Failure of amygdaloid lesions to increase the threshold for self-stimulation of the lateral hypothalamus and ventral tegmental area

It has been proposed that the directly stimulated axons underlying the rewarding effect of medial forebrain bundle (MFB) stimulation originate in the forebrain and descend at least as far as the ventral tegmentum. However, little is known about the location of the somata that give rise to these axons. Among the nuclei that contribute fibers to the descending component of the MFB and project past the lateral hypothalamus (LH) and ventral tegmental area (VTA) are cell groups within the amygdaloid complex. In this study, the rewarding effectiveness of stimulating the LH and VTA was measured before and after the amygdaloid complex was damaged by electrolytic lesions. Changes in rewarding effectiveness were inferred from shifts in the frequency required to sustain a half-maximal rate of lever-pressing at each of 3 currents. Following the lesions, there was no clear evidence of substantial, sustained decreases in rewarding effectiveness at the 14 stimulation sites, although one subject ceased to self-stimulate reliably. Given that the lesions damaged the principal amygdaloid sources of descending MFB fibers, these results suggest that the amygdaloid complex is not a major source of the directly activated fibers responsible for the rewarding effect of MFB stimulation.