SCH 39166, a potential antipsychotic drug, does not evoke movement disorders in cebus monkeys

Subchronic administration of a neuroleptic in cebus monkeys can reliably mimic the abnormal movements produced by these drugs in humans. SCH 39166 is the best candidate to test in this model to determine if selective antagonism at the dopamine D1 receptor is devoid of these side effects. It has superior selectivity for the dopamine D1 site versus several other sites and a significantly longer duration in primates than the prototypical D1 antagonist, SCH 23390. In contrast to haloperidol, weekly administration of SCH 39166 for 14 weeks did not produce abnormal movements but did produce equivalent sedative effects. Thus dopamine D1 antagonists are uniquely different from D2 antagonists with regards to the production of abnormal movements.

Effect of SCH 39166, a novel dopamine D1 receptor antagonist, on [3H]acetylcholine release in rat striatal slices

SCH 39166 is a novel and selective dopamine D1 receptor antagonist. It has been reported to have potential antipsychotic properties and reduced extrapyramidal side-effect liabilities (EPS). The current studies investigated the pharmacological effects of SCH 39166 on striatal cholinergic function in order to further characterize its dopamine D1 receptor selectivity and to address its EPS liability. Electrically stimulated [3H]acetylcholine (ACh) release from rat striatal slices was measured and comparisons were made between SCH 39166, SCH 23390, (-)-sulpiride, haloperidol or apomorphine on their effect on [3H]ACh release. Results indicated that apomorphine inhibited [3H]ACh release from striatal slices (IC50 = 0.31 microM). (-)-Sulpiride and haloperidol completely reversed the inhibition of [3H]ACh release seen with apomorphine. In contrast, SCH 39166, as well as, SCH 23390 did not reverse the inhibition of [3H]ACh release induced by apomorphine. These findings indicate that dopamine D2 receptors are primarily involved in modulation of [3H]ACh release. Furthermore, selective dopamine D1 receptor antagonists, such as SCH 39166, are ineffective in modulating striatal [3H]ACh release, suggesting that striatal cholinergic hyperactivity and possibly EPS will not be a consequence of dopamine D1 receptor blockade.

Selective up-regulation of D-1 dopamine receptors following chronic administration of SCH 39166 in primates

Caudate, putamen and frontal cortex tissues were obtained from rhesus monkeys that had taken part in a toxicology study required by the Food and Drug Administration. These monkeys had received daily oral treatments of SCH 39166 at three different doses (3, 12 and 48 mg/kg) for three consecutive months. Plasma membranes from the caudate and putamen were analyzed for changes in D-1 and D-2 receptor affinity and number using saturation analyses of 3H-SCH 23390 and 3H-spiperone binding, respectively. Saturation studies were performed on membranes from the frontal cortex using 3H-ketanserin to determine if 5HT2 receptor number or affinity were affected by chronic treatment with SCH 39166. Results indicate a significant, dose-dependent up-regulation of D-1 receptor number in both caudate and putamen, with no changes in either D-2 receptors in the striatal regions or 5HT2 receptors in the frontal cortex. These data, therefore, indicate that SCH 39166 is a selective antagonist at D-1 receptors in the CNS of nonhuman primates.

[3H]SCH 39166, a new D1-selective radioligand: in vitro and in vivo binding analyses

SCH 39166 [(-)-trans-6,7,7a,8,9, 13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo-[d]naphtho[2, 1b]azepine] has recently been described as a selective D1 antagonist and has entered clinical trials for the treatment of schizophrenia. The tritiated analogue of this compound, [3H]SCH 39166, has now been synthesized and characterized for its in vitro and in vivo binding profiles. [3H]SCH 39166 binds to D1 receptors in a saturable, high-affinity fashion, with a KD of 0.79 nM. In competition studies, D1-selective antagonists like SCH 23390 displaced the binding of [3H]SCH 39166 with nanomolar affinities, whereas antagonists of other receptors exhibited poor affinity. In vivo, [3H]SCH 39166 bound to receptors in rat striatum in a fashion suggestive of D1 selectivity. Further, when the time course for the binding of [3H]SCH 39166 was compared with the behavioral time course of the unlabeled compound, the two durations of action were virtually indistinguishable. Similar studies were performed for SCH 23390 and its tritiated analogue, but the in vivo binding of this radioligand exhibited a duration of action far greater than the behavioral activity of the unlabeled drug. In concert, these data demonstrate that [3H]SCH 39166 selectively labels D1 receptors in vitro and in vivo, and that this drug is superior for in vivo imaging of the D1 receptor.

In vivo binding of SCH 39166: a D-1 selective antagonist

SCH 39166 [(-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl- 5H-benzo[d]naphtho-[2,1b]-azepine] has been identified previously as a potent and selective D-1 antagonist. These studies demonstrated that SCH 39166 binds to the D-1 receptor in vitro and inhibits the rat conditioned avoidance response, a test predictive of antipsychotic activity. The current study demonstrates that SCH 39166 inhibits the in vivo binding of [125I]SCH 38840 to D-1 receptors in rat striatal tissue with an ED50 of 11.67 nmol/animal or 0.016 mg/kg s.c. SCH 39166 did not inhibit the in vivo binding of [125I]SCH 38840 to rat frontal cortex, suggesting that, unlike other D-1 antagonists, SCH 39166 was not binding to 5-hydroxytryptamine (5-HT)2 receptors in vivo. The in vivo binding of SCH 39166 to D-2 receptors was studied using [3H]raclopride and demonstrated that SCH 39166 did not bind to D-2 receptors up to doses of 100 mumol/animal or approximately 150 mg/kg s.c. Further studies to determine the in vivo selectivity of SCH 39166 utilized N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to inactivate selected neurotransmitter receptors. Preadministration of SCH 39166, at doses as low as 0.01 mg/kg s.c., produced a statistically significant protection of D-1 receptors from EEDQ inactivation. SCH 39166 produced a similar protection of 5-HT2 receptors only at the highest dose tested, 10 mg/kg s.c., whereas there was no protection of D-2 sites even at this high dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of [N-(L-(1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine (SCH32615), a neutral endopeptidase (enkephalinase) inhibitor, on levels of enkephalin, encrypted enkephalins and substance P in cerebrospinal fluid and plasma of primates

In halothane-anesthetized and -ventilated cynomologus macaque monkeys, the effects of administering vehicle (n = 3) or the neutral endopeptidase inhibitor N-[L-(1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine (16 mg/kg, n = 5; or 100 mg/kg, n = 3, intravenously) was examined. Cisternal CSF aliquots were examined by radioimmunoassay: 1) for Met enkephalin; 2) after trypsin and carboxypeptidase B treatment for encrypted enkephalin (X-ENK); 3) for substance P; and 4) for unmetabolized drug. Similar measures were carried out in femoral artery and femoral venous plasma, except that substance P was not assayed. In CSF, prior to drug, low, but measurable levels of enkephalin (61 pg/ml), X-ENK (285 pg/ml) and substance P (16 pg/ml) were observed. Vehicle-injected animals showed no change from baseline levels over a 4-hr sampling period in either plasma or CSF levels. In contrast, following 16 mg/kg, in CSF, there was a significant 9-fold increase in MET and 11-fold increase in X-ENK at 30 min. CSF-substance P levels rose also by a factor of 2, with the peak effect observed at 60 min. All levels displayed a significant reduction by 4 hr. There was no statistical difference between the maximum effects observed with either the 16- or 100-mg/kg dose. Plasma peptide levels of enkephalin and X-ENK were not altered by drug. CSF displayed significant drug levels by 30 min, which were between 0.1 and 1% of levels observed concurrently in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesic and acute central nervous system side effects of the intravenously administered enkephalinase inhibitor SCH 32615

The analgesic and acute central nervous system (CNS) side effect potential of the enkephalinase inhibitor SCH 32615 (N-[L-(1-carboxy-2-phenyl)ethyl]-L-phenyl-alanine-beta-alanine) were evaluated after IV administration to mice, rats and squirrel monkeys. In mice, SCH 32615 caused dose-related suppression of acetic acid-induced writhing (minimal effective dose, MED = 3 mg/kg IV). In rats, SCH 32615 produced dose-related increases in the response latencies in the yeast inflamed-paw test (MED = 10 mg/kg IV). In squirrel monkeys, using a new hot-water bath tail-flick test, SCH 32615 significantly prolonged the escape latencies (MED = 100 mg/kg IV). These results in primates are the first data showing an analgesic action of an enkephalinase inhibitor in a reflex model of pain. When measured for its CNS side effect potential, SCH 32615 had no significant effects in rats (up to 100 times its analgesically active doses) or in monkeys (up to three times). In the mouse, at doses 100 times its minimal effective dose, SCH 32615 produced brief convulsions; these lasted only a minute, resolved quickly, and did not cause lethality. In contrast, in rats and squirrel monkeys, the standard opioid analgesic morphine produced profound CNS side effects; this was particularly notable in monkeys, in which morphine's maximal analgesic effects were associated with near lethal respiratory depression. These data demonstrate that SCH 32615 produces selective analgesic actions and that its acute side effect liability is less than that seen with a clinically used standard.

Studies on the effect of SCH-34826 and thiorphan on [Met5]enkephalin levels and release in rat spinal cord

SCH-34826 and thiorphan are inhibitors of the neutral endopeptidase (NEP; E.C. 3.4.24.11;) that cleaves the opiate peptides [Met5]- and [Leu5]enkephalin at the glycinylphenylalanine bond. These compounds were evaluated for their ability to affect the levels of [Met5]enkephalin-like immunoreactivity (MELI) in the brain and the spinal cord and the release into the extracellular space under resting and K+-evoked conditions. The results showed that oral administration of SCH-34826 (30-100 mg/kg p.o.) or thiorphan (10-30 mg/kg p.o.) had no effect on tissue levels of MELI. In contrast, both agents caused a dose-dependent increase in both the resting and the K+-evoked levels in spinal perfusates, which reached up to 10 times the control values. These data indicate that tissue (presumably intracellular) stores of [Met5]enkephalin are not affected by NEP inhibition and that it is the extracellular effects of the peptide that are potentiated by enzyme blockade. This agrees with the prior results demonstrating that NEP inhibitors require a nociceptive stimulus sufficient to release endogenous stores of [Met5]enkephalins for their actions to be observed.

Synthesis and receptor affinities of some conformationally restricted analogues of the dopamine D1 selective ligand (5R)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl- 1H-3-benzazepin-7-ol

The synthesis of a structurally novel series of 6,6a,7,8,9,13b-hexahydro-5H-benzo[d]naphtho[2,1-b]azepines (2), conformationally restricted analogues of the dopamine D1 antagonist (5R)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin -7-ol (SCH 23390, 1c), is described. Affinity for D1 receptors was determined by competition for rat striatal binding sites labeled by [3H]SCH 23390; affinity for D2 receptors was similarly determined by competition experiments using [3H]spiperone. Compounds in this series having the B/C-trans ring junction (2b and related analogues), where the D ring is unequivocally fixed in an equatorial orientation, possess considerably more D1 receptor affinity and selectivity vs the D2 receptor than the conformationally mobile cis stereoisomers (2a), thus leading to the conclusion that axial substituents at the 4- or 5-positions of the benzazepine nucleus are detrimental to D1 receptor affinity. Resolution and X-ray analysis demonstrated that D1 receptor affinity was preferentially associated with the (-)-6aS,13bR enantiomer of 2b.

Acute extrapyramidal syndrome in Cebus monkeys: development mediated by dopamine D2 but not D1 receptors

The present study assessed the role of dopamine D1 and D2 receptors in the production of an extrapyramidal syndrome (EPS) in Cebus apella monkeys. Previous studies have shown the development of EPS in both old and new world monkeys with haloperidol administration. We now report that repeated weekly administration of a selective D1 antagonist, SCH 23390, does not produce this syndrome in cebus monkeys. Cebus monkeys were treated with either vehicle (n = 6), the specific D2 antagonist haloperidol (0.3 mg/kg p.o., n = 9) or the specific D1 antagonist SCH 23390 (10.0 mg/kg p.o., n = 9) once a week for approximately 1 year and behavioral effects were observed and scored. The drug doses used in this study produced similar sedative scores when given acutely and sedation increased over the first 12 weeks of the study for both treatment groups. However, by the 12th week of dosing with haloperidol all the monkeys showed a profound EPS characterized by limb extensions, head pushing, tongue protrusions and sometimes severe biting movements. In contrast, none of the SCH 23390-treated monkeys showed any abnormal movements, suggesting D1 antagonists have a low EPS side-effect liability. The profile of the incidence of EPS seen with classical neuroleptic drugs in cebus monkeys and their blockade of EPS by anticholinergic drugs mimics the profile seen in humans. The models presented appear to be predictive of the production of the EPS in humans and could be used to screen neuroleptics for EPS liability. Furthermore, the EPS is probably due to the selective blockade of dopamine D2 receptors with its associated enhancement of cholinergic neurotransmission.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological profile of SCH39166: a dopamine D1 selective benzonaphthazepine with potential antipsychotic activity

SCH39166 [(-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl- 5H-benzo[d]naptho-(2,1-b)azepine] is a benzonaphthazepine that has been evaluated as a selective D1 dopamine receptor antagonist. In vitro, SCH39166 (Ki = 3.6 nM) inhibited the binding of [3H]SCH23390 (a D1 specific compound) and blocked dopamine-stimulated adenylate cyclase (Ki = 9.1 nM); in contrast the Ki for SCH39166 to displace [3H]spiperone (D2) was greater than 1 microM and its Ki vs. [3H]-ketanserin (5-hydroxytryptamine2) binding was greater than 300 nM. In vivo, SCH39166 inhibited both rat and squirrel monkey conditioned avoidance responding (minimal effective dose = 10 and 1.78 mg/kg p.o., respectively) and had a duration of at least 6 hr in both species. In addition, SCH39166 antagonized apomorphine-induced stereotypy in rats (minimal effective dose = 10 mg/kg p.o.). These in vivo actions of SCH39166 are similar to the activity of typical dopamine antagonists. However, in contrast to D2-selective antagonists, SCH39166 failed to increase plasma prolactin levels, did not block apomorphine-induced emesis in the dog and had minimal effects on the striatal levels of homovanillic acid or dihydroxyphenylacetic acid. Furthermore, although immobility was seen after p.o. administration of SCH39166 using the inclined screen test, the drug did not cause catalepsy at doses up to 10 times its minimal effective dose in the rat conditioned avoidance response test. Additionally, SCH39166 inhibited apomorphine-induced climbing at lower doses than it inhibited apomorphine-induced sniffing in mice. The results from these latter two tests suggest that SCH39166 may have a reduced liability to produce extrapyramidal side effects. Therefore, based on this profile of activity, SCH39166 is a selective D1 dopamine receptor antagonist both in vitro and in vivo. Additionally, because this compound is longer acting in the primate than previously available D1 antagonists, it has potential utility as a clinically useful drug.

Effects of D1 and D2 antagonists on basal and apomorphine decreased body temperature in mice and rats

In these experiments representative selective antagonists at D1 (SCH 23390) and D2 (haloperidol) receptors were studied for their effects on basal and apomorphine decreased body temperature in mice and rats. In mice, SCH 23390 (up to 3 mg/kg SC) neither affected basal body temperature nor blocked apomorphine-induced hypothermia (AIH). On the other hand, haloperidol alone was hypothermic and paradoxically also blocked AIH in mice. In rats, SCH 23390 alone produced hyperthermia; the mechanism by which this occurred is not known. SCH 23390 also blocked AIH in rats. However, the inhibition of AIH only occurred at doses of SCH 23390 that were themselves hyperthermic. Haloperidol did not alter basal body temperature but did block AIH in rats. These data suggest that apomorphine-induced body temperature changes are D2 mediated.

Pharmacology of SCH 34826, an orally active enkephalinase inhibitor analgesic

SCH 34826 [(S)-N-[N-[1-[[(2,2-dimethyl-1,3-dioxolan-4yl) methoxy]carbonyl]-2-phenylethyl]-L-phenylalanine]-beta-alanine] was synthesized as a p.o. active prodrug enkephalinase inhibitor. In vivo, it is de-esterified to SCH 32615 (N-[L-(-1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine), the active constituent. In vitro, the Ki for SCH 32615 to block the degradation of Met5-enkephalin by isolated enkephalinase is 19.5 +/- 0.9 nM. In contrast, SCH 32615 did not inhibit aminopeptidase or diaminopeptidase III degradation of Met5-enkephalin up to 10 microM and did not affect angiotensin converting enzyme up to 10 microM. In vivo, p.o. administered SCH 34826 potentiated the analgesic effects of D-Ala2-Met5-enkephalinamide in mice (ED50 = 5.3 mg/kg p.o.) and rats [minimal effective dose (MED) = 1 mg/kg p.o.]; SCH 32615 had no effect up to 30 mg/kg p.o., but was active parenterally (ED50 in mice = 1.4 ng/kg sc). Direct, naloxone-reversible analgesic effects of SCH 34826 were demonstrated in the mouse low temperature hot-plate test (MED = 30 mg/kg p.o.), the mouse acetic acid-induced writhing test (MED = 30 mg/kg p.o.), the rat stress-induced analgesia test (MED = 10 mg/kg p.o.) and the modified rat yeast-paw test (MED = 100 mg/kg p.o.). Using the rat D-Ala2-Met5-enkephalinamide potentiation test the duration of action of SCH 34826 was at least 4 hs. No respiratory or gastrointestinal side effects of any consequence were noted at doses up to 100 times those active in the D-Ala2-Met-5-enkephalinamide potentiation test.

Pharmacological and behavioral effects of D1 dopamine antagonists

This work includes the effects of SCH 23390 and related benzazepines on behavior and attempts to relate these effects to their D1 dopamine antagonist action. Effects on conditioned avoidance responding (CAR) in rats were studied under the same conditions in which in vivo binding of the radioiodinated D1 specific benzazepine 125I-SCH 38840 was measured. It was found that there is very close agreement between the time-course for antagonism of CAR and for in vivo displacement of 125I-SCH 38840 from rat striatum. The effect of SCH 23390 in CAR in monkeys was compared with standard anti-psychotics and although its oral potency was reasonable, its duration was very short (1-2 hours at 5 times its minimal effective dose for statistically significant reduction of avoidance). It is concluded from this and prior work that SCH 23390 and other D1 specific benzazepines inhibit CAR at the same doses that bind to D1 receptors in the CNS and that D1 specific antagonists are behaviorally effective at doses that do not produce D2 receptor effects (e.g. increased plasma prolactin levels, catalepsy).

Serotonergic component of SCH 23390: in vitro and in vivo binding analyses

A series of benzazepines related to SCH 23390 were tested for binding to the 5HT-2 receptor. The compounds tested inhibited the binding of 3H-ketanserin with KI values generally greater than those observed for the D-1 receptor, but less than those for the D-2 receptor. When this serotonergic activity was correlated to the D-1 activity, the resulting coefficient was 0.84, indicating a strong correlation between the two activities. Conversely, the 5HT-2 activity did not show a good correlation with the D-2 activity. To further test the significance of the 5HT-2 binding of the SCH 23390, in vivo binding studies were performed using 125I-SCH 38840 in the frontal cortex, an area containing both D-1 and 5HT-2 receptors. The in vivo binding of 125I-SCH 38840 to frontal cortex exhibited peak levels one hour following subcutaneous administration, similar to the time course previously observed in striatum. The binding was both D-1 and tissue specific. Competition studies with selected standards demonstrated that inhibition of the binding to frontal cortex, in contrast to the inhibition observed in the striatum, exhibited a Hill coefficient less than unity, implying interaction at more than one receptor subtype. When SCH 23390 and ketanserin were administered simultaneously, the inhibition of the in vivo binding of 125I-SCH 38840 to striatum was not different than that observed with SCH 23390, alone. However, the inhibition of binding to frontal cortex was significantly greater than that demonstrated with either SCH 23390 or ketanserin, alone, suggesting that 125I-SCH 38840 was binding to both D-1 and 5HT-2 receptors, in vivo.

D1 and D2 dopamine binding site up-regulation and apomorphine-induced stereotypy

Treatments with drugs to up-regulate specific receptors is a strategy often employed in mechanism of action studies. In this type of experiment, changes in the numbers of receptors and concomitant changes in an animal's sensitivity to the drug have been used as evidence for the participation of the binding site in the behavior. In these studies, to test for the role of D1 and D2 receptors in apomorphine-induced stereotypy (AIS), dopamine binding sites were up-regulated by appropriate pre-treatments and the ability of these pre-treatments to alter AIS was subsequently investigated. In the first experiment, 19 days of pre-treatment with SCH 23390 or haloperidol selectively increased by 35 and 40% the numbers of striatal D1 and D2 binding sites, respectively, without affecting their affinities. However, when challenged with apomorphine, only the animals pre-treated with the D2 antagonist showed behavioral supersensitivity. In the second experiment, reserpine pre-treatment (30 mg/kg IP, 24-hr pre-test) increased the numbers of D1 binding sites by 18%, but did not significantly alter the numbers of striatal D2 binding sites. Behaviorally, these rats were supersensitive to apomorphine's stereotypy-inducing effects; however, they also showed an increased sensitivity to the ability of either haloperidol or SCH 23390 to block AIS. Moreover, this blockade was only attenuated by a D2 (but not a D1) agonist. Collectively, these data suggest that AIS is mediated by both D1 and D2 binding sites, but that D2 binding sites have a more important role.

Similarity of clozapine and SCH 23390 in reserpinized rats suggests a common mechanism of action

Clozapine at doses up to 100 mg/kg p.o. did not antagonize apomorphine-induced stereotypy in vehicle pre-treated rats. However, if the animals were injected with reserpine (30 mg/kg i.p.) 24 h prior to the test, then clozapine (3-100 mg/kg p.o.) produced a dose-related blockade of apomorphine-induced stereotypy. The blockade of apomorphine-induced stereotypy in reserpinized rats by clozapine was attenuated by the D-2 selective agonist LY 171555 but not the D-1 selective agonist SKF 38393. This profile of agonist reversal of antagonist blockade of apomorphine-induced stereotypy seen with clozapine was identical to that seen with the selective D-1 antagonist SCH 23390. Presumably, the D-2 agonist was active because D-1 receptor systems were inhibited (either at the receptor or at some other post-synaptic site) by clozapine or SCH 23390; this allowed a partial restoration of apomorphine-induced stereotypy via the D-2 system. Therefore, these data indicate that clozapine and SCH 23390 share a common mechanism of action via D-1 receptor systems.

Pharmacological effects of SCH 30497--a novel analgesic substance

SCH 30497 (2-[3-(1,2,3,6-tetrahydro-4-2(methylphenyl)-pyridin-1-yl) -propyl]-1,2,4-triazolo[4,3-a] pyridin-3-(2H)-one) was tested for analgesic effects in the rat, mouse and squirrel monkey. SCH 30497 showed dose-related analgesic effects in the rat yeast-paw test; at peak times the ED7sec (95% confidence limits) in mg/kg via the oral, subcutaneous, intramuscular and intravenous routes of administration were as follows, respectively: 4.7 (2.1-9.8), 5.7 (3.4-9.6), 6.4 (4.1-9.8) and 1.4 (0.6-3.2). SCH 30497 was also analgesic in the acetic acid-induced writhing test in mice (ED50 = 1.9 mg/kg p.o.) and the squirrel monkey shock titration test (ED50 = 14.5 mg/kg p.o.). It was inactive in the mouse (100 mg/kg p.o.) and rat (40 mg/kg p.o.) tail-flick tests. Thus, SCH 30497 was efficacious versus chemical, mechanical and electrical nociceptive stimuli. Naloxone antagonism of the analgesic effects of SCH 30497 was species specific with significant inhibition observed only in the rat and not in the mouse or monkey. SCH 30497 did not produce Straub tail or hyperactivity in mice. Twice daily dosing at 30 mg/kg p.o. to rats for 5 days failed to produce tolerance; in separate experiments, daily injections for 10 days at 20 or 100 mg/kg p.o. failed to induce signs of dependence following naloxone challenge. SCH 30497-induced analgesia was not attenuated in rats previously made tolerant to narcotics by implantation of a morphine pellet. SCH 30497 showed a weak ability to displace 3H-Met5-enkephalin from its binding sites on rat brain membranes (IC50 = 48 microM). SCH 30497 (100 microM) did not affect prostaglandin synthesis in vitro. In vivo, the drug did not have anti-inflammatory or ulcerogenic effects up to 80 mg/kg p.o. Acute behavioral, neurological and autonomic side effects were primarily depressant in rodents and occurred at doses greater than 15 times those that were analgesically relevant. Moderate doses in the monkey (2.5 times the ED50) and high doses in mice produced convulsions. It is hypothesized that SCH 30497-like drugs represent a new class of analgesics based on this unique pharmacological spectrum of activity.

Comparisons between warm and cold water swim stress in mice

The following experiments evaluated the effects of warm- or cold-water swim stress on tail-flick latencies (TFL) in mice. To first determine the appropriate control group, the TFL's of dry-vs-dunked mice were compared. Dry mice had significantly shorter TFL's than dunked mice, implying that the dampness of the mouse's tail contributed to the increase in the TFL. Therefore, dunked mice were used as the relevant control for the swum mice. Cold water swimming (2 degrees C) produced a significant increase in the TFL; this was not blocked by the opiate antagonist naloxone (3 mg/kg sc) or potentiated by the enkephalinase inhibitor thiorphan (100 mg/kg sc). Warm water swimming (32 degrees C) up to 3 min produced an inconsistent effect on TFL's, implying that the effects were at the threshold of detectability. Naloxone attenuated and thiorphan modestly potentiated the effects of warm water swimming on TFL's. This suggests that warm water swim stress-induced increases in mouse TFL's may involve opioid pathways, whereas cold water swim stress-induced changes in mice TFL's appear not to be opioid mediated.

Potentiation of the hypoglycemic effect of insulin by thiorphan, an enkephalinase inhibitor

Insulin administration to rats produced a dose-related hypoglycemia. When insulin and the enkephalinase (Enk'ase) inhibitor thiorphan (30 or 100 mg/kg s.c.) were co-administered, there was a potentiation of the hypoglycemic response to insulin; these doses of thiorphan alone had no significant effect on plasma glucose. When tested in vitro against isolated Enk'ase, both insulin and its beta-chain inhibited the catabolism of [Met5]enkephalin. Theoretically, thiorphan blocked the catabolism of insulin by inhibiting Enk'ase. Alternatively, thiorphan acted as an inhibitor of another insulin-catabolizing enzyme having similar substrate requirements as Enk'ase.

Subplantar yeast injection induces a non-naloxone reversible antinociception in spontaneously hypertensive rats

Spontaneously hypertensive (SH), Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats were tested for their responsiveness to noxious mechanical pressure before and after a subplantar yeast injection to the right rear paw. Prior to the yeast injection, hypertensive animals were less responsive to pain relative to normotensive animals, as seen in the significantly greater pre-yeast latencies of SH compared to WKY or SD rats. Subplantar yeast injection produced hyperreactivity in the inflamed paws of WKY or SD rats, with no effect on the contralateral non-injected paw. However, identical subplantar yeast injections to hypertensive animals produced a robust, long-lasting antinociceptive effect in both rear paws of SH rats. This effect was not reversed by naloxone (opiate antagonist), labetalol (beta-blocker/vasodilator antihypertensive), or hydralazine (peripheral vasodilator antihypertensive); the antinociception was not potentiated by thiorphan (enkephalinase inhibitor). However, the alpha 2-receptor antagonist yohimbine (0.1-5 mg/kg s.c.), produced a dose-related reversal of the yeast-induced antinociception in SH rats. These results suggest that the subplantar yeast injection is triggering descending noradrenergic pain inhibitory pathways in SH rats.

Enkephalin heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe)-induced bronchospasm in guinea pigs: a non-naloxone reversible phenomenon

Enkephalin heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MEAP ), is an enkephalin-related peptide derived from pre-pro-enkephalin A. Distribution studies have shown that MEAP is localized in the lung; in this organ there is very little Met-enkephalin (ME). To test the direct actions of MEAP in a tissue where it is differentially distributed from ME, MEAP was given intratracheally to anesthetized guinea pigs. MEAP produced a dose-related bronchospasm characterized by a slow onset (30-90 sec) and a duration exceeding 5 min. No significant effects on heart rate or blood pressure were observed concomitant with the pulmonary effects. ME or ethylketocyclazocine, tested at the same doses as MEAP , did not produce a bronchospasm. Moreover, naloxone failed to block MEAP 's effects. Therefore, it appears as if the MEAP -induced bronchospasm does not have an opioid-related mechanism of action. The bronchospasm produced by MEAP was not blocked by an anti-histamine (chlorpheniramine) or by vagotomy. However, inhibition of cyclo-oxygenase by ibuprofen blocked the MEAP -induced bronchospasm, suggesting that an arachidonic acid metabolite was involved in the effect. Blockade of MEAP 's pulmonary effects by an inhibitor of thromboxane synthetase (SQ 80338) and not by a leukotriene antagonist (FPL 55712) implied that MEAP 's mechanism of action may be via bronchospastic thromboxanes. These data are the first to demonstrate a non-opioid, thromboxane-mediated action of an enkephalin-related peptide.

Effect of cyclo-leucyl-glycine in the rat yeast-paw test

Cyclo-leucyl-glycine (CLG) was tested for its ability to produce an antinociceptive effect in the rat yeast-paw test under conditions in which Z-prolyl-(L)leucine (ZPLL) was effective. CLG at doses from 0.1 to 200 mg/kg p.o. given once per day for three days failed to produce an analgesic effect; at 300 mg/kg per day p.o., CLG produced a slight increase in response latencies (less than 1 s). ZPLL (5 mg/kg per day p.o.) tested similarly in the same experiment produced a strong analgesia (greater than 10 s increase in response latencies). Since CLG and ZPLL have both been shown to inhibit opioid tolerance/dependence, whereas only ZPLL has been demonstrated to produce analgesia, it appears as if the mechanisms underlying these two effects are different.

Determination of analgesic drug efficacies by modification of the Randall and Selitto rat yeast paw test

This report describes a modified Randall and Selitto (1957) rat yeast paw test that can evaluate differences in efficacy of different analgesics. The modifications consist of a decrease in the rate of acceleration of the noxious stimulus (mechanical pressure) on the inflamed paw from 20 to 12.5 mmHg/sec and an extension of the cut-off time from 15 to 60 sec. All the narcoticlike drugs tested (morphine, codeine, and pentazocine) increased the response latencies of the inflamed paws to the cut-off time. The nonsteroidal antiinflammatory-like drugs tested (acetylsalicylic acid, acetaminophen, indomethacin, phenylbutazone, and proquazone) showed plateaus in their analgesic effects (i.e., increasing the dose failed to produce significantly greater increases in the response latencies compared to the next lower dose). Zomepirac (80-240 mg/kg p.o.) did not show this plateau effect, but was unable to increase response latencies to greater than 30 sec because of the toxicity of higher doses (320 mg/kg p.o.). Flunixin NMG (the meglumine salt of flunixin), a nonnarcotic analgesic, did not display a plateau effect and increased response latencies to maximum values. The methodology was therefore able to discriminate analgesics active against mild to severe clinical pain (narcoticlike) from those only useful against mild to moderate pain (nonnarcotic-like).

Potentiation of [D-ala2]enkephalinamide analgesia in rats by thiorphan

The effect of thiorphan, an inhibitor of enkephalin dipeptidyl carboxypeptidase, was tested in the rat tail-flick test. When given alone up to 100 mg/kg s.c., thiorphan had no effect. However, thiorphan (30 mg/kg, s.c.) potentiated intraventricularly administered [D-Ala2,Met5]enkephalinamide (DEAEM) and its [Leu5]-derivative, whereas it had no effect on [D-Ala2,Met5]enkephalin, [D-Met2,Pro5]enkephalinamide, or morphine. Potentiation of DAEAM by thiorphan was dose-responsive and naloxone reversible. The most efficient enhancement of the analgesic effects of DAEAM was seen when the two drugs were co-administrated in rapid succession. Thus, thiorphan potentiation of opioid-peptide induced analgesia in the rat is similar to that seen in the mouse.

Potentiation of stress-induced analgesia (SIA) by thiorphan and its block by naloxone

Exposure of rats to inescapable footshock produces an analgesic effect. To determine if endogenously released enkephalins play a role in this phenomenom, rats were treated with the enkephalinase inhibitor thiorphan (T), exposed to inescapable stress, and tested in the tail-flick test for antinociception. T (10-100 mg/kg sc) caused a dose-related potentiation of both the peak effect and the duration of the SIA. This effect was blocked by doses of naloxone (1 mg/kg sc) that did not affect baseline response latencies.

A characterization of kyotorphin (Tyr-Arg)-induced antinociception

Intracerebroventricular administration of kyotorphin (Tyr-Arg) or Tyr-D-Arg to mice or intrathecal administration of kyotorphin to rats resulted in a dose-dependent, long-lasting, naloxone-reversible analgesia as measured by the 48 degrees C hot plate assay. The potency of kyotorphin was equal to that of Met-enkephalin although its duration of action was substantially longer. Cross-tolerance to kyotorphin could be demonstrated in animals made tolerant to morphine by chronic morphine pellet implantation. Kyotorphin was found to be inactive against column purified enkephalinase A, B and aminopeptidase and indirect evidence would suggest a lack of Met-enkephalin-releasing effect. Thus, kyotorphin represents a unique, naturally occurring peptide with in vivo narcotic-like characteristics and an unknown mechanism of action quite distinct from other opioid peptides.

Relationship of yeast-injection induced changes in brain met-enkephalin and analgesia

Subplantar injection of Brewer's yeast induces a hyperalgesia that is associated with an increase in the level of striatal Met-enkephalin (ME); there was no change in the hypothalamus of periaqueductal gray. To test the relationship between striatal ME and analgesia, naloxone (10, 3, 0.5 mg/kg, SC) or thiorphan (100 micrograms, ICV) were administered. Neither drug caused a potentiation or a reduction in the hypersensitivity. These data suggest that an increase in striatal does not result in altered pain sensitivity in this model.

The effects of peptides on the stimulus properties of ethanol

Male Sprague-Dawley rats were trained to discriminate ethanol (2 g/kg, PO: EtOH) from saline (10 ml/kg, PO: SAL) in a two-bar positively reinforced operant task on a VI 15 sec schedule. After the rats reached criterion performance (greater than 90% correct responses on the appropriate lever), thyrotropin releasing hormone (pyroGlu-His-Pro-NH2: TRH), a metabolite of TRH (His-Pro diketopiperazine: HP), and a structural analog of TRH (HPCA-His-ThiaPro-NH2: OHT) were tested for their ability to antagonize the EtOH cue. These peptides were chosen for their reported ability to reverse ethanol-induced narcosis. However, at doses that did not disrupt performance, TRH, HP, and OHT did not affect the stimulus properties of ethanol at any dose tested, nor did they change the stimulus properties of saline. Naloxone and ACTH(1-10)-NH2 were also tested as ethanol antagonists of the training dose. Pretreatment with either of these compounds failed to alter ethanol-appropriate responding. In addition, (DA1a2-Met5)-enkephalin-ol, (DAla2-Met(O)5)-enkephalin-ol, substance P, delta sleep-inducing peptide, and bombesin were tested for their ability to elicit ethanol appropriate responding. The EtOH cue generalized to none of these peptides.

Substance P and opioid interaction on stimulated and non-stimulated guinea pig ileum

In the past, substance P (SP) has been suggested to be both an opiate agonist and an antagonist. It therfore seemed appropriate to examine potential interactions of SP and opioids on guinea pig ileum. On non-stimulated ileal strips SP caused a dose responsive increase in contraction. Pretreatment of the tissue with morphine (3, 30, 300, 3000 nM), enkephalin (1.42, 14.2, 142, 1420 nM), naloxone (5nM), or atropine (0.144 micron) did not significantly alter the spasmogenic effect of SP. On stimulated guinea pig ileum, whereas morphine and enkephalin inhibited the electrically induced twitch, SP adminstration resulted in contraction of the tissue. Additionally, neither strongly effective non sub-threshold doses of SP antagonized the effects of the narcotics. These data are discussed in terms of separate receptors mediating the effects of the opiates and SP on guinea pig ileum.

Brief communication. Generalization of [DAla2]-enkephalinamide but not of substance P to the morphine cue

Rats were trained to discriminate morphine (7.5 mg/kg, IP) from saline in a two bar positively reinforced lever pressing paradigm on a FR4 schedule. Morphine (IP) showed a naloxone reversible dose-related generalization to the training dose. [DAla2]-Methionine enkephalinamide (DAE) at 1 mg/kg and Substance P (SP) at 0.1 and 0.25 mg/kg showed vehicle appropriate responding after IP injection. DAE (5 mg/kg) disrupted responding completely; SP (0.5 and 0.1 mg/kg) disrupted responding in 50% of the rats. The disruption caused by IP injection of DAE was not naloxone reversible. Intraventricular injection of morphine (5 microgram/rat) and DAE (5 microgram/rat) produced generalization to the opiate cue. The effect of DAE was reversed by naloxone (1 mg/kg, SC). SP (500 and 750 ng/rat, IVT) produced saline-like responding; 1 microgram/rat disrupted responding completely. These data demonstrate that morphine and enkephalin, but not Substance P, share similar discriminative properties.

Effect of propranolol on antinociceptive and withdrawal characteristics of morphine

Propranolol at a dose (10 mg/kg) which did not alter tail-flick latency by itself, did not alter the ED50 of morphine when given 10 min prior to the narcotic. Propranolol at doses of 10 and 25 mg/kg given 10 min prior to naloxone challenge did not significantly alter the frequency of naloxone induced jumping 72 hr after morphine pellet implantation. The ED50 of naloxone in morphine pelleted mice was not altered by treatment with propranolol at 0, 24, and 48 hr after pellet implantation. Naloxone caused hyperactivity in mice when administered 72 hr after morphine pellet implantation. An injection of 25 mg/kg propranolol 10 min prior to naloxone did not block this hyperactivity. In addition, administration of 10 mg/kg of propranolol every 8 hr to rats during withdrawal from morphine failed to alleviate the withdrawal syndrome as evidenced by changes in either body weight or water intake. These data suggest that the beta-adrenergic blocking agent, propranolol, does not alter the antinociceptive activity or lessen the withdrawal syndrome of morphine in rodents.