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

Abstract

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.

Erratum

“…In Dolzer et al (2021), some reference citations in Table 3 were missing. Table 3 should be presented as below: The Publisher apologises for this error.…”

Contribution of membrane-associated oscillators to biological timing at different timescales

“…The network’s “day” setpoint maintains the nocturnal animal’s physiological homeostasis of sleep, while the “night” setpoint maintains the physiological homeostasis of activity. Based on our work on peripheral insect circadian clock neurons we hypothesize that these antagonistic dynamic homeostatic setpoints correlate with antagonistic second messenger compositions ( Stengl, 2010 ; Schendzielorz et al, 2012 ; Schendzielorz et al, 2015 ; Dolzer et al, 2021 ; Stengl and Schröder, 2021 ). The high sensory thresholds and slow response kinetics of sensory neurons during sleep correlate with increased intracellular Ca 2+ and cGMP concentrations.…”

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

“…Transduction is instead reliant on G protein-dependent phospholipase C (PLC)-dependent transient receptor potential (TRP)-like channels, which act as the primary transducers of pheromone odorants (Gawalek and Stengl, 2018 ). Based on follow-up studies employing ion current measurements in trichoid sensilla, sensitization and adaptation mechanisms in pheromone sensilla were indeed later shown to be cAMP- and cGMP-dependent, respectively (Dolzer et al, 2021 ). Multiple disparate lines of evidence, therefore, implicate cyclic nucleotides and coupled signaling cascades as important elements in odor transduction and regulators of OR performance events across insects; these and other modes of OR regulation beyond the present scope have been extensively reviewed (Fleischer et al, 2018 ; Wicher, 2018 ; Wicher and Miazzi, 2021 ).…”

“…In fact, to our knowledge, only two indirect and circumstantial references exist in this regard: NOS expression is differentially expressed between antennae of drone and worker honey bees (Jain and Brockmann, 2020 ), and NOS gene transcripts are highly expressed in antennal tissue samples of the M. sexta hawkmoth as surveyed by Northern blot and RT-PCR (Nighorn et al, 1998 ). Given previous evidence that cGMP can directly activate Orco channels (Wicher et al, 2008 ) and that cGMP is involved in olfactory transduction in insect model systems (Flecke et al, 2006 ; Dolzer et al, 2021 ), we opted to explore the NO signaling pathway as a mode of signaling in antennae. In particular, we ask whether NO acts as a functional messenger involved in the regulation and potentiation of olfactory responses in live insect antennae, as is evident in non-insect olfactory sensory systems as well as insect non-olfactory sensory systems.…”

“…The pharmacological aspect is enabled by previous work characterizing Drosophila NOS and NO receptors, the soluble guanyl cyclases, using a wide panel of pharmacological tools capable of inhibiting and activating the entire NO-cGMP pathway, either through the provision of NO itself or by modulating resultant cGMP activity (Nighorn et al, 1999 ; Langlais et al, 2004 ; Morton, 2004 ; Morton et al, 2005 ). Finally, we also perform experiments comparing response effects between cGMP and cAMP, using both long- and short-time-separated odor stimulation experiments, as both cyclic nucleotides have been suggested to exert opposite effects on OSN responses in vivo as crucial elements of metabotropic transduction cascades (Flecke et al, 2006 ; Dolzer et al, 2021 ). Contrary to expectation, we find no response effect during modulation of the NO-cGMP pathway and find no effect of cGMP application on responses using both calcium imaging of OSNs as well as single sensillum recordings.…”