GABAergic tonic currents in excitatory neurons and 1 fast spiking interneurons in layer IV of mouse barrel cortex

25 Pairing tactile stimulation of whiskers with a tail shock is known to result in expansion 26 of cortical representation of stimulated vibrissae and in the increase in synaptic GABAergic 27 transmission. However, the impact of such sensory learning in classical conditioning 28 paradigm on GABAergic tonic currents has not been addressed. To this end we have 29 performed whole-cell patch-clamp slice recordings of tonic currents from neurons (excitatory 30 regular spiking, regular spiking non-pyramidal and fast spiking interneurons) of layer IV of 31 the barrel cortex from naive and trained mice. Interestingly, endogenous tonic GABAergic 32 currents measured from the excitatory neurons in the cortical representation of “trained” 33 vibrissae were larger than in the “naïve” or pseudoconditioned ones. On the contrary, sensory 34 learning markedly reduced tonic currents in the fast spiking interneurons but not in regular 35 spiking non-pyramidal neurons. Changes of tonic currents were accompanied by changes in 36 the input resistances – decrease in regular spiking and increase in fast spiking neurons, 37 respectively. Applications of nipecotic acid, a GABA uptake blocker, enhanced the tonic 38 currents but the impact of the sensory learning remained qualitatively the same as in the case 39 of the tonic currents. Similar to endogenous tonic currents, sensory learning enhanced 40 currents induced by THIP (superagonist for δ subunit-containing GABAARs) in regular 41 spiking neurons while the opposite was observed for the fast spiking interneurons. In 42 conclusion, our data show that the sensory learning strongly affects the GABAergic tonic 43 currents in a cell-specific manner and suggests that the underlying mechanism involves 44 regulation of expression of δ subunit-containing GABAARs. 45