chapter 1 - Bentham Science

Cardioactive or Inactive Neuropeptides of Insects HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 79
thoracic aorta due to a lack of extensive indirect flight musculature in the thorax. They show mostly
unidirectional, forward oriented cardiac pulsations (review [4, 5]).
The most recent findings [6] show that insect hearts may be both structurally and functionally similar to the
human heart. In both cases, the hearts originate and are regulated by similar sets of the genes [8, 9], and in
both cases their functions depend on involuntary, purely myogenic, regulatory mechanisms. The
rhythmicity of the heartbeat is in both cases determined by special depolarization nodi, i.e. terminal
regulatory nodus of the insect heart [6, 7] and a sinoatrial or atrioventricular nodi known from human
electrocardiography (ECG) [10]. In addition, the heart of D. melanogaster evolved a human like, compact
conical chamber with an atrium, distinguished by synchronic (not peristaltic) systolic contractions [6].
Further evidence from comparative physiology and pharmacology shows that both vertebrate and insect
hearts are a common subject of extensive feed-back regulations. These homeostatic or feed-back factors are
mostly derived from the respiratory metabolism (carbonate ions), muscular activity (lactate or pyruvate),
excretory metabolism (adrenaline or noradrenaline), digestion (glutamate) or intermediary metabolism
(serotonine or octopamine). So far, there are no neuropeptides known to influence physiologically the
heartbeat of vertebrate animals or humans, which is in large conflict with the extensive claims for
cardioactive properties of neuropeptides in insects (see below).
The exact physiological nature of insect heartbeat, whether neurogenic or myogenic, has been a matter of
scientific discussions for some time. Earlier observations on the hearts of arthropods (Horseshoe crab, [11])
indicated a possibility of neurogenic regulation. Later studies, performed chiefly on in vitro preparations of
insect hearts explanted into saline, showed that the hearts disconnected from the nervous system were able
to beat for some time, suggesting a myogenic principle for the systolic contractions [reviews 2-5, 12, 13].
Under physiological, in vivo conditions, however, a clearcut distinction between the myogenic, neurogenic
or, eventually, neurohormonal principles of insect heartbeat remained undecided for long time due to a
substantial lack of data from the living body [14]. In connection with the periodic reversal of insect
heartbeat, there appeared assumptions about the possible interplay of two neurogenic cardiac pacemakers.
One was situated in the caudal region for stimulation of the forward-oriented (anterograde) heartbeat and
the second, frontal pacemaker was assumed to stimulate the reciprocal, backward-oriented (retrograde)
peristaltic waves (review [2]).
In accordance with the above described assumptions about the two reciprocal pacemakers, there indeed
appeared claims for the presence of a neurogenic cardiac pacemaker in the frontal ganglion of the central
nervous system. This frontal neurogenic pacemaker was tentatively responsible for the backward-oriented
or retrograde cardiac pulsations in pupae of the mulberry silkworm (Bombyx mori, [15]). The whole story
of two reciprocal pacemakers ended up by analogous claims for the opposite, posterior neurogenic cardiac
pacemaker situated among the neurons of the terminal abdominal ganglionic mass in pupae of a sphingid
moth (Manduca sexta). This regulatory center was assumed to stimulate rhythmicity of the reciprocal,
forward-oriented or anterograde cardiac pulsations [16-30].
A number of pharmacological, cardiostimulating or inhibitory preparations were tested on insect hearts
more than 50 years ago [11, 23, 24]. Unfortunately, these extensive screenings [11] did not provide
consistent data with respect to cardioactive or inhibitory responses on insect heart. In 1979, the first
neuropeptide cardiostimulating concept was created on the basis of the effects of proctolin, using the most
common, in vitro bioassay on the heart of the American cockroach (Periplaneta americana) [14, 17]. Later,
during the 1980’s, cardiostimulating properties were ascribed to several of the newly isolated or
synthesized neuropeptides. For example, there were cardioactive neuropeptides isolated from the nervous
system of M. sexta [18, 19], from the nervous system of crustaceans (Crustacean cardiostimulating peptide
or CCAP [20]) and, especially, from the nervous system of the American cockroach [21]. The last
neuropeptide, named corazonin, was for a long time advertised as "the most potent cardiostimulating
peptide" of insects [4, 22, 31]. Simple in vitro assays of heartbeat were widely used for the easy
determination of the cardiaoctive properties of various neuropeptides [17-21], most often the assays were
made on cockroach hearts removed from the body and explanted into saline [23, 24, review 3, 4].