Elevation in rat brain histamine content after focused microwave irradiation.

Microwave irradiation focused on the head of small rodents is now widely used as a means of more accurately measuring acetylcholine, choline, cyclic AMP, and several other important brain constituents. Because of its probable neurotransmitter role and rapid turnover, a similar approach was taken to study brain histamine. Histamine was measured by a modified radio-enzymatic method and was found to be nearly tripled in brains from microwave treated rats, compared to decapitation controls (124 vs 42 ng/g). Possible explanations include a microwave-induced inactivation of histamine breakdown, a microwave-induced redistribution of previously unmeasured histamine, and microwaveinduced histidine decarboxylation. Brain histamine remained unchanged up to 30 min after decapitation and microwave heated brains from decapitated rats also had elevated histamine levels, indicating that brain histamine levels in decapitated rats do not represent the remainder of a rapidly depleting pool. No evidence for previously unmeasured histamine was found. Furthermore, microwave irradiation did not enhance the formation of [3H]histamine after intraventricular C3H]histidine administration, indicating a lack of microwave-induced histidine decarboxylation. It is concluded that the elevation in rat brain histamine after focused microwave irradiation is probably not artifactual, although the mechanism responsible for this phenomenon remains obscure. THE APPLICATION of focused microwave irradiation to the head of experimental animals facilitates the accurate measurement of several important substances by rapidly inactivating brain protein. G U I ~ ~ T I et al. (1974) demonstrated the importance of this method for determining cyclic nucleotide content. Furthermore, the use of microwave heating is necessary for accurately measuring acetylcholine and choline (GUIwrn et al., 1974; STAVINOHA et al., 1973). More recently, several other putative neurotransmitter levels have been measured using this method (WEINTRAUB et al., 1975; RICHARDSON & SCUDDER, 1976). A variety of evidence exists suggesting histaminergic transmission in many species (SCHWARTZ, 1975; TAYLOR, 1975). Since TAYLOR & SNYDER (1971) reported that brain histamine was elevated after immersion of rats in liquid nitrogen and because a rapid turnover of brain histamine exists (POLLARD et al., 1974; DISMUKES & NYDER, 1974), the effect of focused microwave irradiation on rat brain histamine was studied. Hypotheses regarding the effects observed have been postulated and tested as described in this