ODOR CODING PROPERTIES OF FROG OLFACTORY CORTICAL-NEURONS

Until now, in amphibians, response odor properties of primary cortical neurons had never been investigated. Furthermore, very few data on th is subject are available in other species. This prompted us to explore the functional properties of olfactory cortical neurons at rest and i n response to odors. To achieve this, our experience with odor coding in the first two stages of the frog olfactory system, the olfactory mu cosa and the olfactory bulb, led us to use odor stimuli which were che mical compounds with known stimulating properties, delivered to the mu cosa in controlled conditions over a wide concentration range. Most of the cortical neurons were found to be very silent at rest, their aver age spontaneous activity being significantly lower than that of bulb n eurons recorded previously in the same conditions. Cortical cells disp layed, with all odors combined: 35% excitatory responses and 8% inhibi tory responses. The excitatory response rate was similar to that of th e bulb, while the inhibitory response rate was about 4.5-fold lower. I nterestingly, two functional groups of cortical cells emerged based bo th on differences in response temporal patterning to odors delivered a t increasing concentrations and in qualitative discrimination power. R egarding intensity coding, group 1 cells (53%) displayed ''classical'' temporal pattern evolution, increase of discharge frequencies and dec rease of latency and burst duration, over the concentration range. The responses of group 2 cells (47%) were clearly original, since they co nsisted of a single spike (or more rarely two spikes) occurring with a strictly reproducible latency at a given concentration and a decrease d latency as a function of increasing concentration. The dynamics of c ell recruitment in the cortex show-ed that group 1 cell recruitment mi micked that of mitral cells, group 2 cells being recruited at higher c oncentrations. Thy analysis of qualitative discrimination properties o f cortical cells regarding the eight-odor set revealed that the discri mination power of group 2 cells was similar to that of mitral cells. B y contrast, the qualitative discrimination power of group 1 cells was found to be similar to that of neuroreceptor cells. In conclusion, thi s pioneer approach leads us to report that olfactory cortical neurons of the frog are responsive to odors and can be clearly divided into tw o groups based on functional criteria. Group 1 cells, which were relat ively selective, poorly discriminating but sensitive, may be mainly de voted to intensity coding. By contrast, group 2 cells, which were not very sensitive but were selective and discriminating, were hypothesize d to provide minimal intensity coding and thus to be mainly devoted to qualitative discrimination tasks. Copyright (C) 1996 IBRO.