Effects of body weight loss and taste on VMH-LH electrical activity of rats

Electrical responses of ventromedial (VMH) and lateral (LH) hypothalamus to graded decrease in body weight and to taste were recorded from a conscious rat with chronically implanted macroelectrodes. Graded reduction in body weight was correlated with gradual increase in LH activity and reciprocal decrease in VMH activity, both of which were stable and specific to each gradation in body weight. On gustation of any test solution, basal activity was temporarily altered, if any. On sweet taste of calorically rich sucrose, VMH activity was enhanced and LH showed decrease. But on sweet taste of calorically inert saccharin, VMH activity was increased, though a reciprocal decrease in LH was not shown. Contrastingly, both bitter taste and salt taste caused increased LH activity but no change in VMH activity. Enhanced VMH activity correlated with sweet taste may be due to activation of VMH glucoreceptors. LH activation correlated with bitter and salt taste is a likely response of two distinct groups of LH units; one responding to salt taste and the other to bitter aversive taste.