio-temporal manner on postsynaptic targets in the networks. In addition to GABA, a large fraction of GABAergic cells also express neuromodulator peptides. Somatostatin (SOM) containing interneurons, in particular, have been recognized as key players in several brain circuits, however, the action of SOM and its downstream network effects remain largely unknown. Here, we used optogenetics, electrophysiologic, anatomical and behavioral experiments to reveal that the dendrite-targeting, SOM + GABAergic interneurons demonstrate a unique layer-specific action in the medial entorhinal cortex (MEC) both in terms of GABAergic and SOM-related properties. We show that GABAergic and somatostatinergic neurotransmission originating from SOM + local interneurons preferentially inhibit layer III-V pyramidal cells, known to be involved in memory formation. We propose that this dendritic GABA-SOM dual inhibitory network motif within the MEC serves to selectively modulate working-memory formation without affecting the retrieval of already learned spatial navigation tasks.

Somatostatin is an important mood and pain-regulating neuropeptide, which exerts analgesic, anti-inflammatory, and antidepressant effects via its Gi protein-coupled receptor subtype 4 (SST 4 ) without endocrine actions. SST 4 is suggested to be a unique novel drug target for chronic neuropathic pain, and depression, as a common comorbidity. However, its neuronal expression and cellular mechanism are poorly understood. Therefore, our goals were (i) to elucidate the expression pattern of Sstr4 / SSTR4 mRNA, (ii) to characterize neurochemically, and (iii) electrophysiologically the Sstr4 / SSTR4 -expressing neuronal populations in the mouse and human brains. Here, we describe SST 4 expression pattern in the nuclei of the mouse nociceptive and anti-nociceptive pathways as well as in human brain regions, and provide neurochemical and electrophysiological characterization of the SST 4 -expressing neurons. Intense or moderate SST 4 expression was demonstrated predominantly in glutamatergic neurons in the major components of the pain matrix mostly also involved in mood regulation. The SST 4 agonist J-2156 significantly decreased the firing rate of layer V pyramidal neurons by augmenting the depolarization-activated, non-inactivating K + current (M-current) leading to remarkable inhibition. These are the first translational results explaining the mechanisms of action of SST 4 agonists as novel analgesic and antidepressant candidates.

Optical microelectrodes (optrodes) are used in neuroscience to transmit light into the brain of a genetically modified animal to evoke and record electrical activity from light-sensitive neurons. Our novel micro-optrode solution integrates a light-transmitting 125 micrometer optical fiber and a 9 micrometer carbon monofilament to form an electrical lead element, which is contained in a borosilicate glass sheathing coaxial arrangement ending with a micrometer-sized carbon tip. This novel unit design is stiff and slender enough to be used for targeting deep brain areas, and may cause less tissue damage compared with previous models. The center-positioned carbon fiber is less prone to light-induced artifacts than side-lit metal microelectrodes previously presented. The carbon tip is capable of not only recording electrical signals of neuronal origin but can also provide valuable surface area for electron transfer, which is essential in electrochemical (voltammetry, amperometry) or microbiosensor applications. We present details of design and manufacture as well as operational examples of the newly developed single micro-optrode, which includes assessments of 1) carbon tip length-impedance relationship, 2) light transmission capabilities, 3) photoelectric artifacts in carbon fibers, 4) responses to dopamine using fast-scan cyclic voltammetry in vivo, and 5) optogenetic stimulation and spike or local field potential recording from the rat brain transfected with channelrhodopsin-2. With this work, we demonstrate that our novel carbon tipped single micro-optrode may open up new avenues for use in optogenetic stimulation when needing to be combined with extracellular recording, electrochemical, or microbiosensor measurements performed on a millisecond basis.