Discriminative stimulus properties of β-carbolines characterized as agonists and inverse agonists at central benzodiazepine receptors

The discriminative stimulus properties of three -carboline derivatives were studied in three groups of rats trained, respectively, to discriminate diazepam (2.5 mg/kg IP), chlordiazepoxide (CDP, 5 mg/kg IP) or pentylenetetrazol (PTZ, 15 mg/kg IP) from saline in standard procedures employing two-lever operant chambers. Two -carbolines, ZK 91296 and ZK 93423, substituted for the benzodiazepines in both CDP- and diazepam-trained rats. The neutral benzodiazepine antagonist Ro 15-1788 blocked the diazepam discriminative stimulus and the ability of ZK 91296 to substitute for diazepam. A third -carboline, FG 7142, was not identified as benzodiazepine-like in generalization tests in either diazepam- or CDP-trained rats, but when administered together with CDP antagonized the benzodiazepine discriminative stimulus. In rats trained to discriminate PTZ from saline (a discrimination which is thought to depend on the anxiogenic properties of PTZ) the PTZ cue was antagonized by diazepam and ZK 93423, and partially antagonized by ZK 91296. The PTZ cue generalized to FG 7142 and this generalization was partially antagonized by Ro 15-1788. These results suggest that the three -carbolines provide more than one kind of discriminative stimulus, consistent with the classification of ZK 93423 as an agonist at central benzodiazepine receptors, with ZK 91 296 as a partial agonist, and with FG 7142 as an inverse agonist. Pharmacologically, ZK 93 423 and ZK 91 296 may exhibit anxiolytic qualities, whereas FG 7142 produces anxiogenic effects.     

Chronic treatment with lorazepam and FG 7142 may change the effects of benzodiazepine receptor agonists, antagonists and inverse agonists by different mechanisms

Treatment of mice with lorazepam 10 mg/kg p.o. or FG 7142 40 mg/kg i.p. once a day for 14 days changed the effects of benzodiazepine (BZ) receptor ligands injected acutely on the threshold of pentylenetetrazol (PTZ)-induced seizures. The effects of the two pretreatments differed qualitatively as well as quantitatively. Lorazepam elicited a shift in the effects of all BZ receptor ligands tested, whereby the agonists lorazepam and ZK 93423 now acted like partial agonists given acutely, the partial agonist ZK 91296 acted like an antagonist and the antagonists Ro 15-1788 and ZK 93426 like partial inverse agonists. The proconvulsant effects of the partial inverse agonist FG 7142 and the full inverse agonist DMCM on the PTZ-induced seizures did not change. However, FG 7142 became a full inverse agonist i.e. became convulsant, and DMCM may have increased in potency as a convulsant. After FG 7142 pretreatment lorazepam and ZK 93423 behaved like partial agonists given acutely whereas there was no change in effect for ZK 91296, Ro 15-1788 and ZK 93426. FG 7142 became convulsant (i.e. kindling occurred) and the potency of DMCM as a convulsant was non-significantly increased, while their proconvulsant effects with respect to PTZ-induced seizures were not altered. The fact that the effects of the two very different pretreatments on the BZ receptor ligand continuum were in the same direction may be explainable by assuming two different mechanisms, both of which may involve the GABA receptors.     

Solubilization of a mammalian β-adrenergic receptor

Summary Binding sites for iodohydroxybenzylpindolol with characteristics of a ₂-adrenergic receptor have been identified in a crude membrane fraction from guinea-pig lung. The binding sites could be solubilized by treatment of the membrane fraction with digitonin. Upon solubilization receptors retain their ₂-adrenergic type as indicated by the rank order of potencies of agonists in binding-inhibition experiments. The solubilized receptors demonstrate a marked increase in affinity for agonists compared with particulate receptors whereas antagonist affinity remains unchanged. Solubilized receptors are insensitive to divalent cations (Mg²⁺, Mn²⁺, Ca²⁺) which increase the potency of agonists for particulate receptors. The effects of Mg²⁺ can be reversed by Gpp(NH)p in particulate preparations; Gpp(NH)p is ineffective for the solubilized preparation. These experiments establish that -adrenergic receptors can be solubilized even from crude mammalian membrane preparations. They also show that the mammalian -adrenergic receptor in situ is under constraints with respect to agonist affinity and is modulated by divalent cations and guanyl nucleotides in the intact membrane.with technical assistance of L. Braun and C. KonradThis report is part of a dissertation to be presented by J. K. to the Fachbereich Humanmedizin, Justus Liebig-Universität Giessen, in partial fulfillment of the requirements for a Doctor of Medical Science degree     

Evaluation of the β-carboline ZK 93 426 as a benzodiazepine receptor antagonist

We describe here biochemical and pharmacological effects of the -carboline ZK 93426, a new and potent benzodiazepine (BZ) receptor antagonist. ZK 93426 was compared with Ro 15-1788 and CGS 8216, two compounds previously described as BZ receptor antagonists. Certain effects of ZK 93426, Ro 15-1788 and CGS 8216 were quite similar (e.g., ³H-FNM displacement, GABA ratio, photo-shift). In most pharmacological tests ZK 93426 and Ro 15-1788 lacked overt effects; Ro 15-1788 was a weak agonist in some paradigms, while ZK 93426 exhibited a potent proconflict effect but also a weak anticonvulsant effect. This interesting finding with ZK 93426 suggests that BZ receptor ligands may possess differential efficacy at BZ receptor subtypes. In contrast, CGS 8216 exhibited potent proconvulsant effects in several paradigms in addition to proconflict and pentylenetetrazol generalizing effects. ZK 93426, Ro 15-1788 and CGS 8216 were almost equally potent as antagonists of the effects of BZ receptor agonists, such as diazepam and lorazepam. However, ZK 93426 was the most potent inhibitor of the convulsions produced by the BZ receptor inverse agonist DMCM.     

Adenosine receptor activation by adenine nucleotides requires conversion of the nucleotides to adenosine

Summary Adenine nucleotides cause adenosine receptor-mediated increases in cyclic AMP in the VA13 human fibroblast line. Levels of adenosine accumulated in the medium are insufficient to account for the responses to adenine nucleotides. Since rapid conversion of the nucleotides to adenosine by 5-nucleotidase in the vicinity of the receptor might account for the responses, six experimental methods were developed to distinguish between local conversion and direct action of the nucleotides. Results of all six methods favored local conversion. (1) 5-Nucleotidase inhibitors blocked the accumulations of cyclic AMP elicited by AMP, ADP, and ATP, but did not affect the response to adenosine. The most potent inhibitor of both conversion of AMP and response to AMP was ,-methylene-ADP (APCP). (2) Adenosine deaminase blocked the responses to AMP, ADP, ATP, and adenosine-containing coenzymes. (3) Theophylline, a specific competitive adenosine antagonist, was an insurmountable inhibitor of the increases in cyclic AMP caused by AMP, ADP, and ATP. The insurmountability was presumably due to substrate sataration of the converting enzyme 5-nucleotidase. (4) Although ADP and ATP had partial agonist-like dose-response curves, they did not inhibit the response to adenosine. (5) Nine cell lines which responded to adenosine were tested for response to AMP. Cell lines with high levels of 5-nucleotidase had large responses to AMP, those with intermediate levels of 5-nucleotidase had large or intermediate responses to AMP, and those with low 5-nucleotidase levels did not respond to AMP. (6) Inhibition of the uptake of labelled adenosine was used as an indicator of unlabelled adenosine concentrations near the cell membrane. Unlabelled AMP inhibited uptake nearly as effectively as unlabelled adenosine. APCP reversed the inhibition by AMP but not the inhibition by adenosine.The adenosine receptor is concluded to be an enity distinct from adenine nucleotide receptors.Submitted in partial fulfillment of the requirements for the degree Doctor of Philosophy in Neurosciences, University of California, San Diego. Supported by NIMH DA-00265 and PHS RR 05665. The author has been a NSF Graduate Fellow. An abstract of this material has been published (Bruns 1977)     

β-Carbolines can enhance or antagonize the effects of punishment in mice

Six -carboline ligands at central benzodiazepine (BZ) receptors were tested for their anxiolytic or anxiogenic properties in mice in the four-plate test. ZK 93 423 and ZK 91 296 increased activity which had been suppressed by punishment (1 mA, 60 ms footshock) at doses which exerted no effect on unpunished locomotion. ZK 93 426, ZK 90 886, FG 7142, and DMCM exerted no antipunishment activity themselves, and antagonized the ability of diazepam to increase both punished and unpunished locomotor activity. DMCM, FG 7142, and ZK 90 886, but not ZK 93 426, also enhanced the ability of a reduced level of footshock (0.3 mA) to suppress activity. This propunishment activity of DMCM and ZK 90 886 took place at doses which had no effect on unpunished locomotion. The nature of the effect of the individual -carbolines on punishment was related to the nature of their interaction with the BZ/GABA receptor/chloride channel complex (GBC complex). Thus the antipunishment properties of ZK 93 423 and ZK 91 296 were associated with their ability to increase binding of ³⁵S-t-butylbicyclophosphorothionate (TBPS) to its binding site associated with the chloride channel, whereas DMCM, FG 7142 and ZK 90 886, which exerted propunishment effects, reduced TBPS binding. ZK 93 426, which was neutral with respect to punished activity, had the weakest effect on TBPS binding. These results are discussed in the context of a possible role of the GBC complex in anxiety.     

Withdrawal precipitation by benzodiazepine receptor antagonists in dogs chronically treated with diazepam or the novel anxiolytic and anticonvulsant β-carboline abecarnil

Summary The effects of the benzodiazepine (BZ) receptor antagonists flumazenil (Ro 15-1788) and the -carboline ZK 93426 were compared in dogs before and after chronic treatment with diazepam or the novel BZ receptor ligand abecarnil (ZK 112119). Abecarnil, -carboline, is thought to act as partial (low efficacy) and/or subtype selective agonist at central BZ receptors. Diazepam and abecarnil were administered at doses which, based on previous experiments on anticonvulsant activity, resulted in about equieffective drug concentrations during treatment. In dogs treated with diazepam, 6 mg/kg/day p.o., for 2 weeks, severe abstinence symptoms, including seizures, were precipitated in all animals by i. v. infusion of the BZ receptor antagonists, differences being found in the type of symptoms caused by flumazenil and ZK 93426. In dogs treated with abecarnil, 4 mg/kg/d s. c., for 6 weeks, only relatively mild abstinence symptoms were precipitated by infusion of flumazenil or ZK 93426, although pharmacologically active plasma concentrations of abecarnil had been maintained throughout the period of treatment. This suggests that BZ receptor ligands which act as partial and/or selective agonists might be more favourable than traditional agonists, such as diazepam, regarding the induction of physical dependence. Send offprint requests to W. Löscher at the above address     

A three-state model of the benzodiazepine receptor explains the interactions between the benzodiazepine antagonist Ro 15-1788, benzodiazepine tranquilizers, β-carbolines,and phenobarbitone

Summary The potent benzodiazepine receptor ligands -carboline-3-carboxylic acid ethyl ester (-CCM) and the corresponding methylester (-CCM) administered i.v. depressed segmental dorsal root potentials in spinal cats, reversed the prolongation of dorsal root potentials by phenobarbitone, and abolished the depression of a motor performance task induced by phenobarbitone in mice; -CCE enhanced the low-frequency facilitation of pyramidal population spikes in the hippocampus of anaesthetized rats. These effects of -carbolines reflect a depression of GABAergic synaptic transmission and, thus, are diametrically opposed to the enhancing action of benzodiazepine tranquilizers. The specific benzodiazepine antagonist, Ro 15-1788, while not affecting dorsal root potentials, hippocampal population spikes or phenobarbitone-induced motor performance depression, abolished the effects of -CCE on the three parameters and similar effects of -CCM on the spinal cord and motor performance.A three-state model of the benzodiazepine receptor is proposed in which benzodiazepine tranquilizers act as agonists enhancing the function of the benzodiazepine receptor as a coupling unit between GABA receptor and chloride channel, -carbolines act as inverse agonists reducing this coupling function, and Ro 15-1788 represents a competitive antagonist blocking both the enhancing effect of agonists and the depressant effect of inverse agonists on GABAergic synaptic transmission.     

“Anxiolytic” and “anxiogenic” benzodiazepines and β-carbolines: effects on aggressive and social behavior in rats and squirrel monkeys

Ethopharmacological studies on the behavior of socially housed rats and squirrel monkeys were conducted to explore the role of the benzodiazepine GABA_A-coupled ionophore receptor complex in aggressive and social interactions. Benzodiazepine receptor (BZR) antagonists, ZK 93426 (1–10 mg/kg) and flumazenil (3–10 mg/kg), the partial agonist, ZK 91296 (1–10 mg/kg) and the partial inverse agonists RO 15-4513 (0.3–10 mg/kg), were administered to (1) squirrel monkeys prior to 1 h focal observations within established social groups or to (2) resident male rats before confrontations with a naive male intruder in their home cage for 5 min. Aggression was modified in a similar manner in both species, although squirrel monkeys were more sensitive to BZR challenges. Specifically, resident male rats showed dose dependent reductions in attack bites directed at intruder males that were significant at the highest dose of ZK 93426 (10 mg/kg). In squirrel monkeys, ZK 93426 (3 and 10 mg/kg) reduced aggressive grasps, threats and displays, as well as reducing the duration of being the target of aggression from untreated group members (1–10 mg/kg). The BZR partial agonist, ZK 91296 and the antagonist, flumazenil produced few effects on social behavior, low and high intensity aggression and motor activity in both species. Flumazenil (10–30 mg/kg) and ZK 91296 (10 mg/kg), but not ZK 93426, produced significant increases in foraging and feeding behaviors in squirrel monkeys. The hyperphagic effects of ZK 91296 and flumazenil, that are typical of BZR agonists compounds, were not observed in rats. Similarly, the inverse agonist-like reductions in social interactions produced by ZK 93426 (3–10 mg/kg) were observed only in squirrel monkeys. The partial inverse agonist Ro 15-4513 reduced aggression in rats, but low doses (1 mg/kg) produced tremors or seizures in 80% of the monkeys tested. Decreases in aggressive and social behaviors are often interpreted to reflect anxiogenic drug properties, whereas increased feeding has been associated with anxiolytic actions. The concurrent emergence of these apparent opposites suggests independent actions on social and alimentary functions.     

Typical and atypical neuroleptics are potent antagonists at α1-adrenoceptors of the dorsal lateral geniculate nucleus

Summary The present electrophysiological studies examined the actions of neuroleptics at central ₁adrenoceptors in the rat. In single-cell recording experiments, typical and atypical neuroleptics, when administered systemically or locally (iontophoresis and pressure ejection), were found to be potent antagonists of activating ₁adrenoceptor responses in the dorsal lateral geniculate nucleus (dLGN). Doses of neuroleptics effective as antagonists at ₁adrenoceptors had very weak effects as muscarinic receptors in the dLGN. Since doses of neuroleptics employed in the present study were within the clinical range, it appears likely that central ₁adrenoceptors would be blocked during neuroleptic therapy in humans.     

Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat

Summary Additional experimental evidence was obtained for an inhibitory function of prejunctional ₂-adrenoceptors and/or dopamine receptors located on noradrenergic neurons innervating the heart and resistance vessels of the pithed normotensive rat. Mixed ₂-adrenoceptor receptor agonists, differing in selectivity towards either receptor type, i.e. N,N-di-n-propyldopamine (DPDA), 2-N, N-di-n-propylamino-6, 7-dihydroxy-1,2,3,4-tetrahydronaphthalene (DP-6,7-ADTN), B-HT 920 and B-HT 933 (azepexole) were used.In pithed normotensive rats, DPDA (30 and 100 g/kg/min) dose-dependently inhibited the electrical stimulation-induced increase in diastolic pressure, but did not significantly affect the stimulation-evoked increase in heart rate. The inhibition exerted by DPDA was blocked by haloperidol and sulpiride (0.3 mg/kg of each), but not by yohimbine (1 mg/kg), indicating the involvement of dopamine receptors. In this respect, sulpiride and haloperidol were found approximately equipotent.DP-6,7-ADTN (10 and 30 g/kg/min) impaired both tachycardic and vasoconstrictor responses in a dose-dependent manner. Sulpiride (0.3 mg/kg) only partially restored the DP-6,7-ADTN-depressed stimulation-evoked increase in diastolic pressure, whereas yohimbine (1 mg/kg) alone was without effect. The combination of both antagonists completely prevented the inhibition caused by DP-6,7-ADTN. On the other hand, yohimbine (1 mg/kg), but not sulpiride (0.3 mg/kg), selectively antagonized the DP-6,7-ADTN-induced inhibition of stimulation-evoked tachycardia.B-HT 920 (1, 3 and 10 g/kg/min) very effectively reduced the increase in diastolic pressure and heart rate caused by electrical stimulation. Inhibitory dopamine as well as ₂-adrenoceptors participated in the vascular effects of B-HT 920, whereas ₂-adrenoceptors were only involved in the cardioinhibitory response to this agonist.B-HT 933 (0.6 and 1 mg/kg/min) dose-dependently reduced the stimulation-evoked increase in arterial pressure through selective stimulation of inhibitory ₂-adrenoceptors, dopamine receptors not taking a part.The results confirm and extend the observations that in addition to ₂-adrenoceptors inhibitory dopamine receptors are located on the sympathetic neurons connected with the arterial vasculature of the pithed normotensive rat. The sympathetic nerves innervating the rat heart do not contain inhibitory dopamine receptors; their activity only can be modulated by ₂-adrenoceptor stimulation. In the pithed normotensive rat, activation of prejunctionally located ₂-adrenoceptors more effectively inhibits the sympathetic activity directed to the heart than that to the resistance vessels.     

Benzodiazepine (ω) receptor partial agonists and the acquisition of conditioned fear in mice

It is well established that benzodiazepines can produce anterograde amnesia in humans and interfere with the acquisition of passive avoidance and spatial responses in rodents. However, the extent to which the disruption of learning is a secondary effect of the sedation produced by these drugs has not been clearly established. In order to investigate this question, the effects of several BZ () receptor partial agonists were studied on the acquisition of conditioned fear (passive avoidance learning) in mice. As these drugs have been shown to produce anticonvulsant and anxiolytic-like effects without sedation or depression of motor activity, it was of interest to see whether they could disrupt learning. Clear effects on the acquisition of conditioned fear were produced by imidazenil (0.01–1.0 mg/kg), divaplon (1–60 mg/kg), ZK 91296 (3–60 mg/kg), and Ro 17-1812 (0.1–10 mg/kg). However, bretazenil (0.1–10 mg/kg) did not produce statistically significant effects. Only the high dose of imidazenil (1.0 mg/kg) decreased levels of exploratory behaviour. These results show that BZ () receptor partial agonists without apparent sedative actions can disrupt fear learning, indicating that the effects of this class of drugs on passive avoidance learning can be dissociated from sedation. The reasons for the observed differences between the different compounds studied are unclear at present and may be related to differences in intrinsic activity or receptor subtype selectivity.     

A repeated tests procedure to assess onset and duration of the cue properties of (−)Δ 9-THC, (−)Δ 8-THC-DMH and (+)Δ 8-THC

Rats were trained in a two-lever operant box in a drug discrimination procedure to respond differentially to the effects induced by 3 mg/kg of (-) ⁹-tetrahydrocannabinol and the drug vehicle. Tests with (-) ⁸-THC and the dimethyl-heptyl (DMH) homologue of (-) ⁸-THC indicated that (-) ⁸-THC-DMH was more potent but had a slower onset of action than (-) ⁸-THC. Two ways of testing the onset and duration of action were compared. In one procedure (separate tests) the time course of the drug action was established by testing each time interval on separate days with a new injection each test day, whereas in the other procedure (repeated tests) all intervals were evaluated after a single injection. The results were similar for both procedures. The median time intervals for the decay of the (-) ⁸-THC stimulus were 122 and 127 min for the separate and repeated tests procedures respectively. The median time intervals for the onset of action of the (-) ⁹-THC effects of (-) ⁸-THC were 65 and 62 min for the separate and repeated tests procedures respectively. The median time intervals for the decay of (-) ⁸-THC-DMH (0.30 and 0.56 mg/kg) was between 8 and 24 h after injection. Furthermore, a stereoselective action is indicated, as (+) ⁸-THC (5.6 and 10 mg/kg) did not substitute for (-) ⁹-THC.     

Effects of diazepam and two beta-carbolines on epileptic activity and on EEG and behavior in rats with absence seizures

In the present series of experiments, effects of a full benzodiazepine receptor agonist (diazepam) are described and compared with those of a partial benzodiazepine receptor agonist (ZK 91296) and an inverse partial benzodiazepine receptor agonist (FG 7142), both compounds of the beta-carboline family. In a rat model for generalized absence epilepsy, the anticonvulsant, the hypnotic and the myorelaxant properties were investigated, as well as effects on on-going behavior and effects on the electroencephalogram (EEG). While diazepam showed all behavioral and electrophysiological changes characteristic for the benzodiazepines, the partial agonist ZK 91296 reduced seizure activity without inducing any signs of sedation, sleepiness, myorelaxation and changes in behavior or EEG spectral content. The partial inverse agonist FG 7142 aggrevated epileptic activity, with slightly enhanced immobile behavior, suggesting some anxiogenic properties. The results not only demonstrate that the multiple effects of the benzodiazepines could be separated by these compounds, but also that the anticonvulsant activity is not related to changes in spectral content of the EEG. Because of its selective activity, ZK 91296 appears to be more suitable than diazepam in reducing seizure activity. Finally, FG 7142 seems a genuine partial inverse agonist which has some, but not all, of the inverse effects of a full agonist.     

Antiepileptic action of the beta-carboline ZK 91296 in a genetic petit mal model in rats

The anticonvulsant action of the benzodiazepine (BZ) receptor partial agonist, ethyl 5-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate (ZK 91296) was studied in rats of Wistar origin exhibiting spontaneous bilateral cortical synchronous spike and wave discharges with a symptomatology paralleling that of human petit mal seizures. ZK 91296 1-16 mg/kg i.p.) attenuated the absence seizures without inducing signs of sedation or disorganized EEG patterns at any dose. Diazepam (1-8 mg/kg i.p.) suppressed seizures but also induced sedation and modified EEG background activity in a dose-related manner.     

Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H₃ receptor selective agonists (R)--methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC₅₀ values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)--methyl-histamine to the right (pK_B value 8.38). The inhibitory effect of 1 mol/l (R)--methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC₅₀: 65 nmol/l) and dimaprit (IC₅₀: 8.6 mol/l); however, the effect of (R)--methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mol/l) and not significantly affected by other H₃ receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 mol/l). In conclusion, H₃ receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H₃ receptors may exist. The data suggest that histamine H₃ receptors modulating 5-HT release in pig small intestine do not belong to either H_3A or H_3B receptors as defined in rat tissue. Correspondence to: K. Racke at the above address     

Opposite effects of agonist and inverse agonist ligands of benzodiazepine receptor on self-defensive and submissive postures in the rat

The effects of benzodiazepine receptor ligands on different types of defensive behaviours were examined in intruder male rats confronted with offensive residents. Chronic administration, via a subcutaneous silastic pellet, of a full agonist (diazepam) for 15 days increased self-defensive postures as well as social and non-social behaviour whereas submissive postures and flight were reduced. Acute administration of a partial agonist (ZK 91296) resulted in a similar increase in self-defensive postures and a decrease of submissive and non-social elements. Acute administration of a partial inverse agonist (FG 7142) reduced defensive postures and social behaviour whereas submissive postures were increased. These results show that activation of benzodiazepine receptors by full or partial agonists increased self-defensive responses to attacks by a conspecific, while decreasing submissive postures. On the contrary, inverse activation of these receptors by an inverse agonist increased submissive postures while decreasing self-defensive responses. These data suggest that benzodiazepine receptors are involved in the control of the animal's strategy to respond to an attack of another rat.     

Involvement of presynaptic imidazoline receptors in the α2-adrenoceptor-independent inhibition of noradrenaline release by imidazoline derivatives

Summary An involvement of imidazoline recognition sites in the modulation of transmitter release was investigated in the rabbit pulmonary artery and aorta preincubated with [³H]noradrenaline and superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Electrical impulses were applied transmurally at 0.66 or 2 Hz. In the absence of further drugs, rauwolscine as well as the imidazoline derivatives BDF 6143 [4-chloro-2-(2-imidazoline-2-ylamino)-isoindoline], idazoxan and phentolamine increased the ³H overflow from the pulmonary artery, evoked by electrical stimulation at 2 Hz; the effect was due to the ₂-autoreceptor blocking property of these drugs. The maximum increase in overflow obtainable with the imidazolines was considerably lower than with rauwolscine. The concentration-response curves of the imidazolines were bell-shaped. At 0.66 Hz, BDF 6143 did not facilitate, but concentration-dependently inhibited, whereas idazoxan failed to change the evoked ³H overflow. When, at the stimulation frequency of 2 Hz, presynaptic ₂-adrenoceptors were blocked by rauwolscine and/or pre-exposure to phenoxybenzamine, the electrically evoked ³H overflow from the pulmonary artery and/or aorta was inhibited by the following imidazoline derivatives: the ₂-adrenoceptor antagonists BDF 6143, idazoxan and phentolamine, the ₁-adrenoceptor agonist with ₂-blocking property cirazoline as well as the ₂-adrenoceptor agonists clonidine and moxonidine. The maximum inhibition caused by BDF 6143 was greater than that due to clonidine and moxonidine; the latter two, hence, behaved as partial agonists. At the stimulation frequency of 0.66 Hz, the imidazolines exhibited a higher potency than, but a similar intrinsic activity to that at 2 Hz. Noradrenaline did not affect the evoked ³H overflow. The BDF 6143-induced inhibition of evoked ³H overflow was not modified by metitepine, atropine, theophylline, dipyridamole and indometacin, but was counteracted by the partial agonists clonidine and moxonidine.The results exclude the possibility that ₁- and ₂-adrenoceptors, 5-HT₁ receptors, muscarine receptors, P₁ purinoceptors and prostaglandin receptors are involved in the imidazoline-induced inhibition of noradrenaline release. They provide evidence indicating that the inhibitory effect is mediated by imidazoline receptors on the postganglionic sympathetic nerve terminals of the rabbit pulmonary artery and aorta.     

Involvement of δ-opioid receptors in the effects of morphine on locomotor activity and the mesolimbic dopaminergic system in mice

Naltrindole (NTI) and naltriben (NTB), a benzofuran derivative of NTI, were recently synthesized as highly selective -opioid receptor antagonists. Both NTI and NTB failed to suppress the antinociceptive effect induced by morphine. In contrast, both NTI and NTB significantly suppressed the morphine-induced hyperlocomotion and increase in turnover of dopamine (DA) in the mouse limbic forebrain. These results suggest that -opioid receptors play, at least in part, a role in the morphine-induced hyperlocomotion and excitation of mesolimbic DA systems, but not antinociception.     

Different types of opioid receptors mediating analgesia induced by morphine,DAMGO, DPDPE,DADLE and β-endorphin in mice

Summary The effects of intracerebroventricular (i.c.v.) administration of d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NH₂ (CTOP), a selective mu-opioid receptor antagonist, (Allyl)2-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174864) and (N,N-Bisallyl-Tyr-Gly-Gly--(CH₂S)-Phe-Leu-OH (ICI 154129), selective delta-opioid receptor antagonists on blocking analgesia induced by -endorphin, morphine, d-Ala²-NMePhe⁴-Gly-ol-enkephalin (DAMGO), d-Ala²-d-Leu⁵-enkephalin (DADLE) and d-Pen²-enkephalin (DPDPE) administered i.c.v. were studied in male ICR mice. The analgesia was assessed by the tail-flick and paw-licking (hot-plate) tests. The potencies of opioid agonists injected i.c.v. for producing analgesia were DAMGO > DADLE > -endorphin > morphine > DPDPE. Intracerebroventricular administration of CTOP (0.05 g) selectively antagonized inhibition of the tail-flick and paw-licking response induced by morphine, DAMGO or DADLE but not -endorphin or DPDPE. ICI 174864 (5 g) and ICI 154129 (5 g) injected i.c.v. selectively antagonized analgesia induced by DPDPE or DADLE but not -endorphin, morphine or DAMGO injected i.c.v. These results indicate that analgesia induced by morphine and DAMGO is mediated by the stimulation of mu-opioid receptors while analgesia induced by DPDPE is mediated by the stimulation of delta-opioid receptors. DADLE-induced analgesia is mediated by the stimulation of both mu- and delta-opioid receptors. Analgesia induced by -endorphin is mediated by neither munor delta-opioid receptors.Abbreviations i.c.v. intracerebroventricular - i.t. intrathecal - CTOP d-Phe-Cys-Tyr-d-Try-Orn-Thr-Pen-Thr-NHZ - DAMGO d-Ala²-NMePhe²-Gly-ol-enkephalin - DADLE d-Ala²-d-Leus-enke-phalin - DPDPE dd-Pen²-dd-Pen⁵-enkephalin - ICI 174864 (Allyl)2Tyr-Aib-Aib-Phe-Leu-OH - ICI 154129 (N,N-Bisallyl-Tyr-Gly-Gly-(CH₂S)-Phe-Leu-OH