Role of ionotropic glutamate receptors in odor information processing through mouse olfactory subsystems

Abstract

Information gathered by sensory organs is processed in the specialized pre-cortical and cortical areas, ultimately leading to animals’ behavioural responses. For rodents, olfaction (sense of smell) has been proven to be a vital sensory modality. The mouse olfactory system that detects, discriminates, and memorizes odors differing in their physicochemical properties, consists of two parallel systems – the main and the accessory olfactory system. Anatomically, this can be divided into three broad regions 1) the main olfactory epithelium and the vomeronasal organ (VNO), 2) the main (MOB) and accessory (AOB) olfactory bulbs, and 3) the olfactory cortex. While the MOB processes the information emanating from volatile molecules, AOB is mainly responsible for processing the information collected from pheromones through VNO. In the MOB and AOB, the inhibitory neuronal network plays an important role in refining the sensory information. These inhibitory interneurons express ionotropic glutamate receptors (iGluRs). In this study, we aimed to dissect the role of iGluRs in modulating synaptic physiology in the context of odor and pheromone information processing through olfactory subsystems. To elucidate the role of these iGluRs we targeted one of the abundant inhibitory interneuron populations in main and accessory olfactory systems called GAD65-expressing neurons. GAD65 is a 65 kDa isoform of glutamic acid decarboxylase (GAD). Using the Cre-LoxP system, we deleted the subunits of ionotropic glutamate receptors, the NR1 subunit of N-methyl-Daspartate (NMDA) receptor and the GluA2 subunit of α-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) receptor, from GAD65 interneuron population in different cohorts of mice, using both genetic and stereotaxic injection strategies. The calcium influx and thereby, the synaptic functioning can be bidirectionally modulated by these modifications (1). Pheromone signalling plays a vital role in driving the social and reproductive behaviours of rodents. Learning and memorizing the pheromone locations involve both somatosensory and olfactory systems (2). To probe the neural basis of this behaviour, we trained female heterozygous knockouts (KOs) of GluA2 and NR1, targeting GAD65-expressing interneuron population, in a pheromone place preference learning assay as described in Pardasani et al., 2021. We observed loss of memory towards pheromone locations on early and late recall periods, pointing towards the possible role of iGluRs and thereby the synaptic inhibition in multimodal learning of pheromone locations. Further, correlated changes were observed in the expression levels of activity-regulated cytoskeletal (Arc) protein, which is critical for memory consolidation, in the associated brain areas. Further, to probe the involvement of MOB and AOB microcircuitry in multimodal learning of pheromone locations, we specifically knocked out NR1 and GluA2 from MOB and/or AOB GAD65- expressing interneurons by injecting Cre-dependent AAV5 viral particles stereotaxically. We observed that perturbing the microcircuitry of MOB and AOB or AOB-alone resulted in the loss of pheromone location memory. These results confirm the role of iGluRs and the synaptic inhibition exerted by the interneuron network of AOB in controlling the multimodal learning of pheromone locations. In the next part of the study, we investigated whether the heterozygous knockout of iGluRs from the GAD65 interneuron population resulted in altering the detection and discrimination learning efficiency of volatile odorants. We performed a Go/No-go behavioural task to challenge the animals in detecting and discriminating simple as well as complex odors. We observed lower detection and discrimination abilities at the threshold levels in both GluA2 and NR1 heterozygous KOs. Further, in GAD65-GluA2 heterozygous knockouts, we observed compromised airflow discrimination learning, a novel function of rodent olfactory system we studied in the lab (3), indicating the importance of iGluRs in controlling ‘multimodal’ information processing through MOB circuits. In conclusion, the results reported in the study demonstrate the role of iGluRs in maintaining optimal inhibition while processing odor and pheromone information through olfactory subsystems.