Pioneer axons from the cingulate cortex initiate corpus callosum (CC) development, yet nothing is known about the molecules that regulate their guidance. We demonstrate that neuropilin 1 (Npn1) plays an integral role in the development of the CC. Npn1 is localized to axons of cingulate neurons as they cross the midline, and multiple class 3 semaphorins (Semas) are expressed around the developing CC, implicating these guidance molecules in the regulation of Npn1-expressing axons emanating from the cingulate cortex. Furthermore, axons from the cingulate cortex display guidance errors in Npn1(Sema-) mice, a knockin mouse line in which Npn1 is unable to bind Semas. Analysis of mice deficient in the transcription factor Emx2 demonstrated that the cingulate cortex of these mice was significantly reduced in comparison to wild-type controls at E17 and that the CC was absent in rostral sections. Expression of Npn1 was absent in rostral sections of Emx2 mutants, suggesting that Npn1-expressing cingulate pioneers are required for CC formation. These data highlight a central role for Npn1 in the development of projections from the cingulate cortex and further illustrate the importance of these pioneer axons in the formation of the CC.

The left and right sides of the nervous system communicate via commissural axons that cross the midline during development using evolutionarily conserved molecules. These guidance cues have been particularly well studied in the mammalian spinal cord, but it remains unclear whether these guidance mechanisms for commissural axons are similar in the developing forebrain, in particular for the corpus callosum, the largest and most important commissure for cortical function. Here, we show that Netrin1 initially attracts callosal pioneering axons derived from the cingulate cortex, but surprisingly is not attractive for the neocortical callosal axons that make up the bulk of the projection. Instead, we show that Netrindeleted in colorectal cancer signaling acts in a fundamentally different manner, to prevent the Slit2-mediated repulsion of precrossing axons thereby allowing them to approach and cross the midline. These results provide the first evidence for how callosal axons integrate multiple guidance cues to navigate the midline.

General anesthesia remains a mysterious phenomenon, even though a number of compelling target proteins and processes have been proposed [1]. General anesthetics such as isoflurane abolish behavioral responsiveness in all animals, and in the mammalian brain, these diverse compounds probably achieve this in part by targeting endogenous sleep mechanisms [2, 3]. However, most animals sleep [4], and they are therefore likely to have conserved sleep processes. A decade of neurogenetic studies of arousal in Drosophila melanogaster have identified a number of different neurons and brain structures that modulate sleep duration in the fly brain [5-9], but it has remained unclear until recently whether any neurons might form part of a dedicated circuit that actively controls sleep and wake states in the fly brain, as has been proposed for the mammalian brain [10]. We studied general anesthesia in Drosophila by measuring stimulus-induced locomotion under isoflurane gas exposure. Using a syntaxin1A gain-of-function construct, we found that increasing synaptic activity in different Drosophila neurons could produce hypersensitivity or resistance to isoflurane. We uncover a common pathway in the fly brain controlling both sleep duration and isoflurane sensitivity, centered on monoaminergic modulation of sleep-promoting neurons of the fan-shaped body.