Sodium appetite and cerebrospinal fluid sodium concentration during hypovolemia

A lowered concentration of sodium in the cerebrospinal fluid (CSF) has been suggested as a critical signal for the elicitation of sodium appetite. This was evaluated in naive male Long-Evans rats made hypovolemic by subcutaneous injection of the colloid polyethylene glycol (PEG) in a hyperoncotic concentration. At 2, 5 or 8 h after PEG or vehicle injection, under conditions of availability or non-availability of water and 0.3 M NaCl for ingestion, the rats were anesthetized and samples of CSF and blood were taken. Water intakes of the PEG treated rats were elevated at 5 and 8 h, and 0.3 M NaCl intakes were elevated at 8 h in comparison to vehicle treated rats. When fluids were not available, CSF sodium concentrations were greater in PEG than in vehicle treated rats at 2, 5 and 8 h. When fluids were available, CSF sodium concentrations were greater in PEG than in vehicle treated rats at 5 and at 8 h. There was no correlation between CSF sodium concentration and saline intake. In summary, a lowered CSF sodium concentration is not a necessary condition for sodium appetite in the hypovolemic rat.