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Presynaptic GABAB receptor-mediated network excitation in the medial prefrontal cortex of Tsc2+/- mice
Davide Bassetti1, Heiko J Luhmann2, Sergei Kirischuk2
1Institute of Physiology, University Medical Center of the Johannes Gutenberg University Mainz, Duesbergweg 6, 55128, Mainz, Germany. dbassett@uni-mainz.de.
Abstract:
The TSC1 and TSC2 tumor suppressor genes control the activity of mechanistic target of rapamycin (mTOR) pathway. Elevated activity of this pathway in Tsc2+/- mouse model leads to reduction of postsynaptic GABAB receptor-mediated inhibition and hyperexcitability in the medial prefrontal cortex (mPFC). In this study, we asked whether presynaptic GABAB receptors (GABABRs) can compensate this shift of hyperexcitability. Experiments were performed in brain slices from adolescent wild-type (WT) and Tsc2+/- mice. Miniature and spontaneous postsynaptic currents (m/sPSCs) were recorded from layer 2/3 pyramidal neurons in mPFC using patch-clamp technique using a Cs+-based intrapipette solution. Presynaptic GABABRs were activated by baclofen (10 µM) or blocked by CGP55845 (1 µM). Independent on genotype, GABABR modulators bidirectionally change miniature excitatory postsynaptic current (mEPSC) frequency by about 10%, indicating presynaptic GABABR-mediated effects on glutamatergic transmission are comparable in both genotypes. In contrast, frequencies of both mIPSCs and sIPCSs were suppressed by baclofen stronger in Tsc2+/- neurons than in WT ones, whereas CGP55845 significantly increased (m/s)IPSC frequencies only in WT cells. Effects of baclofen and CGP55845 on the amplitudes of evoked (e)IPSCs confirmed these observations. These data indicate (1) that GABAergic synapses are inhibited by ambient GABA in WT but not in Tsc2+/- slices, and (2) that baclofen shifts the E/I ratio, determined as the ratio of (m/s)EPSC frequency to (m/s)IPSC frequency, towards excitation only in Tsc2+/- cells. This excitatory presynaptic GABABR-mediated action has to be taken into account for a possible medication of mental disorders using baclofen.
Insights
Presynaptic GABAB receptors do not compensate for hyperexcitability in Tsc2+/- mice. Baclofen shifts the excitation/inhibition ratio towards excitation in Tsc2+/- neurons, impacting potential mental disorder treatments.
Area of Science:
- Neuroscience
- Molecular Biology
- Genetics
Background:
- The TSC1/TSC2 genes regulate the mTOR pathway, crucial for cellular processes.
- Dysregulation of this pathway, as seen in Tsc2+/- mice, leads to medial prefrontal cortex (mPFC) hyperexcitability.
- Reduced postsynaptic GABAB receptor inhibition contributes to this hyperexcitability.
Purpose of the Study:
- To investigate if presynaptic GABAB receptors can counteract mPFC hyperexcitability in a Tsc2+/- mouse model.
- To determine the role of presynaptic GABABRs in glutamatergic and GABAergic transmission in wild-type (WT) and Tsc2+/- mice.
Main Methods:
- Patch-clamp recordings from layer 2/3 pyramidal neurons in adolescent WT and Tsc2+/- mouse mPFC brain slices.
- Measurement of miniature and spontaneous excitatory and inhibitory postsynaptic currents (m/sEPSCs and m/sIPSCs).
- Pharmacological manipulation of presynaptic GABAB receptors using baclofen (agonist) and CGP55845 (antagonist).
Main Results:
- Presynaptic GABAB receptor effects on glutamatergic transmission (mEPSC frequency) were comparable between WT and Tsc2+/- mice.
- Baclofen more strongly suppressed GABAergic transmission (m/sIPSC frequency) in Tsc2+/- neurons than in WT.
- CGP55845 increased m/sIPSC frequency only in WT neurons, suggesting altered ambient GABA effects in Tsc2+/- slices.
- Baclofen shifted the excitation/inhibition ratio towards excitation specifically in Tsc2+/- neurons.
Conclusions:
- Presynaptic GABAB receptors do not compensate for the hyperexcitability observed in the Tsc2+/- mPFC.
- Altered GABAergic transmission and an excitatory shift in the E/I ratio by baclofen in Tsc2+/- mice have implications for neurological disorders.
- The findings necessitate consideration of presynaptic GABABR actions in potential baclofen-based therapies for mental health conditions.
