Chronic Hypernatremia Increases the Expression of Vasopressin and Voltage-Gated Na super(+) Channels in the Rat Choroid Plexus

The choroid plexus (CP) epithelium is one of the extrahypothalamic sources of arginine vasopressin (AVP). However, it is unclear whether the regulation of choroidal AVP synthesis in response to pathophysiological stimuli, such as hyperosmotic stress, is similar to that observed in the hypothalamus. In the present study, rats chronically implanted with cisterna magna cannulas, enabling the collection of cerebrospinal fluid (CSF) in freely moving animals, were subjected to salt loading. CSF osmolality increased from the baseline normonatremic levels ranging between 292 plus or minus 0.5 and 295 plus or minus 2 to 309 plus or minus 4 mosm/kg H sub(2)O at 2 days of hypernatremia. This elevated CSF osmolality was maintained at a relatively stable level until the end of a 10-day observation period. Changes in choroidal and hypothalamic AVP expression in response to hyperosmotic stress were assessed by semiquantitative reverse-transcriptase polymerase chain reaction. An increase in hypothalamic AVP expression was accompanied by augmented AVP synthesis in the CP. Compared to normonatremia, choroidal levels of AVP mRNA increased 5- and 10-fold at 2 and 5 days of salt loading, respectively. Salt loading also resulted in increased hypothalamic expression of the alpha -II, beta 1, and beta 2 subunits of voltage-gated Na super(+) channels. Similarly, the choroidal mRNA levels for the alpha -II and beta 1 subunits increased similar to 2-fold after 5 days of salt loading; however, no changes in the beta 2 subunit expression were found in the CPs of hypernatremic rats. These experiments support the hypothesis that the regulation of choroidal AVP synthesis is similar to that observed in the hypothalamus. It is also suggested that the increased expression of voltage-gated Na super(+) channels found in the hypothalamus and CP after salt loading may play a role in the adaptation of AVP-producing cells to chronic hypernatremia.