Influence of pentobarbital and urethane on release from magnocellular neurons

We examined the responses of vasopressin-neurons (VP-neurons) and oxytocin-neurons (OT-neurons) to acute salt-loading in a group of conscious rats (CON, n = 8) and rats under sodium pentobarbital (NEM, 50 mg/kg, i.p., n = 8) or urethane (URE, 1.6 g/kg, i.p. n = 8) anesthesia. Fifteen minutes following the induction of anesthesia, sodium pentobarbital produced an increase in basal plasma osmolality (Posm, 290 +/- 2 to 296 +/- 3 mosm/kg H2O, p less than 0.007) while urethane did not change basal Posm (287 +/- 2 to 289 +/- 2 mosm/kg H2O). Neither anesthetic agent resulted in any significant changes in basal plasma levels of vasopressin-associated neurophysin (VP-RNP) and oxytocin-associated neurophysin (OT-RNP). In response to intravenous infusion of 18% saline, all three groups of rats had similar rises in Posm. The slopes of the relationship between the rise in plasma VP-RNP and the rise in Posm were markedly reduced in both groups of anesthetized animals compared to that observed for conscious animals (CON = 2.54 +/- 0.5; NEM = 1.22 +/- 0.18; URE = 1.17 +/- 0.24 fmol.ml-1.mosm-1.kg H2O-1 p less than 0.0126). The slopes of the relationship between the rise in plasma OT-RNP and the rise in Posm were not significantly (p less than 0.4478) different between the CON group and the NEM group, while the slope for the URE group was significantly (p less than 0.05) smaller than that for the CON group (CON = 10.9 +/- 1.5; NEM = 9.3 +/- 1.5; URE = 6.3 +/- 0.7 fmol.ml-1.mosm-1.kg H2O-1).(ABSTRACT TRUNCATED AT 250 WORDS)