Minimal changes in hypothalamic temperature accompany microwave‐induced alteration of thermoregulatory behavior

Abstract

This study probed the mechanisms underlying microwave-induced alterations of thermoregulatory behavior. Adult male squirrel monkeys (Saimiri sciureus), trained to regulate the temperature of their immediate environment (T_a) behaviorally, were chronically implanted with Teflon reentrant tubes in the medical preoptic/anterior hypothalamic area (PO/AH), the brainstem region considered to control normal thermoregulatory processes. A Vitek temperature probe inserted into the tube measured PO/AH temperature continuously while changes in thermoregulatory behavior were induced by either brief (10-min) or prolonged (2.5-h) unilateral exposures to planewave 2,450-MHz continuous wave (CW) microwaves (E polarization). Power densities explored ranged from 4 to 20 mW/cm² (rate of energy absorption [SAR] = 0.05 [W/kg]/[mW/cm²]). Rectal temperature and four representative skin temperatures were also monitored, as was the T_a selected by the animal. When the power density was high enough to induce a monkey to select a cooler T_a (8 mW/cm² and above), PO/AH temperature rose ∼ 0.3°C but seldom more. Lower power densities usually produced smaller increases in PO/AH temperature and no reliable change in thermoregulatory behavior. Rectal temperature remained constant while PO/AH temperature rose only 0.2–0.3°C during 2.5-h exposures at 20 mW/cm² because the T_a selected was 2–3°C cooler than normally preferred. Sometimes PO/AH temperature increments greater than 0.3°C were recorded, but they always accompanied inadequate thermoregulatory behavior. Thus, a PO/AH temperature rise of 0.2–0.3°C, accompanying microwave exposure, appears to be necessary and sufficient to alter thermoregulatory behavior, which ensures in turn that no greater temperature excursions occur in this hypothalamic thermoregulatory center.