ROLE OF THE OPIOID SYSTEM IN THE HYPODIPSIA ASSOCIATED WITH AGING

Objective: To compare the effects of the opioid antagonist, naloxone, on fluid ingestion in young and older males, in order to estimate the role of the opioid system in hypodipsia of older men. Design: Single-blinded, randomized, cross-over, placebo-controlled study. Setting: Outpatient Department of Veterans Affairs. Study Participants: Sixteen young subjects (aged 23 to 39) and eight older subjects (aged 69 to 75). All subjects were healthy, community-dwelling, non-smoking men, on no medications. Intervention: Subjects were randomized to receive either placebo or naloxone on day one. Whichever substance the subject did not receive was given on day two with days one and two separated by at least a 2-week washout period. Main Outcome Measures: Fluid intake after overnight food and fluid deprivation, with placebo or naloxone injected in the morning. Results: After overnight fluid deprivation, older individuals consumed 29% less fluid in 2 hours compared with younger individuals (ns). After overnight fluid deprivation and injection with naloxone 100-mu-g/kg, fluid intake was diminished by 42% (P < 0.05) in young subjects compared with placebo, but only by 7% (ns) in older subjects. Subjective ratings did not differ significantly between young and old subjects. Conclusions: These preliminary data suggest that the opioid system plays a role in the drinking response in young subjects while failing to alter fluid intake in older subjects. Hypodipsia in older individuals may be due to a deficit in the opioid drinking drive.