A hypothalamic alpha-adrenergic mechanism mediating the thermogenic response to electrical stimulation of the lower brainstem in the guinea pig
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Electrical stimulation of the lower brain stem (ESLB) at sites presumed to be parts of the ascending noradrenergic system was carried out in unanaesthetized young guinea pigs. At neutral ambient temperature ESLB elicited a thermogenic response resembling that evoked by microinjection of noradrenaline into the hypothalamus. The response consisted of a rise in oxygen uptake (to more than 50% of the resting value) and of body temperatures, especially of the interscapular adipose tissue. In some cases shivering was also evoked. The thermogenic response to ESLB was completely blocked by additional microinjection of an adrenergic alpha-receptor blocker, phentolamine, into the hypothalamic area where the noradrenergic fibres were presumed to terminate. Subsequent intrahypothalamic injection of noradrenaline, which had formerly been shown to restore the decreased responses to repeated ESLB, failed to restore the effectiveness of ESLB after phentolamine. It is concluded that the thermogenic responses to ESLB are mediated by a noradrenergic pathway ascending to the hypothalamus and not by direct stimulation of efferent pathways controlling the peripheral target system. The hypothalamic transmission can be prevented by an alpha-adrenergic blockade.
Key wordsElectrical brainstem stimulation Central noradrenergic pathways Adrenergic alpha-receptor blocking agents Central control of thermogenesis
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