Perfusion with cAMP analogue affects pheromone-sensitive trichoid sensilla of the hawkmoth Manduca sexta in a time-dependent manner

Hawkmoth Pheromone Transduction Involves G-Protein-Dependent Phospholipase Cβ Signaling

Evolutionary pressures adapted insect chemosensation to their respective physiological needs and tasks in their ecological niches. Solitary nocturnal moths rely on their acute olfactory sense to find mates at night. Pheromones are detected with maximized sensitivity and high temporal resolution through mechanisms that are mostly unknown. While the inverse topology of insect olfactory receptors and heteromerization with the olfactory receptor coreceptor suggest ionotropic transduction via odorant-gated receptor-ion channel complexes, contradictory data propose amplifying G-protein-coupled transduction. Here, we used in vivo tip-recordings of pheromone-sensitive sensilla of male Manduca sexta hawkmoths at specific times of day (rest vs activity). Since the olfactory receptor neurons distinguish signal parameters in three consecutive temporal windows of their pheromone response (phasic; tonic; late, long-lasting), respective response parameters were analyzed separately. Disruption of G-protein-coupled transduction and block of phospholipase C decreased and slowed the phasic response component during the activity phase of hawkmoths without affecting any other component of the response during activity and rest. A more targeted disruption of Gα subunits by blocking Gαo or sustained activation of Gαs using bacterial toxins affected the phasic pheromone response, while toxins targeting Gαq and Gα12/13 were ineffective. Consistent with these data, the expression of phospholipase Cβ4 depended on zeitgeber time, which indicates circadian clock-modulated metabotropic pheromone transduction cascades that maximize sensitivity and temporal resolution of pheromone transduction during the hawkmoth's activity phase. Thus, discrepancies in the literature on insect olfaction may be resolved by considering circadian timing and the distinct odor response components.

Serotonergic Modulation of Olfactory Processing in Locust Antennae

Insects have sophisticated olfactory systems that enable them to detect and respond to complex exogenous chemical cues. The encoding mechanisms of these chemical signals have been studied both in their peripheral and central nervous systems (CNS). While many neuromodulators have been shown to play significant roles in olfactory processing within the antennal lobes of the brain, their roles in peripheral olfactory sensory systems, such as the antennae, are less understood. This review focuses on the role of serotonin (5-HT) receptor in the locust antenna, specifically the modulatory function of the serotonin receptor2 on odour inputs. We also review recent studies on the modulation of olfaction in the peripheral nervous systems of other insects and discuss potential directions for future research on the role of neuromodulators in insect peripheral olfactory systems.

Cyclic nucleotide-dependent ionic currents in olfactory receptor neurons of the hawkmoth Manduca sexta suggest pull-push sensitivity modulation

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 I_K(Ca ²⁺ ₎ and the sensitive pheromone-dependent K⁺ current I_K(cGMP-) , which both express fast kinetics, were inhibited by 8bcGMP, while a slow K⁺ current, I_K(cGMP+) , was activated by 8bcGMP. Furthermore, application of 8bcAMP blocked slowly activating, zero mV-reversing, non-specific cation currents, I_LL and I_cat(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 I_LL -dependently. Twenty minutes after application, 8bcGMP blocked a TEA-independent K⁺ current, I_K(noTEA) , and a fast cation current, I_cat(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.

The Diacylglycerol Analogs OAG and DOG Differentially Affect Primary Events of Pheromone Transduction in the Hawkmoth Manduca sexta in a Zeitgebertime-Dependent Manner Apparently Targeting TRP Channels

For the hawkmoth Manduca sexta accumulating evidence suggests that pheromone transduction acts via a metabotropic signal transduction cascade, with G-protein-dependent phospholipase C (PLC) activations generating diacylglycerol (DAG) and inositol trisphosphate as the primary events in hawkmoth pheromone transduction. In contrast, ionotropic olfactory receptor (OR) coreceptor (Orco)-dependent mechanisms do not appear to be involved. In hawkmoths pheromones activated a specific sequence of PLC-dependent ion channels of unknown identity. In several sensory systems transient receptor potential (TRP) ion channels were found downstream of PLC as primary transduction channels. Also in the mammalian vomeronasal organ, DAG-dependent TRP channels are employed. Therefore, we hypothesized that TRPs may be downstream targets for DAG also in the hawkmoth pheromone signal transduction pathway. To test this, we employed two DAG analogs, OAG and DOG for in vivo single-sensillum tip-recordings of pheromone-sensitive sensilla. Since olfactory receptor neurons (ORNs) expressed circadian changes in sensitivity throughout the day, we recorded at two different Zeitgebertimes (ZTs), the hawkmoths activity phase at ZT 1 and its resting phase at ZT 9. We found that the DAG analogs targeted at least two different TRP-like channels that underlie the primary events of hawkmoth pheromone transduction daytime-dependently. At both ZTs OAG sped up and increased the Orco-independent phasic action potential response without affecting the Orco-dependent late, long-lasting pheromone response. Thus, OAG most likely opened a transient Ca2+ permeable TRP channel that was available at both ZTs and that opened pheromone-dependently before Orco. In contrast, DOG slowed down and decreased the sensillum potential, the phasic-, and the late, long-lasting pheromone response. Therefore, DOG appeared to activate a protein kinase C (PKC) that closed TRP-like Ca2+ permeable channels and opened Ca2+ impermeable cation channels, which have been previously described and are most abundant at ZT 9. These data support our hypothesis that hawkmoth pheromone transduction is mediated by metabotropic PLC-dependent mechanisms that activate TRP-like channels as the primary event of pheromone transduction. In addition, our data indicate that at different times of the day different second messenger-dependent ion channels are available for pheromone transduction cascades.

Tuning Insect Odorant Receptors

Among the insect olfactory receptors the odorant receptors (ORs) evolved in parallel to the onset of insect flight. A special property of this receptor type is the capability to adjust sensitivity of odor detection according to previous odor contacts. This article presents a current view on regulatory processes affecting the performance of ORs and proposes a model of mechanisms contributing to OR sensitization.

Biogenic Amines in Insect Antennae

Insect antenna is a multisensory organ, each modality of which can be modulated by biogenic amines. Octopamine (OA) and its metabolic precursor tyramine (TA) affect activity of antennal olfactory receptor neurons. There is some evidence that dopamine (DA) modulates gustatory neurons. Serotonin can serve as a neurotransmitter in some afferent mechanosensory neurons and both as a neurotransmitter and neurohormone in efferent fibers targeted at the antennal vessel and mechanosensory organs. As a neurohormone, serotonin affects the generation of the transepithelial potential by sensillar accessory cells. Other possible targets of biogenic amines in insect antennae are hygro- and thermosensory neurons and epithelial cells. We suggest that the insect antenna is partially autonomous in the sense that biologically active substances entering its hemolymph may exert their effects and be cleared from this compartment without affecting other body parts.

No Evidence for Ionotropic Pheromone Transduction in the Hawkmoth Manduca sexta

Insect odorant receptors (ORs) are 7-transmembrane receptors with inverse membrane topology. They associate with the conserved ion channel Orco. As chaperon, Orco maintains ORs in cilia and, as pacemaker channel, Orco controls spontaneous activity in olfactory receptor neurons. Odorant binding to ORs opens OR-Orco receptor ion channel complexes in heterologous expression systems. It is unknown, whether this also occurs in vivo. As an alternative to this ionotropic transduction, experimental evidence is accumulating for metabotropic odor transduction, implicating that insect ORs couple to G-proteins. Resulting second messengers gate various ion channels. They generate the sensillum potential that elicits phasic-tonic action potentials (APs) followed by late, long-lasting pheromone responses. Because it is still unclear how and when Orco opens after odor-OR-binding, we used tip recordings to examine in vivo the effects of the Orco antagonist OLC15 and the amilorides MIA and HMA on bombykal transduction in the hawkmoth Manduca sexta. In contrast to OLC15 both amilorides decreased the pheromone-dependent sensillum potential amplitude and the frequency of the phasic AP response. Instead, OLC15 decreased spontaneous activity, increased latencies of phasic-, and decreased frequencies of late, long-lasting pheromone responses Zeitgebertime-dependently. Our results suggest no involvement for Orco in the primary transduction events, in contrast to amiloride-sensitive channels. Instead of an odor-gated ionotropic receptor, Orco rather acts as a voltage- and apparently second messenger-gated pacemaker channel controlling the membrane potential and hence threshold and kinetics of the pheromone response.

Octopamine regulates antennal sensory neurons via daytime-dependent changes in cAMP and IP3 levels in the hawkmoth Manduca sexta

The biogenic amine octopamine (OA) mediates reward signals in olfactory learning and memory as well as circadian rhythms of sleep and activity. In the crepuscular hawkmoth Manduca sexta, OA changed pheromone detection thresholds daytime-dependently, suggesting that OA confers circadian control of olfactory transduction. Thus, with enzyme-linked immunosorbent assays we searched hawkmoth antennae for daytime-dependent changes in the concentration of OA and its respective second messengers. Antennal stimulation with OA raised cAMP- and IP₃ levels. Furthermore, antennae expressed daytime-dependent changes in the concentration of OA, with maxima at Zeitgebertime (ZT) 20 when moths were active and also maximal concentrations of cAMP occurred. Maximal IP₃ levels at ZT 18 and 23 correlated with maximal flight activity of male moths, while minimal IP₃ levels at dusk correlated with peaks of feeding activity. Half maximal effective concentration (EC₅₀) for activation of the OA-receptor decreased during the moth’s activity phase suggesting daytime-dependent changes in OA receptor sensitivity. With an antiserum against tyramine, the precursor of OA, two centrifugal neurons were detected projecting out into the sensory cell layer of the antenna, possibly mediating more rapid stimulus-dependent OA actions. Indeed, in fast kinetic assays OA receptor stimulation increased cAMP concentrations within 50 msec. Thus, we hypothesize that fast, stimulus-dependent centrifugal control of OA-release in the antenna occurs. Additional slow systemic OA actions might be based upon circadian release of OA into the hemolymph mediating circadian rhythms of antennal second messenger levels. The resulting rhythms of odor sensitivity are suggested to underlie circadian rhythms in odor-mediated behavior.

Bimodal oscillations of cyclic nucleotide concentrations in the circadian system of the Madeira cockroach Rhyparobia maderae

Pigment-dispersing factor (PDF) is the most important coupling factor of the circadian system in insects, comparable to its functional ortholog vasoactive intestinal polypeptide of the mammalian circadian clock. In Drosophila melanogaster, PDF signals via activation of adenylyl cyclases, controlling circadian locomotor activity rhythms at dusk and dawn. In addition, PDF mediates circadian rhythms of the visual system and is involved in entrainment to different photoperiods. We examined whether PDF daytime-dependently elevates cAMP levels in the Madeira cockroach Rhyparobia maderae and whether cAMP mimics PDF effects on locomotor activity rhythms. To determine time windows of PDF release, we searched for circadian rhythms in concentrations of cAMP and its functional opponent cGMP in the accessory medulla (AMe), the insect circadian pacemaker controlling locomotor activity rhythms, and in the optic lobes, as the major input and output area of the circadian clock. Enzyme-linked immunosorbent assays detected PDF-dependent increases of cAMP in optic lobes and daytime-dependent oscillations of cAMP and cGMP baseline levels in the AMe, both with maxima at dusk and dawn. Although these rhythms disappeared at the first day in constant conditions (DD1), cAMP but not cGMP oscillations returned at the second day in constant conditions (DD2). Whereas in light-dark cycles the cAMP baseline level remained constant in other optic lobe neuropils, it oscillated in phase with the AMe at DD2. To determine whether cAMP and cGMP mimic PDF-dependent control of locomotor activity rhythms, both cyclic nucleotides were injected at different times of the circadian day using running-wheel assays. Whereas cAMP injections generated delays at dusk and advances at dawn, cGMP only delayed locomotor activity at dusk. For the first time we found PDF-dependent phase advances at dawn in addition to previously described phase delays at dusk. Thus, we hypothesize that PDF release at dusk and dawn controls locomotor activity rhythms and visual system processing cAMP-dependently.

The role of the coreceptor Orco in insect olfactory transduction

Insects sense odorants with specialized odorant receptors (ORs). Each antennal olfactory receptor neuron expresses one OR with an odorant binding site together with a conserved coreceptor called Orco which does not bind odorants. Orco is necessary for localization of ORs to dendritic membranes and, thus, is essential for odorant detection. It forms a spontaneously opening cation channel, activated via phosphorylation by protein kinase C. Thereafter, Orco is also activated via cyclic adenosine monophosphate (cAMP). Orco forms homo-as well as heteromers with ORs with unknown stoichiometry. Contradictory publications suggest different mechanisms of olfactory transduction. On the one hand, evidence accumulates for the employment of more than one G protein-coupled olfactory transduction cascade in different insects. On the other hand, results from other studies suggest that the OR-Orco complex functions as an odorant-gated cation channel mediating ionotropic signal transduction. This review analyzes conflicting hypotheses concerning the role of Orco in insect olfactory transduction. In conclusion, in situ studies in hawkmoths falsify the hypothesis that Orco underlies odorant-induced ionotropic signal transduction in all insect species. Instead, Orco forms a metabotropically gated, slow cation channel which controls odorant response threshold and kinetics of the sensory neuron.

In situ tip-recordings found no evidence for an Orco-based ionotropic mechanism of pheromone-transduction in Manduca sexta

The mechanisms of insect odor transduction are still controversial. Insect odorant receptors (ORs) are 7TM receptors with inverted membrane topology. They colocalize with a conserved coreceptor (Orco) with chaperone and ion channel function. Some studies suggest that insects employ exclusively ionotropic odor transduction via OR-Orco heteromers. Other studies provide evidence for different metabotropic odor transduction cascades, which employ second messenger-gated ion channel families for odor transduction. The hawkmoth Manduca sexta is an established model organism for studies of insect olfaction, also due to the availability of the hawkmoth-specific pheromone blend with its main component bombykal. Previous patch-clamp studies on primary cell cultures of M. sexta olfactory receptor neurons provided evidence for a pheromone-dependent activation of a phospholipase Cβ. Pheromone application elicited a sequence of one rapid, apparently IP₃-dependent, transient and two slower Ca²⁺-dependent inward currents. It remains unknown whether additionally an ionotropic pheromone-transduction mechanism is employed. If indeed an OR-Orco ion channel complex underlies an ionotropic mechanism, then Orco agonist-dependent opening of the OR-Orco channel pore should add up to pheromone-dependent opening of the pore. Here, in tip-recordings from intact pheromone-sensitive sensilla, perfusion with the Orco agonist VUAA1 did not increase pheromone-responses within the first 1000 ms. However, VUAA1 increased spontaneous activity of olfactory receptor neurons Zeitgebertime- and dose-dependently. We conclude that we find no evidence for an Orco-dependent ionotropic pheromone transduction cascade in M. sexta. Instead, in M. sexta Orco appears to be a slower, second messenger-dependent pacemaker channel which affects kinetics and threshold of pheromone-detection via changes of intracellular Ca²⁺ baseline concentrations.

Time of day changes in cyclic nucleotides are modified via octopamine and pheromone in antennae of the Madeira cockroach

The cockroach Rhyparobia (Leucophaea) maderae expresses a circadian rhythm in pheromone-dependent mating activity that peaks at the late day/early night. In contrast, the circadian rhythm in olfactory sensitivity of the Madeira cockroach is at its minimum during this time. Until now, the reasons for this obvious discrepancy in phase were not understood. Previously, it was shown that cyclic nucleotides modulate olfactory sensitivity in a zeitgeber time (ZT)-dependent manner. In moths' olfactory receptor neurons, adapting pheromone concentrations elevate cGMP levels, which decrease pheromone sensitivity. In contrast, cAMP elevations sensitized pheromone responses. Thus, with immunoassay kits, it was determined whether cAMP and cGMP baseline levels vary in a ZT-dependent manner in antennal lysates of female R. maderae, revealing underlying circadian rhythms in olfactory sensitivity. Furthermore, it was examined whether adapting pheromone exposure elevates cGMP levels in cockroach antennae, possibly overshadowing underlying circadian rhythms in sensitivity via sensory adaptation. It was shown for the first time that cAMP and cGMP baseline levels oscillate in antiphase in a ZT-dependent manner in an insect's antenna, with the maximum in cAMP concentrations coinciding with maximal mating activity during the late day. Moreover, the cAMP baseline level oscillation expressed a circadian rhythm since it persisted under constant darkness in contrast to cGMP baseline levels. Furthermore, while excess exposure to male pheromones increased cGMP and decreased cAMP baseline levels, the stress hormone octopamine increased adenylyl cyclase activity at all ZTs tested. Therefore, it is suggested that cyclic nucleotide-dependent modulation of olfactory sensitivity due to olfactory overstimulation and stress could be responsible for previously measured phase discrepancies between rhythms in mating behavior and pheromone sensitivity.

The stimulatory Gα(s) protein is involved in olfactory signal transduction in Drosophila

Seven-transmembrane receptors typically mediate olfactory signal transduction by coupling to G-proteins. Although insect odorant receptors have seven transmembrane domains like G-protein coupled receptors, they have an inverted membrane topology, constituting a key difference between the olfactory systems of insects and other animals. While heteromeric insect ORs form ligand-activated non-selective cation channels in recombinant expression systems, the evidence for an involvement of cyclic nucleotides and G-proteins in odor reception is inconsistent. We addressed this question in vivo by analyzing the role of G-proteins in olfactory signaling using electrophysiological recordings. We found that Gα(s) plays a crucial role for odorant induced signal transduction in OR83b expressing olfactory sensory neurons, but not in neurons expressing CO₂ responsive proteins GR21a/GR63a. Moreover, signaling of Drosophila ORs involved Gα(s) also in a heterologous expression system. In agreement with these observations was the finding that elevated levels of cAMP result in increased firing rates, demonstrating the existence of a cAMP dependent excitatory signaling pathway in the sensory neurons. Together, we provide evidence that Gα(s) plays a role in the OR mediated signaling cascade in Drosophila.

Pheromone transduction in moths

Calling female moths attract their mates late at night with intermittent release of a species-specific sex-pheromone blend. Mean frequency of pheromone filaments encodes distance to the calling female. In their zig-zagging upwind search male moths encounter turbulent pheromone blend filaments at highly variable concentrations and frequencies. The male moth antennae are delicately designed to detect and distinguish even traces of these sex pheromones amongst the abundance of other odors. Its olfactory receptor neurons sense even single pheromone molecules and track intermittent pheromone filaments of highly variable frequencies up to about 30 Hz over a wide concentration range. In the hawkmoth Manduca sexta brief, weak pheromone stimuli as encountered during flight are detected via a metabotropic PLCβ-dependent signal transduction cascade which leads to transient changes in intracellular Ca²⁺ concentrations. Strong or long pheromone stimuli, which are possibly perceived in direct contact with the female, activate receptor-guanylyl cyclases causing long-term adaptation. In addition, depending on endogenous rhythms of the moth's physiological state, hormones such as the stress hormone octopamine modulate second messenger levels in sensory neurons. High octopamine levels during the activity phase maximize temporal resolution cAMP-dependently as a prerequisite to mate location. Thus, I suggest that sliding adjustment of odor response threshold and kinetics is based upon relative concentration ratios of intracellular Ca²⁺ and cyclic nucleotide levels which gate different ion channels synergistically. In addition, I propose a new hypothesis for the cyclic nucleotide-dependent ion channel formed by insect olfactory receptor/coreceptor complexes. Instead of being employed for an ionotropic mechanism of odor detection it is proposed to control subthreshold membrane potential oscillation of sensory neurons, as a basis for temporal encoding of odors.