The description of column activities in the visual cortex by D. Hubel and T Wiesel in 1959 confirmed the hypothesis formulated by Cajal. The cortical organization and columns converge on the pyramidal cells from layer 6 that project their axons toward other brain areas. Often seen as a computational unit with recurrent feedback, the pyramidal cell receives converging information from its apical tuft and

from dendrites localized on its cell body. An Cyclopamine important particularity of the pyramidal cell is the back propagation of the action potential from the cell body along Inhibitors,research,lifescience,medical the proximal dendrite.32,33 This process was shown to enhance or inhibit the signals transmitted by the apical tuft, and provides an exquisite mechanism for the integration of multiple inputs (Figure 1). This mechanism depends on the firing rate of action potentials in the axon hillock, as well as synchrony of activities, and it was shown Inhibitors,research,lifescience,medical that back propagation can be disrupted by the activation of ion channels expressed along the principal dendrite.34,35 Figure 1. Schematic representation of pyramidal cells with the

en-passant fibers. Activation of the nicotinic receptors located along the main dendrite produce a short circuit of the signals comng from the apical tuft. Nicotinic Inhibitors,research,lifescience,medical receptors are broadly Inhibitors,research,lifescience,medical expressed in the cortical area with some subtypes such as α7 receptors preponderantly expressed in presynaptic areas, whereas heteromeric receptors are expressed on cell bodies, main dendrites, axons, etc.36-38 Nicotine exposure has been shown to enhance attention and working memory by activating nicotinic receptors.

Although our understanding of these effects remains limited, nicotine increases the threshold for synaptic spike-timingdependent-potentiation (STDP) in layer 5 of the prefrontal cortex of the mouse.39 Systemic administration of nicotinic agonists such as SSR180711 or PNU-282987 has shown beneficial cognitive effects and reversal of amphetamine-induced deficits, Inhibitors,research,lifescience,medical thought to be mediated by acting at the microcircuit cortical level.40,41 Importantly, cholinergic projections that diffusely innervate the cortex are thought to make en-passant connections in the area of the principal dendrite of the pyramidal cells from layer 5 and 6.42 Activation Parvulin of these fibers causes the release, in a volumic manner, of acetylcholine that will diffuse and slowly activate receptors expressed on the principal dendrite. Opening of the nicotinic acetylcholine receptors reduces the membrane resistance and attenuates signals coming from the apical dendrites. This reduction of the influence of the apical tuft and external layer contributions modifies the integration and “computation” of the pyramidal cell.