Electrophysiological properties of paraventriculo-spinal neurones in the rat.

In rats anaesthetized with urethane, electrical stimulation of the thoracic cord at T9-10 evoked antidromic response in neurones in the parvocellular portion of the ipsilateral paraventricular nucleus. Estimated conduction velocities in the spinally-projecting axons ranged from 0.6-5.9 m/sec. The majority (97%) of spinally projecting neurones were quiescent. Increases or decreases in the level of baroreceptor stimulation produced by intravenous injection of phenylephrine (1-5 micrograms) or sodium nitroprusside (10-15 micrograms) inhibited and excited amino acid-induced activity in 5/8 and 4/11 neurones respectively. Stimulation of vagal afferent endings by rapid bolus injection of 5-HT, to evoke the Bezold-Jarisch reflex. inhibited amino acid-induced activity in 15/18 cells. Activation of gastric vagal afferents by distension of the stomach had no effect on paraventriculo-spinal cells. It is suggested that at rest the excitability of paraventriculo-spinal neurones is depressed by a tonic inhibitory input which arises from vagal afferent fibres in the heart.