AMPA glutamate receptors and respiratory control in the developing rat: Anatomic and pharmacological aspects

The developmental role of α-amino-3-hydroxy-5-methyl-4- isoxazolepropionate (AMPA) glutamate receptors in respiratory regulation remains undefined. To study this issue, minute ventilation (V̇(E)) was measured in 5-, 10-, and 15-day-old intact freely behaving rat pups using whole body plethysmography during room air (RA), hypercapnic (5% CO₂), and hypoxic (10% O₂) conditions, both before and after administration of the non-N-methyl-D-aspartate (NMDA) receptor antagonist 1,2,3,4-tetrahydro-6- nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide disodium (NBQX; 10 mg/kg ip). In all age groups, V̇(E) during RA was unaffected by NBQX, despite reductions in breathing frequency (f) induced by increases in both inspiratory and expiratory duration. During hypoxia and hypercapnia, V̇(E) increases were similar in both NBQX and control conditions in all age groups. However, tidal volume was greater and f lower after NBQX. To determine if AMPA receptor-positive neurons are recruited during hypoxia, immunostaining for AMPA receptor (GluR2/3) and c-fos colabeling was performed in caudal brain stem sections after exposing rat pups at postnatal ages 2, 5, 10, and 20 days, and adult rats to room air or 10% O₂ for 3 h. GluR2/3 expression increased with postnatal age in the nucleus of the solitary tract (NTS) and hypoglossal nucleus, whereas a biphasic pattern emerged for the nucleus ambiguus (NA). c-fos expression was enhanced by hypoxia at all postnatal ages in the NTS and NA and also demonstrated a clear maturational pattern. However, colocalization of GluR2/3 and c-fos was not affected by hypoxia. We conclude that AMPA glutamate receptor expression in the caudal brain stem is developmentally regulated. Furthermore, the role of non-NMDA receptors in respiratory control of conscious neonatal rats appears to be limited to modest, albeit significant, regulation of breathing pattern.