PARTITIONING NONLINEARITIES IN THE RESPONSE OF HONEY-BEE OLFACTORY RECEPTOR NEURONS TO BINARY ODORS

In many organisms, of which honey bees are one example, a general (i.e ., non-pheromonal) olfactory receptor neuron may respond to some odora nts by increasing its firing rate and to others by decreasing its firi ng rate. In the latter case, this decrease will be with respect to a b ackground firing rate determined by intrinsic (internal noise) and ext rinsic (background odors) factors. To analyse receptor neurons of this complexity, we extend Beidler's model of receptor protein activation dynamics to account for the competition between depolarizing and hyper polarizing pathways and couple the model to a phenomenological descrip tion of the non-linear relationship between the proportion of activate membrane receptors and the receptor cell spike generation rates. We t hen examine the implications of this theory for predicting the respons e of receptor neurons to odor mixtures based on their response to pure odorants at concentrations matched to the mixture. We derive inequali ties that must be satisfied under our normative model, and propose tha t deviations from the model be designated as synergisms and inhibition s, depending on the direction in which various equalities and inequali ties are violated. We then apply our inequalities to identifying syner gisms and inhibitions in data analysed in a different way elsewhere (A kers, R.P. and Getz, W.M. Response of olfactory receptor neurons in ho ney bees to odorants and their binary mixtures. J. Comp. Physiol. (in press)). In these data regarding the response of honey bee placode sen silla to a number of odorants and their binary combinations, we demons trate the presence of synergisms and inhibitions - that is, elevated o r repressed responses that are not due to competitive interactions of mixture component odorants for receptor sites or Beidler (Beidler, L.M ., 1962. Taste receptor stimulation. Prog. Biophys. Biophys. Chem. 12, 107-151) saturation mechanisms.