Evidence for proctolin and a second myotropic peptide in the cockroach, Leucophaea maderae, determined by bioassay and HPLC analysis

Myoinhibitors controlling oviduct contraction within the female blood-gorging insect, Rhodnius prolixus

Muscle activity can be regulated by stimulatory and inhibitory neuropeptides allowing for contraction and relaxation. There are various families of neuropeptides that can be classified as inhibitors of insect muscle contraction. This study focuses on Rhodnius prolixus and three neuropeptide families that have been shown to be myoinhibitors in insects: A-type allatostatins, myoinhibiting peptides (B-type allatostatins) and myosuppressins. FGLa/AST-like immunoreactive axons and blebs were found on the anterior of the dorsal vessel and on the abdominal nerves. FGLa/AST-like immunoreactive axons were also seen in the trunk nerves and on the bursa. The effects of RhoprAST-2 (FGLa/AST or A-type allatostatins) and RhoprMIP-4 (MIP/AST or B-type allatostatins) were similar, producing dose-dependent inhibition of R. prolixus spontaneous oviduct contractions with a maximum of 70% inhibition and an EC50 at approximately 10(-8)M. The myosuppressin of R. prolixus (RhoprMS) has an unusual FMRFamide C-terminal motif (pQDIDHVFMRFa) as compared to myosuppressins from other insects. Quantitative PCR results show that the RhoprMS receptor transcript is present in adult female oviducts; however, RhoprMS does not have an inhibitory effect on R. prolixus oviduct contractions, but does have a dose-dependent inhibitory effect on the spontaneous contraction of Locusta migratoria oviducts. SchistoFLRFamide, the myosuppressin of Schistocerca gregaria and L. migratoria, also does not inhibit R. prolixus oviduct contractions. This implies that FGLa/ASTs and MIP/ASTs may play a role in regulating egg movement within the oviducts, and that the myosuppressin although myoinhibitory on other muscles in R. prolixus, does not inhibit the contractions of R. prolixus oviducts and may play another role in the reproductive system.

Insect peptide hormones, an overview of the present literature

A comprehensive overview of the recent state of the art of insect peptide hormones with chemical structures is presented. An increased interest in insect neuropeptides and dynamic development of that research area has been influenced by a rapid improvement of instrumentation necessary for isolation and structural characterization. Several research teams have studied the relationships between biological properties of insect and vertebrate peptide hormones. Thus hormones from the AKH family can be considered glucagon counterparts, whereas the myotropic hormones such as proctolin and Lem-PK (LPK) are a substance P equivalent. Insect melanization hormones Bom-MRCH in their structural characteristics and properties resemble those of mammal MSH, and leucosulfakinins Lem-SK-I and -II show some similarities with gastrin II and cholecystokinin. Bombyxin-II (Bom-PTTH-II) reveals a structural homology with human insulin and similar biological properties to adenocorticotropic mammal hormone. Allatostatin (Dip-JHS-I) may be compared to somatostatin as it can be inferred from the observations that this peptide modulates JH secretion in cockroach, Blattella germanica. Determination of the primary structure of eclosion hormones Mas-EH and Bom-EH-II as well as the amino acid sequence of allatotropin and allatostatin is a significant contribution to the understanding of the molecular mechanisms of metamorphosis and insect development.

Expression of allatostatin in the oviducts of the cockroach Diploptera punctata

The cockroach allatostatins (Y/FXFGL/Ia ASTs) are a ubiquitous family of peptides in the invertebrates. They affect numerous physiological processes including the inhibition of juvenile hormone III (JH) biosynthesis, inhibition of muscle contraction, inhibition of ovarian ecdysteroid biosynthesis and inhibition of vitellogenin (Vg) release from the fat body. We have developed and optimized a sensitive and specific quantitative competitive reverse transcriptase polymerase chain reaction (QC-RT-PCR) method to quantify Diploptera punctata AST (Dippu-AST) expression. Using this technique we show that tissues of both lateral and common oviducts and the ovary express message for Dippu-AST. Moreover, the pattern of expression observed in the oviducts and ovary is strikingly similar with significant changes occurring during the reproductive cycle. Specifically, expression of AST is drastically reduced during the time of maximal vitellogenin (Vg) uptake, with higher levels measured prior to and following vitellogenesis. Furthermore, using immunocytochemistry, we have shown Dippu-AST-like-immunoreactivity in the terminal abdominal ganglion, as well as in ventral nerve 7, some branches of which innervate the common and lateral oviducts with other branches innervating the bursa copulatrix and brood sac of mated female D. punctata. The pattern of Dippu-AST expression and immunocytochemical staining suggests that ASTs function, in part, to regulate the cycle of vitellogenesis in mated female D. punctata.

Proctolin, an insect neuropeptide

Synthetic, biological and conformational studies on the insect neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) and some of its analogues are reviewed.

Effect of allatostatin and proctolin on antennal pulsatile organ and hindgut muscle in the cockroach, Diploptera punctata

Diploptera punctata allatostatins are brain neuropeptides that inhibit juvenile hormone synthesis by corpora allata. They also occur in nerves of many organs other than corpora allata. The distribution of allatostatins in, and the effect of allatostatins on two other organs, antennal pulsatile organ and hindgut, are demonstrated here. Allatostatin I-like immunoreactive material is present in cells of subesophageal and terminal abdominal ganglia; these ganglia are known to contain the cells that project to antennal heart nerve and proctodeal nerve, respectively. Electron micrographs of both organs show nerve terminals with allatostatic immunoreactive granules along with terminals containing nonimmunoreactive granules. Immunoreactive neurosecretory granules are about 200 nm in largest dimension. In the antennal pulsatile organ, profiles of the nerve terminals are larger in the ampullar wall than in the muscle; in hindgut the terminals with immunoreactive granules are associated with the muscle net below the circular muscle. Hindgut responded to allatostatins I and IV with a dose-dependent decrease in amplitude and frequency of contraction that was reversible, with the threshold concentration for response between 10(-8) and 10(-7) M. In contrast, pulsatile organ muscle showed no such change with either allatostatin at 10(-7)-10(-4) M. However, both organs responded to proctolin with increased amplitude and frequency of contractions. Allatostatins I and IV inhibited the proctolin-induced increase of hindgut contraction, whereas no such effect was seen in antennal pulsatile organ muscle. Extract of antennal pulsatile organ muscle showed proctolin-like bioactivity that comigrated with authentic proctolin on three sequential HPLC systems.

Some pharmacological properties of the oviduct muscularis of the stable fly Stomoxys calcitrans

1. Spontaneous and rhythmic contractions were measured in 80% of the preparations of the stable fly oviduct which were separated from the central nervous system and other tissues. Measurements of the changes in the amplitude and frequency of contractions and changes in the baseline tonus were taken separately, even though they often occurred together during chemical treatments. 2. L-glutamate, at a concentration of 10(-4) to 10(-3) M, caused an increase in the frequency of contractions and in muscle tonus. Contraction frequency showed the highest percentage of change while amplitude and tonus had lower values. 3. The excitation threshold for proctolin on the oviduct was between 10(-13) and 10(-10) M. The initial responses to the peptide showed increases in the amplitude, frequency, and tonus of muscle contractions. However, tonic changes dominated the response profile when the concentration reached 10(-10) or 10(-9) M. In 12 experiments in which the oviduct was exposed to 10(-7) M leucomyosuppressin, no change in the amplitude, frequency, or tonus of muscle contractions was detected. 4. Octopamine caused an inhibition of spontaneous muscle contractions of the oviduct. Spontaneous contractions showed a graded drop in activity over the concentration range tested (10(-8)-10(-6) M). Also 57% of the preparations tested showed a substantial drop in baseline tonus after the addition of octopamine. 5. Stable fly oviducts exposed to extracts from the male reproductive tract showed a noticeable change in the amplitude frequency and tonus of muscle contraction at levels of 2 to 3 equivalents.

Proctolin: a review with emphasis on insects

The distribution, physiological role, mode of action, and pharmacology of the pentapeptide neuroregulator proctolin are reviewed, with special emphasis on insects. Whereas proctolin is distributed extensively throughout arthropods, its presence in molluscs, annelids, or chordates is not well established. In the arthropods, proctolin acts as a neuromodulator and possibly as a neurohormone. It does not appear to function as a conventional neurotransmitter. Two model proctolinergic systems are highlighted: motor control of the visceral muscles of the locust oviduct and of the skeletal muscles of the locust ovipositor. In these preparations proctolin is a cotransmitter acting to enhance neuromuscular transmission and muscular contraction. The mode of action of proctolin is not well understood, although the second messengers cAMP, phosphatidyl inositol, and calcium have been implicated in various systems. Pharmacologically, the proctolin receptor has been examined with structure/activity studies, and the effects of a variety of amino acid substitutions and deletions of the pentapeptide are described. It is unfortunate that no specific antagonists of the proctolin receptor appear to be available and that no receptor-binding studies have been reported. The prospects are good for advances in our understanding of modulatory mechanisms, since proctolin appears to be emerging as the model for studies of this type.

Identification and distribution of a proctolin-like neuropeptide in the nervous system of the gypsy moth, Lymantria dispar, and in other Lepidoptera

Although the neuropeptide proctolin has important functions in many arthropods, it is reported to be absent in Lepidoptera. Its possible occurrence in these insects was reinvestigated by bioassays of HPLC fractions and immunocytochemistry. A proctolin-like substance was recovered from the frontal and subesophageal ganglia of Lymantria dispar. This substance has the same chromatographic retention time as proctolin; enzymatic degradation indicates that it is a peptide; it is bound by proctolin antisera; and thus it is indistinguishable from authentic proctolin. A small subpopulation of proctolin-like immunoreactive (PLI) neurons was stained in the larval CNS of L. dispar, Manduca sexta, Trichoplusia ni, Galleria mellonella, and Vanessa cardui. Most prominent of these cells are median neurosecretory neurons in the brain, paired neurons in the frontal ganglion, two clusters of neurons in the subesophageal ganglion, paired lateral neurons in the thoracic ganglia, and dorsomedial neurons in the abdominal ganglia. Also, varicose PLI axons are found in the corpora cardiaca and perivisceral organs. In L. dispar, PLI cells also were found in the corpora cardiaca. The results of this study indicate that proctolin is of general occurrence in the Lepidoptera, that it has an important role in the stomatogastric nervous system, and that it may be released as a local neurohormone from various neurohemal organs.

A comparative study of the immunohistochemical localization of a presumptive proctolin-like peptide, thyrotropin-releasing hormone and 5-hydroxytryptamine in the rat central nervous system

A proctolin (PROC)-like peptide was studied immunohistochemically in the hypothalamus, lower brainstem and spinal cord of the rat using an antiserum against PROC conjugated to thyroglobulin. Neuronal cell bodies containing PROC-like immunoreactivity (PROC-LI) were observed in the dorsomedial, paraventricular and supraoptic nuclei of the hypothalamus and in the nucleus raphe magnus, nucleus raphe pallidus, nucleus raphe obscurus and nucleus interfascicularis nervi hypoglossi in the medulla oblongata. Fibers containing PROC-LI were seen in the median eminence and in other hypothalamic nuclei, and in the lower brainstem in cranial motor nuclei including the dorsal motor nucleus of the vagus nerve, the motor trigeminal nucleus, the facial nucleus and nucleus ambiguous, and in lower numbers in the nucleus of the solitary tract and locus coeruleus. Fibers containing PROC-LI were also located in the spinal cord, in the intermediolateral cell column at thoracic levels and in the ventral horns at all levels of the spinal cord. After transection of the spinal cord, all PROC-immunoreactive fibers below the lesion disappeared. Following injection of Fast blue into the thoracic spinal cord, retrogradely labeled cells in the nuclei raphe pallidus, obscurus and magnus and nucleus interfasciculari nervi hypoglossi were seen to contain PROC-LI. PROC-LI had a similar distribution as thyrotropin-releasing hormone (TRH)-LI in the above-mentioned areas and coexistence of TRH-LI and PROC-LI was shown in cell bodies in the hypothalamus and medulla oblongata. PROC-LI could also be shown to coexist with 5-hydroxytryptamine (5-HT)-LI in neuronal cell bodies in the lower brainstem. The results demonstrate the occurrence of a PROC-like peptide in the mammalian nervous system, and these neurons seem to be at least largely identical to previously described TRH systems. A possible involvement of the PROC-like peptide in spinal motor control is discussed in relation to the well-established role of PROC in control of motor behavior in insects and invertebrates.

Proctolin, its presence in and action on the oviduct of an insect

Proctolin has been isolated from oviduct extracts of Leucophaea maderae by HPLC. Quantitative bioassay with the hindgut of L. maderae demonstrated a proctolin titre of 0.93 +/- 0.15 ng/oviduct. Exposure to proctolin produced three changes in the spontaneous contractile activity of the oviduct: an increase in muscle tonus, an increase in the amplitude and frequency of phasic contractions. The sensitivity of the oviduct to proctolin was compared with the hindgut and foregut organ preparations from the same insect. Oviducts were responsive to proctolin in a calcium-free medium and the peptide also appeared to facilitate the reentry of calcium after depleted preparations were returned to normal levels of external calcium.

Isolation and partial characterization of a second myotropic peptide from the hindgut of the cockroach, Leucophaea maderae

Proctolin and a second myotropic peptide were extracted from the hindgut of the cockroach Leucophaea maderae with methanol-water-acetic acid (90:9:1). The two peptides were easily separated by HPLC on a mu-Bondapak-phenyl column. Like proctolin, the second peptide was heat stable and was inactivated by the exopeptidases aminopeptidase M and carboxypeptidase Y. The response of the isolated hindgut to the new peptide was distinguishable from the response to proctolin by the following features: (a) a longer interval following application (1-4 min) to reach a maximum contraction, and (b) a much larger amplitude for single phasic contractions. Like proctolin, the new peptide could cause a protracted stimulation of the hindgut for more than 2 hr.

Structure function analysis of an arthropod peptide hormone: proctolin and synthetic analogues compared on the cockroach hindgut receptor

Proctolin is a pentapeptide (arg-tyr-leu-pro-thr) found in nervous tissues throughout the phylum Arthropoda. Initially described as a peptidergic neuromuscular transmitter, it now appears that proctolin is a major arthropod neurohormone modulating nervous activity, muscle tonus and contractile force. Structure-function studies with synthetic analogues demonstrate diverse peptides which retain agonistic activity, but few exhibit a high degree of affinity for the cockroach hindgut receptor compared with proctolin (Kdapp = 2 x 10(-8) M). High affinity agonists (Kdapp less than or equal to 10(-7) M) are limited to [phe2]-proctolin, [lys1]-proctolin and specific N-terminal additions. In this regard the hindgut receptor differs in its ligand specificity from that reported for the locust extensor tibia receptor. Using the analogue studies to predict sequences which may act as agonists, we have examined the known vertebrate peptide hormones for proctolin-like sequences. A possible relationship between vasoactive intestinal peptide, proctolin and erythrophore concentrating hormone is critically evaluated.

The neuropeptide proctolin acts directly on Limulus cardiac muscle to increase the amplitude of contraction

The pentapeptide proctolin increases the amplitude of contraction but not heart beat frequency of the isolated heart of Limulus polyphemus. It acts directly on the heart muscle and has no effects on the neurones of the cardiac ganglion or on the cardiac neuromuscular EJPs. A peptide with molecular weight, enzymatic susceptibilities and physiological effects similar to those of proctolin occurs in the Limulus cardiac ganglion. It is suggested that proctolin, or a family of proctolin-like peptides, may modulate muscle contraction in more than one subphylum of the Arthropoda.