Electrolytic Lesions of the Cortical and Adjacent Nuclei in the Amygdala Differentially Influence Thresholds for Rewarding Medial Forebrain Bundle Stimulation

After electrolytic lesions to the cortical and adjacent amygdaloid subnuclei, thresholds for rewarding medial forebrain bundle (MFB) stimulation were tracked in 19 rats with bilateral implants and 8 with single implants. Results were categorized into 3 groups depending on the magnitude of the lesion effect on ipsilateral frequency thresholds: substantial (> 60%), small (> 26%), or none (< 26%), compared with baseline values. Five rats exhibited threshold increases up to 225%, among the largest reported to the authors' knowledge. Small shifts were observed in another 5 rats, and no change in the remaining 17 rats. Threshold changes in the contralateral electrode mirrored ipsilateral ones. Results suggest that specific amygdaloid subnuclei modulate MFB reward signals through a diffuse collateralized organization of fibers and lend support to the existence of interhemispheric links in MFB reward pathways.