Differential sensitivity of antinociceptive assays to the bradykinin antagonist Hoe 140

1. The antinociceptive activity of the bradykinin (BK) BK2 receptor antagonist D-Arg-[Hyp3,Thi5D-Tic7,Oic8]BK (Hoe 140) was determined in a range of mouse abdominal constriction assays. 2. Hoe 140 potently inhibited the response induced by i.p. injection of 10 micrograms BK/mouse, and 1 microgram BK/mouse in mice pre-sensitized by i.p. injection of prostaglandin E2 (PGE2). The ED50 values in these assays were 1.9 and 3.7 micrograms kg-1 respectively. This confirms that Hoe 140 is a potent antagonist of BK in vivo. 3. Hoe 140 produced potent, but incomplete inhibition of the responses evoked by i.p. injection of kaolin or 0.25% acetic acid. ED25 values in these assays were 2.7 and 16.1 micrograms kg-1, and the maximum inhibition produced was 60% and 70% respectively. 4. At doses up to 1 mg kg-1, Hoe 140 was completely ineffective against the abdominal constriction response induced by zymosan. In contrast, morphine, ibuprofen and indomethacin had similar potencies against zymosan, kaolin and acetic acid-induced abdominal constriction. 5. Although zymosan, acetic acid and kaolin all produce qualitatively similar responses, it is appears that they achieve this by different mechanisms. The extent to which BK is involved as a mediator differs between the various types of abdominal constriction assay.

Structure/activity studies related to 2-(3,4-dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)-1-substituted- ethyl]acetamides: a novel series of potent and selective kappa-opioid agonists

This paper describes the synthesis of a series of N-[2-(1-pyrrolidinyl)ethyl]acetamides 1, variously substituted at the carbon adjacent to the amide nitrogen (C1), and related analogues, together with their biological evaluation as opioid kappa agonists. In the first part of the study, the variants in N-acyl, N-alkyl, and amino functions were explored when the substituent at C1 was 1-methylethyl and the optimum was found to be exemplified by 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(1-methylethyl)-2- (1-pyrrolidinyl)ethyl]acetamide (13). Subsequently, racemic or chiral amino acids were used to introduce other alkyl and aryl substituents at C1 of the ethyl linking moiety. A series of potent compounds, bearing substituted-aryl groups at C1, were discovered, typified by 2-(3,4-dichloro-phenyl)-N-methyl-N-[(1R,S)-1-(3-aminophenyl)-2-(1- pyrrolidinyl)ethyl]acetamide (48), which was 5-fold more active as the racemate than 13 in vitro and exhibited potent naloxone-reversible analgesic effects (ED50 = 0.04 mg/kg sc) in a mouse abdominal constriction model.

2-(3,4-Dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)-1-substituted- ethyl]-acetamides: the use of conformational analysis in the development of a novel series of potent opioid kappa agonists

This paper describes the synthesis of a series of N-[2-(1-pyrrolidinyl)ethyl]acetamides (1), methylated at C1 and/or C2 of the ethyl linking group, and their biological evaluation as opioid kappa agonists. Conformational analysis of corresponding desaryl analogues 2 suggested that only those compounds capable of occupying an energy minimum close to that of the known kappa agonist N-[2-(1-pyrrolidinyl)cyclohexyl] acetamide U-50488 might possess kappa agonist properties. Starting from chiral amino acids, other alkyl and aryl substituents were introduced at C1 of the ethyl-linking moiety, giving compounds capable of adopting the same conformation as U-50488. The most potent of these, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-(1-pyrrolidinyl)ethyl] acetamide (8), was 146-fold more active than U-50488 in vitro in the mouse vas deferens model and exhibited potent naloxone-reversible analgesic effects (ED50 = 0.004 mg/kg sc) in an abdominal constriction model.

The coexistence of adenosine A1 and A2 receptors in guinea-pig aorta

The effects of adenosine, 5'-(N-ethyl)carboxamidoadenosine (NECA), 2-chloroadenosine (2-CA), N6-cyclohexyladenosine (CHA) and N6(R-2-phenylisopropyl)-adenosine (R-PIA) on the tone of phenylephrine-constricted guinea-pig isolated aorta have been examined. For aortic relaxation the analogues exhibited the following rank order of potency: NECA greater than adenosine greater than 2-CA greater than R-PIA greater than CHA. This is consistent with previous reports that relaxation of this tissue is mediated by the adenosine A2 receptor. An unexpected finding was that R-PIA, 2-CA and CHA all induced contractions at concentrations lower than were required for relaxation, giving a biphasic dose-response curve. Neither NECA nor adenosine contracted the aorta. This is consistent with activation of vascular A1 receptors. An A1-selective concentration of the antagonist 1,3-dipropyl-8-cyclopentyl xanthine abolished the contraction elicited by R-PIA in the guinea-pig aorta. This further suggests that the contraction is mediated by A1 receptors.

The xanthene-9-spiro-4'-piperidine nucleus as a probe for opiate activity

A series of novel 1'-methylxanthene-9-spiro-4'-piperidines has been prepared in the search for opiate analgesics with improved pharmacological properties. It has been found that introduction of a hydroxyl group into the 4-position of the xanthenespiropiperidine nucleus produces a potent mu-opiate agonist. The structure-activity relationship of the series has been explored by use of isosteric replacements of the phenolic hydroxyl group. Moreover, the effect of altering the conformation of the piperidine ring has been studied. It was interesting to note that, in compounds lacking the phenolic hydroxyl group, opiate activity could be produced by introduction of the (phenylamino)ethyl group instead of methyl at the 1'-position.

ICI 204448: a kappa-opioid agonist with limited access to the CNS

1. A number of compounds were evaluated in an attempt to identify a kappa-opioid receptor agonist with limited access to the central nervous system. 2. Quaternary derivatives of the kappa-opioid agonists tifluadom, U-50488H and ethylketocyclazocine were essentially devoid of opioid activity in a range of isolated tissue preparations. 3. A novel compound - ICI 204448 - is described which produced a potent and naloxone-reversible inhibition of electrically-evoked contraction of the guinea-pig ileum, mouse vas deferens and rabbit vas deferens preparations. ICI 204448 was shown to displace the binding of the kappa-opioid ligand [3H]-bremazocine from guinea-pig cerebellum membranes. 4. Ex vivo binding studies in mice showed ICI 204448 to be well absorbed following subcutaneous administration. The brain levels achieved by ICI 20448 were substantially lower than those produced by kappa-agonists such as U-50488H and tifluadom. 5. A good correlation was found for a range of opioids between lipophilicity and degree of CNS penetration.

Differential sensitivity of antinociceptive tests to opioid agonists and partial agonists

1. The antinociceptive activity of a range of opioid agonists and agonist-antagonist analgesics was determined in mice by use of the 55 degrees C hot plate and abdominal constriction assays. 2. Opioid agonists were approximately 10 times more effective in the abdominal constriction assay. 3. The agonist-antagonists produced analgesia only in the abdominal constriction assay, and antagonized the antinociceptive action of opioid agonists in the 55 degrees C hot plate test. 4. These differences were shown to be attributable to the different levels of stimulus employed in the two tests. 5. By comparing the antagonist potencies of the agonist-antagonists in the 55 degrees C hot plate test with their antinociceptive ED50 values in the abdominal constriction assay, an index of intrinsic activity was calculated.

A novel series of potent and selective agonists at the opioid kappa-receptor

Three novel opioid agonists are described. These compounds were found to bind with high affinity and selectivity to the kappa-opioid receptor. Isolated tissue studies using the field-stimulated mouse vas deferens and guinea-pig ileum preparations confirmed the high agonist potency and naloxone-reversibility of these agents. All three compounds exhibited potent antinociceptive activity in the mouse abdominal constriction model. These compounds should prove useful as tools to investigate kappa-receptor function.

The physiological relevance of low agonist affinity binding at opioid mu-receptors

1. Inhibition constant (Ki) were determined for a range of opioid standards using two binding assays; [3H]-[D-Ala2, MePhe4, Gly-ol5]enkephalin ([3H]-GLYOL) binding to guinea-pig brain membranes in HEPES buffer and [3H]-naloxone binding to rat whole brain membranes in Krebs/HEPES buffer. 2. These values were compared with affinity measurements determined by antagonism of GLYOL on the rat isolated vas deferens preparation and by the receptor occlusion technique of Furchgott on the guinea-pig ileum longitudinal muscle, myenteric plexus preparation. 3. Agonists demonstrated markedly reduced binding affinity in the [3H]-naloxone binding assay where binding was conducted in the presence of sodium. 4. A strong correlation was obtained between Ki values from the [3H]-naloxone binding assay and affinity values determined in both isolated tissue preparations. Ki values obtained from [3H]-GLYOL binding did not correlate well with affinity data determined by isolated tissue techniques. 5. These findings suggest that functionally relevant receptors exhibit low agonist affinity.

Specificity of the opioid affinity reagent, DALECK, in a set of isolated tissue assay systems

The C-terminal chloromethyl ketone derivative of D-Ala2-Leu5-enkephalin (DALECK) has previously been shown to act as an affinity reagent at opioid receptors. The specificity of this derivative in its reversible interaction with functional opioid receptors has been examined here in a set of four field-stimulated isolated tissue preparations; the mouse, rat and rabbit vas deferens and the guinea pig ileum. Agonist potencies relative to selective reference agonists and Schild analysis were used to elucidate the overall activity of DALECK when interacting reversibly with opiate receptors under normal physiological conditions in the isolated tissue preparations. Under these conditions the ligand shows a very strong mu-selectivity. Data obtained in the guinea pig ileum suggest that DALECK is more potent than Tyr-D-Ala-Gly-N(CH3)Phe-Gly-ol (DAGO) when acting through mu-receptors. In contrast in the mouse vas deferens DALECK is at least 70-fold less potent than the delta-ligand D-Thr2-Leu5-enkephalin-Thr (DTLET). DALECK shows little interaction with kappa-receptors.

The contribution of intrinsic activity to the action of opioids in vitro

The effects of opioids were compared in five field-stimulated isolated tissue models, the guinea-pig ileum and vasa deferentia from rat, rabbit and mice of the Alderley Park and C57BL/6 strains. Although the mu-receptor agonist [D-Ala2, MePhe4, Gly-ol5] enkephalin appeared to act at similar receptors in the guinea-pig ileum, rat vas deferens, mouse vas deferens and C57BL/6 mouse vas deferens preparations, its potency varied considerably between these preparations. Similar potency differences were also observed with the kappa-agonist, ethylketocyclazocine. It is proposed that these variations in potency reflect differences in the number of spare receptors present in each model. The finding that some drugs which have agonist activity in the more sensitive preparations behave as antagonists in the less sensitive tissues supports this proposal and highlights the importance of intrinsic activity in determining the action of opioids. Many of the prototypic opioid agonists were found to be either partial agonists (eg. morphine and bremazocine) or to possess affinity for more than one receptor type (eg. ethylketocyclazocine, Mr 2034).

Characterisation of the delta-opioid receptor on the hamster vas deferens

It has recently been reported that the hamster vas deferens contains only delta-opioid receptors. We have demonstrated that three delta-receptor agonists [D-Ala2, D-Leu5]enkephalin (DADLE), [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Thr2, Leu5, Thr6] enkephalin (DTLET) appear to mediate their effects via the delta-receptor since they are readily reversed by the selective delta-receptor antagonist ICI 174864. In addition, a number of classical mu and k receptor compounds were devoid of activity in this preparation. However it was observed that some compounds such as etorphine and MR 2034 reported to possess delta-receptor affinity in other assay systems were weak or inactive on the hamster vas deferens.

Conversion of dynorphin-(1-9) to [Leu5]enkephalin by the mouse vas deferens in vitro

The inhibitory action of dynorphin-(1-9) on the electrically stimulated mouse vas deferens was seen to be antagonised by the delta-selective opioid antagonist ICI 174864. The observed delta-receptor mediated responses were partially, but not totally, prevented by peptidase inhibitors which protect the C- and N-termini of dynorphin-(1-9). [3H]Dynorphin-(1-9) is rapidly degraded by slices of vasa deferentia of the mouse. The major product of this metabolism co-elutes with [Leu5]enkephalin on reverse phase HPLC. It is concluded that a major component of the inhibitory effects of dynorphin-(1-9) on the mouse vas deferens is mediated by degradation to [Leu5]enkephalin which in turn acts through delta-receptors. It is possible that in other in vitro and in vivo systems, the effects produced by dynorphin-(1-9) might be similarly mediated by delta-receptor activation.

Mu-receptor binding in physiological media: comparison with isolated tissue data

Mu-receptor affinities have been determined for a number of opioid drugs using a combination of isolated tissue and receptor binding techniques. The affinities of antagonists and partial agonists were determined by their ability to antagonise responses to [D-Ala2,MePhe4,Glyol5]enkephalin (GLYOL) on the rat vas deferens preparation. There was little correlation between these results and affinities measured by displacement of [3H]-GLYOL from guinea-pig brain membranes incubated in 50mM Tris buffer. By contrast, affinities measured by displacement of [3H]-naloxone from rat brain membranes incubated in a Krebs/HEPES buffer containing a non-hydrolysable analogue of GTP, agreed very closely with the isolated tissue data.

Failure to demonstrate mu-isoreceptors

Attempts have been made to confirm reports that naloxonazine and Mr2034 are able to selectively antagonise morphine-induced analgesia and respiratory depression respectively. In the present studies these antagonists proved to be equi-effective against both the analgesic and respiratory depressant effects of morphine, suggesting that these two actions are mediated by a common receptor.

Mu-receptors in the C57BL mouse vas deferens

The C57 mouse vas deferens preparation has been shown to be insensitive to mu-opioids. The present study indicates that this tissue possesses mu, d and k-receptors with similar affinities to those found in other isolated tissue preparations, but has a small receptor reserve. Thus partial agonists such as morphine are detected as antagonists.

Multiple opiate receptors in the mouse vas deferens

The agonist potencies of normorphine and [D-Ala2,D-Leu5]enkephalin (DADL) have been determined on the mouse vas deferens preparation under different conditions. Reducing the temperature at which the assay was performed had little effect on the response to DADL whilst the Emax of normorphine was greatly reduced. Tissues rendered tolerant to either agent exhibited little, if any, cross-tolerance to the other drug. The results suggest the presence of at least two types of opiate receptor in the mouse vas deferens.

Divergent structure activity relationships in series of enkephalin agonists and cognate antagonists

Diallylation of the amino group of [Leu]enkephalin methyl ester yields a moderately potent, delta-selective opiate receptor antagonist. The diallyl congeners of a larger range of potent mu-and delta-selective enkephalin agonists have been prepared and were found to be weak, non-selective antagonists as assessed by their ability to antagonise the effects of normorphine and [Leu]enkephalin on the field-stimulated mouse vas deferens. Conversely, whereas [Gly3 psi (CH2S)Phe4,Leu5]enkephalin and [Gly3 psi(CH2S)-D-Phe4,Leu5]enkephalin are virtually inactive as opiate agonists the corresponding diallyl analogues are moderately potent, highly selective delta-antagonists. Analogues of diallyl[Leu]enkephalin in which the Gly2 and Gly3 residues have been replaced by D- and L- Ala have also been prepared and tested as delta-receptor antagonists. In addition, the empiric energy program ECEPP has been used to derive eighteen low energy conformations of diallyl[Leu]enkephalin and to estimate the effect of the D- and L-Ala substitutions on the conformer energies. Two conformers were identified for which there was a partial correlation between the variations in conformational energy and delta-antagonist potency.

Psychological androgyny and stressful life events

Seventy-one college students completed the Bem Sex-Role Inventory, a stressful life-events questionnaire, and some general questions on adjustment. Subjects were also asked to rate the stressful life events they had experienced in the prior 12 months along the dimensions of desirability, amount of change, anticipation, control, and meaningfulness. As hypothesized, androgynous subjects rated their stressful life events as less undesirable than other subjects, but this result only held for females (p less than .01). Undifferentiated subjects rated their stressful life events as less meaningful than other subjects (p less than .01) and androgynous subjects rated themselves as happier than other subjects (p less than .05). No differences were found among androgynous, masculine, feminine, and undifferentiated persons for the dimensions of change, anticipation, or control. Of the five dimensions examined, only meaningfulness was significantly related to happiness (r = .34, p less than .01). One possible reason for the results of this study may be differential access to and/or use of social support systems by persons of different sex-types. Some findings of the present study are presented that support this contention.

Psychological androgyny and stressful life events

Seventy-one college students completed the Bem Sex-Role Inventory, a stressful life-events questionnaire, and some general questions on adjustment. Subjects were also asked to rate the stressful life events they had experienced in the prior 12 months along the dimensions of desirability, amount of change, anticipation, control, and meaningfulness. As hypothesized, androgynous subjects rated their stressful life events as less undesirable than other subjects, but this result only held for females (p less than .01). Undifferentiated subjects rated their stressful life events as less meaningful than other subjects (p less than .01) and androgynous subjects rated themselves as happier than other subjects (p less than .05). No differences were found among androgynous, masculine, feminine, and undifferentiated persons for the dimensions of change, anticipation, or control. Of the five dimensions examined, only meaningfulness was significantly related to happiness (r = .34, p less than .01). One possible reason for the results of this study may be differential access to and/or use of social support systems by persons of different sex-types. Some findings of the present study are presented that support this contention.

Bicuculline-insensitive GABA receptors on peripheral autonomic nerve terminals

The action of gamma-aminobutyric acid (GABA) and related compounds on rat isolated atria and mouse and guinea pig isolated vas deferens has been studied. GABA depressed the evoked but not basal release of [3H]noradrenaline from atria (IC50 4 micro M) and reduced the twitch responses of the vas deferens (IC50 3 micro M) in a dose-dependent manner. These depressant effects were not prevented by recognized GABA antagonists such as bicuculline and picrotoxin. Numerous GABA analogues, in particular 3-aminopropanesulphonic acid, failed to mimic the action of GABA. However, beta-p-chlorophenyl GABA (baclofen) was stereospecifically active. Other related beta-substituted derivatives were also active but to a lesser degree than GABA. Pretreatment of the vas deferens with the neuronal GABA uptake inhibitors 2,4-diaminobutyric acid or cis-3-aminocyclohexanecarboxylic acid potentiated the action of GABA. These data suggest the presence of a bicuculline-insensitive GABA receptor on autonomic nerve terminals. Preliminary observations indicate a lack of chloride ion dependence in the action of GABA at this site.