Transforming growth factor-beta elicits extracellular glutamate overflow in the lateral hippocampus in mice; a novel mechanism for TGF-beta to mediate neurological deficits

Transforming Growth Factor-β (TGF-β) is a multifunctional cytokine expressed in virtually every cell type of the human body. It has various roles throughout the body and is heavily reliant on cellular and environmental context, with major roles in extracellular matrix (ECM) turnover and inflammation. TGF-β is upregulated after neurological insults including trauma and ischemic injury with implications for increased risk for aging-related phenotypes, such as neurodegeneration and cognitive decline. Neurotransmission in the hippocampus (HC) is largely glutamatergic, and exogenous TGF-β has been shown to decrease the expression of glutamate transporters, implicating TGF-β signaling as a potential mechanism for mediating extracellular glutamate homeostasis post-injury, where glutamate clearance dynamics have never been investigated. This study investigates how local TGF-β application to discrete regions within the lateral HC can modulate glutamatergic neurotransmission using in vivo amperometric recordings with glutamate selective microelectrode arrays (MEA) in adult (6M) male C57BL/6 mice (N=5) anesthetized with urethane (1.5g/kg, i.p.). We hypothesized that local application of exogenous TGF-β would yield a decrease in glutamate clearance rates, coinciding with an increase in glutamate clearance times. A MEA/double-barrel micropipette containing 100μM glutamate and 25ng/mL TFG-β loaded in separate barrels for individual application was placed in the dorsal lateral HC (AP: -2.75, ML: +/-3, DV: -1.5), where 4 local applications of glutamate (pre-TGF-β) were applied followed by a physiologically relevant concentration of TGF-β (400nL), followed by 4 local dose matched applications of glutamate (post-TBG-β). The MEA was then lowered (DV) for subsequent recordings at -2.2, -3, and -4mm. Post-TGF-β glutamate recordings were compared to pre-TGF-β glutamate recordings for changes in glutamate clearance kinetics, which demonstrated slower glutamate clearance at all depths measured in the lateral HC. Unexpectedly, TGF-β application yielded immediate and reproducible evoked glutamate overflow that reached peak concentrations of 6.28±1.74mM and remained elevated for 8.53±1.24s before returning to baseline levels in comparison to physiological saline. Results represent the first evidence that TGF-β can directly and indirectly modulate glutamate neurotransmission in the extracellular space of the HC. These are novel mechanisms (in addition to ECM turnover and neuroinflammation) by which injury-induced TGF-β can mediate post-injury neuropathological sequelae and symptoms after an acquired brain injury. T. Curry and T. Currier Thomas are co-senior authors. Midwestern Graduate Funds, NIH-R01NS100793. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.