Presynaptic Ca2+ increases in accessory olfactory bulb mitral cell terminals in the amygdala.

For relevant publications see Mulligan and Delaney, 2001 and Delaney,2002

To obtain these images Ca2+-Green Dextran 3K MW was applied to the accessory olfactory bulb of a frog using a technique described by Gelperin A and Flores J 1997 J. Neurosci. Meth. 72: 97-108 and allowed to diffuse for 18 hours prior to preparation of the telencephalon and midbrain for in vitro stimulation and recording.

The upper image shows the low resting fluorescence characteristic of low resting Ca2+ levels in the cytoplasm of the terminals and axons. (The accessory olfactory bulb is much brighter because it contains a ball of brightly stained somata. Resting Ca is also low here and increases with electrical stimulation). In the lower image silver bipolar wires placed on the ventricular surface of the telencephalon are used to stimulate trains of action potentials (10 Hz for one second to obtain this image) in mitral cell axons that project to and terminate almost exclusively within the amygdala. Field potential recordings or single cell recordings from neurons in the amygdala permit correlations between the Ca influx in terminals (appears as an increase in the fluorescence intensity) with the release of neurotransmitter. Image obtained by Sean Mulligan.

Using a 20x water immersion lens to focus selectively on the terminals in the amygdala fluorescence transients in response to a train of 20 action potentials stimulated at a rate of 10 Hz were recorded with a photomultiplier tube. Note that the Ca transients build up rapidly and reach a plateau. Note also that the first transient is large compared to the second and subsequent transients. These observations suggest that the resting Ca is low (the terminals are healthy), that there is a large influx of Ca to each action potential and there are low amounts of dye in the terminals so normal Ca buffering properties are not strongly affected.

Addition of 10µM CNQX and 50 µM APV to block ionotropic glutamate receptors had no effect on the Ca transients indicating they are entirely presynaptic in origin.