Previous studies have shown that antinociceptive doses of systemic morphine increase extracellular histamine (HA) levels in the rat periaqueductal gray (PAG), although the cellular origin of basal and morphine-induced HA release in the PAG is unknown. Treatment with alpha-fluoromethylhistidine (FMH; 100 mg/kg, i.p.), the irreversible inhibitor of histidine decarboxylase, decreased basal HA release by a maximum of 80% and prevented morphine-induced HA release in the PAG. In addition, perfusion of this area with the sodium channel blocker tetrodotoxin (10(-6) M) decreased basal HA release by a maximum of 57% from baseline levels. When the perfusion medium was modified by substitution of magnesium for calcium, extracellular HA levels in the PAG decreased by a maximum of 72%, and morphine-induced HA release was prevented. Thioperamide (5 mg/kg, i.p.), an H3 antagonist, increased HA release in the PAG to a maximum of 249% within the first 30-60-min period. Taken together, these results suggest that basal and morphine-induced HA release in the rat PAG have a neuronal origin.
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