[³H]-clonidine is a radioactive ligand used for the characterization of alpha-adrenergic receptors and [³H]-clonidine binding was used in this study as an indicator of the availability of alpha-adrenergic receptors in rats during and after microwave-induced hyperthermia. Physiological actions of acetylcholine in brain are most commonly mediated by muscarinic receptors. Specific [³H]-quinuclidinyl benzilate binding was used to assay the concentration of muscarinic binding sites.
Of six brain regions investigated only the hypothalamus showed significant changes in receptor states. This confirms its pivotal role in thermoregulation. Adrenergic receptors showed a 36% decrease in binding following radiation after a 2.5°C increase in body temperature, suggesting a mechanism to facilitate norepinephrine release. Muscarinic receptors showed a 65% increase in binding at the onset of radiation. This may be attributed to the release of acetylcholine in the hypothalamus in response to heat cumulation. The continued elevated binding during the period of cooling after exposure was shut off may suggest the existence of an extra-hypothalamic heat-loss pathway.