While the transduction cascade of pheromone-sensitive olfactory receptor neurons (ORNs) in insects has been thoroughly investigated in vitro (Breer et al., 1988Boekhoff et al., 1990Boekhoff et al., , 1993Stengl et al., 1992;Stengl, 1993Stengl, , 1994Wegener et al., 1993Wegener et al., , 1997, little is known about olfactory adaptation and the mechanisms involved. Adaptation is a universal characteristic of receptor cells of all sensory modalities (Burkhardt, 1961). There are different definitions for the term 'adaptation' but implicit in any definition is a decrease in sensitivity due to the influence of a previous (conditioning) stimulus (Zack, 1979).Moths distinguish pheromone mixtures according to the concentration ratios of the different pheromone components. Thus, differentiation of pheromone concentrations is very crucial for recognition of prospective mates. Turbulences and wind velocities determine the structure of the pheromone filaments that stimulate the antenna of a flying moth. Thus, adapting and non-adapting pheromone stimuli of variable concentrations and stimulus durations reach different parts of the antenna at various time intervals. It is still unknown how the moth can recognize relevant pheromone ratios in various states of adaptation. In our study of pheromone-sensitive ORNs, we examine how adapting pheromone stimuli affect the encoding of different pheromone concentrations (quantity coding) in the intact moth. We distinguish the rapidly (within seconds to minutes) reversible reduction of sensitivity due to prior stimulation (short-term adaptation) from the decline in excitation, as seen during a phasic-tonic response to a stimulus of long duration (desensitization; Zufall and Leinders-Zufall, 2000). Thus, we compare quantity coding in response to short (50·ms) and long (1000·ms) pheromone stimuli, as possibly encountered during flight to the calling female. We do not examine the more slowly occurring (within several minutes to hours) reduction of sensitivity due to In extracellular tip recordings from long trichoid sensilla of male Manduca sexta moths, we studied dose-response relationships in response to bombykal stimuli of two different durations in the adapted and the non-adapted state. Bombykal-responsive cells could be distinguished from non-bombykal-sensitive cells in each trichoid sensillum because the bombykal-responsive cell always generated the action potentials of larger initial amplitude. The bombykal cell, which was recorded at a defined location within a distal flagellar annulus, can resolve at least four log 10-units of pheromone concentrations but is apparently unable to encode all stimulus durations tested. Parameters of the amplitudemodulated sensillar potential and the frequencymodulated action potential responses were examined in different states of adaptation. Evidence is presented for the existence of several mechanisms of adaptation, which affect distinct steps of the transduction cascade. After adapting pheromone stimuli, the sensillar potential rises to a lower ampl...

SUMMARY Pheromone-dependent mate search is under strict circadian control in different moth species. But it remains unknown whether daytime-dependent changes in pheromone sensitivity already occur at the periphery in male moths. Because adapting pheromone stimuli cause rises of cyclic guanosine monophosphate (cGMP) in pheromone-sensitive trichoid sensilla of the night-active hawkmoth Manduca sexta, we wanted to determine whether cGMP decreases pheromone-sensitivity of olfactory receptor neurons in a daytime-dependent manner. Long-term tip recordings from trichoid sensilla were performed at the early day (ZT 1-4), when many moths are still active, and at the middle of the day (ZT 8-11), when moths are resting. A non-adapting pheromone-stimulation protocol combined with perfusion of the sensillum lymph with the membrane-permeable cGMP analogue 8bcGMP adapted the action potential response but not the sensillar potential. Perfusion with 8bcGMP decreased the initial action potential frequency, decreased the numbers of action potentials elicited in the first 100 ms of the pheromone response and attenuated the reduction of action potential amplitude. Furthermore, the decrease in 8bcGMP-dependent action potential frequency was stronger in recordings made at ZT 8-11 than at ZT 1-4. In the control recordings during the course of the day the pheromone responses became increasingly tonic and less phasic. At ZT 8-11 only, this daytime-dependent effect was further enhanced by 8bcGMP application. Thus we hypothesize that during the moths' resting phase,elevated cGMP levels underlie a daytime-dependent decrease in pheromone sensitivity and a decline in the temporal resolution of pheromone pulses.

In the hawkmoth Manduca sexta, pheromone stimuli of different strength and duration rise the intracellular Ca2+ concentration in olfactory receptor neurons (ORNs). While second-long pheromone stimuli activate protein kinase C (PKC), which apparently underlies processes of short-term adaptation, minute-long pheromone stimuli elevate cyclic guanosine monophosphate (cGMP) concentrations, which correlates with time courses of long-term adaptation. To identify ion channels involved in the sliding adjustment of olfactory sensitivity, inside-out patch clamp recordings on cultured ORNs of M. sexta were performed to characterize Ca2+-, PKC-, and cGMP-dependent ion channels. Stepping to positive holding potentials in high intracellular Ca2+ elicits different Ca2+-dependent ion channels, namely small-conductance channels (2–20 ps), medium-conductance channels (20–100 ps), and large-conductance channels (>100 ps). Ion channels of 40, 60, and 70 ps opened after PKC activation, whereas 10- and >100-ps channels were observed less frequently. Application of 8-bromo cyclic guanosine monophosphate opened 55- and 70-ps channels and increased the open probability of >100-ps channels, whereas even in the presence of phorbol ester 40-ps channels were inhibited. Thus, cGMP elevations activate a different set of ion channels as compared with PKC and suppress at least one PKC-dependent ion channel.

Olfactory receptor neurons (ORNs) of the hawkmoth Manduca sexta sensitize via cAMP‐ and adapt via cGMP‐dependent mechanisms. Perforated patch clamp recordings distinguished 11 currents in these ORNs. Derivatives of cAMP and/or cGMP antagonistically affected three of five K+ currents and two non‐specific cation currents. The Ca2+‐dependent K+ current IK(Ca2+) and the sensitive pheromone‐dependent K+ current IK(cGMP−), which both express fast kinetics, were inhibited by 8bcGMP, while a slow K+ current, IK(cGMP+), was activated by 8bcGMP. Furthermore, application of 8bcAMP blocked slowly activating, zero mV‐reversing, non‐specific cation currents, ILL and Icat(PKC?), which remained activated in the presence of 8bcGMP. Their activations pull the membrane potential towards their 0‐mV reversal potentials, in addition to increasing intracellular Ca2+ levels voltage‐ and ILL‐dependently. Twenty minutes after application, 8bcGMP blocked a TEA‐independent K+ current, IK(noTEA), and a fast cation current, Icat(nRP), which both shift the membrane potential to negative values. We conclude that conditions of sensitization are maintained at high levels of cAMP, via specific opening/closure of ion channels that allow for fast kinetics, hyperpolarized membrane potentials, and low intracellular Ca2+ levels. In contrast, adaptation is supported via cGMP, which antagonizes cAMP, opening Ca2+‐permeable channels with slow kinetics that stabilize depolarized resting potentials. The antagonistic modulation of peripheral sensory neurons by cAMP or cGMP is reminiscent of pull–push mechanisms of neuromodulation at central synapses underlying metaplasticity.