The long range goal of this research is to determine the basic neurochemical mechanisms linking chemical drive and ventilation. Unanesthetized, unrestrained rats prepared with chronically indwelling femoral artery catheters will be used to obtain arterial blood gas and acid-base measurements. We have shown that the arterial PCO2 in awake, normoxic, intact rats is 40 torr. As long as CO2 is added, ventilation and metabolic rat must be monitored in a plethysmograph. Our experimental approach is to: 1) deplete endogenous neurochemicals (with the knowledge that this often affects biosynthetic pathways), 2) replete selected neurochemicals, and 3) use antagonists and mimetics. Mulitple agents will be used to minimize nonspecific effects. Pharmacologic action will be verified by neurochemical activity altered by PaCO2 or (H+) to the ventilatory adapations to hypoxia. Hypothesis 1a states that ventilatory acclimatization to hypoxia is due to a diminishing inhibition of carotid chemorecetpor discharge mediated by both endogenous opiates and dopamine. Ventilatory indices will be measured in rats treated with antagonists to dopamine (doperidone, i.p.) or endogenous opiates (naloxone, i.p.) during exposure to up to 7 days hypoxia with or without hypocania. We will test hypothesis 1b that the sustained hyperventilation observed at the onset of deacclimatization from prolonged hypoxia is partially due to an increased CNS dopamine activity resulting from the hypocapnia attendent to the hypoxia. Selected CNS dopamine antogonists (i.p. haloperidol; i.c.v. domperidone or alpha-methyl-rho-tyrosine, AMT, followed by repletion of NE using dihydroxphenylserine, DOPS) will be administered and ventilation measured during acute restoration of normoxia following up to 7 days of hypoxia.
Aim 2 is to determine the relationships between carotid body influenced CNS neurochemical activity and eupneic ventilation. These studies will use a carotid body excised (CBX) rat model. We hve shown that whole body depletion of catecholamines (with AMT) has a minimal ventilatory effect in the intact rat, but procedures a substantial hyperventilation in the CBX rat. Hypothesis 2 a states that carotid body excision allows expression of a net tonic inhbibtory influence on ventialation mediated by CNS norepinephrine (NE). This hypothesis will be tested by measuring ventilation in awake CBX rats: 1) before and after repleting NE with DOPS following AMT treatment, and 2) after treating with whole body NE depletor, FLA- 63. An average-of a) the rate of monoamine buildup after inhibition of monoamine oxidase, and of b) the rate of monoamine breakdown after inhibition of biosynthesis-is the index for monoamine turnover. Based on preliminary studies, it is hypothesized 2b that 5HT activity is increased in the area of respiratory motonuclei in the cervical and thoracic spinal cord of CBX rats due to the lack of carotid body input. This is reflected by an increased 5HT turnover in the spinal cord in CBX rats while the levels and turnovers of the monoamines in the brain stem and cerebral hemispheres are unchanged. We will expand these preliminary studies to identify the specific regions the spinal cord in which CBX rats have an increased 5HT turnover, and plan to examine the mechanism of this effect by measuring spinal cord 5HT turnover in intact and CBX rats maintained for 7 days in 4-5% CO2. These studies will provide basic knowledge applicable to clinical respiratory care in a broad sense including anesthesiology, neonatology, and the care of chronic respiratory disease.
