Anterolateral lesions of the medial forebrain bundle increase the frequency threshold for self-stimulation of the lateral hypothalamus and ventral tegmental area in the rat

Abstract

Psychophysical data suggest that descending reward fibers directly link the lateral hypothalamus (LH) and ventral tegmental area (VTA) (Bielajew & Shizgal, 1982, 1986). As a step toward identifying the nuclei from which these fibers arise, electrolytic lesions were made in the anterior medial forebrain bundle (MFB) and the effect on self-stimulation of both the LH and VTA was examined. Changes in the rewarding effect were inferred from lateral displacements of curves relating the rate of responding to the frequency of the stimulation. Seven rats with lesioning electrodes in the anterior LH and self-stimulation electrodes in a more caudal portion of the LH and/or in the anterior VTA served as subjects. Lesions in 5 of the 7 rats displaced the rate-frequency functions for the LH and/or VTA sites toward higher frequencies, a result consistent with a reduction in the rewarding impact of the stimulation. Both of the ineffective lesions were centered in Compartment C of the anterior MFB, whereas the five effective lesions invaded the more lateral Compartments A, D, and E (Nieuwenhuys, Geeraedts, & Veening, 1982). These data suggest that projections arising in, coursing through, or terminating in the anterolateral MFB play a role in the rewarding effect of stimulating the LH and the VTA.