To ascertain the nature of intracranial self-stimulation (ICSS) interactions between locus coeruleus (LC) and various hypothalamic (HYP) sites, rats were tested with monophasic pulse pair stimulation. When trains of pulse pairs were delivered to a single ICSS site, lower responding occurred at short pulse-pair intervals. However, when the pulses of each pulse pair were split between LC and HYP ICSS sites, response rates at all pulse-pair intervals were elevated well above single-pulse responding, indicating a neurophysiological interaction between these loci. The symmetry of LC and HYP interactions depended upon the HYP electrode placement. Both internal capsule and fields of Forel placements produced asymmetrical LC-HYP interactions in which higher ICSS rates occurred when the LC received the first pulse of each pair than when it received the second pulse of each pair, suggesting a direct, and possibly modulating, influence of the LC upon these HYP placements. Both medial forebrain bundle and perifornical HYP placements produced symmetrical LC-HYP interactions in which similar ICSS rates occurred regardless of site stimulation order, suggesting an indirect, reciprocal influence between the LC and these HYP placements.
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