Manganese-enhanced 3D MRI of established and disrupted synaptic activity in the developing insect brain in vivo

The antennal lobe of the sphinx moth Manduca sexta serves as a model for the development of the olfactory system. Here, the establishment of the glomerular synaptic network formed by the olfactory receptor axons and antennal lobe neurons at pupal stage P12 was followed by transection of the right antenna and – within 24 h – by injection of MnCl2 into the hemolymph. In vivo 3D MRI at 100 and 60 μm isotropic resolution was then performed at P13 to P17. Whereas the left antennal lobe revealed a pronounced increase of the signal-to-noise ratio (SNR) reflecting normal synaptic activity, the observation of only a small SNR increase within the right antennal lobe indicated the disruption of pertinent activity after antennal transection. The accumulation of manganese in the intact antennal system became observable within 3 h and lasted for at least 2 days after injection. Intra-individual comparisons between the right and left side yielded a statistically significant differential SNR increase in the left antennal lobe. Because such an effect was not observed in younger animals studied at pupal stages P10/P11, the MRI findings confirm the development of functional synapses in the antennal lobe of Manduca sexta by P13.