Hyperdipsia in sheep bearing lesions in the medial septal nucleus

Previous experiments in rodents showed that ablation of the septal brain region caused hyperdipsia. We investigated which part of the septal region needs ablation to produce hyperdipsia in sheep, and whether increased drinking was a primary hyperdipsia. Following ablation of the medial septal region (n = 5), but not parts of the lateral septal region (n = 4), daily water intake increased from ~2.5-5 L/day up to 10 L/day for up to 3 months post-lesion. In hyperdipsic sheep, plasma osmolality increased on the first day post-lesion and body weight fell, suggesting that initial hyperdipsia was secondary to fluid loss. However hyperosmolality was not sustained long-term and plasma hypo-osmolality persisted from 0.5 to 3 months post-lesion. Acute dipsogenic responses to intravenous hypertonic saline, intravenous or intracerebroventricular angiotensin II, water deprivation for 2 days, or feeding over 5 h were not potentiated by medial septal lesions, showing that the rapid pre-systemic inhibitory influences that cause satiation of thirst upon the act of drinking were intact. However, hyperdipsic sheep continued to ingest water when hyponatremic (plasma [Na] was 127-132 mmol/l) and plasma osmolality was 262-268 mosmol/kg due to retention of ingested fluid resulting from intravenous infusion of vasopressin administered to maintain a basal blood level of antidiuretic hormone. The results show that septal lesion-induced hyperdipsia is not due to disruption of acute pre-systemic influences associated with drinking water that initiates rapid satiation of thirst. Rather, inhibitory influences of hyponatremia, hypo-osmolality or hypervolemia on drinking appear to be disrupted by medial septal lesions.