Enhanced dendritic activity in awake rats

Abstract

Almost nothing is known about dendritic activity in awake animals and even less about its relationship to behavior. The tuft dendrites of layer 5 (L5) pyramidal neurons lie in layer 1, where long-range axons from secondary thalamic nuclei and higher cortical areas arrive. This class of input is very dependent on active thalamocortical loops and activity in higher brain areas and so is likely to be heavily influenced by the conscious state of the animal. If, as has been suggested, the dendrites of pyramidal neurons actively participate in this process, dendritic activity should greatly increase in the awake state. Here, we measured calcium activity in L5 pyramidal neuron dendrites using the ''periscope'' fiberoptic system. Recordings were made in the sensorimotor cortex of awake and anesthetized rats following sensory stimulation of the hindlimb. Bi-phasic dendritic responses evoked by hindlimb stimulation were extremely dependent on brain state. In the awake state, there was a prominent slow, delayed response whose integral was on average 14-fold larger than in the anesthetized state. Moreover, the dramatic increases in dendritic activity closely correlated to the strength of subsequent hindlimb movement. These changes were confined to L5 pyramidal dendrites and were not reflected in the response of layer 2/3 (L2/3) neurons to air-puff stimuli in general (which actually decreased in the awake state). The results demonstrate that the activity of L5 pyramidal dendrites is a neural correlate of awake behavior.calcium imaging ͉ calcium spike ͉ fiberoptics ͉ layer 5 pyramidal neuron ͉ neocortex T he changes that occur at the neuronal level from the anesthetized to the awake brain states is one of the most fundamental questions remaining in neuroscience (1-3). One leading hypothesis is that during awake sensory processing, thalamo-cortical interactions involving deep-layer pyramidal neurons result in non-specific thalamic nuclei projecting feedback input to the tuft dendrites of L5 pyramidal neurons (4). It is also hypothesized that higher cortical areas (more active during conscious processing) project to the same tuft dendrites in L1 in primary sensory areas (5, 6). Both these hypotheses predict specific input to pyramidal cell dendrites during the awake state. Given the intrinsic excitability of pyramidal tuft dendrites with their ability to sustain regenerative Ca 2ϩ activity (7,8), these hypotheses also predict that dendritic activity in these neurons should increase in the awake state.So far, it has only been possible to test the effectiveness of feedback input to L1 in vitro (9, 10). In vivo recordings of dendritic calcium activity has been restricted to anesthetized animals. In these studies, there is evidence for sensory stimulusevoked dendritic Ca 2ϩ spikes in the apical dendrites (11, 12), which is, however, suppressed by powerful dendritic inhibitory mechanisms (13) that are more active during anesthesia (14). These studies also predict that dendritic activity should increase in the awake preparation; howe...

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