Pharmacological profile of a potent, efficacious fentanyl derivative in rhesus monkeys

The recent synthesis of fentanyl derivatives, some of which appear to have novel profiles of pharmacological effects, has provided compelling evidence that mu opioid efficacy might be altered systematically by modifications in the parent compound fentanyl. In the present study a new 4-(heteroanilido)-piperidine, compound 28, was studied for its effects in rhesus monkeys. In self-administration studies compound 28 maintained rates of lever pressing similar to those maintained by alfentanil; the reinforcing effects of compound 28 were attenuated by the opioid antagonist quadazocine. In drug discrimination studies compound 28 did not substitute for the kappa agonist ethylketocyclazocine and did substitute for the mu agonist alfentanil. In morphine-treated subjects discriminating between saline and naltrexone, compound 28 did not substitute for naltrexone; however, in morphine-abstinent subjects compound 28 reversed naltrexone lever responding. Moreover, this discriminative stimulus effect in morphine-abstinent subjects was antagonized by naltrexone and by quadazocine in a manner consistent with mu receptor mediation. Compound 28 also was an effective analgesic in a warm-water, tail-withdrawal procedure and it decreased markedly respiratory function. The analgesic effects as well as the respiratory depressant effects of compound 28 were antagonized by quadazocine. Together, these results show compound 28 to be a potent, efficacious mu agonist of similar potency to alfentanil. Large differences in apparent efficacy at mu receptors between compound 28 and another compound in this series (mirfentanil), clearly demonstrate that, within this chemical family, small chemical changes can confer significant differences in pharmacologic effect.

Effect of metaphit on dopaminergic neurotransmission in rat striatal slices: involvement of the dopamine transporter and voltage-dependent sodium channel

Metaphit, an isothiocyanate analog of phencyclidine (PCP), increased the basal release of radioactivity (outflow) from perfused rat striatal slices preloaded with [3H]dopamine above levels observed with the dopamine uptake blocker nomifensin. Preperfusing the slices with metaphit, followed by its removal, attenuated the amphetamine- or dopamine-induced outflow. In slices prepared from reserpine-pretreated rats, the metaphit (100 microM)-induced outflow was reduced to that observed with 10 microM nomifensin, suggesting a vesicular releasing effect of metaphit in addition to dopamine uptake blockade. Electrically induced overflow of radioactivity from normal slices was stimulated by nomifensin and PCP, and by metaphit at 3 microM; it was unaffected by metaphit at 10 and 25 microM, and inhibited by higher concentrations of metaphit. Evidence that the latter effect is due to blockade of voltage-dependent sodium channels is as follows. First, metaphit, as did PCP, inhibited the binding of [3H]batrachotoxinin A 20-alpha benzoate to rat striatal synaptoneurosomes by increasing its dissociation rate; the effect of PCP, but not that of metaphit, was reversible by washing. Second, metaphit, as did PCP, inhibited veratridine (5 microM)-induced influx of [14C]guanidinium ion into synaptoneurosomes. Third, metaphit inhibited overflow of radioactivity from [3H]dopamine-preloaded slices induced by 2.5 microM veratridine, as did the sodium channel blocker tetrodotoxin.

Selective antagonism by naltrindole of the antinociceptive effects of the delta opioid agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin in the rat

Spinal delta opioid receptors have been proposed to mediate antinociception in the rat on the basis of 1) the efficacy of a small number of agonists; 2) the lack of effect of mu-selective antagonists; and 3) the lack of cross-tolerance with mu-selective agonists. However, direct evidence to support or refute this postulate has not been obtained in the rat due to a lack of suitable delta-selective antagonists. The present study characterized the ability of Naltrindole (NTI, 17-cyclopropylmethyl-6,7-dehydro-4,5 alpha-epoxy-3,14-dihydroxy-6,7-2',3'-indolomorphinan), a recently discovered delta-selective antagonist, to antagonize the antinocieption produced by intrathecal (i.t.) administration of the prototypic delta-selective agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin (DPDPE) or the mu-selective agonists morphine and [D-Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO) in the rat. Intrathecal coadministration of NTI with DPDPE significantly antagonized the increase in tail-flick latency (TFL) and hot-plate latency (HPL) produced by DPDPE. In the absence of NTI, the ED50 values and 95% CL of DPDPE in the tail-flick and hot-plate tests were 2.8 (1.1-4.7) and 19.5 (13.3-33.7) micrograms, respectively. In the presence of 10 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was unchanged and was increased by 2-fold in the hot-plate test to 35.9 (26.2-60.1) micrograms. In the presence of 30 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was increased by 5-fold to 14.5 (8.5-24.9) micrograms and its antinociceptive effect in the hot-plate test was antagonized completely.(ABSTRACT TRUNCATED AT 250 WORDS)

Electroencephalographic characteristics of audiogenic seizures induced in metaphit-treated small rodents

Adult male mice, rats, and guinea pigs were subjected to intense sound stimulation of an electric bell (100 dB, 12 kHz for 60 s) after a single intraperitoneal (i.p.) injection of metaphit (1-(1-(3 isothiocyanatophenyl)-cyclohexyl)piperidine) (50 mg/kg). When the animals were tested 24 h after administration of metaphit, audiogenic seizures were observed. None of the control saline-injected animals had convulsions. EEG recordings demonstrated the appearance of paroxysmal activity and spike-wave complexes in the trace from cortical and hippocampal electrodes, with frequency and amplitude increasing with time. Behaviorally, myoclonic jerks of facial muscles, ears, and neck appeared, but no correlation was noted between EEG and the motor phenomena. Auditory stimulation was necessary to elicit the full-blown sequence of seizure responses consisting of wild running followed by clonic and then tonic extension. At the time of seizures, repetitive high-amplitude spikes and waves appeared in the EEG, followed by profound EEG and behavioral depression. None of the animals died during or immediately after seizures. The seizure response to sound stimulation of mice, rats, and guinea pigs was phenomenologically similar, with minor differences in quantitative pattern of convulsive components, which suggests that all three animal species share the common property of extreme susceptibility to audiogenic stimulation caused by metaphit administration.

Anticonvulsant activity of the low-affinity uncompetitive N-methyl-D- aspartate antagonist (+-)-5-aminocarbonyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine (ADCI): comparison with the structural analogs dizocilpine (MK-801) and carbamazepine

(+-)-5-Aminocarbonyl-10,11-dihydro-5H-dibenzo[a,d] [a,d]cyclohepten-5,10-imine (ADCI), a tricyclic compound structurally related to dizocilpine (MK-801) and carbamazepine, was a potent anticonvulsant in the mouse maximal electroshock seizure test when administered i.p. (ED50, 8.9 mg/kg) or p.o. (ED50, 23.5 mg/kg), but failed to cause motor impairment except at substantially higher doses (TD50 values, 49.2 mg/kg i.p. and 293 mg/kg p.o.). ADCI was also protective against chemically induced seizures in mice, including those produced by 4-aminopyridine (ED50, 7.1 mg/kg s.c.) and pentylenetetrazol (ED50, 37.4 mg/kg s.c.). In addition, ADCI antagonized the behavioral effects and lethality of s.c. administered N-methyl-D-aspartate (NMDA: ED50, 15.2 mg/kg), but was a weaker antagonist of kainate-induced clonic seizures (ED50, 33.0 mg/kg), indicating that the drug is a selective functional NMDA antagonist. In common with other NMDA antagonists, ADCI retarded the development of amygdaloid kindled seizures in rats, but failed to attenuate the afterdischarge duration in fully kindled animals. Whole cell voltage clamp recordings from cultured hippocampal neurons demonstrated that ADCI selectively blocks inward current responses to NMDA in a use-dependent fashion without affecting responses to kainate or quisqualate, indicating that ADCI is a selective open channel (uncompetitive) blocker of the NMDA receptor-ionophore complex. ADCI blocked NMDA-evoked inward current responses with a potency (IC50, 14 microM) similar to that with which it displaces [3H]-1-[1-(2-thienyl)-cyclohexyl]piperidine from binding to NMDA receptor channels in rat brain homogenates (IC50, 11.3 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

GBR12909 antagonizes the ability of cocaine to elevate extracellular levels of dopamine

Rats were administered various IP doses of the high-affinity dopamine (DA) reuptake inhibitor 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine (GBR12909). The caudate nuclei were removed 60 min after drug administration and stored at -70 degrees C. Striatal membranes were prepared later. The results demonstrated that GBR12909 produced a dose-dependent decrease in the binding of [3H]cocaine or [3H]GBR12935 to the DA transporter (ED50 about 10 mg/kg). Saturation binding studies with [3H]GBR12935 showed that this was due to both an increase in the Kd, due to residual drug, and to a decrease in the Bmax. At a dose of 25 mg/kg IP, GBR12909 produced a 50% decrease in the Bmax, and a 3.4-fold increase in the Kd. In the in vivo microdialysis studies, GBR12909 (25 mg/kg IP) produced a modest, long-lasting and stable elevation of extracellular DA. Administration of cocaine through the microdialysis probe to rats pretreated with either saline or GBR12909 (25 mg/kg IP) produced a dose-dependent increase in extracellular DA in both groups. GBR12909 inhibited cocaine-induced increases in extracellular DA by about 50% at all doses. These data collectively indicate that at a dose sufficient to decrease by 50% the Bmax of [3H]GBR12935 binding sites, GBR12909 antagonizes the ability of cocaine to elevate extracellular DA by 50%. Further studies will be needed to evaluate a possible role for GBR12909 in the medical treatment of cocaine addiction.

Phencyclidine increased release of beta-endorphin from anterior lobe of the pituitary

Phencyclidine (PCP) has been found to affect neuroendocrine function by altering the release of the anterior pituitary hormones, adrenocorticotrophin, luteinizing hormone and prolactin. The purpose of this study was to examine the effect of PCP on release of the two pituitary hormones also derived from the adrenocorticotropin precursor, namely, alpha-melanocyte-stimulating hormone and beta-endorphin (beta-E), synthesized in the neurointermediate and anterior lobes of the pituitary. At behaviorally active doses, PCP administered i.c.v. increased plasma levels of immunoreactive beta-E (i beta-E) without affecting the concentration of immunoreactive alpha-melanocyte-stimulating hormone, suggesting that PCP increased the release of beta-E from only the anterior lobe of the pituitary. Dexamethasone pretreatment blocked the PCP-induced increase in i beta-E which indicated further the anterior lobe effects of PCP. MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate), a selective PCP ligand, at behaviorally active doses also increased the plasma concentration of i beta-E. The dose-response curves for induction of behavior was very different from that for increasing the concentration of i beta-E in plasma. The increase in release of i beta-E was stereoselective as (+)-(1-(1-phenylcyclohexyl)-3 methylpiperidine but not (-)-(1-(1-phenylcyclohexyl)-3 methylpiperidine increased release of i beta-E. The increase in plasma levels of beta-E was not due to an interaction with opioid receptors because naloxone did not block PCP-induced release of beta-E. In vitro, PCP also significantly increased release of i beta-E from anterior lobe of the pituitary.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of drugs that bind to PCP and sigma receptors on punished responding

Several arylcyclohexylamines and opioid benzomorphans that bind to phencyclidine (PCP) receptors were studied for their effects on punished and unpunished responding maintained under fixed-interval schedules of food presentation. All of these drugs increased both punished and unpunished responding, although higher doses decreased responding. The order of potency for increasing punished responding was MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzoa(a,d)-cyclohepten-5,1 0-imine] greater than [1-[1-(2-thienyl)cyclohexyl]piperidine] = PCP greater than (+)-N-allylnormetazocine = (-)-N-allynormetazocine. There was a high correlation (0.95) between the relative potency of these drugs in increasing punished responding and their relative affinity for PCP receptors. Because some of these drugs also bind to sigma receptors, drugs with a high affinity for sigma receptors, such as haloperidol, BD 737 [1S,2R-(-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N- methyl-2-(1-pyrrolidinyl) cyclohexylamine] and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, were also studied for their effects on punished and unpunished responding. These drugs produced only rate-decreasing effects. The correlation between the relative potency of drugs in increasing punished responding and their relative affinity for sigma receptors was low (-0.19). These data suggest that the PCP receptor is involved in some drug-induced increases in punished responding.

[3H]1-[2-(2-thienyl)cyclohexyl]piperidine labels two high-affinity binding sites in human cortex: further evidence for phencyclidine binding sites associated with the biogenic amine reuptake complex

Previous work demonstrated two high-affinity PCP binding sites in guinea pig brain labeled by [3H]TCP (1-(1-[2-thienyl]cyclohexyl)piperidine): site 1 (N-methyl-D-aspartate [NMDA]-associated) and site 2 (dopamine-reuptake complex associated). The present study examined brain membranes prepared from various species, including human, for the presence of site 2, defined as binding in the presence of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d]cyclohepten-5,10-imine maleate ((+)-MK801) minus binding in the presence of 10 microM TCP (nonspecific binding). Studies were conducted in absence of sodium which was found to be inhibitory to [3H]TCP binding. The results demonstrated detectable levels of site 2 in brain membranes of guinea pig, rabbit, pig, mouse, sheep, and human but not in the rat or chicken. Using human cortical membranes, site 2 was the predominant binding site. Detailed studies conducted with human cortical tissue showed that high-affinity dopamine (1-[2- [bis(4-fluorophenyl)-methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909)], [1,2]benzo(b)thiophenylcyclo-hexylpiperidine (BTCP), and serotonin (fluoxetine) uptake inhibitors produced a wash-resistant inhibition of [3H]TCP binding to site 2, but not site 1. Preincubation of guinea pig brain membranes with BTCP was shown to produce an increase in the dissociation rate of [3H]TCP from PCP site 2. Structure activity studies with various uptake inhibitors showed that GBR12909, benztropine, fluoxetine, and BTCP have higher affinity for site 2 than for site 1. (+)-MK801, ketamine, and tiletamine were very selective for site 1, whereas dexoxadrol and TCP were moderately selective for site 1. These results suggest that human cortex possesses high-affinity PCP binding sites associated with biogenic reuptake binding sites, and that guinea pig brain, but not rat brain, may be an appropriate animal model for studying PCP site 2 in human brain.

In vivo study of NMDA-sensitive glutamate receptor by fluorothienylcyclohexylpiperidine [correction of fluorothienylcycloexylpiperidine], a possible ligand for positron emission tomography

As a preliminary to positron emission tomography (PET) studies of excitatory amino acid neurotransmission, N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors of mice and rats were labelled in vivo with [3H]fluorothienylcyclohexylpiperidine [corrected] (FTCP), which binds to the phencyclidine site of the NMDA receptor. After intravenous injection, the half-life of clearance of authentic FTCP from blood was 4.2 min in mice, 12 min in rats and 45 min in a rhesus monkey. In rodent brain, the specific binding of [3H]FTCP, 10 min after intravenous injection, was 10-20% of the total binding and no regional differences were observed. However, if animals were treated with NMDA intraperitoneally (0.68 mmol/kg), 10 min before injection of [3H]FTCP, a three- to five-fold increase in specific binding was observed in hippocampus, cerebral cortex and striatum but not in cerebellum. Thus, specific binding of [3H]FTCP in vivo revealed the physiological status of the NMDA receptor; in fact, preliminary PET studies with [18F]FTCP in monkeys indicated increased binding after activation of NMDA receptors. These data suggest that PET with [18F]FTCP can be a tool to evaluate physiological or pathological modifications of the function of NMDA receptors.

Mirfentanil: pharmacological profile of a novel fentanyl derivative with opioid and nonopioid effects

Mirfentanil [N-(2-pyrazinyl)-N-(1-phenethyl-4-piperidinyl)-2-furamide] was studied for its binding affinity in isolated neuronal membranes, and for its effects in vivo. In binding to opioid receptors in monkey brain membranes, mirfentanil was much more selective for mu sites (7.99 nM) than for either kappa (1428 nM) or delta (480 nM) sites as measured by displacement of [3H]DAMGO. [3H]U-69.593 or [3H]DPDPE, respectively. In morphine-treated pigeons discriminating among naltrexone, saline and morphine, mirfentanil failed to substitute for either training drug; in morphine-abstinent pigeons, mirfentanil reversed responding on the naltrexone key (i.e., reversed withdrawal). In morphine-treated monkeys discriminating between saline and naltrexone, mirfentanil substituted completely for naltrexone, and this effect was attenuated by an acute injection of morphine; mirfentanil also attenuated the withdrawal-reversing effects of alfentanil in morphine-abstinent monkeys. Administered i.v., mirfentanil maintained rates of self-administration responding only slightly below rates maintained by alfentanil, and this effect of mirfentanil was antagonized by quadazocine. Small doses of mirfentanil (0.032-0.32 mg/kg) antagonized the analgesic effects of alfentanil; larger doses of mirfentanil both antagonized the analgesic effects of alfentanil and produced analgesic effects when administered alone. The analgesic effects of mirfentanil were not attenuated by large doses of opioid antagonists. Mirfentanil had modest respiratory depressant effects that were not altered by quadazocine; however, mirfentanil antagonized the respiratory depressant effects of large doses of alfentanil. Both in vivo and in vitro, mirfentanil appears to have selectivity for opioid mu receptors. Moreover, at doses larger than those which exert opioid effects, mirfentanil has nonopioid analgesic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional brain measurement of Bmax and KD with the opiate antagonist cyclofoxy: equilibrium studies in the conscious rat

Brain distribution of the opiate receptor antagonist, cyclofoxy (CF), was evaluated at equilibrium in rats. A combination of i.v. injection and constant i.v. infusion was used to administer CF over a wide dose range (2.4-450 nmol/rat). Kinetic simulations and experimental results showed that this administration schedule accomplishes "true" tissue-blood equilibrium of CF within 60 min. To estimate the receptor-ligand binding parameters, we assumed that the CF concentration at the receptor site is identical to that in plasma water at equilibrium, and can be calculated from measured blood data after corrections for radiolabeled metabolites and plasma protein binding. This assumption was supported by CSF and plasma water measurements at equilibrium. Regional KD, Bmax, and a nonspecific tissue binding equilibrium constant (Keq) were estimated by fitting the tissue and plasma water concentrations to a single receptor model; the estimated values were 1.4-2.9 nM, 15-74 pmol/g of tissue, and 5.2-8.0, respectively. They are in good agreement with previous in vitro measurements (Rothman and McLean, 1988) as well as in vivo estimates from i.v. injection experiments (Sawada et al., 1990c). The conventional method to estimate the receptor-ligand binding parameters using data from cerebellum to approximate nonspecific tissue binding was found to be unacceptable. Although cerebellum is a brain region with no opiate receptors in rats, small differences in nonspecific tissue binding in different brain regions resulted in significant overestimations of KD and Bmax with this method. Receptor-active and -inactive enantiomers [[18F](-)-CF and [3H](+)-CF)] were simultaneously administered to the same animal and the receptor-bound CF concentration could be accurately measured; this method was used to estimate Bmax from a single study in a single animal and has potential for direct application in human studies using positron emission tomography.

Differential antagonism of U69,593- and bremazocine-induced antinociception by (-)-UPHIT: evidence of kappa opioid receptor multiplicity in mice

The effect of pretreatment with the kappa receptor nonequilibrium antagonist, (-)-UPHIT (1S,2S-trans-2-isothiocyanato-4,5-dichloro-N-methyl-N-[2-(1-pyrrol idinyl) cyclohexyl]benzeneacetamide), on U69,593 [(5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1-oxaspiro(4,5) dec-8-yl)benzeneacetamide]- and bremazocine-induced antinociception was examined in mice. Both U69,593 and bremazocine produced antinociception in the warm water tail-flick test after i.c.v. administration. Pretreatment with the kappa antagonist, nor-binaltorphimine, at doses shown not to affect [D-Ala2, NMePhe4, Gly-ol]enkephalin- (mu-agonist) or [D-Pen2, D-Pen5]enkephalin (delta-agonist)-induced antinociception, significantly attenuated the effects of U69,593 and bremazocine, suggesting actions of these agonists at kappa receptors. Furthermore, beta-funaltrxamine (mu antagonist) and ICI 174,864 [N,N,-diallyl-Tyr-(alpha-aminoisobutyric acid)2-Phe-Leu-OH] (delta antagonist), had no effect on U69,593 or bremazocine in this test providing further evidence of kappa receptor-mediated activity. Pretreatment with (-)-UPHIT produced no effect alone and a long-lasting (up to 48 hr) antagonism of U69,593, but not bremazocine, antinociception. The antagonist actions of (-)-UPHIT did not alter the antinociceptive effects of [D-Ala2, NMePhe4, Gly-ol]enkephalin or [D-Pen2, D-Pen5]enkephalin. These data suggest that (-)-UPHIT is a selective, long-lasting kappa antagonist which can differentially antagonize the antinociception produced by these two kappa agonists. These data provide evidence in vivo supportive of kappa receptor subtypes in the mouse, and suggest that (-)-UPHIT may be a useful probe for the exploration of kappa receptor heterogeneity.

Interaction of beta-funaltrexamine with [3H]cycloFOXY binding in rat brain: further evidence that beta-FNA alkylates the opioid receptor complex

beta-Funaltrexamine (beta-FNA) is an alkylating derivative of naltrexone. In addition to acting as an irreversible inhibitor of mu-receptor-mediated physiological effects, intracerebroventricular (i.c.v.) administration of beta-FNA to rat attenuates the ability of selective delta receptor antagonists and naloxone to reverse delta receptor-mediated effects. Moreover, recent work demonstrated that i.c.v. administration of beta-FNA alters the conformation of the opioid receptor complex, as inferred by a decrease in the Bmax of the lower affinity [3H][D-ala2,D-leu5]enkephalin binding site. Consistent with the decreased potency of naloxone as an inhibitor of delta receptor mediated effects, beta-FNA doubled the naloxone IC50 for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site. These data collectively support the hypothesis that the opioid receptor complex postulated to mediate mu-delta interactions in vivo is identical to the opioid receptor complex as defined by vitro ligand binding studies. A direct prediction of this hypothesis is that beta-FNA should increase the Kd of antagonists for the mu binding site (mu cx) of the receptor complex. The data reported in this paper demonstrate that beta-FNA doubled the IC50 of the potent narcotic antagonist, 6-desoxy-6 beta-fluoronaltrexone (cycloFOXY) for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site, and doubled the Kd of [3H]cycloFOXY for its mu binding site, providing additional data that the mu binding site labeled by [3H]cycloFOXY is the mu binding site of the opioid receptor complex. beta-FNA also altered the kappa binding site labeled by [3H]cycloFOXY, and when administered intrathecally to mice, beta-FNA produced a longlasting antinociception in the acetic acid writhing test.

Novel site-directed affinity ligands for GABA-gated chloride channels: synthesis, characterization, and molecular modeling of 1-(isothiocyanatophenyl)-4-tert-butyl-2,6,7-trioxabicyclo[2.2.2]octanes

p-, m-, and o-isothiocyanate derivatives (1-3, respectively) of tert-butylbicycloorthobenzoate (TBOB) were synthesized from 3-tert-butyloxetane-3-methanol (4) as the starting material. While 2 was readily obtained in four steps via catalytic hydrogenation of the m-nitro-tert-butylbicycloorthobenzoate (9) intermediate, 1 and 3 could not be obtained this way. 1 and 3 were instead synthesized by an alternative four-step approach while made use of the stability of the isothiocyanate moiety to strong Lewis acids such as boron trifluoride etherate, conditions that would isomerize isothiocyanato oxetane ester intermediates to their corresponding orthoesters. The p-isothiocyanate derivative of TBOB, compound 1, inhibited [35S]-tert-butylbicyclophosphorothionate (TBPS) binding to rat cortical membranes with a potency (IC50 62 nM) comparable to the parent compound while 2 and 3 were approximately 10-fold less potent (IC50 values 570 and 609 nM, respectively). Preincubating tissue with radioligand further reduced the potencies of 2 and 3 by approximately 1 order of magnitude (IC50 values 5400 and 7500 nM, respectively) while the potency of 1 (IC50 90 nM) was only marginally affected by this procedure. Pretreatment of membranes with 1 and 2 followed by extensive washing resulted in a concentration-dependent inhibition of [35S]TBPS binding. In contrast, preincubating tissues with up to 2.4 microM of 3 did not elicit an apparent acylation of [35S]TBPS binding sites. Molecular modeling of the effective diameters of 1-3 in their thermodynamically most stable conformations indicates a relationship between these diameters and their relative efficacies as site-directed acylators; the smaller the diameter, the more potent the acylator. This hypothesis explains both the relative potencies of these compounds and their differential abilities to acylate the TBPS binding site.

(+)-cis-3-methylfentanyl and its analogs bind pseudoirreversibly to the mu opioid binding site: evidence for pseudoallosteric modulation

Previous studies demonstrated that preincubation of membranes from the brain of the rat with 1 microM (+)-cis-3-methylfentanyl produced a wash-resistant inhibition of mu receptor binding. The present study was designed to: (1) determine the mechanism by which (+)-cis-3-methylfentanyl produced a wash-resistant inhibition of mu receptor binding, and (2) to generate a structure-activity study, using wash-resistant inhibition as the end-point. Pretreatment of membranes with 500 nM (+)-cis-3-methylfentanyl increased the Kd of binding sites for [3H]ohmefentanyl, without altering the Bmax. The increase in the Kd was only partially due to the presence of residual drug and was accompanied by an increase in the dissociation rate of the binding of [3H]ohmefentanyl. Therefore, pretreatment of membranes with (+)-cis-3-methylfentanyl resulted in a lower affinity interaction of [3H]ohmefentanyl with the mu binding site, consistent with a model postulating pseudoallosteric modulation of mu binding sites by (+)-cis-3-methylfentanyl and its analogs. The rank order of potencies for wash-resistant inhibition of the binding of [3H]6 beta-fluoro-6-desoxyoxymorphone or [3H]ohmefentanyl, was lofentanil greater than (+)-cis-3-methylfentanyl greater than ohmefentanyl greater than sufentanil. All other opioids tested (1 microM morphine, 1 microM naloxone, 1 microM fentanyl, 1 microM (+)-cyclazocine, 1 microM (-)-cis-3-methylfentanyl) did not act as wash-resistant inhibitors of mu binding sites. Although the pseudoirreversible IC50 of these agents did not correlate with their ED50 values for producing antinociception, after intravenous administration, the authors speculate that this property, termed "pseudoirreversible inhibition", might contribute to the extraordinary potency of (+)-cis-3-methylfentanyl and its analogs as antinociceptive agents.

Evaluation of U-50,488H analogs for neuroprotective activity in the gerbil

U-50,488H, a kappa (kappa) opioid ligand with moderate potency at sigma (sigma) receptors, protects against mechanical and ischemia-induced injury. The purpose of this study was to evaluate the possibility that sigma-receptors may be involved in mediating the neuroprotective actions of U-50,488H. This possibility was examined by testing the potential of a series of U-50,488H analogs, which are potent sigma-ligands with minimal activity at kappa-opioid receptors, to protect against ischemia-induced neuronal damage in the gerbil. Like U-50,488H, BD-449 (20 mg/kg), the cis-diastereomer of U-50,4888H, protected against ischemia-induced neuronal damage as did BD-737 (50 and 30 mg/kg) and BD-738 (50 mg/kg). All 3 compounds interacted selectively with sigma-receptors. In contrast, BD-601 (50 mg/kg), did not protect against ischemia-induced neuronal damage, although it also interacted potently with sigma-receptors. One difference between the compounds that were neuroprotective and BD-601 is that only BD-601 produced sigma-like behavioral effects in the rat. Thus, it is possible that BD-601 may interact differently or at a different sigma-subtype than BD-449, BD-737 and BD-738 with sigma-receptors. However, these results clearly indicate that an interaction with kappa-opioid receptors is not required for anti-ischemic activity, and that sigma-receptors may play a role in neuroprotection.

Photoaffinity labeling of the dopamine reuptake carrier protein with 3-azido[3H]GBR-12935

A high affinity tritiated azido-diphenylpiperazine derivative, 3-azido[3H]GBR-12935, was synthesized as a potential photoaffinity probe of the dopamine transporter. Initially, the reversible binding of 3-azido[3H]GBR-12935 to crude synaptosomal membranes from the rat striatum was characterized. Specific binding was sodium dependent and inhibited by a variety of drugs that are known to potently inhibit dopamine uptake. Other neurotransmitter uptake inhibitors, as well as cis-flupenthixol, a potent inhibitor of [3H]GBR-12935 binding to piperazine binding sites, failed to inhibit specific binding at concentrations of less than or equal to 10 microM. A good correlation was observed between the relative potencies of these drugs in inhibiting dopamine uptake into synaptosomes and in inhibiting specific 3-azido[3H]GBR-12935 binding to rat striatal membranes (r = 0.95, p less than 0.01). These data suggest that 3-azido[3H]GBR-12935, like other diphenylpiperazines such as [3H]GBR-12935 and [3H]GBR-12909, binds primarily to the dopamine transporter under defined assay conditions. After UV photolysis of crude synaptosomal membranes preincubated with 3-azido[3H]GBR-12935 (1-2 nM), a single radiolabeled polypeptide with an apparent molecular mass of 80 kDa was observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Photoincorporation of 3-azido[3H]GBR-12935 into this polypeptide was inhibited selectively by compounds that inhibit the uptake of dopamine (but not other biogenic amines) and was completely dependent on the presence of Na+. No photolabeled proteins were observed when cerebellar membranes were substituted for striatal membranes. Essentially complete adsorption of the radiolabeled 80-kDa polypeptide to wheat germ agglutinin and elution with N-acetyl-D-glucosamine strongly suggest that the dopamine transporter polypeptide photolabeled by 3-azido[3H]GBR-12935 is glycosylated.

The partial opioid agonist, buprenorphine, protects against lethal effects of cocaine

Buprenorphine (0.3-3.0 mg/kg) produced dose-dependent protection against the lethal effects of cocaine in mice. The (+)-enantiomer of buprenorphine did not protect up to doses over 100 times greater than the lowest effective dose of its (-)-enantiomer. The protective effects were also produced by the opioid agonists morphine and methadone, but not by the opioid antagonist, naltrexone. Low doses of naltrexone (0.3-1.0 mg/kg) blocked the protective effects of buprenorphine. Protection conferred by buprenorphine was not observed in CXBK mice, a recombinant inbred strain relatively devoid of mu-opioid receptors. Thus, buprenorphine appears to protect against the lethal effects of cocaine by a process mediated by mu-opioid receptors. The present results should provide some additional safety assurance in future clinical trials with buprenorphine, especially in outpatient trials where cocaine abuse may continue along with treatment.

Kinetic analysis of transport and opioid receptor binding of [3H](-)-cyclofoxy in rat brain in vivo: implications for human studies

[3H]Cyclofoxy (CF: 17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta-fluoromorp hinan) is an opioid antagonist with affinity to both mu and kappa subtypes that was synthesized for quantitative evaluation of opioid receptor binding in vivo. Two sets of experiments in rats were analyzed. The first involved determining the metabolite-corrected blood concentration and tissue distribution of CF in brain 1 to 60 min after i.v. bolus injection. The second involved measuring brain washout for 15 to 120 s following intracarotid artery injection of CF. A physiologically based model (Sawada et al., 1990a) and a classical compartmental pharmacokinetic model (Wong et al., 1986a) were compared. The models included different assumptions for transport across the blood-brain barrier (BBB); estimates of nonspecific tissue binding and specific binding to a single opiate receptor site were found to be essentially the same with both models. The nonspecific binding equilibrium constant varied modestly in different brain structures (Keq = 3-9), whereas the binding potential (BP) varied over a much broader range (BP = 0.6-32). In vivo estimates of the opioid receptor dissociation constant were similar for different brain structures (KD = 2.1-5.2 nM), whereas the apparent receptor density (Bmax) varied between 1 (cerebellum) and 78 (thalamus) pmol/g of brain. The receptor dissociation rate constants in cerebrum (k4 = 0.08-0.16 min-1; koff = 0.16-0.23 min-1) and brain vascular permeability (PS = 1.3-3.4 ml/min/g) are sufficiently high to achieve equilibrium conditions within a reasonable period of time. Graphical analysis (Patlak and Blasberg, 1985) of the data is inappropriate due to the high tissue-loss rate constant (kb = 0.03-0.07 min-1) for CF in brain. From these findings, CF should be a very useful opioid receptor ligand for the estimation of the receptor binding parameters in human subjects using [18F]CF and positron emission tomography.

Probing the opioid receptor complex with (+)-trans-superfit. I. Evidence that [D-Pen2,D-Pen5]enkephalin interacts with high affinity at the delta cx binding site

A variety of data support the existence of an opioid receptor complex composed of distinct but interacting mu cx and delta cx binding sites, where "cx" indicates "in the complex." The ability of subantinociceptive doses of [Leu5]enkephalin and [Met5]enkephalin to potentiate and attenuate morphine-induced antinociception, respectively, is thought to be mediated via their binding to the delta cx binding site. [D-Pen2,D-Pen5]Enkephalin also modulates morphine-induced antinociception, but has very low affinity for the delta cx binding site in vitro. In the present study, membranes were depleted of their delta ncx binding sites by pretreatment with the site-directed acylating agent, (3S,4S)-(+)-trans-N-[1-[2-(4-isothiocyanato)phenyl)-ethyl]-3-methy l-4- piperidyl]-N-phenylpropaneamide hydrochloride, which permits selective labeling of the delta cx binding site with [3H][D-Ala2,D-Leu5]enkephalin. The major findings of this study are that with this preparation of rat brain membranes: a) there are striking differences between the delta cx and mu binding sites; and b) both [D-Pen2,D-Pen5]enkephalin and [D-Pen2,L-Pen5]enkephalin exhibit high affinity for the delta cx binding site.

Site-specific acylation of GABA-gated Cl- channels: effects on 36Cl- uptake

Radioligand binding studies indicate that p-isothiocyanato-t-butylbicycloorthobenzoate (p-NCS-TBOB) specifically acylates GABA-gated chloride channels. Preincubation of synaptoneurosomes with p-NCS-TBOB followed by washing resulted in a concentration dependent (63-500 nM) inhibition of both muscimol-stimulated chloride uptake and [355]t-butylbicyclophosphorothionate (TBPS) binding. The extent of acylation (assessed by inhibition of [35S]TBPS binding) was highly correlated (r = 0.89; p less than 0.001) with the inhibition of muscimol-stimulated Cl- uptake. Neither basal Cl- uptake nor [3H]muscimol binding to GABAA receptors were affected by p-NCS-TBOB. Preincubation with the nonacylating 'cage' convulsant t-butylbicycloorthobenzoate (500 nM) followed by washing had no effect on either muscimol-stimulated Cl- uptake or [35S]TBPS binding. These findings indicate that p-NCS-TBOB interferes with the efficacy of muscimol promoted channel openings, but does not affect the recognition qualities of GABAA receptors. p-NCS-TBOB should prove useful in electrophysiological and biochemical studies examining the properties of GABA-gated Cl- channels.

Characterization and localization of cannabinoid receptors in rat brain: a quantitative in vitro autoradiographic study

A potent, synthetic cannabinoid was radiolabeled and used to characterize and precisely localize cannabinoid receptors in slide-mounted sections of rat brain and pituitary. Assay conditions for 3H-CP55,940 binding in Tris-HCl buffer with 5% BSA were optimized, association and dissociation rate constants determined, and the equilibrium dissociation constant (Kd) calculated (21 nM by liquid scintillation counting, 5.2 nM by quantitative autoradiography). The results of competition studies, using several synthetic cannabinoids, add to prior data showing enantioselectivity of binding and correlation of in vitro potencies with potencies in biological assays of cannabinoid actions. Inhibition of binding by guanine nucleotides was selective and profound: Nonhydrolyzable analogs of GTP and GDP inhibited binding by greater than 90%, and GMP and the nonhydrolyzable ATP analog showed no inhibition. Autoradiography showed great heterogeneity of binding in patterns of labeling that closely conform to cytoarchitectural and functional domains. Very dense 3H-CP55,940 binding is localized to the basal ganglia (lateral caudate-putamen, globus pallidus, entopeduncular nucleus, substantia nigra pars reticulata), cerebellar molecular layer, innermost layers of the olfactory bulb, and portions of the hippocampal formation (CA3 and dentate gyrus molecular layer). Moderately dense binding is found throughout the remaining forebrain. Sparse binding characterizes the brain stem and spinal cord. Densitometry confirmed the quantitative heterogeneity of cannabinoid receptors (10 nM 3H-CP55,940 binding ranged in density from 6.3 pmol/mg protein in the substantia nigra pars reticulata to 0.15 pmol/mg protein in the anterior lobe of the pituitary). The results suggest that the presently characterized cannabinoid receptor mediates physiological and behavioral effects of natural and synthetic cannabinoids, because it is strongly coupled to guanine nucleotide regulatory proteins and is discretely localized to cortical, basal ganglia, and cerebellar structures involved with cognition and movement.

Labeling by [3H]1,3-di(2-tolyl)guanidine of two high affinity binding sites in guinea pig brain: evidence for allosteric regulation by calcium channel antagonists and pseudoallosteric modulation by sigma ligands

Equilibrium binding studies with the sigma receptor ligand [3H]1,3-di(2-tolyl)guanidine ([3H]DTG) demonstrated two high affinity binding sites in membranes prepared from guinea pig brain. The apparent Kd values of DTG for sites 1 and 2 were 11.9 and 37.6 nM, respectively. The corresponding Bmax values were 1045 and 1423 fmol/mg of protein. Site 1 had high affinity for (+)-pentazocine, haloperidol, (R)-(+)-PPP, carbepentane, and other sigma ligands, suggesting a similarity with the dextromethorphan/sigma 1 binding site described by Musacchio et al. [Life Sci. 45:1721-1732 (1989)]. Site 2 had high affinity for DTG and haloperidol (Ki = 36.1 nM) and low affinity for most other sigma ligands. Kinetic experiments demonstrated that [3H]DTG dissociated in a biphasic manner from both site 1 and site 2. DTG and haloperidol increased the dissociation rate of [3H]DTG from site 1 and site 2, demonstrating the presence of pseudoallosteric interactions. Inorganic calcium channel blockers such as Cd2+ selectively increased the dissociation rate of [3H]DTG from site 2, suggesting an association of this binding site with calcium channels.