CENTRALLY MEDIATED CARDIOVASCULAR RESPONSES TO INTRACISTERNAL INJECTIONS OF SYMPATHOMIMETIC AMINES IN ANESTHETIZED RATS

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The cardiovascular effects resulting from intracisternal (i.c.) injections of sympathomimetic amines were studied in α-chloralose-urethanized rats. Norepinephrine (0.5-5 μg i.c.) caused a typical rise in blood pressure with no significant change in heart rate and a fall in blood pressure with a bradycardia, which were completely blocked after treatment with phentolamine (10-50 μg i.c.). L-isoproterenol (0.05-0.5μg i.c.) and trimetoquinol (0.5-3 μg i.c.), a β-sympathomimetic agent, usually caused a fall in blood pressure with a tachycardia, which was reduced after treatment with propranolol (10-50 μg i.c.), but trimetoquinol was inclined to cause a rise in blood pressure with a tachycardia. Epinephrine (5 μg i.c.) showed both centrally mediated α- and β-sympathomimetic effects. Tyramine (0.5-1 mg i.c.) caused mixed blood pressure responses presumably due to a release of norepinephrine and epinephrine, and these responses were partially blocked after treatment with phentolamine (100 μg i.c.) or propranolol (50 μg i.c.). These observations suggest that both α- and β-sensitive adrenergic zones may exist on the vasomotor center of the pons and medulla in rats, and both norepinephrine and epinephrine might centrally play a physiological role as the neurotransmitters controlling blood pressure in rats.
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