Naloxone causes apparent antinociception and pronociception simultaneously in the rat paw formalin test

Naloxone is known to decrease, increase or have no effect on nociceptive thresholds. Here, using two commonly accepted pain-related behaviors (licking and flinching) associated with injection of noxious formalin into a hind paw in rats, naloxone (0.1-1 mg/kg s.c.) simultaneously decreases and increases nociceptive responding in the same animal. Licking, which is reduced by naloxone, is enhanced by low doses but attenuated by high doses of morphine. However, although licking initially increases with a rise in formalin concentration, at higher concentrations the time spent licking the injected paw actually declines. By contrast, flinching, which is enhanced by naloxone, is only antagonized by morphine and increases linearly with formalin concentration. Both actions of naloxone can be interpreted in terms of a leftward shift in the formalin concentration-response curves. This study demonstrates that naloxone can increase formalin-induced flinching while simultaneously decreasing licking behavior. These findings suggest that, on its own, an unexpected decrease in a single nociceptive index may be an inadequate criterion for demonstrating antinociception.

Buprenorphine and gastrointestinal transit in rats: effect of naloxone on the biphasic dose-response curve

1. Buprenorphine (0.01-10 mg/kg, subcutaneous [s.c.]) slowed the passage of a charcoal meal along the gastrointestinal tract in rats. The dose-response relationship was U-shaped. 2. When rats were pretreated with naloxone (0.30 mg/kg, s.c.), both the descending and ascending components of the buprenorphine dose-response curve were displaced to the right. 3. Buprenorphine-induced delay of transit was maximal at a dose of 0.10 mg/kg. In rats pretreated with naloxone, a 30-fold higher dose of buprenorphine was required for a comparable peak effect. 4. Moderate-high doses of buprenorphine may be acting on a functionally related binding site which non-competitively inhibits the usual buprenorphine-mu opioid receptor interaction.

Effects of intracerebroventricular beta-funaltrexamine on mu and delta opioid receptors in the rat: dichotomy between binding and antinociception

The effects of intracerebroventricular (i.c.v.) beta-funaltrexamine (beta-FNA) pretreatment at -24 or -6 h were studied on mu and delta opioid receptor binding and on antinociception produced by i.c.v. morphine in rats. Mu and delta opioid receptor binding in brain membrane preparations was performed with [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) as radiolabeled ligands, respectively. Effects of i.c.v. beta-FNA (24 h) on mu and delta binding depended on dosage. For [3H]DAGO binding, 3 micrograms beta-FNA did not affect either the Kd or Bmax, whereas 10 micrograms increased the Kd without changing the Bmax. beta-FNA pretreatment for 24 h did not alter [3H]DPDPE binding at 3 micrograms; at 10 micrograms, the Kd was increased with no change in the Bmax. Pretreatment with 10 micrograms beta-FNA for 6 h gave similar results to the 24-h treatment in mu binding, but did not change delta binding. When mu binding was performed on various brain regions, pretreatment with 10 micrograms beta-FNA for 24 h increased the Kd in all regions studied (the periaqueductal gray, thalamus, striatum and cortex). However, this pretreatment decreased the Bmax only in the periaqueductal gray (by 22%) and cortex (by 14%). Pretreatment of rats with beta-FNA (3 or 10 micrograms at -24 h), which by itself caused some hyperalgesia, greatly antagonized the antinociceptive effect of morphine (10 micrograms i.c.v.) in the hot-plate test. Our work with beta-FNA has revealed an apparent discrepancy between binding and behavioral results. This dichotomy may, in part, be the result of the limited distribution of beta-FNA to the periventricular area.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurogenic and tissue-mediated components of formalin-induced edema: evidence for supraspinal regulation

Injection of formalin into a hind paw of rats produces localized inflammation and pain. The nociceptive effect of formalin, recorded as flinching/shaking of the injected paw, is biphasic. The present study shows that formalin-induced inflammation and edema (assessed by measurement of paw volume up to 24 h post-injection) is also biphasic, an early neurogenic component being followed by a later tissue-mediated response. Rapid initiation of edema is closely related to early phase nociception and is dependent on activity in primary afferent neurons and axon reflexes, but not on transmission of the noxious stimulus and the perception of pain itself. The major site responsible for down-regulating the inflammatory response, particularly in the later stages when tissue-mediated components are most heavily involved, appears to be located supraspinally. Down-regulation occurs principally by means of descending neuronal pathways but may also involve a secondary humoral component. The perhaps surprising dependence on neuronal mechanisms which this study demonstrates promotes spinal and peripheral sites as potential therapeutic targets in certain inflammatory conditions.

Tonic pain perception in the mouse: differential modulation by three receptor-selective opioid agonists

The proposition that tonic nociception models are more analogous to clinical pain than traditional acute models prompted our previous development of a modified mouse paw formalin test. To discern possible modulatory roles and site(s) of action of endogenous opioid systems, the receptor-preferring agonists sufentanil (mu), U-50,488H (kappa) and [D-Pen2,5]enkephalin (DPDPE, delta) were evaluated for antinociceptive activity in the formalin paradigm by systemic (except DPDPE), spinal and supraspinal routes. All observations were done under blind conditions. Doses causing overt behaviors that indicated a breach of receptor specificity (during the observation period) were rejected. Higher doses of centrally administered DPDPE (greater than 0.3 micrograms/mouse, intrathecal; greater than 3 micrograms/mouse, intracerebroventricular) induced a behavioral syndrome traditionally associated with mu agonism, and thus were not considered for this study. A50 values from behaviorally acceptable dose ranges for mean percent analgesia (reduction of paw licking compared to controls) were: trans-(+/- )-3,4-dichloro-N-methyl-N-[U-50,488H 2-pyrrolidinyl)cyclohexyl]-benzeneacetamide methanesulfonate,U-50,488H--3200 nmol/kg, subcutaneous, 1100 nmol/kg, intrathecal and 314 nmol/kg, intracerebroventricular; sufentanil--11.1 nmol/kg, subcutaneous, 8.6 nmol/kg, intrathecal; and DPDPE--inactive. On the basis of our dose-response data, we suggest that, in mice, kappa and mu, but not delta, opioid receptors modulate tonic pain perception at both spinal and supraspinal loci. The results also support inclusion of the modified formalin test in preclinical evaluations of potential kappa agonists.

Standardization of the rat paw formalin test for the evaluation of analgesics

Administration of 5% formalin into the rat or guinea pig hind paw evokes two spontaneous responses: flinching/shaking and licking/biting of the injected paw. The temporal and behavioral characteristics of these objective endpoints are described. Additionally, several practical suggestions aimed at standardizing this test for the evaluation of analgesics are presented. The early/acute and late/tonic (0-10 and 20-35 min post-formalin, respectively) phases of flinching were used to quantitate antinociception in the rat. PD 117302, the kappa selective agonist, was three times more potent than morphine against tonic flinching after SC administration. Formalin may therefore be a noxious stimulus of choice in the evaluation of kappa agonists. Morphine was only twice as potent against tonic flinching as against acute flinching or the tail-dip reflex to water (50 degrees C). In contrast, PD 117302 was 27 times less potent on early phase and was inactive in the tail-dip test. Thus, while morphine is essentially equipotent across tests, PD 117302 shows a spectrum of activity with impressive potency and efficacy being obtained against tonic pain. Kappa receptors may therefore be prominently involved in tonic pain states. Aspirin given orally was not consistently antinociceptive in either phase of the formalin test. Spinal transection completely abolished late phase responding but only partly attenuated flinching in the early phase. This suggests that the relative involvement of spinal (as opposed to supraspinal) processing of noxious inputs may, at least in part, be a function of stimulus intensity and underlie the differences in antinociceptive potency observed in this work.

Effects of selective opioid agonists on feline colonic transit

The mu agonist morphine and the non-specific opioid antagonist naloxone both may accelerate feline colonic transit; the effects of morphine are dose dependent. Kappa and delta receptor function was studied in the present work. Colonic transit of a radionuclide marker instilled into the cecum was quantitated for 6 hr in a crossover study. The delta agonist [D-Pen2,D-pen5]enkephalin (1 mg/kg, i.m.) prolonged the cecum and ascending colon half-emptying time by 337% (P less than 0.05), and delayed the progression of the geometric center over time. The kappa agonist U-50,488 (1 mg/kg, i.m.) had no apparent effect on the cecum and ascending colon, but delayed filling of the descending colon. Loperamide, an antidiarrheal agent, also delayed colonic transit. Thus, selective opioid agonists have both site and functional differences in their effect on feline colonic transit.

A view from inside

There is sometimes an aspect of a disease that can only be uncovered by surgical exposure in surgical cases and, in spite of all the modern scanning and pathological investigations, by the assembly of the patient's history and symptoms in medical cases. My knowledge of Parkinson's disease is minimal, but I am a trained observer and I have attempted to present a brief study of the condition from 'inside looking out' in the hope that it will assist in expanding clinical rapport.

Failure of (+)-naloxone to accelerate feline colonic transit

To determine whether the colonic transit accelerating effect of (-)-naloxone (0.3 mg/kg, i.m.) is due to an action at opioid receptors or a direct pharmacologic effect, its enantiomer, (+)-naloxone (0.3 mg/kg, i.m.), was administered to cats and compared to saline control using colonic transit scintigraphy. Transit was not accelerated by (+)-naloxone. The effects of naloxone on colonic transit are thus stereospecific, and are probably mediated by opioid receptors.

Formalin nociception in the mouse does not lead to increased spinal serotonin turnover

The mouse formalin test is a model of tonic (continuous), chemical/inflammatory nociception. To test the hypothesis that bulbospinal serotonergic pathways modulate such nociception, whole spinal cords from mice pretreated with probenecid and sacrificed at 15, 30, 45 and 60 min after injection of 5% formalin or 0.9% saline in the hindpaw were assayed by high performance liquid chromatography with electrochemical detection for the serotonin metabolite, 5-hydroxyindoleacetic acid, as an index of turnover. No difference in serotonin turnover was found between formalin and saline groups, indicating that increased spinal serotonin release is not a normal response to formalin nociception in the mouse.

[D-Pen2, D-Pen5]enkephalin, the standard delta opioid agonist, induces morphine-like behaviors in mice

[D-Pen2, D-Pen5]enkephalin (DPDPE; 3-30 micrograms) and morphine (10 micrograms) both caused Straub tails, increased locomotion, and circling after ICV administration to ICR mice. DPDPE-induced tail stiffening was reduced when mice were pretreated with naloxone (0.5 mg/kg SC) or beta-funaltrexamine (10 micrograms ICV), but not with ICI 174864 (2 mg/kg SC), the selective antagonist at delta opioid receptors. These results point to (a) mu receptors mediating the tail stiffening and (b) the loss of delta receptor selectivity after 10 and 30 micrograms DPDPE.

Statistical analysis of drug-drug and site-site interactions with isobolograms

The use of more than one drug to achieve a desired effect has been a common practice in pharmacologic testing and in clinical practice. For example, combinations of analgesics are frequently prescribed with a view to enhancing pain relief and reducing adverse effects. It is also well established that administration of more than one drug may give effects that are greater than, or less than, the additive effect of each drug given individually. A non-mechanistic method of characterizing the effect resulting from the administration of two compounds is the isobologram. It is relatively simple to draw and interpret isobolograms. However, this graphical technique, which employs equieffective concentrations of individual drugs and combinations of these, obtains the concentrations as random variables from concentration-effect data, usually transformed to a parallel line assay. Thus, statistical confidence limits from such assays, as well as from non-parallel designs, must be expressed on the isobologram if this diagram is to establish superadditive, subadditive, or merely additive effects. We now present a detailed statistical analysis of the isobolographic method illustrated with examples of the statistical procedures, a rational basis for selecting proportions of each drug in the combination, and a relatively novel application of the isobolographic concept, i.e., interactions involving different anatomical sites.

Fully effective contraception in male and female guinea pigs immunized with the sperm protein PH-20

Immunization of male and female animals with extracts of whole sperm cells is known to cause infertility. Also, men and women who spontaneously produce antisperm antibodies are infertile but otherwise healthy. Although the critical sperm antigens are unknown, these observations have led to the proposal that sperm proteins might be useful in the development of a contraceptive vaccine. The guinea pig sperm surface protein PH-20 is essential in sperm adhesion to the extracellular coat (zona pellucida) of the egg, a necessary initial step in fertilization. Here, we report that 100% effective contraception was obtained in male and female guinea pigs immunized with PH-20. Antisera from immunized females had high titres, specifically recognized PH-20 in sperm extracts, and blocked sperm adhesion to the egg zona pellucida in vitro. The contraceptive effect was long-lasting and reversible: immunized females, mated at intervals of six to fifteen months after immunization, progressively regained fertility.

Direct dependence studies in rats with agents selective for different types of opioid receptor

The objective of this study was to describe, quantitate and compare naloxone-induced abstinence syndromes in rats infused centrally (Sylvian aqueduct) with agonists that are currently the most selective for mu [( D-Ala2, MePhe4, Gly-ol5]enkephalin), delta [( D-Pen2, D-Pen5]enkephalin) and kappa (3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]benzeneacetamide) (U-50,488H) opioid receptors, respectively. Morphine, ethylketazocine and dynorphin A served as reference compounds. After 70 hr of infusion from s.c. implanted osmotic minipumps, three levels of abstinence were associated with the injection of naloxone (3 mg/kg s.c.): 1) negligible syndromes (scores of less than 21) were obtained in rats on water or the kappa-directed ligands, U-50,488H and dynorphin A; 2) a low-to-moderate abstinence score (37-38) was recorded with rats receiving [D-Pen2, D-Pen5]enkephalin and ethylketazocine; and 3) a high abstinence score (64-73) was obtained with rats on morphine and DAGO. These results reinforce the concept of developing selective, nonbenzomorphan kappa agonists as clinically useful analgesics and emphasize that, when evaluating new analgesics, high selectivity for delta receptors does not, in itself, guarantee freedom from physical dependence.

Methodological refinements to the mouse paw formalin test. An animal model of tonic pain

The formalin tonic pain test has recently been adapted for use in mice. We have observed two problems with this procedure. First, compared to a vehicle control group, the majority of mice injected in a hind paw with dilute formalin exhibited a significant nociceptive response (paw licking), including peaks of activity, well beyond the end of the recommended observation period; and, second, an unacceptably wide variation in response occurred. We report two modifications that eliminate these difficulties: a) extending the observation period to 20-45 min postformalin injection, and b) lightly anesthetizing the mice with ether before injection of formalin into the paw. The modified protocol was tested by evaluating morphine, pentazocine and zomepirac (analgesic A50 values were 2.1, 23.8, and greater than 100 mg/kg, s.c., respectively). An intrathecal or intracerebroventricular injection of saline, given to the lightly anesthetized mouse, did not interfere with the nociceptive behavior; this finding widens the applicability and usefulness of the test. Collectively, these refinements significantly improve the mouse formalin model of tonic pain.

Purification of the guinea pig sperm PH-20 antigen and detection of a site-specific endoproteolytic activity in sperm preparations that cleaves PH-20 into two disulfide-linked fragments

Previous work has indicated that the guinea pig sperm membrane protein, PH-20, functions in sperm-egg adhesion and that its surface expression is regulated by the acrosome reaction. The PH-20 protein was purified by monoclonal antibody affinity chromatography. Sixty-seven to one hundred percent of the PH-20 antigenic activity present in an octylglucoside (OG) extract of sperm was recovered in the purified protein. From 10(10) sperm, approximately 0.4 mg of PH-20 protein was obtained, which was about 0.24% of the total protein in the OG extract. The purified protein retained the ability to bind the three anti-PH-20 monoclonal antibodies we have isolated. Silver staining of purified PH-20 on overloaded sodium dodecyl sulfate (SDS) gels allowed the estimate that silver-stainable contaminants were present at a level of one part in 2000. The purified PH-20 protein exists in three forms separable on SDS-polyacrylamide gel electrophoresis: a major form with a molecular mass of 64 kDa, a minor form of 56 kDa, and an endoproteolytically cleaved form composed of two disulfide-linked fragments of 41-48 kDa and 27 kDa. Cleveland digests of the 64 kDa and 56 kDa polypeptides indicated that they were structurally related. A proportion of the 64 kDa polypeptide in each purified preparation had undergone endoproteolysis at a specific site, so that it was cleaved into the two disulfide-linked fragments, 41-48 kDa and 27 kDa. It is speculated that the site-specific endoproteolysis of PH-20 may occur during the acrosome reaction and have biological significance.

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.

Supraspinal and spinal potency of selective opioid agonists in the mouse writhing test

Three agonists with the highest degree of selectivity available for mu ([D-Ala2,NMePhe4,Gly-ol]enkephalin, DAGO), delta ([ D-Pen2,D-Pen5]enkephalin, DPDPE) and kappa (U-50,488H, U50) opioid receptors were compared for their activity in inhibiting acetic acid-induced writhing in mice. Additionally, three reference agonists for mu (morphine), delta ([ D-Ala2,D-Leu5]enkephalin, DADLE) and kappa (ketocyclazocine, KC) receptors were also studied in this test. The agonists were given directly into the lateral cerebral ventricle (i.c.v.) or into the lumbar spinal subarachnoid space (intrathecal), and the potency of each compound was compared across injection sites and with data previously obtained in a thermal analgesic test (mouse hot-plate test). The rank order of potency for inhibition of writhing after i.c.v. administration was DAGO greater than DADLE greater than morphine greater than DPDPE; KC and U50 showed no significant activity by this route. After intrathecal administration, the compounds inhibited writhing with a rank potency order of DAGO greater than KC greater than morphine = DADLE greater than DPDPE greater than U50. All compounds were more potent in inhibiting writhing at spinal sites than at supraspinal sites; DPDPE and DAGO were 15 and 24 times more potent after intrathecal than after i.c.v. administration, respectively. The proposed delta agonists DPDPE and DADLE inhibited writhing at both spinal and supraspinal sites. Further, although the proposed kappa-acting compounds KC and U50 were effective at relatively low doses at spinal levels, these compounds lacked activity at supraspinal sites at doses not causing sedation.(ABSTRACT TRUNCATED AT 250 WORDS)