Behavioral measures of conduction velocity and refractory period for reward-relevant axons in the anterior LH and VTA

Psychophysical techniques were used to estimate the conduction velocities and refractory periods of reward-relevant fibers in the anterior lateral hypothalamus (ALH) and ventral tegmental area (VTA). Six male rats of the Long-Evans strain served as subjects. Stimulation consisted of trains of pulse pairs delivered to the ALH and VTA; each stimulation site received one pulse from each pair. Collision of antidromic and orthodromic action potentials along the same reward-relevant axons was inferred from a decrease in the effectiveness of the stimulation at short intra-pair intervals. Significant collision-like effects were obtained for 5 of the 6 subjects, supporting the notion that reward-relevant neurons directly link the ALH and VTA. Estimates of the slowest conduction velocities ranged from 0.4 to 5.5 m/s. Refractory period estimates, obtained by delivering the trains of pulse pairs to a single stimulation site, ranged from 0.5 to 5.4 ms. The overlap between these psychophysical estimates and the electrophysiological estimates obtained in the accompanying paper is consistent with the notion that reward-relevant MFB fibers arise in one or several rostral MFB nuclei.