Hippocampal Interneuron Subtypes Show Distinct Tonic Inhibition Plasticity

The work, published in Frontiers in Molecular Neuroscience, combined whole-cell electrophysiology with morphological reconstruction of biocytin-filled neurons to compare two subclasses of these cells: oriens-lacunosum moleculare (OLM) interneurons and non-OLM interneurons. The findings reveal a clear functional division of labor between the two groups in how they regulate tonic inhibition, the persistent form of GABAergic signaling that operates outside traditional synapses.

Tonic inhibition is mediated by extrasynaptic GABAA receptors, which are activated by ambient GABA rather than by the brief, concentrated transmitter release that occurs at synaptic junctions. This sustained conductance acts as a continuous brake on neuronal excitability and has been implicated in the regulation of neuronal activity, synaptic plasticity, and hippocampus-dependent learning and memory. Although tonic inhibition itself is known to undergo plastic changes, the authors note that the description of this plasticity in distinct subclasses of hippocampal interneurons has remained incomplete, motivating the present investigation.

Under control conditions, the researchers found that tonic current density was significantly larger in OLM interneurons than in non-OLM interneurons. This baseline difference establishes that the two subclasses, both identified as somatostatin-positive, are not equivalent in their resting extrasynaptic GABAergic signaling.

The key experimental intervention was a brief, three-minute administration of NMDA, which activates NMDA-type glutamate receptors. When the team applied this treatment, the response of the two interneuron subclasses diverged sharply. In a subset of non-OLM somatostatin-positive interneurons, identified as the upper median in the cumulative distribution, brief NMDA receptor activation significantly enhanced tonic current density. In contrast, tonic current density in OLM cells showed no plastic changes upon NMDA treatment. This asymmetry demonstrates that susceptibility to activity-dependent modulation of tonic inhibition is not a uniform property of somatostatin-positive interneurons but is instead concentrated in the non-OLM subgroup.

Because GABAergic plasticity in the hippocampus often involves α5-containing GABAA receptors, the researchers turned to a pharmacological tool to probe this receptor subtype. They used L-655,708, a selective inverse agonist at α5-GABAA receptors, to estimate the contribution of these receptors to the tonic currents they recorded. The results showed that NMDA treatment increased the relative contribution of α5-GABAA receptors to tonic currents in non-OLM interneurons. In other words, the plastic enhancement of tonic inhibition in these cells was accompanied by a shift in the receptor composition underlying the current, with α5-containing receptors taking on a larger share after NMDA receptor activation.

Taken together, the data demonstrate a striking reversal of the baseline hierarchy when plasticity is considered. Non-OLM somatostatin-positive interneurons show lower tonic current density than OLM cells under resting conditions, yet the former subgroup is characterized by greater susceptibility to NMDA-induced changes in tonic current density, and these plastic changes are associated with an increased contribution of α5-GABAA receptors.

The authors interpret these observations as evidence of a clear functional specialization of non-OLM and OLM somatostatin-positive interneurons in the regulation of hippocampal network activity.

The study was conducted at Wroclaw Medical University in Poland by Patrycja Droździel, Katarzyna Lebida, and Jerzy Władysław Mozrzymas. It was received on 10 July 2026 and accepted on 5 October 2026, and it appears in the journal’s Molecular Signalling and Pathways section as an original research article.

For the broader field of inhibitory circuit research, the findings underscore that plasticity of tonic inhibition is a subclass-specific phenomenon rather than a uniform response of interneuron populations.

Subject of Research: Plasticity of tonic GABAergic inhibition in OLM and non-OLM somatostatin-positive hippocampal CA1 interneurons

Article Title: Distinct tonic inhibition and its plasticity in hippocampal OLM and non-OLM somatostatin-positive interneurons

Article References: Droździel, P., Lebida, K., & Mozrzymas, J. W. (2026). Distinct tonic inhibition and its plasticity in hippocampal OLM and non-OLM somatostatin-positive interneurons. Frontiers in Molecular Neuroscience, 19, 1934043. https://doi.org/10.3389/fnmol.2026.1934043

Image Credits: AI Generated

DOI: 10.3389/fnmol.2026.1934043

Keywords: tonic inhibition, GABAA receptors, alpha5-GABAA, hippocampus, CA1, interneurons, somatostatin, OLM cells, non-OLM cells, NMDA, synaptic plasticity, electrophysiology