Intracranial self-stimulation: temporal interactions among mesencephalic and diencephalic sites

The monophasic pulse pair technique has been employed to ascertain whether pairs of intracranial self-stimulation (ICSS) sites interact with each other. The present study investigated the interactive properties of ICSS placements in the substantia nigra (SN) and mid-ventral periaqueductal gray (MV) with those hypothalamic ICSS placements within (MFB) or outside of (non MFB) the medial forebrain bundle. ICSS response rates when pulses of each pulse pair were split between two ICSS sites were significantly higher than the sum of rates when each site was stimulated singly with single-pulse trains. Moreover, all interaction conditions yielded higher rates than pulse pairs delivered to the mesencephalic site at an optimal interval, yet similar rates to pulse pair stimulation delivered to the diencephalic site. The symmetry of the interactions depended upon electrode loci: MV/MFB and SN/non MFB interactions were significantly higher when the mesencephalic site received the first pulse of each pair, effects which accounted for 49 and 58% of the variance respectively. Conversely, MV/non MFB and SN/MFB interactions were significantly higher when the mesencephalic site received the second pulse of each pair, effects which accounted for 5 and 14% of the variance respectively. These behavioral ICSS interactions are discussed in terms of interrelated heterogeneous subsystems subserving ICSS behavior.