Fig. 5. Antagonistic brain stem modulatory inputs change the threshold and gain of thermoregulatory responses in guinea pigs. The horizontal line at the metabolic rate (MR) of 5 W/kg denotes the normal value measured at thermoneutral ambient temperature. Intrahypothalamic (i.h.) microinjection of noradrenaline (NA) shifts the threshold temperatures for the onset of cold defense to higher temperatures. Microinjection of serotonin (5-HT) into the hypothalamus shifts the shivering threshold to lower temperatures. Selective lesions of noradrenergic or serotonergic input, respectively, by the neurotoxins 6-hydroxydopamine (6-OHDA) and 5,6 dihydroxytryptamine (5,6-DHT) are denoted by the dotted lines. Both NA and 5-HT microinjections increase the threshold temperatures for heat defense (increase in respiratory evaporative heat loss [REHL]), and these changes could not be reversed by selective neurochemical lesions. Intrahypothalamic microinjections release prostaglandins, which increase threshold temperatures for heat defense reactions. The shaded bars indicate the width of the interthreshold zone (ITZ), which depends on the balance between the modulatory NA and 5-HT inputs. For example, a wide ITZ results when 5-HT input dominates in cold-adapted guinea pigs. Data are from Zeisberger. 168