EVIDENCE IMPLICATING BOTH SLOW-CONDUCTING AND FAST-CONDUCTING FIBERS IN THE REWARDING EFFECT OF MEDIAL FOREBRAIN-BUNDLE STIMULATION

A behavioral version of the collision test was used to determine wheth er reward-relevant neurons directly link self-stimulation sites in the lateral hypothalamic (LII) and ventral tegmental (VTA) areas. Five ma le rats served as subjects. Trains of conditioning (C) and test (T) pu lses were delivered to the two stimulation sites, each site receiving one of the pulses from each pair. The C-T interval was varied from 0.2 -17.3 ms, and the effectiveness of the paired pulse stimulation was es timated by comparing the rate-number curve obtained at each C-T interv al to rate-number curves obtained with trains of evenly spaced single pulses delivered via one electrode. For 4 of the subjects, stimulation effectiveness increased with the C-T interval, and the form of this i ncrease was similar regardless of which electrode delivered the C-puls es. These increases in effectiveness are consistent with recovery from collision block in reward-relevant fibers stimulated at both sites. T he domain of the rising portion of the effectiveness versus C-T interv al curve spanned 2.2-7.7 ms. Such a gradual rise suggests that the dir ectly stimulated substrate is composed of fibers with a wide range of conduction velocities and/or refractory periods. The discrepancy betwe en these gradually rising collision curves and the steeply rising curv es obtained in previous collision studies may have been due to inadequ ate sampling of the rate-number function in the earlier studies.