Uses of double-pulse stimulation behaviorally to infer refractoriness, summation, convergence, and transmitter characteristics of hypothalamic reward systems.

To self-administer trains of pulse pairs, rats with electrodes in the hypothalamic reward system would press a lever at lower current thresholds or faster latencies, the shorter the intrapair interval--unless the interval was so short that each second pulse fell within the refractory period following the first. By delivering all second pulses to the contralateral reward system not only was this refractory period limitation on temporal summation circumvented but spatial summation of the two reward systems was demonstrated. Evidently, they converge somewhere upon common neurons. Nearby nonreward structures did not share in this convergence. Assuming that the temporal-summation decline at longer intrapair intervals reflected the course of transmitter disposal at the synapse, imipramine and diisopropylfluorophosphate were peripherally administered. These drugs, which retard disposal, respectively, in adrenergic and cholinergic synapses, indeed prolonged temporal summation, thus, supporting the assumption and implying that adrenergic and cholinergic mechanisms both mediate self-stimulation.