A fundamental issue in cortical processing of sensory information is whether top-down control circuits from higher brain areas to primary sensory areas not only modulate but actively engage in perception. Here, we report the identification of a neural circuit for top-down control in the mouse somatosensory system. The circuit consisted of a long-range reciprocal projection between M2 secondary motor cortex and S1 primary somatosensory cortex. In vivo physiological recordings revealed that sensory stimulation induced sequential S1 to M2 followed by M2 to S1 neural activity. The top-down projection from M2 to S1 initiated dendritic spikes and persistent firing of S1 layer 5 (L5) neurons. Optogenetic inhibition of M2 input to S1 decreased L5 firing and the accurate perception of tactile surfaces. These findings demonstrate that recurrent input to sensory areas is essential for accurate perception and provide a physiological model for one type of top-down control circuit.

During tactile perception, long-range intracortical top-down axonal projections are essential for processing sensory information. Whether these projections regulate sleep-dependent long-term memory consolidation is unknown. We altered top-down inputs from higher-order cortex to sensory cortex during sleep and examined the consolidation of memories acquired earlier during awake texture perception. Mice learned novel textures and consolidated them during sleep. Within the first hour of non-rapid eye movement (NREM) sleep, optogenetic inhibition of top-down projecting axons from secondary motor cortex (M2) to primary somatosensory cortex (S1) impaired sleep-dependent reactivation of S1 neurons and memory consolidation. In NREM sleep and sleep-deprivation states, closed-loop asynchronous or synchronous M2-S1 coactivation, respectively, reduced or prolonged memory retention. Top-down cortical information flow in NREM sleep is thus required for perceptual memory consolidation.

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Hippocampal pyramidal neurons express various extrasynaptic glutamate receptors. When glutamate spillover was facilitated by blocking glutamate uptake and fast synaptic transmission was blocked by antagonists of AMPA- and NMDA-type glutamate receptors and an ionotropic GABA receptor blocker, repetitive synaptic stimulation evoked a persistent membrane depolarization that consisted of an early Ca(2+)-independent component and a late Ca(2+)-dependent component. The early component, which we refer to as a plateau potential, had a half-width of 770 +/- 160 ms and a steady peak level of -9.54 +/- 3.50 mV. It was accompanied by an increase in membrane conductance, the I-V relationship of which showed a peak at -19.91 +/- 2.18 mV and reversal of the current at -4.32 +/- 2.13 mV, and was suppressed by high concentration of an NMDA receptor (NMDAR) antagonist d-APV, or an NMDAR glycine-binding site antagonist 5,7-dCK. After blocking synaptically located NMDARs using MK801, the potential was still evoked synaptically when spillover was facilitated. A sustained depolarization was evoked by iontophoretic application of glutamate in the presence or absence of a glutamate uptake blocker. This potential was not affected by Na(+) or Ca(2+) channel blockers, but was suppressed by 5,7-dCK, leaving an unspecified depolarizing potential. Iontophoresis of NMDA evoked a sustained depolarization that was blocked by a high concentration of d-APV or 5,7-dCK. The I-V relationship of the current during this potential was similar to that obtained during the synaptically induced plateau potentials. These results show that CA1 pyramidal neurons generate plateau potentials mediated most likely by activation of extrasynaptic NMDARs.

Localized brain tissue damage activates surrounding astrocytes, which significantly influences subsequent long-term pathological processes. Most existing focal brain injury models in rodents employ craniotomy to localize mechanical insults. However, the craniotomy procedure itself induces gliosis. To investigate perilesional astrocyte activation under conditions where the skull is intact, we created focal brain injuries using light exposure through a cranial window made by thinning the skull that does not induce gliosis. The lesion size is maximal at ~12 h and shows substantial recovery over the subsequent 30 days. Two distinct types of perilesional reactive astrocyte, identified by GFAP upregulation and hypertrophy, were found. In proximal regions, the reactive astrocytes proliferated and expressed nestin, whereas in regions distal to the injury core, astrocytes showed increased GFAP expression but did not proliferate, lacked nestin expression, and displayed different morphology. Simply making the window did not induce any of these changes. There were also significant numbers of neurons in the recovering cortical tissue. In the recovery region, reactive astrocytes radially extended processes, which appeared to influence the shapes of neuronal nuclei. The proximal reactive astrocytes also formed a cell layer, which appeared to serve as a protective barrier, blocking the spread of IgG deposition and migration of microglia from the lesion core to surrounding tissue. The recovery was preceded by perilesional accumulation of leukocytes expressing vascular endothelial growth factor. These results suggest that under intact skull conditions, focal brain injury is followed by perilesional reactive astrocyte activities that foster cortical tissue protection and recovery.