The role of cardiovascular and muscle afferent systems in control of body water balance.

Influences of afferent inputs from cardiovascular and muscle receptors on the activities of neurosecretory neurons in the hypothalamus, which secrete vasopressin (ADH) were studied. Recordings were made from identified neurosecretory neurons in the supraoptic (SON) and paraventricular nuclei (PVN) of cats and rats. Activation of baroreceptors in the carotid sinus and aortic arch and atrial receptors inhibited SON and PVN neuron activities, while activation of chemoreceptors in the carotid sinus excited them. Repetitive electrical stimulation of the carotid sinus and aortic nerves showed that weak stimulation produced excitation and stronger stimulation produced inhibition of SON and PVN neurons. Electrical stimulation of these nerves and the nucleus tractus solitarius (NTS) by a single or short train of pulses showed that 'fast' and 'slow' pathways between the NTS and the SON existed, while these two types of pathways were not observed between the NTS and the PVN. Evidence of direct connections from the NTS to the PVN was found by means of antidromic stimulation of the PVN. Electrical stimulations of group I afferent fibers from the gastrocnemius muscle did not change SON neuron discharges, while activation of group III and IV afferent fibers excited them. Injection of chemicals (NaCl, KCl, bradykinin) into arteries supplying the muscle excited SON neurons. The excitation disappeared after section of the muscle nerves. The results indicated that activation of small afferents from the muscle excites the SON neurons, leading to an increase in vasopressin secretion. All these studies show that afferent inputs from receptors in the cardiovascular system and in the muscle have modulatory effects on neurosecretory neurons, and participate in control of body water balance by regulating vasopressin secretion from the neurohypophysis.