“In lophotrochozoan invertebrates, including annelids and mollusks, chemosensory ionotropic receptors detect environmental chemical signals and mediate sensory perception, especially aquatic olfaction and gustation.”

Chemosensory ionotropic receptors function as ligand-gated ion channels that detect environmental chemical signals across diverse animal taxa. The odor-response profile of an IR complex can be predicted by which IR coreceptor is used; for example, IR76b mediates sensitivity to amines while IR8a mediates responses to carboxylic acids, demonstrating the specificity of these receptors in chemical signal detection.

Chemosensory receptors convert an enormous diversity of chemical signals from the external world into a common language of electrical activity in the brain. Insects use ionotropic receptors, which are gated directly by chemical stimuli, thereby leading to neuronal depolarization. Conservation of ionotropic TRP channel repertoire has been variously implicated in the perception of sour, pungent or spicy chemical stimuli in mice and Drosophila, suggesting conserved chemosensory mechanisms across animal classes.

Ionotropic glutamate receptors (iGluRs) are a highly conserved family of ligand-gated ion channels present in animals, plants, and bacteria. A variant subfamily of iGluRs, the Ionotropic Receptors (IRs), was recently identified as a new class of olfactory receptors in the fruit fly Drosophila melanogaster, hinting at a broader function of this ion channel family in detection of environmental, as well as intercellular, chemical signals.

This study reveals the presence of putative variant Ionotropic Receptors (IRs) which are differentially expressed in the olfactory organs of COTS. Despite being annotated as iGluRs, their translated proteins also show high levels of similarity to the variant ionotropic receptors (IRs) that are known to be chemosensory receptors in Drosophila melanogaster. This is the only study to describe the presence of putative variant Ionotropic Glutamate Receptors in any Echinoderm. Furthermore, they were found to be differentially expressed in the olfactory organs of COTS.

Meanwhile, Benton et al. 2009, were first to discover that variant ionotropic receptors were chemosensory in insects and appear to enhance processing speed. Molluscs are a large and diverse group of aquatic and terrestrial animals that rely heavily on chemical communication. Aplysia is an excellent model in which to investigate and develop breakthrough principles into the molecular aspects of chemoreception in molluscs.

Most remarkably, IR25a homologs were also found in the nematode Caenorhabditis elegans, the mollusc Aplysia californica and the annelid Capitella capitata. Thus, these receptor types seem to be under high selective pressure to maintain the primary structure of the protein suggesting an important functional role of the proteins. SgreIR25a-positive cells were also found in some sensilla chaetica which are supposed to serve gustatory/mechanosensory functions. The notion that IR25a may be present in gustatory chemosensory cells was supported by the result of RT-PCR experiments indicating expression of SgreIR25a in mouth parts, which carry hundreds of s. chaetica (labial palps) and peg-like sensilla (maxillary palps); these sensilla are supposed to have a primary gustatory function.

We have established that nuchal organs, palps, antennae and tentacular cirri are chemosensory organs in Platynereis, responding to an alcohol, an ester, an amino acid and a sugar. This conclusion is likely to extend to annelids, for which similar sensory cells have been detected in electron microscopy. We conclude that chemosensation is a major sensory modality for marine annelids and propose early Platynereis juveniles as a model to study annelid chemosensory systems.

The insect chemosensory receptor superfamily, comprising Odorant Receptors (ORs) and Gustatory Receptors (GRs), forms a critical molecular interface between the insect nervous system and the chemical environment that determines their behavior, yet their evolutionary origin remains obscure.

Ionotropic receptors (IRs) are a highly divergent subfamily of ionotropic glutamate receptors (iGluR) and are conserved across Protostomia, a major branch of the animal kingdom that encompasses both Ecdysozoa and Lophotrochozoa. They are broadly expressed in peripheral sensory systems, concentrated in sensory dendrites, and function in chemosensation, thermosensation, and hygrosensation.

Chemoreceptors operate as ionotropic or metabotropic receptors depending on functional requirements. For fast chemosignal processing, ionotropic receptors were preferred, whereas signal amplification by intracellular cascades are advantageous for weak signal strength. The olfactory system is dedicated to detecting and encoding information from volatile chemical signals.

Ionotropic glutamate receptors (iGluRs) are a highly conserved family of ligand-gated ion channels present in animals, plants, and bacteria. Many of the receptors identified are in insects of significant harm to human health, such as the malaria mosquito, indicating the widespread distribution of ionotropic chemosensory mechanisms across protostome invertebrates.

There are two families of glutamate receptors: ionotropic receptors, which can open or close ion channels in response to neurotransmitters.

In this Research Topic, we intended to challenge the binomial distinction of “taste” and “olfaction” by proposing a broader “vision” of the chemosensory receptor systems in invertebrates.

In this review, we summarize current views of IR function, emphasizing recent advances in understanding the contribution of this receptor family.

In Drosophila, olfactory sensory neurons rely primarily on ionotropic receptors (IRs) to detect environmental chemicals including bitter, sweet, and salty tastants, odors, pheromones, humidity, carbon dioxide, and carbonated water. Conservation of IR/iGluR-related proteins in bacteria, plants, and animals suggests that this receptor family represents an evolutionarily ancient mechanism for sensing both internal and external chemical cues.

Ionotropic Receptors (IRs) are a large subfamily of variant ionotropic glutamate receptors present across Protostomia. While these receptors are expressed in a wide variety of tissues, a subset of IRs are expressed in the olfactory organs of diverse protostome taxa, where they function as chemosensory receptors.

In keeping with their ancient origin, Irs have been associated with detection of broadly appealing or noxious stimuli, including acids, amines, and ammonia. More recently, Ir gene expression has been analyzed in gustatory neurons of both adult and larval stages and accords possible roles in taste recognition to several members of the family.

Three major chemosensory receptor gene families, the odorant receptors (ORs), the ionotropic receptors (IRs), and the gustatory receptors (GRs), are involved in the detection of these stimuli. *Manduca sexta*, olfactory and gustatory cues are essential for finding partners, food, and oviposition sites.

This review summarizes the discoveries of the structure of IR complexes and the expression and function of each IR... comprise individual stimulus-specific tuning receptors and one or two broadly expressed coreceptors.

The objective of this paper is to identify olfactory chemoreceptors in Sepia officinalis and to validate their expression in chemosensory organs. We show that the IR25 gene, a highly conserved ionotropic receptor, is expressed in the olfactory organs, suckers, and statocysts of the cuttlefish Sepia officinalis, supporting its role in chemosensation.

Therefore, a plausible evolutionary scenario envisages the structural and functional conservation of “aquatic” chemosensory receptors in air-breathing tetrapods. This theory unifies taste and smell under a common conceptual and functional framework, with implications for sensory ecology and evolution.

While ionotropic receptors have been extensively characterized in arthropods (insects, crustaceans) and some other invertebrate groups, direct experimental evidence for ionotropic receptor-mediated chemosensation in lophotrochozoan phyla such as annelids and mollusks remains limited. Most chemosensory research on these groups has focused on metabotropic G-protein coupled receptors rather than ionotropic mechanisms, though some TRP channel ionotropic receptors have been identified in mollusks.