Muscarinic M1 receptor agonist actions of muscarinic receptor agonists in rabbit vas deferens

In the electrically field-stimulated rabbit vas deferens, muscarinic receptor agonists increase twitch-height by actions at postjunctional M2 receptors and decrease twitch-height by actions at prejunctional M1 receptors. In the present studies, in contrast to previous reports, muscarinic receptor agonists primarily decreased twitch-height, produced minimal increases in twitch-height, and, produced identical responses in both epididymal and prostatic tissue segments, thus permitting a more detailed investigation of the M1 receptor component of action of muscarinic receptor agonists in the rabbit vas deferens. The nonselective muscarinic receptor agonist carbachol produced biphasic effects on twitch-height in the vas deferens: lower concentrations increased twitch-height to only approximately 25-30% over control, whereas higher concentrations inhibited the twitch. The selective M1 receptor antagonist pirenzepine blocked the inhibitory effects of carbachol, and unmasked carbachol-induced increases in twitch-height. Atropine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) and AF-DX 116 (11-2[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido[2,3-b][1,4]benzodiazepin-6-one) blocked both the inhibitory and stimulatory effects of carbachol, but atropine and 4-DAMP were more potent in blocking the inhibitory than the stimulatory effects of carbachol, whereas the reverse was true for AF-DX 116. McN-A-343 (4-hydroxy-2-butynyl)trimethylammonium chloride, m-chlorocarbanilate) and 12 other muscarinic receptor agonists from a variety of chemical classes also produced concentration-dependent decreases in twitch-height. The log IC50s of the muscarinic receptor agonists for decreasing twitch-height were highly correlated with their log Kis for inhibiting [3H]pirenzepine (r = 0.96) and [3H]oxotremorine-M (r = 0.85) binding in rat hippocampal membranes. The present results demonstrate that the muscarinic M1 receptor mediating inhibition of twitch-height in the rabbit vas deferens has pharmacologic properties similar to the muscarinic M1 receptor in rat hippocampus.

D-16949 (anpirtoline): a novel serotonergic (5-HT1B) psychotherapeutic agent assessed by its discriminative effects in the rat

D-16949 [6-chlor-2-(piperidyl-4-thio)-pyridine; Anpirtoline] is a novel centrally acting compound with serotonergic effects. To assess its discriminative stimulus effects, rats were trained to discriminate D-16949 (2.0 mg/kg i.p., 30 min) from no drug. D-16949 induced dose-dependent discriminative stimulus effects (ED50, 0.31 mg/kg), and did not produce sedation. The opioid analgesics codeine, pentazocine and tramadol all failed to substitute for D-16949. The opioid antagonist naltrexone did not antagonize the discriminative stimulus effects of D-16949. Phencyclidine, d-amphetamine, lysergic acid diethylamide and quipazine produced between 0 and 35% responding on the D-16949 lever. 8-Hydroxy-2-(di-n-propylamino)-tetralin substituted partially (45%) for D-16949, whereas 1-(m-trifluoromethylphenyl)-piperazine and RU 24969 completely and dose-dependently substituted for D-16949. The discriminative stimulus effects of D-16949 were not reversed by either cyproheptadine, ketanserin, pirenperone, spiperone or methylsergide. The 5-hydroxytryptamine3 (5-HT3) active antagonists ICS 205-930 and MDL 72222 were also ineffective as D-16949 antagonists. It is concluded that the discriminative stimulus effects of D-16949 are not mediated through opioid or 5-HT2 mechanisms. The present data also do not suggest the involvement of 5-HT3 mechanisms, but that D-16949 produces its discriminative stimulus effects in the rat primarily via agonistic actions at 5-HT1B receptors.

Novel functional M1 selective muscarinic agonists. 2. Synthesis and structure-activity relationships of 3-pyrazinyl-1,2,5,6-tetrahydro-1-methylpyridines. Construction of a molecular model for the M1 pharmacophore

A series of 3-(3-substituted-pyrazinyl)-1,2,5,6-tetrahydro-1-methylpyridines were synthesized and found to have high affinity for central muscarinic receptors. The ability of some of these compounds to inhibit the electrically stimulated twitch of the guinea pig vas deferens indicated that the compounds were M1 agonists. M1 agonist activity was related to the length of the side chain attached to the pyrazine ring, with maximal activity being obtained with the hexyloxy side chain. The (hexyloxy)pyrazine 3f lacked M2 agonist activity as it failed to affect the guinea pig atria and was also relatively devoid of M3 agonist activity as determined by its lack of tremorogenic and sialogogic effects in mice. A comparison of the M1 agonist efficacy of these pyrazines and related 1,2,5-thiadiazoles and 1,2,5-oxadiazoles suggested that M1 efficacy was related to the magnitude of electrostatic potential located over the nitrogens of the respective heterocycles. The heteroatom directly attached to the 3 position of the pyrazine or 1,2,5-thiadiazole heterocycle markedly influenced the M1 efficacy of the compounds by determining the energetically favorably conformers for rotation about the bond connecting the tetrahydropyridyl ring and the heterocycle. A three-dimensional model for the M1-activating pharmacophore was proposed based on computational studies and the model of the muscarinic pharmacophore proposed by Schulman.

Potency and efficacy of dopamine agonists in mouse strains differing in dopamine cell and receptor number

The potency and efficacy of the selective dopamine D2 receptor agonist quinpirole, the mixed D1/D2 agonist apomorphine, and the selective D1 receptor agonist SKF 38393 in producing hypothermia and changes in locomotor activity were evaluated in four strains of mice: CBA/J, C57BL/6J, ICR Swiss and CF1. CBA/J mice previously have been shown to be deficient in dopamine cell and receptor number relative to other strains such as C57BL/6J mice, whereas ICR Swiss and CF1 are commonly used strains of mice. Quinpirole (0.125 to 1.0 mg/kg) was equiefficacious and equipotent in producing hypothermia in all 4 strains. Apomorphine (0.125 to 16 mg/kg) was equiefficacious in producing hypothermia in all 4 strains, but was approximately four-fold less potent in CBA/J mice than in the other strains. SKF 38393 had little effect on body temperature in any of the 4 strains. Basal motor activity was lowest in CBA/J mice, and tended to be highest in ICR Swiss mice. Quinpirole (0.125 to 32 mg/kg) had no effect on motor activity in CBA/J mice, but decreased motor activity in the other 3 strains. Apomorphine (1 to 16 mg/kg) produced modest increases in motor activity in all 4 strains. The magnitude of the changes produced by apomorphine was comparable in all strains when expressed as change from mean control values. SKF 38393 (8 to 64 mg/kg) also increased motor activity in all 4 strains, with comparable increases when expressed as change from mean control values. The present results are consistent with the interpretation that inherited deficiencies in dopamine cell and receptor number in CBA/J mice produce functional decrements in D2, but not D1, dopamine receptor function.

Effects of oral pergolide mesylate on nociceptive spinal cord reflexes in rats

The effects of orally administered pergolide mesylate on the flexor reflex were evaluated in chronic spinal rats. The mixed D1/D2 agonist pergolide (0.1 to 3.0 mg/kg) produced a dose-related decrease in the magnitude of the flexor reflex elicited by a tetanic stimulus. The effects of pergolide were blocked by haloperidol, demonstrating that the effects of pergolide were mediated through dopamine receptors. In contrast, the selective D2 agonist bromocriptine (3.0 to 30 mg/kg) had no effect on the flexor reflex. The present results are consistent with the interpretation that pergolide produces an antinociceptive action at the spinal cord level by stimulating both D1 and D2 dopamine receptors.

Interactions between scopolamine and muscarinic cholinergic agonists or cholinesterase inhibitors on spatial alternation performance in rats

The effects on working memory of the muscarinic cholinergic agonists oxotremorine, arecoline, RS86 and pilocarpine, and the cholinesterase inhibitors physostigmine and tetrahydroaminoacadine were investigated in male F344 rats. Working memory was assessed by behavior maintained under a spatial alternation schedule of food presentation in which the interval between trials was varied from 2 to 32 sec. Under control conditions the percentage of correct responses decreased as the retention interval was varied from 2 to 32 sec. Administered alone the cholinergic agonists oxotremorine (0.01-0.1 mg/kg), arecoline (3-30 mg/kg), RS86 (0.3-3 mg/kg) and pilocarpine (0.3-3.0 mg/kg), and the cholinesterase inhibitors physostigmine (0.01-0.1 mg/kg) and tetrahydroaminoacridine (0.3-3.0 mg/kg) either had no effect on or produced dose-related deficits in working memory and decreases in response rates. The muscarinic antagonist scopolamine (0.1 mg/kg) produced retention interval-dependent decreases in the percentage of correct responding and rates of responding. The cholinergic agonists and tetrahydroaminoacridine failed to reverse the effects of scopolamine. However, physostigmine produced a dose-dependent reversal of the working-memory deficits and response-rate decreasing effects of scopolamine. The present results are consistent with the interpretation that drugs which primarily enhance M2 muscarinic cholinergic transmission are ineffective in enhancing working memory or in reversing scopolamine-induced deficits in working memory.

A comparison of the effects of cholinergic and dopaminergic agents on scopolamine-induced hyperactivity in mice

The effects of cholinesterase inhibitors, cholinergic agonists, dopaminergic agonists and dopaminergic antagonists on the hyperactivity produced by the muscarinic cholinergic antagonist scopolamine were evaluated in mice. Scopolamine (0.3-10 mg/kg) produced a dose-related increase in locomotor activity, with a peak effect at 3.0 mg/kg. The cholinesterase inhibitor physostigmine (0.03-0.175 mg/kg) was without effect on locomotor activity when administered alone, whereas the cholinesterase inhibitor tetrahydroaminoacridine hydrate (0.3-10 mg/kg) decreased locomotor activity. Both physostigmine and tetrahydroaminoacridine hydrate attenuated the effects of scopolamine. Administered alone, the cholinergic agonists oxotremorine (0.01-0.3 mg/kg) and RS86 (0.1-3.0 mg/kg) produced dose-related decreases in locomotor activity, whereas pilocarpine (0.3-10 mg/kg) had no effect on locomotor activity. None of these three muscarinic agonists significantly attenuated the hyperactivity produced by scopolamine. Administered alone, the dopaminergic agonists quinpirole (0.003-0.1 mg/kg), S-(-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine (0.3-10 mg/kg) and SKF 38393 (8-64 mg/kg) had no significant effect on activity, whereas apomorphine (0.3-10 mg/kg) and d-amphetamine (0.1-3.0 mg/kg) increased activity. Quinpirole, apomorphine and S-(-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine produced dose-related reversals of the increases in locomotor activity produced by scopolamine. The hyperactivity effects of d-amphetamine were approximately additive with scopolamine, whereas SKF 38393 did not significantly affect scopolamine. The mixed D1/D2 dopaminergic antagonist haloperidol (0.003-3.0 mg/kg) and the selective D1 antagonist SCH 23390 (0.01-0.3 mg/kg) produced dose-related decreases in locomotor activity when administered alone, and also produced dose-related reversals of the hyperactivity produced by scopolamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of working memory in rats using spatial alternation behavior with variable retention intervals: effects of fixed-ratio size and scopolamine

The effects of fixed-ratio (FR) size, scopolamine, and the interactions between FR size and scopolamine were investigated in male F344 rats on working memory as assessed by spatial alternation behavior maintained under FR schedules of food presentation where the interval between trials was varied among values of 2, 4, 8, 16, and 32 s within each session. The magnitude of the FR size on the correct and incorrect levers was varied systematically from 1 response to 2, 4, 8, or 16 responses in order to determine whether the FR size influenced either the percentage of correct responding, rates of responding, or both. Under the primary baseline condition, that is when the FR size on both the correct and incorrect levers was one response (designated FR1 FR1), the percentage of correct responses decreased with increasing retention interval duration. Increasing the FR size on the correct lever produced FR-dependent increases in the percentage of correct responding as well as in rates of responding. Increasing the FR size on the incorrect lever produced FR-dependent decreases in correct responding, but had little effect on rates of responding. Dose-effect curves for scopolamine were determined on performance maintained under FR values on the correct and incorrect levers, respectively, of FR1 FR1, FR1 FR10, FR10 FR1, and FR10 FR10. In general, scopolamine produced dose-related decreases in the percentage of correct responding, although the magnitude of the effects of scopolamine varied not only with dose, but also with the length of the retention interval and with changes in FR size.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate receptors of the N-methyl-D-aspartate subtype in the myenteric plexus of the guinea pig ileum

The excitatory amino acids L-glutamate and N-methyl-D-aspartate (NMDA) produced contractions of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum over the concentration range of 3 X 10(-6) to 10(-3) M. The contractile response to L-glutamate and NMDA, but not carbamyl choline, was blocked noncompetitively by 0.6 mM Mg++. In the absence of Mg++, concentration-dependent increases in contractile force also were produced by, in order of potency, L-aspartate, L-homocysteate and D-glutamate, but not by quisqualate, kainate or quinolinate. L-Glutamate was competitively antagonized by the selective NMDA receptor antagonists D-2-amino-5-phosphonovalerate and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (3 X 10(-6)-3 X 10(-5) M), as well as by the nonselective excitatory amino acid antagonist gamma-D-glutamylglycine (3 X 10(-4) M). Glutamic acid diethyl ester (3 X 10(-4) M) noncompetitively antagonized L-glutamate. L-Glutamate was not blocked by gamma-D-glutamylaminomethyl sulphonate (3 X 10(-4) M), an antagonist which preferentially antagonizes kainate and quisqualate. In addition, the phencyclidine-like drugs etoxadrol (10(-7)-10(-5) M), dextromethorphan (10(-6)-10(-5) M) and 5-methyl-10,11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imine (10(-9)-10(-7) M) noncompetitively antagonized L-glutamate. The (+) isomer of 5-methyl-10, 11-dihydroxy-5H-dibenzo(a,d)cyclohepten-5,10-imine was approximately 10-fold more potent than the (-) isomer in antagonizing L-glutamate. The present results demonstrate that receptors for the excitatory amino acid L-glutamate are present in the guinea pig myenteric plexus and are of the NMDA subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological mechanisms of action of flupirtine: a novel, centrally acting, nonopioid analgesic evaluated by its discriminative effects in the rat

Rats were trained to discriminate the novel analgesic flupirtine (10.0 mg/kg i.p., 10 min) from no drug under a two-choice fixed-ratio 5 shock-termination schedule. Flupirtine yielded a dose-response curve with an ED50 of 3.87 mg/kg. The opioid analgesics pentazocine, codeine and tramadol failed to produce flupirtine appropriate responding. The opioid antagonist naltrexone did not antagonize the discriminative effects of flupirtine. The mixed alpha-1/alpha-2 adrenergic agonist clonidine and the highly specific alpha-2 adrenergic agonist UK-14304, both partially and dose-dependently produced flupirtine appropriate responding. The mixed alpha-1/alpha-2 antagonist yohimbine and the highly specific alpha-2 antagonists idazoxan and L-654,284 all partially and dose-dependently antagonized flupirtine appropriate responding. Neither of the alpha-1 agonists phenylephrine or ST 587 produced flupirtine appropriate responding, nor did the alpha-1 antagonist prazosin antagonize flupirtine responding. It is concluded that the discriminative effects of flupirtine are neither of opioid nor of alpha-1 adrenergic type, but are primarily mediated through alpha-2 adrenergic mechanisms.

Beta-carbolines as antagonists of the discriminative stimulus effects of diazepam in rats

Rats were trained to discriminate between saline and 1.0 mg/kg of diazepam in a two-choice procedure where responding was maintained under a fixed-ratio, 5-response schedule of stimulus shock termination. beta-Carboline-3-carboxylate-methyl ester (beta CCM), beta-carboline-3-carboxylate-ethyl ester (beta CCE) and beta-carboline-3-carboxylate-t-butyl ester (beta CCtB), compounds with alkylcarboxy substitutions on the 3-position of the beta-carboline ring structure, were effective antagonists of the discriminative effects of diazepam. The 3-hydroxymethyl-substituted compound (3HMC) was relatively ineffective in antagonizing the discriminative effects of diazepam. The order of potency in antagonizing the 1.0 mg/kg training dose of diazepam was beta CCtB greater than beta CCM greater than beta CCE much greater than 3 HMC. The greater potency of beta CCtB likely reflects its resistance to metabolism in vivo. beta CCE and beta CCtB produced dose-related, parallel shifts in the dose-response curve for the discriminative effects of diazepam, but the magnitude of the shifts was limited: the two highest doses of beta CCE and beta CCtB produced shifts that were not significantly different in magnitude. These latter results suggest that these beta-carbolines antagonize only a portion of the component(s) of action of diazepam in producing discriminative stimuli. In contrast, the 7-substituted beta-carbolines harmane, harmol and harmine were ineffective in antagonizing the discriminative effects of diazepam up to doses of the beta-carbolines which disrupted the ability of the animals to respond.(ABSTRACT TRUNCATED AT 250 WORDS)

CGS 8216: agonist and diazepam-antagonist effects in rodents

CGS 8216, a pyrazoloquinolinone benzodiazepine receptor ligand, was administered alone and concomitantly with diazepam in order to assess its agonist and diazepam-antagonist properties on several behaviors in rodents. In mice, CGS 8216 (i.p.) potentiated the convulsant effects of pentylenetetrazole. Moreover, doses of 1.0 to 10 mg/kg of CGS 8216 produced dose-related antagonism of the anticonvulsant effects of diazepam. In rats, CGS 8216 (0.3-3.0 mg/kg) was without effect on the Rotarod, but produced dose-related, nonparallel shifts to the right in the diazepam dose-effect curve. Also in rats, behavior was maintained under a multiple schedule where in one component every 20th response resulted in water presentation (unpunished component) and in a second component every 20th response resulted in both footshock and water presentation (punished component). CGS 8216 produced dose-related decreases in response rates in both components, but was approximately 10-fold more potent in decreasing rates of punished responding. These effects were blocked by the benzodiazepine antagonist Ro 15-1788 (30 mg/kg i.p.). Increasing doses of diazepam (0.1-10 mg/kg p.o.) first increased and then decreased rates of punished responding but only decreased rates of unpunished responding. CGS 8216 produced a dose-related antagonism of the rate-increasing, but had little effect on the rate-decreasing, effects of diazepam. In another group of rats, behavior was maintained under a multiple fixed interval 5-min fixed ratio 20-response schedule of water presentation. CGS 8216 produced a dose-related decrease in response rates in both components, but these effects were not blocked by Ro 15-1788 (30 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacologic and reinforcing properties of phencyclidine and the enantiomers of N-allylnormetazocine in the dog

Experiments were conducted to evaluate the degree of phencyclidine (PCP)-like activity associated with the dextro and levo enantiomers of the sigma agonist N-allylnormetazocine (NANM). In chronic spinal dogs, d- and l-NANM generally produced similar physiologic and gross animal behavior effects which included miosis, tachycardia, hyperthermia, increased secretory activity (lacrimation, rhinorrhea and salivation), nystagmus and stereotyped head movements. For these effects, d- and l-NANM were generally equal in potency and both were about 1/10th as potent as PCP. However, the NANM enantiomers could be differentiated on the basis of their effects on nociceptive reflexes. Comparisons of dose-response curves and efficacies demonstrated that d-NANM was more similar to PCP in its effectiveness in depressing the flexor and skin twitch reflexes than was l-NANM. In addition, naltrexone selectively antagonized or reduced only the effects of l-NANM on reflex activity. In intact dogs, d-NANM and PCP, but not l-NANM maintained self-administration behavior under FR15 or FI900 (FR10:S) schedules of reinforcement. This represented the most stereospecific action of the NANM enantiomers. Additionally, l-NANM failed to maintain self-administration behavior, even following pretreatment with naltrexone, thus suggesting that the opiate activity of l-NANM was not responsible for its lack of reinforcing efficacy. Taken together, the data demonstrate that both d- and l-NANM have PCP-like properties, but d-NANM is pharmacologically more equivalent than l-NANM to PCP and l-NANM has additional activity which is not PCP-like.

Behavioral effects of CGS 8216 alone, and in combination with diazepam and pentobarbital in dogs

Beagle dogs (N = 3) responded under a multiple fixed-interval (FI) 300 sec, fixed-ratio (FR) 30 schedule of food presentation. The pyrazoloquinoline derivative CGS 8216, given either intravenously (0.01-3.0 mg/kg) or orally (0.1-30.0 mg/kg) had little effect on either the rate or temporal pattern of responding during either component. Both diazepam (0.3 to 17.5 mg/kg, PO) and pentobarbital (0.1-17.5 mg/kg, PO) produced qualitatively similar effects on behavior. Rates of responding during the FI components first increased, then decreased with increasing doses; both drugs produced only dose-related decreases in the rate of responding during the FR components. CGS 8216 antagonized some of the behavioral effects of diazepam; FI and FR response rates returned to baseline, however the effects of diazepam on quarter-life values were not appreciably altered by CGS 8216. The effects of pentobarbital on schedule-controlled responding were not antagonized by CGS 8216. These results indicate CGS 8216 is a selective benzodiazepine antagonist that does not produce benzodiazepine-like behavioral effects.

Differential diazepam-antagonist effects of the benzodiazepine receptor ligand CGS 9895 in rodents

CGS 9895, a pyrazoloquinolone benzodiazepine receptor ligand, was administered alone and concomitantly with diazepam in order to assess its agonist and diazepam-antagonist properties on various behaviors in rodents. In mice, CGS 9895 neither potentiated nor blocked the convulsant effects of pentylenetetrazole. However, doses of 3.0 and 10 mg/kg of CGS 9895 i.p. produced dose-related antagonism of the anticonvulsant effects of diazepam against pentylenetetrazole (80 mg/kg i.p.). In rats, diazepam produced dose-related increases in ataxia as measured on the rotarod. CGS 9895 (0.3-10 mg/kg i.p.) was without effect on performance on the rotarod, but produced dose-related parallel shifts to the right in the diazepam dose-effect curve. Also in rats, behavior was maintained under a multiple schedule where in one component every 20th response resulted in water presentation (unpunished behavior) and in a second component every 20th response resulted in both shock and water presentation (punished behavior). CGS 9895 (0.3-30 mg/kg i.p.) was without significant effect on either punished or unpunished responding. Increasing doses of diazepam (0.1-10 mg/kg p.o.) first increased and then decreased rates of punished responding but only decreased rates of unpunished responding. CGS 9895 (3.0 mg/kg i.p.) neither potentiated nor antagonized diazepam. In another group of rats, behavior was maintained under a multiple fixed-interval 5 min fixed-ratio 20 response schedule of water presentation. CGS 9895 (0.3-30 mg/kg i.p.) did not affect performance under this schedule. Diazepam (0.3-30 mg/kg p.o.) primarily decreased rates under the fixed-ratio schedule, but increasing doses first increased and then decreased rates under the fixed-interval schedule.(ABSTRACT TRUNCATED AT 250 WORDS)

Pyrazoloquinoline benzodiazepine receptor ligands: effects on schedule-controlled behavior in dogs

The effects of diazepam and the pyrazoloquinoline benzodiazepine receptor ligands CGS8216, CGS9896, and CGS9895 on schedule-controlled responding were studied in dogs. Responding was maintained under a multiple fixed-interval (FI) 5-min fixed-ratio (FR) 30 response schedule of food presentation. Diazepam (PO) produced dose-related decreases in response rates under FR component. Under the FI, rates first increased and then decreased with increasing doses of diazepam. Diazepam also produced a dose-related disruption of the temporal pattern of responding under the FI as measured by decreases in quarter-life values. CGS8216 IV produced dose-related decreases in response rates under both components. The highest oral dose of CGS8216 also decreased rates in both components. CGS8216 was approximately 100 times more potent by the IV route as compared to the oral route. CGS9896 IV had no significant effect on responding under either component of the multiple schedule. However, with increasing doses of CGS9896 PO, response rates under both components first decreased and then returned to control values. CGS9895 PO was without significant effect on responding. When CGS8216 was administered concomitantly with graded doses of diazepam, the former drug blocked the rate-decreasing effects of diazepam under the FR component, but not the rate-increasing effects of diazepam under the FI. The present results demonstrate that although these three pyrazoloquinolines are benzodiazepine receptor ligands, they do not exhibit diazepam-like effects on schedule-controlled behavior.

beta-Carboline-3-carboxylate-t-butyl ester: a selective BZ1 benzodiazepine receptor antagonist

The effectiveness of beta-carboline-3-carboxylate-t-butyl ester (beta CCtB) in antagonizing the anticonvulsant, ataxic and antipunishment effects of diazepam were evaluated. In mice, beta CCtB at doses of 3 and 10 mg/kg produced a dose-related antagonism of the anticonvulsant effects of diazepam against pentylenetetrazole (80 mg/kg). A dose of 30 mg/kg of beta CCtB did not produce a further shift in the diazepam dose-effect curve, apparently because beta CCtB failed to block the muscle-relaxant effects of diazepam. Further, beta CCtB (30 mg/kg) failed to antagonize the ataxic effects of diazepam in an inverted screen test. Rats responded under a multiple schedule where in one component every twentieth response (FR20) resulted in water presentation (unpunished component) and in another component every twentieth response (FR20) resulted in both shock and water presentation (punished component). Diazepam p.o. (0.1 to 10 mg/kg) first increased and then decreased rates in the punished component but only decreased rates in the unpunished component. beta CCtB had no effect on response rates when administered alone, but antagonized the rate-increasing effects of diazepam in the punished component. beta CCtB did not alter the rate-decreasing effects of diazepam in either component. Thus, beta CCtB selectively antagonized the effects of diazepam on punished behavior as well as the anticonvulsant effects of diazepam, but beta CCtB failed to antagonize the rate-decreasing and ataxic effects of diazepam. These results are consistent with the interpretation that beta CCtB is a selective BZ1 benzodiazepine receptor antagonist.

CGS8216 noncompetitively antagonizes the discriminative effects of diazepam in rats

The benzodiazepine antagonist properties of CGS8216 were evaluated in rats trained to discriminate between saline and 1.0 mg/kg of diazepam in a two-choice, stimulus-shock termination procedure. CGS8216 (0.3 to 100 mg/kg) administered alone, either s.c., p.o. or i.p., occasioned only saline-appropriate responding. When administered concomitantly with a constant 1.0 mg/kg dose of diazepam, CGS8216 produced dose-related decreases in drug-appropriate responding. CGS8216 was most potent by the i.p. route, and approximately tenfold less potent by the oral route. CGS8216 was dermatotoxic after s.c. administration. CGS8216 i.p. had a long duration of action. A dose of 30 mg/kg completely antagonized the discriminative effects of the 1.0 mg/kg training dose of diazepam when the antagonist was administered 8 hr before the start of the test session. In order to determine the type of antagonism by CGS8216, the dose-effect curve for diazepam was redetermined in the presence of varying doses of CGS8216 (0.3 to 3.0 mg/kg, i.p.). CGS8216 produced a dose-related rightward shift in the diazepam dose-effect curve, but also decreased the slope and appeared to decrease the maximal effect. These results are consistent with the interpretation that CGS8216 antagonizes diazepam in a noncompetitive manner. It may do so because either it interacts with a subpopulation of benzodiazepine receptors, it functions as a pseudo-irreversible antagonist due to its high affinity, or because it is an antagonist with agonist properties.

Morphine-like discriminative stimulus effects of buprenorphine and demethoxybuprenorphine in rats: quantitative antagonism by naloxone

The interactions between the opioid antagonist naloxone and the opioids morphine, buprenorphine and demethoxybuprenorphine ( DMB ), an acid-catalyzed rearrangement product of buprenorphine, were evaluated quantitatively in rats trained to discriminate between saline and 3.0 mg/kg of morphine using a two-choice, shock avoidance procedure. Dose-effect curves for each of the three agonists were determined alone and in the presence of varying doses of naloxone (0.01-1.0 mg/kg). Buprenorphine and DMB produced dose-related increases in morphine-appropriate responding which plateaued over at least a 30-fold dose-range for each drug. On a molar basis, buprenorphine was approximately 140 times and DMB approximately 45 times more potent than morphine. Naloxone produced parallel shifts in the dose-effect curves for morphine, buprenorphine and DMB . Schild plots derived from the relative shifts in the dose-effect curves yielded regression lines with slopes which were not significantly different from -1, consistent with naloxone acting as a competitive antagonist of each of the three opioids. The apparent pA2 values for naloxone were 7.85 +/- 0.36 against morphine, 7.48 +/- 0.16 against buprenorphine and 7.17 +/- 0.27 against DMB . Because the 95% CLs overlapped for morphine and buprenorphine, but not for morphine and DMB , these results are consistent with the interpretation that naloxone is acting at the same receptor in antagonizing morphine and buprenorphine but naloxone may be acting in a more complex manner in antagonizing DMB . The effectiveness of naloxone in antagonizing equieffective doses of morphine (3.0 mg/kg), buprenorphine (0.03 mg/kg) and DMB (0.1 mg/kg) were also evaluated when naloxone was administered either 0 or 30 min after the agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulus control by diazepam of behavior maintained under fixed-ratio stimulus-shock termination schedules in rats

The stimulus control of behavior by diazepam (1.0 mg/kg) was investigated where responding was maintained under fixed-ratio (FR) schedules of stimulus-shock termination in rats. The size of the FR requirement was either 1, 5, 10, or 20 responses. At each FR requirement, dose-effect curves were determined for diazepam, flurazepam, pentobarbital and cyproheptadine. Diazepam-like discriminative stimuli were produced by flurazepam and pentobarbital but not by cyproheptadine. The magnitude of the FR requirement had no significant effect on the dose-effect curves for percentage of responses emitted on the diazepam-appropriate choice lever for any of the four drugs. On the other hand, the effects of these drugs on rates of responding depended on the magnitude of the FR requirement. None of the drugs altered response rates under the FR1 schedule. Diazepam tended to increase response rates under the FR5 schedule, but had no effect or decreased rates under the FR10 and FR20 schedules. Flurazepam and pentobarbital predominantly decreased rates at FR requirements of 5, 10 or 20 responses. Cyproheptadine had no significant effect on response rates at any schedule parameter. Together with previous reports, the present results indicate that the discriminative effects of diazepam are similar under schedules employing noxious (this study) or non-noxious (other reports) consequences, even though the effects on response rates of diazepam-like drugs differ depending on the schedule of reinforcement and consequent event maintaining the behavior.

Comparison of behavior maintained by intravenous cocaine and d-amphetamine in dogs

The effectiveness of i.v. injections of cocaine and d-amphetamine in maintaining schedule-controlled behavior was compared directly in dogs. Behavior was maintained under either a fixed-ratio (FR) 15, fixed-interval (FI)5-min, FI5 -min ( FR5 :S), FI15 -min, FI15 -min ( FR5 :S) or FI45 -min ( FR10 :S) schedule of reinforcement during daily experimental sessions where the maximum number of injections per session was limited to 2 to 11, depending on the schedule employed. Each drug injection was followed by a 10-min (FR schedule) or 5-min (interval schedules) timeout period to reduce the direct effects of the drugs on responding. Both cocaine and d-amphetamine maintained temporal patterns of responding characteristic of each of the schedules. Similar rates of responding were maintained by cocaine and d-amphetamine under the FR and longer (15- and 45-min) interval schedules, but cocaine maintained higher rates than did d-amphetamine under the shorter (5-min) interval schedules. Brief stimulus presentations intermittently contiguous with drug injections did not always maintain higher response rates under the second-order schedules as compared to rates maintained under the simple FI schedules, but higher rates were observed more often when stimuli were paired with cocaine than when stimuli were paired with d-amphetamine. These results suggest that cocaine and d-amphetamine can function differently as reinforcers and the differences depend, at least in part, on the schedule of reinforcement under which the drugs are presented.

Behavior maintained under fixed-interval and second-order schedules by intravenous injections of endogenous noncatecholic phenylethylamines in dogs

The effectiveness of i.v. injections of the endogenously occurring amines beta-phenylethylamine (PEA), N-methyl phenylethylamine (NMPEA) and phenylethanolamine in maintaining schedule-controlled behavior was investigated in dogs. Behavior was maintained under either a fixed-interval (FI) 5-min schedule of i.v. drug injection or a second-order FI 5-min schedule where every fifth response (FR 5) resulted in a 2-sec visual stimulus and the first FR 5 completed after the interval elapsed resulted in both the visual stimulus and i.v. drug injection [FI 5-min (FR 5:S)]. Experimental sessions, with 10 intervals per session, were conducted 5 days/week. Each drug injection was followed by a 5-min timeout period to minimize the direct effects of the drugs on responding. As the dose per injection increased, rates of responding maintained under both schedules by PEA and NMPEA first increased and then decreased. When saline was substituted for drug, responding occurred at very low rates. PEA and NMPEA were approximately equieffective and equipotent in maintaining responding under the FI 5-min schedule. PEA maintained somewhat higher rates under the FI 5-min (FR 5:S) schedule; rates maintained by NMPEA under the second-order schedule were comparable to those maintained under the simple FI schedule. Phenylethanolamine failed to consistently maintain responding under either schedule. Injections of PEA and NMPEA controlled overall patterns of positively accelerated responding under both schedules, whereas the local pattern of responding under the second-order schedule was under the control of both the brief stimulus presentations as well as drug delivery.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-activity relationship studies of phencyclidine derivatives in rats

Phencyclidine (PCP), a semirigid molecule containing a cyclohexane ring with vicinally attached aromatic and piperidine rings, produces characteristic discriminative stimulus properties and pupillary miosis in rats. The effectiveness of a series of aromatic and nitrogen substituted analogs of PCP in producing PCP-like discriminative stimuli and changes in pupil diameter was determined in rats trained to discriminate between saline and 3.0 mg/kg of PCP. Dexoxadrol and its optical isomer levoxadrol were also evaluated for purposes of comparison. Analogs in which the electron-density of the aromatic ring was increased (3NH2-PCP) or only slightly reduced (3F-PCP) retained PCP-like activity. A loss of PCP-like activity occurred with analogs in which the electron-density of the aromatic ring was greatly reduced (3NO2-PCP) or extended to a larger system (1NCP and 2NCP). PCP-like activity also was abolished in analogs in which the distance between the aromatic ring and the remainder of the molecule was systematically increased by one, two or three methylene units. In contrast, substitutions on the nitrogen atom altered the potency, but not the efficacy, of such analogs. Dexoxadrol produced PCP-like activity whereas its optical enantiomer levoxadrol was devoid of such activity. These findings suggest a drug receptor surface with multiple domains or subsites which recognize regions of structural overlap among the phencyclidines, dioxolanes and psychotomimetic benzomorphan derivatives.

Discriminative stimulus effects of diazepam in rats: evidence for a maximal effect

Rats were trained to discriminate between saline and either 0.3, 1.0, 3.0 or 6.0 mg/kg of diazepam in a two-choice, discrete-trial avoidance procedure. Diazepam, chlordiazepoxide, flurazepam and pentobarbital occasioned dose-related increases in diazepam-appropriate responding in all four training dose groups. Increasing the training dose of diazepam from 0.3 to 1.0 mg/kg resulted in approximately a 3-fold shift to the right in the dose-effect curves for each of these four drugs. However, increasing the training dose to 3.0 or 6.0 mg/kg did not result in additional, concomitant shifts in these dose-effect curves. Moreover, the dose-effect curves of nine additional benzodiazepine analogs also did not differ markedly in rats trained with either 1.0 or 3.0 mg/kg of diazepam. The nonbenzodiazepines ethanol, phencyclidine, cyproheptadine and ketocyclazocine failed to produce diazepam-like discriminative stimuli in rats trained with either 0.3, 1.0 or 3.0 mg/kg of diazepam. In rats trained with 1.0 mg/kg of diazepam, Ro 11-6896, but not its inactive stereoisomer Ro 11-6893, occasioned diazepam-appropriate responding. Furthermore, the selective benzodiazepine antagonist CGS8216 blocked the effects of diazepam but not the diazepam-like effects of pentobarbital. These results demonstrate that the discriminative effects of diazepam are qualitatively similar across this 20-fold range of training doses; quantitatively, the discriminative effects of diazepam appear to reach a maximum and plateau above a training dose of 1.0 mg/kg in rats.(ABSTRACT TRUNCATED AT 250 WORDS)