Protocerebral Bridge Neurons That Regulate Sleep in Drosophila melanogaster

The central complex is one of the major brain regions that control sleep in Drosophila. However, the circuitry details of sleep regulation have not been elucidated yet. Here, we show a novel sleep-regulating neuronal circuit in the protocerebral bridge (PB) of the central complex. Activation of the PB interneurons labeled by the R59E08-Gal4 and the PB columnar neurons with R52B10-Gal4 promoted sleep and wakefulness, respectively. A targeted GFP reconstitution across synaptic partners (t-GRASP) analysis demonstrated synaptic contact between these two groups of sleep-regulating PB neurons. Furthermore, we found that activation of a pair of dopaminergic (DA) neurons projecting to the PB (T1 DA neurons) decreased sleep. The wake-promoting T1 DA neurons and the sleep-promoting PB interneurons formed close associations. Dopamine 2-like receptor (Dop2R) knockdown in the sleep-promoting PB interneurons increased sleep. These results indicated that the neuronal circuit in the PB, regulated by dopamine signaling, mediates sleep-wakefulness.

Excitatory postsynaptic calcium transients at Aplysia sensory-motor neuron synapses allow for quantal examination of synaptic strength over multiple days in culture

The ability to monitor changes in strength at individual synaptic contacts is required to test the hypothesis that specialized synapses maintain changes in synaptic strength that underlie memory. Measuring excitatory post-synaptic calcium transients through calcium permeable AMPA receptors is one way to monitor synaptic strength at individual synaptic contacts. Using a membrane targeted genetic calcium sensor, we demonstrate that one can measure synaptic events at individual synaptic contacts in Aplysia sensory-motor neuron synapses. These results show that synaptic strength is not evenly distributed between all contacts in these cultures, but dominated by multiquantal sites of synaptic contact. The probability, quantal size and quantal content can be measured over days at individual synaptic contacts using this technique. Surprisingly, most synaptic contacts were not found opposite presynaptic varicosities, but instead at areas of pre- and post-synaptic contact with no visible thickening of membranes. This technique shows promise in being able to address whether specialized synapses maintain synaptic strength underlying memory.

Synaptic counts approximate synaptic contact area in Drosophila

The pattern of synaptic connections among neurons defines the circuit structure, which constrains the computations that a circuit can perform. The strength of synaptic connections is costly to measure yet important for accurate circuit modeling. It has been shown that synaptic surface area correlates with synaptic strength, yet in the emerging field of connectomics, most studies rely instead on the counts of synaptic contacts between two neurons. Here we quantified the relationship between synaptic count and synaptic area as measured from volume electron microscopy of the larval Drosophila central nervous system. We found that the total synaptic surface area, summed across all synaptic contacts from one presynaptic neuron to a postsynaptic one, can be accurately predicted solely from the number of synaptic contacts, for a variety of neurotransmitters. Our findings support the use of synaptic counts for approximating synaptic strength when modeling neural circuits.

Frameshift mutations of YPEL3 alter sensory circuit function in Drosophila

ABSTRACTA frameshift mutation in Yippee-like (YPEL) 3 was recently found from a rare human disorder with peripheral neurological conditions including hypotonia and areflexia. The YPEL gene family is highly conserved from yeast to human, but their functions are poorly defined. Moreover, the pathogenicity of the human YPEL3 variant is completely unknown. To tackle these issues, we generated a Drosophila model of human YPEL3 variant by CRISPR-mediated In-del mutagenesis. Gene-trap analysis suggests that Drosophila YPEL3 (dYPEL3) is predominantly expressed in subsets of neurons, including nociceptors. Analysis on chemical nociception induced by allyl-isothiocyanate (AITC), a natural chemical stimulant, revealed a reduced nociceptive response in dYPEL3 mutants. Subsequent circuit analysis showed a reduction in the activation of second-order neurons (SONs) in the pathway without affecting nociceptor activation upon AITC treatment. Although the gross axonal and dendritic development of nociceptors was not affected, the synaptic contact between nociceptors and SONs were decreased by dYPEL3 mutations. Together, these suggest that the frameshift mutation in human YPEL3 causes neurological conditions by weakening synaptic connection through presynaptic mechanisms.