The feeding stimulus in Rhodnius prolixus is transmitted to the brain by a humoral factor

Neurosecretory cells in the brain of Rhodnius prolixus are known to be the source of an ovulation hormone released at feeding. They were selected to test the hypothesis that feeding brings about the release of another hormone in the abdomen which is transported forward to activate the neuroendocrine axis in the brain, and that severing the aorta interferes with this transport. These cells have previously been shown to exhibit an increase in action potential frequency at the time of release of their hormone. In normal females, the spike frequency of the cells increased after feeding and remained high over at least the next 24 h. In females with the dorsal vessel severed, the spike frequency remained low, at levels near those of unfed females, except for a transitory increase 4 h after feeding. The spike frequency of the neurosecretory cells in females with the dorsal vessel severed increased when hemolymph from fed normal females or from those with their dorsal vessel severed was placed directly on the brains of the test females. Hemolymph taken from unfed females did not produce this response. The activity in the hemolymph was heat-stable and disappeared after pronase or trypsin digestion. Similar activity was present in the fused thoracico-abdominal ganglionic mass, but absent from fore-, mid- and hindgut and from the abdominal neurosecretory organs.