PRESYNAPTIC NETWORKS. Single-cell-initiated monosynaptic tracing reveals layer-specific cortical network modules

Individual cortical neurons can selectively respond to specific environmental features, such as visual motion or faces. How this relates to the selectivity of the presynaptic network across cortical layers remains unclear. We used single-cell-initiated, monosynaptically restricted retrograde transsynaptic tracing with rabies viruses expressing GCaMP6s to image, in vivo, the visual motion-evoked activity of individual layer 2/3 pyramidal neurons and their presynaptic networks across layers in mouse primary visual cortex. Neurons within each layer exhibited similar motion direction preferences, forming layer-specific functional modules. In one-third of the networks, the layer modules were locked to the direction preference of the postsynaptic neuron, whereas for other networks the direction preference varied by layer. Thus, there exist feature-locked and feature-variant cortical networks.

Local and global motion preferences in descending neurons of the fly

For a moving animal, optic flow is an important source of information about its ego-motion. In flies, the processing of optic flow is performed by motion sensitive tangential cells in the lobula plate. Amongst them, cells of the vertical system (VS cells) have receptive fields with similarities to optic flows generated during rotations around different body axes. Their output signals are further processed by pre-motor descending neurons. Here, we investigate the local motion preferences of two descending neurons called descending neurons of the ocellar and vertical system (DNOVS1 and DNOVS2). Using an LED arena subtending 240° × 95° of visual space, we mapped the receptive fields of DNOVS1 and DNOVS2 as well as those of their presynaptic elements, i.e. VS cells 1–10 and V2. The receptive field of DNOVS1 can be predicted in detail from the receptive fields of those VS cells that are most strongly coupled to the cell. The receptive field of DNOVS2 is a combination of V2 and VS cells receptive fields. Predicting the global motion preferences from the receptive field revealed a linear spatial integration in DNOVS1 and a superlinear spatial integration in DNOVS2. In addition, the superlinear integration of V2 output is necessary for DNOVS2 to differentiate between a roll rotation and a lift translation of the fly.

Integration of lobula plate output signals by DNOVS1, an identified premotor descending neuron

Many motion-sensitive tangential cells of the lobula plate in blowflies are well described with respect to their visual response properties and the connectivity among them. They have large and complex receptive fields with different preferred directions in different parts of their receptive fields matching the optic flow that occurs during various flight maneuvers. However, much less is known about how tangential cells connect to postsynaptic neurons descending to the motor circuits in the thoracic ganglion and how optic flow is represented in these downstream neurons. Here we describe the physiology and the connectivity of a prominent descending neuron called DNOVS1 (for descending neurons of the ocellar and vertical system). We find that DNOVS1 is electrically coupled to a subset of vertical system cells. The specific wiring leads to a preference of DNOVS1 for rotational flow fields around a particular body axis. In addition, DNOVS1 receives input from interneurons connected to the ocelli.

Functional neuroanatomy of the rhinophore of Aplysia punctata

Background

For marine snails, olfaction represents a crucial sensory modality for long-distance reception, as auditory and visual information is limited. The posterior tentacle of Aplysia, the rhinophore, is a chemosensory organ and several behavioural studies showed that the rhinophores can detect pheromones, initiate orientation and locomotion toward food. However the functional neuroanatomy of the rhinophore is not yet clear. Here we apply serotonin-immunohistochemistry and fluorescent markers in combination with confocal microscopy as well as optical recording techniques to elucidate the structure and function of the rhinophore of the sea slug Aplysia punctata.

Results

With anatomical techniques an overview of the neuroanatomical organization of the rhinophore is presented. Labelling with propidium iodide revealed one layer of cell nuclei in the sensory epithelium and densely packed cell nuclei beneath the groove of the rhinophore, which extends to about two third of the total length of the rhinophore. Serotonin immunoreactivity was found within the olfactory glomeruli underneath the epithelium as well as in the rhinophore ganglion. Retrograde tracing from the rhinophore ganglion with 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) demonstrated the connection of glomeruli with the ganglion. Around 36 glomeruli (mean diameter 49 μm) were counted in a single rhinophore. Fluorimetric measurements of intracellular Ca²⁺ levels using Fura-2 AM loading revealed Ca²⁺-responses within the rhinophore ganglion to stimulation with amino acids. Bath application of different amino acids revealed differential responses at different positions within the rhinophore ganglion.

Conclusion

Our neuroanatomical study revealed the number and position of glomeruli in the rhinophore and the rhinophore ganglion as processing stage of sensory information. Serotonin-immunoreactive processes were found extensively within the rhinophore, but was not detected within any peripheral cell body. Amino acids were used as olfactory stimuli in optical recordings and induced sensory responses in the rhinophore ganglion. The complexity of changes in intracellular Ca²⁺-levels indicates, that processing of odour information takes place within the rhinophore ganglion. Our neuroanatomical and functional studies of the rhinophore open up a new avenue to analyze the olfactory system in Aplysia.

Adrenocortical function in the very low birth weight infant: improved testing sensitivity and association with neonatal outcome

OBJECTIVE

To evaluate adrenocortical function in ill preterm infants and investigate potential relationships between plasma cortisol concentrations and major neonatal outcomes.

STUDY DESIGN

Randomized trial of adrenocorticotropic hormone (1-24ACTH) stimulation testing, followed by a chart review.

SETTING

Two level III neonatal intensive care units, Sacramento, Calif.

PARTICIPANTS

Sixty-seven very low birth weight infants, born at 32 weeks of gestation or earlier weighing 1500 gm or less, who had endotracheal intubation and indwelling arterial access.

RESULTS

Most infants (76%) had baseline cortisol concentrations < 414 nmol/L (15.0 micrograms/dl), and of those, only 36% responded to stimulation with 1-24ACTH, 0.1 microgram/kg. Raising the 1-24ACTH dose to 0.2 microgram/kg resulted in a response rate of 67% (p = 0.09) but decreased the sensitivity of the test. An elevated mean 11-deoxycortisol/cortisol ratio indicated that decreased 11 beta-hydroxylase activity may limit cortisol production in some infants. Infants with baseline cortisol concentrations less than 414 nmol/L (15.0 micrograms/dl) were more likely to have chronic lung disease (p < 0.002) and less likely to have severe intraventricular hemorrhage (p < 0.02). Response to 1-24ACTH was not associated with a detectable difference in outcome.

CONCLUSION

Many very low birth weight infants have low cortisol and ACTH concentrations and are unable to mount a cortisol response to physiologic doses (0.1 microgram/kg) of 1-24ACTH. These findings suggest that delayed maturation of adrenal response may result in physiologically inadequate cortisol concentrations in stressed very low birth weight infants. This delayed maturation may contribute to the development of chronic lung disease.