Histamine releases norepinephrine in the paraventricular nucleus/anterior hypothalamus of the conscious rat.

These experiments evaluated norepinephrine (NE) release evoked by histamine (HA) in the paraventricular nucleus/anterior hypothalamic (PV/AH) region of conscious rats. In vivo microdialysis techniques were used to estimate extracellular NE concentrations in the PV/AH before, during and after perfusing the microdialysis probes with artificial cerebrospinal fluid containing HA (0, 1, 3 or 10 mg of HA per ml of artificial cerebrospinal fluid). Administration of HA through the microdialysis probe resulted in a significant, dose-related increase in dialysate NE concentration, which returned to control values after HA perfusion. Blood pressure and heart rate were not altered during perfusion with these concentrations of HA. The increase in NE was attenuated by administration of a specific H1 receptor antagonist, chlorpheniramine, but was not altered consistently by treatment with the H2 receptor antagonist, cimetidine. Furthermore, extracellular NE increased when perfusate contained the specific H1 receptor agonist, 2-thiazolyethylamine, but was not increased by similar administration of the H2 receptor agonist, dimaprit. These data demonstrate that HA releases NE in the PV/AH brain region of the conscious rat primarily by activation of H1 receptors. This finding supports an interaction between HA and NE neurons in the hypothalamus.