The Role of Mouse Barrel Cortex in Tactile Trace Eye Blink Conditioning

Abstract

Mouse whisker-related primary somatosensory cortex (also known as barrel cortex, BCx) is required to form an association between a behaviorally relevant tactile stimulus and its consequences, only if the first conditioned stimulus CS (here a single whisker deflection), and the latter unconditioned stimulus US (here a corneal air puff) are separated by a ‘trace’ (brief memory period). I investigated whether tactile trace eye blink conditioning (TTEBC) has a correlate in BCx activity and whether such BCx activity in the two periods, CS and trace are required for learning. I trained three head-fixed mice on TTEBC to assess learning related functional plasticity of BCx by recording LFPs and multi-unit (MU) spiking from 4-shank laminar silicone probes (8 electrodes per shank, inter-shank distance 200μm) spanning the depths of the principal barrel column and its neighbors. Current source density analysis (CSD) showed the known short latency sink (~8ms) in L4 and L5/6 during CS presentation, followed by a weaker current sink during ongoing tactile stimulation, spanning across the column. At the same depth, a novel current source was discovered during the trace period. The latter two currents were consistently attenuated during TTEBC acquisition. Onset MU spike response to the CS (at a latency of <15ms) was stable in most units, while steady state CS-response (50-250ms) typically decreased below the pre-learning level. Spiking during the trace period also depressed during learning. These plastic changes were observed in neighboring shanks at a horizontal distance of up to 400μm. These findings show that BCx is functionally involved in TTEBC acquisition. Matching the lateral spread of the neuronal signal into the neighboring column, I found mice to generalize the CS-US association only to adjacent, but not to near and far whiskers. I next asked whether the involvement of BCx during the trace period has any causal role in TTEBC. I employed the well-established VGAT-ChR2 mouse line that, due to expression of channelrhodopsin-2 in inhibitory neurons (Zhao et al., 2011), blocks virtually all spikes in a column with high temporal precision, using blue light. I found that BCx functionality was required during CS presentation. However, mice learned normally when blocking BCx during the trace period. After learning, BCx activity during CS & trace was entirely dispensable for task performance. In summary, I demonstrate that the barrel column is involved in acquiring the TTEBC association. Nevertheless, the plasticity of the neuronal response in the trace period is a non- causal reflection of learning, and after learning, in the early phase of retention BCx is not needed for task performance. Future research need to establish if BCx assumes a more critical role in late consolidation. Further, the nature and projection of the signals measured during the learning have to be explored on the microscopic network and cellular level.