Responses to prolonged odour stimulation in frog olfactory receptor cells

1. The suction pipette technique was used to record receptor current and sp iking responses from isolated frog olfactory receptor cells during prolonge d odour stimuli. 2. The majority (70 %) of cells displayed 'oscillatory' responses, consisti ng of repeated bursts of spikes accompanied by regular increases in recepto r current. The period of this oscillation varied from 3.5 to 12 a in differ ent cells. The remaining cells responded either with a 'transient' burst of spikes at the onset of stimulation (10 %), or by 'sustained' firing throug hout the odour stimulus (20 %). 3. In cells with oscillatory responses, the Ca2+-activated Cl- channel bloc ker niflumic acid prolonged the period of oscillation only slightly, despit e a 3.8-fold decrease in the receptor current. A 3-fold reduction in the ex ternal Cl- concentration nearly doubled the receptor current, but had littl e effect on the oscillation period. These results imply that the majority o f the receptor current underlying these oscillatory responses is carried by the Ca2+-activated Cl- conductance, suggesting that the intracellular Ca2 concentration oscillates also. 4. In cells with oscillatory responses, the period of oscillation was prolo nged 1.5-fold when stimulated in a low-Na+ solution designed to incapacitat e Na+-Ca2+ exchange, irrespective of whether Na+ was replaced by permeant L i+ or impermeant choline. The dependence of the oscillation period upon ext ernal Na+ suggests that it may be governed by the dynamics of Ca2+ extrusio n via Na+-Ca2+ exchange. 5. Exposure to the membrane-permeable cyclic nucleotide analogue CPT-cAMP e voked a sustained rather than an oscillatory response even in cells with os cillatory responses to odour. The inability of CPT-cAMP to evoke an oscilla tory response suggests that the cAMP concentration is likely to oscillate a lso. 6. Perforated-patch recordings revealed that oscillatory responses could on ly be evoked when the membrane potential was free to change, but not when i t was clamped near the resting potential. Since substantial changes in Ca2-activated Cl- current, and hence odour-induced depolarisation, had little effect upon the period of oscillation, changes in membrane potential are su ggested to play only a permissive role in these oscillatory responses. 7. These results are interpreted in terms of the coupled oscillation of Ca2 + and cyclic nucleotide concentrations within the olfactory cilia during pr olonged odour stimulation.