Effects of volume loading on paraventriculo-spinal neurones in the rat

Abstract

Experiments were carried out to investigate the effects of volume loading on the activity of antidromically identified paraventriculo-spinal neurones in rats anaesthetised with urethane. Intravenous infusion of isotonic saline (1.5–4.2 ml) inhibited amino acid-induced activity in 12/20 cells. Of the remainder, 4 cells were excited and 4 were unaffected by volume loading. In 4/4 neurones inhibited by volume expansion, subsequent activation of vagal afferent nerve endings by i.v. bolus injection of 5-hydroxytryptamine (25–100 μg) also inhibited cell firing. The onset (0.5–7 min) and duration (5–29 min) of the inhibition evoked by volume loading showed considerable variation in different animals. The inhibitory effects of volume loading were absent in bilaterally vagotomised rats ( n = 4). In these preparations the activity of paraventriculo-spinal neurones was facilitated by volume loading. It is suggested that during volume loading inhibitory vagally mediated inputs and excitatory non-vagal influences converge on paraventriculo-spinal neurones.