Unexpected effects of peptide and nonpeptide substance P receptor antagonists on basal prolactin and growth hormone release in vitro

Pharmacological characteristics of endokinin C/D-derived peptides in nociceptive and inflammatory processing in rats

Endokinins designated from the human TAC4 gene consist of endokinin A, endokinin B, endokinin C (EKC) and endokinin D (EKD). EKC/D is a peptide using the common carboxyl-terminal in EKC and EKD and consists of 12 amino acids, and exerts antagonistic effects on the induction of scratching behavior by substance P (SP). Some of SP-preferring receptor antagonists have several d-tryptophan (d-Trp); however, the pharmacological effect of EKC/D-derived peptides with d-Trp remains to be solved. Therefore, to clarify the pharmacological characteristics of EKC/D-derived peptides, effects of pretreatment with these peptides on SP-induced scratching and thermal hyperalgesia, formalin-induced flinching and carrageenan-induced inflammation were evaluated. Intrathecal administration of [d-Trp(8)]-EKC/D and [d-Trp(10)]-EKC/D showed a markedly long inhibitory effect, at least 14 h, whereas the antagonistic effects of [d-Trp(8,10)]-EKC/D and EKC/D without d-Trp disappeared after 1h. Furthermore, the inhibitory effect of [d-Trp(10)]-EKC/D-derived peptides was dependent on the number of amino acids from the amino-terminus, and the more numerous the amino acids, the more marked the antagonistic effect. Thus, these results indicate that the effective duration of EKC/D-derived peptides is dependent on the number of d-Trp in the carboxyl-terminal region and the amino-terminal region regulates the antagonistic effect of EKC/D.

Leucine at the carboxyl-terminal of endokinins C and D contributes to elicitation of the antagonistic effect on substance P in rat pain processing

Endokinins are tachykinin peptides designated from a human preprotachykinin C (PPT-C, TAC4) gene and consist of endokinin A (EKA), endokinin B (EKB), endokinin C (EKC) and endokinin D (EKD). A representative of mammalian tachykinins is substance P (SP), which functions as a neurotransmitter or modulator in the pain system; however, little is known about the role of these endokinins, especially EKC and EKD, in pain processing. Therefore, we evaluated the effects of EKC/D (using the common carboxyl-terminal duodecapeptide in EKC and EKD) on pain processing in rats. Pretreatment with EKC/D prevented induction of scratching behavior and thermal hyperalgesia by intrathecal administration of EKA/B (using the common C-terminal decapeptide in EKA and EKB) and SP and c-Fos expression in laminae I/II and V/VI of the spinal cord by noxious thermal stimulation. A prominent difference between EKC/D and SP is the presence of leucine instead of methionine at the carboxyl-terminal of EKC/D. Thus, to clarify whether leucine at the carboxyl-terminal of EKC/D plays an important role in determining the inhibitory effect of this peptide, we intrathecally administered [Met(12)]-EKC/D in which only leucine of EKC/D is replaced by methionine. This peptide did not exhibit the inhibitory effect on SP-induced scratching behavior or thermal hyperalgesia but conversely caused thermal hyperalgesia. Taken together, these findings indicate that EKC/D has an inhibitory effect on pain processing in the rat spinal cord, and the effect is due to leucine at the carboxyl-terminal of EKC/D.

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.

Effect of nutrient restriction on the somatotropes and substance P-immunoreactive cells in the pituitary of the female ovine fetus

The maternal environment affects fetal development and may influence the physiology of the adult. Fetal growth hormone (GH) is increased by maternal undernutrition but the mechanisms responsible are unknown. This study determined the effect of maternal undernutrition on the development of fetal pituitary somatotropes in the female. Ewes were grouped randomly into control (fed 100% of requirements) or nutrient restricted (fed 50%) from Days 28 to 78 of gestation. At Day 78, the ewes were killed and fetuses collected (Day 78 NR (nutrient restricted): n=6; Day 78C (control): n=6). Remaining ewes were realimented to 100% of nutritional requirements and were killed at Day 135 (Day 135 NR (nutrient restricted): n=6; Day 135 C (control): n=6). Somatotropes were visualized immunocytochemically and the size, mean density, total percentage and proportion colocalized with substance P were determined for each group. Nutrient restriction increased (p<0.01) the density of pituitary cells in Day 78 fetuses but this difference was no longer apparent by Day 135 after realimentation. The density and proportion of somatotropes were not different between treatment groups at Day 78 but were significantly (p<0.05) lower in the nutrient restricted Day 135 fetuses as compared to the Day 135 control animals. Somatotropes from restricted fetuses were significantly (p<0.001) larger at Day 78. Nutrient restriction increased the density (p<0.001) and percentage (p<0.05) of substance P-immunoreactive cells Day 135 fetuses. Similarly, the proportion of somatotropes that expressed substance P was significantly (p<0.05) increased by nutrient restriction in the Day 135 fetuses. Although nearly two thirds of substance P-immunoreactive cells co-expressed GH, there was no significant effect of treatment on this co-expression. Additional studies are required to determine if other components of the neuroendocrine GH axis are affected by this nutritional insult, if the alterations that we have observed, particularly in the tachykinin system, persist into adulthood and, importantly, what are the long-term consequences of an altered GH axis.

Novel orally active growth hormone secretagogues

A novel class of growth hormone-releasing compounds with a molecular weight in the range from 500 to 650 has been discovered. The aim of this study was to obtain growth hormone secretagogues with oral bioavailability. By a rational approach we were able to reduce the size of the lead compound ipamorelin (4) and simultaneously to reduce hydrogen-bonding potential by incorporation of backbone isosters while retaining in vivo potency in swine. A rat pituitary assay was used for screening of all compounds and to evaluate which compounds should be tested further for in vivo potency in swine and oral bioavailability, fpo, in dogs. Most of the tested compounds had fpo in the range of 10-55%. In vivo potency in swine after iv dosing is reported, and ED50 was found to be 30 nmol/kg of body weight for the most potent compound.

Fatty acids and brain peptides

The role of fatty acids (FA) as a mediator and modulator of central nervous system activity in general, and peptides in particular, is only recently becoming understood. This paper reviews numerous findings concerned with the activity of fatty acids, particularly with their interaction with diverse neurochemical systems and their consequences for better understanding neurotransmitters, hormones and peptides. The effects include FA as precursors in the manufacture of neurochemical elements, including enzymes, neurotransmitters, and hormones. Of particular interest is the important changes in neuronal membrane composition that have been attributed to FA. Such changes may account for the changes in thermoregulation, learning, and other functions that accompany dietary manipulation of FA intake. While the total level of FA has been the object of many investigations, this report addresses the need to focus on the ratio of FA, especially alpha-linolenic/linoleic acid, which has been shown to be a critical factor in a number of research studies.

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.