Influence of aminergic and peptidergic substances on heart beat frequency in the stick insect Carausius morosus (Insecta, Phasmatodea).

The dorsal heart of the Indian stick insect, Carausius morosus, is responsible for the anterograde flow of hemolymph to the aorta and into the body cavity. The contraction frequency of the insect heart is known to be influenced by several substances of neural source. Here, a semi-exposed heart assay was employed to study the effect of an aminergic substance (octopamine) and three neuropeptides (C. morosus hypertrehalosemic hormone [Carmo-HrTH], crustacean cardioactive peptide [CCAP], and proctolin) on heart contraction. The contraction frequency was measured as beats per minute in adults ligated between the head and the prothorax. All three investigated neuropeptides had a stimulatory effect on heart contraction that lasted approximately 6 min, after which the normal heart beat rate was restored. Proctolin and CCAP stimulated the rate of heart beat also in unligated stick insects, whereas Carmo-HrTH was active only in ligated insects. The latter could suggest that when the stick insect is not ligated, a competing substance may be released from the head of C. morosus; the competing substance is, apparently, not physiologically active but it binds or blocks access to the receptor of Carmo-HrTH-II, thereby rendering the HrTH peptide "not active." In ligated stick insects, 6.7 × 10-8  M Carmo-HrTH-II significantly increased the heart beat rate; higher doses resulted in no further increase, suggesting the saturation of the HrTH receptor. Octopamine inhibited the rate at which the heart contracted in a dose-dependent manner; inhibition was achieved with 10-4  M of octopamine.