Mechanisms Underlying Long-Term Facilitation in the Carotid Body

Glutamate and γ-aminobutyric acid (GABA) are major modulators of excitatory and inhibitory transmission in the mammalian brain. Glutamate transmission is critical for neural plasticity, learning and memory, whereas GABA plays a fundamental role in regulating neuronal excitability and facilitating the generation of neural oscillations. Our data indicates that both glutamate and GABA are released in the carotid body (CB) to modulates its response to hypoxia. Objectives: We set out to investigate whether repeated stimulation with glutamate and GABA induces lasting effects on CB afferent discharge. Hypothesis: We hypothesised that glutamate/GABA mediates long-term facilitation (LTF) of CB afferent activity. Methods: We mined high-throughput RNA sequence data and used immunohistochemistry to map components underlying neuroplasticity in the CB in Wistar and Spontaneously-hypertensive rats (SHR). CSN discharge was measured as a functional readout of CB function from an ex vivo arterially perfused carotid artery-CB preparation. Data: We catalogue the expression of glutamate receptors (R) and transporters in the rat and mouse CB. We found N-methyl-D-aspartic acid receptor (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR) and transporters to be localised to the CB chemosensory cells. Targeted administration of 15 mM glutamate activated the carotid sinus nerve ( p=0.003). Repetitive [5 times, 2-minute duration, 5-minute intervals] application of 15 mM glutamate evoked LTF of CB afferent discharge as demonstrated by both a 12-fold increase in basal firing frequency (0.78 ± 0.37 vs 1232 ± 596 spikes 1s−1; p = 0.004) over 60 minutes ( p = 0.004) and amplification of the evoked response to chemical hypoxia (CN−; 1.23 μmol bolus) by 2-fold ( p = 0.009). The stimulation paradigm was specific as neither application of glutamate, 5 times for 1-minute at 5-minute intervals, nor a single glutamate exposure evoked LTF. In contrast, repeated application of 15mM GABA [3 times for 1-minute at 5-minute intervals] evoked long-term depression (LTD) of CB afferent activity as seen by a reduced basal discharge from 10.5 to 3.5 spikes 1s−1 ( p = 0.0004) and a 2-fold suppression of the response to chemical hypoxia ( p = 3.71 x 10−5). Summary of the data: Repetitive exposure to glutamate using a specific protocol increased basal discharge of CB afferent output akin to LTF and caused sensitisation of evoked responses to CN−. Repeated exposures of GABA depressed the CB basal discharge akin to LTD. A statement of the conclusions: It is conceivable that processes such as LTP and LTD operate in the CB to modulate the set point of peripheral chemoreceptor sensitivity and its state-dependent tonicity; the role of LTP in peripheral chemoreflex sensitisation in disease remains to be determined. Research was supported by the Health Research Council of New Zealand and the Sidney Taylor Trust. This is the full abstract presented at the American Physiology Summit 2024 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.