ESTIMATES OF THE AXONAL REFRACTORY PERIOD OF MIDBRAIN DOPAMINE NEURONS - THEIR RELEVANCE TO BRAIN-STIMULATION REWARD

Psychophysical studies have shown that the directly activated neurons subserving the rewarding effect produced by electrical stimulation of the medial forebrain bundle (MFB) have refractory periods (RPs) shorte r than those of dopaminergic (DA) neurons; this suggests that the dire ctly stimulated substrate for the rewarding effect does not include DA neurons. Comparison of RP estimates, however, is difficult since thos e of DA neurons were obtained with the standard electrophysiological t echnique that characterizes cell body/initial segment rather than the directly stimulated axonal segment a anesthetized rats, we re-estimate d and compared the RP of DA neurons that project to the MFB and the ve ntral striatum (VST) with two stimulation procedures: one that charact erizes the axonal segment near the stimulation electrode and the stand ard one that characterizes the cell body/initial segment near the reco rding electrode. Results showed that DA axonal RPs range from 1.0 to 2 .2 ms, whereas cell body/initial segment RPs range from 1.0 to 3.0 ms. The axonal RP was equal to or shorter (mean difference = 0.22 ms, n = 15) than the cell body/initial segment RP. Axonal RP estimates for MF B sites range from 1.3 to 2.1 ms, values that slightly overlap with th e late recovery from refractoriness reported in psychophysical studies for reward-relevant neurons. Axonal RP estimates obtained for VST sit es were very similar (mean = 1.66, LH and 1.62 ms, VST) suggesting tha t the excitability of DA axons does not differ along their path. These results further support the hypothesis that DA axons are unlikely to constitute a major component of the directly-stimulated reward-relevan t axons in the MFB. They also suggest that the direct contribution of DA axons to reward produced by VST stimulation is more important than by the MFB.