Brain serotonin blockade and paradoxical salt intake in rats.

Serotonin antagonism in the lateral parabrachial nucleus (LPBN) enhances sodium appetite induced by hypovolaemia and angiotensin-mineralocorticoid activation, but produces no sodium intake in euhydrated animals. In the present work, male adult rats (n=21) that received bilateral injections of the serotonergic antagonist methysergide (4 microg/0.2 microl) into the LPBN combined to intragastric load of 2 M NaCl (2 ml/rat), ingested hypertonic NaCl (ingestion of 4.3 +/- 1.6 ml/2 h of 0.3 M NaCl versus vehicle into LPBN: 0.2 +/- 0.2 ml/2 h, P<0.05). Methysergide- and vehicle-treated animals also ingested water (9.5 +/- 0.7 and 7.2+/-0.5 ml/2 h, respectively, P>0.05) as expected from the state of cell dehydration produced by the load. Ingestion of water (11.0 +/- 1.2 ml/2 h), and of 0.3 M NaCl (1.1 +/- 0.7 ml/2 h) were not altered by methysergide in NaCl loaded rats with misplaced LPBN injections (n=15).The ingestion of hypertonic NaCl by rats with serotonergic blockade in the LPBN suggests that the circuits subserving sodium appetite are activated, but at the same time strongly inhibited through the LPBN, during cell dehydration.