μ- AND K-OPIATE RECEPTOR AGONISTS REDUCE PLASMA NEUROHYPOPHYSIAL HORMONE CONCENTRATIONS IN WATER-DEPRIVED AND NORMALLY HYDRATED RATS

SUMMARY 1. The effects of μ‐ and K‐opiate receptor agonists on plasma concentrations of immunoreactive (ir) oxytocin (OXY) and arginine vasopressin (AVP) in normally hydrated and water‐deprived rats were studied. 2. Water‐deprivation for 36 h elevated both plasma ir‐OXY and ir‐AVP concentrations of Long‐Evans rats. These elevated levels were lowered in a dose‐dependent manner after subcutaneous administration of bremazocine (3–10 μg/kg), M320 (10–100 μg/kg) or morphine (0.1–10 mg/kg). Comparable reductions of plasma concentrations of ir‐AVP and ir‐OXY were observed. 3. Plasma concentrations of ir‐OXY and ir‐AVP of normally hydrated Long‐Evans rats were lower after subcutaneous administration of bremazocine (10 μg/kg), M320 (10 μg/kg) and morphine (1.0 mg/kg). 4. These data suggest that both μ‐ and K‐opiate receptor agonists inhibit both OXY and AVP release from the neurohypophysis of rats.