FREQUENCY-RESPONSE CHARACTERISTICS OF AUTONOMIC NERVOUS-SYSTEM FUNCTION IN CONSCIOUS RATS

To characterize the efferent pathway from the hypothalamic paraventric ular nucleus (PVN) to peripheral autonomic neurons and finally to sele cted effector organs, we stimulated the PVN in 10 conscious rats at fr equencies ranging from 0.05 to 2.0 Hz. Simultaneously, blood pressure, heart rate, splanchnic sympathetic nerve activity, and mesenteric art ery blood flow were measured. The sinus node of the heart responded to PVN stimulation via the parasympathetic pathway (during beta(1)-adren ergic blockade) up to a stimulation frequency of 2.0 Hz, whereas the s ympathetically mediated response (during muscarinic blockade) was limi ted to stimulation frequencies <0.5 Hz. The splanchnic nerve responded to PVN stimulation with synchronous discharges up to stimulation freq uencies of 2.0 Hz, whereas the oscillatory component of the vasoconstr ictor response of the mesenteric artery was negligible beyond stimulat ion frequencies of 1.0 Hz. We conclude that sympathetic transmission t o the heart is at least four times slower than parasympathetic transmi ssion. In addition, the time-limiting step in sympathetic transmission from the hypothalamus to vascular smooth muscle contraction and pacem aker activity of the sinus node may be located at the site of synaptic transmission to the adrenergic receptors.