The role of neurogenesis in olfaction-dependent behaviors.

Newly born neurons continuously migrate into the main and accessory olfactory bulbs and modulate the output of projection neurons. Despite some contradictory results, it is becoming clear that these newly born neurons play an important role in the response to some odorant cues. In this minireview, we discuss the recent findings surrounding the functional significance of adult neurogenesis in olfaction-dependent behaviors

Genetic Methods to Identify and Manipulate Newly Born Neurons in the Adult Brain

Although mammalian neurogenesis is mostly completed by the perinatal period, new neurons are continuously generated in the subventricular zone of the lateral ventricle and the subgranular zone of the hippocampal dentate gyrus. Since the discovery of adult neurogenesis, many extensive studies have been performed on various aspects of adult neurogenesis, including proliferation and fate-specification of adult neural stem cells, and the migration, maturation and synaptic integration of newly born neurons. Furthermore, recent research has shed light on the intensive contribution of adult neurogenesis to olfactory-related and hippocampus-mediated brain functions. The field of adult neurogenesis progressed tremendously thanks to technical advances that facilitate the identification and selective manipulation of newly born neurons among billions of pre-existing neurons in the adult central nervous system. In this review, we introduce recent advances in the methodologies for visualizing newly generated neurons and manipulating neurogenesis in the adult brain. Particularly, the application of site-specific recombinases and Tet inducible system in combination with transgenic or gene targeting strategy is discussed in further detail

Attenuation of Synaptic Potentials in Dendritic Spines

Dendritic spines receive the majority of excitatory inputs in many mammalian neurons, but their biophysical properties and exact role in dendritic integration are still unclear. Here, we study spine electrical properties in cultured hippocampal neurons using an improved genetically encoded voltage indicator (ArcLight) and two-photon glutamate uncaging. We find that back-propagating action potentials (bAPs) fully invade dendritic spines. However, uncaging excitatory post-synaptic potentials (uEPSPs) generated by glutamate photorelease, ranging from 4 to 27 mV in amplitude, are attenuated by up to 4-fold as they propagate to the parent dendrites. Finally, the simultaneous occurrence of bAPs and uEPSPs results in sublinear summation of membrane potential. Our results demonstrate that spines can behave as electric compartments, reducing the synaptic inputs injected into the cell, while receiving bAPs are unmodified. The attenuation of EPSPs by spines could have important repercussions for synaptic plasticity and dendritic integration

A Distinct Role of the Queen in Coordinated Workload and Soil Distribution in Eusocial Naked Mole-Rats

We investigated how group members achieve collective decision-making, by considering individual intrinsic behavioural rules and behavioural mechanisms for maintaining social integration. Using a simulated burrow environment, we investigated the behavioural rules of coordinated workload for soil distribution in a eusocial mammal, the naked mole-rat (Heterocephalus glaber). We tested two predictions regarding a distinct role of the queen, a socially dominant individual in the caste system: the presence of a queen would increase the workload of other caste individuals, and the cues by a queen would affect the soil distribution. In experiment 1, we placed four individuals of various castes from the same colony into an experimental burrow. Workers exhibited the highest frequency of workload compared to other castes. The presence of a queen activated the workload by other individuals. Individuals showed a consistent workload in a particular direction so as to bias the soil distribution. These results suggest that individuals have a consensus on soil distribution and that the queen plays a distinct role. In experiment 2, we placed the odour of a queen in one of four cells and observed its effect on other individuals’ workload and soil distribution. Relative to other cells, individuals frequently dug in the queen cell so the amount of soil in the queen cell decreased. These results suggest that queen odour is an important cue in coordinated workload and soil distribution in this species

Optical Characterization of Potential Dampening in Dendritic Spine Electrophysiology

This study aims to quantitatively characterize the electrophysiology of the dendritic spine as compared to that of its adjacent dendritic shaft, by imaging artificially induced back-propagating action potentials using a variety of different genetically encoded voltage indicators. We performed whole cell patch clamp and current injection recordings with simultaneous voltage imaging of neonatal mouse hippocampal neurons, which were transfected to express ‘ArcLight’ or one of two variants of ‘Archaerhodopsin 3 (Arch)’ known as ‘QuasAr1’ and ‘QuasAr2’. With ArcLight, we coupled electrophysiological current injection recordings with fluorescence imaging and compared the individual peak fluorescence change () value of each spine with the peak  value of its adjacent dendritic shaft in response to induced back-propagating action potentials. The results from ArcLight do not indicate a statistically significant dampening in membrane potential from the dendrite shaft across an adjacent spine, but do suggest a difference in membrane potential fluctuations between long pulse (100msec) and short pulse (20msec) current injections. With Arch, we quantified the  values from current injection recordings and voltage imaging sessions of neuronal soma for ‘QuasAr1’ and ‘QuasAr2’. Preliminary current injection and soma imaging results initially exhibited negligible signal-to-noise ratios of  in response to induced action potentials, possibly due to technical maladjustments. A subsequent, more general characterization of Arch using voltage clamp and voltage step manipulations with QuasAr1 at different laser intensities indicates that QuasAr1 shows large changes in fluorescence at much weaker laser intensities (~10 mW) than was used for aforementioned current injection voltage imaging (~110 mW). We intend to further optimize and apply QuasAr1 and QuasAr2 to spine imaging, and subsequently investigate the difference in peak  values for different current injection pulse durations as suggested by ArcLight imaging data