Interhemispheric links in brain stimulation reward.

The MFB substrate of self-stimulation (SS) has generally been viewed as a unilateral system. We re-examined this belief with pairs of moveable SS electrodes placed bilaterally in the MFB. Rats barpressed for trains of single or twin cathodal pulses of fixed intensity and width and of variable frequency. The first (C) and second (T) pulse of each pair was delivered through the left and right electrode or inversely. C-T intervals ranging from 0.2 to 5.0 ms were tested. The frequency of C pulses required for criterial bar-pressing was used to plot the stimulation efficacy (SE), as a function of the C-T interval and pulse presentation order. The electrodes were subsequently moved and the same procedure repeated for more ventral sites. With some pairs of contralateral hypothalamic (H) sites, the SE was independent of the C-T interval. However, with other pairs of contralateral H sites, the SE increased with C-T interval in a manner resembling a collision effect, with the important exception that no conduction time (CT) was apparent in the data. The absence of CT excludes the presence of a genuine collision effect. When one pulse was sent to the H and another to the contralateral ventral tegmentum (VT), the H-VT curve rose always earlier than the VT-H curve, thus resembling a transynaptic collision effect. However, the C-T interval at which the VT-H curve began rising (always 1.0 ms or less) fails to support the contention that the electrodes activated fibers separated by a synapse. Finally, a typical collision effect was noted with ipsilateral H-VT electrode placements, confirming the presence of direct linkage between ipsilateral MFB sites. Computer-generated data based on two parsimonious assumptions were found to match the empirical results. These assumptions were that each electrode activated a different branch of the same reward neuron and that conduction failure occurred at the branchpoint. The model, which posits that a large number of MFB reward neurons send branches to the other hemisphere, is testable and makes clear-cut predictions about the effects of lesions. In a preliminary test, we recorded the H and contralateral VT threshold frequencies before and after lesioning the H. The H threshold increased more when using small pulse current and remained constant throughout the 4-week testing period. The VT threshold was elevated more for intermediate pulse current and kept increasing with time.(ABSTRACT TRUNCATED AT 250 WORDS)