EFFECTS OF EXCITOTOXIC LESIONS OF THE BASAL FOREBRAIN ON MFB SELF-STIMULATION

Electrolytic lesions of the anterior medial forebrain bundle (MFB) hav e been shown to attenuate the rewarding impact of stimulating more cau dal MFB sites, In the present study, excitotoxic lesions were employed to determine the relative contribution of somata or fibers of passage contributing to that effect. Changes in reward efficacy were inferred , at three currents, from lateral displacements of the curve relating the rate of responding to the number of stimulation pulses per train. After baseline data were collected from stimulation sites in the later al hypothalamus (LH) and the ventral tegmental area (VTA), 70 nmol of N-methyl-D-aspartic acid was injected via cannulae aimed at basal fore brain sites. Three subjects were injected with vehicle and served as c ontrols. In 5 out of 15 cases, lesions encompassing the lateral preopt ic area, anterior LH, and substantia innominata resulted in long-lasti ng, large increases (0.2-0.47 log,, units) in the number of pulses req uired to maintain half-maximal rates of self-stimulation for low curre nts delivered via the LH electrode; smaller increases (0.08-0.33 log(1 0) units) were noted at moderate and high currents. Seven rats with si milar or more dorsally located damage showed moderate or transient inc reases in the number of pulses required to maintain half-maximal rates of LH or VTA self-stimulation. Vehicle injections did not affect beha viour. Varying degrees of demyelination were seen, mostly removed from the electrode tip, and in locations that varied substantially across subjects manifesting similar changes in self-stimulation, These result s support the notion that somata in the basal forebrain give rise to s ome of the directly activated fibers subserving self-stimulation of th e MFB.