High and asymmetric somato-dendritic coupling of V1 layer 5 neurons independent of visual stimulation and locomotion

Active dendrites impact sensory processing and behaviour. However, it remains unclear how active dendritic integration relates to somatic output in vivo. We imaged semi-simultaneously GCaMP6s signals in the soma, trunk and distal tuft dendrites of layer 5 pyramidal neurons in the awake mouse primary visual cortex. We found that apical tuft signals were dominated by widespread, highly correlated calcium transients throughout the tuft. While these signals were highly coupled to trunk and somatic transients, the frequency of calcium transients was found to decrease in a distance-dependent manner from soma to tuft. Ex vivo recordings suggest that low-frequency back-propagating action potentials underlie the distance-dependent loss of signals, while coupled somato-dendritic signals can be triggered by high-frequency somatic bursts or strong apical tuft depolarization. Visual stimulation and locomotion increased neuronal activity without affecting somato-dendritic coupling. High, asymmetric somato-dendritic coupling is therefore a widespread feature of layer 5 neurons activity in vivo.

Adrenergic Modulation Regulates the Dendritic Excitability of Layer 5 Pyramidal Neurons In Vivo

The excitability of the apical tuft of layer 5 pyramidal neurons is thought to play a crucial role in behavioral performance and synaptic plasticity. We show that the excitability of the apical tuft is sensitive to adrenergic neuromodulation. Using two-photon dendritic Ca²⁺ imaging and in vivo whole-cell and extracellular recordings in awake mice, we show that application of the α2A-adrenoceptor agonist guanfacine increases the probability of dendritic Ca²⁺ events in the tuft and lowers the threshold for dendritic Ca²⁺ spikes. We further show that these effects are likely to be mediated by the dendritic current I_h. Modulation of I_h in a realistic compartmental model controlled both the generation and magnitude of dendritic calcium spikes in the apical tuft. These findings suggest that adrenergic neuromodulation may affect cognitive processes such as sensory integration, attention, and working memory by regulating the sensitivity of layer 5 pyramidal neurons to top-down inputs.

Glutathione transferase theta in apical ciliary tuft regulates mechanical reception and swimming behavior of Sea Urchin Embryos

An apical tuft, which is observed in a wide range of embryos/larvae of marine invertebrates, is composed of a group of cilia that are longer and less motile than the abundant lateral cilia covering the rest of the embryonic surface. Although the apical tuft has been thought to function as a sensory organ, its molecular composition and roles are poorly understood. Here, we identified a glutathione transferase theta (GSTT) as an abundant and specific component of the apical tuft in sea urchin embryos. The expression of GSTT mRNA increases and becomes limited to the animal plate of the mesenchyme blastula, gastrula, and prism larva. Electron microscopy and tandem mass spectrometry demonstrated that the apical tuft contains almost every axonemal component for ciliary motility. Low concentrations of an inhibitor of glutathione transferase bromosulphophthalein (BSP) induce bending of apical tuft, suggesting that GSTT regulates motility of apical tuft cilia. Embryos treated with BSP swim with normal velocity and trajectories but show less efficiency of changing direction when they collide with an object. These results suggest that GSTT in the apical tuft plays an important role in the mechanical reception for the motility regulation of lateral motile cilia in sea urchin embryos.

Contributions of voltage-gated Ca2+ channels in the proximal versus distal dendrites to synaptic integration in prefrontal cortical neurons

The electrogenesis of synaptically activated dendritic Ca2+-mediated potentials, which may contribute to synaptic signal integration in pyramidal cells, was examined in rat layers V-VI prefrontal cortical (PFC) neurons in vitro. Intrasomatically recorded suprathreshold synaptic responses evoked by stimulation of the distal dendrites were attenuated by focal Cd2+ application to the proximal apical dendritic stem (100-200 micron from soma), but not to the apical dendritic tuft (>500 micron from soma). With use of intracellular QX-314 and Cs+ to block Na+ and K+ currents, intrasomatic recordings revealed that the Cd2+-induced attenuation of synaptic responses was attributable to the blockade of a dendritic Ca2+-mediated "hump" potential and high-threshold Ca2+ spike activated by NMDA EPSPs. The hump potential was not blocked by bath application of Ni2+ (100 microM) but was blocked by focal application of Cd2+ to the proximal but not distal apical dendrites, suggesting that it was generated by Ca2+ channels located in the proximal dendrites. Direct patch-clamp recordings made from the distal apical tuft of layers V-VI PFC neurons revealed that layers I-II synaptic stimulation or intradendritic depolarizing current pulses evoked tetrodotoxin- and QX-314-sensitive Na+ spikes. Unlike in the stem of the apical dendrite, Ca2+ spikes were not easily evoked in the distal apical tuft when Na+ channels were blocked. When triggered, the Cd2+-sensitive Ca2+ spikes in the dendritic tuft were nonregenerative and had very high activation thresholds (approximately +10 mV). These results suggested that the high voltage-activated Ca2+ potentials that amplify distal EPSPs are primarily generated in the proximal stem of the apical dendrite and not within the fine dendritic branches of the apical tuft of layers V-VI PFC neurons.