Assessment of the neural substrate for intracranial self-stimulation by the postreinforcement pause technique.

The poststimulation excitability of neurons mediating intracranial self-stimulation (ICSS) was evaluated by the paired-pulse method. Stimulus effectiveness was assessed by the postreinforcement pause (PRP) and by frequency threshold (FT) determinations in 7 rats performing ICSS in the medial forebrain bundle (MFB) and in the ventral tegmental area (VTA). Stimulus effectiveness values were minimal at conditioning-test (C-T) pulse intervals of 0.6 and 0.8 ms for MFB and VTA animals, respectively, because of neuronal refractoriness. Local potential summation could account for the increase in effectiveness at very short C-T intervals, and an additional peak of enhanced effectiveness at a C-T interval of 2.0 ms, perhaps reflecting synaptic events, was observed only in VTA animals with the PRP method. Important advantages of the PRP method were that the C-T interval was the only stimulus parameter that was varied, and the behavioral output of the animal remained relatively constant.