(D-Phe12) bombesin and substance P analogues function as central bombesin receptor antagonists

Sensitization of neonatal rat lumbar motoneuron by the inflammatory pain mediator bradykinin

Bradykinin (Bk) is a potent inflammatory mediator that causes hyperalgesia. The action of Bk on the sensory system is well documented but its effects on motoneurons, the final pathway of the motor system, are unknown. By a combination of patch-clamp recordings and two-photon calcium imaging, we found that Bk strongly sensitizes spinal motoneurons. Sensitization was characterized by an increased ability to generate self-sustained spiking in response to excitatory inputs. Our pharmacological study described a dual ionic mechanism to sensitize motoneurons, including inhibition of a barium-sensitive resting K⁺ conductance and activation of a nonselective cationic conductance primarily mediated by Na⁺. Examination of the upstream signaling pathways provided evidence for postsynaptic activation of B₂ receptors, G protein activation of phospholipase C, InsP3 synthesis, and calmodulin activation. This study questions the influence of motoneurons in the assessment of hyperalgesia since the withdrawal motor reflex is commonly used as a surrogate pain model.

DOI:http://dx.doi.org/10.7554/eLife.06195.001

When we accidentally place our hand on a hot stove, we normally experience a painful sensation that starts with the sensory nerves under our skin. These nerves respond by transmitting electrical impulses to our brain, where the painful sensation is then processed. At the same time, these impulses are also transmitted to the motor nerves that control the muscles in our hand to trigger an immediate reflex to withdraw the hand from the hot stove. Pain therefore has a useful role as it can reduce how bad an injury is.

People with a condition called hyperalgesia have an increased sensitivity to pain. This condition can result from a chemical called bradykinin ‘sensitizing’ the sensory nerves, causing them to transmit more electrical impulses in response to pain than normal. This makes the injury feel much more painful, and can make the pain last for longer than is beneficial.

It was less clear whether bradykinin also affects motor nerves and so triggers a withdrawal reflex. By recording the electrical activity of motor nerve cells taken from the spinal cords of newborn rats, Bouhadfane et al. now show that these motor nerves become more active when exposed to bradykinin.

Nerve cells generate electrical signals when ions—such as potassium, sodium, and calcium ions—move through channels in the membranes of the cell. Therefore, to investigate how bradykinin influences the electrical activity of motor nerves, Bouhadfane et al. exposed the cells to drugs that inhibit particular ion channels. This revealed that bradykinin sensitizes the motor nerves by blocking a type of potassium ion channel and activating another ion channel that mainly transports sodium ions. Furthermore, Bouhadfane et al. were able to identify the signaling pathways that allow bradykinin to affect the motor nerve cells.

The study implies that the neuronal circuitry for pain does not rely exclusively on sensory nerve cells but should also integrate motor nerve cells. A future challenge remains in developing a protocol to resolve the contribution of motor nerve cells to hyperalgesia assessed by reflex withdrawal.

DOI:http://dx.doi.org/10.7554/eLife.06195.002

Gastrin-releasing peptide microinjected into the amygdala inhibits feeding

Bombesin (BN)-like peptides including gastrin-releasing peptide (GRP) are known to inhibit feeding. In the amygdaloid body BN receptors have been found in moderate to high densities. The central part of the amygdala (ACE) is essentially involved in the regulation of feeding and body weight. In the present experiments GRP was injected into the ACE and liquid food intake, general behavioural activity, as well as core temperature, were examined in male CFY rats. Food intake was measured every 5 min for 30 min and at the 40th and the 60th min following GRP or vehicle microinjections. Bilateral application of 50, 100 or 150 ng GRP resulted in transient inhibition of food intake while bilateral injection of 25 or 300 ng GRP did not modify feeding. Effect of GRP was eliminated by prior application of BN receptor antagonist [Leu(13)-psi(CH(2)NH)-Leu(14)]BN. After GRP or vehicle treatments animals were video-monitored and food intake, the first meal latency (FML), intermeal intervals (IMI), the time spent feeding (FT), grooming, resting and exploration were analysed at 5-min intervals for 30 min. However, FML did not change after GRP, the first IMI increased and intake, FT and intake/FT significantly decreased during the first 5 min. Duration of resting gradually increased after GRP and animals spent less time with exploration after GRP treatment than after vehicle injection. These differences were significant during the 25-30-min period. In body temperature, no significant changes were observed. Our results show that GRP in the ACE inhibits feeding and that GRP may decrease the efficiency of eating and may act as a satiety signal.

Bombesin and its family of peptides: prospects for the treatment of obesity

Bombesin, its family of bombesin-like peptides, and many other peptides/hormones modulate biological and behavioral functions in animals. Among the wide variety of functions influenced by bombesin/bombesin-like peptides, the most prominent may be their role in feeding-related behavior. Over many years, intensive psychopharmacological studies have addressed the mechanisms by which these peptides induce feeding suppression, and the results suggest the applicability of bombesin/bombesin-like peptides for the treatment of eating disorders and/or obesity in humans. Recent studies using gene-knockout mice also shed new light on the relationship between bombesin/bombesin-like peptides and feeding behavior. In addition, genetic analyses of the possible links between bombesin/bombesin-like peptides/receptors and human obesity have also been undertaken. Here, we briefly review the literature pertaining to the relationship between bombesin/bombesin-like peptides and feeding behavior-with particular attention to human subjects-and discuss the pharmacotherapeutic potential of bombesin/bombesin-like peptides with regard to obesity.

Bombesin receptor antagonists block the effects of exogenous bombesin but not of nutrients on food intake

The endogenous, meal-contingent release of bombesin (BN)-like peptides is thought to contribute to the termination of a meal. In the following experiments the potency of BN receptor antagonists to attenuate the ability of nutrients to suppress food intake was tested. First, the effectiveness of BN receptor subtype antagonists was verified by testing their ability to block the effects of exogenous BN on food intake. Rats were administered intraperitoneal (i.p.) injections of either saline or 0.1 mg/kg [D-Phe12,Leu14]BN (binds both GRP and NMB receptors), [D-Phe6]BN(6-13) ethyl amide (binds GRP > NMB), and cyclo-SS-octa (BIM-23042; binds NMB > GRP). Five minutes later rats were administered 8 micrograms/kg BN (i.p.) and milk intake was measured. Injections of [D-Phe12,Leu14]BN and [D-Phe6]BN(6-13) ethyl amide reliably attenuated the ability of BN to suppress milk intake whereas BIM-23042 was ineffective. The results show that the antagonists were behaviorally effective and that exogenous BN may exert its effects on food intake primarily through the GRP receptor subtype. Next, the antagonists were administered either 5 min prior to or 5 min after an intragastric nutrient load or no load in both overnight-deprived and nondeprived rats, and milk intake was then measured. Stomach loads reduced intake and this effect was not attenuated by BN receptor antagonists. Finally, rats were allowed to prefeed and the milk was then removed. Rats were then administered a BN receptor antagonist (0.1 and 1.0 mg/kg) or saline either immediately after the prefeed, 10 min later, or 20 min later. Milk diet was then returned and intake was measured. Peripheral injections of the BN receptor antagonist had no effect compared to saline on milk intake. Collectively, the results indicate that the blockade of peripheral Bn peptide receptors is not sufficient to attenuate the safety signals generated by stomach loads or prefeeding.

Desmethionine-bombesin receptor antagonist blocks bombesin-induced inhibition of alcohol intake

[D-Phe6,Des-Met14]bombesin(6-14), ethyl amide (D-BN) is a specific, competitive receptor antagonist of bombesin, a neuropeptide that inhibits alcohol and food intake. We tested the effects of IP injected D-BN (4-400 micrograms/kg) on bombesin-induced (4 micrograms/kg) reduction of caloric intake. In the first experiment, ad lib-fed female and male rats (Ns = 18) were deprived of water for 23 h, injected with peptides or saline in randomized sequences of doses, and immediately given access to 5% w/v ethanol for 30 min, followed by 30 min of water. In a second experiment, male rats (N = 10) were injected with the antagonist at 10 or 20 min prior to bombesin injection and alcohol access, and behaviors were observed and quantified once a minute with an instantaneous time-sampling technique. D-BN injection blocked the bombesin-induced reduction in alcohol intake (> or = 40 micrograms/kg) and food intake (> or = 200 micrograms/kg). When injected 20 min prior to access, D-BN alone (200 micrograms/kg) initially elevated alcohol drinking and later increased feeding behaviors and decreased resting, relative to saline injection. Results indicate bombesin-induced reduction of alcohol intake depends on a specific peptidergic receptor process, and endogenous bombesin-like peptide could act physiologically to elicit satiation with ethanol and food.

A novel synthetic phyllolitorin analogue [desTrp3,Leu8]phyllolitorin inhibits scratching behavior induced by neuromedin C in rats

[DesTrp3,Leu8]phyllolitorin (DTP) (pGlu-Leu-Ala-Val-Gly-Ser-Leu-Met-NH2) was synthesized as an analogue of phyllolitorins, a new member of bombesin family, and examined if it antagonized neuromedin C (NMC)-induced scratching. DTP inhibited dose-dependently the scratching behavior by NMC (1 microgram), whereas it did not alter any element of other grooming behaviors. DTP (6 micrograms) alone was found to be neither toxic nor active in inducing both scratching and grooming, which were comparable to vehicle alone. Assuming that the scratching behavior is commonly and specifically elicited by bombesin family peptides, DTP might be classified as a new type of bombesin antagonist.

Galanin receptor antagonists M40 and C7 block galanin-induced feeding

Two peptide antagonists of the galanin receptor, M40 (galanin[1-13]-Pro-Pro-[Ala-Leu]2-Ala amide) and C7 (galanin[1-13]-spantide amide), significantly inhibited galanin-induced consumption of a palatable wet cookie mash, when microinjected intraventricularly to satiated rats. Antagonists were effective at doses equimolar to or less than the active doses of galanin. Feeding induced by an overnight fast was not significantly different in rats microinjected with saline as compared to M40 or C7, at doses which inhibited galanin-induced feeding. The activity of the chimeric compound, C7, did not appear to be linked to the properties of its C-terminal spantide-like sequence, as C7 did not induce barrel rolling at doses which inhibited galanin-induced feeding. The IC50 for displacement of 125I-[Tyr26]-porcine galanin 1-29 binding in rat hypothalamic membranes was 15 nM for M40, and 0.2 nM for C7, as compared to 0.8 nM for unlabelled porcine galanin(1-29). These two structurally different galanin antagonists, both demonstrating antagonist activity in vivo in awake, behaving rats, provide promising tools for further analyses of the functional activity of galanin in the mammalian brain.

Biological effects and receptor binding affinities of new pseudononapeptide bombesin/GRP receptor antagonists with N-terminal D-Trp or D-Tpi

In an attempt to produce more powerful (effective) bombesin/GRP receptor antagonists, the D forms of Trp or Trp analog (Tpi) were introduced at position 6 in two pseudononapeptides, Leu13 psi (CH2NH)Leu14-bombesin(6-14) and Leu13 psi(CH2NH)Phe14-bombesin (6-14). These antagonists were tested for their ability to inhibit basal and gastrin releasing peptide (GRP) (14-27)-induced amylase release from rat pancreatic acini in a superfusion assay. They were also assessed for the inhibition of 125I-Tyr4-bombesin binding to Swiss 3T3 and small cell lung carcinoma cell line H-345 and the mitogenic response of Swiss 3T3 cells induced by GRP(14-27). The peptides, when given alone, did not stimulate amylase secretion, but were able to inhibit gastrin releasing peptide (14-27)-induced amylase release. All of the antagonists showed strong binding affinities for Swiss 3T3 and H-345 cells and suppressed the GRP(14-27)-induced increase of [3H]thymidine incorporation into DNA of Swiss 3T3 cells at nanomolar concentrations. Antagonist D-Tpi6,Leu13 psi (CH2NH)Leu14-bombesin (6-14)(RC-3095) was slightly more potent in these assays than D-Trp6,Leu13 psi (CH2NH)Leu14-bombesin (6-14)(RC-3125). Nevertheless, D-Trp6,Leu13 psi (CH2NH)Phe14-bombesin (6-14) showed the highest binding affinity for Swiss 3T3 and H345 cells and it was the most potent inhibitor of GRP(14-27)-induced amylase secretion. This antagonist RC-3420 was particularly effective in inhibiting the growth of Swiss 3T3 cells, exhibiting an IC50 value less than 1 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fourth ventricle bombesin injection on meal-related parameters and grooming behavior

Injections of bombesin (BN) into the vicinity of the caudal brainstem suppress food intake in rats. In the present study, the food intake parameters [meal size (MS), intermeal interval (IMI), satiety ratio (SR)] affected by 4th ventricle BN injections were determined. Following a 15-h food deprivation, rats were administered 4th ventricle injections of saline (0.15 M) and BN in doses of 1, 5, 10, and 20 ng BN, and were then given access to sweetened milk. The animals' behaviors (feeding, resting, grooming, exploring) were scored every one min and milk intake every five min for 60 min following the injections. Fourth ventricle injections of 5 ng BN and greater reliably suppressed milk. intake. This reduction was reflected in a significant reduction in the MS. The IMI was not affected. As a result, the SR (IMI2/MS1), which is thought to represent the satiating property of food, was reliably greater following BN than following saline administration. The reduced food intake was accompanied by a significant increase in grooming behavior and a corresponding decrease in exploring. The amount of time spent resting (inactive) was similar following saline and all but the highest dose of BN. To demonstrate that the behavioral effects of BN were mediated by specific caudal brainstem BN receptors, 4th ventricle injections of [D-Phe12,Leu14]BN, a BN receptor antagonist, or saline preceded the 4th ventricle injection of 5 ng BN. Pretreatment with [D-Phe12,Leu14]BN reliably blocked the effects of BN on food intake and grooming.

N-acetyl-GRP(20-26)-O-CH3 reverses intracisternal bombesin-induced inhibition of gastric acid secretion in rats

Intracisternal injection of 19 pmoles of bombesin in light-ether-anesthetized rats, five minutes after intracisternal vehicle, produced a 75% and 63% inhibition in gastric acid output and concentration, respectively, in 2-hour pylorus-ligated rats. Pretreatment of rats with the characterized peripheral bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 (1 nmole) reversed the inhibitory effect of bombesin on gastric acid output and concentration. In contrast, the related bombesin antagonist N-acetyl-GRP-O-CH2-CH3 (1 nmole) was ineffective in this system. In urethane-anesthetized, acute gastric fistula rats infused with pentagastrin, intracisternal N-acetyl-GRP(20-26)-O-CH3 protected against the inhibition in gastric acid output produced by intracisternal bombesin (19 pmoles). Thus the recently characterized peripheral bombesin antagonist N-acetyl-GRP(20-26)-O-CH3 also appears to be effective in antagonizing central bombesin-induced inhibition in gastric acid secretion in two models. This represents a first report of a synthetic bombesin antagonist effective in reversing central bombesin-induced effects on gastric function.

Progress in the development of potent bombesin receptor antagonists

Bombesin and the mammalian-related peptides gastrin-releasing peptide (GRP), GRP and neuromedin B have been shown to have numerous actions in the CNS, gastrointestinal tract and on growth. However, the role of the peptides in various physiological processes has remained unclear because of the lack of potent antagonists. Recent in vitro studies have described four different classes of bombesin receptor antagonist, some of which are active in the nanomolar range and in vivo. Robert Jensen and David Coy describe recent insights into peptide structural determinants of biological activity. Evidence from structure-function studies have resulted in identification of some analogues that function as potent antagonists in all systems examined. Furthermore, various subtypes of bombesin receptors can now be differentiated by these various classes of antagonist.

Receptor heterogeneity for bombesin-like peptides in the rat central nervous system

As an initial characterization of bombesin binding sites in the rat central nervous system, we examined the pharmacological specificity of binding of bombesin (BBS) and several BBS analogs to rat cortex and compared these results to those for rat pancreas. In addition, we used in vitro receptor autoradiography to evaluate binding of 125I-[Tyr4]bombesin and 125I-Bolton-Hunter neuromedin B (NMB) to several regions of the rat brain. The results of the pharmacological study indicated that the pancreas and cortex had different binding affinities for BBS-like peptides. While cortical binding sites had a high affinity for NMB, pancreatic binding sites had almost no affinity for NMB. Results from the autoradiographic study demonstrated that BBS receptor heterogeneity exists in individual nuclei in the rat brain. Some nuclei have a high affinity for NMB, similar to cortical BBS binding sites, other regions have a low affinity similar to pancreatic BBS binding sites. These results provide evidence that subtypes of BBS receptors are present in different tissues and within discrete regions of the rat central nervous system.

Effects of capsaicin treatment on immunoglobulin secretion in the rat: further evidence for involvement of tachykinin-containing afferent nerves

Neonatal capsaicin treatment has previously been shown to diminish the primary antibody response of adult rats to the subcutaneously administered T-dependent antigen, sheep red blood cells, as measured using a modification of the Cunningham plaque-forming cell assay technique. We have now studied the kinetics of this response in adult normal, neonatally capsaicin-pretreated and neonatally capsaicin-pretreated substance P-infused rats, and examined the effects of the tachykinin antagonist Spantide, on the plaque-forming cell response. Capsaicin pretreatment did not affect the antigen-specific plaque-forming cell response over the first 4 days following antigen injection. At days 5, 6 and 7 of the response, there was a statistically significant decrease in the number of plaque-forming cells secreting antigen-specific IgM, an effect not observed in capsaicin-pretreated rats which were given a subcutaneous infusion of substance P at the time of antigen injection. The tachykinin antagonist Spantide inhibited the plaque-forming cell response in normal rats after in vivo infusion at the time of antigen injection by more than 70%. This effect of Spantide was dose dependent, occurred with maximal effect at 10 microM, and appeared to be independent of any histamine-mediated action. The results of this study provide further evidence for a receptor-mediated immunomodulatory role of tachykinin-containing primary afferent nerves.

Effect of substitutions in position 12 of bombesin on antagonist activity

Recent studies show that substitutions for the His in position 12 of bombesin (Bn) are important in determining antagonist activity. The present study was designed to investigate the chemical properties of the substitution in position 12 of Bn that determined antagonist activity and affinity. Nine [Leu14]Bn analogues with a single amino acid substitution and two analogues with multiple substitutions in addition to position 12 were synthesized. Replacing His12 with Phe12 resulted in an agonist with 100-fold decrease in potency and as reported previously, replacement with D-Phe12 resulted in an antagonist, but with a 10,000-fold decrease in affinity. Substitution of D-beta-naphthylalanine (D-Nal12), a larger and more hydrophobic group than D-Phe, produced a complete loss of antagonist activity, whereas substitution of D-pyridylalanine (D-Pal12), a group more hydrophilic and similar in size to D-Phe, converted the analogue to a very weak agonist with 300-fold lower affinity than the D-Phe analogue. Antagonist activity depended on the nature of the aromatic moiety, with a D-Trp12 resulting in an inactive analogue, and with a D-Tyr12 resulting in a weak antagonist being 100-fold less potent than the D-Phe12 substitution. The addition of an electron withdrawing group to the D-Phe substitution (D-Cpa12) resulted in a minimal decrease in antagonist activity, whereas the addition of an electron donating group (p-hydroxy in D-Tyr12) resulted in a 30-fold decrease in antagonist activity. The addition of a basic group (D-Arg12 or D-Pal12) resulted in weak agonists.(ABSTRACT TRUNCATED AT 250 WORDS)