Effect of 6-Hydroxydopamine on Baroreceptor-Heart Rate and Nasopharyngeal Reflexes of the Rabbit

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The baroreceptor-heart rate reflex was studied in conscious rabbits by deriving sigmoid mean arterial pressure (MAP)-heart period (HP, pulse interval) curves before and at different times after intracisternal (i.c.) injection of either vehicle or 6-hydroxydopamine (6-OHDA). Each curve was characterized by the HP range (HPR) between upper and lower HP plateau levels, median blood pressure (BP50), and average gain (G) about BP50. Intracisternal vehicle was without effect on these parameters at 2 and 4 hr and on day 7 after injection. After giving 400 or 600 μg/kg 6-OHDA, there were similar significant rises at 2 and 4 hr in HPR and G to about 200% of control, and a rise in BP50 of about 10 mm Hg. In pontine (decerebrate) rabbits there were similar rises in HPR and G. but BP50. decreased. Chronic changes after 6-OHDA included a rapid weight loss necessitating artificial feeding. After 400 μg/kg weight was regained by day 7 and BP50 and threshold for eliciting bradycardia increased significantly by 13.7 $ 2.3 mm Hg. Absence of significant parameter changes after 600 μg/kg on day 7 was considered to be due to residual fluid balance disturbances in this group. These appeared to be no longer present in 6 rabbits given a second i.c. dose of 600 μg/kg on day 7 and studied 1 week later, when changes in reflex properties consisted of a rise in BP50 (10.9 $ 3.7 mm Hg), reduction in G to 52 $ 17.8% of control, a small reduction in HPR, and impairment in the cardiac sympathetic component of the HP response studied after vagal block. The nasopharyngeal reflex, consisting of a rise in HP and maintenance of MAP close to resting, was evoked by graded inhalation of formalin vapor. The responses were not significantly altered after i.c. vehicle. After 600 μg/kg 6-OHDA, MAP was significantly less well maintained at 2 and 4 hr and on day 7 than before injection, and there were also significant changes in HP response. Neither MAP nor HP responses were further impaired by a second dose of 6-OHDA on day 7. and both were virtually restored 1 week later. The results suggest that catecholaminergic neurons normally participate in the baroreceptor-heart rate reflex and the nasopharyngeal heart rate and constrictor reflexes and that different groups of neurons probably modulate each response.

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