α4β2^* nicotinic acetylcholine receptors drive human temporal glutamate/GABA balance toward inhibition

Heteromeric nicotinic acetylcholine nAChRs (nAChRs) containing the α4 and β2 subunits (α4β2^* nAChRs) modulate neurotransmitter release in several regions of the brain. In temporal lobe epilepsy, inhibitory GABAergic neurotransmission is altered, whereas no evidence of nicotinic dysfunction has been reported. Here, we investigated, in human epileptic cortical tissues, the ability of α4β2^* nAChRs to modulate synaptic transmission. An increased expression of α4 and β2 subunits was observed in the temporal cortex of epileptic patients. We then recorded excitatory and inhibitory postsynaptic currents from layer 5 pyramidal neurons in the cortex of temporal lobe epilepsy patients, before and during selective modulation of α4β2^* nAChRs by desformylflustrabromine (a selective α4β2^* positive allosteric modulator). We observed a decrease in both frequency and amplitude of spontaneous excitatory postsynaptic currents, along with an increase in spontaneous inhibitory postsynaptic current frequency. Both these effects were blocked by dihydro-β-erythroidine, a selective α4* antagonist. α4β2^* activation enhanced the excitability of interneurons (but not of layer 5 pyramidal neurons) by lowering the action potential threshold. Moreover, upon block of action potential propagation by TTX, α4β2^* activation did not alter miniature inhibitory postsynaptic currents recorded from pyramidal neurons, at the same time as reducing the release at glutamatergic synapses by a GABA_B-dependent process. KEY POINTS: Heteromeric nicotinic acetylcholine receptors containing the α4 and β2 subunits (α4β2^* nAChRs) increase GABA release in several regions of the brain. We observe an increase of α4β2^* nAChRs expression in the temporal cortex of patients with temporal lobe epilepsy (TLE, the most represented human focal epilepsy). When selectively activated with the positive allosteric modulator desformylflustrabromine (dFBr), α4β2^* nAChRs increase the frequency of GABA release and decrease the glutamate release onto pyramidal neurons in the layer 5 of human TLE cortex. The increase of GABA release is related to an α4β2^*-mediated enhanced excitability of cortical interneurons; instead, the decrease of glutamate release involves a presynaptic GABA_B-mediated mechanism, being abolished by a selective GABA_B blocker. Our findings show that the activation of α4β2^* nAChRs induce an increase of the inhibitory tone in human TLE cortex and candidate nicotinic positive allosteric modulators as new pharmacological tools to treat TLE.