The effect of sensory deprivation on anatomical and physiological properties in two genetically defined types of layer 6 corticothalamic pyramidal cells in mouse somatosensory barrel cortex was investigated using in vitro electrophysiology. The two types analysed were the L6-Ntsr1 subtype, found preferentially in the upper region of layer 6 and projecting to both ventral posterior medial nucleus of the thalamus and posterior medial nucleus of the thalamus, and the L6-Drd1a subtype, located mostly in the lower regions of layer 6 and projecting to posterior medial nucleus. We found that the apical dendrite in L6-Ntsr1 cells is longer and more branched, compared with L6-Drd1a cells, and that the increase in firing frequency with increasing current stimulation is steeper in L6-Drd1a cells. Sensory deprivation was achieved clipping one row of whiskers from birth until the day of experiment (16 AE 2 days). Mice of this age are actively exploring. In L6-Ntsr1, but not in L6-Drd1a cells, sensory deprivation decreased apical and basal dendrite outgrowth, and calcium influx evoked by backpropagating action potentials. These results contribute to the ongoing functional characterisation of corticothalamic layer 6 cells and indicate differences in the postnatal cortical refinement of two distinct corticothalamic circuits.Abbreviations: aCSF, artificial cerebrospinal fluid; AP, action potential; bAP, backpropagating action potential; BP, bandpass filter, in optical setups; BS, beam splitter, in optical setups; CC, corticocortical, neurons located in the cortex with main axonal projections to cortical regions; CT, corticothalamic, neurons located in the cortex with axons reaching the thalamus; dLGN, dorsal lateral geniculate nucleus, part of visual thalamus; Drd1a, dopamine receptor D1; naming convention updated to Drd1; used here as name for mouse-line only; e.g., Latin for 'for example'; i.e., Latin for 'that is'; L2/3, layers 2 and 3 of the neocortex; L4, layer 4 of the neocortex; L5, layer 5 of the neocortex; L6, layer 6 of the neocortex, here primary somatosensory cortex; L6-Drd1a CT cells, singular term to describe cells positive for Drd1a, located in layer 6 of the neocortex and having their main axonal projection reach the thalamus. Here used as name for cell population.; L6-Ntsr1 CT cells, singular term to describe cells positive for Ntsr1, located in layer 6 of the neocortex and having their main axonal projection reach the thalamus. Here used as name for cell population.; NDD, nondescanned detectors, in optical setups; NonDep, cell group: from not directly affected regions through sensory deprivation; nRT, thalamic reticular nucleus; part of somatosensory thalamus; Ntsr1, neurotensin receptor type 1; gene; used here as name for mouse-line only; P14, describes the age of the mouse in days. P means postnatal day, starting from P0, the day of birth and counting up; PB, phosphate buffer solution; PFA, para-formaldehyde solution; PIA, pia mater; POm, posterior medial nucleus; part of somatosensory thalamus; SD, standard d...

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