In vivo evidence for multiple opiate receptors mediating analgesia in the rat spinal cord.

In rats implanted with chronic catheters in the spinal subarachnoid space, intrathecal injections of SKF 10047 and dynorphin did not produce any elevation of the nociceptive threshold as defined by hot-plate and tail-flick tests. In contrast, intrathecal ethylketocyclazocine (EKC) and (D-Ala2,D-Leu5)-enkephalin (DADL) administration resulted in a dose-dependent antinociceptive effect which was reversible with intraperitoneal naloxone. Calculation of the Schild dose-ratio plots for the data derived from systemically administered naloxone reveals a slope of--1 and a calculated pA2 value of 6.8 for EKC and 6.2 for DADL. Also, animals made tolerant to systemic morphine showed a diminished analgesic response to intrathecal morphine and EKC when compared to naive animals. There was, however, no significant change in the dose response curve of intrathecal DADL. Thus, these experiments suggest that in addition to mu receptors a separate subpopulation of delta but not kappa or sigma receptors are involved with spinally mediated analgesia.

A comparison of (+)SK&F 10047 and MK-801 on cortical spreading depression.

MK-801 and (+)SK&F 10047 produced a dose-related inhibition of the EEG suppression and cortical hyperemia associated with cortical spreading depression (CSD) and reduced the CSD propagation rate; ED50 = 1 mg/kg, i.v. and 15 mg/kg, i.v., respectively. MK-801 had a delayed onset of action (inversely related to dose) and a prolonged duration of action at all doses (> 2 h). In contrast, (+)SK&F 10047 had a rapid onset of action (< 30 min) and a predictable dose-related duration of action. These results suggests that an efficacious compound acting with moderate affinity as a non-competitive antagonist at the NMDA-receptor channel may possess a preferable time-course and toxicity profile when compared to agents acting similarly, but with high affinity.

Involvement of haloperidol-sensitive sigma-sites in antitussive effects.

The effects of selective sigma-ligands on the capsaicin-induced cough reflex in rats were studied. Intraperitoneal injection of (+)-N-allylnormetazocine ((+)-SKF-10,047) and N,N'-di(ortho-tolyl)guanidine (DTG) in doses that ranged from 0.3 to 3.0 mg/kg decreased the number of coughs dose dependently. The antitussive effects of these sigma-ligands were significantly attenuated by pretreatment with haloperidol. Pretreatment with haloperidol also markedly reduced the antitussive effects of (+/-)-pentazocine and dextromethorphan. These results suggest that haloperidol-sensitive sigma-sites may be involved in the regulation of coughs.

Oral self-administration of N-allylnormetazocine (SKF-10,047) stereoisomers in rhesus monkeys: substitution during phencyclidine self-administration and withdrawal.

Orally-delivered N-allylnormetazocine (NANM) and its isomers were tested for their ability to function as reinforcers by substituting them for phencyclidine (PCP). Two monkeys were trained to self-administer PCP (0.25 mg/ml) and water under concurrent fixed-ratio (FR) 16 schedules during 3-hr sessions. Liquid deliveries were contingent upon lip-contact responses on solenoid-operated drinking devices. When the dextro (+)-isomer of NANM (0.062-1 mg/ml) was substituted for PCP, response rates increased and then decreased in an inverted U-shaped concentration-response function with peak response rates comparable to those maintained by PCP. Drug intake ranged from 2.8 to 25.7 mg/kg across the two monkeys and five concentrations. Water-maintained responding was considerably lower than drug-maintained behavior indicating that NANM was functioning as a reinforcer. As previously reported for PCP, almost all of the (+)-NANM was self-administered during the first half of the session. Substitution of the levo (-)-isomer of NANM resulted in an immediate decline to low response rates that were not distinguishable from those maintained by water. The racemic form (+/-) of NANM was also not self-administered in excess of concurrent water. In the second experiment concurrent PCP- and water-maintained responding were reestablished under FR 8 schedules during three 6.5-hr sessions daily. Food (6 g/pellet) was available under FR 64 and FR 80 schedules during three 1-hr sessions immediately preceding the liquid components. Water was then substituted for PCP for four days and PCP, (+)-, (-)- or (+/-)-NANM were reinstated in subsequent replications of the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)

Reinforcing properties of stereoisomers of the putative sigma agonists N-allylnormetazocine and cyclazocine in rhesus monkeys.

There are many similarities in the effects of phencyclidine (PCP) and opioids with sigma agonist activity such as N-allylnormetazocine (SKF-10,047) and cyclazocine. Yet PCP and sigma agonists differ in reinforcing properties. PCP is reliably self-administered in animal models and has abuse liability in humans, whereas (+/-)-N-allylnormetazocine and (+/-)-cyclazocine are not self-administered or abused. Inasmuch as other research has demonstrated differences in the selectivity of PCP-like effects of the stereoisomers of opioids, this study compared the reinforcing properties of PCP to the reinforcing properties of isomers of N-allylnormetazocine and cyclazocine. Rhesus monkeys were trained to lever-press on a fixed-ratio 10 schedule for i.v. cocaine (50 micrograms/kg/injection). During substitution tests, cocaine was replaced with various doses of test drugs or vehicle. The drugs tested were PCP, (+/-)-N-allylnormetazocine, (-)-N-allylnormetazocine, (+)-N-allylnormetazocine, (+/-)-cyclazocine, (-)-cyclazocine and (+)-cyclazocine. PCP and the (+)-isomers of both benzomorphans, at one or more doses, were self-administered at rates which exceeded rates maintained by vehicle and which equaled or exceeded cocaine-maintained base-line rates. Neither the racemates nor the (-)-isomers of N-allylnormetazocine or cyclazocine were self-administered by the monkeys at rates which were greater than vehicle control levels of responding. Reinforcing doses of the three compounds resulted in injections evenly distributed across the 60-min session in a pattern similar to cocaine self-administration, whereas the racemic mixtures and the (-)-isomers produced negatively accelerated patterns of injections which resembled typical saline patterns of extinction responding. These data provide further evidence that the (+)-isomers of sigma agonists are more selective for PCP-like effects than the (-)-isomers in monkeys.

Identification of multiple opiate receptors through neuroendocrine responses. I. Effects of agonists.

The effects of the systemic administration of three prototypic multiple opiate receptor agonists, morphine sulfate (MS), ethylketocyclazocine methanesulfonate (EKC) and N-allylnormetazocine hydrochloride (NANMT), on the release of anterior pituitary hormones were studied in the rat. The serum levels of corticosterone, growth hormone, prolactin and luteinizing hormone were measured by radioimmunoassay 30 min after s.c. injection of the drugs. The three opiate compounds elicited different patterns of release of the four hormones. MS, EKC and NANMT elicited rises in the serum levels of corticosterone whereas only MS and EKC induced elevations in growth hormone. MS stimulated but NANMT inhibited the release of prolactin. The administration of the lowest dose of EKC stimulated the release of prolactin whereas higher doses were without effect suggesting biphasic dose-response relationship. The administration of MS, EKC and the highest dose of NANMT elicited a fall in serum luteinizing hormone levels suggesting that NANMT possesses agonist activity at the receptor mediating luteinizing hormone release. The data support the hypothesis that multiple opiate receptors are involved in the mechanism of action of opiate-induced changes in anterior pituitary hormone release.

Opiate-induced enhancement of the effects of naloxone on serum luteinizing hormone levels in the male rat: specificity for Mu agonists.

We have shown previously that acute morphine administration markedly enhances naloxone-induced increases in serum luteinizing hormone (LH) levels in the male rat. The purposes of the present studies were to determine whether this effect was opiate-specific and, if so, whether it was mediated by mu, kappa or sigma opiate receptors. In agreement with our previous reports, we found that naloxone-induced increases in serum LH levels were markedly enhanced (greater than 400%) in morphine-pretreated rats, relative to controls, 6 to 8 hr after a single injection; furthermore, similar effects were observed with all mu agonists assessed with the order of potency being etorphine greater than levorphanol greater than morphine greater than methadone greater than codeine. In contrast, we were unable to demonstrate any enhancement of the effects of naloxone on serum LH levels by ketocyclazocine, cyclazocine or SKF 10,047, prototypic ligands for kappa and sigma binding sites in brain. Finally, we observed that neither ethanol nor Nembutal induced a period of supersensitivity to the effects of naloxone on LH, even though both compounds transiently depressed serum LH levels over a time course similar to that observed for the opiates. On the basis of these results, it appears that the phenomenon of opiate-induced enhancement of the effects of naloxone on serum LH levels is opiate specific and, most importantly, is a unique feature of mu opiate agonists. The mechanisms underlying this phenomenon are unclear, but our results suggest that as yet unidentified events occurring within the hypothalamus must be responsible.

Effects of acute and chronic administration of (+)-SKF 10,047 on body temperature in the rat: cross-sensitization with phencyclidine.

The current study investigated the effects of acute and chronic administration of (+)-SKF 10,047 on body temperature in rats. The effect of phencyclidine on body temperature in chronically (+)-SKF 10,047-treated rats was also investigated. The acute administration of (+)-SKF 10,047 at doses of 5 to 40 mg/kg s.c. did not alter body temperature; however, 80 mg/kg produced hypothermia. In contrast, chronic administration of (+)-SKF 10,047 (5-20 mg/kg) produced dose-dependent hyperthermia when tested on day 7 and 10 of chronic treatment. Moreover, sensitization to the hyperthermic effects occurred as the degree of hyperthermia was greater on day 10 compared to day 7. Phencyclidine (20 mg/kg s.c.) produced hypothermia in rats chronically treated with saline for 13 days, but hyperthermia in rats chronically treated with 20 mg/kg of (+)-SKF 10,047 for the same duration. The hyperthermic effect of chronic (+)-SKF 10,047 treatment is similar to the previously reported dose-dependent hyperthermia in chronically phencyclidine-treated animals. The cross-sensitization of chronically (+)-SKF 10,047-treated rats to the hyperthermic effects of phencyclidine supports the hypothesis that there may be common mechanisms underlying the chronic effects of these drugs on body temperature.

Differential effects of prototype opioid agonists on the acquisition of conditional discriminations in monkeys.

In each of two components of a multiple schedule, patas monkeys were required to respond on a right or left lever depending upon the stimulus combination (a color and a geometric form) presented. Reinforcement of a response in the presence of one stimulus (the form) was conditional upon the other stimulus (the color). The completion of a two-member chain of discriminations produced a food pellet. Errors produced a brief time-out. One component of the multiple schedule was a repeated-acquisition task where the discriminative stimuli for left- and right-lever responses changed each session (learning). In the other component, the discriminative stimuli were the same each session (performance). Morphine, ketocyclazocine and SKF 10,047 each produced dose-related decreases in overall rate of responding in both components of the multiple schedule. However, each drug affected the patterning of responding in a different manner. Across a range of doses which decreased response rate, neither morphine nor ketocyclazocine affected accuracy of the discriminations. In contrast, SKF 10,047 and its enantiomers produced a dose-dependent disruption of accuracy; errors were increased in the acquisition component at doses lower than those required to disrupt the discrimination in the performance component. The effects of the l-isomer of SKF 10,047 were generally comparable to those of the racemate. In comparison to the racemate, the d-isomer of SKF 10,047 was found to produce greater error-increasing effects in the acquisition component and smaller rate-decreasing effects in both components of the multiple schedule. In general, the effects of cyclazocine and pentazocine on both accuracy and rate of responding were comparable to those obtained with SKF 10,047. The results suggest that in patas monkeys, opioids with activity at the putative sigma receptor such as SKF 10,047, cyclazocine and pentazocine exert a dose-dependent disruptive effect on the accuracy of discriminations, an action not shared by prototypical mu and kappa opioid agonists, at doses which produce approximately equivalent rate-decreasing effects.

Subclasses of opioids based on body temperature change in rats: acute subcutaneous administration.

Morphine and a number of opioid agonists and agonist-antagonists were injected into rats to examine their effects on body temperature after acute systemic administration. Dose- and time-response curves were constructed for each drug alone and in the presence of the antagonist naloxone. Based on these data, the opioids could be subdivided into several groups. The first group, made up of morphine, heroin, l-methadone, etorphine, fentanyl and levorphanol, caused hyperthermia at lower doses and hypothermia at higher ones. Both effects could be blocked by naloxone. A second group, consisting of buprenorphine, nalbuphine and l-pentazocine, produced only a naloxone-sensitive increase in body temperature, whereas the group comprising ethylketazocine, ketazocine, l-cyclazocine and normeperidine caused only a decrease. This fall in temperature was relatively less sensitive to naloxone blockade. Still another group of drugs (meperidine, normorphine and d-pentazocine) had little effect themselves but, in combination with naloxone, induced hypothermia. The fifth group (N-allylnormetazocine, d-cyclazocine, dextrorphan and d-methadone) had no effect whether alone or in the presence of the antagonist. These findings can be explained in terms of a two-receptor model by ascribing distinct thermoregulatory functions to each receptor type.

Effects of some opiates and opioid peptide eyedrops on ocular melatonin regulation in rabbits.

Levels of melatonin in rabbit eye tissues were detected by radioimmunoassay. Solutions of met-enkephalin, leu-enkephalin, alpha-endorphin and beta-endorphin were given topically. Met-Enkephalin and alpha-endorphin lowered levels of melatonin in the iris, iris root-ciliary body, retina and choroid; leu-enkephalin raised levels in the retina and lowered them in other tissues. beta-Endorphin only lowered levels in the iris root-ciliary body. DAGO (a mu agonist) given i.v. lowered levels of melatonin in the iris, iris root-ciliary body and retina. The delta and sigma agonists given i.v. only lowered levels in the iris root-ciliary body, and a kappa agonist given i.v. raised levels in the ciliary body. No opiate binding sites could be detected in the rabbit iris or iris root-ciliary body for any class of receptor. Our data suggest opioids may be useful for treating glaucoma.

Receptor mechanisms and circuitry underlying NMDA antagonist neurotoxicity.

NMDA glutamate receptor antagonists are used in clinical anesthesia, and are being developed as therapeutic agents for preventing neurodegeneration in stroke, epilepsy, and brain trauma. However, the ability of these agents to produce neurotoxicity in adult rats and psychosis in adult humans compromises their clinical usefulness. In addition, an NMDA receptor hypofunction (NRHypo) state might play a role in neurodegenerative and psychotic disorders, like Alzheimer's disease and schizophrenia. Thus, understanding the mechanism underlying NRHypo-induced neurotoxicity and psychosis could have significant clinically relevant benefits. NRHypo neurotoxicity can be prevented by several classes of agents (e.g. antimuscarinics, non-NMDA glutamate antagonists, and alpha(2) adrenergic agonists) suggesting that the mechanism of neurotoxicity is complex. In the present study a series of experiments was undertaken to more definitively define the receptors and complex neural circuitry underlying NRHypo neurotoxicity. Injection of either the muscarinic antagonist scopolamine or the non-NMDA antagonist NBQX directly into the cortex prevented NRHypo neurotoxicity. Clonidine, an alpha(2) adrenergic agonist, protected against the neurotoxicity when injected into the basal forebrain. The combined injection of muscarinic and non-NMDA Glu agonists reproduced the neurotoxic reaction. Based on these and other results, we conclude that the mechanism is indirect, and involves a complex network disturbance, whereby blockade of NMDA receptors on inhibitory neurons in multiple subcortical brain regions, disinhibits glutamatergic and cholinergic projections to the cerebral cortex. Simultaneous excitotoxic stimulation of muscarinic (m(3)) and glutamate (AMPA/kainate) receptors on cerebrocortical neurons appears to be the proximal mechanism by which the neurotoxic and psychotomimetic effects of NRHypo are mediated.