Neurotransmitter mechanisms in the enhancement of the hypoxic ventilatory response by antecedent hyperoxia in the anesthetized rat
A brief period of antecedent oxygen breathing enhances the ventilatory response to hypoxia. The mechanisms of this phenomenon are uncertain and have been variably linked to the central glutamatergic or nitrergic pathways. In the present study we put a question of how blockade of either neurotransmitter pathway would compare with the concurrent blockade of them both in terms of the enhancement of posthyperoxic hypoxic ventilation. The study was performed on the anesthetized, vagotomized, spontaneously breathing rats divided into the following experimental groups: control NaCl-treated, glutamate blocker 2-amino-5-phosphonopentanoic acid (AP5)-treated, nitric oxide synthase blocker 7-nitroindazol (7NI)-treated, and AP5+7NI-treated. The protocol consisted of measuring the ventilatory response to 12% O2, a steady-state poikilocapnic hypoxia, undertaken in three consecutive conditions in each animal: the initial control, 25 min after injection of a given chemical agent, and then after a 15-min period of oxygen breathing. Respiration was evaluated from the diaphragmatic EMG signal. We found that the posthyperoxic hypoxic ventilatory enhancement was but partially dampened by either AP5 or 7NI. Concurrent administration of the two blockers further diminished, but did not abolish, the hypoxic ventilatory enhancement. We conclude that although the glutamate-NO system accounts for an appreciable part of the posthyperoxic hypoxic ventilatory enhancement, other, as yet unclear, mechanisms contribute as well. These mechanisms may be worth exploring given the substantial enhancing effect the antecedent oxygen has on hypoxic hyperventilation.