The vagina muscles of the blood-sucking insect Rhodnius prolixus as a model for exploring the physiological effects of proctolin.

Proctolin is a neuroactive pentapeptide first isolated from the cockroach Periplaneta americana in which it has an excitatory effect on contractions on visceral muscles of the hindgut. Subsequently, proctolin is reported in a wide variety of invertebrates, and considerable efforts have been made to determine its mode of action. Its primary role appears to be that of a neuromodulator rather than a classical neurotransmitter, and it may also serve as a neurohormone, depending on the muscles examined. The present study identifies the vagina muscles of the blood-sucking insect Rhodnius prolixus (Stål) as a proctolinergic system. Physiological doses of proctolin generate prolonged contractions that closely mimic the effects of motor nerve stimulation. This preparation is convenient and robust, warranting its use as an experimental system to further understand the role of proctolin in the regulation of muscle contractions in insects. Moreover, these muscles are innervated by an identifiable inhibitory component providing a means to investigate the interaction between proctolin excitation and neural inhibition.