Identifying the building blocks for learning and memory in the carotid body
Authors: Gold Olivia , Pauza Audrys , Paton Julian 
Published: May 2023

Glutamate is a major excitatory neurotransmitter with well-established roles for inducing neural plasticity, learning and memory in the central nervous system. In the carotid body, an organ derived in part from the neural crest, evidence suggests glutamatergic signalling is involved in its sensing of hypoxia. We hypothesized that altered glutamatergic signalling may underpin enhanced chemoreflex responses in carotid body in disease states. Therefore, we have initiated an investigation to assess the presence and location of glutamate receptors expressed in the carotid body of Wistar rats. We mined two independent RNA-sequencing (RNAseq) datasets to identify glutamate signalling components. Digital droplet PCR (ddPCR) was performed to validate the expression of selected glutamatergic target genes. Protein localisation was detailed using immunofluorescence and confocal microscopy in 12–16-week-old normotensive male Wistar rats. Neuronal phenotypes were identified using tyrosine hydroxylase (type I or glomus cells) and glial fibrillary acidic protein antibodies (type II cells). In carotid body extracts, RNAseq data revealed the presence of novel glutamatergic signalling components, not previously described. ddPCR analysis confirmed the expression of AMPA, NMDA and excitatory amino acid transporters. Glutamate ionotropic receptor AMPA type subunit 1 ( Gria1) showed co-localisation with the membrane of type I glomus cells. Glutamate ionotropic receptor NMDA type 1 ( Grin1) showed co-localisation with the nuclei membrane [JP1] of glomus cells. NMDA receptor type subunit 2C ( GRIN2C) showed co-localisation with the membranes of both glomus cells and type II cells. Expression of these receptors was not found on all glomus cells within a cluster suggesting functional heterogeneity within the carotid body. Synaptic Ras GTPase-activating protein 1 ( Syngap), a protein that interacts with glutamate synapses involved in learning, was expressed in both plasma and nuclear membranes of glomus cells. The excitatory amino acid transporter 5 ( EAAT5) showed co-localisation with both type I and type II cells. Additionally, we have observed that a local arterial injection of glutamate (via the internal carotid artery) evoked excitatory activity, followed by an inhibitory period as recorded from the carotid sinus nerve. Our results identified expression and location of glutamatergic AMPA and NMDA receptors in the carotid body[JP2]. We also found the presence of excitatory amino acid transporters, and proteins involved in chaperoning glutamate receptors for learning and memory, All told, we have identified the substrate for glutamate as a transmitter in the carotid body. Whether it plays a role in the learning and memory of carotid body function and/or sensitisation in disease remains to be elucidated. Funding: Health Research Council of New Zealand. Nothing to disclose. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.