Time-dependent modulation of pheromone-sensitive trichoid sensilla of the hawkmoth Manduca sexta
The pheromone release and also the male responsiveness to pheromone in nocturnal moths are controlled by circadian pacemakers. In Manduca sexta the circadian rhythm in the calling behaviour of female moths with a maximum at the end of the scotophase is correlated with peaks in the male flight activi...
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|Summary:||The pheromone release and also the male responsiveness to pheromone in nocturnal moths are controlled by circadian pacemakers. In Manduca sexta the circadian rhythm in the calling behaviour of female moths with a maximum at the end of the scotophase is correlated with peaks in the male flight activity. Furthermore circadian changes in the concentration of the biogenic amine octopamine (OA) in the hemolymph and brain modulate the moth´s sensitivity and responsiveness to pheromone. In addition the injection of OA into the hemolymph improved pheromone-source finding in wind tunnel experiments time-dependently. The OA-dependent sensitization of the male behavior might be partly due to central effects and partly due to the augmentation of the pheromone transduction. To investigate the time-dependent modulation of the pheromone transduction of olfactory receptor neurons (ORNs), OA and its precursor tyramine (TA) were applied via the recording electrode in long-term tip recordings of single trichoid sensilla of M. sexta. To search for time-dependent differences the recordings were performed at Zeitgebertime (ZT) 22-1, 1-4 or 8-11 (ZT 0 = lights on). The trichoid sensilla were stimulated with the main pheromone component bombykal (BAL) in a non-adapting protocol for 180 min. The perfusion of the sensillar lymph with OA and TA increased both the sensillar potential (SP) amplitude and the action potential (AP) frequency at ZT 8-11 but not at ZT 22-1. Furthermore the ORNs are adapted on the level of the AP response with beginning of the photophase and the application of OA and TA disadapts the ORNs at ZT 8-11. In addition the endogenous adaptation in the AP distribution in BAL responses at ZT 8-11 was antagonized by OA and TA. OA but not TA also increased the spontaneous AP frequency by increasing the number of bursts and by prolonging the bursts, without affecting the generation of single APs. Furthermore the OA-receptor antagonist epinastine (EPI) decreased the AP frequency strongly at ZT 8-11 and to a lesser extent at ZT 22-1, without affecting the SP. Also the spontaneous AP frequency was reduced by EPI to the same extent at ZT 22-1 and ZT 8-11. At ZT 22-1 EPI shifted the AP distribution in BAL responses from phasic to tonic responses. These results show that OA is the key factor in the time-dependent modulation of the pheromone-sensitivity of ORNs and that OA could be obligatory for the generation of strong and phasic BAL responses and thus could be necessary for a high pulse resolution of the ORNs. Because the majority of OA receptors is positively coupled to adenylyl cyclases it was determined whether cAMP mimics the OA-dependent disadaptation. The membrane-permeable cAMP analogue 8-bromo cAMP (8bcAMP) increased the SP amplitude in recordings at ZT 1-4 and ZT 8-11 but not at ZT 22-1. Furthermore 8bcAMP antagonized an endogenous adaptation in the mean SP amplitude at ZT 1-4 and partly at ZT 8-11. In contrast, 8bcAMP did not affect the normalized initial AP frequency or the distribution of APs in responses and did not antagonize the endogenous shift to lower mean AP frequencies at ZT 8-11. In addition, 8bcAMP increased the spontaneous AP frequency in recordings at ZT 8-11. Furthermore, the ORNs are also adapted on the level of the spontaneous activity with beginning of the photophase and 8bcAMP disadapts the ORNs at ZT 1-4 and ZT 8-11 by increasing both the number of bursts and spikes. In contrast, OA strongly increased the mean spontaneous AP frequency at ZT 8-11 by predominantly increasing the number of bursts. Thus, 8bcAMP only partly mimics the OA-dependent effects. Our results suggest that OA at least activates an adenylyl cyclase, additional OA-dependent effects on the Ca2+-levels are discussed. Because adapting pheromone stimuli cause rises of cyclic guanosine monophosphate (cGMP) in trichoid sensilla of M. sexta, we wanted to determine whether cGMP decreases the pheromone-sensitivity of ORNs in a time-dependent manner. The membrane-permeable cGMP analogue 8-bromo cGMP (8bcGMP) adapted the AP response but not the SP. Perfusion with 8bcGMP decreased the initial AP frequency and the numbers of APs in the first 100 ms of the BAL response. Furthermore, the decrease in the 8bcGMP-dependent AP frequency was stronger in recordings at ZT 8-11 than at ZT 1-4. In addition, at ZT 8-11 8bcGMP enhanced the endogenous shift from phasic to tonic responses. Furthermore 8bcGMP altered the waveform of spontaneous APs by increasing their peak-to-peak amplitude and by prolonging the negative phase. Thus, we hypothesize that additionally to changes in the OA concentration during the photophase, elevated cGMP levels underlie a daytime-dependent decrease in pheromone sensitivity and a decline in the temporal resolution of pheromone pulses.|