Review article: alginate-raft formulations in the treatment of heartburn and acid reflux.

Alginate-based raft-forming formulations have been marketed word-wide for over 30 years under various brand names, including Gaviscon. They are used for the symptomatic treatment of heartburn and oesophagitis, and appear to act by a unique mechanism which differs from that of traditional antacids. In the presence of gastric acid, alginates precipitate, forming a gel. Alginate-based raft-forming formulations usually contain sodium or potassium bicarbonate; in the presence of gastric acid, the bicarbonate is converted to carbon dioxide which becomes entrapped within the gel precipitate, converting it into a foam which floats on the surface of the gastric contents, much like a raft on water. Both in vitro and in vivo studies have demonstrated that alginate-based rafts can entrap carbon dioxide, as well as antacid components contained in some formulations, thus providing a relatively pH-neutral barrier. Several studies have demonstrated that the alginate raft can preferentially move into the oesophagus in place, or ahead, of acidic gastric contents during episodes of gastro-oesophageal reflux; some studies further suggest that the raft can act as a physical barrier to reduce reflux episodes. Although some alginate-based formulations also contain antacid components which can provide significant acid neutralization capacity, the efficacy of these formulations to reduce heartburn symptoms does not appear to be totally dependent on the neutralization of bulk gastric contents. The strength of the alginate raft is dependant on several factors, including the amount of carbon dioxide generated and entrapped in the raft, the molecular properties of the alginate, and the presence of aluminium or calcium in the antacid components of the formulation. Raft formation occurs rapidly, often within a few seconds of dosing; hence alginate-containing antacids are comparable to traditional antacids for speed of onset of relief. Since the raft can be retained in the stomach for several hours, alginate-based raft-forming formulations can additionally provide longer-lasting relief than that of traditional antacids. Indeed, clinical studies have shown Gaviscon is superior to placebo, and equal to or significantly better than traditional antacids for relieving heartburn symptoms. Alginate-based, raft-forming formulations have been used to treat reflux symptoms in infants and children, and in the management of heartburn and reflux during pregnancy. While Gaviscon is effective when used alone, it is compatible with, and does not interfere with the activity of antisecretory agents such as cimetidine. Even with the introduction of new antisecretory and promotility agents, alginate-rafting formulations will continue to have a role in the treatment of heartburn and reflux symptoms. Their unique non-systemic mechanism of action provides rapid and long-duration relief of heartburn and acid reflux symptoms.

An ICAM-1 antisense oligonucleotide prevents and reverses dextran sulfate sodium-induced colitis in mice.

Mice treated p.o. with 5% dextran sodium sulfate develop a mild to moderate colitis characterized by focal areas of inflammation and crypt abscesses. Immunohistological analysis of colons from dextran sodium sulfate-treated mice revealed an increased expression of intercellular adhesion molecule 1 (ICAM-1) and infiltration of lymphocyte function antigen 1-positive cells. A murine-specific antisense oligonucleotide, ISIS 3082, was used to determine the role of ICAM-1 expression in the development of colitis. Prophylactic treatment of dextran sodium sulfate-treated mice with ISIS 3082 reduced the clinical signs of colitis in a dose-dependent manner, with maximal effects occurring at a dose of 1 mg/kg/day. Reductions in ICAM-1 immunostaining and infiltrating leukocytes were observed in colons of animals treated with 1 mg/kg ISIS 3082. Scrambled control oligonucleotides failed to modify the course of the disease. The ICAM-1 oligonucleotide also diminished the clinical severity of colitis in mice with established colitis. The toxicity of ISIS 3082 was assessed in normal CD-1 mice by administering the oligonucleotide intravenously every other day for 2 weeks. At pharmacologically relevant doses of ISIS 3082 (1 and 10 mg/kg), there were no signs of toxicity with respect to body and organ weights, clinical chemistry or hematology. At a dose of oligonucleotide 20- to 100-fold greater than maximal pharmacological doses, the oligonucleotide produced an increase in liver and spleen weights; a mild chronic inflammation in liver, lung and lymph nodes; monocytosis and an elevation of serum liver transaminases. These data suggest that an antisense oligonucleotide that reduces ICAM-1 expression could be effective in the therapy of inflammatory bowel disease in humans and that such an oligonucleotide would be safe at pharmacologically relevant doses.

Characterization of endothelin-induced nociception in mice: evidence for a mechanistically distinct analgesic model.

The behavioral response elicited in mice by an i.p. injection of endothelin-1 (ET-1) (0.1 mg/kg) was differentiated from that elicited by standard agents such as acetylcholine (ACh) (5.5 mg/kg) or phenyl-p-quinone (PpQ) (1.25 mg/kg). First, there was lack of two-way "cross-tolerance' between test paradigms. That is, at equieffective doses, a 60-min prior i.p. injection of ET-1 blocked the behavioral response to a subsequent i.p. injection of ET-1 or PpQ, but not of ACh, whereas a 60-min prior injection of ACh or of PpQ had no effect on a subsequent i.p. injection of ACh, PpQ or ET-1. Second, differential antagonism of ET-1-, ACh- or PpQ-induced responses was observed in an examination of 36 test compounds. For example, cyclo-oxygenase inhibitors such as indomethacin and ibuprofen did not block the ET-1-induced response at > 10 times the doses that blocked ACh- or PpQ-induced responses, whereas other compounds (such as certain benzodiazepines) inhibited ET-1-induced, but not ACh- or PpQ-induced, responses. These findings suggest that ET-1 produces a novel nociceptive stimulus, mechanistically distinct from ACh and PpQ. Hence, the ET-1-induced behavioral response in mice serves as a rapid and convenient measure of in vivo endothelin activity. In addition, this test might be a model for clinical pains not adequately treated by present analgesic agents or adequately tested by preclinical antinociceptive screens using ACh or PpQ. As such, it is a potentially valuable model for the identification of novel analgesic and other agents.

Endothelin-induced nociception in mice: mediation by ETA and ETB receptors.

Endothelins (ET-1, ET-2 or ET-3) or endothelin precursors (big-ET-1[1-38], big-ET-2[1-37] or big-ET-3[1-41]) injected i.p. in mice have previously been shown to elicit a characteristic nociceptive behavioral response. In this study, we investigated the endothelin receptor type (ETA or ETB) that mediates this behavioral response. Mice were injected i.p. with ET-1, ET-2, ET-3, big-ET-1[1-38], big-ET-2[1-37], big-ET-3[1-41], sarafotoxin S6a, sarafotoxin S6b, sarafotoxin S6c, ET-1 with Ala substitutions for Cys3 and Cys11 or His-Leu-Asp-Ile-Ile-Trp, and quantal dose-response curves were obtained for each of the compounds (except the latter). Co-administration of enzyme inhibitors with the big-endothelins was used to establish the requisite conversion to endothelins and big-ET-1[22-38], big-ET-2[22-37] and ET-3[22-41] amide, and the ETA-selective antagonist cyclo[-D-Asp-Pro-D-Val-Leu-D-Trp-] was used to determine receptor specificity. The ED50 values were 2.9, 3.3 and 23.9 micrograms/kg i.p. for ET-1, ET-2 and ET-3, respectively, 0.6, 0.6 and 13.1 micrograms/kg i.p. for sarafotoxin S6a, sarafotoxin S6b and sarafotoxin S6c, respectively, and 5.3 micrograms/kg i.p. for ET-1 with Ala substitutions for Cys3 and Cys11. Big-ET-1[22-38], big-ET-2[22-37], big-ET-3[22-41] amide and ET-C produced less than 25% effect up to 2000 micrograms/kg. The big-ET-1-induced effects were blocked by the enzyme inhibitors phosphoramidon and thiorphan (ID50 = 0.9 mg/kg) but not by ubenimex (bestatin), captopril or perindopril. Cyclo[-D-Asp-Pro-D-Val-Leu-D-Trp-] blocked ET-1- and ET-2-induced effects but not ET-3-, ACh- or phenyl-p-quinone-induced effects. These results suggest that endothelin-induced nociceptive behavioral response in mice can be mediated via both ET receptor types, ETA and ETB. Further, the ET-1 carboxy-terminal hexapeptide is insufficient to produce the effect, and the Cys3-Cys11 disulfide bridge of ET-1 is not required.

A Gi2alpha antisense oligonucleotide differentiates morphine antinociception, constipation and acute dependence in mice.

In the same mice in which the intracerebroventricular (i.c.v.) administration of antisense oligodeoxyribonucleotide (oligo) directed against the Gi2alpha (but not Gi1alpha, Gi3alpha or G(s)alpha) G-protein subunits attenuated i.c.v. morphine-induced antinociception in the tail-flick test, none of the oligos altered naloxone-precipitated jumping (acute dependence). Likewise, none of the oligos significantly altered morphine-induced constipation. Hence, i.c.v. morphine-induced antinociception might be preferentially mediated via transduction pathway(s) different from constipation or acute dependence, offering novel opportunities for drug discovery.

Complementary and synergistic antinociceptive interaction between the enantiomers of tramadol.

The explanation for the co-existence of opioid and nonopioid components of tramadol-induced antinociception appears to be related to the different, but complementary and interactive, pharmacologies of its enantiomers. The (+) enantiomer had Ki values of only 1.33, 62.4 and 54.0 microM at mu, delta and kappa receptors, respectively. The (-) enantiomer had even lower affinity at the mu and delta sites (Ki = 24.8, 213 and 53.5 microM, respectively. The (+) enantiomer was the most potent inhibitor of serotonin uptake (Ki = 0.53 microM) and the (-) enantiomer was the most potent inhibitor of norepinephrine uptake (Ki = 0.43 microM). Basal serotonin release was preferentially enhanced by the (+) enantiomer and stimulation-evoked norepinephrine release was preferentially enhanced by the (-) enantiomer. The (+) and (-) enantiomers each independently produced centrally mediated antinociception in the acetylcholine-induced abdominal constriction test (ED50 = 14.1 and 35.0 micrograms i.t., respectively). Racemic tramadol was significantly more potent (P < .05) than the theoretical additive effect of the enantiomers (antinociceptive synergy). Synergy was also demonstrated (P < .1) in the mouse 55 degrees C hot-plate test (i.p. route) and (P < .05) the rat Randall-Selitto yeast-induced inflammatory nociception model (i.v. and i.p. routes). Critically, the enantiomers interacted less than synergistically in two side-effects of inhibition of colonic propulsive motility and impairment of rotarod performance. The racemate and the (+) enantiomer were active in a chronic (arthritic) inflammatory pain model. Taken together, these findings provide a rational explanation for the coexistence of dual components to tramadol-induced antinociception and might form the basis for understanding its clinical profile.

Vasorelaxant effect of the potassium channel activator, RWJ 29009, is tissue selective.

Potassium channel activators have potential cardioprotective properties, in part due to their ability to increase coronary blood flow. We compared the vasorelaxant properties of potassium channel activators, a calcium channel blocker (nicardipine) and a direct smooth muscle relaxant (sodium nitroprusside) in the canine coronary artery, the femoral artery and the saphenous vein precontracted with 0.03 microM endothelin-1. In the circumflex coronary artery, RWJ 29009, a novel and potent potassium channel activator, maximally relaxed the precontracted rings with an EC50 of 1.9 nM. Cromakalim (EC50 = 220 nM) and nitroprusside (EC50 = 109 nM) were also active. Nicardipine (EC50 = 16.6 nM) produced only a 70% relaxation at 1 microM concentration. In both femoral artery and saphenous vein, all agents relaxed the precontracted rings only at much higher concentrations, and the relaxations were only 75% of maximal relaxation. The results show that while all vasodilators preferentially relax the coronary artery, potassium channel activators appear to be the most selective and potent of these agents.

Endothelin-1, -2 and -3 directly and big-endothelin-1 indirectly elicit an abdominal constriction response in mice.

When injected intraperitoneally into mice, endothelins ET-1, ET-2, ET-3 and big-endothelin-1[1-38] (big-ET-1[1-38]) produced a dose-related, robust and easily quantified abdominal constriction response within 20 min. The ED50 values for this response were 0.026, 0.005, 0.131, and 0.043 mg/kg, respectively. Hence, this test could provide a convenient in vivo endpoint for endothelin activity. The results also imply that ET-1, ET-2, ET 3 or big-ET-1[1-38] may be nociceptive under certain conditions. Morphine (4 mg/kg, s.c.) administered 30 min prior completely blocked the response produced by ET-1. Thus, in conjunction with other indicators, the test may also serve as an in vivo screen for agents useful in the treatment of abdominal or visceral pain. The effect of big-ET-1[1-38], but not ET-1, was blocked by pretreatment with the enzyme inhibitor phosphoramidon (10 mg/kg, s.c., 30 min prior), implying that the big-ET-1[1-38] must first be enzymatically cleaved, presumably to ET-1, in order to elicit the abdominal constriction response. This test might also serve as a discriminative antinociceptive screen, because the response to ET-1 was not blocked by acetaminophen (400 mg/kg, p.o.), ibuprofen (75 mg/kg, p.o.) or indomethacin (1.0 mg/kg, p.o.).

Endothelin-1-induced nociception.

Intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of morphine to mice antagonized the abdominal constriction induced by an i.p. injection of endothelin-1 (ET-1; 0.1 mg/kg). The ED50 values (95% confidence intervals) were 39.3 (16.5-80.2) ng and 1.5 (0.8-4.9) ng, respectively. The antagonism of ET-1-induced abdominal constriction by morphine was blocked by naloxone (1.0 mg/kg, s.c.) or by 24 h pretreatment with beta-funaltrexamine (beta-FNA; 8.84 micrograms, i.c.v.). These results demonstrate for the first time that the stimulus resulting from an i.p. injection of ET-1 is transmitted via ascending (pain) pathways that are subject to attenuation by opioid (mu) receptor activation. Hence, ET-1-induced abdominal constriction is a new pain model which, given the other pharmacology of ET-1, might represent a unique model with potential specific utility for anginal or other visceral pain.

Central and peripheral administration of serotonin produces opposite effects on mouse colonic propulsive motility.

The effects of central or peripheral administration of serotonin on colonic expulsion time (CE) of a glass bead were evaluated after i.p. or free hand i.c.v. administration to mice. Serotonin (5-HT) caused an inhibition of CE when administered centrally but stimulated propulsion after i.p. administration. Several selective serotonin agonists were then tested. Inhibition after i.c.v. administration was produced by 8-OH-DPAT (5-HT1a), RU-24969 (5-HT1b), and 2-methyl serotonin (5-HT3), but not DOI (5-HT2) which augmented propulsion. Relative potencies for inhibition (ED50S) were RU (0.9 micrograms, 3.9 nM) greater than 8-OH-DPAT (3 micrograms, 9.1 nM) greater than 5-HT (7.8 micrograms, 20.1 nM) greater than 2-methyl serotonin (43 micrograms, 140 nM). After i.p. administration 5-HT stimulated propulsive motility (ED50 = 16.1 micrograms, 41.4 nM) while 8-OH-DPAT (ED50 = 55 micrograms, 167 nM) and RU-24969 (ED50 = 54 micrograms, 236 nM) inhibited. DOI and 2-MS had no dose-related activity. The finding that several of the serotonin receptor agonists were capable of inhibiting propulsive motility either by i.p. or i.c.v. administration is a new finding and may help to explain drug-induced constipating activity in man. No selective agonist completely mimicked the effect of serotonin.

Effect of Phe-D-Met-Arg-Phe-NH2 and other Phe-Met-Arg-Phe-NH2-related peptides on mouse colonic propulsive motility: a structure-activity relationship study.

The effect of several i.c.v.-administered FMRFamide (Phe-Met-Arg-Phe-NH2)-like peptides (FaRPs) on mouse colonic propulsive motility was examined. Dose-related inhibition of propulsive motility (measured as an increase in the time of colonic bead expulsion) was produced by analogs with either the sequence (-)F[X]RFamide or with the FMRFamide sequence containing D-amino acid substitutions. The C-terminal dipeptide sequence Arg-Phe-NH2 was sufficient to produce this effect. D-amino acid substitution in the second position, i.e., [D-Met2]-FMRFamide (DMFa), conferred significantly enhanced activity (nearly maximal obtainable response under the test conditions) in this preparation (ED25 = 2.3 micrograms = 3.8 nmol). DMFa did not block the action of morphine but, like morphine, was blocked by the opioid antagonist naloxone and was attenuated by 24-hr pretreatment with the selective mu-1 opioid antagonist naloxonazine (35 mg/kg s.c.). It is concluded that a variety of FaRPs, particularly those with a nonpolar residue in the first position and with Arg in the third position, behave as opioid-like agonists, not antagonists, on mouse colonic propulsive motility. DMFa is identified as the most active FaRP studied to date on this endpoint.

A-18-famide and F-8-famide, endogenous mammalian equivalents of the molluscan neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2), inhibit colonic bead expulsion time in mice.

Morphine and the two endogenous mammalian FMRFamide (Phe-Met-Arg-Phe-NH2)-related peptides known as morphine-modulating neuropeptides, F-8-Famide (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2) and A-18-Famide (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe -NH2), were administered intracerebroventricularly (ICV) to mice and the effect of each on colonic bead expulsion time was measured. Each of the three compounds delayed expulsion of a 3 mm glass bead placed in the distal colon. A-18-Famide was more potent than F-8-Famide [ED 50 = 2.3 micrograms (1.2 nmole) and 13.9 micrograms (13.0 nmole), respectively]. A-18-Famide: 1) did not block morphine-induced delay of bead expulsion time, and 2) was blocked by simultaneous administration (ICV) of 1.0 microgram of the competitive opiate antagonist naloxone. These data demonstrate apparent opioid modulatory or agonist-like, rather than antagonist-like, properties of A-18-Famide and F-8-Famide.

FMRFamide enhances acetylcholine-induced contractions of guinea pig ileum.

Isolated guinea pig ilea were contracted with acetylcholine (ACh) in the absence and presence of the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2). FMRFamide (0.17-17 microM) enhanced ACh-induced contractions (observed as a leftward shift of the dose-response curve and increase in Emax) with maximal effect at 1.7 microM. FMRFamide had no effect when administered alone. These results extend the demonstration of a FMRFamide/ACh interaction to mammalian tissue and support the concept that FMRFamide, or mammalian equivalents, could play a modulatory role in mammals.

The effect of Phe-Met-Arg-Phe-NH2 (FMRFamide) on morphine-induced inhibition of colonic propulsive motility in mice.

Morphine and the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) were administered to mice alone or in combination intracerebroventricularly (i.c.v.) and the effect on colonic propulsive motility was measured. Both morphine (1.0 microgram, i.c.v.) and FMRFamide (10 and 50 micrograms, i.c.v.) delayed expulsion of a 3 mm glass bead placed in the distal colon of mice compared to vehicle-treated controls. The inhibitory effects of morphine and FMRFamide on expulsion time were additive at the doses used and individually blocked by naloxone. These data suggest that FMRFamide does not antagonize this nonanalgesic effect of morphine, but appears to have opioid agonist properties.

Colonic bead expulsion time in normal and mu-opioid receptor deficient (CXBK) mice following central (ICV) administration of mu- and delta-opioid agonists.

A method utilizing the insertion of a 3 mm glass bead into the distal colon was used to evaluate the activity of intracerebroventricularly (ICV) administered mu- and delta-opioid agonists on colonic bead expulsion time in mice. Specifically, the ability of two mu-opioid receptor agonists, morphine and [D-Ala2,NMePhe4, Gly-ol5]-enkephalin (DAGO) and a selective delta-opioid receptor agonist, [D-Pen2,L-Pen5]-enkephalin (DPLPE), to inhibit colonic bead expulsion time was measured in normal (Swiss) and mu-opioid deficient (CXBK) mice. All three compounds maximally inhibited colonic bead expulsion time in normal mice. All three compounds also inhibited colonic bead expulsion time in CXBK mice, but none maximally. These results are in contrast to previous work in which clear differential analgesic sensitivity of CXBK mice to centrally administered mu- and delta-opioid receptor agonists was observed in the tail-flick test. Taken together, the results suggest (a) that mu-, and possibly delta-, opioid receptors can mediate supraspinal inhibition of colonic bead expulsion in mice and (b) that the genetic deficits of mu-receptor number or genetically-induced alteration in receptor function in CXBK mice do not equally affect inhibition of colonic bead expulsion and tail-flick antinociception.

Neurokinin-induced salivation in the anesthetized rat: a three receptor hypothesis.

Substance P (3 micrograms/kg), neurokinin A (20 micrograms/kg), neurokinin B (6 micrograms/kg) and acetylcholine (875 micrograms/kg) all produced salivation upon i.v. infusion in the anesthetized rat. Against single equivalent agonist doses, atropine (135 micrograms/kg i.v.) blocked both acetylcholine- and neurokinin B-, but not substance P- or neurokinin A-induced salivation. [D-Pro2,D-Trp7,9]-substance P (1 mg/kg i.v.), a putative substance P antagonist, reduced responses to mammalian neurokinins but caused a 2-fold potentiation of acetylcholine-induced salivation. [D-Pro2,D-Trp6,8,Nle10]-Neurokinin B (1 mg/kg i.v.), a novel putative neurokinin B antagonist, significantly reduced substance P- and neurokinin B- but not acetylcholine- or neurokinin A-induced salivation. The three agonists (at doses that produced salivation) and [D-Pro2,D-Trp6,8,Nle10]-neurokinin B (1 mg/kg i.v.) lowered blood pressure in anesthetized rats by 35 to 40%. [D-Pro2,D-Trp7,9]-Substance P (1 mg/kg i.v.) had no significant effect on blood pressure. Hydralazine at 0.60 mg/kg (i.v.), a dose which lowered blood pressure by 47%, did not reduce substance P-induced salivation. Thus, blockade of neurokinin-induced salivation by [D-Pro2,D-Trp6,8,Nle10]-neurokinin B was probably not due to hypotension. Based on the differential effects of the three antagonists on neurokinin- and acetylcholine-induced salivation, we hypothesize the existence of three distinct neurokinin receptors in rat salivary gland, and suggest that neurokinin B receptors reside presynaptically.

Antisecretory activity of human, dog, and rat metabolites of fenoctimine.

Fenoctimine (1a), a nonanticholinergic inhibitor of gastric acid secretion in dogs and rats, was evaluated as a gastric antisecretory agent in humans. In humans it exhibited weak antisecretory activity and caused anticholinergic-like side effects such as dry mouth and nasal passages. Studies of the metabolic fate of fenoctimine in humans, dogs, and rats provided structures of the resultant metabolites. These were synthesized and tested for antisecretory and anticholinergic activity. The human metabolites were all less active than fenoctimine as antisecretory agents, and some displayed significant anticholinergic activity. These results suggest that the unexpectedly weak effect of fenoctimine as a gastric antisecretory agent in humans, as well as anticholinergic effects, may be due to its extensive metabolism, which is different from that seen in dog and rat.

Differentiation of multiple neurokinin receptors in the guinea pig ileum.

We have studied the selectivity and competitiveness of three neurokinin antagonists and atropine against substance P, neurokinin A, and neurokinin B. DPDTNLE-NB, [D-Pro2, D-Trp6,8, Nle10]-neurokinin B is a competitive antagonist of neurokinin B (pA2 = 5.5), but not substance P or neurokinin A. DPDT-SP ([D-Pro2,Trp7,9]-substance P), competitively blocks substance P (pA2 = 6.9) and neurokinin B (pA2 = 6.8), but not neurokinin A. Spantide ([D-Arg1, D-Trp7,9, Leu11]-substance P) competitively blocks substance P (pA2 = 6.7) and at a log unit higher concentration blocks neurokinin A (pA2 = 5.8), but does not block neurokinin B. Atropine is a competitive antagonist of neurokinin B (pA2 = 9.0) at ten times the concentration needed to block acetylcholine (pA2 = 10.1), but does not inhibit the other neurokinins. These results support the hypothesis of multiple neurokinin receptors in the guinea pig ileum and indicate that the site of neurokinin B, but not substance P or neurokinin A is predominantly on intramural neurons. This indirect stimulation appears to be dependent on the release of acetylcholine. Neurokinin B also has activity on smooth muscle receptors since the contractile response could not be completely antagonized by atropine. There appear to be two smooth muscle neurokinin receptors on the basis of results obtained with DPDT-SP and spantide, one predominantly responsive to substance P and the other to neurokinin A. Only spantide appeared to have any effect on the neurokinin A receptor and that was at a much higher concentration than that needed to block substance P.

A differentiation between peripheral and central neurokinin receptors using phenoxybenzamine.

Concentrations of phenoxybenzamine ranging from 0.33-33 micron produced a competitive block of kassinin-, neurokinin A- and neurokinin B-induced contractions of the guinea-pig ileum with pA2 values of 6.6, 5.6 and 6.2, respectively. Physalaemin- and substance P-induced contractions were insensitive to phenoxybenzamine treatment. Differences in sensitivity to phenoxybenzamine and pA2 values suggest the existence of at least two and possibly three neurokinin receptors in the guinea-pig ileum. Injected intrathecally to mice, phenoxybenzamine blocked neurokinin-induced, but not bombesin- or somatostatin-induced, reciprocal hind limb scratching. Phenozybenzamine was 6-32 times more effective in blocking neurokinin B-induced scratching than substance P, kassinin, physalaemin or neurokinin A-induced scratching. These results suggest that multiple peripheral and central neurokinin receptors can be differentiated from one another by phenoxybenzamine treatment. They also suggest the existence of a distinct neurokinin B receptor in the mouse spinal cord and the apparent identification of a third neurokinin receptor in the guinea-pig ileum.