Signal transmission in lobster olfactory receptor cells: functional significance of electrotonic structure analysed by a compartmental model

Relation between stimulus and response in frog olfactory receptor neurons in vivo

The spiking activity of receptor neurons was recorded extracellularly in the frog olfactory epithelium in response to four odourants applied at precisely controlled concentrations. A set of criteria was formulated to define the spikes in the response. Four variables - latency, duration, number of interspike intervals and frequency - were determined to quantify the responses. They were studied at the single neuron, neuron population and ciliary membrane levels. The dose-response curves were determined using specific functions and their characteristics were evaluated. The characteristic molar concentrations at threshold or at maximum duration and the characteristics of variables, e.g. minimum latency or maximum frequency, have asymmetric histograms with peaks close to the origin and long tails. Dynamic ranges have even more asymmetric histograms, so that a significant fraction of neurons presents a much wider range than their one-decade peak. From these histograms, response properties of the whole neuron population can be inferred. In general, location along the concentration axis (thresholds), width (dynamic ranges) and heights of dose-response curves are independent, which explains the diversity of curves, prevents their global categorization and supports the qualitative coding of odourants. No evidence for odourant-independent types of neurons was found. Finally, receptor activation and ciliary membrane conductance were reconstructed in the framework of a model based on firing data, known mucus biochemical and neuron morpho-electrical characteristics. It is in agreement with independent determinations of Kd of odourant-receptor interaction and of conductance characteristics, and describes their statistical distributions in the neuron population.

A simple analytical method for determining the steady-state potential in models of geometrically complex neurons

A method is presented for solving the cable equation for a spiking neuron below firing threshold or a nonspiking neuron of arbitrary geometry under constant stimulation. The neuron structure is considered as a tree composed of a set of cylinder cables of three types (terminal, intermediate and branching) characterized by their lengths, diameters and linear membrane properties. The stimulation can result from either a uniform conductance-change over a whole cable segment or a point injection of a current. Other special segments are considered (synapses, voltage clamp, lumped soma). Equations are given for replacing any segment with its Thévenin equivalent, i.e. resistance and electromotive force. The step by step use of these elementary equations allows one to find the Thévenin equivalent of the whole neuron and to determine the steady-state membrane potential at any point.

Mixture suppression without inhibition for binary mixtures from whole cell patch clamp studies of in situ olfactory receptor neurons of the spiny lobster

Whole cell patch clamping was used to investigate mechanisms of mixture suppression for in situ olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus. We used a set of single compounds and binary mixtures that have been used in previous biochemical studies of receptor-odorant binding, electrophysiological studies of spiking output from ORNs, and behavioral studies. These odorants were adenosine 5'-monophosphate (AMP), betaine (Bet), L-cysteine (Cys), L-glutamate (Glu), taurine (Tau), ammonium chloride, D,L-succinate, binary mixtures of these compounds, as well as a 33-component artificial oyster mixture (AOM). For the 40 ORNs studied, these stimuli more frequently elicited inward than outward currents. AMP, Glu, Tau and Bet evoked the largest and most numerous inward currents; Cys most commonly evoked outward currents. Na+ was an important charge-carrying ion for the Glu-evoked response in one ORN and the Bet-evoked response in another ORN. Mixture suppression, defined conservatively in this study as cases where the response to a binary mixture was less than the response to the more excitatory component of that mixture, was observed in 6 ORNs. In all 6 cases, neither component of the mixture evoked an outward conductance (i.e. neither was inhibitory). Five of these cases of mixture suppression involved a mixture containing two excitatory compounds (i.e. producing inward conductances): four ORNs were excited by both Glu and AMP, and one ORN was excited by both Tau and Glu. One case of mixture suppression occurred for a compound (Tau) tha did not produce a current when presented alone but which when added to Bet suppressed the inward current generated by Bet. Mechanisms for these suppressions are discussed, including inhibition of receptor binding by the components of a binary mixture and effects on second messengers or ion channels.