The role of galanin in respiratory chemoreception

Chronic respiratory diseases including chronic obstructive pulmonary disease, obesity hypoventilation syndrome and obstructive sleep apnoea involve living in a chronic hypercapnic state and are associated with high mortality rate, poor prognosis and low quality of life. Affected individuals exhibit a reduction in CO2-stimulated ventilatory responses, indicating plasticity in ventilatory behaviour. This thesis investigates whether neurons mediating the central respiratory chemoreflex are involved in this plasticity. The retrotrapezoid nucleus (RTN) contains chemoreceptor neurons that modulate ventilation via glutamatergic innervation of the ventral respiratory column (VRC), the central pattern generator. Recently, RTN neurons were found to express inducible neuropeptides including the inhibitory neuropeptide galanin. Microinjections of galanin into the VRC blunts CO2-mediated chemoreflex responses, however the function of galanin in the respiratory chemoreflex circuit is not clear. Hence, this thesis hypothesised that galaninergic RTN-VRC circuitry contributes to centrally mediated adaptation to long-term hypercapnia (LH) and is involved in the pathophysiology of associated respiratory disorders. The first aim explored neuropeptide gene expression in mouse RTN following room air, short-term hypercapnia (SH, 6 or 8 hrs, 5 or 8% CO2) or LH (10 days, 8% CO2). The results revealed a biphasic pattern of neuropeptide mRNA expression, with a decrease following SH and increase following LH. The second aim investigated CO2-responsiveness of chemoreceptor populations following LH (c-Fos immunoreactivity). LH blunted responsiveness of all regions to AH (1 hr, 10% CO2) except for the RTN which retained its CO2-sensitivity. The last aim used retrograde tracing to identify all galaninergic neurons projecting to the VRC, and whether VRC neurons express galanin receptors (GalR), using fluorescent in situ hybridisation. It was revealed that 29 brain regions project to the VRC, amongst which 5 were galaninergic and only RTN neurons were CO2-responsive (increased c-Fos). Finally, VRC neurons contained ~850 GalR1+ neurons with 27% co-labelled with glycine transporter 2. In conclusion, this thesis characterises galaninergic VRC circuitry and suggests that galaninergic neurotransmission from the RTN contributes to diminished hyperventilatory response observed during adaptation to LH.