Re-evaluation of the role of dopamine in intracranial self-stimulation using in vivo microdialysis

Abstract

Rats were implanted with an electrode-microdialysis assembly in order to test the hypothesis that the reward signal elicited by medial forebrain bundle stimulation is relayed by the meso-accumbens dopamine cells. We first obtained the strength-duration function of self-stimulation, that is, a family of behaviorally equivalent stimuli (pulse intensity and pulse duration pairs yielding a constant self-stimulation rate). We then collected the self-stimulation-bound intra-accumbens dopamine for several pairs of intensity and duration, selected from within the strength-duration function. Our reasoning was that if the reward signal travels along the meso-accumbens dopaminergic neurons, the release of dopamine should not depend on the stimulus parameters because behaviorally equivalent stimuli should produce a constant output in all neural stages carrying the reward signal. The results showed that short duration/high intensity pulses induced considerably larger increases in dopamine levels than long duration/low intensity pulses, despite the fact that these stimuli maintained a constant self-stimulation rate. Among the interpretations envisaged, the most parsimonious one seems to be that the MFB rewarding signal is not relayed exclusively by meso-accumbens dopaminergic cells and that the latter may play a permissive-facilitator role at some transmission stage of the reward signal.