Olfactory sensitivity exhibits daily fluctuations. Several studies have suggested that the olfactory system in insects is modulated by both biogenic amines and neuropeptides. However, molecular and neural mechanisms underlying olfactory modulation in the periphery remain unclear since neuronal circuits regulating olfactory sensitivity have not been identified. Here, we investigated the structure and function of these signaling pathways in the peripheral olfactory system of the American cockroach, Periplaneta americana, utilizing in situ hybridization, qRT-PCR, and electrophysiological approaches. We showed that tachykinin was co-localized with the octopamine receptor in antennal neurons located near the antennal nerves. In addition, the tachykinin receptor was found to be expressed in most of the olfactory receptor neurons in antennae. Functionally, the effects direct injection of tachykinin peptides, dsRNAs of tachykinin, tachykinin receptors, and octopamine receptors provided further support for the view that both octopamine and tachykinin modulate olfactory sensitivity. Taken together, these findings demonstrated that octopamine and tachykinin in antennal neurons are olfactory regulators in the periphery. We propose here the hypothesis that octopamine released from neurons in the brain regulates the release of tachykinin from the octopamine receptor neurons in antennae, which in turn modulates the olfactory sensitivity of olfactory receptor neurons, which house tachykinin receptors.

Olfaction as an important sensory modality in insects is essential for identification of hosts, mates, oviposition sites, and food resources in nature. In the cockroach, both olfactory sensitivity in the antennae and the formation of shortand long-term olfactory memories exhibit daily fluctuations that are regulated by the circadian system. An important problem is to characterize the signalling systems and molecules that are involved in this regulation of olfactory reception and olfactory behaviour. Recent results suggest that insect olfactory systems are modulated by both biogenic amines and neuropeptides. However, it remains elusive how these molecules modulate olfactory system in the peripheral systems. In the present study, our aim was to characterize the structure and organization of these signalling systems in the peripheral olfactory system of the American cockroach, Periplaneta americana. This work illuminated that tachykinin and its receptors regulate olfactory sensitivity in the antennae of the cockroach. Injections of tachykinin peptides caused decreases in the amplitude of the electroantennoogram (EAG), cells that produce tachykinin were localized in the antennae, and olfactory receptor neurons expressed tachykinin receptors. Interestingly, the tachykinin expressing cells also express receptors for the biogenic amine, octopamine and injections of octopamine also cause reductions in EAG amplitude. These results suggest that both octopaminergic and tachykinin peptide signalling pathways are important regulators of olfactory reception in the cockroach. We propose the hypothesis that octopamine regulates the release of tachykinin from cells in the antennae that, in turn, modulate the sensitivity of olfactory receptor neurons.

An olfactory system is one of the complicatedly-equipped sensory facilities in the insect sensory systems, which is most essential for insect olfactory-driven behaviors relevant to survival such as finding hosts, mates, oviposition sites, and food resources. These behaviors are mostly controlled by circadian rhythm. The american cockroach, Periplaneta americana, has been an ideal model to extensively study olfactory system associated with complex behavioral repertoires and circadian controls of certain behaviors, respectively. Even though it is known that olfactory-related physiology in peripheral and central olfactory systems seems to be highly variable by circadian rhythms, little is known about how these are controlled at the neuronal and molecular levels. It has been reported that the plasticity in the olfactory system is modulated by a set of neuropeptides. However, it remains still elusive how these neuropeptides and neuroendocrine system interact in the peripheral systems to change olfactory responses in cockroaches. Here, current study focuses on the localization of neuropeptides and their receptors by using in situ hybridization and immunostaining methods. Also, expression level of these genes are evaulated by qRT-PCR methods. Circadian fluctuation of these genes seem to be important neurotransmission machineries in the periphery. Our current study suggests that microcircuits of neuronal systems in the peripheral olfactory organ play an important in olfactory modulation by circadian rhythm