Medial forebrain bundle units in the rat: dependence of refractory period estimates on pulse duration

The slope of some psychophysically-derived curves describing recovery from refractoriness in medial forebrain bundle (MFB) reward neurons decreases as the pulse duration is lengthened. This effect has been attributed to increased recruitment of small-diameter fibers by the long-duration pulses. Alternatively, this effect might be due to increases in the refractory periods of a fixed population of neurons. By means of extracellular recording, we measured the effect of increasing the pulse duration on recovery from refractoriness in 26 neurons antidromically activated by MFB stimulation. In all cases, recovery from refractoriness was delayed as the pulse duration was increased from 0.1 ms to 2.0 ms. The delay was more pronounced at near-threshold currents than at higher currents and was generally largest in neurons that recovered most rapidly from refractoriness when stimulated with brief pulses. These effects of long duration pulses on recovery of excitability in a fixed population of neurons may have contributed to the delay in recovery implied by the psychophysical data. Thus, delayed recovery does not necessarily imply preferential recruitment of small-diameter fibers by the long-duration pulses.