Short-term synaptic plasticity at the main and vomeronasal olfactory receptor to mitral cell synapse in frog.

Synaptic responses resulting from stimulation of the main olfactory and vomeronasal (VN) nerves were measured in main and accessory olfactory bulb (AOB) of frog, Rana pipiens, to test the hypothesis that properties of these synapses would reflect the distinct differences in the time course of odour delivery to each of these olfactory structures. Paired-pulse depression dominated responses to repetitive stimulation of the main olfactory nerve for interstimulus intervals (ISI) up to several seconds. Inhibition of voltage-gated Ca(2+) channels by GABAb receptors contributes significantly to this inhibition of transmitter release, particularly for ISI > 0.5 s. In contrast, the monosynaptic connection between VN sensory neurons and mitral cells in the AOB showed enhancement with pairs or short trains of stimuli for ISI of 0.5 to > 10 s. A small inhibitory effect of GABAb receptors on presynaptic Ca(2+) influx and release was only evident when a large proportion of the VN axons were stimulated simultaneously but even with inhibition present an overall enhancement of release was observed. Increasing the number of conditioning stimuli from one to five increased residual [Ca(2+)] and enhancement but a direct correlation between residual [Ca(2+)] and either the magnitude or the time course of enhancement was not observed. Enhanced transmitter release from VN afferent terminals results in effective integration of sustained low-frequency activity, which may play a role in the detection of low-intensity odourant stimuli by the VN system.