Interaction between norepinephrine and prostaglandin E2 in the preoptic area of guinea pigs.

The release of norepinephrine (NE) and prostaglandin E2 (PGE2) in the preoptic-anterior hypothalamus (POA) by systemically administered pyrogens suggests that both substances may mediate the febrile response. To investigate their possible interaction, we measured directly the levels of PGE2 in the extracellular fluid of the POA of conscious guinea pigs microdialyzed intrapreoptically with exogenous NE over the entire course of their febrile response to endotoxin. Acidified and buffered NE (NEa, NEb), artificial cerebrospinal fluid (aCSFa, aCSFb), and vehicle (Veha, Vehb) were tested. All but aCSFb depressed the febrile response to endotoxin. The microdialysis of aCSFa, aCSFb, Veha, Vehb, and NEa did not change basal preoptic PGE2 levels. However, NEb, at a dose that by itself did not affect body temperature (Tb), caused a large elevation in preoptic PGE2. The intravenous injection of endotoxin increased the level of PGE2 in the POA. NEb potentiated this increase, whereas NEa, aCSFa, and Vehb reduced it; Veha reduced it for the first 60 min and enhanced it for the last 90 min of the experiment. Thus these data suggest that the low pH of the NE solute and/or its Veh may confound the observed effects of NE on the Tb and preoptic PGE2 induced by endotoxin. We surmise that this is due to a neurotoxic action of the antioxidants and the acidity of the solution on thermosensitive neurons in the POA. Hence, the results of experiments using exogenous, usually acidified, NE preparations that often also contain additives should be interpreted with caution.