Odor processing in the frog olfactory system

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. From the responses of primary second-order neurons, neuroreceptor and mitral cells, odor stimuli could be classified in qualitative groups, revealing that neuronal discriminative mechanisms are partly based on the structure of odor molecule. In the olfactory bulb, thanks both to the anatomical convergence of primary afferences and intrinsic network properties, mitral cells have been demonstrated to gain in odor discrimination and detection power abilities. GABAergic bulbar interneurons were found to be involved in the control of mitral cell excitability, adjusting response thresholds and duration and promoting a progressive increase of burst discharges with stimulus concentration. Otherwise, dopamine was observed to shunt off mitral cell spontaneous activity without altering their odor responsivity properties. Dopamine was demonstrated to act through D2 receptors. Matching anatomical and electrophysiological data, D2 receptors are assumed to be localized on mitral cells. The frog olfactory cortex neurons, silent at rest, could be segregated in two functional groups basing on their odor response properties. The first group shared most intensity coding properties with mitral cells while showing a lower discriminative power, similar to that of neuroreceptor cells. By contrast, the second group provided only minimal intensity coding and, basing on its high discrimination power, was assumed to be mainly devoted to odor discrimination. Thus, along the olfactory pathways, intensity and quality odor parameters which are simultaneously encoded by a neuroreceptor or mitral cell, become specified by two distinct populations in the cortex.