Salt ingestion responses to diencephalic electrical stimulation in the unrestrained conscious sheep

Abstract

In 20 conscious sheep, evidence was sought for localized brain mechanisms controlling ingestion of sodium salts, using electrical stimulation of diencephalic and neighbouring regions. The sheep lost saliva continuously through parotid fistulae. They were permitted to replace resultant sodium deficits episodically by drinking NaHCO 3 solution (300 mEquiv./l). Focal cathodal stimulation at 40 Hz was delivered through needle electrodes, permanently fixed or roving in depth, when the sheep were behaviourally satiated for available salt, water and food. Response maps were constructed for approximately 1,000 brain loci tested. On stimulation of particular sites in fimbria, fornix columns, perifornical hypothalamus, or medial forebrain bundle, 6 sheep ingested salt specifically, but similar loci in other elicited only licking or arousal. In some sheep, selection between NaHCO 3 and water for elicited drinking was influenced by situational factors. Licking was elicited from widespread loci in the diencephalon, fornix system, preoptic region and strio-pallidum, interspersed with excitatory or inactive points. The results suggest that hypothalamic mechanisms controlling salt ingestion exist, but are not reliably activated at comparable loci in different animals. It is alternatively possible that stimulated ingestion results from interactions between generalized arousal, excitation of unspecific alimentary circuits, and concurrent patterns of afferent input.