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

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 micrograms) 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.