Pharmacokinetics of extracellular melatonin in Siberian hamster forebrain.

In vivo brain microdialysis was used to characterize the pharmacokinetics of subcutaneously injected melatonin in the anterior hypothalamic-preoptic area (AH-POA) of the male Siberian hamster. Animals with a microdialysis probe implanted in the AH-POA were treated with a subcutaneous melatonin injection at 0900 h (3 h after lights-on) or 2000 h (2 h prior to lights-off). Treatment with 2.5 or 0.25 mg/kg dosages of melatonin in saline vehicle induced peak concentrations of melatonin in AH-POA microdialysates within 20 min of injection (165 +/- 34 and 18 +/- 8 pg/20 min, respectively). For the 2.5 and 0.25 mg/kg dosages, the half-life of the absorbed melatonin (t 1/2 elimination) was less than 20 min, and the concentrations fell to baseline within 60 min after injection. There were no significant time of day effects on the kinetic profile of extracellular melatonin associated with either of these dosages. These results confirm the rapid accumulation and clearance of extracellular melatonin in the vicinity of its putative target tissues.