The dynamics of somatic exocytosis in monoaminergic neurons.

Some monoaminergic neurons can release neurotransmitters by exocytosis from their cell bodies. The amount of monoamine released by somatic exocytosis can be comparable to that released by synaptic exocytosis, though neither the underlying mechanisms nor the functional significance of somatic exocytosis are well understood. A detailed examination of these characteristics may provide new routes for therapeutic intervention in mood disorders, substance addiction, and neurodegenerative diseases. The relatively large size of the cell body provides a unique opportunity to understand the mechanism of this mode of neuronal exocytosis in microscopic detail. Here we used three photon and total internal reflection fluorescence microscopy to focus on the dynamics of the pre-exocytotic events and explore the nature of somatic vesicle storage, transport, and docking at the membrane of serotonergic neurons from raphe nuclei of the rat brain. We find that the vesicles (or unresolved vesicular clusters) are quiescent (mean square displacement, MSD ∼0.04 µm(2)/s) before depolarization, and they move minimally (<1 µm) from their locations over a time-scale of minutes. However, within minutes of depolarization, the vesicles become more dynamic (MSD ∼0.3 µm(2)/s), and display larger range (several µm) motions, though without any clear directionality. Docking and subsequent exocytosis at the membrane happen at a timescale (∼25 ms) that is slower than most synaptic exocytosis processes, but faster than almost all somatic exocytosis processes observed in endocrine cells. We conclude that, (A) depolarization causes de-tethering of the neurotransmitter vesicles from their storage locations, and this constitutes a critical event in somatic exocytosis; (B) their subsequent transport kinetics can be described by a process of constrained diffusion, and (C) the pre-exocytosis kinetics at the membrane is faster than most other somatic exocytosis processes reported so far.

CREB regulates excitability and the allocation of memory to subsets of neurons in the amygdala.

The mechanisms that determine how information is allocated to specific regions and cells in the brain are important for memory capacity, storage and retrieval, but are poorly understood. We manipulated CREB in a subset of lateral amygdala neurons in mice with a modified herpes simplex virus (HSV) and reversibly inactivated transfected neurons with the Drosophila allatostatin G protein-coupled receptor (AlstR)/ligand system. We found that inactivation of the neurons transfected with HSV-CREB during training disrupted memory for tone conditioning, whereas inactivation of a similar proportion of transfected control neurons did not. Whole-cell recordings of fluorescently tagged transfected neurons revealed that neurons with higher CREB levels are more excitable than neighboring neurons and showed larger synaptic efficacy changes following conditioning. Our findings demonstrate that CREB modulates the allocation of fear memory to specific cells in lateral amygdala and suggest that neuronal excitability is important in this process.

Fluorescence correlation microscopy with real-time alignment readout.

In confocal fluorescence correlation microscopy (FCM) it is important to ensure that the correlation measurement is actually performed at the chosen location of the three-dimensional image of the specimen. We present a confocal FCM design that provides an automatic real-time readout of the location in the confocal microscopic image, which is aligned with the detector of the fluorescence correlation spectrometer. The design accomplishes this without using any special positioning device. The design is based on an apertured fluorescence detector placed close to the back aperture of the objective lens and can be easily incorporated into virtually any confocal microscope. We demonstrate the method by performing FCM measurements of a dye diffusing on a cell membrane.