Ceramide, a key intermediate in sphingolipid metabolism, is synthesized by acylation of sphinganine followed by dehydrogenation of dihydroceramide to ceramide. Using radioactive sphinganine, we have examined the site and topology of dihydroceramide synthesis in well-characterized subcellular fractions from rat liver. [4,5-3H]Sphinganine was introduced as a complex with BSA and was metabolized to [4,5-3H]dihydroceramide upon incubation of rat liver homogenates or microsomes with fatty acyl CoA. Conditions were established in a detergent-free system in which dihydroceramide synthesis was not limited by either substrate availability or by amounts of microsomal protein or reaction time. The distribution of dihydroceramide synthesis was found to exactly parallel that of an endoplasmic reticulum (ER) marker upon subfractionation of microsomes, and no endogenous activity was detected in either purified Golgi apparatus or plasma membrane fractions. Limited protease digestion demonstrated that sphinganine N-acyltransferase is localized at the cytosolic surface of intact ER-derived vesicles. These results are discussed with regard to the subsequent transport of (dihydro)-ceramide from the ER to sites of further metabolism in a pre-Golgi apparatus compartment and in the cis and medial cisternae of the Golgi apparatus.

Since the development of transcranial magnetic stimulation (TMS) in the early 1980s, a range of repetitive TMS (rTMS) protocols are now available to modulate neuronal plasticity in clinical and non-clinical populations. However, despite the wide application of rTMS in humans, the mechanisms underlying rTMS-induced plasticity remain uncertain. Animal and in vitro models provide an adjunct method of investigating potential synaptic and non-synaptic mechanisms of rTMS-induced plasticity. This review summarizes in vitro experimental studies, in vivo studies with intact rodents, and preclinical models of selected neurological disorders—Parkinson’s disease, depression, and stroke. We suggest that these basic research findings can contribute to the understanding of how rTMS-induced plasticity can be modulated, including novel mechanisms such as neuroprotection and neurogenesis that have significant therapeutic potential.

Repetitive transcranial magnetic stimulation (rTMS) is increasingly used as a treatment for neurological and psychiatric disorders. Although the induced field is focused on a target region during rTMS, adjacent areas also receive stimulation at a lower intensity and the contribution of this perifocal stimulation to network-wide effects is poorly defined. Here, we examined low-intensity rTMS (LI-rTMS)-induced changes on a model neural network using the visual systems of normal (C57Bl/6J wild-type, n = 22) and ephrin-A2A5^−/− (n = 22) mice, the latter possessing visuotopic anomalies. Mice were treated with LI-rTMS or sham (handling control) daily for 14 d, then fluorojade and fluororuby were injected into visual cortex. The distribution of dorsal LGN (dLGN) neurons and corticotectal terminal zones (TZs) was mapped and disorder defined by comparing their actual location with that predicted by injection sites. In the afferent geniculocortical projection, LI-rTMS decreased the abnormally high dispersion of retrogradely labeled neurons in the dLGN of ephrin-A2A5^−/− mice, indicating geniculocortical map refinement. In the corticotectal efferents, LI-rTMS improved topography of the most abnormal TZs in ephrin-A2A5^−/− mice without altering topographically normal TZs. To investigate a possible molecular mechanism for LI-rTMS-induced structural plasticity, we measured brain derived neurotrophic factor (BDNF) in the visual cortex and superior colliculus after single and multiple stimulations. BDNF was upregulated after a single stimulation for all groups, but only sustained in the superior colliculus of ephrin-A2A5^−/− mice. Our results show that LI-rTMS upregulates BDNF, promoting a plastic environment conducive to beneficial reorganization of abnormal cortical circuits, information that has important implications for clinical rTMS.